Competitive Gold-Activation Modes in Terminal Alkynes: An Experimental and Mechanistic Study

The competition between - and dual sigma,-gold-activation modes is revealed in the gold(I)-catalyzed heterocyclization of 1-(o-ethynylaryl)urea. A noticeable effect of various ligands in gold complexes on the choice of these activation modes is described. The cationic [Au(IPr)](+) (IPr=2,6-bis(diisopropylphenyl)imidazol-2-ylidene) complex cleanly promotes the activation of terminal alkynes, whereas [Au(PtBu3)](+) favors intermediate sigma, species. In this experimental and mechanistic study, which includes kinetic and cross-over experiments, several sigma-gold, sigma,-gold, and other gold polynuclear reaction intermediates have been isolated and identified by NMR spectroscopy, X-ray diffraction, or MALDI spectrometry. The ligand control in the simultaneous or alternative - and sigma,-activation modes is also supported by deuterium-labeling experiments.