PROCESS FOR THE REDUCTION OF CARBONYL COMPOUNDS

Abstract: A process for the preparation of a primary or secondary alcohol, which comprises hydrogenating respectively an aldehyde or ketone in the presence of a catalyst system comprising a platinum compound, an acid having a pKa in the range of from 2 to 10.5 and a phosphorus compound of general formula R¹R²POH (I) in which R¹ and R² independently represent an alkyl group or an optionally substituted phenyl group, at a temperature in the range of from 20 to 150 o C and a pressure in the range of from 1 to 100 bar.

Description: The present invention relates to a process for the preparation of primary or secondary alcohols by the reduction of aldehydes or ketones. In particular it relates to a hydrogenation process in which a homogeneous catalyst system comprising a platinum compound, a phosphorus compound, and an acid is used. P.W.N.M. van Leeuwen et al , J. Chem. Soc., Chem. Commun., 1986, 31 to 33, describes the hydroformylation of certain alkenes using a catalyst system comprising a platinum compound and a phosphorus compound of formula Ph₂POH. The products of the hydroformylation are mixtures of aldehydes or ketones and alcohols. Presumably the alcohols are formed by the hydrogenation of carbonyl compounds. Surprisingly, it has now been found that aldehydes and ketones can advantageously be hydrogenated using a platinum compound, a phosphorus compound and an acid. Accordingly, the present invention provides a process for the preparation of a primary or secondary alcohol, which comprises hydrogenating respectively an aldehyde or ketone in the presence of a catalyst system comprising a platinum compound, an acid having a pKa in the range of from 2 to 10.5 and a phosphorus compound of general formula R¹R²POH (I) in which R¹ and R² independently represent an alkyl group or an optionally substituted phenyl group, at a temperature in the range of from 20 to 150°C and a pressure in the range of from 1 to 100 bar. The process according to the invention affords the alcohol products at a very high rate, which is very surprising. Preferably the aldehyde or ketone is a compound of general formula R³CHO (II) or R³COR⁴ (III) in which each of R³ and R⁴ is independently selected from optionally substituted alkyl, alkenyl, aryl, cycloalkyl and cycloalkenyl, or R³ and R⁴ together form an optionally substituted alkylene chain optionally containing one or more carbon-carbon double bonds. In this specification, unless otherwise stated, any alkyl or alkenyl group preferably has from 1 to 10, more preferably from 3 to 7 carbon atoms. Any aryl group is preferably a phenyl group. Any cycloalkyl or cycloalkenyl group preferably has from 5 to 7 carbon atoms. An alkylene chain optionally containing one or more carbon-carbon double bonds preferably has from 4 to 6 carbon atoms. Optional substituents, when present, are preferably selected from alkyl, alkoxy, halogen, alkoxyalkyl and alkoxycarbonyl groups. Very surprisingly, it has been found that when compounds containing a non-conjugated carbon-carbon double bond are hydrogenated according to the process of the invention, the carbon-carbon double bond is remarkably resistant to reduction. Accordingly, the process according to the invention is particularly advantageous for the selective reduction of aldehydes and ketones containing a non-conjugated carbon-carbon double bond. It is particularly surprising that aldehydes may successfully be reduced by the process according to the invention. Thus ruthenium and rhodium based catalysts can be poisoned by carbon monoxide which may be liberated during hydrogenation. The catalyst system used in the process according to the invention may be generated either by mixing each of the individual components, or by mixing substances which do not correspond to each of the individual components, but which in combination generate the catalyst system. The platinum compound may conveniently be a salt of platinum, for example a platinum carboxylate such as acetate, or a complex of platinum, for example platinum