Enantioselective Oxidative Gold Catalysis Enabled by a Designed Chiral P,N-Bidentate Ligand

H-Bonded Counterion-Directed Catalysis: Enantioselective Gold(I)-Catalyzed Addition to 2-Alkynyl Enones as a Case Study

H-bonded counterion-directed catalysis (HCDC) is a strategy wherein a chiral anion that is hydrogen-bonded to the achiral ligand of a metal complex is responsible for enantioinduction. In this article we present the application of H-bonded counterion-directed catalysis to the Au(I)-catalyzed enantioselective tandem cycloisomerization-addition reaction of 2-alkynyl enones. Following the addition of C-, N- or O-centered nucleophiles, bicyclic furans were obtained in moderate to excellent yield and enantioselectivity (28 examples, 59-96 % yield, 62 : 38 to 95 : 5 er). The optimal catalytic system, comprising a phosphinosquaramide Au(I) chloride complex and a BINOL-derived phosphoramidate Ag(I) salt, was selected in a combinatorial fashion from a larger library with the help of high-throughput screening. An enantioselectivity switch of ca. 120 Δee% was observed upon addition of the achiral Au(I) component to the Ag(I) salt.

New-Generation Ligand Design for the Gold-Catalyzed Asymmetric Activation of Alkynes

Gold(I) catalysts are ideal for the activation of alkynes under very mild conditions. However, unlike allenes or alkenes, the triple bond of alkynes cannot be prochiral. In addition, the linear coordination displayed by gold(I) complexes places the chiral ligand far away from the substrate resulting in an inefficient transfer of chiral information. This poses a significant challenge for the achievement of high enantiocontrol in gold(I)-catalyzed reactions of alkynes. Although considerable progress on enantioselective gold(I)-catalyzed transformations has recently been achieved, the asymmetric activation of non-prochiral alkyne-containing small molecules still represents a great challenge. Herein we summarize recent advances in intra- and intermolecular enantioselective gold(I)-catalyzed reactions involving alkynes, discussing new chiral ligand designs that lie at the basis of these developments. We also focus on the mode of action of these catalysts, their possible limitations towards a next-generation of more efficient ligand designs. Finally, square planar chiral gold(III) complexes, which offer an alternative to chiral gold(I) complexes, are also discussed.

Wolff Rearrangement of Oxidatively Generated α-Oxo Gold Carbenes: An Effective Approach to Silylketenes

Gold-catalyzed oxidations of alkynes by N-oxides offer direct access to reactive α-oxo gold carbene intermediates from benign and readily available alkynes instead of hazardous diazo carbonyl compounds. Despite various versatile synthetic methods developed based on this strategy, one of the hallmarks of α-oxo carbene/carbenoid chemistry, that is, the Wolff rearrangement, has not been realized in this context. This study discloses the first examples that show the Wolff rearrangement can be readily realized by α-oxo gold carbenes oxidatively generated from TBS-terminated alkynes (TBS=tert-butyldimethylsilyl). The thus-generated silylketenes can be either isolated pure or subsequently trapped by various internal or external nucleophiles in one pot to afford α-silylated carboxylic acids, their derivatives, or TBS-substituted allenes.

Diastereospecific Gold(I)-Catalyzed Cyclization Cascade for the Controlled Preparation of N- and N,O-Heterocycles

The reaction of oxime-tethered 1,6-enynes with a cationic gold(I) catalyst demonstrates a great potential for the synthesis of a range of heterocycles in a diastereospecific fashion. The control of the configuration of the oxime and the alkene of the enyne moiety is the key to selectively obtain dihydro-1,2-oxazines, isoxazolines or dihydropyrrole-N-oxides as single diastereoisomers. As supported by DFT calculations, these cascade reactions proceed stepwise, by the intramolecular addition of the O or N atom of the oxime onto cyclopropyl gold(I) carbene intermediates. In this study, a rare [3,3]-sigmatropic rearrangement of nitrones is also observed.

Brønsted Acid Catalyzed Oxygenative Bimolecular Friedel-Crafts-type Coupling of Ynamides

A non-metal approach for accessing α-oxo carbene surrogates for a C-C bond-forming bimolecular coupling between ynamides and nucleophilic arenes was developed. This acid-catalyzed coupling features mild temperature, which is critical for the required temporal chemoselectivity among nucleophiles. The scope of nucleophiles includes indoles, pyrroles, anilines, phenols and silyl enolethers. Furthermore, a direct test of S_N 2' mechanism has been provided by employing chiral N,N'-dioxides which also enlightens the nature of the intermediates in related metal-catalyzed processes.

C,O-Chelated BINOL/Gold(III) Complexes: Synthesis and Catalysis with Tunable Product Profiles

Unprecedented stable BINOL/gold(III) complexes, adopting a novel C,O-chelation mode, were synthesized by a modular approach through combination of 1,1'-binaphthalene-2,2'-diols (BINOLs) and cyclometalated gold(III) dichloride complexes [(C^N)AuCl₂ ]. X-ray crystallographic analysis revealed that the bidentate BINOL ligands tautomerized and bonded to the Au^III atom through C,O-chelation to form a five-membered ring instead of the conventional O,O'-chelation giving a seven-membered ring. These gold(III) complexes catalyzed acetalization/cycloisomerization and carboalkoxylation of ortho-alkynylbenzaldehydes with trialkyl orthoformates.

Gold-Catalyzed Ring Expansion of Alkynyl Heterocycles through 1,2-Migration of an Endocyclic Carbon-Heteroatom Bond

A mild and efficient gold-catalyzed oxidative ring-expansion of a series of alkynyl heterocycles using pyridine-N-oxide as the oxidant has been developed, which affords highly valuable six- or seven-membered heterocycles with wide functional group toleration. The reaction consists of a regioselective oxidation and a chemoselective migration of an endocyclic carbon-heteroatom bond (favored over C-H migration) with the order of migratory aptitude for carbon-heteroatom bonds being C-S>C-N>C-O. In the absence of an oxidant, polycyclic products are readily constructed through a ring-expansion/Nazarov cyclization reaction sequence.

A Desulfonylative Approach in Oxidative Gold Catalysis: Regiospecific Access to Donor-Substituted Acyl Gold Carbenes

Donor-substituted acyl gold carbenes are challenging to access selectively by gold-promoted intermolecular oxidation of internal alkynes as the opposite regioisomers frequently predominate. By using alkynyl sulfones or sulfonates as substrates, the oxidative gold catalysis in the presence of substituted pyridine N-oxides offers regiospecific access to acyl/aryl, acyl/alkenyl, and acyl/alkoxy gold carbenes by in situ expulsion of sulfur dioxide. The intermediacies of these reactive species are established by their reactivities, including undergoing further oxidation by the same oxidant, cyclopropanation of styrenes, engaging in a [3+2] cycloaddition with α-methylstyrene, and conversion into dienones.