Skraba-Joiner Sarah L, Johnson Richard P, Agarwal Jay
Department of Chemistry and Materials Science Program, University of New Hampshire , Durham, New Hampshire 03824, United States.
Center for Computational Quantum Chemistry, University of Georgia , Athens, Georgia 30602, United States.
J Org Chem. 2015 Dec 4;80(23):11779-87. doi: 10.1021/acs.joc.5b01488. Epub 2015 Oct 7.
The conceptual dehydrogenation of pericyclic reactions yields dehydropericyclic processes, which usually lead to strained or reactive intermediates. This is a simple scheme for inventing new chemical reactions. Computational results on two novel dehydropericyclic reactions are presented here. Conjugated enynes undergo a singlet-state photoisomerization that transposes the methylene carbon. We previously suggested excited-state closure to 1,2-cyclobutadiene followed by thermal ring opening. CCSD(T)//DFT computations show two minima of similar energy corresponding to 1,2-cyclobutadiene, one chiral and closed shell and the second a planar diradical. The chiral structure has a low barrier to ring opening and may best explain results on enyne photoisomerization. The first examples of 1,3-diyne + yne cycloadditions to give o-benzynes were reported in 1997. Computations on intramolecular versions of this tridehydro (-3H2) Diels-Alder reaction support a concerted mechanism for the parent triyne (1,3,8-nonatriyne); however, a slight electronic advantage in the concerted path may be outweighed by the difference in entropy of activation for sequential vs simultaneous formation of two new ring bonds.
周环反应的概念性脱氢产生脱氢周环过程，这通常会生成张力中间体或活性中间体。这是一种发明新化学反应的简单方案。本文给出了两个新型脱氢周环反应的计算结果。共轭烯炔发生单线态光异构化，使亚甲基碳发生转位。我们之前提出激发态闭环生成1,2 - 环丁二烯，随后发生热开环。CCSD(T)//DFT计算表明，对应于1,2 - 环丁二烯有两个能量相近的极小值，一个是手性闭壳结构，另一个是平面双自由基结构。手性结构的开环势垒较低，可能最能解释烯炔光异构化的结果。1,3 - 二炔与炔烃的环加成反应生成邻苯炔的首例报道于1997年。对该三脱氢（-3H2）狄尔斯 - 阿尔德反应分子内版本的计算支持母体三炔（1,3,8 - 壬三炔）的协同机理；然而，协同路径中微小的电子优势可能会被依次形成与同时形成两个新环键时活化熵的差异所抵消。
