Nuclear magnetic resonance studies of isoprenyllithium derived from 1,1-diphenyl-n-butyllithium-3,4-d5, and isoprene

The equimolar reactions of 1,1-diphenyl-n-butyllithium-3,4-d₅ (RLi) with isoprene (I) and isoprene-1,4-d₄ (Id) were carried out in benzene-d₆ quantitatively to give isoprenyllithiums, RILi and RIdLi, respectively. From the NMR spectrum of the RILi it was proposed that the isoprene unit had cis-1,4, cis-4,1 and some unknown structures in benzene-d₆. When RILi was prepared in the presence of about one equivalent of THF to RILi, the anion was considered to include an isoprene unit in cis-1,4, trans-1,4, cis-4,1, and probably 3,4 structures. The same anion was obtained even if an equimolar THF was added afterward to the RILi prepared in benzene-d₆ The RLi was reproduced by the reverse reaction from RILi, when a large excess of THF was added or the temperature of the solution was elevated. The results obtained were correlated with those of anionic polymerizations of isoprene by lithium initiators.