Question

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

Answer 1

`"9-BBN"` is more regioselective in hydroboration of alkenes.

Explanation:

Hydroboration involves the addition of a `"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 `"B"` atom adding to the less substituted carbon ("anti-Markovnikov" addition).

The regioselectivity is enhanced if a bulky dialkylborane, `"R"_2"BH"`, is the reagent.

`"9-BBN"` (9-borabicyclo[3.3.1]nonane is a dialkylbrane with the structure

(from http://www.hull.ac.uk/php/chsanb/alkenes/boron.pdf)

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

`"9-BBN"` is highly regioselective in its addition to alkenes.

(from http://www.hull.ac.uk/php/chsanb/alkenes/boron.pdf)

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

`"9-BBN"` is also highly sensitive to electronic factors.

(from http://www.hull.ac.uk/php/chsanb/alkenes/boron.pdf)

`"9-BBN"` is a highly versatile dialkylborane. Most alkenes react with it within 1 h at the usual reaction temperatures of 60-80 °C.