Method for preparing alpha-(4-isobutylphenyl)propionic acid or its precursor

Abstract: A method for preparing α-(4-isobutylphenyl)propionic acid or its precursor is here disclosed which comprises a step (I) of dehydrogenating p-isobutylethylbenzene in a gaseous phase in the presence of a dehydrogenating metal catalyst to form p-isobutylstyrene and at least one unsaturated hydrocarbon compound selected from a group A defined in Claim 1; a step (II) of reacting p-isobutyl­ styrene obtained in the step (I) with carbon monoxide and hydrogen or with carbon monoxide and water or a lower alcohol in the presence of a transition metal complex carbonylating catalyst to form α-(4-isobutylphenyl)propionic acid or its precursor; and a step (III) of hydrogenating at least one unsaturated hydrocarbon compound selected from the group A obtained in the dehydrogenation step (I) to form p-isobutylethylbenzene, and recycling the thus formed p-isobutylethylbenzene through the step (I) as the raw material of the step (I).

Description: (i) Field of the Invention The present invention relates to a method for preparing α-(4-isobutylphenyl)propionic acid or its precursor, i.e., an alkyl α-(4-isobutylphenyl)propionate, α-(4-isobutylphenyl)-propionaldehyde or 2-(4-isobutylphenyl)propanol economically and in a high purity. More specifically, the present invention relates to an ecomonical method for preparing α-(4-isobutylphenyl)propionic acid or its precursor which comprises a dehydrogenation step of dehydrogenating p-isobutylethylbenzene in a gaseous phase in the presence of a dehydrogenating metal catalyst to form olefins including p-isobutylstyrene and by-products; a carbonylation step of reacting the formed p-isobutylstyrene with carbon monoxide and water or an alcohol in the presence of a transition metal complex carbonylating catalyst to form α-(4-isobutylphenyl)propionic acid or its precursor; and a hydrogenation step of hydrogenating the formed olefin by-products to convert the same into p-isobutylethylbenzene and hydrogenating the other by-products formed in the carbonylation step to form α-(4-isobutylphenyl)propionic acid or its precursor. This α-(4-isobutylphenyl)propionic acid is a useful medicine (trade name Ibuprophen) having an alleviating effect on fever and pain and an antiphlogistic effect, as described in British Patent No. 971700 and French Patent No. 1549758. On the other hand, it is known that the alkyl α-(4-isobutylphenyl)propionate can be easily converted into α-(4-isobutylphenyl)propionic acid by hydrolysis with an acid or an alkali in a known manner. Similarly, it is also known that α-(4-isobutylphenyl)propionaldehyde and α-(4-isobutylphenyl)propanol can be easily converted into α-(4-isobutylphenyl)propionic acid by oxidation in a known manner. Therefore, these compounds can all be considered to be the precursors of α-(4-isobutylphenyl)propionic acid. (ii) Description of the Prior Art Heretofore, α-(4-isobutylphenyl)propionic acid and its precursor have been synthesized from an extremely great number of compounds as starting materials by various methods. However, in order to synthesize α-(4-isobutylphenyl)-propionic acid and its precursor at a low cost and in a high purity, the following requirements are essential: (a) The starting materials should be simple compounds. (b) The intermediates formed in each step of a method used should also be as simple and stable as possible. (c) In place of expensive reagents, inexpensive reagents or catalysts should be employed. (d) It is necessary that by-products can be effectively utilized. (e) The number of steps for the synthesis should be as few as possible. (f) Since the isobutyl group is susceptible to isomerization, it is necessary to use a reaction in which the isomerization and other undesirable side reactions are inhibited as much as possible. For example, in Japanese Patent Laid-open Publication No. 52-65243 and USP 3959364 which suggest synthetic methods for producing α-(4-isobutylphenyl)propionic acid or its alkyl esters, complicated and expensive compounds or Grignard reagents which are unstable and difficult to handle are utilized as starting materials. Therefore, these suggested methods are not always considered to be economical from an industrial viewpoint. Additionally, in Japanese Patent Laid-open Publication No. 50-4040 which discloses methods for the preparation of α-(4-isobutylphenyl)propionic acid, isobutylbenzene is used as the starting material, and aluminum chloride is used as a catalyst, and thus, the isobutyl group tends to isomerize. In addition, expensive reagents are used. In methods described in French Patent No. 1549758, British Patent Nos. 1160725 and 1549140, US Patent Nos. 3965161 and 4143229, Japanese Patent Laid-open Publication Nos. 52-57338, 52-97930, 53-18535 and 56-154428, p-isobutylacetophenone is used as the starting material. However, p-isobutylacetophenone is not considered to be an inexpensive compound for the following reason. In the most economical synthesis of p-isobutylacetophenone isobutylbenzene is used as the starting material, but it is not preferable from an economical viewpoint to convert isobutylbenzene into p-isobutylacetophenone. In order to convert the latter into p-isobutylacetophenone, it is indispensable to make use of acetyl chloride which is an expensive and unstable material, and in addition, anhydrous aluminum chloride which is very sensitive to water must be used as a reaction catalyst at least in an amount equimolar to acetyl chloride, i.e., in a large amount. For example, even if this conversion reaction proceeds stoichiometrically in a yield of 100%, as much as 700 kg anhydrous aluminium chloride must be used to manufacture 1 ton of p-isobutylacetophenone. Moreover, after the end of the reaction, 410 kg of aluminum hydroxide and 750 kg of a chloride ions, result from the inactivation of anhydrous aluminum chloride, and thus it is additionally necessary to treat 1,160 kg of wastes, the amount of which is much greater than that of the manufactured p-isobutylacetophenone. For this reason, needless to say, p-isobutylacetophenone itself as the starting material is expensive. Furthermore, the conversion of p-isobutylacetophenone into α-(4-isobutylphenyl)propionic acid or its alkyl esters proceeds via intricate intermediates, and it is fair to say that the known method is not always economical from an industrial viewpoint. Japanese Patent Laid-open Publication Nos. 52-97930 and 59-10545 and US Patent No. 4329507 suggest methods for preparing α-(4-isobutylphenyl)propionic acid from p-isobutylstyrene through a hydroformylation reaction or a Reppe reaction. These methods using p-isobutylstyrene are economically excellent as techniques for preparing α-(4-isobutylphenyl)propionic acid, because p-isobutylstyrene which is the starting material is simple and stable, and because the hydroformylation reaction and the Reppe reaction do not require expensive reagents. However, the conventional methods for producing p-isobutylstyrene require a complex reaction route or expensive reagents, so that the above-mentioned advantages are lost. It was known that ethylbenzene may be dehydrogenated to produce styrene. FR-A-23 81 005 discloses a specific technique for dehydrogenating ethylbenzene, in which a specific combination of heating and separation steps is carried out. JP-56-16423 discloses a dehydrogenation step, in which an alkylbenzene is contacted with a specific catalyst comprising a molten alloy which contains potassium. US-A-42 46 421 refers to an improved catalyst for the dehydrogenation of an alkyl aromatic compound to the corresponding alkenyl aromatic compound which, according to this reference, is used for the dehydrogenation of ethylbenzene to styrene, diethylbenzene to divinylbenzene and methylethyl pyridine to methylvinyl pyridine. Furthermore, it was known that olefinic double bonds are susceptible to carbonylation. In the case of p-prenylstyrene the carbonylation reaction may be carried out selectively (JP-58-35140). US Patent No. 4694100 discloses a method which comprises subjecting isobutylbenzene and acetaldehyde to a condensation reaction in the presence of a sulfuric acid catalyst to form 1,1-bis(p-isobutylphenyl)ethane, catalytically decomposing the latter to p-isobutylstyrene by the use of an acid catalyst, reacting the resultant compound with carbon monoxide and water or with carbon monoxide and an alcohol in the present of a carbonylation complex catalyst in order to obtain α-(4-isobutylphenyl)propionic acid or its alkyl ester. However, since the above-mentioned method employs sulfuric acid, the sulfonation reaction of isobutylbenzene which is the valuable raw material for the manufacture of 1,1-bis(p-isobutylphenyl)ethane cannot be avoided, so that a part of isobutylbenzene is lost as a sulfonated compound, which means that the method is economically unpreferable. In addition, since this condensation reaction is a dehydration reaction, sulfuric acid which is used as the catalyst is diluted with the resulting water. In order to reuse the catalyst, the diluted sulfuric acid must be treated by, for example, high-temperature distillation, in which devices are liable to corrode. Additionally, a great part of the sulfonated compound is dissolved in the sulfonic acid phase, and therefore the catalyst cannot be easily recovered by the simple distillation. In consequence, the resultant water must be removed through chemical reaction by the use of anhydrous sulfuric acid or fuming sulfuric acid, with the result that the cost of the catalyst increases. As discussed above, the conventional techniques regarding the manufacture of α-(4-isobutylphenyl)propionaldehyde or its alkyl ester are not considered yet to be economical. As already described, p-isobutylstyrene is a useful intermediate in manufacturing α-(4-isobutylphenyl)propionic acid, and if this p-isobutylstyrene is utilized, α-(4-isobutylphenyl)propionic acid can be prepared inexpensively and easily. Accordingly, a novel method to produce p-isobutylstyrene is desired. As one technique of manufacturing p-isobutylstyrene at a low cost, the dehydrogenation of p-isobutylethylbenzene can be considered. However, in the product obtained by subjecting p-isobutylethylbenzene to the dehydrogenation reaction, olefinic by-products are inevitably present to some extent together with p-isobutylstyrene. In addition, the inventors of the present application have found that some compounds of the above-mentioned olefinic by-products are sensitive to hydroformylation, hydrocarbonylation or hydroesterification, and therefore they cause troubles in the carbonylation of p-isobutylstyrene. In view of the object of the present invention that a medicine or its precursor is prepared, means must be taken as to eliminate the unpreferable effects of these olefinic by-products in the carbonylation step. SUMMARY OF THE INVENTION The present invention is directed to a technique for preparing α-(4-isobutylphenyl)propionic acid or its precursor at a low cost and in a high purity by effectively utilizing both the olefinic by-products formed in the dehydrogenation of p-isobutylethylbenzene and the by-products formed in the step of carbonylating these compounds. A first embodiment (claim 1) of the present invention relates to a method for preparing α-(4-isobutylphenyl)propionic acid or an alkyl α-(4-isobutylphenyl)propionate or α-(4-isobutylphenyl)propionaldehyde, which comprises the following steps (I), (II) and (III) : the step (I) of dehydrogenating p-isobutylethylbenzene in the gaseous phase in the presence of a dehydrogenation metal catalyst to form p-isobutylstyrene and at least one unsaturated hydrocarbon compound selected from the following group A; group A: 4-(2'-methyl-1'-propenyl)ethylbenzene, 4-(2'-methyl-1'-propenyl)vinylbenzene, 4-(2'-methyl-2'-propenyl)ethylbenzene, and 4-(2'-methyl-2'-propenyl)vinylbenzene; the step (II) of reacting p-isobutylstyrene obtained in said step (I) with carbon monoxide and hydrogen or with carbon monoxide and water or a lower alcohol in the presence of a transition metal complex