METAL-CATALYZED ALKYNE CYCLOTRIMERIZATIONS IN SUPERCRITICAL WATER

This work demonstrates that supercritical water can be utilized as a benign solvent for traditional, synthetic organometallic chemistry. The cobalt(I) complex CpCo(CO)2 catalyzed cyclotrimerizations of 1-hexyne, phenylacetylene, and 2-butyne to their respective benzene derivatives in water at 374-degrees-C. Product yields and regiochemistries were in good agreement with those reported for analogous cyclizations in organic solvents. Participation by water did not interfere with the cyclizations, and hydrolysis of the alkynes was observed only in the presence of acid. Unreacted alkyne was recovered in the absence of added catalyst. Cyclizations attempted in water at 140-degrees-C proceeded slowly and gave a multitude of products. Thus, alkyne cyclization proved to be a viable reaction in supercritical water but not in the lower temperature water. Pyridine synthesis via the coupling of alkynes and acetonitrile under supercritical conditions was also observed but was less effective due to relatively facile hydrolysis of the acetonitrile.