AuI/AuIII Catalysis: An Alternative Approach for CC Oxidative Coupling

Photosensitizer-free visible light-mediated gold-catalysed cis-difunctionalization of silyl-substituted alkynes

A new photosensitizer-free visible light-mediated gold-catalysed cis-difunctionalization reaction is developed.

A new photosensitizer-free visible light-mediated gold-catalysed cis-difunctionalization reaction is developed. The reaction was chemoselective towards silyl-substituted alkynes with excellent regioselectivity and good functional group compatibility, giving a series of silyl-substituted quinolizinium derivatives as products. The newly synthesized fluorescent quinolizinium compounds, named JR-Fluor-1, possessed tunable emission properties and large Stokes shifts. With unique photophysical properties, the fluorophores have been applied in photooxidative amidations as efficient photocatalysts and cellular imaging with switchable subcellular localization properties.

Gold Redox Catalysis through Base-Initiated Diazonium Decomposition toward Alkene, Alkyne, and Allene Activation

The discovery of photoassisted diazonium activation toward gold(I) oxidation greatly extended the scope of gold redox catalysis by avoiding the use of a strong oxidant. Some practical issues that limit the application of this new type of chemistry are the relative low efficiency (long reaction time and low conversion) and the strict reaction condition control that is necessary (degassing and inert reaction environment). Herein, an alternative photofree condition has been developed through Lewis base induced diazonium activation. With this method, an unreactive AuI catalyst was used in combination with Na2 CO3 and diazonium salts to produce a AuIII intermediate. The efficient activation of various substrates, including alkyne, alkene and allene was achieved, followed by rapid AuIII reductive elimination, which yielded the C-C coupling products with good to excellent yields. Relative to the previously reported photoactivation method, our approach offered greater efficiency and versatility through faster reaction rates and broader reaction scope. Challenging substrates such as electron rich/neutral allenes, which could not be activated under the photoinitiation conditions (<5 % yield), could be activated to subsequently yield the desired coupling products in good to excellent yield.

Gold(iii)-arene complexes by insertion of olefins into gold-aryl bonds

The synthesis and characterization of the first gold(iii)-arene complexes are described. Well-defined (P,C)-cyclometalated gold(iii)-aryl complexes were prepared and characterized by NMR spectroscopy. These complexes swiftly and cleanly reacted with norbornene and ethylene to provide cationic gold(iii)-alkyl complexes, in which the remote phenyl ring was η2-coordinated to gold. The interaction between the aromatic ring and the gold(iii) center was thoroughly analyzed by NMR spectroscopy, X-ray diffraction, and DFT calculations. The π-arene coordination was found to significantly influence the stability and reactivity of low coordinated gold(iii) alkyl species.

Hypervalent Iodine Reagents in High Valent Transition Metal Chemistry

Over the last 20 years, high valent metal complexes have evolved from mere curiosities to being at the forefront of modern catalytic method development. This approach has enabled transformations complimentary to those possible via traditional manifolds, most prominently carbon-heteroatom bond formation. Key to the advancement of this chemistry has been the identification of oxidants that are capable of accessing these high oxidation state complexes. The oxidant has to be both powerful enough to achieve the desired oxidation as well as provide heteroatom ligands for transfer to the metal center; these heteroatoms are often subsequently transferred to the substrate via reductive elimination. Herein we will review the central role that hypervalent iodine reagents have played in this aspect, providing an ideal balance of versatile reactivity, heteroatom ligands, and mild reaction conditions. Furthermore, these reagents are environmentally benign, non-toxic, and relatively inexpensive compared to other inorganic oxidants. We will cover advancements in both catalysis and high valent complex isolation with a key focus on the subtle effects that oxidant choice can have on reaction outcome, as well as limitations of current reagents.

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.

Anchoring Triazole-Gold(I) Complex into Porous Organic Polymer To Boost the Stability and Reactivity of Gold(I) Catalyst

Stability and reactivity have been recognized as some critical issues for gold(I) catalysts. Such issues can be well-circumvented by anchoring the gold(I) complex onto the backbones of porous organic polymer (POP) followed by coordination with a triazole ligand as illustrated in the present work via a series of gold(I)-catalyzed reactions. In this strategy, 1,2,3-triazole was used as the special "X-factor" to avoid the formation of solid AgCl involved in typical gold-activation processes. The catalyst could be readily recycled without loss of reactivity. Moreover, compared with the PPh₃-modified polystyrene beads, the POP support was advantageous by providing high surface area, hierarchical porosity, and better stabilization of cations. In some cases, significantly improved reactivity was observed, even more so than using the homogeneous system, which further highlighted the great potential of this heterogeneous gold catalyst.

