Gold-Catalyzed Oxidative Acyloxylation of Arenes

RhRu Bimetallic Oxide Cluster Catalysts for Cross-Dehydrogenative Coupling of Arenes and Carboxylic Acids

Noble-metal-based bimetallic oxide clusters are promising novel catalysts. In this study, we developed carbon-supported RhRu bimetallic oxide clusters (RhRuOx/C) with a mean diameter of 1.2 nm, which showed remarkable catalytic activity for the cross-dehydrogenative coupling (CDC) of arenes and carboxylic acids with O2 as the sole oxidant. RhRu bimetallic oxide cluster formation was confirmed by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy and synchrotron X-ray absorption spectroscopy. Kinetic isotope and substituent effects indicated that arene C-H bond cleavage was the rate-determining step and proceeded via electrophilic concerted metalation-deprotonation mechanism, with a carboxylate as an internal base. Density functional theory calculations supported the proposed mechanism and indicated that the active center for C-H bond activation was Rh(V) rather than Rh(III), while Ru enhanced the electrophilicity of the Rh(V) site by decreasing the negative charge of the surrounding oxygen atoms. Electron-rich arenes showed relatively high reactivity for the RhRuOx/C-catalyzed CDC reaction, and both aliphatic and aromatic carboxylic acids were applicable to the reaction. The RhRuOx/C catalyst is promising for the CDC reaction of arenes and carboxylic acids to produce aryl esters. This work promotes the development of noble-metal-based bimetallic oxide clusters for C-H bond activation reactions.

Catalytic Gold Chemistry: From Simple Salts to Complexes for Regioselective C-H Bond Functionalization

Gold coordinated to neutral phosphines (R₃ P), N-heterocyclic carbenes (NHCs) or anionic ligands is catalytically active in functionalizing various C-H bonds with high selectivity. The sterics/electronic nature of the studied C-H bond, oxidation state of gold and stereoelectronic capacity of the coordinated auxiliary ligand are some of the associated selectivity factors in gold-catalyzed C-H bond functionalization reactions. Hence, in this review a comprehensive update about the action of different types of gold catalysts, from simple to sophisticated ones, on C-H bond reactions and their regiochemical outcome is disclosed. This review also highlights the catalytic applications of Au(I)- and Au(III)-species in creating new opportunities for the regio- and site-selective activation of challenging C-H bonds. Finally, it also intends to stress the potential applications in selective C-H bond activation associated with a variety of heterocycles recently described in the literature.

Cobalt-Catalyzed C-H Acetoxylation of Phenols with Removable Monodentate Directing Groups: Access to Pyrocatechol Derivatives

An efficient cobalt-catalyzed C-H acetoxylation of phenols has been developed by using PIDA (phenyliodine diacetate) as a sole acetoxy source to synthesize pyrocatechol derivatives for the first time. The key feature of this method is the use of earth-abundant metal cobalt as the green and inexpensive catalyst for the acetoxylation of C(sp²)-H bonds under neutral reaction conditions. Furthermore, the gram-scale reaction and late-stage functionalization demonstrated the usefulness of this method.

Cross-dehydrogenative coupling reactions between arenes (C-H) and carboxylic acids (O-H): a straightforward and environmentally benign access to O-aryl esters

Transition-metal catalyzed cross-dehydrogenative-coupling reactions encompass highly versatile and atom economical methods for the construction of various carbon-carbon and carbon-heteroatom bonds by combining two C(X)-H (X = heteroatom) bonds. Along this line, direct acyloxylation of C-H bonds with carboxylic acids has emerged as a powerful and green approach for the synthesis of structurally diverse esters. In this focus-review we will describe recent progress in direct esterification of aromatic C-H bonds with special emphasis on the mechanistic features of the reactions. Literature has been surveyed until the end of February 2019.

Oxidant-free oxidative gold catalysis: the new paradigm in cross-coupling reactions

The construction of C–C and C–X (X = hetero atom) bonds is the core aspect for the assembly of molecules. This feature article critically presents an overview of all the redox neutral cross-coupling reactions enabled by gold catalysis, which we believe would stimulate further research activities in this promising area.

Dual gold and photoredox catalysed C-H activation of arenes for aryl-aryl cross couplings

A mild and fully catalytic aryl-aryl cross coupling via gold-catalysed C-H activation has been achieved by merging gold and photoredox catalysis. The procedure is free of stoichiometric oxidants and additives, which were previously required in gold-catalysed C-H activation reactions. Exploiting dual gold and photoredox catalysis confers regioselectivity via the crucial gold-catalysed C-H activation step, which is not present in the unselective photocatalysis-only counterpart.

