| Abstract
| - Both symmetrical and nonsymmetrical trans-2,3-diaryloxiranes are regio- and stereoselectivelyopened 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 °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 β-carbon with respectto 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 onprotonated epoxide structures, supports the formation of a cationic acyclic intermediate. Applicationof the method on ortho-methoxy and ortho-nitro 2,3-diaryloxiranes afforded the syn-bromohydrinsin excellent yield, via regio- and stereoselective opening at either α- or β-carbon, respectively.
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