PROCESS FOR THE PREPARATION OF AN ESTER OF A 3-ARYL-SUBSTITUTED ACRYLIC ACID

Abstract: Process for the preparation of an ester of a 3-aryl-­ substituted acrylic acid by reacting (a) a substituted aromatic hydrocarbon containing a 1-alkenyl group with one or two hydrogen atoms in beta-position, (b) CO and (c) an alcohol in the presence of a Group 8 noble metal or a compound thereof, a quinone and a redox agent.

Description: The present invention relates to a process for the preparation of an ester of a 3-aryl-substituted acrylic acid. It is known from U.S. patent specification no. 3,530,168 to contact an olefin, carbon monoxide and oxygen with an alcoholic solution of a catalyst comprising a Group 8 noble metal, a biphyllic ligand and a redox agent to prepare mono- and diesters of saturated and unsaturated carboxylic acids having one more carbon atom than the olefin reactant. The examples in this patent specification show that esters of monocarboxylic acids were obtained in a relatively low yield, calculated on starting olefin. According to the non pre-published European patent specification no. 231044, an optionally substituted ethylenically unsaturated hydrocarbon is reacted with carbon monoxide and an alcohol in the presence of a catalyst comprising a Group 8 noble metal, a quinone and a redox agent, to prepare diesters of dicarboxylic acids. This process is preferably carried out at a pressure in the range from 5 to 200 bar, so as to obtain said diesters in a very high yield. U.S. patent specification no. 3,755,421 discloses a process for the preparation of esters of unsaturated carboxylic acids, beta-alkoxy substituted carboxylic acids and dicarboxylic acids, which comprises reacting an olefin with carbon monoxide and an alcohol in the presence of a catalyst system comprising a Group 8 noble metal catalyst, a quinone and optionally a redox agent. It has now, surprisingly, been found that by using special ethylenically unsaturated hydrocarbons as starting materials at a relatively low partial pressure of carbon monoxide, esters of mono-carboxylic acids are obtained in a high yield. Accordingly, the present invention relates to a process for the preparation of an ester of a 3-aryl-substituted acrylic acid which comprises reacting (a) a substituted aromatic hydrocarbon containing a 1-alkenyl group with one or two hydrogen atoms in beta-position, (b) carbon monoxide and (c) an alcohol, at a partial pressure of carbon monoxide below 10 bar, in the presence of: 1) a noble metal of Group 8 of the Periodic Table of the Elements and/or a compound thereof, 2) a quinone, and 3) a redox agent, and isolating said ester from the reaction mixture thus obtained. The process according to the present invention is carried out at a partial pressure of carbon monoxide which is below 10 bar so as to insure a high yield of esters of 3-aryl-substituted acrylic acids. This partial pressure is preferably between 0.1 and 4.5 bar. The noble metals which are used in the process according to the present invention are platinum, rhodium, ruthenium, palladium, iridium and/or osmium. These metals may be used in metallic form or as compounds. Mixtures of compounds of the same or different of such noble metals or mixtures of such noble metals in metallic form may be used. The noble metals may be used as finely divided metals, not supported on a carrier, or supported on a carrier, for example on activated carbon, pumice or graphite. The present process is preferably carried out in the presence of palladium and/or a compound of palladium. Very good results have been obtained with compounds of palladium. Examples of suitable compounds of Group 8 noble metals are salts, such as nitrates, sulphates, halides, (fluorides, chlorides, bromides and iodides) and carboxylates. Among the carboxylates salts of alkanoic acids having not more than 12 carbon atoms per molecule are preferred, particularly the palladium (II) salts. Palladium (II) acetate is most preferred. Further examples of suitable palladium compounds are palladium complexes such as bis(2,4-pentanedionato)palladium, bis(picolinato)palladium, tetrakis(triphenylphosphine)palladium, tris(dibenzylideneacetone)dipalladium, bis(triphenylphosphine)(1,4-benzoquinone)palladium, tetrakisacetonitrilepalladium tetrafluoroborate, bis(tri-o-tolylphosphine)palladium acetate, bis(triphenylphosphine)palladium sulphate, palladium olefin complexes, for instance di-m-chloro-dichlorobis(ethylene)dipalladium([Pd · C₂H₄ · Cl₂]₂), and di-m-chlorodichlorobis(propylene)dipalladium([Pd · C₃H₆ · Cl₂]₂), and palladiumhydride complexes. Palladium may also be used in complex combination with phosphites, such as triphenyl phosphite or tributyl phosphite. The quinone applied in the process according to the present invention may be an ortho-quinone or a para-quinone and may be, for example, a benzoquinone, a naphthoquinone, an anthraquinone or a