Copper-Catalyzed Regioselective C-H Sulfonyloxylation of Electron-Rich Arenes with p-Toluenesulfonic Acid and Sulfonyloxylation of Aryl(mesityl)iodonium Sulfonates

Copper-catalyzed regioselective C-H sulfonyloxylation of electron-rich arenes with p-toluenesulfonic acid has been developed. Electron-rich benzene derivatives and heteroarenes can undergo this C-H sulfonyloxylation reaction to generate aryl tosylates. Furthermore, sulfonyloxylation of aryl(mesityl)iodonium sulfonates has also been investigated. Both aryl(mesityl)iodonium tosylates and triflates can react smoothly to get aryl sulfonates. The formed aryl sulfonates can be converted to phenols, as well as used as good partners of cross-coupling reactions.

Advances in Synthetic Applications of Hypervalent Iodine Compounds

The preparation, structure, and chemistry of hypervalent iodine compounds are reviewed with emphasis on their synthetic application. Compounds of iodine possess reactivity similar to that of transition metals, but have the advantage of environmental sustainability and efficient utilization of natural resources. These compounds are widely used in organic synthesis as selective oxidants and environmentally friendly reagents. Synthetic uses of hypervalent iodine reagents in halogenation reactions, various oxidations, rearrangements, aminations, C-C bond-forming reactions, and transition metal-catalyzed reactions are summarized and discussed. Recent discovery of hypervalent catalytic systems and recyclable reagents, and the development of new enantioselective reactions using chiral hypervalent iodine compounds represent a particularly important achievement in the field of hypervalent iodine chemistry. One of the goals of this Review is to attract the attention of the scientific community as to the benefits of using hypervalent iodine compounds as an environmentally sustainable alternative to heavy metals.

Palladium-catalyzed direct and regioselective C–H acyloxylation of indolizines

A direct and highly regioselective C1-acyloxylation of indolizines was developed via palladium-catalyzed C–H functionalization. In this reaction, the regioselectivity was achieved in the absence of a directing group.

Regioselective gold-catalyzed oxidative C-N bond formation

A novel protocol for the regioselective intermolecular amination of various arenes has been developed. By using an I(III) oxidant in the presence of a Au(I) catalyst, a direct and novel route for regioselectively accessing a variety of substituted aniline moieties has been achieved with yields as high as 90%. Mechanistic insight suggests that regioselectivity can be predicted based on electrophilic aromatic metalation patterns.

Gold-catalyzed oxidative coupling of arylsilanes and arenes: origin of selectivity and improved precatalyst

The mechanism of gold-catalyzed coupling of arenes with aryltrimethylsilanes has been investigated, employing an improved precatalyst (thtAuBr3) to facilitate kinetic analysis. In combination with linear free-energy relationships, kinetic isotope effects, and stoichiometric experiments, the data support a mechanism involving an Au(I)/Au(III) redox cycle in which sequential electrophilic aromatic substitution of the arylsilane and the arene by Au(III) precedes product-forming reductive elimination and subsequent cycle-closing reoxidation of the metal. Despite the fundamental mechanistic similarities between the two auration events, high selectivity is observed for heterocoupling (C-Si then C-H auration) over homocoupling of either the arylsilane or the arene (C-Si then C-Si, or C-H then C-H auration); this chemoselectivity originates from differences in the product-determining elementary steps of each electrophilic substitution. The turnover-limiting step of the reaction involves associative substitution en route to an arene π-complex. The ramifications of this insight for implementation of the methodology are discussed.

Gold-catalyzed direct arylation

Biaryls (two directly connected aromatic rings, Ar(1)-Ar(2)) are common motifs in pharmaceuticals, agrochemicals, and organic materials. Current methods for establishing the Ar(1)-Ar(2) bond are dominated by the cross-coupling of aryl halides (Ar(1)-X) with aryl metallics (Ar(2)-M). We report that, in the presence of 1 to 2 mole percent of a gold catalyst and a mild oxidant, a wide range of arenes (Ar(1)-H) undergo site-selective arylation by arylsilanes (Ar(2)-SiMe(3)) to generate biaryls (Ar(1)-Ar(2)), with little or no homocoupling (Ar(1)-Ar(1)/Ar(2)-Ar(2)). Catalysis proceeds at room temperature and tolerates a broad range of functional groups, including those incompatible with cross-coupling. These features expedite biaryl preparation, as demonstrated by synthesis of the nonsteroidal anti-inflammatory diflunisal.