Carbonyl Cobalt-Catalyzed Cyclotrimerization of Terminal Alkynes in Supercritical Carbon Dioxide

Abstract

Atom-efficient [2+2+2] cycloaddition reaction of alkynes in green solvent supercritical carbon dioxide (ScCO2) is an environmentally friendly reaction process that conforms to the principles of green chemistry. Cyclotrimerization of terminal alkynes catalyzed by Co2(CO)8 in pure ScCO2 has been studied to obtain 1,2,4-trisubstituted benzene derivatives with excellent selectivity. The reaction conditions for the cyclotrimerization were optimized, such as concentration of catalyst, CO2 pressure, reaction temperature and time. The solubility and phase behavior of the reaction materials and catalysts in ScCO2 medium were discussed, and the mechanism of Co2(CO)8 catalyzed cyclotrimerization of terminal alkynes was assumed. The reaction substrate was extended from C≡C (alkyne) to C≡N (nitrile), and the alkyne-nitrile cycloaddition reaction in ScCO2 was preliminary explored. Our optimized catalytic system for the cyclotrimerization of terminal alkynes exhibited wide substrate scope and high product selectivity, in which no organic co-solvent or additives were added. It provided a green synthetic method for 1,2,4-trisubstituted benzenes.