Trans-dimethylborolane

Masamune subsequently developed the trans-dimethylborolane 104, which furnishes optically pure alcohols starting from cis- and trans-disubstituted alkenes as well as their trisubstituted counterparts [44]. The inherent structural simplicity of 104 as a reagent for asymmetric hydroboration is, unfortunately, overshadowed by the fact that preparation of the chiral borane 104 is rather labor-intensive. 

The stereoisomer of trans-2,5-dimethylborolane shown here has the R configuration at both stereocenters. Reaction of this chiral borane with rrans-2-butene produces 2-butanol in 71% yield. The 2-butanol is almost enantiomerically pure. It consists of 99.5% of the ( -enantiomer and only 0.5% of the (R)-enantiomer. This occurs because the transition state leading to the (50-enantiomer is much more favorable on steric grounds than is the transition state leading to the (R)-enantiomer. 

Preparative Methods a ca. 1 1 mixture of cis- and trans-B-methoxy-2,5-dimethylborolanes is first obtained by reaction of the Grignard reagent derived from 2,5-dibromohexane with diethylaminodichloroborane, followed by acidic methanolysis (eq 1). ... 

Garcia J, Kim B, Masamune S. Asymmetric addition of (Eland (Z)-crotyl-trans-2,5-dimethylborolanes to aldehydes. J. Org. Chem. 1987 52 4831 832. 

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