Free radical mechanisms in coordination chemistry

Abstract

Transition metal complexes exhibit several distinctive types of reactions leading to the formation of free radicals, notably: LM + RX —'LnMX + R• L M-R —L N. + R n n L N-H + C=C —'L M + HC-C n n The thermodynamic, kinetic and mechanistic aspects of these reactions and their roles in homogeneous catalysis are discussed. INTRODUCTION. The theme of free radical mechanisms in coordination chemistry is an old one as manifested by such long familiar, examples as the catalytic roles of metal salts (cobalt, manganese, copper, etc.) in the autoxidation of organic compounds. However, research and developments in the fields of coordination chemistry and of homogeneous catalysis during the past several decades have tended to deemphasize this theme and to focus for the most part, instead, on processes such as hydrogenation, carbonylation, hydroformylation and certain oxIdation reactions which proceed through non-radical mechanisms of the type depicted in Table 1, namely mechanisms involving organometallic intermediates (typically 16or 18-electron species) and characteristic sequences of twoelectron component steps such as oxidative addition, migratory insertion and reductive elimination (1-4). The emphasis on such processes has been reinforced by the highly novel chemistry associated with them, as well as by important and distinctive applications, notably homogeneous catalytic processes of unusual selectivity such as asymmetric catalytic hydrogenation (5). In view of this, it is noteworthy that some of the most recent developments in the fields of homogeneous catalysis and coordination chemistry have once again focussed attention on free radical mechanisms and suggest that such mechanisms are more important and widespread than previously suspected, not only in new contexts but also in the context of processes, for example certain hydrogenation and hydroformylation reactions, that have previously been interpreted in terms of non-radical mechanisms (6-9). The present paper de-. scribes and discusses some of these developments and their implications. GENERATION OF FREE RADICALS BY REACTIONS OF METAL COMPLEXES Three distinctive and widespread processes in which free radicals are generated through reactions of metal complexes are depicted schematically by eqs. 1, 2 and 3, respectively. LM. +R-X——--LM-X+R• (1) n n LM-R —LM' + R (2) n n LM-H + C=CZ LnM + HC-CZ (3)