Journal
CHEMISTRY-A EUROPEAN JOURNAL
Volume 20, Issue 12, Pages 3463-3474Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.201303737
Keywords
alkynes; click chemistry; copper; nitrenes; oxazoles; reactive intermediates
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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.
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