carbonylation catalyst to form α-(4-isobutylphenyl)propionic acid or its precursor which is an alkyl α-(4-isobutylphenyl)propionate or α-(4-isobutylphenyl)propionaldehyde; and the step (III) of hydrogenating at least one unsaturated hydrocarbon compound selected from said group A obtained in said dehydrogenation step (I) to form p-isobutylethylbenzene, and recycling the thus formed p-isobutylethylbenzene to said step (I) as the raw material of said dehydrogenation step (I). BRIEF DESCRIPTION OF THE DRAWING Fig. 1 shows the relation between the conversion of p-isobutylethylbenzene and the selectivity of of p-isobutylstyrene in the dehydrogenation reaction of the present invention. In this drawing, solid lines indicate results of Experimental Examples 1 to 10 of the present invention. DETAILED DESCRIPTION OF THE INVENTION The present invention will be described in detail together with the other features of the present case. Dehydrogenation Step In the dehydrogenation step (I) of the method of the present invention, p-isobutylethylbenzene is dehydrogenated in the presence of a dehydrogenating metal catalyst to form p-isobutylstyrene. More specifically, this dehydrogenation step intends to prepare p-isobutylstyrene by selectively dehydrogenating only the ethyl group of p-isobutylethylbenzene. p-isobutylethylbenzene which is the raw material in the dehydrogenation step (I), may be prepared by any manufacturing process. For example, p-isobutylethylbenzene can be used which has been manufactured by reacting isobutylbenzene with ethylene in the presence of an acid catalyst. The dehydrogenation catalyst is a metal catalyst containing a metal selected from the groups VIB, VIIB, VIII, IB and IIB of the periodic table. Typical examples of the catalyst include the metals or compounds of iron, copper, zinc, nickel, palladium, platinum, cobalt, rhodium, iridium, ruthenium, chromium and molybdenum, and combinations of these metals or their compounds may be also used effectively. These metals can be used in the form of the free metal or in the form of an oxide, a chloride, a sulfide or a hydrogen-treated compound. The preferable catalyst contains at least one metal selected from iron, copper and chromium. In particular, the iron oxide catalyst and the copper-chromium catalyst containing copper oxide and chromium oxide are effective in the dehydrogenation step (I) of the present invention, since they have the high selectivity of p-isobutylstyrene. Usually, the dehydrogenation catalyst loses its activity gradually due to coking and the like, while used for a long period of time. The initial activity of the catalyst can be recovered by decoking at a high temperature of, e.g., 500°C with air or the like. Alternatively, the activity can be also recovered, if necessary, by subjecting the catalyst in a hydrogen flow at a temperature of 200 to 500°C to a hydrogen treatment. The reaction temperature of the dehydrogenation depends upon the composition of the catalyst, the contact time and the grade of the dilution, but it is usually from 300 to 650°C, preferably 400 to 650°C. When the reaction temperature is above this range, secondary reactions such as decomposition reactions and the further dehydrogenation of formed p-isobutylstyrene occur vigorously, so that the selectivity of p-isobutylstyrene decreases noticeably. In consequence, a great part of p-isobutylethylbenzene is lost, and the distribution of the products is complicated, with the result that it is difficult to separate p-isobutylstyrene and p-isobutylethylbenzene by distillation or the like. When the reaction temperature is less than the above-mentioned range, the reaction rate lowers noticably, which is not economical, though the selectivity coefficient of p-isobutylstyrene is high. Olefins formed by the dehydrogenation reaction are polymerizable, thus if they are kept at a high temperature in a high concentration in a reaction zone, a part of the p-isobutylstyrene is polymerized and lost. In order to effectively avoid this undesirable phenomenon, it is effective that the concentration of olefins is diluted with a non-reducing gas such as nitrogen gas, helium gas, argon gas, steam or oxygen gas. In addition, the olefins may be diluted with a solvent such as benzene which is hardly dehydrogenated. Furthermore, in order to maintain the catalyst activity for the dehydrogenation, steam can be fed to the reaction layer in the course of the dehydrogenation. In this case, the amount of steam is not particularly limited. The reaction system in the dehydrogenation step (I) may be any of a fixed bed, a moving bed and a fluidized bed. The reaction pressure for the dehydrogenation is not particularly limited, as long as it permits vaporizing p-isobutylstyrene produced under the above-mentioned reaction conditions. Nevertheless, the reaction pressure is usually 50 bar (kg/cm²) or less, and a range of from atmospheric pressure to 10 bar (kg/cm²) preferable and economical. The time of contact with the raw material p-isobutylethylbenzene is in the range of 0.005 to 20 seconds, preferably 0.01 to 10 seconds, more preferably 0.05 to 5 seconds. When the contact time is less than the above-mentioned range, the reaction efficiency is inconveniently low. When it is more than the above-mentioned range, the produced p-isobutylstyrene is further dehydrogenated, and the selectivity coefficient of p-isobutylstyrene lowers unpreferably. The contact time can be suitably altered in the above-mentioned range in accordance with the selected reaction system, the composition of the reaction gas, the composition of a catalyst, the reaction temperature, the preheating temperature of the raw material gas and the like. Needless to say, the dehydrogenation step (I) can be carried out by a continuous system or a batch system. The researches of the present inventors have elucidated that in the present invention, the influence of the reaction conditions and factors on the reaction can be represented by the conversion of p-isobutylethylbenzene and the selectivity coefficient of p-isobutylstyrene. That is, the relation between the optional conversion x of p-isobutylethylbenzene obtained under the above-mentioned reaction conditions and the selectivity coefficient y of p-isobutylstyrene can be represented by the linear function y = ax + b wherein a and b are inherent constants of the catalyst. Fig. 1 shows the relation between the conversion of p-isobutylethylbenzene and the selectivity coefficient of p-isobutylstyrene obtained in the examples (hereinafter referred to as "dehydrogenation performance straight line"). For example, if certain reaction conditions are fixed, a point on the dehydrogenation performance straight line corresponding to a certain conversion indicates the selectivity coefficient of p-isobutylstyrene which can be actually obtained. Therefore, the reaction conditions can be chosen so as to obtain the conversion of p-isobutylethylbenzene corresponding to the desired selectivity coefficient in accordance with the dehydrogenation performance straight line of the used dehydrogenation catalyst. For example, in the case of the copper-chromium catalyst, it is suitable in the present invention that the conversion of p-isobutylethylbenzene is maintained at 80% by weight or less, preferably 60% by weight or less, more preferably 50% by weight. Furthermore, in the case of the iron oxide catalyst, it is suitable in the present invention that the conversion of p-isobutylethylbenzene is maintained preferably at 80% by weight or less, more preferably 70% by weight or less. If the conversion is in excess of the range, the selectivity coefficient of p-isobutylstyrene deteriorates rapidly and diverges from the dehydrogenation performace straight line, so that not only by-products but also cracked products increase unpreferably. In the case that the conversion is in the above-mentioned range, the lower the conversion, the higher is the selectivity coefficient. However, the productivity of p-isobutylstyrene is the product of the conversion and the selectivity coefficient, and therefore the employment of the low conversion is unpreferable, because the separation and recovery operation of unreacted p-isobutylethylbenzene by the subsequent distillation is very burdensome. From an economical viewpoint, it is desirable that the conversion is maintained at a level of 5% by weight or more. As discussed above, if p-isobutylethylbenzene is dehydrogenated in the dehydrogenation step of the present invention, the ethyl group is exclusively dehydrogenated against conventional expectation, so that it becomes possible to prepare p-isobutylstyrene in a surprisingly high selectivity. However, depending on the reaction conditions by the above-mentioned dehydrogenation step olefinic by-products mentioned below are formed to some extent. In this dehydrogenation step (I), the olefinic by-products are any of the olefins of the following group A: group A: 4-(2'-methyl-1'-propenyl)ethylbenzene, 4-(2'-methyl-1'-propenyl)vinylbenzene, 4-(2'-methyl-2'-propenyl)ethylbenzene, and 4-(2'-methyl-2'-propenyl)vinylbenzene; Hydrogenation Any olefinic by-product can be converted into the raw material isobutylethylbenzene by hydrogenating the by-product. Therefore, after the dehydrogenation step, p-isobutylstyrene is separated and recovered by distillation, and the by-product which is the remaining unsaturated hydrocarbon compound in the group A is hydrogenated in a usual manner, so that the carbon-carbon double bond in a substituted propenyl group or a vinyl group of the by-product compound is hydrogenated to form p-isobutylethylbenzene. A part or all of the thus formed p-isobutylethylbenzene can be recycled as the raw material in the above-mentioned dehydrogenation step (I). In this way, the olefinic by-product of the dehydrogenation step can be utilized effectively. In the hydrogenation step, an expensive hydrogenating agent is not necessary, and the preferable hydrogenation can be achieved by reacting the olefinic by-product with hydrogen in the presence of a conventional known hydrogenating metal catalyst. The hydrogenation catalyst in the hydrogenation step can be suitably selected from the conventional known hydrogenation metallic catalysts which hydrogenate the ethylenic carbon-carbon unsaturated double bond but do not hydrogenate the nucleus of an aromatic ring. Typical examples of these catalysts include metal catalysts containing metals such as Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, Re, Mo, W, Cr and Ta. These metal catalysts, when used, can be supported on a suitable carrier such as silica, silica-alumina, pumice stone or carbon. The reaction product depends upon the kind of catalyst, conditions of the hydrogenation reaction and the like, but the reaction product can be selected considering the activity of the catalyst under the condition that the ethylenic carbon-carbon double bond is hydrogenated but the nucleus of the aromatic ring is not hydrogenated. In general, the reaction temperature of the hydrogenation is from ordinary temperature to 300°C, and a hydrogen pressure is from atmospheric pressure to 300 bar (kg/cm²). A solvent for the hydrogenation of the present invention can be used without any limitation, so long as it does not impede the purpose of this step. Carbonylation In the step (II) of the present invention, p-isobutylstyrene obtained in the dehydrogenation step (I) is carbonylated to form α-(4-isobutylphenyl)-propionic acid or its precursor. This carbonylation, includes a hydroformylation by a reaction with carbon monoxide and hydrogen to produce an aldehyde, hydrocarboxylation by a reaction with carbon monoxide and water to produce an acid, and hydroesterification by a reaction with carbon monoxide and a lower alcohol to produce an ester. In the above-mentioned p-isobutylethylbenzene dehydrogenation step, p-isobutylstyrene is formed as the main component, but any of the four olefinic by-products of the group A is also formed to some extent depending on the reaction condition. Needless to say, in the carbonylation step, it is preferable that the