hydroboration-oxidation

Hydroboration-oxidation is the go-to method for adding water across an alkene the anti-Markovnikov way, putting the hydroxyl on the less-substituted carbon. It runs in two parts: first borane (BH3, usually as a complex) adds across the double bond (hydroboration), then a basic hydrogen peroxide workup (H2O2 with NaOH) swaps the boron for an OH (oxidation). The result is the mirror-image regiochemistry of acid hydration or oxymercuration.

Two features make this reaction special, and they come from a single concerted step. When BH3 adds, the boron and a hydrogen attach to the two alkene carbons at the same instant, on the same face — so the addition is syn. And because boron is the electron-poor (less electronegative) atom, it bonds to the less-hindered, less-substituted carbon, putting hydrogen on the more-substituted one; that is the anti-Markovnikov outcome. There is no carbocation, so there is no rearrangement and no Markovnikov bias. The oxidation step then replaces boron with OH while keeping that same position, with retention.

Hydroboration-oxidation is one of the most useful single transformations in the alkene toolkit precisely because it does what the ionic methods cannot: it delivers the less-substituted alcohol, cleanly and stereospecifically syn. Paired with oxymercuration (Markovnikov) it gives a chemist full control over which carbon bears the new hydroxyl — a beautiful example of how choosing the mechanism chooses the product.