GB1598270A - Process for the preparation of oxalate esters - Google Patents

Description

(54) PROCESS FOR THE PREPARATION OF OXALATE ESTERS (71) We, ATLANTIC RICHFIELD COMPANY, a corporation organised under the laws of the State of Pennsylvania, United States of America, of ARCO Plaza, 515 S. Flower Street, Los Angeles, State of California, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the preparation of oxalate esters.

In our copending patent application No. 20026/77 accepted as Serial no. 1570901, we described and claim a process for the preparation of an oxalate di-ester of an alcohol which comprises reacting, under substantially anhydrous liquid phase conditions, a monohydric saturated aliphatic, alicyclic or aralkyl alcohol containing from 1 to 20 carbon atoms, and which may contain at least one inert substituent (as hereinafter defined), with a mixture of carbon monoxide and oxygen at a partial pressure of carbon monoxide of between 500 psig and 3000 psig and a temperatue in the range of about 40"C and 1500C in the presence of an effective amount of a catalytic mixture of (a) a palladium, rhodium, platinum, copper or cadmium metal salt compound or mixtures thereof, (b) an aliphatic, cycloaliphatic, aromatic or heterocyclic amine or ammonia, (c) a copper (II) or iron (III) oxidant salt compound with a counterion other than a halide, and (d) from 0.1 to 40 per cent by weight, based on the weight of alcohol, of an ammonium or substituted ammonium salt compound with a counterion other than a halide, and recovering the desired oxalate ester.

By an inert substituent is meant a substituent which will not interfere with the reaction.

By the term "oxidant salt compound" is meant a salt compound which will oxidise the reduced form of the palladium, rhodium, platinum, copper or cadmium salt catalyst which occurs in the reaction; i.e. a redox agent.

We have now found in accordance with one aspect of the present invention that the counterion of component (c) of the catalytic mixture may be a halide and that copper (I) and iron (II) salts, added as such to the reaction mixture, are also suitable for use as component (c). It is understood that the salt chosen for component (c) must differ from that chosen for component (a).

In accordance with an alternative or additional aspect of the present invention, we have also found that the process may be operated with a carbon monoxide partial pressure in excess 3000 psi, up to 5000 psi.

We have further found that the minimum value for the preferred range of concentration of alcohol in the reaction mixture may be reduced from 62% to 50%, making a preferred range of 50% to 99.7% by weight of the reaction mixture.

Preferably, a catalytic amount of a ligand selected from halides and organic mono- and poly-dentate ligands having at least one atom containing a free electron pair, the atom being selected from phosphorous, arsenic and antimony, or of a co-ordination complex of said ligand and a metal selected from palladium, rhodium, platinum, copper and cadmium, is also included in the reaction mixture.

The ammonium or substituted ammonia salt compound may be added as such to the reaction mixture or formed in situ in the reaction mixture by reaction with an acid, such as sulfuric acid, added to the reaction mixture. Thus, for example, triethylamine can be employed initially in sufficient amounts and sulfuric acid added to form triethylammonium sulfate in the desired catalytic quantities. The addition of the amine (or ammonia) and the acid to form the ammonium or substituted ammonium salt must be closely controlled in order to establish a balance between the amine or ammonia base and the ammonium or substituted ammonium salt compound.

The reaction between the alcohol, carbon monoxide, and oxygen may be carried out in an autoclave or any other high pressure reactor. Although the order of addition of reactants and the components forming the catalyst mixture may vary, a general procedure is to charge the alcohol, amine or ammonia, (substituted) ammonium salt (or the required amount of amine or ammonia and acid), metal salt compound and component (c) into the reaction vessel, and if desired the ligand or coordination complex, then introduce the proper amount of carbon monoxide and oxygen to the desired reaction pressure and then heat the mixture to the desired temperature for the appropriate period. The reaction can be carried out batchwise or as a continuous process and the order of addition of the reactants may also be varied to suit the particular apparatus employed. The addition of the oxygen or oxygen-containing gas, such as air, can be a pulsed or continuous addition to the reaction system. The reaction products are recovered and treated by any conventional method such as distillation and/or filtration, etc. to effect separation of the oxalate ester from unreacted materials, amine or ammonia, component (c), (substituted) ammonium salt, metal salt compound, by-products, etc.

