锆氢化反应研究Ⅵ.环氧化合物的选择还原

Hydrozirconation Ⅵ. Selective Reduction of Epoxides

环氧化合物还原成醇,是较为常见的反应。其区域选择性取决于所使用的还原剂。一般规律是:LiAlH4类型的亲核性复合金属氢化物从空间障碍较小的部位接近氧,从而获得取代基较多的醇;而硼烷（尤其在BF3存在之下）或铝烷类型的亲电性金属氢化物则使环氧化合物从相反方向开环并主要获得取代基较小的醇。锆氢试剂（Cp2ZrHCl）对羰基化合物的还原已经详细研究,本文将研究环氧化合物的还原及其区域选择性,并进一步认识锆氢试剂在反应中的作用及其本质。

Epoxides undergo hydrozirconation under mild condition. The reaction intermedia- tes, alkoxyl bis(cyclopentadienyl)zirconium chlorides, are unstable and easily decompose by absorbing moisture from air. After hydrolysis, the intermediates afford alcohols. The hydrozirconation of epoxides to alcohols shows high regioselectivity which can be higher than 92% for aromatic epoxides. In contrast with LiA1H_4, the results of hydrozirconation of epoxides to alcohols are similar to electrophilic metal hydrides which open the ring in the opposite direction and give predominantly less substituted alcohols. Addition of Cp_2- MCl_2 (M=Ti, Zr), or Ti (OBu^n)_4 improved the regioselectivity of hrdrozirconation but worsened the hydrogenation selectivity with LiA1H_4.