Origin of solvent acceleration in organolithium metal-halogen exchange reactions

Quantum chemical studies (B3LYP) of metal-halogen exchange in the model system LiCH=CH2/MeI are consistent with a T-shaped hypervalent iodine species of the type I(vinyl)(Me)(Li) being the transition state. In support of prior proposals by Collum and others that solvent acceleration of organolithium reactions does not result primarily from solvent-induced changes in. the aggregation state of the lithium reagent, we find the solvent substantially accelerates the reaction. even for a monomeric nonaggregated transition state. Introduction of a model solvent molecule suggests that solvent acceleration is the result of transition state stabilization by tighter binding of the solvent to lithium in the transition state as well as a destabilization. of the reactant adduct, (vinylI).(LiMe), relative to the separated reagents.