Bimetallic catalysis for C-C and C-X coupling reactions

Bimetallic catalysis represents an alternative paradigm for coupling chemistry that complements the more traditional single-site catalysis approach. In this perspective, recent advances in bimetallic systems for catalytic C-C and C-X coupling reactions are reviewed. Behavior which complements that of established single-site catalysts is highlighted. Two major reaction classes are covered. First, generation of catalytic amounts of organometallic species of e.g. Cu, Au, or Ni capable of transmetallation to a Pd co-catalyst (or other traditional cross-coupling catalyst) has allowed important new C-C coupling technologies to emerge. Second, catalytic transformations involving binuclear bond-breaking and/or bond-forming steps, in some cases involving metal-metal bonds, represent a frontier area for C-C and C-X coupling processes.

Arene C–H activation by gold(iii): solvent-enabled proton shuttling, and observation of a pre-metallation Au–arene intermediate

Au–C bond cleavage by H⁺ and C–H activation by Au(iii) are reversible through solvent-enabled proton shuttling. In the absence of a donor solvent, a gold(iii)–arene intermediate is detected.

Exploiting the dual role of ethynylbenziodoxolones in gold-catalyzed C(sp)–C(sp) cross-coupling reactions

Reported herein is the gold-catalyzed alkynylation of terminal alkynes using ethynylbenziodoxolones (EBXs), where EBXs serve a dual role as oxidants as well as alkyne transfer agents to access unsymmetrical 1,3-diynes. Hence, the catalytic system requires no external oxidants and is compatible with a broad range of substrates, including those with polar functional groups such as NH, OH and B(OH)₂.

Catalytic domino amination and oxidative coupling of gold acetylides and isolation of key vinylene digold intermediates as a new class of ditopic N-heterocyclic carbene complexes

A catalytic domino amination and oxidative coupling of in situ prepared gold acetylides has been developed for the synthesis of abnormal NHC (aNHC) gold complexes, and key vinylene digold intermediates are isolated.

Gold-catalyzed domino cyclization–alkynylation reactions with EBX reagents: new insights into the reaction mechanism

New insights into the mechanism of gold-catalyzed domino reactions with hypervalent iodine reagents revealed by computation and confirmed by experiments.

Dual gold/photoredox-catalyzed bis-arylative cyclization of chiral homopropargyl sulfonamides with diazonium salts: rapid access to enantioenriched 2,3-dihydropyrroles

A novel dual gold/photoredox-catalyzed bis-arylative cyclization of chiral homopropargyl sulfonamides with diazonium salts is presented.

