Aggregation and Reactivity of the Lithium Enolate of 2-Biphenylylcyclohexanone in Tetrahydrofuran¹

Deprotonation of biphenylylcyclohexanone (BPCH) with a lithium base in THF occurs preferentially at the secondary enolate position to give the unconjugated lithium enolate which is gradually converted to the more stable conjugated enolate by further proton transfer from the tertiary enolate position of the ketone. The unconjugated lithium enolate is present dominantly as the tetramer, but it is the monomer that reacts with ketone to give the conjugated enolate, LiBPCH. The conjugated enolate is present as a monomer−dimer mixture with K_1,2 = 4300 M^-1. The equilibrium constant changes only slightly at lower temperatures, indicating that dimerization is primarily entropy-controlled. The ion pair pK of LiBPCH, 12.3, is 6.0 units less than that of the corresponding cesium enolate, CsBPCH. Alkylation of LiBPCH with methyl brosylate or benzylic bromides occurs at the tertiary carbon, and k_M ≫ k_D. These reactions are faster than those of LiPhIBP, although LiPhIBP has a higher ion pair pK value. The ion pair displacement reaction has a relatively low activation energy but a normal entropy of activation.