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 3 ( L1 ), P( t Bu) 3 ( 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 4 –, 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 4 – 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 4 – 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.