bis(cyclo-octa-1,5-diene) or platinum tris(ethylene). The acid having a pKa in the range of from 2 to 10.5 may be an inorganic acid such as phosphoric acid or an organic acid such as a carboxylic acid or phenol. Preferably the acid is a carboxylic acid. When the acid having a pKa in the range of from 2 to 10.5 is a carboxylic acid, it may be an aliphatic carboxylic acid such as an alkanoic acid, e.g., ethanoic acid, propanoic acid, butanoic acid or 2-methylpropanoic acid; or an aromatic carboxylic acid such as benzoic acid. The phosphorus compound of general formula (I) is preferably diphenylphosphinous acid. Preferably the process according to the invention is effected at a temperature in the range of from 80 to 120°C. The pressure is preferably in the range of from 20 to 60 bar. The molar ratio of phosphorus compound of formula (I) to platinum compound is conveniently in the range of from 1:1 to 4:1, but could be higher. Preferably the ratio is in the range of from 2:1 to 4:1. In generating the catalyst system, the platinum compound and the phosphorus compound of general formula (I) may be introduced separately or in the form of a single complex. For example they may be introduced in the form of a complex of general formula (R¹R²PO)(R¹R²POH)₂PtH (IV) in which R¹ and R² are as defined for the general formula (I). In this case the molar ratio of phosphorus compound to platinum compound is, of course, exactly 3:1. The molar ratio of acid having a pKa in the range of from 2 to 10.5 to platinum compound may conveniently be in the range of from 1:1 to 100:1. It will be appreciated that aldehydes tend to react with air to afford carboxylic acids. Consequently impure samples of aldehydes often contain carboxylic acids. When such impure samples of aldehyde are used in the process according to the invention, it may be unnecessary to use any additional acid. The process is optionally effected in the presence of a solvent. Suitable solvents include aromatic hydrocarbons such as toluene, haloalkanes such as dichloromethane, ethers such as dioxan or tetrahydrofuran, esters such as ethyl ethanoate, and alcohols such as ethanol. In certain cases it is advantageous to use an additional pi-acceptor ligand in the catalyst system. The additional pi-acceptor ligand may be, for example, carbon monoxide, ethene, triphenylphosphine, tri(p-tolyl)phosphine or tricyclohexylphosphine. Additional gaseous pi-accetor ligands may conveniently be present at a pressure in the range of from 5 to 10 bar. Phosphines may conveniently be present at a molar ratio of phosphine to platinum of below 2. The alcohol products of the process according to the invention may be recovered using techniques well known in the art of homogeneous catalysis, such as distillation under reduced pressure. Advantageously platinum may be recovered from the reaction product by chromatography over phosphine-carrying resins, and the phosphine oxides may be removed by percolation over alumina. The following examples illustrate the invention. Example 1 Hydrogenation of 3-(4-Methyl-3-cyclohexen-1-yl)-1-butanal A 100ml autoclave (Hastelloy C) was charged with 20ml of toluene, 15 mmol of 3-(4-methyl-3-cyclohexen-1-yl)butanal (prepared by the hydroformylation of limonene, as described in EP-A-54986), 0.05 mmol of (Ph₂PO)(Ph₂POH)₂PtH and 0.5 mmol of CH₃COOH. The autoclave was then pressurized with 40 bar H₂ and then heated to 95°C. The reaction mixture was kept at this temperature for 0.5 hours. The mixture was then allowed to cool and was analysed thereafter using gas-liquid chromatography. The conversion of 3-(4-methyl-3-cyclohexen-1-yl)butanal was 37.5%, with a selectivity to 3-(4-Methyl-3-cyclohexen-1-yl)-1-butanol of 90%, i.e. less than 10% of the olefinic double bond had been hydrogenated. From the conversion, amount of catalyst and the reaction time an average turnover per hour is calculated of 200 mol of product per mol of catalyst per hour. Examples 2 to 11 The method of Example 1 was repeated using different aldehydes and ketones. The amounts of substances used and the rates of reaction are