Formation of Oxazoles from Elusive Gold(I) α-Oxocarbenes: A Mechanistic Study.

The gold(I) catalyzed reaction between phenylacetylene, pyridine N-oxide and acetonitrile leading, via a putative gold-α-oxocarbene intermediate, towards an oxazole product has been investigated. A novel mass spectrometric method called "delayed reactant labeling" is used to track consecutive and parallel reactions. It clearly shows that the intramolecular formation of a pyridine adduct of gold-α-oxocarbene is in competition with the formation of the oxazole product. The reaction mechanism most probably corresponds to competition between acetonitrile and pyridine in an almost barrierless reaction with putative gold-α-oxocarbene within the solvent cage. The detected ionic species have been characterized by helium tagging infrared photodissociation spectroscopy.