The alcohols which may be employed in concentrations of from about 50 to 99.7 weight per cent, preferably 77 to 94 weight per cent, and suitable for use in the process of the present invention are monohydric saturated aliphatic, alicyclic or aralkyl alcohols having from 1 to 20 carbon atoms and may contain inert substituents such as amido, alkoxy, amino, carboxy or cyano radicals in addition to the hydroxyl group. The substituents, in general, do not interfere with the reaction of the invention.

The alcohols which are employed may be primary, secondary or tertiary alcohols and conform to the general formula ROH, wherein R is an optionally substituted aliphatic, aralkyl or alicyclic 'group containing from 1 to 20 carbon atoms. Unsubstituted monohydric saturated aliphatic alcohols containing from 1 to 8 carbon atoms are preferred.

Representative alcohols especially suitable for use in this invention are saturated monohydric alcohols such as methyl, ethyl, n-, iso-, sec-, and tert-butyl, amyl, hexyl, octyl, lauryl, n- and iso-propyl, cetyl, benzyl, chlorobenzyl and methoxy-benzyl alcohols as well as, for example, tolylcarbinol, cyclohexanol, heptanols, decanols, undecanols, 2-ethyl hexanol, nonanol, myristyl alcohol, stearyl alcohol, methyl cyclohexanol, pentadecanol, oleyl and eicosyl alcohols. The preferred alcohols are the primary and secondary monohydric saturated aliphatic alcohols, such as methanol, ethanol, 1- and 2-propanol, n-butyl alcohol etc., up to 8 carbon atoms.

The amines employed in the catalyst component mixture in at least catalytic quantities in the process of the invention, in addition to ammonia, may be primary, secondary or tertiary amines and are aliphatic, cycloaliphatic, aromatic and heterocyclic amines or mixtures thereof. The amines may be unsubstituted or contain other substituents such as halides, alkyl, aryl, hydroxy, amino, alkylamino or carboxy. The amines are suitably employed as part of the catalyst system in the reaction in concentrations of from 0.1 to 10 weight per cent and preferably at a concentration of from 0.3 to 4 weight per cent as the free amine, based on the weight of alcohol.

Representative amines as hereinabove describe include for example, mono-, di- and tri-methyl, ethyl and propyl amines, iso- and diisopropylamines, allyl amines, mono-, di-, tri-, iso- and diisobutyl amines, 1-methylpropyl amine, 1 ,1-dimethylethyl amine, amyl amines, cyclohexyl amine, dicyclohexylamine, 1,3-dimethyl-butyl amine, 2ethylhexylamine, 1-cyclopentyl-2-amino propane, 1,1,1,3-tetramethylbutylamine, aniline, ethylene diamine, methylene diamine, ethanolamine, octylamines, n-decyl amine, do-, tetra-, hexa-, oct-, dido-, ditetra-, diocta-, trido- and triocta-decylamines, chloro-anilines, nitro-anilines, toluidines, naphthylamine, N-methyl and N-ethyl, and N,N-dimethyl and N,N-diethyl aniline, di- and triphenylamines, N,N-diamylaniline, benzyl dimethyl amine, piperidine and pyrrolidine. The preferred amines are the tertiary amines such as triethylamine and tributyl amine.

The metal salt compounds employed in the process of this invention in the catalyst mixture are the palladium, platinum, rhodium, copper and cadmium salts or mixtures thereof. Among the chemical forms of the metal salt compounds which can be used as such or as mixtures or formed in the reaction system from the metals per se are the palladium, platinum, cadmium and rhodium, halides, sulfates, oxalates and acetates and the copper halides, preferably the palladium (II) sulfate and palladium (II) and copper (I) or (II) halides such as palladium (II) iodide, and copper (I) iodide. Representative catalytic metal salt compounds include, for example palladium (II) chloride, copper (II) chloride, rhodium (III) chloride, copper (I) iodide, palldium (II) sulfate, palladium (II) acetate, palladium (II) iodide, rhodium (III) bromide, platinum (II) chloride, platinum (II) sulfate, cadmium chloride. As indicated above the metals as such may be added to the reaction as part of the catalyst mixture, the salt compound being formed in situ from at least a portion of the metal under reaction conditions.

