ORIGIN OF SOLVENT ACCELERATION IN ORGANOLITHIUM METAL-HALOGEN EXCHANGE-REACTIONS

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