H眉ckel and M枚bius Bond-Shifting Routes to Configuration Change in Dehydro[4n+2]annulenes

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• 作者：Mitchell V. Santander ; Michael B. Pastor ; Jordan N. Nelson ; Claire Castro ; William L. Karney
• 刊名：The Journal of Organic Chemistry
• 出版年：2013
• 出版时间：March 1, 2013
• 年：2013
• 卷：78
• 期：5
• 页码：2033-2039
• 全文大小：446K
• 年卷期：v.78,no.5(March 1, 2013)
• ISSN：1520-6904

文摘

Computational investigation of the potential energy surfaces of dehydro[10]- and dehydro[14]annulenes revealed that mechanisms involving H眉ckel and M枚bius 蟺-bond shifting can explain the observed or proposed configuration change reactions. Unlike the case of annulenes, in which bond-shift midpoints correspond to transition states, for transformations of dehydroannulenes with 螖trans = 0, 鈥渉idden鈥?H眉ckel bond shifts occur on the side of an energy hill, on the way to a cumulenic, purely conformational transition state. For example, interconversion between CTCCTC-dehydro[14]annulene (1a) and CCTCTC-dehydro[14]annulene (2a) has a CCSD(T)/cc-pVDZ//BHLYP/6-31G* barrier of 18.7 kcal/mol, consistent with experimental observations, and proceeds via a conformational transition state, with H眉ckel 蟺-bond shifts occurring both before and after the transition state. However, when 螖trans = 1, a true M枚bius 蟺-bond shift transition state was located. The isomerization of CCTC-dehydro[10]annulene (10) to CCCC-dehydro[10]annulene (11) occurs by an initial 鈥渉idden鈥?H眉ckel bond shift, followed by passage through a M枚bius bond-shift transition state to 11, with an overall barrier of 29.8 kcal/mol at the CASPT2(12,12)/cc-pVDZ//(U)BHLYP/6-31G* level of theory. This is the lowest energy pathway between 10 and 11, in contrast to a cyclization/ring-opening route via a bicyclic allene described in previous reports.