Borylation

In organic chemistry, metal-catalyzed C–H borylation reactions constitute a family of organic reactions in which a transition metal catalyst activates aliphatic or aromatic C–H bonds to produce an organoboron compound (i.e., a C–B bond). They are thus functionalization reactions and a form of carbon–hydrogen bond activation. Compared to traditional polar reactions that introduce boron into a molecule, they obviate preparatory functionalization, eschew toxic byproducts, and (in some cases) functionalize a different part of the same substrate.

The net reaction for a borylation is generally a single displacement of the form

R₂B−BR₂ + R'H → R₂BH + R'BR₂

The R₂B−BR₂ structure is rather unusual, but known in bis(pinacolato)diboron (B₂Pin₂), bis(catecholato)diborane (B₂Cat₂), diboron tetrachloride, and tetrahydroxydiboron.

Primary versus secondary metal-alkyl complex

Mechanistically, the reaction resembles a noble-metal-catalyzed coupling reaction, and proceeds through cycles of oxidative addition followed by reductive elimination. First, the catalyst inserts into the boron-boron bond, forming a diboryl complex before eliminating one hydroboronic acid equivalent. Then it inserts into a carbon-hydrogen bond, forming a heteroleptic hydride-boryl-alkyl complex, before eliminating the product.