given in Table 1. The rate is the average rate in mol product per mol of catalyst per hour measured over 30-50% conversion. Example 12 The method of Example 1 was repeated using 20.0 mmol of 5-formyl-bicyclo[2,2,1]-2-heptene (exo/endo, prepared by the Diels Alder addition of acroleine to cyclopentadiene) as the aldehyde, 0.05 mmol of (Ph₂PO)(Ph₂POH)₂PtH and 1 mmol of PPh₃ as additional ligand. The reaction temperature was 110°C. The pressure applied was 38 bar of H₂ and 2 bar of CO. After 1 hour of reaction time 50% was converted to 5-hydroxymethyl-bicyclo- [2,2,1]-2-heptene with a selectivity of 98%. The calculated rate is 200 mol per mol per hour. Examples 13 to 18 These Examples show the effect of varying the presence and/or using additional pi-acceptor ligands on the hydrogenation of 5-formyl-bicyclo[2,2,1]-2-heptene. The amounts of reactants and solvent and the temperature were the same as in Example 12. Example 19 10-decenal (5ml) was hydrogenated using 0.05 mmol of (Ph₂PO)(Ph₂POH)₂PtH, 25ml of toluene, and 0.5 mmol of isobutyric acid at a pressure of 40 bar H₂ and a temperature of 95°C. After 55 minutes the conversion was 56%, with a selectivity of 59% to the unsaturated aldehyde according to gas chromatographic analysis. Example 20 The method of Example 19 was repeated but with (PPh₂OH)(PPh₂O)PtH(P(p-tolyl₃))as the platinum compound. The conversion was 63% after 70 minutes with a selectivity of 52%. Example 21 The method of Example 20 was repeated but using a platinum compound of formula (PPh₂OH)(PPh₂O)PtH(P-(cyclohexyl)₃). After 60 minutes a conversion was obtained of 81% with a selectivity of 59%. Example 22 The method of Example 21 was repeated except that carbon monoxide was additionally supplied at a pressure of 2 bar. The selectivity increased to 98%, at 120°C, and at 17% conversion after one hour. Example 23 to 25 Hydrogenation of 6-methyl-5-hepten-2-one was carried out with the catalysts and conditions of Examples 19 to 22.

Claims: A process for the preparation of a primary or secondary alcohol, which comprises hydrogenating respectively an aldehyde or ketone in the presence of a catalyst system comprising a platinum compound, an acid having a pKa in the range of from 2 to 10.5 and a phosphorus compound of general formula R¹R²POH (I) in which R¹ and R² independently represent an alkyl group or an optionally substituted phenyl group, at a temperature in the range of from 20 to 150°C and a pressure in the range of from 1 to 100 bar. A process as claimed in claim 1, in which the aldehyde or ketone is a compound of general formula R³CHO (II) or R³COR⁴ (IV) in which each of R³ and R⁴ is independently selected from optionally substituted alkyl, alkenyl, aryl, cycloalkyl and cycloalkenyl, or R³ and R⁴ together form an optionally substituted alkylene chain optionally containing one or more carbon-carbon double bonds. A process as claimed in claim 1 or claim 2, in which the aldehyde or ketone contains a non-conjugated carbon-carbon double bond. A process as claimed in any one of claims 1 to 3, in which an aldehyde is used. A process as claimed in any one of claims 1 to 4, in which the platinum compound is platinum bis(cyclo-octa-1,5-diene), platinum acetate or platinum tris(ethylene). A process as claimed in any one of claims 1 to 5, in which the acid having a pKa in the range of from 2 to 10.5 is a carboxylic acid. A process as claimed in claim 6, in which the carboxylic acid is selected from ethanoic acid, propanoic acid, 2-methylpropanoic acid and butanoic acid. A process as claimed in any one of claims 1 to 7, in which the phosphorus compound is diphenylphosphinous acid. A process as claimed in any one of claims 1 to 8, in which the temperature is in the range of from 80 to 120°C. A process as claimed in any one of claims 1 to 9, in which the pressure is in the range of from 20 to 60 bar.

Inventors: ROOBEEK, CORNELIS FRANCISCUS, VAN LEEUWEN, PETRUS WILHELMUS NICOLAAS MARIA
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1989
Main classification: C07C29
IPC codes: C07C29, C07C31, C07C35, B01J31, C07C33, C07B61