Interplay of Structure and Reactivity in a Most Unusual Furan Diels-Alder Reaction

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• 作者：Gerry A. Griffith ; Ian H. Hillier ; Andrew C. Moralee ; Jonathan M. Percy ; Ricard Roig ; Mark A. Vincent
• 刊名：Journal of the American Chemical Society
• 出版年：2006
• 出版时间：October 11, 2006
• 年：2006
• 卷：128
• 期：40
• 页码：13130 - 13141
• 全文大小：317K
• 年卷期：v.128,no.40(October 11, 2006)
• ISSN：1520-5126

文摘

Difluorinated alkenoate ethyl 3,3-difluoro-2-(N,N-diethylcarbamoyloxy)-2-propenoate reacts rapidlyand in high yield with furan and a range of substituted furans in the presence of a tin(IV) catalyst. Non-fluorinated congener 2-(N,N-diethylcarbamoyloxy)-2-propenoate fails to react at all under the sameconditions. These reactions have been explored using density functional theory (DFT) calculations. Theyreveal a highly polar transition state, which is stabilized by the Lewis acid catalyst SnCl₄ and by polarsolvents. In the presence of both catalyst and solvent, a two-step reaction is predicted, corresponding tothe stepwise formation of the two new carbon-carbon bonds via transition states which have similar energiesin all cases. Our experimental observations of the lack of reaction of the non-fluorinated dienophile, thestereochemical outcomes, and the rate acceleration accompanying furan methylation are all well predictedby our calculations. The calculated free energy barriers generally correlate well with measured reactionrates, supporting a reaction mechanism in which zwitterionic character is developed strongly. An in situring opening reaction of exo-cycloadduct ethyl exo-2-(N,N-diethylcarbamoyloxy)-3,3-difluoro-7-oxabicyclo[2.2.1]hept-5-enyl-2-endo-carboxylate, which results in the formation of cyclic carbonate ethyl 4,4-difluoro-5-hydroxy-2-oxo-5,7a-dihydro-4H-benzo[1,3]dioxole-3a-carboxylate by a Curtin-Hammett mechanism, hasalso been examined. Substantial steric opposition to Lewis acid binding prevents carbonate formation from2-substituted furans.