Grignard reagents, radical mechanisms

An alternative radical mechanism is formulated as the transfer of a single electron from the Grignard reagent 2 onto the carbonyl group (single electron transfer... 

The mechanism of Grignard reagent formation involves free radicals," and... 

The mechanism for the formation of Grignard reagents might involve radicals ... 

Kinetic experiments have been performed on a copper-catalyzed substitution reaction of an alkyl halide, and the reaction rate was found to be first order in the copper salt, the halide, and the Grignard reagent [121]. This was not the case for a silver-catalyzed substitution reaction with a primary bromide, in which the reaction was found to be zero order in Grignard reagents [122]. A radical mechanism might be operative in the case of the silver-catalyzed reaction, whereas a nucleophilic substitution mechanism is suggested in the copper-catalyzed reaction [122]. The same behavior was also observed in the stoichiometric conjugate addition (Sect. 10.2.1) [30]. 

The addition of RLi and other nucleophiles to carbonyl functions in general proceeds via one of the two possible reaction pathways, polar addition (PL) and electron transfer (ET)-radical coupling (RC) sequence (equation 5). Current reaction design for the synthetic purpose of additions of common nucleophiles to aldehydes and ketones is mostly based on the polar mechanism, but apparently the ET process is involved in some reactions of, for example, Grignard reagents Mechanistically there are three possible variations the PL pathway, the ET rate-determining ET-RC route and the RC rate-determining ET-RC route. 

The mechanisms are complex, particularly in the organomagnesium (Grignard) reactions. Several reactive species are present, and the product metal alkoxide can complex with unreacted organometallic. Furthermore, trace transition metal impurities in the magnesium used to prepare Grignard reagents appear to facilitate electron transfer and may cause the reaction to proceed at least partly by a radical pathway. See (a) J. Laemmle, E. C. Ashby, and H. M. Neumann, J. Amer. Chem. Soc., 93, 5120 (1971) (b) E. C. Ashby, J. Laemmle, and H. M. Neumann, Accts. Chem. Res., 7,272 (1974). 

A concerted electron transfer mechanism, with formation of an alkyl radical and quinone radical anion, has been proposed to account for the products of reaction of benzophenone with alkyllithium or Grignard reagents 92 the ratio of addition to reduction products is dependent on the alkyl group and not on the metal. 

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