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.

The cesium (1-Cs, CsPhPAT) and lithium (1-Li, LiPhPAT) enolates of 2,6-diphenyl-R-tetralone, 1, and the lithium enolate (2-Li, LiPhAT) of 2-phenyl-R-tetralone, 2, are present in dilute THF solution as monomers and dimers with K 1,2 ) 1810 (1-Cs, CsPhPAT), 2650 (1-Li, LiPhPAT), and 1930 (2-Li, LiPhAT) M -1 . These values were obtained by singular value decomposition analysis of the UV spectra and by the dependence of ion pair pK's with concentration. On the ion pair pK scales previously defined, the monomers have pK ) 17.80 (1-Cs, CsPhPAT) and 11.14 (1-Li, LiPhPAT). The monomers are much faster than dimers in alkylation reactions; reaction products from alkyl halides are those of C-alkylation, but 1-Cs (CsPhPAT) with methyl sulfonates gives large amounts of O-alkylation. Comparison with previous results shows that the reactivity of cesium enolates parallels their basicity but that lithium enolates show no correlation between ion pair pK and alkylation reactivity.

[reaction: see text] The lithium enolate 1-Li of 6-phenyl-alpha-tetralone forms a monomer-tetramer equilibrium in THF at 25 degrees C with K(1,4) = 4.7E+10 M(-3). The lithium enolate 2-Li, however, forms a monomer-dimer equilibrium with K(1,2) = 3800 M(-1). In both cases reaction with benzyl bromide is dominantly with the monomer. The results support an earlier conjecture of House that alkylation of an enolate is frequently accompanied by extensive polyalkylation because the less substituted enolates are more aggregated.

The lithium pK scale has been extended to 25 indicators with a pK range of 9.7-24.4. The resulting scale is compared with the cesium ion pair acidities and to ionic pK's in DMSO and aqueous DMSO.Key words: ion pair acidity, organolithium compounds, indicator, acidity scale.

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The cesium enolate of 2-biphenylylcyclohexanone (CsBPCH), generated by deprotonation of 2-biphenylylcyclohexanone with diphenylmethylcesium, is shown to be a mixture of monomer and dimer in THF, with a dimerization constant of 1.9 x 10(3) M(-)(1). The ion pair acidity corresponds to pK = 19.30 +/- 0.05. Even in the presence of excess dimer, only monomeric CsBPCH is found to react with methyl tosylate (MeOTs), methyl p-bromobenzenesulfonate (MeOBs), hexyl tosylate (HexOTs), hexyl bromide (HexBr) and p-tert-butylbenzyl chloride (BBCl). The relative reactivities of these alkylating reagents toward monomeric CsBPCH are BBCl, 1.00; MeOTs, 1.21; MeOBs 6.7; HexBr, 0.10; HexOTs, 0.050. CsBPh(4) has essentially no effect on the rates.

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The lithium (1) and cesium (2) enolates of α-phenylcyclohexanone, PCH, have been studied in THF at 25°C. Both form monomer-dimer equilibria with K1,2 = 2.8 × 103 M-1 for 1 and 1.8 × 103 M-1 for 2. The ultraviolet-visible (UV-vis) spectra of the monomers and dimers were derived from the spectra of the enolates at a series of concentrations by singular value decomposition. Measurements of the ion pair pK values gave pK = 12.7 for 1 and 19.8 for 2. The monomers of 1 and 2 are alkylated by benzyl halides faster than are the dimers.Key words: carbon-acid ionization, enolate ions, ion pairs, ion pair acidity, aggregation, dimerization constants, alkylation.