Silver-N-Heterocyclic Carbenes in π-Activation : Synergistic Effects between the Ligand Ring Size and the Anion

A series of 12 silver(I)-N-heterocyclic carbene (NHC) complexes were prepared featuring five- (both saturated and unsaturated backbone), six-, and seven-membered ring ligand scaffolds. The N-substituents of the NHCs were diisopropylphenyl in all cases, while the anion was varied between bromide, acetate, and triflate. The complexes were evaluated as catalysts in the spirocyclization of 1-(1H-indol-3-yl)-4-phenylbut-3-yn-2-one to give a spirocyclic indolenine product. To our knowledge, it is the first time that a systematic study has been conducted to examine the effects of both NHC ring size and anion in this type of silver-catalyzed reaction. While the acetate and triflate complexes catalyzed the reaction to 100% conversion, the bromide complexes exhibited a significant ligand/anion effect. Reactions catalyzed by both complexes bearing the five-membered ring NHC ligands and the complex bearing the seven-membered ring NHC ligand stalled after approximately two turnovers. However, the bromide complex bearing the six-membered ring NHC ligand catalyzes the reaction to almost full conversion, similarly to the acetate and triflate complexes. This demonstrates that the NHC ligand ring size can have a dramatic effect in these types of reactions and does not necessarily display a linear correlation.