Geometrical isomerism in benzyl carbanions α-substituted by 2- or 4-pyridyl or quinolyl groups

Abstract

¹ H and ¹³C n.m.r. spectra of the sodium salt of the carbanion of 4-benzylpyridine in Me₂SO show that the anion is non-symmetric at room temperature, because of slow rotation around the bond linking the carbanionic carbon with C(4) of the heterocycle. To establish unambiguously the stereochemistry of the parent system, a model in which rotation was prevented, the sodium salt of the anion of 3-methyl-4-benzylpyridine was studied. An analogous approach was followed for the anion of 2-benzylpyridine. The stereochemistry in these systems was assessed by measuring three stereodependent parameters: (i)³J_C,H between the appropriate carbon atom of the heterocycle and the proton bonded to the carbanionic carbon; (ii) nuclear Overhauser effects; and (iii) the effect on the ¹³C shifts of the heterocycle of a further phenyl group, as in the anions of diphenyl-methylpyridines. The study was extended to 2- and 4-benzylquinoline. The data obtained suggest that in the anions of the 2-benzylheteroaromatic compounds the geometry with the phenyl ring cis to nitrogen is preferred; interference with the nitrogen lone pair is preferred to that with the aromatic C–H bond.