Wurtz coupling Zinc chloride

The reagent is described as inexpensive, readily prepared and nonpyrophoric, but air-sensitive. Sodium dispersed on titanium dioxide in a similar way has been used to reduce zinc chloride to active zinc for reaction with RX to give RZnX. This method produces fewer by-products from Wurtz coupling.18... 

The initiation of the zinc-mediated reaction could then be attributed to the formation of an allyhc radical anion of the type [CH2=CH-CH2-X] on the metal surface. This radical surface could then react with the carbonyl group to give an alkoxide radical, which could add an electron and form the alcohols (Einhom and Luche, 1987). This process takes place with an increase of the pH value of the reaction mixture (Sjohohn et al, 1994). Likewise, Wurtz coupling products were detected with cinnamyl chloride, which was supposed to stem from an allylic radical anion intermediate. Moreover, in one experiment without aldehyde, Sjdholm et al. (1994) observed that all cinnamyl chloride was transformed into dicinnamyl. 

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). 

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