Process for the preparation of indigo compounds

Abstract: Disclosed is a process for preparing an indigo compound in improved yield by reacting an indole compound having no substituent at the 2- and 3-positions with an organic hydroperoxide in the presence of an additive selected from the group consisting of (1) a carboxyl-containing compound and (2) a boric acid ester compound. The yield of the indigo compound can further be increased by using a metallic compound catalyst capable of oxidizing carbon atom at the 3-­ position of the indole compound, such as a compound of a metal of group 4A, 5A or 6A of the periodic table.

Description: This invention relates to a process for the preparation of an indigo compound. More specifically, it relates to a process for the preparation of an indigo compound by reacting an indole compound having no substituent at the 2- and 3-positions with an organic hydroperoxide in the presence of a specific additive and a catalyst. Indigo compounds are important compounds that are useful as dyes. The presently employed industrial processes for the preparation of indigo comprise forming an N-phenylglycine salt from aniline and chloroacetic acid, or from aniline, cyanic acid and formaldehyde, converting this salt into an indoxyl compound by alkali fusion at elevated temperature, and then oxidizing this compound with air. However, these processes are not only complicated involving many steps, but also require the use of large amounts of potassium hydroxide and sodium hydroxide. Moreover the recovery and reuse of used potassium hydroxide and sodium hydroxide has the disadvantage of consuming much energy and requirin special equipment. Therefore, conversion to a simpler process has been desired. Meanwhile, it has been reported in the field of synthetic organic chemistry that a small amount of indigo was formed by oxidation of indole. Specifically, Obata et al. have reported that, when peracetic acid, which is a percarboxylic acid, was produced from hydrogen peroxide and acetic acid in the reaction system and reacted with indole, a trimer of indole, or 2,2-diindyl-ψ-indoxyl, was obtained and in addition a small amount of indigo was formed as a by-product (Bull. Agr. Chem. Soc. Japan, Vol. 20, pp. 80-83, 1956). Moreover, B. Witkop et al. have reported that, when perbenzoic acid, which is a percarboxylic acid, was reacted with indole in chloroform by allowing the reaction mixture to stand in a refrigerator overnight, a considerable amount of indigo was formed together with a variety of other products (Justus Liebigs Annalen der Chemie, Vol. 558, pp. 91-98, 1947). Furthermore, A.K. Sheinkman et al. have reported that, when hydrogen peroxide, which is an inorganic peroxide, was reacted with indole in methanol, the trimer 2,2-diindyl-ψ-indoxyl was obtained in high yield, as was the case with the reaction using peracetic acid, and the formation of indigo was only detected by chromatography (Khim, Geterotsikl. Soedin., Vol. 11, pp. 1490-1496, 1978). More recently still EP-A-0330419 (published 30th August 1989) and EP-A-0368508 (published 16th May 1990) have described more efficient procedures for preparing indigo, both documents constitute prior art within the meaning of Article 54(3) EPC. The former describes the preparation of indigo compounds by reacting indole compounds unsubstituted in the 2- and 3-positions with organic hydroperoxides, preferably secondary or tertiary alkyl hydroperoxides, preferably in the presence of a metallic compound catalyst capable of oxidising the indole 3-carbon atom (for example compounds of titanium, vanadium, molybdenum and tungsten) and preferably in the presence of at least one solvent selected from water, aliphatic and alicyclic hydrocarbons, aromatic hydrocarbons, aliphatic and aromatic halogen compounds, ethers, alcohols, ketones, esters, carbonates and aromatic nitro compounds. The latter describes the preparation of indigo compounds by reacting a corresponding indole compound having no substituents at the 2- and 3-positions with a percarboxylic acid in an alcohol solvent or by heating to above 60°C in an aprotic solvent, preferably selected from aliphatic and alicyclic hydrocarbons, aromatic hydrocarbons, aliphatic and aromatic halogen compounds, ethers, ketones, esters, carbonates, aliphatic and aromatic nitro compounds and nitriles and preferably in the presence of a metallic compound catalyst capable of oxidising the carbon atom at the 3-position of the indole, especially a compound of