Abstract
Gold catalysis has become one of the fastest growing fields in chemistry, providing new organic transformations and offering excellent chemoselectivities under mild reaction conditions. Methodological developments have been driven by wide applicability in the synthesis of complex structures, whereas the mechanistic understanding of Au(III)-mediated processes remains scanty and have become the Achilles’ heel of methodology development. Herein, the systematic investigation of the reactivity of bis(pyridine)-ligated Au(III) complexes is presented, based on NMR spectroscopic, X-ray crystallographic, and DFT data. The electron density of pyridines modulates the catalytic activity of Au(III) complexes in propargyl ester cyclopropanation of styrene. To avoid strain induced by a ligand with a nonoptimal nitrogen–nitrogen distance, bidentate bis(pyridine)–Au(III) complexes convert into dimers. For the first time, bis(pyridine)Au(I) complexes are shown to be catalytically active, with their reactivity being modulated by strain.
Highlights
Despite humankind’s fascination with gold ever since ancient times, Au catalysis has lagged behind the chemistry of other transition metals, such as palladium, rhodium, and platinum
Significant efforts have been put into the establishment and understanding of Au(I)-catalyzed processes,[1,2] whereas Au(III)-mediated reactions have received only minute interest.[3−5] They have sometimes been simplistically rationalized as Au(I) catalytic processes, with Au(III) being a precatalyst,[6] or as intermediates in a catalytic redox cycle starting with Au(I).[7−10] This lag is likely explained by the challenging lability of Au(III) complexes by the largely unexplored potential of their ligation and by the lack of experimental evidence necessary for understanding of the mechanism of Au(III)-mediated processes.[11]
Bis(pyridine)Au(III) complexes are applicable as catalysts as shown by using the cyclopropanation of styrene (6) with propargyl ester (5) as a model reaction
Summary
Despite humankind’s fascination with gold ever since ancient times, Au catalysis has lagged behind the chemistry of other transition metals, such as palladium, rhodium, and platinum. A frequent motif in transition metal chemistry,[33,34] have been explored for ligation of Au(III)[35−38] and have shown to yield an improved catalytic performance and stability[39] as compared to simpler Au(III) salts, e.g., K/ NaAuCl4, AuCl3, and AuBr3, presumably due to activation and stabilization of the Au(III) center. Whereas some initial data exist, the catalytic activity of pyridine complexes has not yet been systematically investigated, and the mechanism of pyridine-ligated Au(III)-mediated reactions remains unexplored. Motivated by this knowledge gap, we studied bis(pyridine)Au(III) complexes, evaluating the influence of electronic and steric effects on their reactivity and investigating their coordination mode by using NMR spectroscopic, X-ray crystallographic, and computational (DFT) techniques
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