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

Abstract

Following up on our previous observation that the rate of formation of a β-alkoxy-substituted α-sulfonyl carbanion depends on the stereochemistry of the alkoxy group, we have found similar behaviour when the β-substituent is R2N, RS, or R3N+. With each substituent, the variation of kN (defined by kN = (kexch)X /(kexch)model) is consistent with an equation of the form log kN = a + b cos2θ, where θ 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.Key words: negative hyperconjugation, substituent effects, stereoelectronic factors, sulfonyl carbanions, anomeric effect.