titanium, vanadium, molybdenum or tungsten, and/or a promoter, for example a dehydrating agent, a radical scavenger, a ligand of a catalytic metal, a boric acid ester, a metaboric acid ester, a salt of an alkaline metal or alkaline earth metal or a salt of a rare earth metal. The present inventors have made exhaustive investigations on a wide variety of oxidizing agents and additives, with a view to developing a reaction system which can produce an indigo compound efficiently and simply by using, as a starting material, an indole compound which can readily be obtained industrially. As a result, it has now been found that, if an organic hydroperoxide is used as an oxidizing agent and reacted with an indole compound in the presence of a specific additive and of a metal compound catalyst an indigo compound can readily be obtained in a single step while achieving a higher yield and formation rate than achievable in the above-described prior art processes using other oxidizing agents or than achievable in the case without using any additive. The present invention has been completed on the basis of these findings. According to the present invention, there is provided a process for the preparation of an indigo compound which comprises reacting an indole compound having no substituent at the 2- and 3-positions with an organic hydroperoxide in the presence of an additive, the organic hydroperoxide being selected from secondary and tertiary alkyl hydroperoxides, the additive being selected from (1) carboxyl-containing compounds selected from carboxylic acids, carboxylic anhydrides and metallic carboxylates and (2) boric acid ester compounds, and the reaction being carried out in the presence of a metallic compound catalyst capable of oxidizing the indole 3-carbon atom and of at least one solvent selected from water, aliphatic and alicyclic hydrocarbons, aromatic hydrocarbons, aliphatic and aromatic halogen compounds, ethers, alcohols, ketones, esters, carbonates and aromatic nitro compounds. The indole compound having no substituent at the 2- and 3-positions, which is used as one of the starting materials in the process of the present invention, is selected from indole; alkylindoles having 1 to 4 alkyl groups of 1 to 10 carbon atoms, such as 1-methylindole, 4-ethylindole, 5-methylindole, 6-methylindole, 6-isopropylindole, 7-methylindole and 4,5-dimethylindole; cycloalkylindoles having 1 to 4 cycloalkyl groups of 3 to 12 carbon atoms, such as 4-cyclohexylindole and 5-cyclopentylindole; arylindoles having 1 to 4 unsubstituted or alkyl-substituted aryl groups of 6 to 30 carbon atoms, such as 5-phenylindole and 6-β-naphthylindole; halogenated indoles having 1 to 4 halogen atoms, such as 4-chloroindole, 5-chloroindole, 5,7-dichloroindole, 5-bromoindole, 6-bromoindole, 5,7-dibromoindole and 4-chloro-5-bromoindole; hydroxyindoles having 1 to 4 hydroxyl groups, such as 4-hydroxyindole, 5-hydroxyindole and 4,5-dihydroxyindole; alkoxyindoles having 1 to 4 alkoxy groups of 1 to 10 carbon atoms, such as 4-methoxyindole and 5-benzyloxyindole; phenoxyindoles having 1 to 4 phenoxy groups of 6 to 30 carbon atoms, such as 5-phenoxyindole; halogenated alkylindoles having 1 to 3 halogen atoms and 1 to 3 alkyl groups of 1 to 10 carbon atoms, such as 4-chloro-5-ethylindole, 6-chloro-4-methylindole, 4-bromo-5-ethylindole and 5-bromo-4-methylindole; nitroindoles having 1 to 4 nitro groups, such as 4-nitroindole, 5-nitroindole and 7-nitroindole; acylindoles having 1 to 4 acyl groups of 2 to 20 carbon atoms, such as 1-benzoylindole and 4-acetylindole; acyloxyindoles having 1 to 4 acyloxy groups of 2 to 20 carbon atoms, such as 1-acetoxyindole and 4-benzoyloxyindole; indolecarboxylic acids, such as indole-5-carboxylic acid, and esters thereof; N,N-dialkylaminoindoles having 1 to 4 N,N-dialkylamino groups in which each alkyl group contains 1 to 10 carbon atoms, such as 5-N,N-dimethylaminoindole; and sulfonated indoles. Of course, these indole compounds should not have the above-described substituent groups at the 2- and 3-positions. In addition, indole compounds having a combination of two or more types of substituent groups as described above are also useful in the process of the present invention. At positions other than the 2-and 3-positions, these indole compounds may have any substituent that does not interfere with the reaction. Among these indole compounds, indole is especially preferred. The organic hydroperoxide, which is used as the other starting material in the process of the present invention, is