The palladium, platinum, rhodium, copper and cadmium compounds employed may be in a homogeneous state in the reaction mixture at reaction conditions. Thus, the compounds may be present in solution, or suspension and may also be on support materials such as alumina, silica gel, aluminosilicates, activated carbon or zeolites. The compounds may be partially or completely soluble under reaction conditions. The reaction is carried out in the presence of a catalytic proportion of the metal salt compound and will proceed with small amounts of the metal salt compounds hereinabove described. Generally the proportions of the metal salt compound used in the reaction will be equivalent to from 0.001 to 5 weight per cent of the alcohol employed and are preferably employed in amounts of from 0.01 to 2 per cent by weight of the alcohol employed. Larger or smaller amounts may be employed at varied pressures and temperatures.

As mentioned hereinabove, a ligand of specified kind or a coordination complex compound of said ligand may be included in the reaction mixture and thereby provide a pronounced increase in the selectivity for the oxalate ester. The ligands that are suitable for use are halides and organic mono- and poly- dentate ligands in which at least one of the electron-donating atoms is an atom of phosphorus, arsenic or antimony containing a, lone pair of electrons. Examples include alkyl and aryl phosphines, arsines and stibines, and halide salts such as lithium iodide, ammonium chloride and triethylammonium iodide.

Suitable mono-dentate ligands include alkyl phosphines such as trimethylphosphine and tributylphosphine, aryl-phosphines such as diethylphenylphosphine and radicals derived from such phosphines, for example the radical having the formula -P(CH3)2. Hydrocarbyloxy phosphines, i.e. phosphites, such as triphenyl phosphite may also be employed.

Suitable polydentate ligands include tetramethyl di-phosphinoethane and tetraphenyl diphosphinoethane. Exactly analogous derivatives arsenic and antimony may be used; however, because of their greater ease of preparation and stability of the derived complexes, the hydrocarbyl derivatives of phosphorus are preferred. It is preferred to employ the alkali metal iodides, e.g. lithium iodide, or ammonium or substituted ammonium iodides, e.g. triethyl ammonium iodide.

The complexes of these ligands which are suitable for use in the process of the present invention are the complex compounds of palladium, platinum, rhodium, cadmium and copper. The complex compounds may contain one or more atoms of the said metals in the molecule and when more than one such atom is present, the metals may be the same or different.

These complex compounds may contain in the molecule, in addition to the ligands discussed above, one or more other atoms, groups or molecules, which are chemically bonded to the metal atom or atoms. Atoms which may be bonded to the metal include, for example, hydrogen, nitrogen and halogen atoms; groups which may be bonded to the metal include, for example hydrocarbyl, hydrocarbyloxy, carbonyl, nitrosyl, cyano and SnCl3groups; molecules which may be bonded to the metal include, for example, organic isocyanides and isothiocyanates. Examples of suitable complex compounds are those represented by the following formulae: RhBr3(PPhEt2) Rh(CO)Cl(AsEt3)2 RhCl(CO) (PPhEt2)2 RhCl(CO) (PEt3)2 Rh(Ph2PCH2CH2PPh2)2Cl PdC12(PPh3)2 Rh[(PhO)3P]3Cl PdI2(PPh3)2 Li2PdI4 PtCl2(p-ClC6H4PBu2)2

The complex compounds employed may be introduced into the reaction mixture as such, or they may be formed in situ from a suitable metal or metal compound noted above and the desired ligand.

The ligand or complex compounds may be used in catalytic amounts of from 0 to 3 per cent, preferably from 0.1 to 1 per cent, by weight of the alcohol to be reacted although larger or smaller amounts may be employed at varied pressures or reaction rates.

