Abstract
Gold(III) complexes are versatile catalysts offering a growing number of new synthetic transformations. Our current understanding of the mechanism of homogeneous gold(III) catalysis is, however, limited, with that of phosphorus-containing complexes being hitherto underexplored. The ease of phosphorus oxidation by gold(III) has so far hindered the use of phosphorus ligands in the context of gold(III) catalysis. We present a method for the generation of P,N-chelated gold(III) complexes that circumvents ligand oxidation and offers full counterion control, avoiding the unwanted formation of AuCl4–. On the basis of NMR spectroscopic, X-ray crystallographic, and density functional theory analyses, we assess the mechanism of formation of the active catalyst and of gold(III)-mediated styrene cyclopropanation with propargyl ester and intramolecular alkoxycyclization of 1,6-enyne. P,N-chelated gold(III) complexes are demonstrated to be straightforward to generate and be catalytically active in synthetically useful transformations of complex molecules.
Highlights
Whereas gold had barely been applied in organic synthesis before the 21st century, over the past 2 decades, gold catalysis has grown into a distinct subfield, with its wide applicability having already been demonstrated.[1−8] In spite of its young age, gold catalysis has provided new organic transformations and has been shown to offer chemoselectivity under mild conditions,[9,10] often with higher tolerance toward moisture and oxygen than the more established transitions metals, such as palladium, platinum, rhodium, cobalt, and nickel
Conventional phosphine ligands have so far scarcely been explored in gold(III) chemistry because phosphorus gets oxidized in the presence of gold(III)
By systematic NMR spectroscopic, X-ray crystallographic, and computational assessment of the synthesis of a series of structurally closely related P,N-chelated gold(III) complexes, we describe the mechanism of their formation and show it to involve dimeric gold(I) and gold(III) intermediates
Summary
Whereas gold had barely been applied in organic synthesis before the 21st century, over the past 2 decades, gold catalysis has grown into a distinct subfield, with its wide applicability having already been demonstrated.[1−8] In spite of its young age, gold catalysis has provided new organic transformations and has been shown to offer chemoselectivity under mild conditions,[9,10] often with higher tolerance toward moisture and oxygen than the more established transitions metals, such as palladium, platinum, rhodium, cobalt, and nickel. Homogeneous gold(I) catalysts are comparably well-developed and understood,[11] as evidenced by the number of ligated gold(I) complexes in use and by the variety of gold(I)-catalyzed transformations and mechanistic studies available in the literature.[12−18] In contrast, gold(III) catalysis mainly uses the initially developed inorganic gold(III) salts, without a stabilizing ligand.[19,20] The successful development of widely applicable synthetic techniques and access to complex structures has hitherto been demonstrated, yet without a detailed mechanistic understanding.[21−23] A few examples of ligated gold(III) complexes used as catalysts are known and have revealed that ligation provides increased catalyst lifetime and tuneability.[24] These have enabled mechanistic studies that were previously impossible because of the instability of gold chlorides.[25−35] Ligands encompassing phosphorus donor(s) are common in gold(I) catalysis,[6,36−38] which include P,N-chelated gold(I) complexes.[39] conventional phosphine ligands have so far scarcely been explored in gold(III) chemistry because phosphorus gets oxidized in the presence of gold(III). Inorganic Chemistry pubs.acs.org/IC are rare,[51] the use of P,N ligands for gold(III) have
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