Catalytic ring-closing reactions of gold compounds containing bis(phosphino)ferrocene ligands

Abstract

The efficiency of various gold compounds containing bis(phosphino)ferrocene ligands for catalyzing ring-closing reactions was examined. Six commercially available bis(phosphino)ferrocene ligands: 1,1′-bis(ditert-butylphosphino)ferrocene (dtbpf), 1,1′-bis(diphenylphosphino)ferrocene (dppf), 1,1′-bis(dicylohexylphosphino)ferrocene (dcpf), 1,1′-bis(diiso-propylphosphino)ferrocene (dippf), 1-diphenylphosphino-1′-di-tert-butylphosphinoferrocene (dppdtbpf) and 1,1′-bis(5-methyl-2-furanylphosphino)ferrocene (dfurpf) were employed in this study. In addition to the previously reported [Au2Cl2(μ-PP)] (PP = dtbpf, dppf, dcpf, dippf or dppdtbpf) compounds, [Au2Cl2(μ-dfurpf)] was synthesized and characterized by NMR spectroscopy, cyclic voltammetry and X-ray crystallography. All of these gold compounds react with Na[BArF24] (BArF24 = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) to remove a chloride and yield the cationic monochlorides, [Au2(μ-Cl)(μ-PP)]+. The effectiveness of the [Au2Cl2(μ-PP)] and [Au2(μ-Cl)(μ-PP)][BArF24], either pre-formed or generated in situ, compounds for the catalytic intramolecular alkyne hydroalkoxylation of (Z)-3-methylpent-2-en-4-yn-1-ol was examined in CDCl3 and toluene-d8. The ring-closing of N-(prop-2-yn-1-yl)benzamide was also examined and was efficiently carried out in toluene-d8 using [Au2(μ-Cl)(μ-PP)][BArF24] (PP = dtbpf, dppf, dcpf, dippf, dppdtbpf or dfurpf). In addition, [Au2Cl2(μ-dppe)] and [Au2(μ-Cl)(μ-dppe)][BArF24] were examined as catalysts for these ring-closing reactions in order to determine if the ferrocene backbone of the bis(phosphino)ferrocene ligands was important for catalysis.