Protection of the Gold(I) Catalyst by AsPh3 in Reactions of Enynes

Neutral gold complexes with hydrogen-bond-supported heterocyclic carbene (HBHC) and nitrogen acyclic carbene (NAC) ligands have been synthesized by the reactions of isocyanogold derivatives [AuCl(CNR)] with amines. Cationic [Au(carbene)(AsPh3)][SbF6] complexes have also been prepared. The catalytic activity of both types of complex (for the former, AgSbF6 is used to extract the halide ligand) in the skeletal rearrangement and methoxycyclization of enynes has been studied. The cationic complexes with AsPh3 are active but slower; advantageously, they do not decompose during the catalysis. In contrast, the catalysts formed in situ from the neutral halide complexes are very fast but undergo decomposition. An interesting trade-off was found by adding substoichiometric amounts of AsPh3 (e.g., 10 mol-%) relative to the gold catalyst {[Au(carbene)Cl] + AgSbF6}, which prevents or dramatically reduces the decomposition. This protecting ligand promises to prevent or minimize the undesired decomposition of gold catalysts.