Internal B–O Bond-Facilitated Photoisomerization of Boranes: Ring Expansion Versus Oxyborane Elimination/Intramolecular Diels–Alder Addition

Boron compounds (1–4) containing an internal B–O bond have been found to undergo facile multistructural transformations upon irradiation at 365 or 410 nm, generating rare 8-membered B,O-heterocycles (1c–4c). In addition, 2 and 3 also undergo an intramolecular Diels–Alder addition and oxyborane elimination concomitantly, via intermediates 2b/3b, producing 2d/3d. The pathways to isomer c and product d were found to be a thermal process and a photo process, respectively.