Twisted C=C Double Bonds with Very Low Rotational Barriers in Dioxanediones and Isoxazolones Determined by Low-Temperature Dynamic NMR Spectroscopy and Computational Chemistry
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- 作者：Curt Wentrup ; Rainer Koch and Erich Kleinpeter
- 刊名：European Journal of Organic Chemistry
- 出版年：2016
- 出版时间：October 2016
- 年：2016
- 卷：2016
- 期：29
- 页码：4985-4990
- 全文大小：923K
- ISSN：1099-0690
文摘
Extremely low rotational barriers for ethylenic C=C double bonds in the range 5–17 kcal/mol have been measured by low temperature 1H and 13C NMR spectroscopy and confirmed by calculations at the M06-2X level for 5-methylene-1,3-dioxane-4,6-diones (Meldrum's acid derivatives) and 4-methyleneisoxazol-5(4H)-ones. The barriers are ascribed to the push-pull (donor-acceptor) nature of the alkenes. A correlation between the calculated C=C bond lengths and the ratios of electron occupancies in the ethylenic π and π* orbitals (the π/π* quotient) constitutes a useful measure of the push-pull character.