Manganese-Catalyzed Benzene Synthesis by [2+2+2] Coupling of 1,3-Dicarbonyl Compound and Terminal Acetylene

Treatment of a mixture of a 1,3-dicarbonyl compound such as a beta-ketoester or 1,3-ketone and a terminal acetylene with a catalytic amount of MnBr(CO)5 in heated toluene produces a benzene derivative by a [2+2+2] coupling reaction incorporating the enol part of the dicarbonyl compound and two moles of the acetylene. When the reaction was carried out using phenylacetylene derivatives, the reaction was completely regioselective, producing p-terphenyl compounds in good to excellent yield. Aliphatic terminal acetylenes also reacted readily but gave a mixture of regioisomers. The reaction features high atom economy, neutral conditions, and functional group tolerance, and will be useful for materials-oriented studies.