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

Abstract

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.

Aggregation and C−N Rotation of the Lithium Salt of N,N-Dimethyldiphenylacetamide

“…These results are summarized in Figure . Δ H ° is relatively small; dimerization is primarily an entropy-driven process with an entropy change comparable to that for a lithium enolate . The dimerization constant K 1,2 extrapolated to room temperature is 75 M -1 and is consistent with results reported earlier .…”

A Computational Study of Lithium Ketone Enolate Aggregation in the Gas Phase and in THF Solution

“…Both of those species have been observed by ultraviolet-visible spectroscopy. [15][16][17][18] (40) Bauer, W.; Laube, T.; Seebach, D. Chem. Ber.…”

Perspectives on Computational Organic Chemistry

“… (a) DMA = N,N-dimethylacetamide (b) Obtained by extrapolation (c) Ref 38 (d) Ref 39 (e) Ref 40 (f) Unpublished results (g) Ref 41 (h) Ref 42 (i) Ref 43 (j) Ref 44 (k) Ref 45 (l) Ref 46 (m) Ref 47 (n) Ref 48 (o) Ref 34, per hydrogen (p) For phenylacetylene (q) For aniline (r) For cyclohexyl phenyl ketone (s) For 1-indanone …”