Asymmetric hydroboration of styrenes catalyzed by cationic chiral phosphine-rhodium(I) complexes

Article abstract of DOI:10.1016/S0957-4166(00)86112-1

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(diphenylphosphino)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.