Merging Visible Light Photoredox and Gold Catalysis

Since the beginning of this century, π-Lewis acidic gold complexes have become the catalysts of choice for a wide range of organic reactions, especially those involving nucleophilic addition to carbon-carbon multiple bonds. For the most part, however, the gold catalyst does not change oxidation state during the course of these processes and two-electron redox cycles of the kind implicated in cross-coupling chemistry are not easily accessible. In order to address this limitation and expand the scope of gold catalysis beyond conventional hydrofunctionalization, extensive efforts have been made to develop new oxidative reactions using strong external oxidants capable of overcoming the high potential of the Au^I/Au^III redox couple. However, these processes typically require superstoichiometric amounts of the oxidant and proceed under relatively harsh conditions. Moreover, to date, gold-catalyzed oxidative coupling reactions have remained somewhat limited in scope because, for many systems, the desired cross-coupling does not favorably compete with homodimerization or conventional hydrofunctionalization. In 2013, we disclosed a new concept for gold-catalyzed coupling reactions that, rather than involving external oxidants, employs aryl radicals that act as both the oxidant and the coupling partner in overall redox-neutral transformations. For this, we developed a dual catalytic system combining homogeneous gold catalysis with the emerging field of visible light photoredox catalysis. Using aryldiazonium salts, which are known to act as sources of aryl radicals upon activation with reducing photocatalysts, we could achieve intramolecular oxy- and aminoarylations of alkenes upon irradiating the reaction mixtures with visible light. Further studies on this transformation, in which nucleophilic addition onto a gold-activated alkene is followed by C(sp³)-C(sp²) bond formation, expanded the scope of the process to intermolecular, three-component oxyarylation, while inexpensive organic dyes and user-friendly diaryliodonium salts could be employed as alternative photocatalysts and aryl radical sources, respectively. The potential of dual gold/photoredox catalysis was quickly realized by several research groups and a range of diverse new coupling reactions involving nucleophilic addition to π-systems and even P-H and C(sp)-H functionalization have been developed. In addition to the ambient reaction conditions and the simple setup using household light sources or even sunlight, a key advantage of dual gold/photoredox catalysis results from the simultaneous oxidation of gold(I) and coordination of the coupling partner, which results in high levels of selectivity for the cross-coupled products over homodimers. Furthermore, when gold complexes that are not catalytically active prior to oxidation by the aryl radical are employed, background reactions not involving coupling can be suppressed. Notably, this feature has allowed for the successful use of allenes and alkynes, for which conventional hydrofunctionalization pathways are highly favored, opening the door to new transformations involving the most common substrate classes for gold catalysis. In this Account, we provide an overview of dual gold/photoredox catalysis and highlight the potential of this concept to greatly expand the scope of homogeneous gold catalysis and enable the efficient construction of complex organic molecules. Moreover, recent studies on the visible light-promoted synthesis of novel gold(III) complexes suggest that photoredox activation could yet find further applications in gold chemistry beyond coupling.

Nucleophile promoted gold redox catalysis with diazonium salts: C-Br, C-S and C-P bond formation through catalytic Sandmeyer coupling

Gold-catalyzed C-heteroatom (C-X) coupling reactions are evaluated without using sacrificial oxidants. Vital to the success of this methodology is the nucleophile-assisted activation of aryldiazonium salts, which could be an effective oxidant for converting Au(i) to Au(iii) even without the addition of an assisting ligand or photocatalyst. By accelerating the reaction kinetics to outcompete C-C homo-coupling or diazonium dediazoniation, gold-catalyzed Sandmeyer reactions were achieved with different nucleophiles, forming C-Br, C-S and C-P bonds in high yields and selectivities.

Preparation of an Aurylated Alkylthiophene Monomer via C-H Activation for Use in Pd-PEPPSI-iPr Catalyzed-Controlled Chain Growth Polymerization

In the search for new synthetic routes toward greener and more facile syntheses of conjugated polymers, C-H functionalization provides a promising solution by minimizing the production and processing of aryl halide monomer precursors used in traditional organometallic coupling reactions. In this paper, we investigate the use of Au(I) and its ability to directly C-H activate 2-bromo-3-hexylthiophene to form a reactive monomer species, bypassing the typical Grignard monomer formation from a dihalogenated thiophene. Addition of Pd-PEPPSI-iPr as a palladium catalyst source in the presence of the resultant aurylated thiophene monomer yielded poly(3-hexylthiophene) as observed by both NMR and GPC. Studies on the growth of these polymers show linear dependence between M_n and monomer conversion, low dispersities, as well as M_n predicted by catalyst loading, which is supportive of a living-type chain growth mechanism. This Au-Pd system represents a novel methodology for incorporating C-H activation into the synthesis of P3HT with control over M_n.

Catalytic role of dinuclear σ,π-acetylide gold(I) complexes in the hydroamination of terminal alkynes: theoretical insights