The compounds which may be employed in an anhydrous condition as component (c) in the catalyst component mixture, and preferably in catalytic amounts of from 0.1 to 10 weight per cent, preferably 2 to 6 weight per cent, based on the weight of alcohol, are the iron (II), iron (III), copper (I) and copper (II) salts such as the halides, sulfates, trifluoroacetates, oxalates, naphthenates or acetates; preferably the copper (II) sulfate and trifluoroacetate, copper (I) iodide and iron (III) sulfate. Representative salts include, for example, copper (II) sulfate, copper (II) trifluoroacetate, copper (II) acetate, copper (II) oxalate, copper (II) trifluoromethane sulfonate and copper (II) fluorosulfonate, copper (I) chloride, copper (I) sulfate, iron (III) sulfate, iron (II) iodide, iron (II) chloride, iron (III) acetate, iron (III) oxalate and iron (III) trifluoroacetate.

While halides may be employed in the process of this invention, excess halides in the form of component (c) salts, ligand or coordination complex compounds, or metal salt compounds may be detrimental to the reaction system of the present invention giving a low yield of oxalates esters.

The ammonium or substituted ammonium salts having a counter-ion other than a halide, and which are an important and required part of the catalytic component mixture, and are employed in an anhydrous condition and in a catalytic amount of from 0.1 to 40 weight per cent, and preferably 2 to 20 weight per cent, based on the weight of alcohol, in the process of the invention, include, for example, the ammonium and substituted ammonium sulfates, formates, trifluoroacetates and acetates, preferably the tertiary amine sulfates such as triethylammonium hydrogen sulfate and triethylammonium sulfate. The salt may be present in the reaction system only in equilibrium amounts. Representative salts include, for example, diethylammonium sulfate, ethylammonium sulfate, butylammonium sulfate, ammonium sulfate, trimethylammonium sulfate, monomethylammonium sulfate, trimethylammonium hydrogen sulfate, ammonium acetate, triethylammonium formate, ammonium trifluoroacetate, and methyl-, ethyl- and butyl-ammoniumtrifluoroacetate.

The ammonium or substituted ammonium salts may be added as such or formed in situ in the required amounts upon the addition of an acid, such as sulfuric, benzene sulfonic, phosphoric, o-boric, p-toluene sulfonic, acetic or trifluoroacetic, to the reaction mixture while using greater than the required quantities of the amine or ammonia base. The acids which may be used to form the salt include those which do not form a complex with the metal salt compound of the catalytic mixture or the metal oxidant salt compounds inactivating the catalyst and oxidant. As indicated hereinabove, the acids must be of sufficient strength, i.e., stronger than water, and such that the anion will not substantially complex with the metal salt or oxidant salt employed in the catalytic mixture. The salts which may be formed in situ may in themselves not necessarily be isolable and may exist in equilibrium in the reaction mixture under carbonylation reaction conditions. Thus, such salts could not be added per se but, as indicated above, may be formed in situ upon the addition of a suitable acid to the reaction mixture containing an excess of an amine or ammonia.

Although not required, solvents, if desired, which are chemically inert to the components of the reaction system may be employed. Suitable solvents include, for example, organic esters such as methyl formate, ethyl acetate, n-propyl formate, isopropyl acetate, sec- and iso-butyl acetate, amyl acetate, cyclohexyl acetate, n-propyl benzoate and lower alkyl phthalates; the alkyl sulfones and sulfoxides such as propyl ethyl sulfoxide, dissopropyl sulfone and diisooctyl sulfoxide; acetone and cyclohexanone.

As indicated above the reaction can be suitably performed by introducing the oxygen and carbon monoxide at a desired pressure into contact with the alcohol/catalyst mixture comprising the metal salt compound, amine or ammonia, (substituted) ammonium salt and salt employed as component (c) and possibly a ligand or coordination complex and heating to the desired temperature. A carbon monoxide partial pressure of 500 psi to 5000 psi, and preferably from 900 psi to about 2200 psi, is employed. Stoichiometric quantities of carbon monoxide are generally employed. However, an excess of carbon monoxide may be employed, for example, in continuous processes. Where large excess of or high carbon monoxide requirements are generally utilized, a suitable recycle of the unreacted carbon monoxide may be employed. The reaction will proceed at temperatures of from 40"C to 1500C. It is generally preferred to operate the process at temperatures in the range of 60"C. to 1000C. to obtain a convenient rate of reaction. Heating and/or cooling means may be employed interior and/or exterior of the reaction to maintain the temperature within the desired range.

