Asymmetric synthesis using organoboranes. Relative effectiveness of the B-Halobis(terpenyl)boranes for the enantioselective halogenative cleavage of representative meso-epoxides

A comparative study of the relative effectiveness of various Ter BX, such as EapBX, Eap BX, 2-IcrBX, 4-Icr BX, and CleapBX along with IpcBX 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-Icr BCl (78?80%) demonstrated significant improvement in enantiomeric excess over a previously explored reagent, IpcBCl (41%), especially for meso-cyclohexene oxide. Although all these three reagents, IpcBBr, EapBBr, and 2- IcrBBr provided comparable enantiomerically enriched 2-bromocyclohexan-1-ol (76?86%) from meso-cyclohexene oxide, the carene-based reagent, 2-IcrBBr 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, IpcBBr (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. © CSIRO 2007.