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.