Dianions of 4-phenylbut-1-ene. Evidence for complex-induced proximity effects in the double metallation of hydrocarbons

Abstract

Metallation of 2-methyl-4-phenylbut-1-ene 1 resulted in the formation of a dianion linearly conjugated with the phenyl ring rather than the cross-conjugated dianion often seen in these systems. Examination of the reaction mixtures at varying periods of time revealed that the initial deprotonation was not at the benzylic carbon. Complexation of n-butyllithium–tetramethylethylenediamine (TMEDA) with the aromatic ring of 1(a complex-induced proximity effect, CIPE) enhances the rate of metallation at the carbon β to the aromatic ring and α to the double bond of 1, as demonstrated in competitive metallations of 1 with ethylbenzene and of 1 with hexa-1,5-diene. This is the first report of a complex induced proximity effect in the double metallation of a hydrocarbon in which complexation occurred with the neutral hydrocarbon.