At least stoichiometric amounts of oxygen or an oxygen-containing gas such as air may be employed and at any oxygen partial pressure such that the explosive range is avoided. Thus, the concentrations of oxygen should be low enough so that the reaction mixture is not potentially explosive. The Handbook of Chemistry and Physics, 48th Edition, 1967 indicates that the explosive limits of pure oxygen in carbon monoxide are 6.1 to 84.5 volume per cent and air in carbon monoxide are 25.8 to 87.5 volume per cent.

The reaction time is generally dependent upon the alcohol being reacted, temperature, pressure and on the amount and type of the catalyst mixture being charged as well as the type of equipment being employed. Reaction times will vary dependent on whether the process is continuous or batch.

The following Examples are provided to illustrate the invention in accordance with the principles of this invention but are not to be construed as limiting the invention in any way except as indicated by the appended claims.

Example I A solution of triethylamine (197.6 mmoles), concentrated (96.4 per cent) sulfuric acid (68.0 mmoles), and n-butyl alcohol (2.19 moles) was charged to a 500 ml stainless steel stirred autoclave. Palladium (II) iodide (2.0 mmoles), copper (I) iodide (4.0 mmoles) as component (c), and lithium sulfate monohydrate (8.0 mmoles) were charged to the autoclave as solids. Carbon monoxide was charged to the autoclave to 1070 psig, and the autoclave was heated to 70"C. with stirring at a rate of 1500 rpm. Stirring was discontinued while air was charged to give a total pressure of 1800 psig. A carbon monoxide flow rate of 4.0 llmin. and an air flow rate of 2.7 I/min. were established at 1800 psig. Stirring was started. The contents were allowed to react for 45 minutes, during which time gas samples of the effluent gases were collected periodically throughout the run and were analyzed for carbon dioxide.

The autoclave was cooled to ambient temperaure with tap water. The gas flows were stopped, and the reactor was vented. The vent gases were collected and analyzed for carbon dioxide. The liquid product was analyzed by gas-liquid chromatography after vacuum filtration to separate the precipitated solids from the liquid product. Analysis showed 0.023 mole dibutyl carbonate, and 0.267 mole dibutyl oxalate. The gases contained 0.165 mole carbon dioxide. Other oxalate-containing salts (0.040 mole) were detected in the liquid product.

Examples 2 to 6 Examples 2 to 6 which follow show, inter alia, the advantage of employing a (substituted) ammonium salt in the catalytic mixture of the invention. Example 2 is comparative in that no such salt, or acid to form same, is employed. Examples 3 to 6 employ such a salt (triethylammonium sulfate) in a range of weight per cent concentrations.

In Examples 2 to 6 the procedure of Example 1 was repeated employing the same conditions with the exception that the reaction was carried out for 60 minutes. In each of the Examples a total of 62 mmoles of triethylamine was present in the catalyst mixture as free amine.

The amount of catalyst charged in millimoles, weight per cent amine salt in solution, reactants and yield of dibutyl carbonate, dibutyl oxalate and carbon dioxide (CO2) are summarized in the Table.

TABLE Catalyst charged (moles) Wt.% (Et3NH)2SO4(6) Products (moles) EX. in Charge n-BuOH(3) No. PdI2 CuI Et3N(1) H2SO4 TEAHS(2) Solution (moles) DBC(4) DBO(5) CO2 2 2.0 4.0 61.6 --- --0.00 2.19 0.009 0.098 0.105 3 2.0 4.0 63.6 1.0 --- 0.17 2.19 0.013 0.136 0.116 4 2.0 4.0 73.4 11.8 --- 2.1 2.19 0.015 0.165 0.121 5 2.0 4.0 652 295 --- 34.2 2.19 0.019 0.228 0.136 6 2.0 4.0 212 --- 150 21.0 2.19 0.041 0.291 0.197 (1) Et3N - triethylamine (2) TEAHS - triethylammonium hydrogen sulfate (3) n-BuOH - n-butyl alcohol (4) DBC - dibutyl carbonate (5) DBO - dibutyl oxalate (6) (Et3NH)2SO4 - triethylammonium sulfate

