Rate of Enolate Formation Is Not Very Sensitive to the Hydrogen Bonding Ability of Donors to Carboxyl Oxygen Lone Pair Acceptors; A Ramification of the Principle of Non-Perfect Synchronization for Gen
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- 作者：Zhenlin Zhong ; Timothy S. Snowden ; Michael D. Best ; and Eric V. Anslyn
- 刊名：Journal of the American Chemical Society
- 出版年：2004
- 出版时间：March 24, 2004
- 年：2004
- 卷：126
- 期：11
- 页码：3488 - 3495
- 全文大小：124K
- 年卷期：v.126,no.11(March 24, 2004)
- ISSN：1520-5126
文摘
Two series of structures (1 and 2) possessing intramolecular hydrogen bonds to the lone-pairelectrons of carbonyl oxygens have been examined to reveal the influence of the pKa of the hydrogen-bond donor on the rate of general-base-catalyzed enolate formation. The geometry of the hydrogen bondsis well accepted to be appropriate for intramolecular hydrogen-bond formation. Yet, as revealed by Br
nstedplots, both series show very little dependence of the rate of enolate formation on the hydrogen-bond donorability. The intramolecular hydrogen bonds give rate enhancements only on the order of 10-100-fold, andcorrected Brnsted 
-values are slightly below 0.1. The results can be understood by interpreting them inlight of the Principle of Non-Perfect Synchronization. The results are consistent with the proton transferoccurring through an asynchronous transition state with the developing negative charge localized on carbon.We postulate that catalysts of enolate formation will be most effective if the binding groups are focused onstabilizing negative charge that is forming on the enolate carbon rather than on the enolate oxygen.
nstedplots, both series show very little dependence of the rate of enolate formation on the hydrogen-bond donorability. The intramolecular hydrogen bonds give rate enhancements only on the order of 10-100-fold, andcorrected Brnsted -values are slightly below 0.1. The results can be understood by interpreting them inlight of the Principle of Non-Perfect Synchronization. The results are consistent with the proton transferoccurring through an asynchronous transition state with the developing negative charge localized on carbon.We postulate that catalysts of enolate formation will be most effective if the binding groups are focused onstabilizing negative charge that is forming on the enolate carbon rather than on the enolate oxygen.