a secondary or tertiary alkyl hydroperoxide preferably having 3 to 30 carbon atoms in the alkyl moiety, such as tert-butyl hydroperoxide, 1-phenylethyl hydroperoxide, 1-methyl-1-phenylethyl hydroperoxide (cumene hydroperoxide), bis-(1-methylethyl)phenyl hydroperoxides, 1-methyl-1-(4-methylcyclohexyl)ethyl hydroperoxide, 2,5-dimethylhexane-2,5-dihydroperoxide or 1,1,3,3-tetramethylbutyl hydroperoxide. Among these compounds, tert-butyl hydroperoxide, 1-phenylethyl hydroperoxide and 1-methyl-1-phenylethyl hydroperoxide are especially preferred. These organic hydroperoxides may be used alone, or two or more of them may be used in admixture or in sequence. Alternatively, it is also possible to use a suitable combination of components (e.g., isopropylbenzene and an oxygen-containing gas) which can produce such an organic hydroperoxide in the reaction system. Although the amount of organic hydroperoxide used is not critical, it is usually in the range of 0.01 to 100 moles, preferably 0.1 to 20 moles, per mole of the indole compound. The additive used in the process of the present invention is selected from (1) various carboxyl-containing compounds and (2) boric acid ester compounds. The carboxyl-containing compound used as the additive is a compound selected from carboxylic acids, carboxylic acid anhydrides and metallic carboxylates. Examples of useful carboxylic acids include saturated aliphatic carboxylic acids such as formic acid, acetic acid, propionic acid, butyric acid, isobutyric acid, valeric acid, isovaleric acid, pivalic acid, caproic acid, caprylic acid, lauric acid, stearic acid, phenylacetic acid, diphenylacetic acid, 3-phenylpropionic acid, succinic acid, adipic acid and sebacic acid; unsaturated aliphatic carboxylic acids such as oleic acid, linoleic acid, linolenic acid, acrylic acid, methacrylic acid, cinnamic acid and fumaric acid; alicyclic carboxylic acids such as cyclohexanecarboxylic acid and cyclohexane-1,4-dicarboxylic acid; aromatic carboxylic acids such as benzoic acid, phthalic acid, isophthalic acid, terephthalic acid, trimesic acid, α-naphthalenecarboxylic acid and naphthalene-1,8-dicarboxylic acid; alkyl-substituted aromatic carboxylic acids such as p-methylbenzoic acid, m-isopropylbenzoic acid and toluene-3,5-dicarboxylic acid; halogen-substituted aromatic carboxylic acids such as o-chlorobenzoic acid, m-chlorobenzoic acid, p-chlorobenzoic acid, m-bromobenzoic acid, 2,4-dichlorobenzoic acid and 6-bromonaphthalene-1-carboxylic acid; hydroxy-substituted aromatic carboxylic acids such as m-hydroxybenzoic acid, p-hydroxybenzoic acid and 6-hydroxynaphthalene-1-carboxylic acid; alkoxy-substituted aromatic carboxylic acids such as o-methoxybenzoic acid, m-methoxybenzoic acid, p-ethoxybenzoic acid, 5-methoxynaphthalene-2-carboxylic acid and ethoxyterephthalic acid; and heterocyclic aromatic carboxylic acids such as nicotinic acid, isonicotinic acid and quinoline-3-carboxylic acid. These compounds may additionally have any substituent group that does not interfere with the reaction. Examples of useful carboxylic acid anhydrides include the anhydrides of the above-described carboxylic acids. Examples of useful metallic carboxylates include the salts derived from the above-described carboxylic acids and metals such as alkali metals, alkaline earth metals, zinc, silver, iron family metals (i.e., iron, cobalt and nickel) and rare earth metals (e.g., lanthanum and cerium). Moreover, it is also possible to use a suitable compound, or combination of compounds, which can produce such a carboxyl-containing compound in the reaction system. The boric acid ester compound used as the additive is selected from orthoboric acid esters and metaboric acid esters. Examples of useful orthoboric acid esters include trialkyl borates such as trimethyl borate, triethyl borate, triisopropyl borate, tri-sec-butyl borate, tri-tert-butyl borate, tricyclohexyl borate, trimenthyl borate and tribenzyl borate; triaryl borates such as triphenyl borate; and other boric acid esters such as menthoxyboric acid, 2-acetylacetonato-l,3,2-benzodioxaborole and 2-hexafluoroacetylacetonato-1,3,2-benzodioxaborole. Examples of useful metaboric acid esters include alkyl metaborates such as isopropyl metaborate, n-butyl metaborate, sec-butyl metaborate, tert-butyl metaborate, cyclohexyl metaborate and menthyl metaborate; and aryl metaborates such as phenyl metaborate and (2,6-di-tert-butyl-4-methyl)phenyl metaborate. Moreover, it is also possible to use a suitable combination of compounds