INFLUENCE OF AN AROMATIC CARBOXYLIC-ACID AS COCATALYST IN THE PALLADIUM-CATALYZED REDUCTIVE CARBONYLATION OF AROMATIC NITRO-COMPOUNDS

The addition of carboxylic acids such as benzoic acid derivatives as c ocatalyst has been found to enhance the catalytic activity and selecti vity of a Pd(phen)(2)(OTf)(2) catalyst system in the reductive carbony lation of aromatic nitro compounds. A strong dependency of the catalyt ic performance on the amount of cocatalyst in the reaction mixture was found, yet no influence of the pK(a)-value of the particular cocataly st could be discovered. Though the accelerating influence of the acidi c cocatalyst is mainly caused by the protons of the cocatalyst since p roton transfer plays a crucial role in the catalytic cycle, the positi ve 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 anio ns result in a decrease of catalyst activity. The formation of azoxybe nzene was suppressed under the influence of the protons of the aromati c carboxylic acid, leading to an improvement of the catalyst selectivi ty. Based on the results a schematic mechanism for the palladium-catal ysed reductive carbonylation of aromatic nitro compounds is proposed.