Boryls, their compounds and reactivity: a structure and bonding perspective

Abstract

Boryls and their compounds are important due to their diverse range of applications in the fields of materials science and catalysis. They are an integral part of boron chemistry, which has attracted tremendous research interest over the past few decades. In this perspective, we provide an in-depth analysis of the reaction chemistry of boryl compounds from a structure and bonding perspective. We discuss the reactivity of boryls in various transition metal complexes and diborane(4) compounds towards different substrate molecules, with a focus on their nucleophilic and electrophilic properties in various reaction processes. Additionally, we briefly discuss the reactivity of boryl radicals. Our analysis sheds new light on the unique properties of boryls and their potential for catalytic applications.

Fig. 1. Boryls in various compounds. (a) Organoboron compounds with a boryl group bonded to an organic group. (b) Metal boryl complexes featuring a boryl ligand coordinated to a metal centre. (c) Diborane(4) compounds showcasing two boryl groups bonded via a B–B σ bond.

Fig. 2. Three possible reaction scenarios when a boryl group reacts with a substrate molecule.

Fig. 3. Sketches of the transition state structures for the two competing pathways (S_N2 versus halogen-abstraction) in the reaction of lithium boryl with organohalides.

Fig. 4. Synthetic routes of magnesium(ii) diboronate and boryl complexes.

Scheme 1. Mechanism of the Cu-boryl-catalyzed diboration reaction of aldehydes.

Fig. 5. Illustration of the Cu–B σ bonding pair of electrons in boryl ligand nucleophilicity.

Fig. 6. Illustration of how a nucleophilic boryl initiates and is involved in the activation of unsaturated molecules.

Scheme 2. Mechanism of the Pt-boryl-catalyzed diboration reaction of acrolein.

Scheme 3. Insertion of a ketone into the Rh(i)–Bpin bond.

Fig. 7. DFT-predicted reactivity of various transition metal boryl complexes towards benzaldehyde.

Fig. 8. Illustration of the dual reactivity of diborane(4) toward the activation of unsaturated molecules.

Scheme 4. Mechanism of the reaction of pinB-BMes₂ with CO.

Scheme 5. Mechanism of the reaction of pinB-BMes₂ with 1 equiv. of Xyl-NC (bottom) and 2 equiv. of Xyl-NC (top).

Scheme 6. Mechanism of the reaction of B₂(o-tol)₂ and tBu–NC.

Scheme 7. Mechanism of the reaction of B(An)₂BF₂ and Ph-N₃.

Fig. 9. Illustration of the diverse reaction modes of diborane(4) compounds for unsaturated organic substrates.

Fig. 10. Illustration of the 5e and 7e boryl radical species.

Scheme 8. Reactions of a Lewis base ligated boryl radical with unsaturated and saturated molecules.

Xueying Guo

Zhenyang Lin