Dibromoborane-Dimethyl Sulfide and Monobromoborane-Dimethyl Sulfide as Superior Reagents for the Opening of Oxiranes to Bromohydrins

Abstract

BHBr2-DMS and BH2Br-DMS have been found to be highly promising, mild, stereo- and regioselective reagents useful in the opening of epoxides to the corresponding halohydrins at 0 °C to ambient temperature. Except for styrene oxide, all the terminal epoxides were regioselectively cleaved by borderline SN2-type mechanisms favoring the formation of primary vs secondary bromides. In contrast to B-bromobis(dimethylamino)borane (BDB), the polarity of the solvent has very little or no effect on the regioselectivity achieved by BH2Cl-DMS and BH2Br-DMS. In addition, the regioselectivities were not greatly affected by variation of temperature in the case of BH2Cl-DMS and BH2Br-DMS. BH2Br-DMS and BHBr2-DMS are very useful hydroborating agents with their own regio- and chemospecific reactivity. BH2Br-DMS cleaves epoxides chemo- and regioselectively in the presence of reactive functional groups such as olefin and phenylalkyl ether, especially cleanly at lower temperatures.