hydroborations

Hydroboration refers to the addition of boron and hydrogen across unsaturated carbon–carbon bonds, most commonly to alkenes and alkynes, using borane reagents such as borane–tetrahydrofuran (BH3·THF), borane–dimethyl sulfide (BH3·SMe2), or more stable boron sources like pinacolborane (HBpin) and catecholborane. The classic application is hydroboration–oxidation, in which hydroboration of an alkene is followed by oxidation (usually H2O2/NaOH) to give anti-Markovnikov alcohols. The reaction is notable for its syn addition and high regioselectivity.

Mechanistically, hydroboration proceeds through a concerted, four‑membered cyclic transition state in which boron attaches to the less substituted carbon and hydrogen attaches to the more substituted carbon. This syn addition preserves the alkene geometry and explains the anti‑Markovnikov regioselectivity observed after oxidation. With alkynes, hydroboration delivers vinyl boranes with similarly syn, anti‑Markovnikov addition; oxidation then yields aldehydes from terminal alkynes and ketones from internal alkynes.

Beyond alcohol synthesis, organoboron products serve as versatile intermediates for cross‑coupling reactions (most notably Suzuki coupling) and other transformations. The broad utility has spurred development of milder boron reagents (e.g., HBpin) and catalytic, enantioselective variants using transition‑metal catalysts with chiral ligands to generate enantioenriched boron‑containing intermediates, which can be converted to chiral alcohols or other motifs.

Typical scope includes a wide range of alkenes and functional groups. Limitations include slower reactions with highly substituted alkenes and substrates that coordinate strongly to boron or catalyst. Overall, hydroboration remains a cornerstone method for site‑selective, mild installation of boron and subsequent diversification of simple unsaturated substrates.