Variation of the ease of α-sulfonyl carbanion formation with the orientation of different β-substituents:: Experimental evidence for the generality of negative hyperconjugation as an important substituent effect

Following up on our previous observation that the rate of formation of a beta-alkoxy-substituted alpha-sulfonyl carbanion depends on the stereochemistry of the alkoxy group, we have found similar behaviour when the beta-substituent is R2N, RS, or R3N+. With each substituent, the variation of k(N) (defined by k(N) = (k(exch))(X) /(k(exch))(model)) is consistent with an equation of the form log k(N) = a + b cos(2) theta, where theta is the H-C-C-X torsion angle. We propose that the a term describes the polar (field plus inductive) effect and the b term the negative hyperconjugative effect of the substituent; we show how the variations in a and b may be readily accommodated within this framework. Some features of the trialkylammonio group previously postulated in the literature are discussed in the light of our results.