Coupling, organometallic Wurtz reaction

Silicon heterocycles with different ring sizes have been synthesized by variations of the organometallic Wurtz coupling reaction. The method requires synthesis of intermediate organosilicon derivatives that can involve several steps. 

Reduction of the C—N bond 0-86 Coupling of alkyl halides (Wurtz) 0-87 Coupling of alkyl halides with or-ganometallic reagents 0-89 Reaction between organometallic reagents and alkyl sulfates or sulfonates... 

However, their nonstatistical distribution as well as retention of stereochemistry if alkenyl organometallics were used (see Scheme 10.14),12 suggested that free radicals were not intermediates in such reactions. Such Wurtz-type couplings therefore probably proceeded by transmetallation and then recombination (Scheme 10.1). 

The formation of organometallic intermediates, however, can be initiated by an allylic radical anion on the metal surface (Figure 4.3). Noteworthy is the high value of the electron affinity of allylic bromide and, to a lesser extent, chloride (Moyano et al, 1990). As a matter of fact, in tin-mediated reactions, Wurtz coupling products were sometimes detected (Kim et al, 1993), but, unlike the zinc-mediated allylation, no dimerization of the carbonyl compounds was observed in the absence of the halide (Einhom and Luche, 1987). 

Reactions with Metals and Organometallics. Difficulties encountered in the preparation of benzylic metal compounds with active metals are due primarily to the tendency of these compounds to undergo Wurtz coupling (self condensation). Benzylmagne-sium bromide may nevertheless be prepared from benzyl bromide and used under standard or Barbier conditions. Benzyllithium cannot be obtained practically from benzyl bromide. Benzylzinc bromide and the cyanocuprate BnCu(CN)ZnBr have both been... 

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