Mechanisms of aromatic lithiation: Influence of aggregation and directing groups

Abstract

High level ab initio calculations (MP2(fc)/6-31+ G*/MP2(fc)6-31G*Δ+ZPE/6-31IG* show that the reaction of aromatic compounds with methyllithium is not an electrophilic substitution, but a hydrogen transfer along an almost linear path, with the (coordinatively highly unsaturated) “active” lithium cation bridging the methyl carbanion and the carbon atom being lithiated. Aggregation (modelled with lithium hydride) does not change the mechanism. However, intra-aggregate bonding stabilizes the “active” lithium ( i.e., the one directly undergoing transfer). The mixed dimer of methyllithium and lithium hydride is predicted to be more reactive than methyllithium monomer in the lithiation of benzene, due to interaction of the “passive” lithium with the aromatic π-system and the carbon atom being lithiated. The accelerating and ortho-directing effect of Lewis base substituents is due to stabilization of the “active” lithium in the transition state by intramolecular coordination and favorable electrostatic interactions. These stabilizing effects are strongest for reaction of a monomer, but are also significant in the reaction of anisole with the mixed dimer of methyllithium and lithium hydride.