olefinic by-product is not contained in p-isobutylstyrene. Therefore, according to one procedure of the present invention, the olefinic by-product is separated and removed by distillation or the like, and high-purity p-isobutylstyrene is then fed to the subsequent carbonylation step. This procedure is preferable, because when the thus separated olefin by-product is hydrogenated, p-isobutylethylbenzene is formed which can be recycled as the raw material in the dehydrogenation step. However, the boiling point of the component to be separated is close to that of p-isobutylstyrene, and therefore the means, e.g., distillation for separating the olefinic by-product from the dehydrogenated reaction product is not always effective. Here, the present inventors have found that even if a mixture of p-isobutylstyrene and any of the above-mentioned four olefinic by-products is carbonylated together, α-(4-isobutylphenyl)propionic acid or its precursor can be prepared at a low cost and in a high purity, whereby the object of the present invention can be achieved. That is, p-isobutylstyrene obtained in the previous dehydrogenation step (I) can be fed to the subsequent carbonylation step in the form of a mixture containing at least one olefinic by-product or if necessary, only after a simple distillation, without separating and removing the olefin by-product completely. Therefore a preferable material to be fed to the next step, i.e the carbonylation step (II) in the present invention is a mixture comprising p-isobutylstyrene and at least one unsaturated hydrocarbon compound selected from the group consisting of 4-(2'-methyl-1'-propenyl)ethylbenzene and 4-(2'-methyl-2'-propenyl)ethylbenzene, a mixture comprising p-isobutylstyrene and at least one unsaturated hydrocarbon compound selected from the group consisting of 4-(2'-methyl-1'-propenyl)vinylbenzene and 4-(2'-methyl-2'-propenyl)vinylbenzene, or a mixture thereof. Hydroformylation In the first place, reference will be made to the hydroformylation reaction of the carbonylation step (II) in which p-isobutylstyrene is reacted with carbon monoxide and hydrogen. In the hydroformylation step of the present invention, p-isobutylstyrene obtained in the above-mentioned dehydrogenation step (I) is converted into α-(4-isobutylphenyl)-propionaldehyde by a hydroformylation using carbon monoxide and hydrogen in the presence of a transition metal complex catalyst. It has also surprisingly been found that specific carbonylation conditions function to inhibit the activity of the propenyl substituent group in 4-(2'-methyl-1'-propenyl)ethylbenzene, 4-(2'-methyl-1'-propenyl)vinylbenzene, 4-(2'-methyl-2'-propenyl)ethylbenzene or 4-(2'-methyl-2'-propenyl)vinylbenzene which is the olefinic by-product in the group A, more than that of a vinyl group in the olefinic by-product. This fact will be further described in detail: The compounds 4-(2'-methyl-1'-propenyl)ethylbenzene and 4-(2'-methyl-2'-propenyl)ethylbenzene of the four olefinic by-products scarcely react under the specific carbonylation conditions of the present invention. On the other hand, under the same carbonylation conditions, 4-(2'-methyl-1'-propenyl)vinylbenzene and 4-(2'-methyl-2'-propenyl)vinylbenzene are substantially hydroformylated only on the vinyl group thereof, but the propenyl group present as a substituent substantially scarcely reacts. That is, under the specific hydroformylation conditions of the present invention, 4-(2'-methyl-1'-propenyl)vinylbenzene is hydroformylated to α-[4-(2'-methyl-1'-propenyl)-phenyl]propionaldehyde, and 4-(2'-methyl-2'-propenyl)vinylbenzene is hydroformylated to α-[4-(2'-methyl-2'-propenyl)-phenyl]propionaldehyde. In the case that a simple distillation has been used to separate the olefinic by-product, the above-mentioned reaction results can be obtained similarly both when the olefin by-product is carbonylated singly and when the mixture of the four olefin by-products in which p-isobutylstyrene may be present is carbonylated. Needless to say, when p-isobutylstyrene is carbonylated in the form of the olefin mixture containing any of the olefinic by-products, p-isobutylstyrene can be similarly converted into α-(4-isobutylphenyl)propionaldehyde efficiently in the hydroformylation step. The transition metal complex carbonylation catalyst used in the hydroformylation contains a transition metal such as palladium, rhodium, iridium or ruthenium. The usable transition metal can have an oxidation number of from 0 to a maximum value, and the usable complex contains, as a ligand, a halogen atom, a trivalent phosphorus compound, a π-allyl group, an amine, a nitrile, an oxime, an olefin, carbon monoxide or hydrogen. Typical examples of the catalyst include bistriphenylphosphinedichloro complex, bistributylphosphinedichloro complex, bistricyclohexylphosphinedichloro complex, π-allyl-triphenylphosphinedichloro complex, triphenylphosphine-piperidinedichloro complex, bisbenzonitriledichloro complex, biscyclohexyloximedichloro complex, 1,5,9-cyclododecatrienedichloro complex, bistriphenylphosphinedicarbonyl complex, bistriphenylphosphine acetate complex, bistriphenylphosphine dinitrate complex, bistriphenylphosphine sulfate complex, tetrakistriphenylphosphine complex, chlorocarbonylbistriphenylphosphine complex having carbon monoxide as a part of the ligands, hydridocarbonyltristriphenylphosphine complex, bischlorotetracarbonyl complex and dicarbonylacetyl acetonate complex. The catalyst can be used by feeding the same to the reaction system in the form of the complex, or alternatively, a compound which will be a ligand is separately fed to the reaction system, and the complex is formed therefrom in the system and then used as the carbonylation catalyst. This procedure can be carried out by simultaneously adding, to the reaction system, an oxide, a sulfate or a chloride of the above-mentioned transition metal and a compound, which will be the ligand, such as a phosphine, a nitrile, an allyl compound, an amine, an oxime, an olefin, carbon monoxide or hydrogen. Examples of the phosphine are triphenylphophine, tritolylphophine, tributylphosphine, tricyclohexylphosphine and triethylphosphine; examples of the nitrile include benzonitrile, acrylonitrile, propionitrile and benzylnitrile; examples of the allyl compound are allyl chloride and allyl alcohol; examples of the amine are benzylamine, pyridine, piperazine and tri-n-butylamine; and examples of the oxime are cyclohexyloxime, acetoxime, benzoaldoxime; and examples of the olefin are 1,5-cyclooctadiene and 1,5,9-cyclododecatriene. The amount of the complex catalyst or the compound which is used to produce the complex is in the range of 0.0001 to 0.5 mol, preferably 0.001 to 0.1 mol per mol of an olefin such as p-isobutylstyrene. Furthermore, the amount of the compound which will be the ligand is in the range of 0.8 to 10 mols, preferably 1 to 4 mols per mol of a transition metal, which will be the nucleus of the complex, such as palladium, rhodium, iridium or ruthenium. For the purpose of accelerating the reaction, it is possible to add an inorganic halide such as hydrogen chloride or boron trifluoride, or an organic iodide such as methyl iodide. The amount of the halide to be added is in the range of 0.1 to 30 moles, preferably 1 to 15- moles per as much as one mole of the complex catalyst or the compound used to form the complex. When the amount of the halide is less than 0.1 mole, the effect of the added halide cannot be perceived, depending upon the kind of catalyst. Inversely, when it is more than 30- moles, the activity of the catalyst declines unexpectedly, and the halogen is added to the double bond of p-isobutylstyrene, which inhibits the intended reaction. The hydroformylating reaction is performed at a reaction temperature of 40 to 150°C, preferably 55 to 110°C. When the reaction temperature is less than 40°C, a reaction rate is too low to be practical; when it is more than 150°C, secondary reactions such as polymerization and hydrogenation as well as the decomposition of the complex catalyst tend to occur inconveniently. The reaction pressure can be suitable selected, in so far as it is 10 bar (kg/cm²) or more. When the reaction pressure is less than 10 bar (kg/cm²), the reaction rate is too low to be practical. The higher the reaction pressure is, the higher is the reaction rate. However, when the pressure is too high, it is required to heighten pressure resistance of a reaction vessel, and therefore limitation is naturally present. In practice, the upper limit of the pressure is usually 600 bar (kg/cm²). The reaction is performed until the mixed gas of carbon monoxide and hydrogen is not absorbed any more, and usually the reaction time is in the range of 4 to 20 hours. Carbon monoxide and hydrogen which are necessary for the reaction can be fed to the reaction vessel in the state of a mixed gas or separately. The molar ratio between carbon monoxide and hydrogen can be suitably selected, when they are fed to the reaction system. However, in the hydroformylating reaction in the hydroformylation step of the present invention, carbon monoxide and hydrogen are absorbed and consumed in a molar ratio of 1:1 accurately. Therefore, the feed of carbon monoxide and hydrogen in a molar ratio of 1:1 is most effective, depending upon the size of the reaction vessel and the system of the reaction. In the hydroformylation step of the present invention, a solvent which is inert to the hydroformylation can be used with the view of removing reaction heat, and the like. Examples of the solvent which is inert to the hydroformylation include polar solvents such as ethers, ketones and alcohols as well as nonpolar solvents such as paraffins, cycloparaffins and aromatic hydrocarbons. Even under solvent-free conditions, however, sufficiently satisfactory effects can usually be obtained. After completion of the hydroformylation reaction, the reaction product is separated by distillation preferably under reduced pressure, so that the solvent can be easily removed therefrom. The thus recovered complex catalyst can be used again. It is important that the hydroformylation is carried out at the reaction temperature and under reaction pressure in the above-mentioned respective ranges. When these conditions deviate from these ranges, particularly when the reaction temperature or reaction pressure is higher than the above limited range, 4-(2'-methyl-1'-propenyl)ethylbenzene and 4-(2'-methyl-2'-propenyl)ethylbenzene of the olefin by-products in the group A react sometimes, and furthermore 4-(2'-methyl-1'-propenyl)vinylbenzene and 4-(2'-methyl-2'-propenyl)vinylbenzene are carbonylated unpreferably not only on the vinyl groups thereof but also on the propenyl groups thereof. If p-isobutylstyrene is fed to the carbonylation step (II) in the form of its mixture containing any one of the olefinic by-products in the group A, an unsaturated aldehyde having an ethylenic double bond of the following group B is formed as a by-product in addition to α-(4-isobutylphenyl)-propionaldehyde: Group B: α-[4-(2'-methyl-1'-propenyl)phenyl]propionaldehyde and α-[4-(2'-methyl-2'-propenyl)phenyl]propionaldehyde. As described above, 4-(2'-methyl-1'-propenyl)ethylbenzene and 4-(2'-methyl-2'-propenyl)ethylbenzene scarcely react under the carbonylation reaction conditions of the present invetnion, and as a result, they are present as unreacted components in the carbonylated product. The unreacted olefins which contain 4-(2'-methyl-1'-propenyl)ethylbenzene and 4-(2'-methyl-2'-propenyl)ethylbenzene and in which the unreacted p-isobutylstyrene is present can be easily separated and recovered, since the boiling points of these components are different from those of the produced aldehydes. All of these unreacted olefins can be converted into p-isobutylethylbenzene by hydrogenation in a usual manner. This hydrogenation can be achieved by the same procedure as in the hydrogenation of the olefinic by-products in the group A which has been already described in the previous hyrogenation step. p-Isobutylethylbenzene obtained by hydrogenating the unreacted