Hydroboration dimethylborolane

Among chiral dialkylboranes, diisopinocampheylborane (8) is the most important and best-studied asymmetric hydroborating agent. It is obtained in both enantiomeric forms from naturally occurring a-pinene. Several procedures for its synthesis have been developed (151—153). The most convenient one, providing product of essentially 100% ee, involves the hydroboration of a-pinene with borane—dimethyl sulfide in tetrahydrofuran (154). Other chiral dialkylboranes derived from terpenes, eg, 2- and 3-carene (155), limonene (156), and longifolene (157,158), can also be prepared by controlled hydroboration. A more tedious approach to chiral dialkylboranes is based on the resolution of racemates. /n j -2,5-Dimethylborolane, which shows excellent enantioselectivity in the hydroboration of all principal classes of prochiral alkenes except 1,1-disubstituted terminal double bonds, has been... 

Dimethylborolane. Excellent results in asymmetric hydroborations are obtained with Masamune s tran5-2,5-dimethylborolane reagents. The C(2)-symmetry of the reagent ensures that both faces of the borolane ring are equivalent, which reduces the number of competing anri-Markovnikov transition states. Unfortunately, the reagent is not readily available. 

Masamune and cowotkers48 designed a set of chiral rrans-2,5-dimethylborolane-based reagents that show remarkable enantioselectivities in hydroboration,49 ketone reduction50 and crotyl addition to aldehydes.51 The chirality transfer also turns out to be highly efficient in the aldol reaction. Initial experiments were aimed at defining the structural parameters which affect the E(0) Z(0) ratios of the enolates prepared from various alkanethioates with 2,5-dimethylborolanyl triflate (90) under standard conditions... 

Limonylborane (LimBH) hydroborates all major classes of alkenes, except terminal disubstituted alkenes, with moderate asymmetric inductions of 45-66% ee. Similar to tram-2,5-dimethylborolane and in contrast to other dialkylboranes, it handles (E)- and (Z)-2-butene with comparable enantioselectivity.10 70. However, both dilongifolylborane and limonylborane are less selective than the best reagents developed for a particular class of olefins and are therefore not widely utilized. 

In summary, only frorw-2,5-dimethylborolane, which is not readily available, consistently gives products of high optical purity with three major classes of olefins. In addition, relatively unhindered (Z)-olefms arc the only class handled well by diisopinocampheylborane. Consequently, the range of organoboranes directly produced in high optical purity by asymmetric hydroboration is rather limited. Fortunately, mono- and diisopinocampheylborane derivatives have several features which allow their optical purity to be upgraded to essentially 100% ee. 

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. 

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