hydroboranes

Hydroboranes are chemical compounds that contain boron–hydrogen bonds. The class includes the simplest boron hydride, borane (BH3), which primarily exists as stabilized adducts such as borane–tetrahydrofuran (BH3·THF) or borane–dimethyl sulfide (BH3·SMe2) because free BH3 is highly reactive. More complex hydroboranes include diborane (B2H6) and larger borane clusters. Collectively, hydroboranes encompass discrete molecules, oligomers, and polyhedral boron hydrides that feature B–H bonds.

Bonding in hydroboranes often involves electron-deficient boron centers. In many polyboranes, multicenter bonds such as three-center two-electron B–H–B bridges accommodate electron deficiency. Larger boranes can adopt closo- or other polyhedral structures in which multiple B–H vertices contribute to multicenter bonding.

Preparation and reagents: BH3 is typically generated and stabilized as adducts (BH3·THF, BH3·SMe2). B2H6 arises by dimerization of BH3. Reagents for hydroboration include 9-borabicyclo[3.3.1]nonane (9-BBN), catecholborane (HBcat), and borane complexes with pyridine or THF. These hydroborating reagents are often used under inert atmosphere because hydroboranes can be air- and moisture-sensitive.

Applications: Hydroboration adds B–H across carbon–carbon multiple bonds in a syn fashion, yielding organoboranes. Upon oxidation with hydrogen peroxide in base, the boron group is converted to alcohols in anti-Markovnikov fashion. Hydroboration is a key step in many synthetic routes and is compatible with diverse functional groups; boron substituents can be transformed into boronates for cross-coupling reactions such as Suzuki–Miyaura.

Safety and handling: Many hydroboranes are air- and moisture-sensitive; some, notably diborane, are highly reactive and toxic, requiring careful handling under inert atmosphere and appropriate safety precautions.