2,8 (4,6)-二取代半瞬烯及其BCO衍生物的Cope重排

运用从头计算量化方法计算了一系列Cope重排的2,8(或4,6)-二取代半瞬烯(1和2), 以及2,8:4,6-四取代半瞬烯(3和4)的过渡态结构、活化能、能量差和核独立化学位移. 在2,8(或4,6)-二取代半瞬烯体系里, π电子基均使得Cope重排向2进行, 这与Hoffmann早年对此的定性预测相反. 它们的过渡态是离域双同芳香性的. 本文重点讨论了π电子基与BCO共同取代的2,8:4,6-四取代半瞬烯体系, 由于BCO的稳定效应和π电子基共轭效应, π电子基均使得结构4具有优先稳定性, 其Cope重排活化能也大大降低甚至消除. 随着卤素主量子数的变化, 它们的取代半瞬烯Cope重排相关参数也有相应的规律性变化.

Cope Rearrangement of 2,8 (4,6)-Disubstituted Semibullvalenes and Their BCO-derivatives

Transition structures, activation energies, energy differences and nucleus independent chemical shifts (NICS) values were calculated by ab initio quantum mechanical methods for the Cope rearrangements of a series of 2,8 (4,6)-disubstituted semibullvalenes (1 and 2) and 2,8:4,6-tetrasubstituted semibullvalenes (3 and 4). In considering π-electron donating or accepting substituents at positions 2,8 or 4,6 of semibullvalenes, it was found that the Cope rearrangements were all shifted toward 4,6-disubstituted semibullvalenes (2), which does not agree with Hoffmann’s early argument. However, their transition states are delocalized and bishomoaromatic. In 2,8:4,6-tetrasubstituted semibullvalenes, which we emphasized on, π-electron groups all shift the equilibrium toward 4, and their Cope barriers are reduced drastically and even eliminated. That can be ascribed to the BCO stabilization effects and the conjugative effects of π-electron groups. The halogen substituent parameters of the Cope rearrangement also have corresponding varieties with the change of the main quantum number.