chrysenequinone. Preference is given to optionally substituted benzoquinones, particularly to p-benzoquinones. According to a preferred embodiment of the present invention in which halogen-substituted p-benzoquinones are present, very high yields of diesters of dicarboxylic acids are being obtained. In this embodiment one or more fluorine, chlorine, bromine and/or iodine atoms are attached to the aromatic nucleus of the p-benzoquinone. Examples of such quinones are 2-iodo-, 2-bromo-, 2-chloro- and 2-fluoro-p-benzoquinone, 2,6-diiodo- 2,6-dibromo-, 2,6-dichloro- and 2,6-difluoro-p-benzoquinone, 2,3,6-triiodo-, 2,3,6-tribromo-, 2,3,6-trichloro- and 2,3,6-trifluoro-p-benzoquinone. Tetrahalo-p-benzoquinones are preferred, particularly tetrachloro-p-benzoquinone (also referred to as "chloranil"). According to another preferred embodiment unsubstituted p-benzoquinone is applied in the present process. Further examples of suitable quinones are 9,10-anthraquinone, 1,4-naphthoquinone, 5,6-chrysenequinone and alkyl-substituted p-benzoquinones such as 2-methyl-p-benzoquinone and 2,6-dimethyl-p-benzoquinone. Mixtures of quinones may be present, for example of p-benzoquinone and a halogen-substituted p-benzoquinone. Redox agents are well known in the art. It is generally a compound of copper, iron, vanadium, cobalt or manganese. Mixtures of such compounds may be used. These five metals are preferably used in the form of salts of acids having a pKa below 2.0, measured in aqueous solution at a temperature of 25°C, such as chlorides, nitrates, sulphates, perchlorates and sulphonates, for example benzenesulphonates and p-tosylates. Other examples of suitable salts are carboxylates, preferably alkanoates and, more preferably, those alkanoates having not more than 12 carbon atoms per molecule. Among the metal compounds cupric compounds are preferred, particularly cupric tosylate and cupric perchlorate. Further examples of suitable redox agents are cobalt (II) complexes of organic ligands, cupric acetate, cupric butyrate, cupric chloride, cupric bromide, ferrous propionate, ferric acetate, ferric chloride, ferric bromide and manganese chloride. The process according to the present invention may be carried out using a molar ratio noble metal of Group 8 and/or a compound thereof to aromatic hydrocarbon which is not critical and may vary within wide ranges. This molar ratio is suitably in the range from 10⁻² to 10⁻⁶. The process according to the present invention may be carried out using a molar ratio redox agent to noble metal of Group 8 and/or a compound thereof which is not critical and may vary within wide ranges. This molar ratio is suitably in the range from 0.5 to 1000 and preferably from 1 to 200. The process according to the present invention results in the formation of an ester of a 3-aryl-substituted acrylic acid and of a hydroquinone. The hydroquinone may be isolated from the reaction mixture and, if desired, purified. The isolated and optionally purified hydroquinone may be used for any suitable purpose but is preferably oxidized in a suitable manner to the corresponding quinone which quinone is preferably used in the process according to the present invention. It has, furthermore, been found that the yield of esters of 3-aryl-substituted acrylic acids is further enhanced by carrying out the process in the presence of molecular oxygen. Molecular oxygen may be supplied, for example, as pure oxygen, in air enriched with oxygen, in air or diluted with an inert gas such as argon. Oxygen is supplied using a molar ratio molecular oxygen to carbon monoxide which is not critical and may vary within wide ranges, preferably in the range from 1: 3 to 1: 6, the stoichiometric ratio being 1: 4. Oxygen may be supplied in one portion but may, for safety reasons, be supplied in two or more than two portions. The process according to the present invention can be carried out using a molar ratio quinone to aromatic hydrocarbon which is not critical and which may vary within wide limits. This molar ratio may vary, for example, in the range from 0.001 to 5. The process according to the present invention may be carried out using a molar ratio carbon monoxide to aromatic hydrocarbon which is not critical and may vary within wide limits, preferably in the range from 0.5: 1 to 10: 1 and particularly from 1: 1 to 3: 1, the stoichiometric ratio being 2. The process according to the present invention can be carried out in wide ranges of temperature, preferably in the range from 20°C to 200°C, more preferably from 50°C to 125°C. The alcohol can be applied in a molar ratio alcohol to aromatic hydrocarbon which is not critical and may vary within wide limits. This molar ratio is suitably at least the stoichiometric molar ratio which is 1 and may be, if desired, in the range from 1 to 1000. The process according to the present invention can be carried out either in the absence or, which is preferred, in the presence of a solvent which does not inhibit the reaction. The alcohol which is used as a reactant may be used as a solvent. Very good results have been obtained with ethers. Examples of suitable ethers are methyl ethyl ether, diethyl ether, dipropyl ether, tetrahydrofuran, dimethyl ether of diethylene glycol (also referred to as "diglyme"), methyl tert.