Free Radical Mechanisms in Homogeneous Catalysis
The theme of free radical mechanisms in homogeneous catalysis 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 field 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 I, namely mechanisms involving organometallic intermediates (typically 16- or 18-electron species) and characteristic sequences of “two-electron” 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 organometallic chemistry have once again focussed attention on free radical mechanisms and suggest that much 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 describes and discusses some of these developments and their implications.
KeywordsMetal Hydride Bond Dissociation Energy Homogeneous Catalytic Process Homogeneous Catalysis Free Radical Mechanism
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