Activating Azides and Alkynes for the Click Reaction with [Cu(aNHC)2I] or [Cu(aNHC)2]+ (aNHC = Triazole‐Derived Abnormal Carbenes): Structural Characterization and Catalytic Properties

Abstract

AbstractNeutral, iodido‐containing copper(I) complexes [Cu(aNHC)2I] {aNHC = 1‐benzyl‐3‐methyl‐4‐phenyl‐1,2,3‐triazol‐5‐ylidene (for 6) and 3‐methyl‐1‐[2‐(methylthio)phenyl]‐4‐phenyl‐1,2,3‐triazol‐5‐ylidene (for 7)} and cationic, halide‐free copper(I) complexes [Cu(aNHC)2](BF4) {aNHC = 1‐benzyl‐3‐methyl‐4‐phenyl‐1,2,3‐triazol‐5‐ylidene (for 8), 3‐methyl‐1,4‐diphenyl‐1,2,3‐triazol‐5‐ylidene (for 9), 3‐methyl‐1‐[2‐(methylthio)phenyl]‐4‐phenyl‐1,2,3‐triazol‐5‐ylidene (for 10), and 1‐mesityl‐3‐methyl‐4‐phenyl‐1,2,3‐triazol‐5‐ylidene (for 11)}, both containing two monodentate abnormal‐carbene ligands (aNHC), were synthesized from [Cu(CH3CN)4](BF4) and the corresponding triazolium salts. It was possible to selectively synthesize both kinds of complexes by simply variing the counter‐anion of the triazolium salts and keeping the metal precursor the same. All complexes were characterized by elemental analysis and spectroscopic methods. 6 and 11 were studied with single‐crystal X‐ray diffraction analyses. In 6, the copper(I) center is tricoordinated, and its geometry is in between trigonal planar and T‐shaped. In halide‐free 11, the copper(I) center is linearly coordinated by two abnormal‐carbene ligands. All complexes were tested as catalysts in the Huisgen [3+2] cycloaddition reaction between azides and alkynes, and they showed excellent efficiencies under neat conditions. A comparison between the efficiencies of the halide‐containing complexes 6 and 7 and the halide‐free cases 8–11, shows that the halide‐free Cu–aNHC complexes are significantly more efficient than their halide‐containing counterparts. The best catalyst was used for a substrate screening by utilizing a variety of azides and a couple of alkynes. The efficiency of the catalyst was maintained with loadings as low as 0.005 mol‐%. Mechanistic studies were carried out as well.