-butyl ether, dichloroethyl ether, ethyl phenyl ether, diethylene glycol diethyl ether and 1,4-dioxane. Other examples of suitable solvents are halogenated hydrocarbons such as chloroform, chlorobenzene, carbon tetrachloride and perfluoroalkanes; esters such as the methyl and ethyl esters of formic acid, acetic acid, adipic acid, succinic acid, propionic acid, oxalic acid and benzoic acid; sulphones such as dimethyl sulphone, methyl butyl sulphone and tetrahydrothiophone 1,1-dioxide (also referred to as "sulfolane"); aromatic hydrocarbons such as benzene, toluene and the three xylenes; cycloalkanes such as cyclohexane; nitrobenzene. The aryl group in the starting aromatic hydrocarbon may be, for example, monocyclic, which is preferred, or fused polycyclic. In the former case, the aryl group is derived from benzene. In the latter case, the aryl group may be derived from, for example, indene, naphthalene, anthracene, phenanthrene, fluorene or picene. The aryl group either may or may not carry substituents other than the 1-alkenyl group. Examples of such other substituents are alkyl groups, preferably those having not more than five carbon atoms, and halogen atoms, viz. fluorine, chlorine, bromine and iodine atoms. The carbon atoms in the 1-alkenyl substituent having the alpha- and beta-positions with respect to the aromatic nucleus may each carry one substituent, provided that at least one hydrogen atom is present in beta-position, preferably an alkyl group having less than five carbon atoms. Preferably, the 1-alkenyl group is an unsubstituted vinyl group, the starting aromatic hydrocarbon being styrene. Other examples of aromatic hydrocarbons are b-methylstyrene, b-pentylstyrene, p-methylstyrene, p-methoxystyrene, p-chlorostyrene and b-methyl-p-isopropylstyrene. A wide variety of alcohols may be used in the process according to the present invention; the alcohol may be mono- or polyhydric, may be primary, secondary or tertiary and may be aliphatic, cycloaliphatic or aromatic. Monohydric alcohols having from 1 to 20 carbon atoms per molecule are preferred. Very good results have been obtained with methanol. Other examples of suitable alcohols are ethanol, propanol, 2-propanol, butanol, tert-butyl alcohol, pentanol, hexanol, heptanol, octanol, nonanol, decanol, ethylene glycol, diethylene glycol, propylene glycol, ethylene glycol monoalkyl ethers (the alkyl group having up to, for example, 10 carbon atoms), 1,3-butanediol, cyclohexanol, phenol, benzyl alcohol, 2-naphthol and 2-phenanthrol. Compounds which can decompose during the reaction and liberate alcohols may be applied, for example acetals, ketals, carboxylic acid orthoesters and orthoboric acid esters. The process according to the present invention is an example of the so-called "oxidative carbonylation". The acid portion of the ester obtained according to the present invention is derived from the aromatic hydrocarbon and the alcohol portion of the ester is derived from the alcohol. Accordingly, styrene, methanol and carbon monoxide are converted into methyl cinnamate, α-methylstyrene, ethanol and carbon monoxide into ethyl α-methylcinnamate; 1-propenylbenzene, methanol and carbon monoxide into methyl β-methylcinnamate, and 1-butenylbenzene, methanol and carbon monoxide into methyl b-ethylcinnamate. The process according to the present invention is particularly important for the preparation of methyl cinnamate which is a precursor of perfumes, agricultural chemicals and sweeteners. The process according to the invention can be carried out batchwise, semi-continuously or continuously. The reaction time varies in relation to the temperature used and is usually between 0.5 and 20 hours. The ester of a 3-aryl-substituted acrylic acid obtained by the process according to the present invention may be isolated by conventional separating means, such as distillation or extraction. The following Examples further illustrate the invention. Examples 1-5 The experiments described below were carried out in a 300 ml autoclave made of Hastelloy C ("Hastelloy" is a Registered Trade Mark) provided with a magnetically driven stirrer. In each experiment the autoclave was charged with palladium (II) acetate (0.1 mmol), methanol (10 ml, 247 mmol), diglyme (50 ml) and styrene (15 ml, 131 mmol). The autoclave was further charged with chloranil and a redox agent as detailed in the Table hereinafter, flushed with carbon monoxide, charged with carbon monoxide until a partial pressure thereof of 5 bar was obtained, heated to a temperature of 100°C