Reactivity of 2-benzylpyridyl lithium toward benzonitrile derivatives: Addition versus elimination

Abstract

This work investigated the reactivity of 2-benzylpyridyl lithium (2-Pyr)C(Ph)(R)Li (R = SiMe3, Li1; R = H, Li2) toward benzonitrile derivatives. Based on the different products, the reaction between lithium salts and nitriles might involve in addition, elimination and bimolecular coupling pathways, respectively. Treatment of Li1 with ArCN (Ar = Ph, p-Tolyl, o-Tolyl, p-OMePh) yielded an addition intermediate pyridyl-1-aza-allyl-lithium [{(2-Pyr)C(Ph)C(Ar)N(SiMe3)}Li]2 (1, Ar = Ph) and its corresponding hydrolysis product 2-benzylpyridyl-ketone 2–5, respectively, in which the reaction involved in a 1,3-shift of -SiMe3 group to form a dimeric pyridyl-1-aza-allyl-lithium then followed by acidic hydrolysis. The MeOLi elimination reaction between Li2 and p-MeO(C6H4)CN resulted in formation of 4-(2-benzylpyridyl)benzonitrile 6. The reaction of Li2 with p-Me(C6H4)CN in the presence of TMEDA generated a 1:2 hydrolysis adduct 2-benzylpyridyl-enaminone 7, however, in the absence of TMEDA it afforded a coupling product of bimolecular nitriles, 1-(4-methylphenyl)-2-cyanophenyl-ethanone 8. We speculated the reaction mechanisms in sequence. The crystal structures of 1 and 5–8 were analyzed.