非对称环氧乙烷的区域选择性亲核开环反应

本文总结了常用亲核试剂对非对称环氧乙烷的亲核开环反应及其区域选择性.强亲核性的亲核试剂通常只受空间效应影响,进攻非对称环氧乙烷位阻小的碳原子,对于烯基取代环氧乙烷还可以进攻烯基的β-碳原子发生Sn2'开环反应,其他亲核试剂同时受空间效应和电子效应的影响,对于烷基环氧乙烷通常进攻其取代少的碳原子,空间效应起主导作用,而对...

Regioselective Nucleophilic Ring Opening Reactions of Unsymmetric Oxiranes

Nucleophilic ring opening reactions of unsymmetric oxiranes and their regioselectivity with widely used nucleophiles are reviewed. Strong nucleophiles attack the less substituted carbon atom of unsymmetric oxiranes, whatever alkyl, alkenyl, and aryloxiranes, controlled by the steric hindrance only. They can undergo an S_N2' ring-opening reaction with alkenyloxiranes via the attack on the β-carbon atom of their alkenyl group. Other nucleophiles generally attack the less substituted carbon atom for alkyloxiranes, controlled by the steric hindrance, but attack the arylmethyl and allyl carbon atom for aryl and alkenyloxiranes, controlled by the electronic effect. In the presence of proton acids or strong Lewis acids, although monoalkyloxiranes are attacked on their less substituted carbon atom with nucleophiles (steric hindrance control), aryl, alkenyl, and geminal dialkyloxiranes are attacked on their more substituted carbon atom with weak nucleophiles (electronic effect control). The regioselectivity of intramolecular nucleophilic ring opening reaction of oxiranes is controlled by the ring size of products. The favorable order is five-membered ring > six-membered ring > seven-membered ring. Thus, the regioselectivity is controlled by a balance between the steric hindrance and electronic effect of oxiranes and nucleophiles.