Reaction of Alkynes and Azides: Not Triazoles Through Copper-Acetylides but Oxazoles Through Copper-Nitrene Intermediates

Well-defined copper(I) complexes of composition [Tpm*Cu-,Cu-Br(NCMe)]BF4 (Tpm*(,Br)=tris(3,5-dimethyl-4-bromo-pyrazolyl)methane) or [Tpa(*)Cu]PF6 (Tpa(*)=tris(3,5-dimethyl-pyrazolylmethyl)amine) catalyze the formation of 2,5-disubstituted oxazoles from carbonyl azides and terminal alkynes in a direct manner. This process represents a novel procedure for the synthesis of this valuable heterocycle from readily available starting materials, leading exclusively to the 2,5-isomer, attesting to a completely regioselective transformation. Experimental evidence and computational studies have allowed the proposal of a reaction mechanism based on the initial formation of a copper-acyl nitrene species, in contrast to the well-known mechanism for the copper-catalyzed alkyne and azide cycloaddition reactions (CuAAC) that is triggered by the formation of a copper-acetylide complex.