Transition-metal-promoted hydroboration of alkenes: A unique reversal of regioselectivity

When hydroboration of 1-octene is carried out in the presence of catalytic amounts of rhodium trichloride followed by the usual oxidation hydrogen peroxide in aqueous alkali), only minor proportions of 1-octanol (2.4%) are formed accompanied by very significant amounts of 2- (17.4%, 3- (36.9%), and 4-octanol (43.3%). These product compositions are obtained in good overall yield when the borane-THF complex is slowly added to a stirred solution of 1-octene in THF solvent containing the rhodium trichloride. Isomerization of 1-octene to 2-, 3-, and 4-octene in the presence of rhodium trichloride alone is far too slow to account for the foregoing results. The mechanism likely involves multiple and reversible addition/elimination of a Rh-activated B-H species across the double bonds.