Oxidative Addition to Gold(I) by Photoredox Catalysis: Straightforward Access to Diverse (C,N)-Cyclometalated Gold(III) Complexes

A Hemilabile NHC-Gold Complex and its Application to the Redox Neutral 1,2-Oxyarylation of Feedstock Alkenes

We describe a AuI complex of a hemi-labile (C^N) N-heterocyclic carbene ligand that is able to mediate oxidative addition of aryl iodides. Detailed computational and experimental investigations have been undertaken to verify and rationalize the oxidative addition process. Application of this initiation mode has resulted in the first examples of "exogenous oxidant-free" AuI /AuIII catalyzed 1,2-oxyarylations of ethylene and propylene. These demanding yet powerful processes establish these commodity chemicals as nucleophilic-electrophilic building blocks in catalytic reaction design.

Au(III) Cyclometallated Compounds with 2-Arylpyridines and Their Derivatives or Analogues: 34 Years (1989–2022) of NMR and Single Crystal X-ray Studies

A review paper on Au(III) cyclometallated compounds with 2-arylpyridines (2-phenylpyridine, 2-benzylpyridine, 2-benzoylpyridine, 2-phenoxypyridine, 2-phenylsulfanylpyridine, 2-anilinopyridine, 2-(naphth-2-yl)pyridine, 2-(9,9-dialkylfluoren-2-yl)pyridines, 2-(dibenzofuran-4-yl)pyridine, and their derivatives) and their analogues (2-arylquinolines, 1- and 3-arylisoquinolines, 7,8-benzoquinoline), with 113 references. A total of 554 species, containing κ2-N(1),C(6′)*-Au(III), or analogous moiety (i.e., chelated by nitrogen of the pyridine-like ring and the deprotonated ortho- carbon of the phenyl-like ring) and, thus, possessing a character intermediate between metal complexes and organometallics, studied in the years 1989–2022 by NMR spectroscopy and/or single crystal X-ray diffraction (207 X-ray structures), are described. The compounds for which biological or catalytic activity and the luminescence properties were studied are also quoted.

Heart of gold: enabling ligands for oxidative addition of haloorganics in Au(I)/Au(III) catalysed cross-coupling reactions

The field of Au-catalysis has been an area rich with new discoveries due to the unique properties of the lustrous element. In the past decade, developments in Au(I)/Au(III) cross-coupling methodology have been made possible with the use of external oxidants that facilitate the challenging oxidation of Au(I) to Au(III) in a stable and catalytically competent fashion. Until recently, Au-chemistry was not known to undergo catalytic transformations that feature oxidative addition of haloarenes like those that were made famous by transition metals such as Pd and Ni. The discovery that ligand modification could facilitate the oxidative addition of Au(I) with haloorganics to provide Au(III) intermediates that are competent in other areas of catalysis (i.e. Lewis acid catalysis) has revolutionized this field and has led to the invention of new cross-coupling methodology. The recent advances at the leading edge in the emerging field of Au(I)/Au(III) catalysis under redox-neutral conditions are highlighted.

On the mercuration, palladation, transmetalation and direct auration of a C^N^C pincer ligand

The C^N^C ligand 2,6-bis(2,3-dialkoxyphenyl)pyridine forms dimercury and orthopalladated complexes, both of which may be transmetallated to gold(III) complexes; the gold complexes may also be formed directly in a Rh(III)-catalysed process, hence it is possible to circumvent the use of organomercury intermediates in the synthesis of this important class of compound.

Ligand-enabled oxidation of gold(i) complexes with o-quinones

Chelating P^P and hemilabile P^N ligands were found to trigger the oxidation of Au(i) complexes by o-benzoquinones. The ensuing Au(iii) catecholate complexes have been characterized by NMR spectroscopy, single crystal X-ray diffraction and X-ray absorption spectroscopy. They adopt tetracoordinate square-planar structures. Reactivity studies substantiate the reversibility of the transformation. In particular, the addition of competing ligands such as chloride and alkenes gives back Au(i) complexes with concomitant release of the o-quinone. DFT calculations provide insight about the structure and relative stability of the Au(i) o-quinone and Au(iii) catecholate forms, and shed light on the 2-electron transfer from gold to the o-quinone.

Stable Au(I) catalysts for oxidant-free C-H Functionalization with Iodoarenes

The development of oxidant-free gold-catalyzed cross coupling reactions involving aryl halides have been hamstrung by the lack of gold catalysts capable of performing oxidative addition at Au(I) centers. Herein, we report the development of novel tricoordinate Au(I) catalysts supported by N,N-bidentate ligands and ligated by phosphine or arsine ligands for C-H functionalization without external oxidants to form biaryls with no homocoupling. The unsymmetrical character of the Au(I) catalyst is critical to facilitating this necessary orthogonal transformation. This study unveils yet another potential of Au(I) catalysis in biaryl synthesis.

Rapid Construction of Polysubstituted “Caged” Oxa-Bishomocubane Framework from Vinylidenecyclopropanes through a Sequential Dual Catalysis of Copper(I) and Visible-Light-Induced Photosensitization

This context describes a sequential dual catalytic transformation involving copper(I)-catalyzed cyclization/isomerization/migration-dimerization and visible-light photo-induced intramolecular [2+2] cycloaddition of vinylidenecyclopropanes for the rapid construction of polysubstituted “caged” oxa-bishomocubane products. The reaction...

Merging Gold/Copper Catalysis and Copper/Photoredox Catalysis: An Approach to Alkyl Oxazoles from N-Propargylamides

Here, we report a mild and highly efficient approach to alkyl oxazoles through merging gold/copper catalysis and copper/photoredox catalysis. Various alkyl oxazoles are synthesized from N-propargylamides with alkyl halides in good to excellent yields with wide functional-group compatibility under blue-light irradiation. Significantly, a copper catalyst plays a dual role in this transformation: as a powerful cocatalyst to accelerate protodeauration of vinyl gold intermediates and improve photoredox catalysis.