olefins can be recycled partially or wholly as the raw material in the dehydrogenation step (I). After the hydroformylation, if necessary, α-(4-isobutylphenyl)propionaldehyde and the aldehydes in the group B may be separated by distillation. The separated α-(4-isobutylphenyl)propionaldehyde can be easily converted into α-(4-isobutylphenyl)propionic acid by oxidation in a usual manner which will be described hereinafter. With regard to the aldehydes of group B, these may be subjected to hydrogenation, so that the ethylenic double bond only in the propenyl substituent group of the aldehyde is selectively hydrogenated. In this way, α-(4-isobutylphenyl)propionaldehyde which is the desired compound of the hydroformylation step can be prepared from the aldehyde by-product of the carbonylation step. This selective hydrogenation can be carried out in the presence of a suitable hydrogenating agent, for example, a hydrogenation metal such as lithium aluminium hydride in a usual manner. Alternatively, the hydrogenation can be also performed by the reaction with hydrogen in the presence of the hydrogenation metal catalyst, as in the case of the technique of hydrogenating the olefinic by-product of the above-mentioned group A. If a hydrogenation process in which the selectivity is not so high is employed at the time of the hydrogenation of the unsaturated aldehyde, the carbon-carbon double bond of the aldehyde is predominantly reduced, but simultaneously the aldehyde group also tends to be reduced. Similarly in the case that the unsaturated aldehyde in the group B is reacted with hydrogen in the presence of the above-mentioned hydrogenation metal catalyst, the unsaturated aldehyde is hydrogenated not only on the carbon-carbon double bond in the propenyl substituent group thereof but also on the carbon-oxygen double bond in the formyl group thereof by employing high-pressure hydrogen or a high hydrogenation temperature to form 2-(4-isobutylphenyl)propanol, depending upon the kind of metal catalyst. For example, when the compound in the group B is hydrogenated with Ni-Cr₂O₃-acidic china clay or 5% Pd-Al₂O₃, α-(4-isobutylphenyl)propionaldehyde can be obtained in a high yield, and if reaction conditions are made more rigorous, the production of 2-(4-isobutylphenyl)propanol increases. However, the ability of Pd to hydrogenate the formyl group is usually poor, and therefore α-(4-isobutylphenyl)propionaldehyde is mainly produced, but the ability of Pt, Ni or Cu to hydrogenate the formyl group is high, and thus 2-(4-isobutylphenyl)propanol increases. In this connection, under such conditions that the unsaturated aldehyde is severely reduced as far as 2-(4-isobutylphenyl)-propanol, α-(4-isobutylphenyl)propionaldehyde is also usually reduced and converted into 2-(4-isobutylphenyl)-propanol. However, in the present invention, the production of 2-(4-isobutylphenyl)propanol involves no problem, because 2-(4-isobutylphenyl)propanol can be easily converted into α-(4-isobutylphenyl)propionic acid by oxidizing its hydroxyl group in a known manner, in other words because this alcohol is the precursor of the α-(4-isobutylphenyl)propionic acid. Conveniently, this oxidation can be accomplished by the same procedure as in the oxidation technique of α-(4-isobutylphenyl)propionaldehyde to an acid. Thus, the hydrogenation can be applied to the mixture of α-(4-isobutylphenyl)propionaldehyde and the unsaturated aldehyde in the group B. Therefore, in order to hydrogenate the unsaturated aldehyde of the group B, there can be also utilized a method in which the selectivity to the reduction of the carbon-carbon double bond is not so high, for example, the above-mentioned way of hydrogenating the olefinic by-product of the group A. In consequence, the preferable hydrogenation method in the present invention comprises directly hydrogenating the carbonylated material without any treatment, or alternatively first recovering α-(4-isobutylphenyl)propionaldehyde in the form of a mixture containing the unsaturated aldehyde in the group B by simple distillation, and then hydrogenating this mixture. This hydrogenation is carried out so as to hydrogenate at least carbon-carbon double bond in the substituted propenyl group of the aldehyde in the group B. A more preferable hydrogenation manner comprises selectively hydrogenating the carbon-carbon double bond alone in the substituted propenyl group of the aldehyde in the group B present in the mixture. If the hydrogenation is selectively performed in such a manner, the resulting product is only α-(4-isobutylphenyl)propionaldehyde. As the method of oxidizing the hydroxyl group or formyl group of α-(4-isobutylphenyl)propionaldehyde or 2-(4-ispbutylphenyl)propanol obtained in the carbonylation step or the subsequent hydrogenation step in order to form α-(4-isobutylphenyl)-propionic acid, any conventional oxidation procedure can be used. Since the 2-(4-isobutylphenyl)propanol is a primary alcohol, the oxidation of the alcohol can be achieved by employing a usual oxidation technique by which such a kind of primary alcohol is oxidized and converted into a carboxylic acid. In particular, a method using an oxidizing agent such as K₂Cr₂O₇, KMnO₄, NaOCl, NaOBr or NaOI is preferable, since in that case the yield of α-(4-isobutylphenyl)propionic acid is high. In the case that the phase of the oxidation reaction mixture becomes a non-uniform liquid-liquid phase, depending upon the kinds of oxidizing agent and solvent, a phase transfer catalyst such as methyltrioctylammonium chloride is also effective in the oxidation reaction of the present invention. Moreover, a method of oxidizing α-(4-isobutylphenyl)propionaldehyde so as to covert the latter into α-(4-isobutylphenyl)propionic acid is also utilized, in place of the oxidation of 2-(4-isobutylphenyl)propanol. In this case, the amount of the oxidizing agent is about half the amount of an oxidizing agent necessary to oxidize 2-(4-isobutylphenyl)propanol to α-(4-isobutylphenyl)propionic acid. In any case, the reaction temperature in the oxidation step is usually less than room temperature, preferably 0°C or less. When the reaction temperature is higher than this level, secondary reactions such as the oxidation of an isobutyl group increase unpreferably. Carbonylation (Hydrocarboxylation or hydroesterification) Next, reference will be made to the method for converting p-isobutylstyrene into α-(4-isobutylphenyl)propionic acid or its alkyl esters by hydrocarboxylation or hydroesterification. In the hydrocarboxylation reaction, p-isobutylstyrene is reacted with carbon monoxide and water to form α-(4-isobutylphenyl)propionic acid. Moreover, in the hydroesterification reaction, p-isobutylstyrene is reacted with carbon monoxide and optionaly a lower alcohol having a lower alkyl group to form an alkyl ester of α-(4-isobutylphenyl)-propionic acid. For example, when methyl alcohol is used, methyl α-(4-isobutylphenyl)propionate is formed. Under specific reaction conditions of the hydrocarboxylation or hydroesterification, the olefinic by-product in the group A reacts as follows: 4-(2'-Methyl-1'-propenyl)ethylbenzene and 4-(2'-methyl-2'-propenyl)ethylbenzene scarcely react. On the other hand, under the specific reaction conditions of the present invention, 4-(2'-methyl-1'-propenyl)vinylbenzene and 4-(2'-methyl-2'-propenyl)vinylbenzene are substantially hydrocarboxylated or hydroesterified only on the vinyl group thereof, and the substituted propenyl group thereof scarcely changes. That is, 4-(2'-methyl-1'-propenyl)vinylbenzene is hydrocarboxylated or hydroesterified to α-[4-(2'-methyl-1'-propenyl)phenyl]propionic acid or its alkyl esters, and 4-(2'-methyl-2'-propenyl)vinylbenzene is hydrocarboxylated or hydroesterified to α-[4-(2'-methyl-2'-propenyl)phenyl]-propionic acid or its alkyl esters. Whether the target of the hydrocarboxylation or hydroesterification is the olefinic by-product alone in the above-mentioned group A or an olefin mixture of p-isobutylstyrene and the olefinic by-product, similar reaction results will be obtained under the specific reaction conditions of the present invention. Examples of the transition metal complex catalyst used in the above-mentioned hydrocarboxylation or hydroesterification are complex catalysts of transition metals such as palladium, rhodium and iridium, and a particularly preferable catalyst is a complex of palladium. The transition metal has a ligand, and therefore it is used in the form of the transition metal containing the ligand. Examples of the ligand are halogen atoms, trivalent phosphorus compounds and carbon monoxide, but carbon monoxide is used in the form of a carbonyl complex. the transition metal can be used as described above,, but in the case of palladium, any valency of 0 to 2 can be used. Typical examples of the catalyst for the hydrocarboxylation or hydroesterification include bistriphenylphosphinedichloro complex, bistributylphosphinedichloro complex, bistricyclohexylphosphinedichloro complex, π-allyltriphenylphosphinedichloro complex, triphenylphosphinepiperidinedichloro complex, bisbenzonitriledichloro complex, biscyclohexyloximedichloro complex, 1,5,9-cyclododecatriene-dichloro complex, bistriphenylphosphinedicarbonyl complex, bistriphenylphosphine acetate complex, bistriphenylphosphine dinitrate complex, bistriphenylphosphine sulfate complex, tetrakistriphenylphosphine complex, chlorocarbonylbistriphenylphosphine complex having carbon monoxide as a part of a ligand, hydridocarbonyltristriphenylphosphine complex, bischlorotetracarbonyl complex and dicarbonylacetyl acetonate complex. The catalyst can be used by feeding the same to the reaction system in the form of the complex, or alternatively, a compound which will be a ligand is separately fed to the reaction system, and the complex is formed therefrom in the system and then used as the catalyst. The amount of the complex catalyst or the compound capable of forming the complex within the reaction system is from 0.0001 to 0.5 mole, preferably 0.001 to 0.1 mole, with respect to 1 mole of an olefin such as p-isobutylstyrene. Furthermore, the amount of the compound which will be the ligand is from 0.8 to 10 moles, preferably 1 to 4 moles with respect to 1 mole of the transition metal which will be the nucleus of the complex such as palladium, rhodium or iridium. For the purpose of accelerating the reaction, an inorganic halide such as hydrogen chloride or boron trifluoride may be added to the reaction system. The amount of such a halide is from 0.1 to 30 moles, preferably 1 to 15 moles in terms of a halogen atom per 1 mol of the complex catalyst or the compound capable of forming the complex. When the amount of the halide is less than 0.1 mole, the effect of the added halide is not perceptible sometimes, depending upon the kind of catalyst. When it is in excess of 30 moles, the activity of the catalyst deteriorates reversely, and a halogen atom is added to the double bond of p-isobutylstyrene, so that the intended reaction is inhibited unpreferably. The hydrocarboxylation or hydroesterification reaction is carried out at a temperature of 40 to 250°C, preferably 70 to 120°C. When the reaction temperature is less than 40°C, the reaction rate is too low to actually achieve the hydrocarboxylation or hydroesterification. When it is more than 250°C, polymerization reaction and the decomposition of the complex catalyst occur unpreferably. The reaction pressure is 5 bar (kg/cm²) or more. When the pressure is less 5 bar (kg/cm²), a reaction rate is too low to acturally achieve the hydrocarboxylation or hydroesterification. The higher the pressure of carbon monoxide is, the faster the reaction proceeds, but when the pressure is too high, it is required to sufficiently heighten the pressure resistance of a reactor. Therefore, it is natural that the upper limit of the reaction pressure is present, and it is 600 bar (kg/cm²) in practice. The reaction is allowed to proceed until carbon monoxide is not absorbed any more, and the reaction time is usually in the range of 4 to 20 hours. Examples of the usable alcohol include lower