Borylenes constitute a particularly reactive class of compounds due to the presence of a lone pair of electrons and two vacant p orbitals at the boron center. They can either be stabilized in the coordination sphere of a transition metal or as metal-free borylenes bearing supporting donor ligands. The latter owe their stability predominantly to the excellent σ donor and π acceptor properties of one or two cyclic alkyl(amino)carbene (CAAC) ligands, which enable delocalization of the electron density of the low-valent boron center. Recently, a few related stable metal-free borylenes including aryl-, cyano- and haloborylenes have been reported, which were also stabilized by π-accepting donor ligands. In this work, we present the two-electron reduction of the CAAC-stabilized boron isothiocyanates 1R (R = Me, Cy) in the presence of the N-heterocyclic carbene IiPr yielding the corresponding mixed-Lewis-base-stabilized borylenes 2R. In contrast, reduction in the absence of a donor ligand afforded the deep blue bis(CAAC)-stabilized thiazaborolo[5,4-d]thiazaboroles 3R, formally resulting from a borylene dimerization product with C–C coupling (Scheme 1). The borylenes 2R and the B,N,S-heterocycles 3R were fully characterized by X-ray crystallographic analysis, NMR, IR and UV-vis spectroscopy as well as computational studies.

Highly colored boron-doped thiazolothiazoles from the reductive dimerization of boron isothiocyanates

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.