Radicals, reduction with benzylic hydrogens

Any substituted benzyl- ions formed in the course of the reduction will yield eventually a polystyrene, and indeed, a small amount of polymer was found in the reduction products of styrene (17). However, the reduction of compounds which give radicals of higher electron affinity leads to a substantial amount of carbanions. i.e. with those compounds the electron-transfer to a radical competes efficiently with a hydrogen transfer from NH2, e.g. 1,1-diphenyl ethylene gives Pl C-CH3 ion under conditions which yield ethyl benzene from styrene (17). 

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 ... 

Photolysis of diethyl bromomalonate in benzene in the presence of compounds with a benzylic hydrogen as possible H-atom source leads to reduction of the ester and bromina-tion of the benzylic compound263. The malonyl radical and not the Br atom turns out to be the H-atom abstracting agent. 

For tertiary, secondary, and primary chlorides the reduction becomes increasingly difficult due to shorter chain lengths. On the other hand, the replacement of a chlorine atom by hydrogen in polychlorinated substrates is much easier. Table 4.2 shows the rate constants for the reaction of (TMS)3Si radical with some chlorides [32]. The comparison with the analogous data of Table 4.1 shows that for benzyl and tertiary alkyl substituents the chlorine atom abstraction is 2-3 orders of magnitude slower than for the analogous bromides. 

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