A detailed density functional theory (DFT) investigation of the hydroamination of 1-octyne with aniline mediated by a σ,π-digold(I) bulky phosphine-based complex was undertaken in order to shed light on the mechanistic aspects of such processes. With the purpose to probe whether the performance that the cationic digold complexes exhibit is superior to those of mononuclear complexes, the same hydroamination reaction was explored by considering separately the reaction of aniline with both the monogold(I) complexes formed by π- and σ-coordination of 1-octyne to the dialkylbiarylphosphine Au(I) precatalyst. The outcomes of the computational analysis presented here show that, when the σ,π-digold alkynide complex can be formed, the reaction is not necessarily assisted by such a complex, as the computed energy barrier is almost equal to that found when the π-coordinated alkyne mononuclear gold complex is involved in the hydroamination process. The catalytic assistance of the Au(I)-σ-alkynyl complex, instead, can be surely excluded as the hydroamination product is formed by overcoming an energy barrier significantly higher than that computed when both σ,π-digold and π-coordinated alkyne monogold complexes assist the reaction. Moreover, regardless of the implicated gold(I) species, the investigated mechanism accounts for the Markovnikov selectivity of the reaction, confirming the experimental evidence. The proposed mechanisms for the conversion of Au(I) π-coordinated alkyne complexes into the corresponding σ,π-digold alkynide complexes were also explored.

Alkyne Difunctionalization by Dual Gold/Photoredox Catalysis

Highly selective tandem nucleophilic addition/cross-coupling reactions of alkynes have been developed using visible-light-promoted dual gold/photoredox catalysis. The simultaneous oxidation of Au(I) and coordination of the coupling partner by photo-generated aryl radicals, and the use of catalytically inactive gold precatalysts allows for high levels of selectivity for the cross-coupled products without competing hydrofunctionalization or homocoupling. As demonstrated in representative arylative Meyer-Schuster and hydration reactions, this work expands the scope of dual gold/photoredox catalysis to the largest class of substrates for gold catalysts and benefits from the mild and environmentally attractive nature of visible-light activation.

Dual gold photoredox C(sp2)–C(sp2) cross couplings – development and mechanistic studies

A new oxidant- and base-free dual catalysed C(sp²)–C(sp²) coupling between arylboronic acids and aryldiazonium salts has been developed. Mechanistic investigations reveal two divergent pathways depending on the gold catalyst employed.

Arenediazonium salts as electrophiles for the oxidative addition of gold(i)

Arenediazonium salts have been found to behave as efficient electrophiles for the oxidation of gold(i). Starting from anilines, a one pot cross-coupling reaction of anilines with silver acetylides mediated by gold has been developed.

Palladium-catalyzed decarboxylative alkoxycarbonylation of potassium aryltrifluoroborates with potassium oxalate monoesters

An efficient method for the preparation of aryl esters has been developed via Pd-catalyzed decarboxylative coupling of potassium aryltrifluoroborates and potassium oxalate monoesters.

Dual gold/photoredox-catalyzed C(sp)-H arylation of terminal alkynes with diazonium salts

The arylation of alkyl and aromatic terminal alkynes by a dual gold/photoredox catalytic system is described. Using aryldiazonium salts as readily available aryl sources, a range of diversely-functionalized arylalkynes could be synthesized under mild, base-free reaction conditions using visible light from simple household sources or even sunlight. This process, which exhibits a broad scope and functional group tolerance, expands the range of transformations amenable to dual gold/photoredox catalysis to those involving C-H bond functionalization and demonstrates the potential of this concept to access AuI/AuIII redox chemistry under mild, redox-neutral conditions.

Gold-catalysed cross-coupling between aryldiazonium salts and arylboronic acids: probing the usefulness of photoredox conditions

Combining gold catalysis and photoredox processes allowed the synthesis of biaryl compounds from diazonium salts and boronic acids under mild conditions.

Gold-catalyzed bis(stannylation) of propiolates

Bis(stannylation) of propiolates with hexabutylditin is achieved under mild conditions using a cationic gold(i) catalyst.

Pyridylidene ligand facilitates gold-catalyzed oxidative C-H arylation of heterocycles

Triaryl-2-pyridylidene effectively facilitates the gold-catalyzed oxidative C-H arylation of heteroarenes with arylsilanes as a unique electron-donating ligand on gold. The employment of the 2-pyridylidene ligand, which is one of the strongest electron-donating N-heterocyclic carbenes, resulted in the rate acceleration of the C-H arylation reaction of heterocycles over conventional ligands such as triphenylphosphine and a classical N-heterocyclic carbene. In situ observation and isolation of the 2-pyridylidene-gold(III) species, as well as a DFT study, indicated unusual stability of gold(III) species stabilized by strong electron donation from the 2-pyridylidene ligand. Thus, the gold(I)-to-gold(III) oxidation process is thought to be facilitated by the highly electron-donating 2-pyridylidene ligand.