ASYMMETRIC HYDROBORATION OF STYRENES CATALYZED BY CATIONIC CHIRAL PHOSPHINE-RHODIUM(I) COMPLEXES

Reaction of styrene with catecholborane in the presence of 1 mol % of a cationic phosphine-rhodium catalyst prepared in situ from [Rh(COD)2]BF4 and 1,4-bis(diphenyl-phosphino)butane proceeded regioselectively to give, after oxidation, 1-phenylethanol in a quantitative yield. The regioselectivity forming benzylic alcohols was also observed in the reaction of substituted styrenes. Use of (R)-2,2'-bis(diphenylphosphino)-1,1'-binaphthyl (BINAP) as a chiral ligand for the rhodium-catalyzed hydroboration of substituted styrenes (ArCH = CH2) gave optically active (R)-1-arylethanols (ArCH(OH)Me) in high yields. The enantiomeric purities of the alcohols are 96% ee, 94% ee, 91% ee, 85% ee, 89% ee, and 82% ee for Ar = Ph, 4-MeC6H4, 4-ClC6H4, 3-ClC6H4, 4-MeOC6H4, and 2-MeOC6H4, respectively.