alcohols having 1 to 4 carbon atoms such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol and isobutyl alcohol, and above all, methyl alcohol is preferable. After completion of the hydrocarboxylation or hydroesterification reaction, the catalyst can be easily separated from the reaction product by extraction or distillation. The recovered complex catalyst can be reused. It is important that the hydrocarboxylation or hydroesterification is carried out under conditions of the reaction temperature, reaction pressure and the like in the above-mentioned respective ranges. When these conditions deviate from these ranges, particularly when the reaction temperature or reaction pressure is higher than the above limited range, 4-(2'-methyl-1'-propenyl)ethylbenzene and 4-(2'-methyl-2'-propenyl)ethylbenzene of the olefin by-products in the group A react sometimes, and furthermore 4-(2'-methyl-1'-propenyl)vinylbenzene and 4-(2'-methyl-2'-propenyl)vinylbenzene are carbonylated unpreferably not only on the vinyl groups thereof but also on the propenyl groups thereof. In the hydrocarboxylation step, α-(4-isobutylphenyl)-propionic acid is obtained from p-isobutylstyrene. Furthermore, in the hydroesterification step, α-(4-isobutylphenyl)propionic acid obtained from p-isobutylstyrene is esterified to form an alkyl ester, and the latter is then easily converted into α-(4-isobutylphenyl)propionic acid by hydrolysis in the presence of an acid or alkali catalyst in a usual manner. In the case that the olefin mixture of p-isobutylstyrene and the olefinic by-product in the group A is hydrocarboxylated or hydroesterified under the specific conditions of the present invention, any one of unsaturated compounds in the following group C is formed in addition to α-(4-isobutylphenyl)propionic acid or its alkyl ester: Group C: α-[4-(2'-methyl-1'-propenyl)phenyl]propionic acid or its alkyl ester and α-[4-(2'-methyl-2'-propenyl)phenyl]propionic acid or its alkyl ester. Therefore, p-isobutylstyrene in the olefin mixture is hydrocarboxylated or hydroesterified, and α-(4-isobutylphenyl)propionic acid or its alkyl ester is then recovered. On the other hand, the unsaturated propionic acid or its alkyl ester of the by-products in the group C is hydrogenated on the substituted propenyl group thereof so as to be easily converted into α-(4-isobutylphenyl)propionic acid or its alkyl ester which is the desired compound of this carbonylation step. This procedure is beneficial, since the unsaturated by-product in the group C can be also converted into the desired compound. Moreover, also the mixture containing α-(4-isobutylphenyl)propionic acid or its alkyl ester and the unsaturated compound in the group C may be subjected to hydrogenation This hydrogenation can be achieved by the same procedure as in the hydrogenation of the olefinic by-product in the above-mentioned group A. That is, the hydrogenation is carried out- by the use of a suitable hydrogenating agent such as a metal hydride, but in general, by reaction with hydrogen in the presence of a hydrogenation metal catalyst. The hydrogenation catalyst in the hydrogenation step can be suitably selected from the conventional known hydrogenating metal catalysts which hydrogenate an ethylenic carbon-carbon unsaturated double bond but are inert to the nucleus of an aromatic ring. Typical examples of these catalysts include metal catalysts containing at least one of metals such as Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, Re, Mo, W, Cr and Ta. These metal catalysts, when used, can be supported on a suitable carrier such as silica, silica-alumina, pumice stone or carbon. The conditions of the hydrogenation can be suitably selected, taking an activity of the catalyst into consideration, provided that the nucleus of the aromatic ring is not hydrogenated. In general, the reaction temperature of the hydrogenation is from ordinary temperature to 300°C, and the hydrogen pressure is from atmospheric pressure to 300 bar (kg/cm²). As described above, in the carbonylation step of the olefin mixture, 4-(2'-methyl-1'-propenyl)ethylbenzene and 4-(2'-methyl-2'-propenyl)ethylbenzene do not react substantially, and as a result, they are present as unreacted components in the reaction product. These unreacted components can be easily separated and recovered by usual distillation, since boiling points of these components are different from a boiling point of the produced acid or ester. All of these unreacted olefins can be converted into p-isobutylethylbenzene by hydrogenation. This hydrogenation can be achieved by the same procedure as in the hydrogenation of the olefin by-products in the group A which has been already described. The thus recovered p-isobutylethylbenzene can be recycled partially or wholly as the raw material in the dehydrogenation step. After the hydrogenation, if necessary, α-(4-isobutylphenyl)propionic acid or its alkyl esters is recovered by distillation. The alkyl α-(4-isobutylphenyl)propionate is then hydrolyzed in a usual manner in order to obtain α-(4-isobutylphenyl)propionic acid, as described above. The thus obtained α-(4-isobutylphenyl)propionic acid can be suitably recrystallized. The present invention permits industrially and economically preparing α-(4-isobutylphenyl)propionic acid and its precursor, i.e., alkyl α-(4-isobutylphenyl)propionate or α-(4-isobutylphenyl)propionaldehyde by selectively dehydrogenating the ethyl group of p-isobutylethylbenzene; converting the latter into p-isobutylstyrene effectively; and selectively carbonylating the obtained p-isobutylstyrene fraction. When the dehydrogenation of p-isobutylethylbenzene is carried out under specific conditions in the dehydrogenation step of the present invention, p-isobutylstyrene can be prepared with a high selectivity coefficient. Therefore, high-purity p-isobutylstyrene and unreacted p-isobutylethylbenzene can be prepared by merely subjecting the dehydrogenated reaction mixture obtained by the method of the present invention to two or three simple unit operations such as the separation from a water layer, drying and distillation. Furthermore, the unreacted p-isobutylethylbenzene can be recovered and reused as the raw material for the dehydrogenation. On the other hand, olefinic by-products of the dehydrogenation step, i.e., 4-(2'-methyl-1'-propenyl)-ethylbenzene, 4-(2'-methyl-1'-propenyl)vinylbenzene, 4-(2'-methyl-2'-propenyl)ethylbenzene and 4-(2'-methyl-2'-propenyl)vinylbenzene are, after separation, converted into p-isobutylethylbenzene by the hydrogenation, and this product can be reused as the raw material for the dehydrogenation in the dehydrogenation step (I). The carbonylated reaction mixture obtained in the carbonylation step (II) can be separated and recovered by a simple distillation under reduced pressure or extraction, so that high-purity α-(4-isobutylphenyl)propionic acid and its precursor, i.e., alkyl α-(4-isobutylphenyl)propionate or α-(4-isobutylphenyl)propionaldehyde can be separated and recovered. In the present invention, an olefin mixture containing an olefin by-product of group A in addition to the p-isobutylstyrene fraction can be fed to the carbonylation step (II). In this case, a reaction mixture obtained in the carbonylation step (II) can be hydrogenated directly without any treatment or after a suitable simple distillation. Secondary carbonylated product which is formed in the carbonylation of the olefin mixture containing the secondary olefin, i.e., α-[4-(2'-methyl-1'-propenyl)phenyl]propionic acid or its alkyl ester or α-[4-(2'-methyl-2'-propenyl)-phenyl]propionic acid or its alkyl ester can be easily converted, by the hydrogenation, into α-(4-isobutylphenyl)-propionic acid or its alkyl ester which is the desired compound in the carbonylation step. When α-[4-(2'-methyl-1'-propenyl)phenyl]propionaldehyde and α-[4-(2'-methyl-2'-propenyl)phenyl]propionaldehyde which are the by-products in the hydroformylation step are selectively hydrogenated, α-(4-isobutylphenyl)propionaldehyde is obtained. Furthermore, such by-products may be converted into 2-(4-isobutylphenyl)propanol by the hydrogenation. α-(4-Isobutylphenyl)propionaldehyde or 2-(4-isobutylphenyl)propanol can be easily similarly oxidized, so that α-(4-isobutylphenyl)propionic acid which is the desired compound can be obtained. Therefore, in the present invention, it is not necessary to accurately distill p-isobutylstyrene after the dehydrogenation. In addition, the by-products in the dehydrogenation and the carbonylation step can be also recovered effectively as the desired compound. EXAMPLES Method for Preparing α-(4-Isobutylphenyl)propionaldehyde: As described in the undermentioned examples, dehydrogenation step, hydroformylation step and hydrogenation step were carried out. Preparation of p-isobutylstyrene: Experimental Example 1 An iron oxide dehydrogenation catalyst containing potassium and chromium as promotors (trade name G-64A; made by Nissan Gardlar Co., Ltd.) was regulated so as to have a grain diameter of 1 to 2 mm, and a stainless steel pipe having an inner diameter of 12 mm and a length of 1 m was filled with 20 ml of the dehydrogenation catalyst. p-Isobutylethylbenzene (hereinafter referred to as "PBE" at times) was dehydrogenated by passing PBE itself and water through a preheating pipe and the catalyst layer at a temperature of 550°C (a contact time with the catalyst was 0.2 second, and a molar ratio of steam to p-isobutylethylbenzene was 93), flow rates of PBE and water being 10 ml/hour and 90 ml/hour, respectively. The dehydrogenated material was then cooled, and a gas and water were separated out. Afterward, the resulting organic phase was inspected by gas chromatography to confirm the conversion of p-isobutylethylbenzene and the selectivity of p-isobutylstyrene (hereinafter referred to as "PBS" at times). The organic phase of the dehydrogenated material was principally composed of PBE, PBS, 4-(2'-methyl-1'-propenyl)-ethylbenzene (hereinafter referred to as "1-MPE" at times), 4-(2'-methyl-2'-propenyl)ethylbenzene (hereinafter referred to as "2-MPE" at times), 4-(2'-methyl-1'-propenyl)vinylbenzene (hereinafter referred to as "1-MPV") and 4-(2'-methyl-2'-propenyl)vinylbenzene (hereinafter referred to as "2-MPV"), and the composition of the same is as follows: Component Content PBE 69.3 wt% PBS 24.7 wt% 1-MPE 0.6 wt% 2-MPE 1.6 wt% 1-MPV 0.9 wt% 2-MPV 2.1 wt% Unidentified 0.8 wt% It was confirmed by the above data that the conversion of PBE was 31% and the selectivity of PBS was 83%, which means that PBS was formed by the dehydrogenation in the high selectivity. The respective components were separated from the dehydrogenated material and then identified by mass spectrometry, IR and proton NMR. As a result, it was confirmed that p-isobutylethylbenzene was all the same as what had been used as the raw material and the production of sec-butylbenzene and tert-butylbenzene was not perceived, which means that secondary reaction such as the isomerization of an isobutyl group did not take place. As for PBS, its butyl group was an isobutyl group, which was present at the p-position. Experimental Examples 2 to 5 Following the same procedure as in Experimental Example 1, dehydrogenation reaction was carried out, changing reaction temperature. The obtained results are set forth in Table 2 together with the results of Experimental Example 1. Experimental Example 2 3 1 4 5 Reaction Temp. (°C) 450 500 550 600 650 Contact Time (sec.) 0.2 0.2 0.2 0.2 0.2 Steam Ratio 93 94 92 93 94 PBE Conversion (%) 1 6 31 75 96 PBS Selectivity (%) 99 98 83 51 7 Experimental Examples 6 to 10 Following the same procedure as in Experimental Example 1, dehydrogenation reaction was carried out, changing contact time. The obtained results are set forth in Table 3. Experimental Example 6 7 8 9 10 Reaction Temp. (°C) 550 550 550 550 550 Contact Time (sec.) 0.06 0.10 0.21 0.28 0.38 Steam Ratio 96 98 96 94 96 PBE Conversion (%) 21 33 37 47 54 PBS Selectivity (%) 89 84 79 73 69 Experimental Examples 11 to 15 Following the same procedure as in Experimental Example 1, dehydrogenation reaction was carried