Highly Convenient Regioselective Intermolecular Hydroamination of Alkynes Yielding Ketimines Catalyzed by Gold(I) Complexes of 1,2,4-triazole Based N-heterocyclic Carbenes

A series of highly efficient gold(I) precatalysts of 1,2,4-triazole based N-heterocyclic carbenes, [1-R-4-R'-1,2,4-triazol-5-ylidene]AuCl [R = (CH2COBu)-Bu-t, R' = CH2Ph (1c); R = (CH2CONHBu)-Bu-t, R' = CH2Ph (2c); R = (CH2COBu)-Bu-t, R' (CH2COBu)-Bu-t (3c), and R = C6H10OH, R' = CH2Ph (4c)] are reported for the hydroamination of terminal alkynes with a variety of sterically demanding o/p-substituted aryl amines yielding the corresponding ketimines in air. The gold 1c-4c complexes exhibited extremely high activity in comparison to the silver analogues 1b-4b, thereby highlighting the role of gold as a metal in the catalysis of the hydroamination reaction. Additionally, the 1,2,4-triazole based 1c-4c precatalysts showed significantly superior activity in comparison to the two representative imidazole analogues, namely, [1-(benzyl)-3-(N-t-butylacetamido)imidazol-2-ylidene]AuCl and [1-(2-hydroxy-cyclohexyl)-3-(benzyl)imidazol-2-ylidene]AuCl, thereby underscoring the importance of the 1,2,4-triazole based N-heterocyclic carbenes over the imidazole based ones in designing the gold(1) precatalysts for the hydroamination reaction. The gold(I) complexes (1c-4c) were synthesized by transmetalation reaction of the silver analogues 1b-4b with (SMe2)AuCl in 60-76% yield while the silver 1b-4b complexes in turn were synthesized from the respective 1,2,4-triazolium halide salts by treatment with Ag2O in 43-64% yield.