which can produce such a boric acid ester in the reaction system. The above-described carboxyl-containing compounds and boric acid ester compounds, which are useful as additives, may be used alone or in admixture of two or more. These additives may usually be used in an amount of not greater than 50 moles, preferably 0.001 to 20 moles, per mole of the indole compound. In the process of the present invention, no particular limitation is placed on the method by which the reaction is carried out. There may be employed any method that permits at least the indole compound, organic hydroperoxide and additive to be effectively mixed or contacted with each other, and the reaction may be carried out in any of batch, semibatch and continuous operations. More specifically, there may be employed a method in which the indole compound, organic hydroperoxide and additive are charged into a reactor all at once, a method in which one of the materials is continuously or intermittently added to a mixture of the other two, a method in which two of the materials are similarly added to the other, or a method in which the three materials are continuously or intermittently fed to a reactor. The reaction temperature and the reaction time may vary according to the types and amounts of the indole compound, organic hydroperoxide and additive used. However, the reaction temperature is usually in the range of -10°C to 200°C. If the reaction temperature is too low, the reaction will become unduly slow, while if it is too high, the reaction may be attended with danger because of violent decomposition of the organic hydroperoxide. Preferably, the reaction temperature is in the range of 10°C to 150°C. The reaction time is usually within 50 hours and preferably in the range of 0.01 to 20 hours. According to circumstances, the reaction may be carried out under reduced, atmospheric or superatmospheric pressure. In the process of the present invention, the reaction may be carried out under an atmosphere of an inert gas or in the presence of molecular oxygen such as air. In the process of the present invention, the yield of the indigo compound and the reaction rate are enhanced by additionally using a metallic compound catalyst capable of oxidizing carbon atom at the 3-position of the indole compound. The term "metallic compound catalyst capable of oxidizing carbon atom at the 3-position of the indole compound" comprehends compounds of metals which, in the reaction of the indole compound with the organic hydroperoxide, can cause carbon atom at the 3-position of the indole compound to be oxidized by an oxygen atom. For example, this catalyst is at least one compound selected from compounds of the metals of groups 4A, 5A and 6A of the periodic table. Specifically, useful metallic compounds include various compounds of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten. More specifically, they include inorganic compounds of the aforesaid metals, such as halides, oxyhalides, oxides, mixed oxides, sulfides, borides, phosphides, hydroxides, oxyhydroxides, cyano complexes, inorganic acid salts (e.g., sulfates, nitrates and phosphates), metallic oxyacids (e.g., titanic acid, molybdic acid and tungstic acid) and salts thereof, and heteropoly-acids (e.g., phosphomolybdic acid and silicotungstic acid) and salts thereof; compounds of the aforesaid metals having an organic group in at least a portion thereof, such as organic acid salts (e.g., acetates, oxalates, benzoates and naphthenates), alkoxides (e.g., those derived from ethyl alcohol and isopropyl alcohol), phenoxides (e.g., those derived from phenol and m-chlorophenol), and halogen compounds having an alkoxy or phenoxy group; complex compounds of the aforesaid metals, such as carbonyl complexes, amine complexes, pyridine complexes (e.g., those derived from pyridine and bipyridyl), oxo complexes, thiolate complexes (e.g., those derived from cysteine and dithiocatechol), sulfide complexes, dithiocarbamate complexes, thiocyanate complexes, isocyanate complexes, nitrosyl complexes, phosphine complexes (e.g., those derived from triphenylphosphine and 1,2-diphenylphosphinoethane), phosphoryl complexes, phthalocyanine complexes, porphyrin complexes, nitrile complexes, ether complexes, ketone complexes, β-ketocarbonyl complexes (e.g., those derived from acetylacetone), alkyl and allene complexes, olefin complexes and cyclopentadienyl complexes; and compounds of the aforesaid metals coming under two or more of the foregoing categories. These metallic compounds