Benzylic halides Barbier-type reactions

Indium-mediated Barbier-type coupling between carbonyl compounds and allyl halides has been revealed to proceed effectively in diverse reaction media. Even under solvent-free conditions, allylation works well, although no reaction is observed with benzyl bromide and a-halo carbonyl compounds.59 Various aldehydes react with allyl bromide mediated by indium in liquid carbon dioxide to give homoallylic alcohols (Scheme 1). In contrast to the corresponding neat allylation, the liquid C02-mediated reaction can allylate solid aldehydes successfully.60 Indium-mediated allylations of carbonyl compounds with allyl bromide proceed in room temperature ionic liquids. In [bmim][BF4] and [bmim][PF6] (bmin l-butyl-3-methylimidazolium), the desired homoallylic alcohols are formed with good levels of conversion.61 Homoallyllic alcohols are also prepared by the reaction of resin-bound aldehydes (Equation (l)).62... 

Benzyl halides and allyl (propargyl) halides are structurally similar but have drastically different chemical reactivities in the aqueous Barbier-Grignard-type reactions. Although tribenzyl and dibenzyltin derivatives have been prepared in aqueous conditions since the 1960s, they do not add onto carbonyls, most likely because it is not possible to form a six-membered cyclic transition state with the carbonyl group in a two-componenf fashion. Still, zinc-mediated benzylation of carbonyl compounds in aqueous media was reported by Bieber et al. recently. The benzylation of 4-nitrobenzaldehyde could be controlled chemoselectively by using various phase transfer catalysts and metal reductants in water (Eq. 4.41). 2... 

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