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 R-hydrogens, (k exch ) OR , in a set of 19 β-alkoxy sulfones of known, fixed (or strongly preferred) H-C R -C β -OR torsion angles have been measured. Those of corresponding model compounds lacking the alkoxy group, (k exch ) model , were also measured, thereby providing the ratio, k N ) (k exch ) OR /(k exch ) model ; the k N 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 k N ) a + b cos 2 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 σ C-O / 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 R-sulfonyl carbanion formation to the Taft equation, to provide an equation yielding torsion-angle-dependent Taft σ* constants for the alkoxy group: (σ θ / ) OR ) 0.35 + 0.55 cos 2 θ. 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.

Heteroatoms and substituent effects: The importance of heteroatom hyperconjugation

“…As we have shown elsewhere [8], the concept of the variation of substituent effect with torsion angle establishes the E1cB mechanism without recourse to further hypotheses.…”

“…Our first substituent was the alkoxy group [7,8]. The full array of ␤-alkoxy sulfones along with the appropriate unsubstituted analogues lacking the alkoxy group ("model" compounds) are shown as structures 1-25.…”

“…Of the remaining points five (8)(9)(10)(11)(12) are believed to reflect two effects: (a) a steric acceleration deriving from a strong nonbonding eclipsing interaction between 21, 24, 25). Filled circles and filled black diamonds show the effect of the alkoxy substituent without requiring any correction for steric or other effects.…”

“…Noting that the rate of the H-D exchange in PhSO 2 CH 2 CH 2 OMe vs that of PhSO 2 CH 2 CH 3 in our most generally used system (D 2 O-dioxane-d 8 , 25…”

The cis‐trans Isomerisation of Homologous 2‐Hydroxycycloalkanecarboxylic Acids under Basic Conditions

“…The intermediate was prepared through the removal of a proton from the carbon atom bearing the carboxylate anion by the hydroxyl ion. It was done because it was proven experimentally 11,17 that it is the proton that is removed under strongly basic conditions.…”