Hydroboration-Oxidation

Hydroboration is an important process in organic chemistry in which a hydrogen-boron bond is introduced to specific carbon double and triple bonds. The outcome of hydroboration is the formation of organoborane intermediates, which are versatile in reacting with diverse reagents to yield valuable compounds like alcohols, amines, and alkyl halides. Notably, the oxidation of organoboranes, typically using hydrogen peroxide, is a renowned reaction that leads to the production of alcohols.

​

Characteristically, hydroboration exhibits anti-Markovnikov selectivity, meaning the hydrogen atom attaches to the more substituted carbon in the double bond. This unique regiochemistry is a consequence of the B-H bond polarity. The reaction advances through a four-membered transition state with hydrogen and boron adding to the same side of the double bond. In this concerted mechanism, the C-B bond forms marginally quicker than the C-H bond, resulting in a transition state where boron carries a slight negative charge and the more substituted carbon a slight positive charge, which is stabilized by the latter’s substitution.

​

In cases of trisubstituted alkenes, boron predominantly attaches to the less substituted carbon atom. The C-B bonds created through hydroboration are highly reactive with various reagents, with hydrogen peroxide being the most common. The stereospecific nature of H-B addition to olefins ensures that the subsequent oxidation reaction is diastereoselective, particularly when dealing with trisubstituted alkenes.