Experimental Evidence for Negative Hyperconjugation as a Component of the Polar Effect: Variation of the Ease of -Sulfonyl Carbanion
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- 作者：James F. King ; Rajendra Rathore ; Zhenrong Guo ; Manqing Li ; and Nicholas C. Payne
- 刊名：Journal of the American Chemical Society
- 出版年：2000
- 出版时间：October 25, 2000
- 年：2000
- 卷：122
- 期：42
- 页码：10308 - 10324
- 全文大小：227K
- 年卷期：v.122,no.42(October 25, 2000)
- ISSN：1520-5126
文摘
We report evidence for a strongly geometry-dependent substituent effect. The rate constants forH-D exchange of the 
-hydrogens, (kexch)OR, in a set of 19 
-alkoxy sulfones of known, fixed (or stronglypreferred) H-C-C-OR torsion angles have been measured. Those of corresponding model compoundslacking 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 ismade for steric and other influences, our observations are consistent with an equation of the form log kN = a+ b cos2 
(where a = 1.70 ± 0.17 and b = 2.62 ± 0.20). It is further shown that the observed rate constantratios 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 hyperconjugationor the generalized anomeric effect, specifically a torsion-angle-dependent donation of the partial negative chargeof the incipient carbanion into the 
orbital. This effect is largest with torsion angles of 0 and 180
and atthese torsion angles constitutes the major source of stabilization of the incipient carbanion by the -alkoxygroup. The present observation of a torsion-angle-dependent substituent effect may be combined with theknown adherence of the specific rate of -sulfonyl carbanion formation to the Taft equation, to provide anequation yielding torsion-angle-dependent Taft 
* constants for the alkoxy group: (
)OR = 0.35 + 0.55cos2. The idea of torsion angle dependence is usefully applied to the long-standing problem of the mechanismsof base-promoted elimination with 2-tosyloxycyclohexyl p-tolyl sulfones.
-hydrogens, (kexch)OR, in a set of 19 -alkoxy sulfones of known, fixed (or stronglypreferred) H-C-C-OR torsion angles have been measured. Those of corresponding model compoundslacking 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 ismade for steric and other influences, our observations are consistent with an equation of the form log kN = a+ b cos2 (where a = 1.70 ± 0.17 and b = 2.62 ± 0.20). It is further shown that the observed rate constantratios 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 hyperconjugationor the generalized anomeric effect, specifically a torsion-angle-dependent donation of the partial negative chargeof the incipient carbanion into the orbital. This effect is largest with torsion angles of 0 and 180 and atthese torsion angles constitutes the major source of stabilization of the incipient carbanion by the -alkoxygroup. The present observation of a torsion-angle-dependent substituent effect may be combined with theknown adherence of the specific rate of -sulfonyl carbanion formation to the Taft equation, to provide anequation yielding torsion-angle-dependent Taft * constants for the alkoxy group: ()OR = 0.35 + 0.55cos2. The idea of torsion angle dependence is usefully applied to the long-standing problem of the mechanismsof base-promoted elimination with 2-tosyloxycyclohexyl p-tolyl sulfones.© 2004-2018 中国地质图书馆版权所有 京ICP备05064691号 京公网安备11010802017129号 地址：北京市海淀区学院路29号 邮编：100083 电话：办公室：(+86 10)66554848；文献借阅、咨询服务、科技查新：66554700 |