hydroborating

Hydroboration is the addition of boron and hydrogen across a multiple bond, most commonly an alkene or an alkyne, to form organoborane intermediates. The reaction proceeds with syn addition and generally exhibits anti-Markovnikov regioselectivity: boron attaches to the less substituted carbon of the double or triple bond, while hydrogen adds to the more substituted carbon. The process typically occurs in organic solvents at modest temperatures and proceeds through a concerted four-center transition state, giving stereospecific outcomes in many cases.

Common boron sources include borane-tetrahydrofuran complex (BH3·THF), borane in various solvents, and bulky boranes such as disiamylborane or 9-borabicyclo[3.3.1]nonane (9-BBN), which can influence selectivity. After hydroboration, oxidation of the organoborane with hydrogen peroxide in basic solution (H2O2/NaOH) converts it to an alcohol, providing anti-Markovnikov hydration of alkenes. For terminal alkenes, this yields primary alcohols; for internal alkenes, secondary or tertiary alcohols can result depending on substitution.

Hydroboration is valuable for its chemoselectivity and the versatility of the boron moiety, which can be transformed into various other functionalities, including through cross-coupling reactions. It also extends to alkynes, where hydroboration forms vinyl boranes that can be oxidized to aldehydes (for terminal alkynes) or ketones under appropriate conditions. Safety considerations include the air sensitivity and pyrophoric nature of many borane reagents, requiring proper handling and inert-atmosphere techniques.