Process for the hydroformylation of certain acrylic acid derivatives

0001The present invention relates to a process for the hydroformylation of certain acrylic acid derivatives.

0002British patent specification number 1,586,805 discloses a process for the preparation of C₁₋₁₈ alkyl α-formylpropionates by the hydroformylation of C₁₋₁₈ alkyl acrylates at a pressure of from 100 to 700 bars and a temperature of from 50 to 200°C, in the presence of a rhodium complex which contains, as a ligand bound to rhodium, a phosphorus compound of the general formula <maths id="math0001" num=""><math display="block"><mrow><msub><mrow><mtext>P[(O)</mtext></mrow><mrow><mtext>n</mtext></mrow></msub><mtext>R]₃</mtext></mrow></math><img file="EP0306094B1_D0001.tif" /></maths> where the radicals R are identical or different alkyl, aromatic, aralkyl or cycloalkyl radicals and n is 1 or zero. Triphenylphosphine and triphenylphosphite are specifically disclosed on page 2, lines 31 to 34 as suitable phosphorus compounds. However, the two Examples only illustrate the use of triphenylphosphine. Both Examples illustrate the hydroformylation of methyl acrylate at a pressure of 280 bars and a temperature of 105°C or above.

0003In a review paper entitled "Advances in the Hydroformylation of Olefins Containing Functional Groups", published in the Journal of Molecular Catalysis, 40 (1987) 129-182, it is stated on page 164 that alkyl acrylates have been frequently hydroformylated by rhodium catalysts at high pressure. However, it is also stated that low pressure hydroformylation of ethyl acrylate (≦ 0.4MPa) using HRh(CO)(PPh₃)₃ in the presence of an excess of PPh₃ at 80°C failed remarkably.

0004One skilled in the art would be inclined to conclude that alkyl acrylates can only be hydroformylated at high pressures when using the rhodium catalysts described in British patent specification number 1,586,805.

0005Surprisingly it has now been found that certain acrylic acid derivatives, including C₁₋₁₈ alkyl acrylates, may be hydroformylated at low pressure using a catalyst comprising a rhodium compound and a triphenylphosphite.

0006Accordingly, the present invention provides a process for the hydroformylation of a compound of general formula <chemistry id="chem0001" num="0001"><img file="EP0306094B1_D0002.tif" /></chemistry> wherein R₁ represents a hydrogen atom or an alkyl group having from 1 to 6 carbon atoms and R₂ represents an alkyl group having from 1 to 18 carbon atoms, which comprises hydroformylating the compound of general formula I with carbon monoxide and hydrogen in the liquid phase in the presence of a homogeneous catalyst system comprising <ul id="ul0001" list-style="none"><li>a) a rhodium compound and</li><li>b) one or more triphenylphosphites, the phenyl groups of which may optionally be substituted by halogen, alkyl having from 1 to 6 carbon atoms, alkoxy having from 1 to 6 carbon atoms, carboxy or cyano, at a temperature in the range of from 50 to 100°C and a pressure in the range of from 3 to below 100 bars.</li></ul>

0007The process according to the invention has been shown to afford aldehydes at a very high conversion rate (≧ 5200 mol aldehyde/g at Rh.hr). Since high pressures are not required by the process according to the invention, it is thus particularly advantageous.

0008In view of the disclosure in British patent specification number 1,586,805 and Journal of Molecular catalysis, 40 (1987) 129-182, the process according to the invention is very surprising. Indeed when the conditions of the process of the invention were performed using triphenylphosphine or tri-(n)-butylphosphite instead of a triphenylphosphite, only very low conversion rates were observed.

0009The process according to the invention has been shown to proceed with very high selectivity for the iso-aldehyde. It is thus particularly advantageous for preparing alkyl α-formylpropionates which are useful in the preparation of fine chemicals and/or pharmaceuticals.

0010Preferably R₁ represents a hydrogen atom.

0011When R₁ represents a hydrogen atom, R₂ preferably represents an ethyl, n-propyl, isopropyl, isobutyl, n-butyl, or more preferably, a methyl group.

0012The rhodium compound used in the process according to the invention may be, for example, rhodium acetate, rhodium di(carbonyl)acetylacetonate, 1,5-cyclooctadiene-rhodium (I) acetate, 1,5-cyclooctadiene-rhodium (I) acetyl acetonate, dirhodium octacarbonyl, rhodium nitrate, rhodium chloride, tetrarhodium dodeca carbonyl or hexa rhodium hexadeca-carbonyl.

0013Preferably the number of gram atoms of rhodium used per mole of compound of general formula I is in the range of from 0.001-0.1.

0014Preferably the or each triphenylphosphite used in the process according to the invention is selected from triphenylphosphite, di(phenyl)p-chlorophenylphosphite di(p-chlorophenyl)phenyl phosphite, tri(p-chlorophenyl) phosphite, di(phenyl)p-cyanophenyl phosphite, di(p-cyanophenyl) phenylphosphite, tri(p-cyanophenyl)phosphite, di(phenyl)p-methoxyphenylphosphite, di(p-methoxyphenyl)phenylphosphite, tri(p-methoxyphenyl)phosphite, di(phenyl)tolylphosphite, phenyl di(tolyl)phosphite, tri(tolyl)phosphite, di(phenyl)p-carboxyphenylphosphite, di(phenyl)o-chlorophenylphosphite, phenyl di(o-chlorophenyl)-phosphite and tri(o-chlorophenyl)phosphite.

0015More preferably, the or each triphenylphosphite used is selected from triphenylphosphite, diphenyl(p-chloro- phenyl)phosphite, diphenyl(p-methoxyphenyl)phosphite, diphenyl(p-cyanophenyl)phosphite and diphenyl(p-tolyl)phosphite. Triphenylphosphite itself is most preferred.