Claims (39)

1. WHAT WE CLAIM IS:1. A modification of the process for the preparation of an oxalate di-ester of an alcohol by reacting under substantially anhydrous liquid phase conditions a monohydric saturated aliphatic, alicyclic or aralkyl alcohol containing from 1 to 20 carbon atoms, and which may contain at least one inert substituent (as hereinbefore defined), with a mixture of carbon monoxide and oxygen at a partial pressure of carbon monoxide of between 500 psi and 3000 psi and at a temperature in the range of 40"C to 1500C in the presence of an effective amount of a catalytic mixture of (a) a palladium, rhodium, platinum, copper or cadmium metal salt compound or mixtures thereof, (b) an aliphatic, cycloaliphatic, aromatic or heterocyclic amine or ammonia, (c) a copper (II) or iron (III) oxidant salt compound with a counterion other than a halide, and (d) from 0.1 to 40 per cent by weight, based on the weight of alcohol, of an ammonium or substituted ammonium salt compound with a counterion other than a halide, and recovering the desired dioxalate ester, in which: (i) component (c) of the catalytic mixture, is selected from copper (II) and iron (III) halides and copper (I) and iron (II) salts added as such to the reaction zone, and is different from component (a), and/or (ii) the carbon monoxide partial pressure is above 3000 psi up to 5000 psi.
2. 2. A process as claimed in Claim 1 in which the alcohol forms from 50 to 99.7% by weight of the reaction mixture.
3. 3. A process for the preparation of an oxalate di-ester of an alcohol by reacting under substantially anhydrous liquid phase conditions a monohydric saturated aliphatic, alicyclic or aralkyl alcohol containing from 1 to 20 carbon atoms, and which may contain at least one inert substituent (as hereinbefore defined), with a mixture of carbon monoxide and oxygen at a partial pressure of carbon monoxide of between 500 psi and 3000 psi and at a temperature in the range of 40"C to 1500C in the presence of an effective amount of a catalytic mixture of (a) a palladium, rhodium, platinum, copper or cadmium metal salt compound or mixtures thereof, (b) an aliphatic, cycloaliphatic, aromatic or heterocyclic amine or ammonia, (c) a copper (II) or iron (III) oxidant salt compound with a counterion other than a halide, and (d) from 0.1 to 40 per cent by weight, based on the weight of alcohol, of an ammonium or substituted ammonium salt compound with a counterion other than a halide, and recovering the desired dioxalate ester; and wherein the alcohol forms from 50 to 99.7% by weight of the reaction mixture.
4. 4. A process according to Claim 1, Claim 2 or Claim 3 wherein the alcohol is an unsubstituted monohydric saturated aliphatic alcohol containing from 1 to 8 carbon atoms.
5. 5. A process accoding to Claim 4, wherein the alcohol is selected from the group consisting of methanol, ethanol, propanol, a butanol and octanol.
6. 6. A process according to Claim 5, wherein the alcohol is methanol.
7. 7. A process according to Claim 5, wherein the alcohol is ethanol.
8. 8. A process accoding to Claim 5 wherein the alcohol is 2-propanol.
9. 9. A process according to Claim 5 wherein the alcohol is normal butyl, iso-butyl or sec-butyl alcohol.
10. 10. A process according to any one of Claims 1 to 9 wherein the metal salt compound is selected from palladium, rhodium, platinum and cadmium, halides, oxalates, sulfates and acetates and copper halides or mixtures thereof.
11. 11. A process according to Claim 10 wherein the metal salt compound is selected from palladium iodide, palladium sulfate, palladium chloride, palladium bromide, platinum acetate, platinum chloride, platinum bromide, copper iodide, cadmium chloride, cadmium iodide, and rhodium chloride.
12. 12. A process according to Claim 11 wherein the metal salt compound is palladium iodide.
13. 13. A process according to Claim 11 wherein the metal salt compound is palladium sulfate.
14. 14. A process according to Claim 11 wherein the metal salt compound is copper iodide.
15. 15. A process according to any one of Claims 1 to 14, wherein the amine is a primary, secondary or tertiary amine and is employed in a concentration of from 0.1 to 10 weight per cent, based on the weight of the alcohol.