and kept at this temperature for two hours. The partial pressure of carbon monoxide was kept constant at a value of 5 bar during this period. After cooling to ambient temperature the conversion of styrene and the selectivity to methyl cinnamate were determined. This selectivity is defined as 100 × a: b, in which "a" is the amount of styrene that has been converted into methyl cinnamate and "b" is the total amount of styrene that has been converted. Among the by-products obtained in each of the experiments according to the Examples dimethyl phenylsuccinate was present in a very minor amount. The results presented in the Table hereinbefore show that methyl cinnamate has been obtained in a very high yield. Comparative Experiment A The experiment of Example 1 was repeated with the exception that the partial pressure of carbon monoxide was 10 bar instead of 5 bar. The conversion of styrene, after 1 h, was 40% and the selectivity to methyl cinnamate was 45%, the yield of methyl cinnamate, calculated on chloranil, being 47%. Dimethyl phenylsuccinate was the main by-product. Comparative Experiment B Comparative Experiment A was repeated with the exception that the partial pressure of carbon monoxide was 20 bar instead of 10 bar. The conversion of styrene, after 1 h, was 45% and the selectivity to methyl cinnamate was 31%, the yield of methyl cinnamate, calculated on chloranil, being 37%. Dimethyl phenylsuccinate was the main by-product. Comparative Experiment C The experiment of Example 1 was repeated with the exception that no redox agent was present. The conversion of styrene, after 2 h, was 50% and the selectivity to methyl cinnamate was only 55%, the yield of methyl cinnamate, calculated on chloranil, being 72%. Methyl 3-phenylpropionate was the main by-product.

Claims: 1. Process for the preparation of an ester of a 3-aryl-substituted acrylic acid which comprises reacting (a) a substituted aromatic hydrocarbon containing a 1-alkenyl group with one or two hydrogen atoms in beta-position, (b) carbon monoxide and (c) an alcohol, at a partial pressure of carbon monoxide below 10 bar, in the presence of: 1) a noble metal of Group 8 of the Periodic Table of the Elements and/or a compound thereof, 2) a quinone, and 3) a redox agent, and isolating said ester from the reaction mixture thus obtained. 2. Process as claimed in claim 1 which is carried out in the presence of palladium and/or a compound of palladium. 3. Process as claimed in claim 2 in which said palladium compound is a palladium (II) salt of an alkanoic acid having not more than 12 carbon atoms per molecule. 4. Process as claimed in claim 3 which is carried out in the presence of palladium (II) acetate. 5. Process as claimed in any one of the preceding claims in which the quinone is an optionally substituted benzoquinone. 6. Process as claimed in claim 5 in which the optionally substituted benzoquinone is a halogen-substituted p-benzoquinone. 7. Process as claimed in any one of the preceding claims in which said redox agent comprises a compound of copper, iron, vanadium, cobalt and/or manganese. 8. Process as claimed in claim 7 in which said redox agent is a salt of an acid having a pKa below 2.0, measured in aqueous solution at a temperature of 25°C. 9. Process as claimed in claim 8 in which said redox agent is cupric tosylate or ferrous perchlorate. 10. Process as claimed in any one of the preceding claims in which a molar ratio noble metal of Group 8 and/or a compound thereof to aromatic hydrocarbon in the range from 10⁻² to 10⁻⁶ is used. 11. Process as claimed in any one of the preceding claims in which a molar ratio redox agent to noble metal of Group 8 and/or a compound thereof in the range from 1 to 200 is used. 12. Process as claimed in any one of the preceding claims which is carried out in the presence of molecular oxygen. 13. Process as claimed in claim 12 in which a molar ratio molecular oxygen to carbon monoxide in the range from 1: 3 to 1: 6 is applied. 14. Process as claimed in any one of the preceding claims in which a molar ratio carbon monoxide to aromatic hydrocarbon in the range from 0.5: 1 to 10: 1 is applied. 15. Process as claimed in any one of the preceding claims which is carried out in the presence of a solvent. 16. Process as claimed in claim 15 in which the solvent comprises an ether. 17. Process as claimed in any one of the preceding claims wherein a temperature is maintained in the range from 20°C to 200°C. 18. Process as claimed in any one of the preceding claims in which said aromatic hydrocarbon is monocyclic and said 1-alkenyl substituent is a vinyl group. 19. Process as claimed in any one of the preceding claims in which the alcohol is monohydric and has in the range from 1 to 20 carbon atoms per molecule.

Inventors: DRENT, EIT
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1988
Main classification: C07C69
IPC codes: C07C69, C07B61, C07C67, B01J31