Asymmetric ring opening of meso-epoxides with B-halobis(2-isocaranyl)boranes 2-dIcr2BX

Hydroboration of commercially available (+)-2-carene (96% ee) with either BH2Cl·SMe2 or BCl3/Me3SiH, provides chemically pure B-chlorobis(2-isocaranyl)borane (2-dIcr2BCl) whereas B-bromobis(2-isocaranyl)borane (2-dIcr2BBr) could only be prepared by Matteson’s BBr3/Me3SiH procedure in high chemical yield and purity. The enantiomeric excess achieved with 2-dIcr2BCl (78%), was significantly higher than those realized with the previously explored reagent, dIpc2BCl (41%), especially for meso-cyclohexene oxide. The new reagent, 2-dIcr2BBr also 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 the previously reported reagent, dIpc2BBr (57% and 61%, respectively).

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