may be used alone or in admixture of two or more. It is also possible to use a suitable combination of components which can produce any of these metallic compounds in the reaction system. Although these metallic compounds are preferably soluble in the reaction mixture, they may be partially or totally insoluble therein. These metallic compounds are usually used in an amount of not greater than 0.5 mole, preferably 0.00001 to 0.1 mole, per mole of the indole compound. The process of the present invention is carried out in the presence of a solvent. For this purpose, there may be used any solvent that does not interfere with the reaction. Useful solvents include, for example, water; aliphatic and alicyclic hydrocarbons such as n-hexane, n-pentane and cyclohexane; aromatic hydrocarbons such as benzene, toluene, ethylbenzene and cumene; aliphatic and aromatic halogen compounds such as dichloromethane, chloroform, chlorobenzene and dichlorobenzene; ethers such as diethyl ether, diphenyl ether, tetrahydrofuran and ethylene glycol diethyl ether; alcohols such as methanol, ethanol, tert-butanol cyclohexanol, benzyl alcohol and propylene glycol; ketones such as acetone, methyl ethyl ketone and acetophenone; esters such as ethyl acetate and ethyl propionate; carbonates such as dimethyl carbonate; and aromatic nitro compounds such as nitrobenzene. These solvents may be used alone or in admixture of two or more. When these solvents are used, it does not matter whether the reaction mixture forms a homogeneous system or a heterogeneous system consisting of a plurality of phases. In the process of the present invention, the desired indigo compound can be obtained by working up the resulting reaction mixture in the usual manner. On completion of the reaction, most of the formed indigo compound has usually separated out. Therefore, the indigo compound can easily be recovered as a solid according to a conventional solid-liquid separation technique such as filtration, centrifugation or decantation. Where the amount of the precipitated indigo compound is insufficient, it is also possible to concentrate the reaction mixture and then recover the resulting increased amount of precipitate therefrom. The present invention is further illustrated by the following examples. Example 1 A three neck flask having a capacity of 500 ml and fitted with a stirrer, a thermometer and a cooling coil was charged with 10.0 g (85.4 mmoles) of indole, 1.04 g (8.5 mmoles) of benzoic acid as an additive, 225 mg (0.85 mmole) of molybdenum hexacarbonyl as a catalyst, 300 g of cumene as a solvent, and 47.0 g (256.3 mmoles as cumene hydroperoxide) of an 83 wt.% solution of cumene hydroperoxide in cumene (the CHP solution), all at once. This reaction mixture was heated at 80°C on an oil bath and stirred under an atmosphere of air for 5 hours to effect the reaction. After completion of the reaction, this reaction mixture was filtered. The solid so separated was washed with small amounts of cumene and methanol, and then dried at 50°C under reduced pressure to obtain 7.91 g of indigo. The indigo yield was 70.7% and the indigo yield per hour was 14.1%. Comparative Example 1 Reaction and after-treatment were carried out in the same manner as in Example 1, except that the use of benzoic acid was omitted. Thus, there was obtained 5.91 g of indigo. The indigo yield was 52.8% and the indigo yield per hour was 10.6%. The omission of the additive resulted in a low yield and a slow formation rate. Examples 2-25 Reaction and after-treatment were carried out in the same manner as in Example 1, except that the benzoic acid was replaced by each of the additives whose type and amount are shown in Table 1. The results thus obtained, together with those of Example 1 and Comparative Example 1, are shown in Table 1. In all of these examples, the results show an improvement in yield and formation rate as compared with the results of Comparative Example 1 using no additive. Example 26 A three neck flask having a capacity of 100 ml and fitted with a stirrer, a thermometer and a cooling coil was charged with 1.0 g (8.5 mmoles) of indole, 58 mg (0.43 mmoles) of p-methyl benzoic acid as an additive, 2.3 mg (0.0087 mmole) of molybdenum hexacarbonyl as a catalyst, 30 g of diphenyl ether as a solvent, and 4.68 g (25.5 mmoles as cumene hydroperoxide) of an 83 wt.