Mechanisms of organic oxidation and reduction by metal complexes.
The mechanism of many organic oxidation and reduction reactions can be described in terms of the formation and reaction of free radicals with metal complexes. Redox (trace-metal) catalysis also involves the oxidation and reduction of radical intermediates with a metal species which oscillates between several oxidation states (4). The oxidation and reduction of free radicals with metal complexes follow two general mechanisms, electron transfer and ligand transfer. Direct analogy exists with wholly inorganic descriptions of outer-sphere and innersphere processes. In an electron transfer or outersphere mechanism the redox process is derived largely by transfer of an electron from reductant to oxidant, with only indirect contributions from the solvent and ligand. Carbonium ion intermediates and transition states are important considerations, and the scission of the beta-hydrogen bond is minor during oxidation of alkyl radicals to alkenes. In contrast, ligand transfer or inner-sphere mechanism demands maximum involvement of the ligand in the transition state. Free-radical character prevails; cationic contributions from the organic moiety are minimal. Oxidation and reduction are conjugate processes. In an electron transfer mechanism the oxidation of alkyl radicals to carbonium ions is conceptually represented by a microscopic reverse reaction in which a carbonium ion is reduced to an alkyl radical. A similar duality exists in the interconversion of carbanions and free radicals by metal complexes. The reversibility of the ligand transfer process is easier to observe. For example, the chlorine-transfer oxidation of alkyl radicals is represented by a microscopic reverse reduction of alkyl chlorides to alkyl radicals by cuprous chlorides. A ligand transfer counterpart of the reduction of radicals R*+Cu(II)Cl(n)<---->R-Cl+Cu(I)Cl(n-1) can also be described. Hopefully, these simple redox mechanisms will be utilized in rationalizing complex reactions and formulating new syntheses. The limited number of examples cited in this short review represent only an introduction to the vast area of chemical research to be tapped in the study of the mechanisms and the synthetic utility of oxidation-reduction reactions and catalysis.
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