Process for the preparation of esters of carbonic acid

The present invention refers to a process for the preparation of esters of carbonic acid. Esters of carbonic acid are known which are useful as solvents and polymerization agents in the production of polycarbonates by transesterification with glycols and diphenols. Alkyl, alkyl-aryl or aryl carbonates are employed in this case.

According to the known art the above esters are obtained by reacting an alcohol or a glycol with phosgene or chloroformiates in the presence of bases selected from the group consisting either of hydroxides, alkaline or alkaline-earth carbonates, or of pyridine or other organic bases. In the first case the reaction is carried out in the presence of solvents and at a controlled temperature in order to get the final products out of the hydrolysis by the same bases and the water anyhow present in the reaction vessel.

We have now found it possible to prepare esters of carbonic acid by reacting the alcohol for esterification with carbon monoxide and oxygen in the presence of suitable catalysts. The reaction goes on up to quantitative yields with respect to the fed reactants.

The process according to the present invention may be substantially schematized by the following way: ##STR1## wherein R is a hydrocarbon radical selected from the alkyl, aryl or cycloalkyl radicals.

The reaction is catalyzed by complexes of metals (M) of the 1st B, 2nd B and 8th group of the periodic system. Very suitable metals are Cu, Ag, Au, Zn, Cd, Hg, Fe, Co, Ni. i.e., metals able to exist in two different valence conditions by means of redox reactions. The corresponding metal ions are able to carry out the above reaction only with very low yields and through very unfavourable kinetics. Very suitable catalysts are the complex molecules having the formula MX n L m wherein n ranges between 1 and 3, m ranges between 5 and 3. X is an anion and L is a neutral ligand. The complex may be a saline complex too as [ML m ] + n n X - . The n + m sum is generally equal to six. i.e. the complexes may exist at the most in a hexacoordination state, which may dissociate in solution. The more suitable anions are halide ions, CN, C10 4 - , complex anions as BF 4 - and the like. The L ligands are selected from the group consisting of organic bases as pyridine, dipyridyl, imidazole, phenanthroline, alkyl or aryl phosphines, dimethylsulfoxide, dimethylformamide, quinuclidine, CO; suitable ligands are also the nitriles as CH 3 CN, C 6 H 5 CN, and the bidentate ligands as malonitrile, succinodinitrile, adiponitrile and the like.

The inventive reaction will be illustrated by the unrestrictive employment of complexes of Cu, Co and Au. It is preferable that the complexes contain the aforesaid metals in their lower oxidation state; from a kinetic point of view it is interesting to use the carbonyl compounds obtained from the above complexes; they are prepared before being introduced into the reaction vessel and then are put therein.

The reaction is carried out in a solvent consisting either of alcohol to be oxidized or of a mixture of the alcohol with an inert solvent. The latter is selected with the view of removing of the reaction water by means of an azeotropic distillation. Therefore the more suitable solvents are C Cl.sub. 4, C 6 H 6 , CH 3 --C 6 H 5 and the like.

The complex may be obtained by starting from the metal ion dissolved in the solvent to which a stoichiometric amount of the complexing base is added. It is generally prepared in advance, characterized and then introduced into the reaction vessel.

The inventive process may be carried out in a wide range of pressures and temperatures and conveniently runs out of atmospheric pressure and room temperature too. Both factors may be usefully changed in order to advantageously influence the reaction kinetics and their limit values depend only on the boiling temperature of the mixture or on the firmness of the introduced catalyst. The temperature generally ranges between -20° C. and +110° C. and preferably between 50° and 70° C. The inventive reaction satisfactorily runs at room pressure, but it is preferable to increase the partial pressure of CO to values higher than the atmospheric one up to 3 + 4 atmospheres. This fact kinetically favours the reaction owing to the stabilization of the carbonyl derivatives which are the process catalysts.

The invention is now illustrated by the following unrestrictive examples: