Aggregation and Alkylation of Enolates of 2-Phenyl-α-tetralone and 2,6-Diphenyl-α-tetralone¹

Abstract

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.

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

“…The results were also in qualitative agreement with an early study that used a combination of DFT and semiempirical calculations, and also used dimethyl ether to represent THF solvation. 19 Lower enolate aggregates have been reported in very dilute THF solutions, [15][16][17][18] and this can be qualitatively explained from eq 10. As the solution becomes more dilute, the concentration of THF increases relative to that of the enolate.…”

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

Palladium‐Catalyzed Asymmetric Allylic Alkylation of Ketone Enolates

“…The lithium enolate of α‐tetralone has been shown to be predominately a monomer–tetramer equilibrium in THF, but with the addition of a 2‐isopropyl group the enolate is predominately a dimer 25. Studies of 2‐phenyl‐1‐tetralone show that the lithium enolate exists as a monomer–dimer mixture and that the equilibrium lies in favor of the dimer (>90 %) at concentrations usually used in synthesis (>0.1 M ) 26. However, in the case of 2‐phenyl‐1‐tetralone, the dimer was shown to be much less reactive in alkylation reactions than the monomer.…”

7 Reaction mechanisms. Part (i) Polar reactions

“…The aggregation and alkylation of enolates 163 (M = Li, Cs; R = Ph) and 163 (M = Li; R = H) were investigated in dilute THF solution. 138 At the concentrations of reactants used in alkylation reactions the dimeric form of enolate was the most abundant, but the monomeric enolate was found to be the most reactive towards alkylation. Experimental and theoretical 13 C and 17 O kinetic isotope effects were determined for the chlorotrimethylsilane mediated addition of lithium dibutylcuprate to cyclohexenone in THF.…”