Influence of an aromatic carboxylic acid as cocatalyst in the palladium-catalysed reductive carbonylation of aromatic nitro compounds

Abstract

The addition of carboxylic acids such as benzoic acid derivatives as cocatalyst has been found to enhance the catalytic activity and selectivity of a Pd(phen) 2(OTf) 2 catalyst system in the reductive carbonylation of aromatic nitro compounds. A strong dependency of the catalytic performance on the amount of cocatalyst in the reaction mixture was found, yet no influence of the p K a-value of the particular cocatalyst could be discovered. Though the accelerating influence of the acidic cocatalyst is mainly caused by the protons of the cocatalyst since proton transfer plays a crucial role in the catalytic cycle, the positive effects could not be achieved by the use of other carboxylic acids or sulphonic acids as cocatalyst. Buffered systems showed that traces of weakly coordinating anions should be present in order to stabilise various catalytic intermediates, whereas higher concentrations of anions result in a decrease of catalyst activity. The formation of azoxybenzene was suppressed under the influence of the protons of the aromatic carboxylic acid, leading to an improvement of the catalyst selectivity. Based on the results a schematic mechanism for the palladium-catalysed reductive carbonylation of aromatic nitro compounds is proposed.