Regioselectivity of hydroborations

The observed regioselectivity of hydroboration results in part from steric factors — the bulky boron-containing group can approach the less substituted carbon atom more easily. 

Organoborane intermediates can also be used to synthesize alkyl halides. Replacement of boron by iodine is rapid in the presence of base.150 The best yields are obtained with sodium methoxide in methanol.151 If less basic conditions are desirable, the use of iodine monochloride and sodium acetate gives good yields.152 As is the case in hydroboration-oxidation, the regioselectivity of hydroboration-halogenation is opposite to that observed for direct ionic addition of hydrogen halides to alkenes. Terminal alkenes give primary halides. 

Alkylidene borane 360 undergoes reaction with BH3-THF 361 to yield a dimerized cycloaddition product 362. BH3 hydroborates the B—C bond in two molecules of methylidene borane 360. The regioselectivity of hydroboration is governed by the electronic factors which facilitate the attack of boron (from BH3) on the two terminal carbon atoms, thus generating the B-C-B-C-B chain. The final product is obtained by the binding of two boryl ends via two B-H-B three-center, two-electron bridged bonds (Equation 20) <2004ZFA508>. 

Scheme 2. Regioselectivity of hydroboration reactions between AT-alkylsubstituted borazines and vinylsilanes.

These bulkier boranes enhance the regioselectivity of hydroboration of trisubstituted alkenes in particular and may also lead to high diastereoselectivity when there is a stereogenic centre next to the alkene. In this next example, an allylic alcohol is hydroborated with thexyl borane. Oxidation reveals complete regioselectivity and a 9 1 stereoselectivity in favour of hydroboration on the same side as the OH group. 

The carbon-carbon double bond is symmetrically substituted in cix-2-butene, and so the regioselectivity of hydroboration-oxidation is not an issue. Hydration of the double bond gives 2-butanol. 

Step 1 is consistent with the regioselectivity of hydroboration. Boron, with its attached substituents is more sterically demanding than hydrogen, and bonds to the less crowded carbon of the double bond hydrogen bonds to the more crowded one. Electronic effects are believed to be less important than steric ones, but point in the same direction. Hydrogen is transferred with a pair of electrons to the carbon atom that bears more of the positive charge in the tt complex, namely, the one that bears the methyl group. 

As is the case in hydroboration-oxidation, the regioselectivity of hydroboration-halogenation is opposite to that observed for direct addition of hydrogen halides to alkenes. Terminal alkenes give primary halides. 

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