Nitrobenzyl halides dimerization

Halomalonic acid derivatives, 1-halo-l-nitroalkanes [186], and related electrophiles can, upon treatment with a base, also dimerize to yield substituted ethylenes, in the same way as nitrobenzyl halides (see above). The reaction conditions required for this dimerization do not differ much from those required for successful nucleophilic substitution (Scheme 4.46), and if substitution is desired a low concentration of the electrophile should be maintained during the reaction to minimize dimerization. 

Nitrobenzyl halides are reduced in a 1 e-process to radical anions, which rapidly lose halide ion to form the neutral nitrobenzyl radical. The rates for this dissociation were calculated from cyclic voltammetry data to be k = 2,5 for m-nitrobenzyl chloride and k = 2 109 secfor o-nitrobenzyl bromide. The nitrobenzyl radicals predominantly dimerize (90%), whereas a small amount yields nitrotoluenes (< 10%) by hydrogen abstraction 4541. From a series of substituted benzyl bromides those with the more positive reduction potential form bibenzyl in 25—74% yield, whereas from the less easily reducible ones dibenzyl-mercury derivatives are obtained (50—60%) 485). Reduction of benzyl chloride at the plateau of the first wave yields dibenzylmercury 4 By reduction of diphenyliodonium hydroxide at -1,6 V 51% diphenylmercury is obtained 488 ... 

Dinitrobibenzyls are key intermediate species in certain drug syntheses [21, 22]. They can be formed by the electrochemical reduction of nitrobenzyl halides in various solvents [23-25]. In liquid ammonia at — 60 °C, the mechanism involves a one-electron reduction and dechlorination followed by coupling of the neutral radical intermediate species (see Fig. 4). Once the dimer is formed, it can undergo two one-electron reductions at more negative... 

Benzylic electrophiles bearing electron-withdrawing groups at the arene do not always yield the expected products of nucleophilic substitution on treatment with a nucleophile. One important side reaction is the dimerization of these compounds to yield 1,2-diarylethenes (stilbenes). This dimerization does not require such highly activated systems as the example sketched in Scheme 4.28, but can even occur with, for example, 2- or 4-nitrobenzyl chloride [120, 121]. The latter compounds are converted into the corresponding stilbenes by treatment with KOH in ethanol [120]. Di-arylmethyl halides behave similarly and can yield tetraarylethenes on treatment with a base. These reactions presumably proceed via the mechanism sketched in Scheme 4.27, in which the amphiphilic character of the nitro group plays a decisive role (metalated nitroalkanes or 4-nitrobenzyl derivatives can act as nucleophiles and as electrophiles). 

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