out, using a copper-chromium dehydrogenating catalyst which was composed of 43% by weight of CuO, 42% by weight of Cr₂O₃ and 15% by weight of SiO₂, and changing reaction temperature. The obtained results are set forth in Table 4. Experimental Example 11 12 13 14 15 Reaction Temp. (°C) 450 500 550 600 650 Contact Time (sec.) 0.2 0.2 0.2 0.2 0.2 Steam Ratio 93 94 92 93 94 PBE Conversion (%) 5 8 20 50 92 PBS Selectivity (%) 80 79 74 58 5 Experimental Examples 16 to 20 Following the same procedure as in Experimental Example 1, dehydrogenation reaction was carried out, using a copper-chromium dehydrogenating catalyst which was composed of 18% by weight of Cr₂O₃, 39% by weight of CuO and 38% by weight of ZnO. The obtained results are set forth in Table 5. Experimental Example 16 17 18 19 20 Reaction Temp. (°C) 450 500 550 600 650 Contact Time (sec.) 0.2 0.2 0.2 0.2 0.2 Steam Ratio 93 93 94 93 93 PBE Conversion (%) 2 6 12 21 45 PBS Selectivity (%) 78 76 72 64 47 Experimental Example 21 Following the same procedure as in Experimental Example 1 with the exception that the metal of the dehydrogenating catalyst was replaced with each of metals in the following table, dehydrogenation reaction was carried out. In every case, the metal was used in the form of an oxide and supported on silica. The obtained results are set forth in the following table. Metal Conversion (%) Selectivity (%) Ag 31 62 Cd 12 64 Cr 22 61 Zn 13 52 Mo 16 53 W 11 59 Mn 11 61 Tc 12 60 Re 20 57 Ru 17 68 Os 12 70 Co 21 59 Rh 32 48 Ir 25 51 Ni 48 41 Pd 46 43 Pt 44 40 Preparation of α-(4-isobutylphenyl)propionaldehyde and hydrogenation of olefin product: Experimental Example 22 One kilogram of the dehydrogenated reaction mixture obtained in Experimental Example 4 was separated by distillation into 833 g of the fraction of PBE and PBS, 116 g of the fraction of ethylbenzene and vinylbenzene, having a substituted propenyl group respectively, and 51 g of a residue. This fraction of ethylbenzene and vinylbenzene having the substituted propenyl group was analyzed by gas chromatography, and the results are set forth in Table 6. Component Content PBS 10.3 wt% 1-MPE 8.6 wt% 2-MPE 21.6 wt% 1-MPV 11.2 wt% 2-MPV 29.3 wt% Unidentified 19.0 wt% In a 200-milliliter autoclave were placed 100 g of this fraction of ethylbenzene and vinylbenzene having the substituted propenyl group and 5 g of palladium black (a catalyst in which 5% of palladium was supported), and reaction was then performed at a reaction temperature of 50°C under a hydrogen pressure of 20 bar (kg/cm²), until hydrogen had not been absorbed any more. Afterwards the used catalyst was removed from the resulting reaction mixture by filtration, and the obtained filtrate was then analyzed by gas chromatography. The results are set forth in Table 7. Component Content PBE 80.7 wt% Unidentified 19.3 wt% The mixture which has undergone the hydrogenation reaction was then distilled to prepare 71.2 g of p-isobutylethylbenzene having a purity of 99.7%. Experimental Example 23 In a 200-milliliter autoclave equipped with a stirrer were placed 121.5 g of the dehydrogenated reaction solution obtained in Experimental Example 1 and 0.3 g of rhodiumhydridcarbonyltristriphenylphosphine, and the solution was then heated up to 60°C with stirring. Afterward, an equimolar mixed gas of hydrogen and carbon monoxide was fed to the solution so that a pressure of 50 kg/cm² might be reached, and reaction was then continued until the mixed gas had not been absorbed any more. After completion of the reaction, the reaction mixture was cooled to room temperature and then recovered, and it was then analyzed by gas chromatography. As a result, it was found that the conversion of p-isobutylstyrene was 99.8% and the selectivity of α-(4-isobutylphenyl)propionaldehyde was 87.8%. The total amount of hydroformylated compounds which were formed by hydroformylating substituted propenyl groups of 4-(2'-methyl-1'-propenyl)ethylbenzene, 4-(2'-methyl-2'-propenyl)ethylbenzene, 4-(2'-methyl-1'-propenyl)-vinylbenzene and 4-(2'-methyl-2'-propenyl)vinylbenzene in the reaction mixture was 1% by weight or less. Afterwards the reaction mixture was subjected to simple distillation under reduced pressure to separate the used catalyst therefrom, and the resulting distillate was then put in a 200-milliliter autoclave together with 5 g of palladium black (a catalyst in which 5% of palladium was supported). Successively, reaction was carried out at a reaction temperature of 50°C under a hydrogen pressure of 20 bar (kg/cm²) until hydrogen had not been absorbed any more. After completion of the reaction, the catalyst was removed from the reaction mixture by filtration, and the resulting filtrate was then analyzed by gas chromatography. The results are set forth in Table 8. Component Content PBE 68.1 wt% α-(4-isobutylphenyl) propionaldehyde 28.0 wt% Balance 3.9 wt% The total amount of 1-MPE, 2-MPE, 1-MPV and 2-MPV as well as hydroformylated compounds which were formed by hydroformylating substituted propenyl groups of 1-MPE, 2-MPE, 1-MPV and 2-MPV was 1% by weight or less. The above-mentioned filtrate was distilled under reduced pressure to obtain 28 g of α-(4-isobutylphenyl)-propionaldehyde having a boiling point of 70 to 76°C/3 mmHg. The purity of this α-(4-isobutylphenyl)propionaldehyde was 99.8% by weight. Furthermore, the structure of the product was confirmed by comparing the results of IR analysis with standards. Experimental Example 24 The same procedure as in Experimental Example 23 was repeated with the exception that rhodiumhydridcarbonyltristriphenylphosphine was replaced with 0.1 g of rhodium oxide and 0.6 g of triphenylphosphine, in order to perform hydroformylation. In this case, the conversion of p-isobutylstyrene was 99.8%, and the selectivity of α-(4-isobutylphenyl)propionaldehyde was 87.2%. The total amount of hydroformylated compounds which were formed by hydroformylating substituted propenyl groups of 4-(2'-methyl-1'-propenyl)ethylbenzene, 4-(2'-methyl-2'-propenyl)ethylbenzene, 4-(2'-methyl-1'-propenyl)vinylbenzene and 4-(2'-methyl-2'-propenyl)vinylbenzene in the reaction mixture was 1% by weight or less. Experimental Example 25 One kilogram of the dehydrogenated reaction mixture obtained in Experimental Example 4 was separated by simple distillation into 52.2 g of the PBE fraction, 441 g of the fraction of PBS, and ethylbenzene and vinylbenzene having a substituted propenyl group, and 35 g of a residue. The fraction of PBS and ethylbenzene and vinylbenzene having the substituted propenyl group was then analyzed by gas chromatography, and the results are set forth in Table 9. Component Content PBE 4.8 wt% PBS 72.6 wt% 1-MPE 2.0 wt% 2-MPE 5.7 wt% 1-MPV 2.9 wt% 2-MPV 7.5 wt% Unidentified 4.5 wt% In a 200-milliliter autoclave were placed 100 g of this fraction and 1.0 g of rhodiumhydridcarbonyltristriphenylphosphine, and they were then heated up to 60°C with stirring. Afterwards, an equimolar mixed gas of hydrogen and carbon monoxide was fed to the solution so that a pressure of 50 bar (kg/cm²) might be reached, and reaction was then continued until the mixed gas had not been absorbed any more. After completion of the reaction, the reaction mixture was cooled, recovered, and then subjected to simple distillation under reduced pressure to separate the used catalyst therefrom. Afterward, the resulting distillate was put in a 200-milliliter autoclave together with 5 g of palladium black (a catalyst in which 5% of palladium was supported), and reaction was then carried out at a reaction temperature of 50°C under a hydrogen pressure of 20 kg/cm² until hydrogen had not been absorbed any more. After completion of the reaction, the catalyst was removed from the reaction mixture by filtration, and the resulting filtrate was then analyzed by gas chromatography. The results are set forth in Table 10. Component Content PBE 10.0 wt% α-(4-isobutylphenyl) propionaldehyde 75.7 wt% Balance 14.3 wt% The total amount of 1-MPE, 2-MPE, 1-MPV and 2-MPV as well as hydroformylated compounds which were formed by hydroformylating substituted propenyl groups of 1-MPE, 2-MPE, 1-MPV and 2-MPV was 1% by weight or less. Experimental Example 26 Following the same procedure as in Experimental Example 25, 100 g of the fraction regarding Table 9 was hydroformylated, and the resulting reaction mixture was recovered and then subjected to simple distillation under reduced pressure to separate the used catalyst therefrom. The resulting distillate was then put in a 200-milliliter autoclave together with 5 g of a copper-chromium hydrogenating catalyst reduced with hydrogen at 200°C for 24 hours (trade name N201; made by Nikki Kagaku Co., Ltd.). Successively, reaction was carried out at a reaction temperature of 80°C under a hydrogen pressure of 20 kg/cm² until hydrogen had not been absorbed any more. After completion of the reaction, the catalyst was removed from the reaction mixture by filtration, and the resulting filtrate was then analyzed by gas chromatography. The composition of the reaction mixture is set forth in Table 11. Component Content PBE 9.7 wt% 2-(4-isobutylphenyl) propanol 76.0 wt% Balance 14.2 wt% The total amount of 1-MPE, 2-MPE, 1-MPV and 2-MPV as well as hydroformylated and hydroxylated compounds which were formed by hydroformylating and hydroxylating substituted propenyl groups of 1-MPE, 2-MPE, 1-MPV and 2-MPV was 1% by weight or less. Preparation of α-(4-isobutylphenyl)pronionic acid from α-(4-isobutylphenyl)propionaldehyde or 2-(4-isobutylphenyl)-propanol by oxidation: Experimental Example 27 In a 200-milliliter flask with a stirrer were placed 25 g of α-(4-isobutylphenyl)propionaldehyde having a boiling range of 70 to 76°C/3 mmHg obtained in Experimental Example 23, and 1 g of concentrated hydrochloric acid and 40 ml of acetone as a solvent were further put therein and these materials were cooled to -15°C. Next, while the temperature was maintained at a level of -12°C to -16°C, 36 g of a 10% aqueous sodium hypochlorite solution was gradually added dropwise thereto. After completion of the addition, reaction was further performed with stirring for 1 hour. After the reaction had been over, a 5% aqueous sodium hydroxide solution was added to the solution so as to adjust its pH to 8.5. The mixture was allowed to stand, and a separated lower aqueous phase was then washed with normal hexane. To this aqueous phase was added 5% hydrochloric acid to adjust its pH to 2, and a separated oil portion was extracted with normal hexane, followed by water washing. The used normal hexane was vaporized out under reduced pressure in order to obtain 26.8 g of light yellow crude α-(4-isobutylphenyl)propionic acid crystals. The thus obtained crude α-(4-isobutylphenyl)propionic acid was then recrystallized from a normal hexane solvent to prepare 22.6 g of white purified α-(4-isobutylphenyl)propionic acid (melting point 75-76°C) crystals. The results of spectra and the like were in accord with standards. Experimental Example 28 In a 200-milliliter flask with a stirrer was placed 40 g of the hydrogenated reaction mixture obtained in Experimental Example 25, and the same procedure as in Experimental Example 27 was repeated to perform oxidation, extraction and the like, so that 27.2 g of light yellow crude α-(4-isobutylphenyl)propionic acid crystals was obtained. The thus obtained crude α-(4-isobutylphenyl)propionic acid was recrystallized from a normal hexane solvent in order to prepare 24.1 g of white purified α-(4-isobutylphenyl)propionic acid (melting point 75-76°C) crystals. Experimental Example 29 In a 200-milliliter flask with a stirrer was placed 40 g of the hydrogenated reaction mixture obtained in Experimental Example 26, and 2 g of concentrated hydrochloric acid and 80 ml of acetone as a solvent were further put therein and they were cooled to -15°C. Next, while temperature was maintained at a level of -12 to -16°C, 72 g of a 10% aqueous sodium hypochlorite solution was gradually added dropwise thereto. After completion of the addition, reaction was further performed with stirring for 1 hour. After the reaction had been