Regioselectivity Brown hydroboration reaction

The hydroboration reaction is generally highly, but not completely, regioselective. For example, reaction of 1-hexene (47) with diborane, followed by oxidation, produces 1-hexanol (48) in high yield, with only a small amoimt of 2-hexanol (49, equation 9.46). Brown determined that the preference for the boron atom to add to the less-substituted carbon atom is about 94% for monosubstituted alkenes such as 1-pentene, 99% for em-disubstituted al-kenes such as 2-methyl-l-butene, and 98% for trisubstituted alkenes such as 2-methyl-2-butene. ... 

Brown et a/ [1] have elegantly hydroborated a variety of heterocyclic olefins, which on oxidation yield the corresponding alcohols in excellent yields. In the case of heterocyclic olefin containing a double bond a to the heteroatom, the hydroboration reaction is highly regioselective, placing boron at the (3-carbon atom. The use of 9-BBN for the synthesis of variety of heterocyclic alcohols is given in Chart 6.21. 

Borane 2 adds to carbon-carbon double bonds without the need of catalytic activation. This reaction has been discovered and thoroughly investigated by H. C. Brown, and is called hydroboration.It permits a regioselective and stereospecific conversion of alkenes to a variety of functionalized products. 

Later, Brown and co-workers developed the method described above for the preparation of enantiomerically pure Ipc2BH (>99% ee) and applied the reagent in the asymmetric hydroboration of prochiral alkenes. Oxidation of the trialkylboranes provided optically active alcohols. In the case of cis-alkenes, secondary alcohols were obtained in excellent enantiomeric purity (Figure 1). The reaction is general for most types of cw-alkene, e.g. C(S-2-butene forms (R)-2-butanol in 98.4% ee, and c(s-3-hexene is converted to (R)-3-hexanol in 93% ee. However, the reagent is somewhat limited in reactions with unsymmetrical alkenes e.g. c/s-4-methyl-2-pentene yields 4-methyl-2-pentanol with 96% regioselectivity but only 76% ee (Figure 1). ... 

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