Fuss Werner, Schmid Wolfram E, Trushin Sergei A, Billone Paul S, Leigh William J
Max-Planck-Institut für Quantenoptik, 85741 Garching, Germany.
Chemphyschem. 2007 Mar 12;8(4):592-8. doi: 10.1002/cphc.200600639.
Photochemical pericyclic reactions are believed to proceed via a so-called pericyclic minimum on the lowest excited potential surface (S(1)), which is common to both the forward and backward reactions. Such a common intermediate has never been directly detected. The photointerconversion of 1,3-butadiene and cyclobutene is the prevailing prototype for such reactions, yet only diene ring closure proceeds with the stereospecificity that the Woodward-Hoffmann rules predict. This contrast seems to exclude a common intermediate. Using ultrafast spectroscopy, we show that the excited states of two cyclobutene/diene isomeric pairs are linked by not one, but by two common minima, p* and ct*. Starting from the diene side (cyclohepta-1,3-diene and cycloocta-1,3-diene), electrocyclic ring closure passes via the pericyclic minimum p*, whereas ct* is mainly responsible for cis-trans isomerization. Starting from the corresponding cyclobutenes (bicyclo[3.2.0]heptene-6 and bicyclo[4.2.0]octene-7), the forbidden isomer is formed from ct*. The path branches at the first (S(2)/S(1)) conical intersection towards p* and ct*. The fact that the energetically unfavorable ct* path can compete is ascribed to a dynamic effect: the momentum in C=C twist direction, acquired--such as in other olefins--in the Franck-Condon region of the cyclobutenes.
光化学周环反应被认为是通过最低激发势能面（S(1)）上的所谓周环最低点进行的，这对于正向和逆向反应都是相同的。这样一个共同的中间体从未被直接检测到。1,3 - 丁二烯和环丁烯的光互变是这类反应的主要原型，但只有二烯闭环反应具有伍德沃德 - 霍夫曼规则所预测的立体专一性。这种对比似乎排除了共同中间体的存在。利用超快光谱技术，我们表明两个环丁烯/二烯异构体对的激发态是由两个而非一个共同最低点p和ct连接的。从二烯一侧（环庚 - 1,3 - 二烯和环辛 - 1,3 - 二烯）开始，电环化闭环反应通过周环最低点p进行，而ct主要负责顺反异构化。从相应的环丁烯（双环[3.2.0]庚烯 - 6和双环[4.2.0]辛烯 - 7）开始，禁阻异构体由ct形成。路径在第一个（S(2)/S(1)）锥形交叉点处分支为p和ct*。能量上不利的ct*路径能够竞争这一事实归因于一种动力学效应：在环丁烯的弗兰克 - 康登区域获得的——如同在其他烯烃中一样——C = C扭转方向的动量。