over, a 5% aqueous sodium hydroxide solution was added to the solution so as to adjust its pH to 8.5. The mixture was allowed to stand, and a separated lower aqueous phase was then washed with normal hexane. To this aqueous phase was added 5% hydrochloric acid to adjust its pH to 2, and a separated oil portion was extracted with normal hexane, followed by water washing. The used normal hexane was vaporized out under reduced pressure in order to obtain 26.2 g of light yellow crude α-(4-isobutylphenyl)propionic acid crystals. The thus obtained crude α-(4-isobutylphenyl)propionic acid was then recrystallized from a normal hexane solvent to prepare 20.9 g of white purified α-(4-isobutylphenyl)propionic acid (melting point 75-76°C) crystals. Comparative Example 1 Following the same procedure as in Experimental Example 1, p-sec-butylethylbenzene (purity 97.5% by weight) was subjected to dehydrogenation reaction. The results are set forth in Table 12. Next, a hydrocarboxylation or hydroesterification step and a hydrogenation step followed. Preparation of α-(4-isobutylphenyl)propionic acid or its alkyl ester: Experimental Example 30 (hydrocarboxylation) In a 500-milliliter autoclave were placed 50 g of p-isobutylstyrene having a purity of 97.8% by weight which had been purified by distilling the dehydrogenated reaction mixture prepared in Experimental Example 1, 5.5 g of bisdichlorotriphenylphosphine palladium, 80 g of a 10% aqueous hydrochloric acid solution and 80 ml of toluene as a solvent. Afterward, carbon monoxide was fed to the autoclave with stirring at ordinary temperature so that a pressure of 100 kg/cm² might be reached therein, and while the solution was then heated up to 120°C, the pressure in the autoclave was increased up to 300 bar (kg/cm²). Reaction was performed until carbon monoxide had not been absorbed any more, and additionally the reaction was further continued for 24 hours. After completion of the reaction, the reaction mixture was recovered and then separated into an oil layer and an aqueous layer by the use of a separatory funnel. Afterward, the oil layer was extracted three times with 50 ml of a 8% aqueous sodium hydroxide solution, and the resulting aqueous extract was mixed with the above aqueous layer. Hydrochloric acid was then added to the mixture so as to adjust a pH of the latter to 2. Next, the mixture was extracted three times with 500 ml of chloroform, and the resulting extract was exposed to conditions of reduced pressure in order to distill off chloroform, thereby obtaining 52.3 g of light yellow crystals of α-(4-isobutylphenyl)propionic acid. In this case, the conversion of p-isobutylstyrene was 100%, and the selectivity of α-(4-isobutylphenyl)propionic acid was 89.0%. Experimental Example 31 In a 500-milliliter autoclave were placed 202.43 g of the dehydrogenated material obtained in Experimental Example 1, 5.5 g of bisdichlorotriphenylphosphine palladium and 80 g of a 10% aqueous hydrochloric acid solution. Afterward, carbon monoxide was fed to the autoclave with stirring at ordinary temperature so that a pressure of 100 bar (kg/cm²) might be reached therein, and while the solution was then heated up to 120°C, the pressure in the autoclave was increased up to 300 bar (kg/cm²). Reaction was performed until carbon monoxide had not been absorbed any more, and additionally the reaction was further continued for 24 hours. After completion of the reaction, the reaction mixture was recovered and then separated into an oil layer and an aqueous layer by the use of a separatory funnel. Afterward, the oil layer was extracted three times with 50 ml of a 8% aqueous sodium hydroxide solution, and the resulting aqueous extract was mixed with the above aqueous layer. Hydrochloric acid was then added to the mixture so as to adjust a pH of the latter to 2. Next, the mixture was extracted three times with 500 ml of chloroform, and the resulting extract was exposed to conditions of reduced pressure in order to distill off chloroform, thereby obtaining 50.2 g of light yellow crystals of α-(4-isobutylphenyl)propionic acid. In this case, the conversion of p-isobutylstyrene was 100%, the selectivity of α-(4-isobutylphenyl)propionic acid was 87.3%, the hydrocarboxylation ratio of the substituted propenyl group of 4-(2'-methyl-1'-propenyl)ethylbenzene was 0%, the hydrocarboxylation ratio of the substituted propenyl group of 4-(2'-methyl-2'-propenyl)ethylbenzene was 0.8%, the hydrocarboxylation ratio of the substituted propenyl group of 4-(2'-methyl-1'-propenyl)vinylbenzene was 0%, and the hydrocarboxylation ratio of the substituted propenyl group of 4-(2'-methyl-2'-propenyl)vinylbenzene was 0.6%. Experimental Example 32 (hydroesterification) In a 200-milliliter autoclave were placed 70.4 g of p-isobutylstyrene having a purity of 97.8% by weight which had been purified by distilling the organic phase of the dehydrogenated material prepared in Experimental Example 1, 25.5 ml of methanol, 40 ml of toluene as a solvent, 0.0756 g of PdCl₂ as a catalyst, 0.0292 g of CuCl₂ as a promotor and 0.2161 g of triphenylphosphine as a ligand. Afterwards, the temperature in the autoclave was elevated up to 90°C with stirring and the pressure in the autoclave was maintained at 70 kg/cm² by feeding carbon monoxide thereto, and reaction was performed for 8 hours. After completion of the reaction, the reaction mixture was analyzed through gas chromatography, and as a result, the conversion of p-isobutylstyrene was 99.6%, and the selectivity of methyl α-(4-isobutylphenyl)propionate was 90.9%. Experimental Example 33 In a 500-milliliter autoclave were placed 285.0 g of the organic phase of the dehydrogenated material prepared in Experimental Example 1, 25.5 ml of methanol, 0.0756 g of PdCl₂ as a catalyst, 0.0292 g of CuCl₂ as a promotor and 0.2161 g of triphenylphosphine as a ligand. Afterwards the temperature in the autoclave was elevated up to 90°C with stirring and the pressure in the autoclave was maintained at 70 kg/cm² by feeding carbon monoxide thereto, and reaction was performed for 8 hours. After completion of the reaction, the reaction mixture was analyzed through gas chromatography, and as a result, the conversion of p-isobutylstyrene was 99.8%, the selectivity of methyl α-(4-isobutylphenyl)propionate was 88.9%, the hydroesterification ratio of the substituted propenyl group of 4-(2'-methyl-1'-propenyl)ethylbenzene was 0%, the hydroesterification ratio of the substituted propenyl group of 4-(2'-methyl-2'-propenyl)ethylbenzene was 0.6%, the hydroesterification ratio of the substituted propenyl group of 4-(2'-methyl-1'-propenyl)vinylbenzene was 0%, and the hydroesterification ratio of the substituted propenyl group of 4-(2'-methyl-2'-propenyl)vinylbenzene was 0.3%. Experimental Example 34 In a 500-milliliter autoclave were placed 100 g of the fraction regarding Table 9, 9.4 g of bisdichlorotriphenylphosphine palladium, 137 g of a 10% aqueous hydrochloric acid solution and 140 ml of toluene as a solvent. Afterwards carbon monoxide was fed to the autoclave at ordinary temperature with stirring so that a pressure of 100 bar (kg/cm²) might be reached therein, and the pressure was further increased up to 300 bar (kg/cm²), while the temperature in the autoclave is elevated up to 120°C. Reaction was performed until carbon monoxide had not been absorbed any more, and additionally the reaction was further continued for 24 hours. After completion of the reaction, the reaction mixture was cooled, recovered, and separated into an oil layer and an aqueous layer by the use of a separatory funnel. The oil layer was dried with anhydrous sodium sulfate and then filtered, and the resulting filtrate was then placed in a 500-milliliter autoclave together with 5 g of palladium black (a catalyst wherein 5% of palladium was supported). Reaction was performed at a reaction temperature of 50°C under a hydrogen pressure of 20 kg/cm², until hydrogen had not been absorbed any more, and the used catalyst was removed from the reaction mixture by filtration. The resulting filtrate was then analyzed through gas chromatography, and contents of components other than toluene were set forth in Table 13. Component Content PBE 9.8 wt% α-(4-isobutylphenyl) propionic acid 74.1 wt% Others 16.1 wt% In the hydrogenated reaction mixture, the total amount of 1-MPE, 2-MPE, 1-MPV and 2-MPV as well as hydrocarboxylated compounds formed by hydrocarboxylating substituted propenyl groups of 1-MPE, 2-MPE, 1-MPV and 2-MPV was 1% by weight or less. Experimental Example 35 In a 500-ml autoclave were placed 100 g of the fraction regarding Table 9, 31 ml of methanol, 50 ml of toluene as a solvent, 0.0921 g of PdCl₂ as a catalyst, 0.0355 g of CuCl₂ as a promotor and 0.2637 g of triphenylphosphine as a ligand. The temperature in the autoclave was elevated-up to 90°C with stirring, and carbon monoxide was fed to the autoclave so that the pressure therein might be maintained at 70 bar (kg/cm²), and reaction was performed for 8 hours. After the reaction, the reaction mixture was cooled and recovered, the used catalyst was separated and removed by distillation under reduced pressure. Afterwards, the reaction mixture was then placed in a 500-milliliter autoclave together with 10 g of palladium black (a catalyst wherein 5% of palladium was supported), reaction was then performed at a reaction temperature of 50°C under a hydrogen pressure of 20 bar (kg/cm²), until hydrogen had not been absorbed any more. The palladium black catalyst was removed from the reaction mixture by filtration, and the resulting filtrate was analyzed through gas chromatography, and contents of components other than toluene were as shown in Table 14. Component Content PBE 9.4 wt% α-(4-isobutylphenyl) propionic acid 74.2 wt% Others 16.4 wt% In the hydrogenated reaction mixture, the total amount of 1-MPE, 2-MPE, 1-MPV and 2-MPV as well as hydrocarboxylated compounds formed by hydrocarboxylating substituted propenyl groups of 1-MPE, 2-MPE, 1-MPV and 2-MPV was 1% by weight or less. Preparation of α-(4-isobutylphenyl)propionic acid by hydrolysis of methyl α-(4-isobutylphenyl)propionate: Experimental Example 36 Thirty grams of methyl α-(4-isobutylphenyl)propionate obtained by distilling the reaction mixture of Experimental Example 32 under reduced pressure and 150 ml of a 10% aqueous sodium hydroxide solution were refluxed with stirring in order to carry out hydrolysis for about 3 hours. After cooling, the mixture was allowed to stand, and a separated lower aqueous phase was then washed with normal hexane. Afterwards, 5% hydrochloric acid was added to the aqueous phase so as to adjust its pH to 2, and a separated oil portion was extracted with normal hexane and then washed with water. Normal hexane was vaporized out under reduced pressure in order to obtain 23.9 g of light yellow crude α-(4-isobutylphenyl)propionic acid crystals. The crude α-(4-isobutylphenyl)propionic acid was recrystallized from a normal hexane solvent, thereby obtaining 20.7 g of white purified α-(4-isobutylphenyl)-propionic acid (melting point 75-76°C) crystals. Spectra of this product were in accord with those of a control. Experimental Example 37 One hundred grams of the hydroesterified reaction mixture of Experimental Example 33 and 150 ml of a 10% aqueous sodium hydroxide solution were refluxed with stirring in order to carry out hydrolysis for about 3 hours. After cooling, the mixture was allowed to stand, and a separated lower aqueous phase was then washed with normal hexane. Afterwards, 5% hydrochloric acid was added to the aqueous phase so as to adjust its pH to 2, and a separated oil portion was extracted with normal hexane and then washed with water. Normal hexane was vaporized out under reduced pressure in order to obtain 22.4 g of light yellow crude α-(4-isobutylphenyl)propionic acid crystals. The crude α-(4-isobutylphenyl)propionic acid was then recrystallized from a normal hexane solvent, thereby obtaining 19.9 g of white purified α-(4-isobutylphenyl)-propionic acid (melting point 75-76°C) crystals. Spectra of this product were in accord with those of a control.

