AN ELECTROCHEMICAL AND SPECTROELECTROCHEMICAL (IR) INVESTIGATION OF THE REDUCTION OF RCO(II)TPP (R = BENZYL OR BUTYL, TPP = TETRAPHENYLPORPHYRIN) - MECHANISTIC IMPLICATIONS IN THE COTPP CATALYZED ELECTROCARBOXYLATION OF ALKYL-HALIDES

The electrochemical reduction of the intermediates generated from the reaction of Co(I)TPP- (TPP = tetraphenyl porphyrin) with benzyl chlori de (PhCH2Cl) or butyl bromide (BuBr) have been studied by use of elect rochemistry and IR-spectroelectrochemistry. The rates of formation of PhCH2Co(III)TPP, and of decomposition of the reduction product [PhCH2C o(II)TPP](-) are much faster than those found for the corresponding Bu Br system. The decomposition of [RCo(II)TPP](-) proceeds via homolytic Co-C bond cleavage, to generate the corresponding organic radical at negative potential, where RCo(III)TPP can be reduced to [RCo(II)TPP](- ). A mechanism involving the reaction of organic radicals with the por phyrin ring and transfer from cobalt to a nitrogen of the pyrrole ring , leading to the formation of an N-alkyl cobalt porphyrin complex is p roposed, which is consistent with the destruction of catalyst during e lectrocatalytic carboxylation and reduction of an organic halide. (C) 1998 Elsevier Science S.A. All rights reserved.