% solution of CHP solution, all at once. This reaction mixture was heated at 100°C on an oil bath and stirred under an atmosphere of air for 5 hours to effect the reaction. The reaction mixture was homogeneous at the start of the reaction, but a deep blue solid gradually precipitated out with the progress of the reaction. After completion of the reaction, this reaction mixture was filtered. The solid so separated was washed with a small amount of toluene and then dried at 50°C under reduced pressure to obtain a deep blue solid product. Elemental analysis and IR spectroscopic analysis revealed that this product was indigo. The indigo yield was 62.3% and the indigo yield per hour, which serves as a measure of its formation rate, was 12.5%. Examples 27-33 Reaction and after-treatment were carried out in the same manner as in Example 26, except that the amounts of indole and the CHP solution used, the types and amounts of additive and solvent used, the reaction temperature and the reaction time were altered as shown in Table 2, and that each of the catalysts listed in Table 2 was used in the indicated amount. The results thus obtained are shown in Table 2. Comparative Examples 2-9 The procedures of Examples 26-33 were repeated except that the additive used in each of the examples was omitted. The results thus obtained, together with those of Examples 26-33, are shown in Table 2. In all of these comparative examples, the omission of the additive resulted in a low yield and a slow formation rate. Example 34 Reaction was carried out in the same manner as in Example 26 except that 8.7 mg (0.085 mmole) of lithium acetate dihydrate was used in place of the p-methylbenzoic acid, 22.5 mg (0.085 mmole) of molybdenum hexacarbonyl was used as a catalyst, and the reaction time was 5 hours. After completion of the reaction, the reaction mixture was filtered. The solid so separated was washed with a small amount of toluene, and then washed with water and methanol, and dried at 50°C under reduced pressure. As a result, the indigo yield was 55.9% and the indigo yield per hour was 11.2%. Comparative Example 10 Reaction and after-treatment were carried out in the same manner as in Example 34, except that the use of lithium acetate dihydrate was omitted. As a result, the indigo yield was 50.9% and the indigo yield per hour was 10.2%. Examples 35-39 Reaction and after-treatment were carried out in the same manner as in Example 34, except that the lithium acetate dihydrate was replaced by each of the additives whose type and amount are shown in Table 3. The results thus obtained are shown in Table 3. Examples 40-44 Reaction and after-treatment were carried out in the same manner as in Example 26, except that the p-methylbenzoic acid was replaced by each of the additives whose type and amount are shown in Table 3, 22.5 mg (0.085 mmole) of molybdenum hexacarbonyl was used as a catalyst, and the reaction time was 5 hours. The results thus obtained, together with those of Examples 34-39 and Comparative Example 10, are shown in Table 3. Example 45 A four neck flask having a capacity of 100 ml and fitted with a stirrer, a thermometer, a dropping funnel and a cooling coil was charged with 1.0 g (8.5 mmoles) of indole, 104.2 mg (0.85 mmole) of benzoic acid as an additive, 11.3 mg (0.043 mmole) of molybdenum hexacarbonyl as a catalyst, and 30 g of toluene as a solvent. While this mixture was being heated at 80°C on an oil bath and stirred under an atmosphere of air, 5.6 g (42.9 mmoles as tert-butyl hydroperoxide) of a 69 wt.% aqueous solution of tert-butyl hydroperoxide was added dropwise thereto over a period of one hour. Thereafter, the reaction was continued for 5 hours under the same conditions. The resulting reaction mixture was worked up in the same manner as in Example 26. As a result, the indigo yield was 29.3% and the indigo yield per hour was 5.9%. Examples 46-48 Reaction and after-treatment were carried out in the same manner as in Example 45, except that the amounts of indole and benzoic acid used were altered as shown in Table 4 and the 69 wt.