Toward optimizing the performance of homogeneous L-Au-X catalysts through appropriate matching of the ligand (L) and counterion (X-)

The effects of the ligand (L) and counterion (X-) are considered the two most important factors in homogeneous gold catalysis, but a rational understanding of their synergy/antagonism is still lacking. In this work, we synthesized a set of 16 gold complexes of the type L-Au-X that differ as follows: (i) L = PPh<inf>3</inf> (L1), P(tBu)<inf>3</inf> (L2), tris(3,5-bis(trifluoromethyl)phenyl)phosphine (PArF, L3), and 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (NHC, L4), with the deliberate purpose of varying the electron withdrawing ability of the ligand, and (ii) X- = BF<inf>4</inf>-, OTf-, OTs-, and TFA-, which have various coordinating abilities, basicities, and hydrogen bond acceptor powers. All these catalysts were tested in two different model reactions: the cycloisomerization of N-(prop-2-ynyl)benzamide to 2-phenyl-5-vinylidene-2-oxazoline and the methoxylation of 3-hexyne. The main results are that the choice of the most efficient L-Au-X catalyst for a given process should not be made by evaluating the properties of L and X- alone, but rather based on their best combination. For NHC-Au-X, the noncoordinating and weakly basic anions (such as BF<inf>4</inf>- and OTf-) have been recognized as the best choice for the cycloisomerization of N-(prop-2-ynyl)benzamide. On the other side, the intermediate coordinating ability and basicity of OTs- provide the best compromise for achieving an efficient methoxylation of 3-hexyne. A completely different trend is found in the case of complexes bearing phosphanes: OTs- and TFA- have been found to accelerate the cycloisomerization of N-(prop-2-ynyl)benzamide, and BF<inf>4</inf>- and OTf- are suitable for the methoxylation of 3-hexyne. A possible explanation of the observed differences between phosphane and NHC ancillary ligands might be found in the higher affinity of the counterion (especially OTs-) for the gold fragment for phosphane instead of NHC. © 2015 American Chemical Society.