Pinacolboronate

Pinacolboronate refers to a class of boronate esters in which boron is bound to the diol pinacol, forming a cyclic boronate ester commonly abbreviated as B(pin). The pinacol group is 2,3-dimethyl-2,3-butanediol, and the resulting B(pin) moiety is a widely used protecting and masking group for boron in organic synthesis.

Structurally, a pinacolboronate consists of a boron atom coordinated to two oxygen atoms from the pinacol-derived diol, yielding a five-membered cyclic boronate ester. This arrangement imparts notable stability to air and moisture compared with other boronic acids or esters, while still allowing conversion back to boronic acids under appropriate conditions.

Preparation and properties: Pinacol boronates are typically prepared by transesterification or condensation of boronic acids or boron reagents with pinacol. They are generally isolable as relatively stable, often crystalline solids. They can be hydrolyzed to the corresponding boronic acids under aqueous or acidic/basic workup, enabling recovery of the reactive boron species when needed.

Applications: In organic synthesis, pinacol boronates serve as stable equivalents of boronic acids, enabling storage and handling of boron-containing substrates. They are widely used in Suzuki–Miyaura cross-coupling and related transformations, where the boron unit is activated in situ to participate in transmetallation. Pinacol boronates also function as protected boron moieties during multi-step sequences and can be converted into various organoboron derivatives through selective deprotection or transesterification. Related reagents, such as bis(pinacolato)diboron (B2pin2), provide boron for borylation reactions and underscore the prominence of the pinacol boronate motif in modern boron chemistry.