Twisted C=C Double Bonds with Very Low Rotational Barriers in Dioxanediones and Isoxazolones Determined by Low-Temperature Dynamic NMR Spectroscopy and Computational Chemistry

摘要

Extremely low rotational barriers for ethylenic C=C double bonds in the range 5-17 kcal/mol have been measured by low temperature H-1 and C-13 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 pi and pi* orbitals (the pi/pi* quotient) constitutes a useful measure of the push-pull character.