Ring Expansion Reactions of Electron-Rich Boron-Containing Heterocycles

The potassium salts of the dianions of isomeric compounds bis-benzocycloborabutylidene, 1, and the ladder diborole 2 were reacted with carbon dioxide (CO₂) and carbon monoxide (CO) and the ring expansion products fully characterized. Both dianions 1 and 2 react rapidly with carbon dioxide to form the same insertion product, 3, in which the boron-containing rings are expanded to six-membered rings. Compound 3 is a B–O analogue of binaphtholate. Only dianion 2 reacts cleanly with the weaker electrophile CO, producing a product (4) in which only one of the boron-containing rings is expanded through formal insertion of CO into a B–C bond. The X-ray structures of both 3 and 4 are reported, and reasonable paths to their formation are proposed.