Solladié-Cavallo Arlette, Lupattelli Paolo, Bonini Carlo
Laboratoire de Stéréochimie Organométallique associé au CNRS, ECPM/Université L. Pasteur, 25 rue Becquerel, 67087-Strasbourg, France.
J Org Chem. 2005 Mar 4;70(5):1605-11. doi: 10.1021/jo048045w.
Both symmetrical and nonsymmetrical trans-2,3-diaryloxiranes are regio- and stereoselectively opened by the LiBr/Amberlyst 15 system. In the case of symmetrical trans-stilbene oxide, the syn- versus anti-bromohydrins ratio ranged between 88/12 and 30/70, by varying the reaction temperature from 20 to -30 degrees C. In the case of nonsymmetrical para-substituted trans-2,3-diaryloxiranes, the regioselectivity is determined by electronic effects. If one phenyl bears a strong electron-withdrawing group (as NO2 or CF3), the nucleophilic attack is totally on the beta-carbon with respect to the substituted phenyl ring. With one phenyl bearing a strong electron-releasing group (OCH3), the regioselectivity is reversed. Ab initio calculation at the DFT/B3LYP/6-31G level, run on protonated epoxide structures, supports the formation of a cationic acyclic intermediate. Application of the method on ortho-methoxy and ortho-nitro 2,3-diaryloxiranes afforded the syn-bromohydrins in excellent yield, via regio- and stereoselective opening at either alpha- or beta-carbon, respectively.
对称和不对称的反式 -2,3 - 二芳基环氧乙烷均可被LiBr/ Amberlyst 15体系区域选择性和立体选择性地开环。对于对称的反式二苯乙烯氧化物，通过将反应温度从20℃变化至 -30℃，顺式与反式溴醇的比例在88/12至30/70之间。对于不对称的对位取代反式 -2,3 - 二芳基环氧乙烷，区域选择性由电子效应决定。如果一个苯基带有强吸电子基团（如NO2或CF3），亲核进攻完全发生在相对于取代苯环的β - 碳上。当一个苯基带有强供电子基团（OCH3）时，区域选择性则相反。在质子化环氧化合物结构上进行的DFT/B3LYP/6 - 31G水平的从头算计算支持了阳离子无环中间体的形成。该方法应用于邻甲氧基和邻硝基2,3 - 二芳基环氧乙烷时，分别通过在α - 或β - 碳上的区域选择性和立体选择性开环，以优异的产率得到顺式溴醇。
