Carbon-metal bonds reductive formation

Burchill and Hickling, 1970). Organic radicals have been shown to be oxidized by a variety of other inorganic ions. The most commonly used oxidants were Cu2 + and Fe(CN)6 (see e.g. Garrison, 1968 Haysom et al., 1972 Bhatia and Schuler, 1974). However, evidence has been presented also for the reduction of such radicals by Fe2 + and Ti3 + (Behar et al., 1973) and the process has been suggested to involve intermediate formation of a carbon-metal bond [reactions (60) and (61)]. Intermediates containing carbon-metal bonds have... 

Hosomi, A., Miura, K. Palladium-catalyzed carbon-metal bond formation via reductive elimination. Handbook of Organopalladium Chemistry for Organic Synthesis 2002, 1,1107-1119. 

Where the nature of the metal or experimental conditions render such auxiliary complexation with either the substrate or the solvent less favorable, there is an increased tendency towards homolysis of the carbon-metal bond. Treatment of organolithium and organomagnesium alkyls with transition metal halides leads to metal halide reduction and hydrocarbon, suggestive of the transitory formation of metal alkyls (45), decomposing thereupon into a lower salt and free alkyl radicals ... 

III.3.4 Palladium-Catalyzed Carbon-Metal Bond Formation via Reductive Elimination... 

This section mainly deals with Pd-catalyzed reactions of aryl, alkenyl, benzyl, alkyl, and acyl electrophiles with metal nucleophiles M = Si, Ge, Sn, B, and transition metals) leading to carbon-metal bond formation (Scheme 1). Allylic and related metallations are described in Sect. V.2.3.3, The reaction mechanism is generally believed to involve, firstly, oxidative addition of R—X to a Pd(0) species second, transmetallation of the resulting Pd(II) species with M—M and finally, reductive elimination forming R—M and the active Pd(0) species. However, another catalytic cycle via oxidative addition of M—M to Pd(0) species is also proposed. ... 

A critical examination of all the proposed mechanisms shows that the carbon — metal bond formation mechanism depends to a considerable extent on the nature of the initial reactants. Above all it should be noted that in all probability the radical mechanism does not hold in the formation of organometallic compoimds durii electrochemical reduction of unsaturated and l logen-containing compounds. 

Metal-induced reductive dimerization of carbonyl compounds is a useful synthetic method for the formation of vicinally functionalized carbon-carbon bonds. For stoichiometric reductive dimerizations, low-valent metals such as aluminum amalgam, titanium, vanadium, zinc, and samarium have been employed. Alternatively, ternary systems consisting of catalytic amounts of a metal salt or metal complex, a chlorosilane, and a stoichiometric co-reductant provide a catalytic method for the formation of pinacols based on reversible redox couples.2 The homocoupling of aldehydes is effected by vanadium or titanium catalysts in the presence of Me3SiCl and Zn or A1 to give the 1,2-diol derivatives high selectivity for the /-isomer is observed in the case of secondary aliphatic or aromatic aldehydes. 

Of great synthetic potential are demetallations with simultaneous formation of C-Sn bonds (Figure 2.20) [137,146,230,281,282]. These reactions presumably proceed via a heterobimetallic intermediate containing an Sn-M-C group. Reductive elimination of the metal M from this intermediate leads to formation of the carbon-tin bond. The resulting alkyl- or vinylstannanes are valuable synthetic intermediates. 

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