Carbanions. 22. Tautomerism between 2-p-Biphenylyl-1,1,2-trimethylpropyl and 1,1,2,2-Tetramethyl-6-phenylspiroocta-4,6-dienyl Anions. Alkali Metal Cation Effects

Article abstract of DOI:10.1021/ja00388a034

Reactions of Cs-K-Na alloy and lithium metal with 2-p-biphenyl-3-chloro-2,3-dimethylbutane, labeled with perdeuterio-gem-dimethyl groups in a ratio α/β (8/9) of 2.1/1, were investigated to study <1,2>sigmatropic migration of the p-biphenyl group.Reaction with Cs-K-Na in THF at -75 deg C gave upon carbonation a 2.8/1 ratio of spiro acid to open acid (11/10).While the perdeuterio-gem-dimethyl group was randomly distributed cis or trans to the carboxyl group in the spiro acid, the open acid had α/β perdeuterio-gem-dimethyl groups in a ratio of 1/1.6 (10a/10b).These ratios were invariant with time of reaction (26 s to 10 min) or the α/β ratio of perdeuterio-gem-dimethyl groups in the starting chloride.Reaction of the product from Cs-K-Na alloy with water or trimethylsilyl chloride gave detectably only open products, but these had an α/β ratio of perdeuterio-gem-dimethyl groups essentially like that in the open acid from carbonation.The results are interpreted on the basis of a facile equilibrium between open organocesium compounds 2 and 4 via the spiro anion 3.The rather large secondary β-deuterium isotope effect of 1.6 is characteristic of the equilibrium between 2 and 4 and is attributable to anionic hyperconjugation.The reaction of the labeled chloride (8 + 9) with lithium in THF at -75 deg C gave upon carbonation only the open acids (10a +10b) but in the same ratio as from reaction with Cs-K-Na alloy.The similarity of behavior of the present tertiary chloride with Cs-K-Na alloy and with lithium contrasts with that of the primary chloride, 1-chloro-2-p-biphenylylethane, and suggests that the carbon-lithium bond in (2-p-biphenylyl-1,1,2-trimethylpropyl)lithium in THF is stretched because of steric repulsions between the gem-dimethyl group α to the anionic center and the peripherally solvated lithium counterion.