Microsolvation, aggregation, and pseudomonomolecular, ionic sp2-stereoinversion mechanism of two exocyclic β,β-di-tert-alkyl-α-arylvinyllithiums

Abstract

A THF-solvated, crystalline, exocyclic alkenyllithium, (tert-alkyl)2CC(Li)–Ph, was synthesized and shown to be a disolvated dimer in the solid state and in toluene as the solvent; increasing amounts of the monomeric species emerged on cooling the toluene solution. In THF as the solvent, only the trisolvated monomer was present and identified as a contact ion pair (CIP) through its scalar 13C,6Li NMR coupling. This ground-state needs only one further THF ligand as a catalyst for breaking the C–Li bond with formation of a tetrasolvated, solvent-separated ion pair (SSIP) on the way to the transition state of cis/trans sp2-stereoinversion. The ensuing pseudomonomolecular, ionic mechanism is confirmed by low pseudoactivation parameters: enthalpy ΔHψ‡ = 6.9(3) kcal mol−1; entropy ΔSψ‡ = −23.3(9) cal mol−1 K−1. Similar parameters were found with 2,6-dimethylphenyl in place of Ph.