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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.

Primary and secondary acids can be readily decarboxylated to alkenes by PbIV tetraacetate in the presence of catalytic amounts of cupric acetate. Terminal alkenes are formed quantitatively from primary acids, but conversions are limited by competitive decarboxylation of acetate. Synthetic procedures can be developed by examination of the effects of acid, PbIV and catalysts on the rates of decarboxylation and conversions of acids. Unreacted acid is recovered readily from the mixture, and recycling should enable high conversions to alkenes. A convenient preparation of ω-alkenoic acids from dibasic acids, such as 6-heptenoic acid from suberic acid, is described. Secondary acids can be converted to alkenes in high conversions since competitive decarboxylation of acetate is not important. Decarboxylations can be carried out either thermally at 80° or photochemically at 30° with essentially the same results. The latter is the preferred method to employ with heat sensitive compounds.

Cobalt(III) trifluoroacetate is formed by the equilibration of di-μ-hydroxodiacetatocobalt(III) in trifluoroacetic acid. Acid catalysis in the cleavage of the hydroxo bridges in the dimer is made irreversible by the use of trifluoroacetic anhydride as a dehydrating agent. Cobalt(III) species formed by ligand exchange in mixtures of trifluoroacetic acid and acetic acid are discussed. Promotion of ion-pair dissociation of cobalt(III) trifluoroacetate in pure trifluoroacetic acid is used to account for its powerful properties as an electron-transfer oxidant. The facile oxidation of such usually inert hydrocarbons as benzene, ethylene and cyclohexane is described.

Cuprous acetate complexes are stable in acetonitrile-acetic acid solutions, and procedures for simple preparations of these solutions are described. Cuprous compounds are useful for the direct initiation of copper-catalyzed decompositions of peroxides at the low temperatures often required to obviate competing ionic rearrangements. Thus, these cuprous solutions can be effectively employed at 0° and lower temperatures to catalyze the homolytic chain decomposition of diacyl peroxides and alkyl hydroperoxides. The copper-catalyzed reactions of peresters with alkenes and other substrates can also be initiated at room temperature simply by stirring with a mixture of copper powder and cupric acetate in acetonitrile-acetic acid solutions. The stoichiometries, rates and kinetic chain lengths of these reactions are delineated with regard to a common mechanism which generally prevails in copper catalysis of peroxide reactions.

The decomposition of tetraethyllead is strongly catalyzed by copper(I) species in acetic acid. Ethane and triethyllead acetate are formed in stoichiometric amounts. The kinetics are first order in tetraethyllead and Cu I but reach limiting values of the latter due to the partial solubility of Cu I acetate. The different kinetic isotope effects for the uncatalyzed and the copper-catalyzed, acetolysis indicates that the latter occurs via a metastable intermediate. A mechanism is proposed in which alkyl transfer from tetraalkylead to Cu I is rate-limiting. The subsequent rapid protonolysis of the alkylcopper intermediate is discussed.

The stoichiometry and the kinetics of tho reaction between various Grignard reagents and alkyl halides with a copper catalyst have been examined in tetrahydrofuran solutions. The catalytically active species is an organocopper(I) complex produced by the rapid motathesis between copper(I) and -(II) halides and Grignard reagents. Primary alkyl bromides react with various Grignard reagents under these conditions to produce cross-coupled dimers exclusively. A mechanism is proposed in which coupling occurs between the alkyl halide and the alkylcopper(I) intermediate by displacement of the halide. On the other hand, only disproportionation products are produced from secondary and tertiary alkyl halides due to the unfavorable displacement process. The rate and the mechanism of the thermal decomposition of alkylcopper(I) species under the reaction conditions as well as sidereactions involved in the coupling reaction are described.

Aus aquimolaren Mischungen von Hexamethylbenzol (I) und dem Salz (II) in-Trifluoressigsaure last sich unter kontrollierten Bedingungen ein 1:1-Elektronendonator-Akzeptor-Komplex (III) in kristalliner Form isolieren.

La stabilisation de divers complexes formyl peut etre realisee par addition de (t-Bu) 3 SnH et autres donneurs de H. Mecanisme

Etude de deux radicaux du cation bipyridinium (MV + et DQ + ) dans la zeolite Y par spectrometrie RPE et spectrometrie d'absorption en comparant avec les spectres de reflexion diffuse des zeolites, colorees en bleu et vert