Heteroatoms and substituent effects: The importance of heteroatom hyperconjugation

Abstract

AbstractWe have found that the specific rate of α‐sulfonyl carbanion formation in a β‐substituted sulfone shows a sizable dependence on the HCαCβX torsion angle. Defining kN = (kexch)X/(kexch)model (where the model has X = H or an alkyl group) we observed for a collection of β‐alkoxy sulfones (X = OR) acceptable agreement with the expression log kN = a + b cos2 θ (where a = 1.70 and b = 2.62). Extension to other β‐substituents (X = RS, R2N, and R3N+) yields the same pattern, with the last showing very large dependence of kN on the torsion angle (b = 6.3). These observations are ascribed to the presence (in addition to the inductive and field effects) of negative hyperconjugation responsible for accelerations of 1000‐fold and more, deriving from donation of the incipient negative charge on carbon into the σ*CX orbital in the transition state. These observations reflect, and at the same time underline, the importance of the low‐lying antibonding orbitals present in heteroatomic molecules. © 2002 Wiley Periodicals, Inc. Heteroatom Chem 13:397–405, 2002; Published online in Wiley Interscience (www.interscience.wiley.com). DOI 10.1002/hc.10067