Wang Yi, Yu Zhi-Xiang
Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry, Peking University, Beijing 100871, China.
Org Biomol Chem. 2017 Sep 13;15(35):7439-7446. doi: 10.1039/c7ob01628j.
A quantum chemical study of [1,j] sigmatropic proton shifts in polyenyl anions and related conjugated systems has been performed. We found that the Woodward-Hoffmann rules can be applied to understand the stereochemical outcome of these sigmatropic rearrangements, showing that [1,j] sigmatropic proton shift occurs antarafacially when j = 4n + 2, while suprafacial proton shift is symmetry-allowed when j = 4n. The activation barriers for [1,j] proton shifts in polyenyl anions CH are 48.2 (j = 2), 32.8 (j = 4), 21.0 (j = 6), 40.5 (j = 8), and 49.1 (j = 10) kcal mol, respectively. This trend can be explained by the trade-off between stereoelectronic requirement and ring strain in the proton shift transition structure. Among these reactions, only the [1,6] proton shift with the lowest activation barrier can occur intramolecularly under mild reaction conditions. The others are unlikely to take place in a direct manner. Consequently, proton shuttles are generally required to facilitate these sigmatropic proton shifts through a protonation/deprotonation mechanism.
我们对多烯基阴离子及相关共轭体系中[1,j] 迁移质子重排进行了量子化学研究。我们发现伍德沃德-霍夫曼规则可用于理解这些迁移重排的立体化学结果，即当j = 4n + 2时，[1,j] 迁移质子重排以异面方式发生，而当j = 4n时，同面质子重排是对称允许的。多烯基阴离子CH中[1,j] 质子迁移的活化能垒分别为48.2（j = 2）、32.8（j = 4）、21.0（j = 6）、40.5（j = 8）和49.1（j = 10）kcal/mol。这种趋势可以通过质子迁移过渡结构中立体电子需求和环张力之间的权衡来解释。在这些反应中，只有活化能垒最低的[1,6] 质子迁移在温和反应条件下能以分子内方式发生。其他反应不太可能直接发生。因此，通常需要质子穿梭体通过质子化/去质子化机制促进这些迁移质子重排。