Claims: A method for preparing α-(4-isobutylphenyl)propionic acid or an alkyl α-(4-isobutylphenyl)propionate or α-(4-isobutylphenyl)propionaldehyde, which comprises the following steps (I), (II) and (III) : the step (I) of dehydrogenating p-isobutylethylbenzene in a gaseous phase in the presence of a dehydrogenation metal catalyst to form p-isobutylstyrene and at least one unsaturated hydrocarbon compound selected from the following group A; group A: 4-(2'-methyl-1'-propenyl)ethylbenzene, 4-(2'-methyl-1'-propenyl)vinylbenzene, 4-(2'-methyl-2'-propenyl)ethylbenzene, and 4-(2'-methyl-2'-propenyl)vinylbenzene; the step (II) of reacting p-isobutylstyrene obtained in said step (I) with carbon monoxide and hydrogen or with carbon monoxide and water or a lower alcohol in the presence of a transition metal complex carbonylation catalyst to form α-(4-isobutylphenyl)propionic acid or its precursor which is an alkyl α-(4-isobutylphenyl)propionate or α-(4-isobutylphenyl)propionaldehyde; and the step (III) of hydrogenating at least one unsaturated hydrocarbon compound selected from said group A obtained in said dehydrogenation step (I) to form p-isobutylethylbenzene, and recycling the thus formed p-isobutylethylbenzene to said step (I) as the raw material of said dehydrogenation step (I). A method according to claim 1 for preparing α-(4-isobutylphenyl)propionaldehyde, which comprises the steps (I), (II)and (III) : wherein step (II) comprises reacting a mixture of p-isobutylstyrene obtained in said step (I) and one unsaturated hydrocarbon compound of said group A with carbon monoxide and hydrogen at a reaction temperature of 40 to 150°C under a carbon monoxide/hydrogen mixed pressure of 10 to 600 bar (kg/cm²) in the presence of a transition metal complex carbonylation catalyst to form α-(4-isobutylphenyl)propionaldehyde and at least one unsaturated aldehyde selected from the following group B; group B: α-[4-(2'-methyl-1'-propenyl)phenyl]propionaldehyde and α-[4-(2'-methyl-2'-propenyl)phenyl]propionaldehyde; and step (III) comprises hydrogenating at least one unsaturated compound selected from the following group C and recycling at least a part of the latter to said dehydrogenation step (I) as the raw material of said dehydrogenation step (I); group C: 4-(2'-methyl-1'-propenyl)ethylbenzene and 4-(2'-methyl-2'-propenyl)ethylbenzene. A method according to claim 1 for preparing α-(4-isobutylphenyl)propionic acid or its alkyl esters which comprises the steps (I), (II) and (III), wherein step (II) comprises reacting a mixture of p-isobutylstyrene obtained in said step (I) and at least one unsaturated hydrocarbon compound selected from said group A with carbon monoxide and water or a lower alcohol at a reaction temperature of 40 to 250°C under a pressure of 10 to 600 bar (kg/cm²) in the presence of a transition metal complex carbonylation catalyst to form α-(4-isobutylphenyl)propionic acid or its alkyl esters and at least one unsaturated acid or its ester selected from the following group B; group B: α-[4-(2'-methyl-1'-propenyl)phenyl]propionic acid or its alkyl esters and α-[4-(2'-methyl-2'-propenyl)phenyl]propionic acid or its alkyl esters and step (III) comprises hydrogenating at least one unsaturated compound from said step (II) selected from the following group C to form p-isobutylethylbenzene, and recycling at least a part of the latter to said dehydrogenation step (I) as the raw material; the group C: 4-(2'-methyl-1'-propenyl)ethylbenzene and 4-(2'-methyl-2'-propenyl)ethylbenzene. A method for preparing α-(4-isobutylphenyl)propionaldehyde which comprises the following steps (I), (II) and (III): the step (I) of dehydrogenating p-isobutylethylbenzene in the gaseous phase in the presence of a dehydrogenation metal catalyst to form p-isobutylstyrene and at least one unsaturated hydrocarbon compound selected from the following group A; group A: 4-(2'-methyl-1'-propenyl)ethylbenzene, 4-(2'-methyl-1'-propenyl)vinylbenzene, 4-(2'-methyl-2'-propenyl)ethylbenzene, and 4-(2'-methyl-2'-propenyl)vinylbenzene; the step (II) of reacting a mixture of p-isobutylstyrene obtained in said step (I) and at least one unsaturated hydrocarbon compound selected from said group A with carbon monoxide and hydrogen at a reaction temperature of 40 to 150°C under a carbon monoxide/hydrogen mixed pressure of 10 to 600 bar (kg/cm²) in the presence of a transition metal complex carbonylation catalyst to form α-(4-isobutylphenyl)-propionaldehyde and at least one unsaturated aldehyde selected from the following group B; group B: α-[4-(2'-methyl-1'-propenyl)phenyl]propionaldehyde and α-[4-(2'-methyl-2'-propenyl)phenyl]propionaldehyde; and the step (III) of selectively hydrogenating at least one unsaturated aldehyde selected from said group B obtained in said step (II) only on the ethylenic carbon-carbon double bond thereof to prepare α-(4-isobutylphenyl)propionaldehyde. A method according to claim 4 for preparing α-(4-isobutylphenyl)propionaldehyde and 2-(4-isobutylphenyl) propanol, wherein in said step (III) the hydrogenation of at least one unsaturated aldehyde selected from said group B is conducted to form 2-(4-isobutylphenyl)propanol. A method according to claim 4 for preparing α-(4-isobutylphenyl)propionaldehyde and 2-(4-isobutylphenyl) propanol, wherein in said step (III) the hydrogenation of one unsaturated aldehyde selected from said group B is conducted to form α-(4-isobutylphenyl)propionaldehyde and 2-(4-isobutylphenyl)propanol. A method for preparing α-(4-isobutylphenyl)propionic acid or its alkyl esters which comprises the following steps (I), (II) and (III): the step (I) of dehydrogenating p-isobutylethylbenzene in the gaseous phase in the presence of a dehydrogenation metal catalyst to form p-isobutylstyrene and at least one unsaturated hydrocarbon compound selected from the following group A; group A: 4-(2'-methyl-1'-propenyl)ethylbenzene, 4-(2'-methyl-1'-propenyl)vinylbenzene, 4-(2'-methyl-2'-propenyl)ethylbenzene, and 4-(2'-methyl-2'-propenyl)vinylbenzene; the step (II) of reacting a mixture of p-isobutylstyrene obtained in said step (I) and at least one unsaturated hydrocarbon compound selected from said group A with carbon monoxide and water or a lower alcohol at a reaction temperature of 40 to 250°C under a pressure of 10 to 600 bar (kg/cm²) in the presence of a transition metal complex carbonylation catalyst to form α-(4-isobutylphenyl)propionic acid or its alkyl esters and at least one unsaturated acid or its esters selected from the following group B; group B: α-[4-(2'-methyl-1'-propenyl)phenyl]propionic acid or its alkyl esters and α-[4-(2'-methyl-2'-propenyl)phenyl]propionic acid or its alkyl esters; and the step (III) of hydrogenating at least one unsaturated acid or its esters selected from said group B obtained in said step (II) to form α-(4-isobutylphenyl)propionic acid or its alkyl esters. A method according to any one of Claims 1 to 7 wherein said dehydrogenation metal catalyst used in said step (I) contains a metal selected from the group consisting of metals of the groups VIB, VIIB, VIII, IB and IIB of the periodic table, preferably selected from the group consisting of iron, copper, zinc, nickel, palladium, platinum, cobalt, rhodium, iridium, ruthenium, chromium and molybdenum. A method according to any one of Claims 1 to 7 further containing the step of oxidizing the obtained α-(4-isobutylphenyl)propionaldehyde or 2-(4-isobutylphenyl)propanol to form α-(4-isobutylphenyl)propionic acid, and/or further containing the step of hydrolyzing the obtained alkyl α-(4-isobutylphenyl)propionate to form α-(4-isobutylphenyl)-propionic acid. A method according to any one of Claims 1 to 9 wherein said step (I) is the following step (I): the step (I) of dehydrogenating p-isobutylethylbenzene in the gaseous phase in the presence of a dehydrogenation metal catalyst containing a metal selected from the group consisting of metals in the groups IB, IIB, VIB, VIIB and VIII of the periodic table under conditions of a reaction temperature of 300 to 650°C, a reaction pressure of 50 bar (kg/cm²) or less, a contact time of 0.005 to 20 seconds and a p-isobutylethylbenzene conversion of 80% by weight or less, in order to form p-isobutylstyrene and at least one unsaturated hydrocarbon compound selected from the following group A; group A: 4-(2'-methyl-1'-propenyl)ethylbenzene, 4-(2'-methyl-1'-propenyl)vinylbenzene, 4-(2'-methyl-2'-propenyl)ethylbenzene and 4-(2'-methyl-2'-propenyl)vinylbenzene. A method according to Claim 1 wherein said step (II) is achieved by reacting p-isobutylstyrene with carbon monoxide and hydrogen at a reaction temperature of 40 to 150°C under a carbon monoxide/hydrogen mixed pressure of 10 to 600 bar (kg/cm²) in the presence of a transition metal complex carbonylation catalyst or, alternatively, by reacting p-isobutylstyrene with carbon monoxide and water or a lower alcohol at a reaction temperature of 40 to 250°C under a carbon monoxide pressure of 10 to 600 bar (kg/cm²) in the presence of a transition metal complex carbonylation catalyst. A method according to any one of Claims 1 to 11 wherein said hydrogenation is achieved by using hydrogen in the presence of a hydrogenating metal catalyst, which preferably contains at least one metal selected from the group consisting of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, Re, Mo, W, Cr and Ta. A method according to Claim 12 wherein said hydrogenation is achieved by using hydrogen at a reaction temperature of from ordinary temperature to 300°C under a hydrogen pressure of from atmospheric pressure to 300 bar (kg/cm²) in the presence of a hydrogenation metal catalyst. A method for preparing α-(4-isobutylphenyl)propionic acid, its alkyl esters or α-(4-isobutylphenyl) propionaldehyde, which comprises the following steps (I) and (II) : the step (I) of dehydrogenating p-isobutylethylbenzene in the gaseous phase in the presence of a dehydrogenation metal catalyst to obtain a mixture of p-isobutylstyrene and at least one unsaturated hydrocarbon compound selected from the group consisting of 4-(2'-methyl-1'-propenyl)ethylbenzene and 4-(2'-methyl-2'-propenyl)ethylbenzene; and the step (II) of reacting a mixture of p-isobutylstyrene and at least one unsaturated hydrocarbon compound selected from the group consisting of 4-(2'-methyl-1'-propenyl)ethylbenzene and 4-(2'-methyl-2'-propenyl)ethylbenzene, with carbon monoxide and water or a lower alcohol or with carbon monoxide and hydrogen in the presence of a transition metal complex carbonylation catalyst at a reaction temperature of 40 to 250°C under a pressure of 10 to 600 bar (kg/cm²) to form selectively α-(4-isobutylphenyl)propionic acid or an alkyl α-(4-isobutylphenyl)propionate or α-(4-isobutylphenyl) propionaldehyde. The method according to claim 14, which additionally comprises the steps (III) and (IV) : the step (III) of recovering said unsaturated hydrocarbon compound from the reaction mixture obtained in said step (II); and the step (IV) of hydrogenating said recovered unsaturated hydrocarbon compound of step (III) to obtain p-isobutylethylbenzene, and recycling at least a part of the latter to said dehydrogenation step (I) as the raw material of said dehydrogenation step (I). A method for preparing α-(4-isobutylphenyl)propionic acid or its alkyl ester, which comprises the following steps (I) and (II) : the step (I) of dehydrogenating p-isobutylethylbenzene in the gaseous phase in the presence of a dehydrogenation metal catalyst to obtain a mixture of p-isobutylstyrene and at least one unsaturated hydrocarbon compound selected from the group consisting of 4-(2'-methyl-1'-propenyl)vinylbenzene and 4-(2'-methyl-2'-propenyl)vinylbenzene; and the step (II) of reacting a mixture of p-isobutylstyrene and at least one unsaturated hydrocarbon compound selected from the group consisting of 4-(2'-methyl-1'-propenyl)vinylbenzene and 4-(2'-methyl-2'-propenyl)vinylbenzene, with carbon monoxide and water or a lower alcohol in the presence of a transition metal complex carbonylation catalyst at a reaction temperature of 40 to 250°C under a pressure of 10 to 600 bar (kg/cm²) to form α-(4-isobutylphenyl)propionic acid or an alkyl α-(4-isobutylphenyl)propionate and at least one unsaturated acid or its esters selected from the group consisting of α-[4-(2'-methyl-1'-propenyl)phenyl]propionic acid or its alkyl esters and α-[4-(2'-methyl-2'-propenyl)-phenyl]propionic acid or its alkyl esters. A method according to claim 16, which comprises the additional step (III) of hydrogenating said unsaturated acid or its alkyl esters obtained in said step (II) to form α-(4-isobutylphenyl)propionic acid or its alkyl esters. A method for preparing α-(4-isobutylphenyl)propionaldehyde which comprises the following steps (I) and (II): the step (I) of dehydrogenating p-isobutylethylbenzene in the gaseous phase in the presence of a dehydrogenation metal catalyst to obtain a mixture of p-isobutylstyrene and at least one unsaturated hydrocarbon compound selected from the group consisting of 4-(2'-methyl-1'-propenyl)vinylbenzene and 4-(2'-methyl-2'-propenyl)vinylbenzene; and the step (II) of reacting a mixture of p-isobutylstyrene and at least one unsaturated hydrocarbon compound selected from the group consisting of 4-(2'-methyl-1'-propenyl)vinylbenzene and 4-(2'-methyl-2'-propenyl)vinylbenzene, with carbon monoxide and hydrogen in the presence of a transition metal complex carbonylation catalyst at a reaction temperature of 40 to 150°C under a carbon monoxide/hydrogen mixed pressure of 10 to 600 bar (kg/cm²) to form α-(4-isobutylphenyl)-propionaldehyde and at least one unsaturated aldehyde selected from the group consisting of α-[4-(2'-methyl-1'-propenyl)phenyl]propionaldehyde and α-[4-(2'-methyl-2'-propenyl)phenyl]propionaldehyde. The method according to claim 18 for preparing α-(4-isobutylphenyl)propionaldehyde and optionally 2-(4-isobutylphenyl)propanol, which comprises the additional step (III) of hydrogenating said unsaturated aldehyde obtained in said step (II) to form 2-(4-isobutylphenyl)propanol or α-(4-isobutylphenyl)propionaldehyde.

Inventors: MATSUMURA YASUO, SHIMIZU ISOO, TOKUMOTO YUICHI, UCHIDA KAZUMICHI, MATSUMURA, YASUO, SHIMIZU, ISOO, TOKUMOTO, YUICHI, UCHIDA, KAZUMICHI
Applicants: NIPPON PETROCHEMICALS CO LTD, NIPPON PETROCHEMICALS COMPANY, LIMITED
Application year: 1989
Main classification: C07C57
IPC codes: C07C51, C07C45, C07C5