% aqueous solution of tert-butyl hydroperoxide was replaced by each of the organic hydroperoxides whose type and amount are shown in Table 4. The results thus obtained, together with those of Example 45, are shown in Table 4. All of these examples gave improved results in yield and formation rate as compared with the results obtained with no additive. Example 49 Reaction was carried out in the same manner as in Example 26, except that 3.0 g (19.8 mmoles) of 5-chloroindole was used in place of the indole, the CHP solution was used in an amount of 18.0 g (98.2 mmoles as cumene hydroperoxide), 309.8 mg (1.98 mmoles) of p-chlorobenzoic acid was used in place of the p-methylbenzoic acid, 22.5 mg (0.085 mmole) of molybdenum hexacarbonyl was used as a catalyst, and the reaction time was 5 hours. The resulting reaction mixture was filtered. The solid so separated was washed with a small amount of methanol and then dried at 50°C under reduced pressure to obtain 1.64 g of the corresponding indigo compound, or 5,5'-dichloroindigo. The molar yield of 5,5'-dichloroindigo as based on the charged 5-chloroindole was 50.1%. Example 50 Reaction was carried out in the same manner as in Example 26, except that 1.0 g (7.6 mmoles) of 6-methylindole was used in place of the indole, the CHP solution was used in an amount of 7.0 g (38.2 mmoles as cumene hydroperoxide), 103.8 mg (0.76 mmole) of m-methylbenzoic acid was used in place of the p-methylbenzoic acid, 22.5 mg (0.085 mmole) of molybdenum hexacarbonyl was additionally used as a catalyst, and the reaction time was 5 hours. The resulting reaction mixture was filtered. The solid so separated was washed with a small amount of methanol and then dried at 50°C under reduced pressure to obtain 0.64 g of the corresponding indigo compound, or 6,6′-dimethylindigo. The molar yield of 6,6'-dimethylindigo as based on the charged 6-methylindole was 57.8%. Example No. Additive Indigo yield (%) Indigo yield per hour (%) Type Amount used (mg) Example 34 Lithium acetate dihydrate 8.7 55.9 11.2 Example 35 Potassium acetate 16.8 54.2 10.8 Example 36 Magnesium acetate tetrahydrate 18.3 53.7 10.7 Example 37 Barium acetate 21.8 55.4 11.1 Example 38 Cerium acetate monohydrate 28.6 58.1 11.6 Example 39 Lanthanum acetate 27.0 54.8 11.0 Example 40 Nickel acetate tetrahydrate 21.2 54.4 10.9 Example 41 Silver acetate 14.2 55.4 11.1 Example 42 Zinc acetate dihydrate 37.3 53.8 10.8 Example 43 Zinc benzoate 13.1 54.1 10.8 Example 44 Acetic anhydride 87.1 61.7 12.3 Comparative Example 10 None 50.9 10.2

Claims: A process for the preparation of an indigo compound which comprises reacting an indole compound having no substituent at the 2- and 3-positions with an organic hydroperoxide in the presence of an additive, the organic hydroperoxide being selected from secondary and tertiary alkyl hydroperoxides, the additive being selected from (1) carboxyl-containing compounds selected from carboxylic acids, carboxylic anhydrides and metallic carboxylates and (2) boric acid ester compounds, and the reaction being carried out in the presence of a metallic compound catalyst capable of oxidizing the carbon atom at the 3-position of the indole compound and of at least one solvent selected from water, aliphatic and alicyclic hydrocarbons, aromatic hydrocarbons, aliphatic and aromatic halogen compounds, ethers, alcohols, ketones, esters, carbonates and aromatic nitro compounds. A process according to claim 1, wherein the indole compound is indole. A process according to claim 1 or claim 2, wherein the organic hydroperoxide is tert-butyl hydroperoxide, 1-phenylethyl hydroperoxide or 1-methyl-1-phenylethyl hydroperoxide. A process according to any one of claims 1 to 3, wherein the organic hydroperoxide is used in an amount of 0.01 to 100 moles per mole of the indole compound. A process according to any one of claims 1 to 4, wherein the boric acid ester compound is selected from orthoboric acid esters and metaboric acid esters. A process according to any one of claims 1 to 5, wherein the additive is used in an amount not greater than 50 moles per mole of the indole compound. A process according to any one of claims 1 to 6, wherein the catalyst is a compound of a metal of groups 4A, 5A, and 6A of the periodic table. A process according to claim 7, wherein the catalyst is a compound of titanium, vanadium, molybdenum or tungsten. A process according to any one of claims 1 to 8, wherein the catalyst is used in an amount not greater than 0.5 mole per mole of the indole compound. A process according to any one of claims 1 to 9, wherein the reaction is carried out at a temperature of -10° to 200°C. A process according to any one of claims 1 to 10, wherein the indigo compound is recovered as a solid from the liquid reaction mixture.

Inventors: AOKI SHINOBU, HARA ISAO, TAKAKI USAJI, YAMAMOTO YOSHIHIRO, AOKI, SHINOBU, HARA, ISAO, TAKAKI, USAJI, YAMAMOTO, YOSHIHIRO, AOKI, SHINOBU 2-11, MITSUI TOATSU KOSUGAYA APAATO, HARA, ISAO 4-18, MITSUI TOATSU APAATO
Applicants: MITSUI TOATSU CHEMICALS, MITSUI TOATSU CHEMICALS, INC.
Application year: 1989
Main classification: C07D209
IPC codes: C09B7, C07D209