0016It will be appreciated that the hydrogen and carbon monoxide used in the process according to the invention may be undiluted or diluted with one or more inert gases, such as nitrogen, helium and carbon dioxide.

0017The molar ratio of hydrogen to carbon monoxide is preferably in the range of from 2: 1 to 1: 2, more preferably about 1: 1.

0018Generally speaking the amount of the triphenylphosphite based on rhodium as central atom of the active complex, that must be present for attractive results is at least the stoichiometric amount, which corresponds to the preferred rhodium complexes HRhCO[P(O Phenyl)₃]₃ and ClRhCO[P-(O Phenyl)₃]₂.

0019However, an excess of the triphenylphosphite over the stoichiometric amount for complex formation i.e. over the amount actually complexed, has an advantageous effect on the yield of the desired isoaldehyde compound. This excess of the triphenylphosphite in free form, amounts advantageously to from 2-10, preferably from 2-6 and more preferably from 3-5 moles per mole of rhodium compound.

0020The reaction temperature is preferably in the range of from 55-75°C. The pressure is preferably in the range of from 15 to 60 bars.

0021The residence period can vary from about a couple of minutes to several hours, and as will be appreciated, this variable will be influenced to a certain extent by the reaction temperature, the choice of the specific reactant, the specific catalyst composition, the total synthesis gas pressure and the partial pressure exerted by its components.

0022A solvent is not generally essential in the process according to the invention, since usually one of the reactants or products can also serve as solvent. However, in certain circumstances the use of a solvent is desirable and practical. One can employ any normally-liquid organic solvent which does not interfere to any substantial degree with the desired hydroformylation reaction under the operative conditions employed. More preferably an apolar solvent is used, if any. Examples of such solvents include the saturated hydrocarbons, such as pentanes, naphtha, kerosene, cyclohexane etc. as well as the aromatic hydrocarbons, ethers, ketones, amides and nitriles. More specific examples are benzene, xylenes, toluene, diethyl ether, acetophenone, cyclohexanone, benzonitrile, N,N-dimethylacetamide, N,N-dimethylformamide and p-dioxan. More preferably toluene, benzene or xylenes will be used.

0023The preparation of the catalyst system to be used in the process of the present invention may be carried out by applying conventional techniques.

0024One suitable method is to combine the rhodium salt of an organic acid with the ligand, e.g. triphenylphosphite in the liquid phase. The valence state of rhodium may then be reduced by hydrogenating the solution prior to the use of the catalysts therein.

0025Alternatively, the catalysts may be prepared from a carbon monoxide complex of rhodium. E.g. one could start with dirhodium octacarbonyl and by heating this substance with the ligand, the ligand will replace one or more of the carbon monoxide molecules, thus producing the desired catalyst. It is also possible to start with the ligand of choice and rhodium metal or an oxide of rhodium and prepare the active catalyst species in situ during the hydroformylation reaction.

0026The process of the present invention may be carried out batchwise, semi continuously or continuously. When operating batchwise, the catalytic system, the starting acrylate compound and solvent are charged to a reactor to form a liquid phase therein, the reactor is pressurized with carbon monoxide and hydrogen and heated to the desired temperature. When operating continuously, the liquid components can be charged to the reactor continuously to form a liquid phase therein and the carbon monoxide/hydrogen continuously introduced into the reactor to contact the liquid phase containing the catalyst. The gaseous reactants can be withdrawn from the reactor as a separate effluent, cooled depressurized and the carbon monoxide and hydrogen may be recycled.

0027The following examples illustrate the invention.

0028All experiments were carried out in a 300 ml magnetically stirred Hastelloy C autoclave ("Hastelloy" is a trade mark). The reaction mixtures obtained were analyzed by means of gas-liquid chromatography.

Example 1

0029The autoclave was charged with rhodium di(carbonyl) acetyl acetonate (0.1 mmol), 20 ml methyl acrylate, 40 ml toluene and 0.3 mmol triphenylphosphite.

0030The autoclave was flushed with an equimolar mixture of hydrogen and carbon monoxide and then pressurized to 60 bar. The reaction mixture was heated up to 70°C, whilst maintaining the before-mentioned pressure.

0031After a total reaction time of 0.5 hr, the reaction mixture was cooled to room temperature and analyzed. The methyl acrylate conversion rate was calculated as being 5800 molCO/gat Rh.hr, while the selectivity for total aldehydes was 90% and for isoaldehyde was 93%.

Example 2

0032In rather the same way as described under example 1, an experiment was carried out, using 1 mmol triphenylphosphite instead of 0.3 mmol. A conversion rate of 5250 molCO/gat Rh.hr was calculated, while the selectivity for total aldehydes was 90% and for isoaldehyde was 96%.

Example 3

0033In rather the same way as described under example 1, an experiment was carried out, except that the total pressure was kept constant at 18 bar, while the temperature was 70°C.

0034After a total reaction time of 3.5 hr, a methyl acrylate conversion rate was calculated as being 1200 mol aldehyde/gat Rh.hr. The selectivity for total aldehydes was 95% and the selectivity for isoaldehyde was 88%.

Comparative Example A-C

0035In rather the same way as described under example 1, experiments were carried out, using 0.3 mmol triphenylphosphine, 0.3 mmol tri-(n)-butyl phosphite and 0.2 mmol diphenyl(pyridyl) phosphine respectively instead of 0.3 mmol of triphenylphosphite, showing after a reaction time of 5, 4 and 5 hours respectively, a calculated conversion rate of 100, 180 and 300 mol/gat Rh.hr, while selectivities for total aldehydes of 90%, > 30% and 80% were found and selectivities for isoaldehyde of 90%, 70% and 90% respectively were found.