A 1H and 13C NMR Study of the Structure of Sulfur-Stabilized Lithiated Allylic Carbanions

NMR studies of the solution structures of lithiated (E)-1-(t-butylthio)but-2-ene (4) and lithiated (E)-1-(phenylthio)but-2-ene (6) are reported. The structure of lithiated (E)-1-(t-butylthio)but-2-ene (4) is best described as a transoid carbanion with the allylic carbons C1, C2, and C3 having intermediate sp2–sp3 hybridization. In (4) the heteroatom and non-allylic substituent do not play any significant role in carbanion stabilization. Lithiated (E)-1-(phenylthio)but-2-ene (6) differs from (4) in that it exhibits cis geometry about the C1–C2 bond and the phenylthio group participates in helping to stabilize allylic charge. There is a discrepancy between the geometry about the C1–C2 bond in the solution and solid structures of (4) in the presence of TMEDA.