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

1 Answer
Ernest Z.
Jan 19, 2016

#"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.

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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

9BBN
(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.

Regio
(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.

Electronic
(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.

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