When propene is treated with a solution containing BH 3· THF, the boron hydride adds successively to the double bonds of three molecules of the alkene to form a trialkylborane (Scheme 15).

Scheme 15

In the oxidation step, hydroperoxide anion (HOO-) adds to the trivalent boron atom and forms an unstable intermediate that has a formal negative charge on the boron. Migration of an alkyl group with a pair of electrons from the boron to the adjacent oxygen leads to neutralization of the charge on boron and displacement of a hydroxide anion. The alkyl migration takes place with retention of configuration at the migrating carbon. Repetition of the hydroperoxide anion addition and migration steps occurs twice more until all of the alkyl groups have become attached to oxygen atoms, resulting in a trialkyl borate ester, B(OR)₃ . The borate ester then undergoes basic hydrolysis to produce three molecules of the alcohol and an inorganic borate anion (Scheme 16).

Scheme 16

The simultaneous addition of boron and hydrogen to the double bond leads to a syn- addition : Boron and hydrogen add across the double bond on the same side of the molecule. The stereochemistry of the hydroboration-oxidation of propene is shown (Scheme 17). Boron and hydrogen add to the same face of the double bond ( syn ) to form a trialkylborane. Oxidation of the trialkylborane replaces boron with a hydroxyl group in the same stereochemical position.

Scheme 17

Hydroboration of alkenes is another example of a stereospecific reaction , in which different stereoisomers of the starting compound react to give different stereoisomers of the product. It is also regioselective and gives anti -Markovnikov product (Scheme 18).