Hydroboration-oxidation of alkenes has been a very valuable laboratory method for the stereoselective and regioselective addition of alkenes, without any rearrangement.

The borane complex

Borane itself exists naturally as a very toxic gas in the form of a dimer with the general formula B₂H₆ (diborane), which ignites spontaneously in air. Borane is commercially available in ether and tetrahydrofuran (THF).  In these solutions the borane exists as a Lewis acid-base complex, which allows boron to have an complete octet.

The mechanism

Elementary step #1

• Part #1: Hydroboration of the alkene. In this first step the addition of the borane to the alkene is initiated and proceeds as a concerted reaction because bond breaking and bond formation occur at the same time. This step involves the vacant 2p orbital of the boron electrophile pairing with the electron pair of the π-bond of the alkene nucleophile.

Transition state

* Note that a carbocation is not formed. Therefore, no rearrangement takes place.

• In the product, the boron has added to the less substituted carbon of the alkene, which then places the hydrogen on the more substituted carbon. Both, the boron and the hydrogen add simultaneously on the same face of the double bond (syn addition).

Oxidation of the trialkylborane by hydrogen peroxide

You will not be required to know this part of the mechanism for this course, but further information is available in the links below.

Stereochemistry of hydroboration

The hydroboration reaction is among the few simple addition reactions that proceed cleanly in a syn fashion. As noted above, this is a single-step reaction. Since the bonding of the double bond carbons to boron and hydrogen is concerted, it follows that the geometry of this addition must be syn. Furthermore, rearrangements are unlikely inasmuch as a discrete carbocation intermediate is never formed. These features are illustrated for the hydroboration of α-pinene.

Since the hydroboration procedure is most commonly used to hydrate alkenes in an anti-Markovnikov fashion, we also need to know the stereoselectivity of the second oxidation reaction, which substitutes a hydroxyl group for the boron atom. Independent study has shown this reaction takes place with retention of configuration so the overall addition of water is also syn.

The hydroboration of α-pinene also provides a nice example of steric control in a chemical reaction. In the less complex alkenes used in earlier examples the plane of the double bond was often a plane of symmetry, and addition reagents could approach with equal ease from either side. In this case, one of the methyl groups bonded to C-6 (colored blue in the equation) covers one face of the double bond, blocking any approach from that side. All reagents that add to this double bond must therefore approach from the side opposite this methyl.