Ring Expansion Reactions of Electron-Rich Boron-Containing Heterocycles

Abstract

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