Synthesis of strained, air-stable boracycles via boron-carbon-centred diradicals.

Boracycles are important functional scaffolds, often exhibiting superior or unique performance compared with their carbon analogues. Five-membered oxaboracycles are key pharmacophores in Food and Drug Administration-approved boron drugs. Meanwhile, six-membered boron-doped polycyclic aromatic hydrocarbons enhance the diversification and functionality of molecular materials. However, boron-containing four-membered rings are less studied owing to limited preparative approaches. Their inherent ring strain makes their synthesis thermodynamically unfavourable and hinders the exploration of their properties and applications. Here we report a triplet energy transfer catalysis for crafting air-stable benzoboretenes through intramolecular coupling of boron-carbon-centred diradicals. In addition, by modulating substrate π-conjugation structures and excitation energies, boron-carbon-centred diradicals can undergo formal 1,6- and 1,5-cyclization to deliver dihydroborinine and dihydrocyclopropaborole derivatives, respectively. The metal-free neutral reaction conditions ensure a broad reaction scope, resulting in structurally diverse boracycles that are stable enough to be purified via column chromatography. Further modification of the boracycles enables the facile synthesis of oxaborabicycles and dihydroborinine-fused polycyclic aromatic hydrocarbons with unique optoelectronic properties.