Experimental Evidence for Negative Hyperconjugation as a Component of the Polar Effect: Variation of the Ease of α-Sulfonyl Carbanion Formation with the Orientation of a β-Alkoxy Substituent

Abstract

We report evidence for a strongly geometry-dependent substituent effect. The rate constants for H−D exchange of the α-hydrogens, (kexch)OR, in a set of 19 β-alkoxy sulfones of known, fixed (or strongly preferred) H−Cα−Cβ−OR torsion angles have been measured. Those of corresponding model compounds lacking the alkoxy group, (kexch)model, were also measured, thereby providing the ratio, kN = (kexch)OR/(kexch)model; the kN values so obtained range over more than 4 orders of magnitude. We show that when due allowance is made for steric and other influences, our observations are consistent with an equation of the form log kN = a + b cos2 θ (where a = 1.70 ± 0.17 and b = 2.62 ± 0.20). It is further shown that the observed rate constant ratios are not consistent with a substituent effect consisting only of the inductive effect and the field effect, and that they are fully consonant with the additional presence of a third effect, namely negative hyperconjugation or the generalized anomeric effect, specifically a torsion-angle-dependent donation of the partial negative charge of the incipient carbanion into the orbital. This effect is largest with torsion angles of 0 and 180° and at these torsion angles constitutes the major source of stabilization of the incipient carbanion by the β-alkoxy group. The present observation of a torsion-angle-dependent substituent effect may be combined with the known adherence of the specific rate of α-sulfonyl carbanion formation to the Taft equation, to provide an equation yielding torsion-angle-dependent Taft σ* constants for the alkoxy group: ( )OR = 0.35 + 0.55 cos2θ. The idea of torsion angle dependence is usefully applied to the long-standing problem of the mechanisms of base-promoted elimination with 2-tosyloxycyclohexyl p-tolyl sulfones.