Conical Intersection Control of Heterocyclic Photochemical Bond Scission^1,2

The photochemistry of the heterocycle 5,6-dihydro-1-methyl-5,5-diphenylpyridin-2(1H)-one (compound 1 in the text) leads to two competitive reactions (the reactions are depicted in the Introduction to the article). These arise from fission of bond a, between the nitrogen (N-6) and C-1, and bond b, between C-4, with the two phenyl substituents, and C-5, adjacent to the nitrogen. Scission of bond a alone leads to a zwitterionic intermediate which can be trapped by nucleophiles, while cleavage of bonds a and b together affords two fragmentsa ketene and an imine. The ketene could be intercepted with nucleophiles and the imine trimerized. Computation reveals little weakening of bonds a and b. But as stretching begins, conical intersections are encountered, leading to ground-state products.杂环化合物5,6-二氢-1-甲基-5,5-二苯基吡啶-2(1H)-酮（文中记为化合物1）的光化学过程可发生两种竞争性反应，该类反应详见本文引言部分。这两种反应均源于两类键的断裂：一是氮原子（N-6）与C-1之间的a键，二是连有两个苯基取代基的C-4与氮原子相邻的C-5之间的b键。仅断裂a键可生成两性离子中间体，该中间体可被亲核试剂捕获；而同时断裂a键与b键则会得到两个片段——烯酮与亚胺。烯酮可被亲核试剂捕获，亚胺则可发生三聚反应。理论计算结果显示，a键与b键本身的键强并无显著弱化，但当键开始拉伸时，体系会遭遇锥形交叉点，最终生成基态产物。