Organic Chemistry/Introduction to reactions/Hydroboration

Hydroboration basics

By reacting an alkene with borane, a useful reaction intermediate called an organoborane can be formed. An organoborane can be oxidized to form a terminal alcohol, or it can be protonated to form an alkane.

(Basic examples of reduction and protonolysis here)

History

Hydroboration reactions were developed by Herbert C. Brown during his research at Purdue University. It earned him the 1979 Nobel Prize in Chemistry (along with Georg Wittig) because of the practical applications to synthesis, namely pharmaceuticals.

Formation of borane reagents

Borane is not stable, it must be generated in situ from a borohydride salt (eg NaBH₄) at the time of organoborane formation. Diborane exists in equilibrium with borane.

Formation of diborane (and borane) from sodium borohydride

Hydroboration/Protonolysis

After one has formed an organoborane, it can be be reacted with an organic acid to obtain an alkane. The two hydrogens are added with syn stereospecificity

Hydroboration/Oxidation

Organoboranes can be oxidized in a basic peroxide solution to form alcohols. The alcohol that is formed is the opposite product that would be formed by oxymercuration or hydration. The hydrogen and hydroxide groups add syn to each other.

Regiochemistry of products

Boration reactions generate anti-Markovnikov products because hydrogen adds second as a hydride nucleophile. (In most other reactions, hydrogen usually adds first as an electrophile.) The borane adds first, which is an honor usually reserved for the strongly electrophilic proton.