Copper(I) Complexes of Normal and Abnormal Carbenes and Their Use as Catalysts for the Huisgen [3+2] Cycloaddition between Azides and Alkynes

Abstract

AbstractComplexes of general formula [(NHC)Cu(μ‐I)2Cu(NHC)] (NHC = 1‐benzyl‐3‐butylbenzimidazolin‐2‐ylidene for 7; NHC = 1‐benzyl‐3‐isopropylbenzimidazolin‐2‐ylidene for 8 NHC = 1‐butyl‐3‐isopropylbenzimidazolin‐2‐ylidene for 9) and [(aNHC)CuI] {aNHC = 1‐benzyl‐3‐methyl‐4‐phenyl‐1H‐1,2,3‐triazol‐3‐ium‐5‐ylidene for 10; 3‐methyl‐1‐[2‐(methylthio)phenyl]‐4‐phenyl‐1H‐1,2,3‐triazol‐3‐ium‐5‐ylidene for 11} were synthesized from CuI and the corresponding azolium salts. All complexes were fully characterized by spectroscopic methods, and single‐crystal X‐ray structural analyses were performed for 7 and 11. Complex 7 exhibits an iodido‐bridged dicopper(I) form in the solid state. The copper centers in 7 adopt a trigonal coordination. The intramolecular Cu–Cu distance is 2.896(2) Å. In contrast to 7, 11 is mononuclear, and the copper center has a linear coordination. All the complexes were tested for their efficiency as catalysts in the [3+2] cycloaddition reaction between azides and alkynes. The benzimidazolin‐2‐ylidene‐containing complexes 7–9 as well as the triazol‐3‐ium‐5‐ylidene‐containing complexes 10–11 turned out to be highly efficient catalysts for the 1,3‐cycloaddition reaction, both under neat conditions and in water. Short reaction times as well as low catalyst loadings could be achieved, and all the manipulations were carried out under proper “click” conditions with simple product isolation and no purification steps. The efficiency of the catalysts seems to correlate well with the σ‐donor ability of the NHCs. Cycloaddition reaction with an internal alkyne was also achieved with these catalysts. Finally a bulky azide was used as a substrate to prepare compound 1‐(2,6‐dimesityl)phenyl‐4‐(2‐pyridyl)‐1,2,3‐triazole (14) with the above mentioned copper catalysts. Such bulky azides are usually unreactive towards cycloaddition reactions with alkynes in the presence of the CuII/ascorbate/tbta catalytic system.