Toward Optimizing the Performance of Homogeneous L‑Au‑X Catalysts through Appropriate Matching of the Ligand (L) and Counterion (X–)

• Published in: Organometallics Vol. 34; no. 9; pp. 1759 - 1765
• Main Authors: Biasiolo, Luca, Del Zotto, Alessandro, Zuccaccia, Daniele
• Format: Journal Article
• Language: English
• Published: American Chemical Society 11.05.2015
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/acs.organomet.5b00308

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 = PPh3 (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– = BF4 –, 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 BF4 – 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 BF4 – 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.