Ketone enolization by lithium hexamethyldisilazide: structural and rate studies of the accelerating effects of trialkylamines

Mechanistic studies of the enolization of 2-methylcyclohexanone mediated by lithium hexamethyldisilazide (LiHMDS; TMS2NLi) in toluene and toluene/amine mixtures are described. NMR spectroscopic studies of LiHMDS/ketone mixtures in toluene reveal the ketone-complexed cyclic dimer (TMS2NLi)2(ketone). Rate studies using in situ IR spectroscopy show the enolization proceeds via a dimer-based transition structure, (TMS2NLi)2(ketone)]++. NMR spectroscopic studies of LiHMDS/ketone mixtures in the presence of relatively unhindered trialkylamines such as Me2NEt reveal the quantitative formation of cyclic dimers of general structure (TMS2NLi)2(R3N)(ketone). Rate studies trace a >200-fold rate acceleration to a dimer-based transition structure, (TMS2NLi)2(R3N)(ketone)]++. Amines of intermediate steric demand, such as Et3N, are characterized by recalcitrant solvation, saturation kinetics, and exceptional (>3000-fold) accelerations traced to the aforementioned dimer-based pathway. Amines of high steric demand, such as i-Pr2NEt, do not observably solvate (TMS2NLi)2(ketone) but mediate enolization via (TMS2NLi)2(R3N)(ketone)]++ with muted accelerations. The most highly hindered amines, such as i-Bu3N, do not influence the LiHMDS structure or the enolization rate. Overall, surprisingly complex dependencies of the enolization rates on the structures and concentrations of the amines derive from unexpectedly simple steric effects. The consequences of aggregation, mixed aggregation, and substrate-base precomplexation are discussed.