Hydroboration-Oxidation of Alkenes

Hydroboration-oxidation proceeds with anti-Markovnikov regioselectivity, adding the hydroxyl group to the less substituted carbon. In contrast, oxymercuration-reduction follows Markovnikov's rule, adding the hydroxyl group to the more substituted carbon. Both reactions convert alkenes to alcohols but differ in regiochemistry and stereoselectivity.

Borane is electron-deficient with a vacant 2p orbital and incomplete octet, making it highly reactive and unstable. Monomeric borane readily dimerizes into diborane. Tetrahydrofuran donates an electron pair into borane's vacant orbital, forming a stable boron-ether complex that can be safely handled and used as a reagent.

The alkene's π electrons attack borane in a concerted manner, forming a cyclic transition state where boron bonds to the less substituted, sterically hindered carbon. This concerted process results in syn-addition of BH2 and hydrogen across the double bond, producing an alkylborane intermediate with both atoms adding from the same face.

Hydroxide ion deprotonates hydrogen peroxide to form hydroperoxide, which attacks the trialkylborane. Alkyl groups migrate from boron to adjacent oxygen atoms, expelling hydroxide ions and forming alkoxyborane intermediates. This process repeats until a trialkoxyborane forms, which undergoes nucleophilic attack by hydroxide, followed by protonation to yield the final alcohol product.

After the first alkene adds to borane forming an alkylborane, the remaining B-H bonds can react with additional alkene molecules. A second alkene produces a dialkylborane, and a third alkene yields a trialkylborane. This sequential addition occurs because borane contains three B-H bonds available for reaction with alkenes.

Hydroboration-oxidation is syn-stereoselective, forming enantiomeric alcohol products with both hydroxyl and hydrogen adding from the same face of the alkene. This contrasts with regioselectivity and stereochemistry of acid catalyzed hydration, which lacks stereoselectivity and follows Markovnikov's rule, producing both syn and anti configurations.

Borane has only six electrons in its valence shell with an unoccupied 2p orbital perpendicular to the plane containing boron and three hydrogens at 120° angles. This electron-deficient structure makes borane highly electrophilic, resembling a carbocation without charge. This electrophilicity enables borane to readily attack the π electrons of alkenes in hydroboration.