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

摘要

Well-defined copper(I) complexes of composition [Tpm~(*,Br)Cu(NCMe)]BF_4 (Tpm~(*,Br)=tris(3,5-dimethyl-4-bromopyrazolyl) methane) or [Tpa~*Cu]PF_6 (Tpa~*=tris(3,5-dimethylpyrazolylmethyl) 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.