Y-Conjugated dianions: a comparison between the 1,1-dibenzylethylene and diphenylacetone-derived dianions

Abstract

The lithium salts of the two title dianions have been synthesised and investigated by ¹H and ¹³C n.m.r. spectroscopy. Whereas the dibenzylidenemethylenemethane dianion salt exists exclusively in the exo,exo conformation, its oxa-analogue is a mixture of two isomers, exo,endo and a symmetrical form, probably exo,exo. The benzyl α,β-CC bond rotation barrier in the ketone dianion salt in THF is 10.8 ± 0.5 kcal mol^–1. The phenyl rotation barrier in the hydrocarbon dianion salt is similar (10.9 kcal mol^–1), as are the charge distributions in the benzylidene groups of the two dianions.