Gibson Christopher M, Havenith Remco W A, Fowler Patrick W, Jenneskens Leonardus W
Department of Chemistry, University of Sheffield , Sheffield, S3 7HF, U.K.
J Org Chem. 2015 Feb 6;80(3):1395-401. doi: 10.1021/jo502179k. Epub 2015 Jan 9.
In contrast to the equilibrium structure of the homoaromatic C(s) homotropenylium cation, C8H9(+) (1), which supports a pinched diatropic ring current, the C(2v) transition state (2) for inversion of the methylene bridge of 1 is antiaromatic and supports a two-lobe paratropic π current, as detected by plotting B3LYP/6-31G** ipsocentric current maps. Participation of the bridge CH bonds is crucial for the change in global character of the current in the transition state, as shown by the quenching of its paratropicity on substitution of H by F. Orbital-based arguments allow rationalization of this transition between homoaromaticity and hyper(conjugative) antiaromaticity. More generally, the hyperconjugative ring current in a family of C(2v) planar-constrained geometries of (CR2)C(N-1)H(N-1)(q) homoannulenes (R = H, F) can be switched from paratropic (antiaromatic) to diatropic (aromatic) by variation of ring size, charge, and bridge substituent. An orbital-based counting rule accounts for these systematic trends.
与支持收缩的抗磁环电流的同芳香性C(s)均环戊烯鎓阳离子C8H9(+) (1)的平衡结构相反，通过绘制B3LYP/6 - 31G**以原子为中心的电流图检测到，1的亚甲基桥反转的C(2v)过渡态(2)是反芳香性的，并支持双叶顺磁π电流。桥连CH键的参与对于过渡态中电流整体性质的变化至关重要，这通过用F取代H时其顺磁性的猝灭得以体现。基于轨道的论据能够解释这种同芳香性与超（共轭）反芳香性之间的转变。更普遍地，在(CR2)C(N - 1)H(N - 1)(q)同环烯（R = H, F）的C(2v)平面受限几何结构家族中，超共轭环电流可以通过改变环大小、电荷和桥连取代基，从顺磁（反芳香性）切换为抗磁（芳香性）。基于轨道的计数规则解释了这些系统趋势。
