# Which is more highly regioselective: a reaction of an alkene with BH_3 or with 9-BBN?

Jan 19, 2016

$\text{9-BBN}$ is more regioselective in hydroboration of alkenes.

#### Explanation:

Hydroboration involves the addition of a $\text{B-H}$ bond to an alkene or alkyne.

It is often followed by oxidation with alkaline hydrogen peroxide to form an alcohol.

The reaction is regioselective, with the $\text{B}$ atom adding to the less substituted carbon ("anti-Markovnikov" addition).

The regioselectivity is enhanced if a bulky dialkylborane, $\text{R"_2"BH}$, is the reagent.

$\text{9-BBN}$ (9-borabicyclo[3.3.1]nonane is a dialkylbrane with the structure

In equations, it is often written as the blue shorthand symbol.

$\text{9-BBN}$ is highly regioselective in its addition to alkenes.

This sensitivity to steric effects arises because the rigid bicyclic structure prevents internal rotation to relieve steric hindrance in the transition state.

$\text{9-BBN}$ is also highly sensitive to electronic factors.

$\text{9-BBN}$ is a highly versatile dialkylborane. Most alkenes react with it within 1 h at the usual reaction temperatures of 60-80 °C.