2024/02/21 by Otto Seppänen, Anna Lenarda, Martin Nieger +1 · 1 citation
Chemistry · #Catalytic Alkyne Reactions #Catalytic Cross-Coupling Reactions #Cyclopropane Reaction Mechanisms
paper · pdf · doi:10.1021/acs.organomet.3c00520
Cyclometalated N-heterocyclic carbene (NHC)-NTs-AuCl 2 complexes of type [(NHC)Au(N)Cl 2 ] are presented as a stable precatalyst, easily activated in situ by an acid (HX) for cationic [(NHC)AuCl 2 ]X catalysis. The “ethyl-and propyl-tosylamide arm” tethered NHC-Au(III)Cl 3 complexes can be smoothly converted to cyclometalated (-HCl) analogues by K 2 CO 3 treatment, and, via the addition of a stoichiometric amount of Brønsted acid (HX), can be further converted to [NHC-Au(III)Cl 2 ]X carrying various counteranions (X = CCl 3 CO 2 –, CF 3 CO 2 –, MsO –, TsO –, BF 4 –, TfO –, and Tf 2 N – ). We studied this concept in catalysis using the cycloisomerization of 2-alkynylphenol to benzofurans as a probe reaction. Excellent yields were obtained in 2 h with an equimolar 1 mol % loading of the Mes-NHC-propyl-NTs-AuCl 2 precatalyst and MsOH as the acid additive under ambient conditions in CHCl 3 . The single crystal structure for a cyclometalated NHCAu(III)-complex was characterized by X-ray analysis. 1 H NMR monitoring of reaction kinetics suggests that the in situ formed Au(III) complex is the catalytically active species.