16. 16. A process according to Claim 15 wherein the amine is employed in a concentration of from 0.3 to 4 weight per cent, based on the weight of the alcohol.
17. 17. A process according to Claim 15 wherein the amine is triethylamine.
18. 18. A process according to any one of Claims 1 to 17 wherein component (c) is a copper (I), copper (II), iron (II) or iron (III) halide, oxalate, sulfate, acetate, naphthenate, or trifluoroacetate.
19. 19. A process according to Claim 18 when appendent to Claim 1 or Claim 2 or any one of Claims 4 to 17 when appendent to Claim 1 or Claim 2, wherein component (c) is copper (I) iodide.
20. 20. A process according to Claim 18 wherein component (c) is copper (II) sulfate.
21. 21. A process according to Claim 18 wherein component (c) is copper oxalate.
22. 22. A process according to any one of Claims 1 to 21 wherein the ammonium or substituted ammonium salt compound is selected from ammonium and substituted ammonium sulfates, trifluoroacetates and acetates.
23. 23. A process according to Claim 22 wherein the ammonium or substituted ammonium salt compound is triethylammonium sulfate.
24. 24. A process according to Claim 22 wherein the ammonium or substituted ammonium salt compound is triethylammonium hydrogen sulfate.
25. 25. A process according to any one of Claims 1 to 24 wherein the ammonium or substituted ammonium salt compound is employed in concentration of from 2 to 20 weight per cent, based on the weight of alcohol.
26. 26. A process according to any one of Claims 1 to 25 wherein the ammonium or substituted ammonium salt compound is formed in situ upon the addition of an acid to the reaction mixture containing an excess of amine or ammonia, said acid being of a strength stronger than water and such that the anion will not substantially complex with the metal salt or component (c) in said catalytic mixture.
27. 27. A process according to Claim 26 wherein said acid is sulfuric acid.
28. 28. A process according to any one of Claims 1 to 27 wherein the reaction is carried out in the presence of a catalytic amount of a ligand selected from halides and organic monoand poly-dentate ligands having at least one atom containing a free electron pair, the atom being selected from phosphorus, arsenic and antimony, or of a co-ordination complex of said ligand and a metal selected from palladium, rhodium, platinum, copper and cadmium.
29. 29. A process according to Claim 28 wherein the ligand is triphenylphosphine.
30. 30. A process according to Claim 28 wherein the ligand is lithium iodide.
31. 31. A process according to Claim 2 or Claim 3 wherein the partial pressure of carbon monoxide is between about 900 psi and 2200 psi and the temperature is in the range of 60"C to 100"C.
32. 32. A process according to Claim 1, Claim 2 or Claim 3 wherein the alcohol is selected from the group consisting of methanol, ethanol, 2-propanol and butanol, the metal salt compound is palladium, iodide, the amine is triethylamine, component (c) is copper (II) sulfate, and the ammonium salt compound is triethylammonium sulfate.
33. 33. A process according to Claim 32 wherein a catalytic amount of lithium iodide is added to the reaction mixture.
34. 34. A process according to any one of Claims 1 to 33 wherein air is employed as a source of oxygen for the reaction.
35. 35. A process according to any one of Claims 1 to 34 wherein the metal salt compound is supported.
36. 36. A process according to Claim 26 wherein the alcohol is n-butyl alcohol, the metal salt compound is palladium iodide, the amine is triethylamine, component (c) is copper (I) iodide, and the ammonium or substituted ammonium salt compound is triethylammonium sulfate which is formed by the addition of concentrated sulfuric acid.
37. 37. A process as claimed in Claim 1, Claim 2 or Claim 3, substantially as hereinbefore described, with particular reference to Examples 1 and 3 to 6.
38. 38. A process as claime in Claim 1, Claim 2 or Claim 3, substantially as shown in any one of Examples 1 and 3 to 6.
39. 39. An oxalate ester when prepared by the process of any one of the preceding claims.