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Aggregation and Reactivity of the Lithium Enolate of 2-Biphenylylcyclohexanone in Tetrahydrofuran1
journal contribution
posted on 1999-06-16, 00:00 authored by Andrew Streitwieser, Daniel Ze-Rong WangDeprotonation 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 K1,2 = 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 kM ≫ kD. 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.