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.
Solladié-Cavallo
Arlette
A
Laboratoire de Stéréochimie Organométallique associé au CNRS, ECPM/Université L. Pasteur, 25 rue Becquerel, 67087-Strasbourg, France.
Lupattelli
Paolo
P
Bonini
Carlo
C
D001592:Benzyl Alcohols / Q000138:chemical synthesis*; D001965:Bromides / Q000737:chemistry*; D005027:Ethylene Oxide / Q000031:analogs & derivatives / Q000138:chemical synthesis / Q000737:chemistry*; D018020:Lithium Compounds / Q000737:chemistry*; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D010084:Oxidation-Reduction; D013237:Stereoisomerism
D001592:Benzyl Alcohols; D001965:Bromides; D018020:Lithium Compounds; C040949:lithium bromide; D005027:Ethylene Oxide