Rh(III)催化的2-羟基苯乙烯与炔烃的[5+2]/[3+2]环加成反应的理论计算

Seoane及其合作者近年来报道了两例新的Rh(III)催化的环加成反应:1)2-羟基苯乙烯与炔烃进行5+2]环加成生成七元含氧杂环和2)2-异烯丙基苯酚和炔烃进行3+2]环加成生成五元螺环.这两例反应使用的催化剂和反应条件相同, 底物相似,但产物完全不同.本文采用密度泛函方法对这两例反应的催化机理进行深入的对比研究,以探究造成不同选择性的原因.研究结果表明,两反应均由O-H键断裂、C-H键活化、炔烃迁移插入及还原消除4步组成.在还原消除中,当空间位阻影响大于去芳香性影响时生成五元螺环产物;反之,则生成七元含氧杂环.还发现反应中C-H键活化的选择性是由C-H键活化及炔烃迁移插入过渡态中的环张力大小决定的.

Studies on Rh(III)-catalyzed[5+2]/[3+2]cycloadditions of 2-hydroxystyrenes with alkynes

Recently, Seoane et al. reported two new types of Rh(III)-catalyzed cycloadditons:1) the hetero-5+2] cycloaddition of 2-hydroxystyrenes with alkynes leading to benzoxepines and 2)the carbo-3+2] cycloaddition of 2-alkenylphenols with alkynes leading to spirocyclic products. Interestingly,in the two reactions the same catalyst and similar substrates were used but completely different products were obtained. To gain insight into the catalytic mechanisms and to unravel the origins of chemoselectivity differences, we herein present a comparative DFT mechanistic study on the two representative reactions. The results show that both cycloadditions consist of four steps,O—H deprotonation, C—H bond activation, alkyne migratory insertion, and reductive elimination. The reductive elimination step controls the chemoselectivity. When the steric effect is greater than the dearomatization effect, the reaction leads to the five-membered spirocyclic product. Otherwise the reaction produces the seven-membered heterocyclic product. We also found that the regioselectivity of C—H bond activation is determined by the ring tension in the transition states of C—H bond activation and alkyne insertion.