RHODIUM-IODIDE CATALYZED CARBONYLATION OF METHYL FORMATE INTO ACETALDEHYDE OR METHYL ACETATE - MECHANISTIC ASPECTS

Under CO pressure, the rhodium/ionic-iodide system catalyzes either th e reductive carbonylation of methyl formate into acetaldehyde or its h omologation into methyl acetate. The influence of the reaction conditi ons on the selectivity of these two react-ions was investigated and it was found that the former occurs selectively only in N-methylpyrrolid one (NMP) (or related solvents), for low I-/Rh ratio, low substrate co ncentration, and high CO pressure, whereas methyl acetate is preferent ially formed, in the same solvent, at higher I- and substrate concentr ations and under lower CO pressure. By using labeled methyl formate (( HCO2CH3)-C-13) it was also shown that the carbonyl group of acetaldehy de or methyl acetate does not result from that of methyl formate. In s itu IR studies conducted under catalytic conditions (high-pressure, hi gh-temperature) have not enabled the identification of any other catal ytic species than RhI2(CO)(2)(-) (which is also the active species in methanol or methyl acetate carbonylations), whatever the reaction cond itions ([I-], P-CO...). Plausible mechanisms are proposed for these re actions in which the essential role played by NMP in controlling the C H3I content in the reaction medium is clarified and taking into accoun t these experimental data. (C) 1997 Academic Press.