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