C–C versus C–H Activation: Understanding How the Carbene π‑Accepting Ability Controls the Intramolecular Reactivities of Mono(carbene)-Stabilized Borylenes

Highly reactive dicoordinate (i.e., mono-base-stabilized) borylenes of the form [LBR] (L = Lewis base) have recently shown intriguing reactivity with inert small molecules, including N2. However, this reactivity is complicated by competing intramolecular C–H and C–C bond activation reactions. In this work, density functional theory (DFT) calculations are performed to study the mechanisms of the selective intramolecular activation of ortho C­(sp2)–C­(sp3) and benzylic C­(sp3)–H bonds of the N-bound aryl group of N-heterocyclic carbene (NHC) and cyclic (alkyl)­(amino)­carbene (CAAC) ligands observed experimentally in mono­(carbene)-stabilized aminoborylenes. We focus our efforts on how different cyclic carbene ligands steer the selectivity of the reaction pathways, as well as the reaction products. The electronic structure analyses reveal that the “empty” p orbital and the “lone pair” of electrons located on the mono­(carbene)-stabilized borylene boron center play important roles in determining the selectivity. The computational results indicate that the electron-pair acceptor capability of the former and the electron-pair donor capability of the latter are significantly affected by the attachment of different NHC ligands. The detailed mechanisms obtained lead to an in-depth understanding of the nature of the cleavage of ortho C­(sp2)–C­(sp3), C­(sp2)–H, and benzylic C­(sp3)–H bonds and the reactivity of different mono­(carbene)-stabilized borylene compounds. The results have direct relevance to the exciting recent discoveries in the field of single-site main-group ambiphiles and their potential for small-molecule activation and catalysis.