Reaction of alkynes and azides: Not triazoles through copper-acetylides but oxazoles through copper-nitrene intermediates

Well-defined copper(I) complexes of composition [Tpm* ,BrCu(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. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.