Heteroatoms and substituent effects: The importance of heteroatom hyperconjugation

✍ Scribed by James F. King; Manqing Li; Allan Z. Cheng; Vinod Dave

• Publisher: John Wiley and Sons
• Year: 2002
• Tongue: English
• Weight: 210 KB
• Volume: 13
• Category: Article
• ISSN: 1042-7163
• DOI: 10.1002/hc.10067

✦ Synopsis

We have found that the specific rate of ␣-sulfonyl carbanion formation in a ␤-substituted sulfone shows a sizable dependence on the H C ␣ C ␤ X torsion angle. Defining k N = (k exch ) X /(k exch ) model (where the model has X = H or an alkyl group) we observed for a collection of ␤-alkoxy sulfones (X = OR) acceptable agreement with the expression log k N = a + b cos 2 θ (where a = 1.70 and b = 2.62). Extension to other ␤substituents (X = RS, R 2 N, and R 3 N + ) yields the same pattern, with the last showing very large dependence of k N 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.