Asymmetric Synthesis using Organoboranes. Relative Effectiveness of the B-Halobis(terpenyl)boranes for the Enantioselective Halogenative Cleavage of Representative meso-Epoxides

Abstract

A comparative study of the relative effectiveness of various Ter2BX, such as dEap2BX, lEap2BX, 2-dIcr2BX, 4-dIcr2BX, and lCleap2BX along with dIpc2BX for the asymmetric ring opening of three representative meso-epoxides (cyclohexene, cyclopentene, and cis-2,3-butene oxides) is reported. Among all the reagents studied, 2-dIcr2BCl (78–80%) demonstrated significant improvement in enantiomeric excess over a previously explored reagent, dIpc2BCl (41%), especially for meso-cyclohexene oxide. Although all these three reagents, dIpc2BBr, dEap2BBr, and 2-dIcr2BBr provided comparable enantiomerically enriched 2-bromocyclohexan-1-ol (76–86%) from meso-cyclohexene oxide, the carene-based reagent, 2-dIcr2BBr showed considerable improvements in enantiomeric excesses in the cases of meso-cyclopentene oxide (67%) and meso-cis-2,3-butene oxide (78%) than those achieved with previously reported reagent, dIpc2BBr (57 and 61% respectively). The enantioselectivity of the reaction was observed to be highly substrate dependent. The present study represents a significant advance in asymmetric synthesis using the chiral organoborane chemistry.