Reaction of ketones with lithium hexamethyidisilazide: Competitive enolizations and 1,2-additions

Reaction of 2-methylcyclohexanone with lithium hexamethyldisilazide (LiHMDS, TMS2NLi) displays highly solvent-dependent chemoselectivity. LiHMDS in THF/toluene effect enolization. Rate studies using in situ IR spectroscopy are consistent with a THF concentration-dependent monomer-based pathway. LiHMDS in pyrrolidine/toluene affords exclusively 1,2-addition of the pyrrolidine fragment to form an a-amino alkoxide-LiHMDS mixed dinner shown to be a pair of conformers by using Li-6, N-15, and C-13 NMR spectroscopies. Rate studies are consistent with a monomer-based transition structure [(TMS2NLi) (ketone)(pyrrolidine)(3)](double dagger). The partitioning between enolization and 1,2-addition is kinetically controlled.