Regio- and stereoselective ring opening of 2,3-diaryl oxiranes by LiBr/Amberlyst 15: a new stereocontrolled access to 1,2-diaryl-2-bromo alcohols.

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.