Vinylboranes

Vinylboranes are organic compounds in which a vinyl group (ethenyl, –CH=CH2) is bonded directly to boron. They are a subset of alkenylboranes and include vinylborane (CH2=CH–BH2) as well as more substituted derivatives where boron carries alkyl or alkoxy substituents. The simplest member, vinylborane, is highly reactive and is usually generated in situ or stored under strictly inert conditions.

Preparation and structure: Vinylboranes are commonly prepared by hydroboration of alkynes with borane reagents such as BH3·THF or B2H6, affording anti-Markovnikov vinylboranes with the boron attached to the less substituted vinyl carbon. Substituted vinylboranes can be generated by hydroboration of substituted alkynes or by subsequent functionalization of simple vinylboranes (e.g., conversion of BH2 to B(OR)2 or to boronates). The boron center is typically three-coordinate in the borane form and can be converted to more stable boronates or boronic esters.

Reactivity and applications: As organoboron reagents, vinylboranes participate in typical boron-based transformations. They serve as precursors to vinyl boronates used in Suzuki–Miyaura cross-coupling to forge carbon–carbon bonds, and can be oxidized or transesterified to access various vinyl boronates and vinyl alcohol derivatives. Hydroboration of alkynes followed by subsequent oxidation or cross-coupling is a common route to elaborated alkenes and conjugated systems.

Safety and handling: Vinylboranes and related reagents are typically air- and moisture-sensitive, requiring inert-atmosphere techniques and appropriate protective equipment.