Altering the Allowed/Forbidden Gap in Cyclobutene Electrocyclic Reactions: Experimental and Theoretical Evaluations of the Effect of Planarity Constraints

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• 作者：Patrick S. Lee ; Shogo Sakai ; Peter H&ouml ; rstermann ; Wolfgang R. Roth ; E. Adam Kallel ; and K. N. Houk
• 刊名：Journal of the American Chemical Society
• 出版年：2003
• 出版时间：May 14, 2003
• 年：2003
• 卷：125
• 期：19
• 页码：5839 - 5848
• 全文大小：302K
• 年卷期：v.125,no.19(May 14, 2003)
• ISSN：1520-5126

文摘

The allowed conrotatory cyclobutene ring-opening has a distinctly nonplanar carbon skeleton.Classic experiments by Brauman and Archie, and by Freedman et al., placed the allowed/forbidden gap atgreater than 15 kcal/mol. Wolfgang Roth proposed that a system forced to planarity might have a smallerpreference for the conrotatory mode than unconstrained systems. Such systems have now been studiedtheoretically and experimentally, and results that confirm Roth's postulate are presented here. Theexperiments were performed in Bochum, and the calculations were carried out in Osaka and Los Angeles.As the cyclobutene ring-opening transition structure approaches planarity, the energy gap between allowedconrotatory and the forbidden disrotatory pathways decreases. For the ring-opening of a cyclobutene fusedto norbornene, the energy gap between the forbidden and the allowed transition state is only 4.1 kcal/molby CASSCF and 8.0 kcal/mol by CAS-MP2 as compared to 13.4 and 19.2 kcal/mol, respectively, for theparent cyclobutene. Experimental studies of 3,4-dimethylcyclobutenes fused to various ring systems arereported, and a trend is found toward a reduced allowed/forbidden gap as the planarity of the cyclobuteneis enforced.