Metallaphotoredox: The Merger of Photoredox and Transition Metal Catalysis

The merger of photoredox catalysis with transition metal catalysis, termed metallaphotoredox catalysis, has become a mainstay in synthetic methodology over the past decade. Metallaphotoredox catalysis has combined the unparalleled capacity of transition metal catalysis for bond formation with the broad utility of photoinduced electron- and energy-transfer processes. Photocatalytic substrate activation has allowed the engagement of simple starting materials in metal-mediated bond-forming processes. Moreover, electron or energy transfer directly with key organometallic intermediates has provided novel activation modes entirely complementary to traditional catalytic platforms. This Review details and contextualizes the advancements in molecule construction brought forth by metallaphotocatalysis.

A Systematic Study of the Effects of Complex Structure on Aryl Iodide Oxidative Addition at Bipyridyl-Ligated Gold(I) Centers

A combined theoretical and experimental approach has been used to study the unusual mechanism of oxidative addition of aryl iodides to [bipyAu(C2 H4 )]+ complexes. The modular nature of this system allowed a systematic assessment of the effects of complex structure. Computational comparisons between cationic gold and the isolobal (neutral) Pd0 and Pt0 complexes revealed similar mechanistic features, but with oxidative addition being significantly favored for the group 10 metals. Further differences between Au and Pd were seen in experimental studies: studying reaction rates as a function of electronic and steric properties showed that ligands bearing more electron-poor functionality increase the rate of oxidative addition; in a complementary way, electron-rich aryl iodides give faster rates. This divergence in mechanism compared to Pd suggests that Ar-X oxidative addition with Au can underpin a broad range of new or complementary transformations.

Monitoring of the Pre-Equilibrium Step in the Alkyne Hydration Reaction Catalyzed by Au(III) Complexes: A Computational Study Based on Experimental Evidences

The coordination ability of the [(ppy)Au(IPr)]²⁺ fragment [ppy = 2-phenylpyridine, IPr = 1,3-bis(2,6-di-isopropylphenyl)-imidazol-2-ylidene] towards different anionic and neutral X ligands (X = Cl⁻, BF₄⁻, OTf⁻, H₂O, 2-butyne, 3-hexyne) commonly involved in the crucial pre-equilibrium step of the alkyne hydration reaction is computationally investigated to shed light on unexpected experimental observations on its catalytic activity. Experiment reveals that BF₄⁻ and OTf⁻ have very similar coordination ability towards [(ppy)Au(IPr)]²⁺ and slightly less than water, whereas the alkyne complex could not be observed in solution at least at the NMR sensitivity. Due to the steric hindrance/dispersion interaction balance between X and IPr, the [(ppy)Au(IPr)]²⁺ fragment is computationally found to be much less selective than a model [(ppy)Au(NHC)]²⁺ (NHC = 1,3-dimethylimidazol-2-ylidene) fragment towards the different ligands, in particular OTf⁻ and BF₄⁻, in agreement with experiment. Effect of the ancillary ligand substitution demonstrates that the coordination ability of Au(III) is quantitatively strongly affected by the nature of the ligands (even more than the net charge of the complex) and that all the investigated gold fragments coordinate to alkynes more strongly than H₂O. Remarkably, a stabilization of the water-coordinating species with respect to the alkyne-coordinating one can only be achieved within a microsolvation model, which reconciles theory with experiment. All the results reported here suggest that both the Au(III) fragment coordination ability and its proper computational modelling in the experimental conditions are fundamental issues for the design of efficient catalysts.

Modular and Selective Arylation of Aryl Germanes (C-GeEt3 ) over C-Bpin, C-SiR3 and Halogens Enabled by Light-Activated Gold Catalysis

Selective Csp 2 -Csp 2 couplings are powerful strategies for the rapid and programmable construction of bi- or multiaryls. To this end, the next frontier of synthetic modularity will likely arise from harnessing the coupling space that is orthogonal to the powerful Pd-catalyzed coupling regime. This report details the realization of this concept and presents the fully selective arylation of aryl germanes (which are inert under Pd0 /PdII catalysis) in the presence of the valuable functionalities C-BPin, C-SiMe3 , C-I, C-Br, C-Cl, which in turn offer versatile opportunities for diversification. The protocol makes use of visible light activation combined with gold catalysis, which facilitates the selective coupling of C-Ge with aryl diazonium salts. Contrary to previous light-/gold-catalyzed couplings of Ar-N2 + , which were specialized in Ar-N2 + scope, we present conditions to efficiently couple electron-rich, electron-poor, heterocyclic and sterically hindered aryl diazonium salts. Our computational data suggest that while electron-poor Ar-N2 + salts are readily activated by gold under blue-light irradiation, there is a competing dissociative deactivation pathway for excited electron-rich Ar-N2 + , which requires an alternative photo-redox approach to enable productive couplings.

Gold Catalyzed Decarboxylative Cross-Coupling of Iodoarenes

This report details a decarboxylative cross-coupling of (hetero)aryl carboxylates with iodoarenes in the presence of a gold catalyst (>25 examples, up to 96% yield). This reaction is site specific, which overcomes prior limitations associated with gold catalyzed oxidative coupling reactions. The reactivity of the (hetero)aryl carboxylate correlates qualitatively to the field effect parameter (F_ortho). Reactions with isolated gold complexes and DFT calculations support a mechanism proceeding through oxidative addition at a gold(I) cation with decarboxylation being viable at either a gold(I) or a silver(I) species.

Stereoselective Formation of Facial Tris-Cyclometalated PtIV Complexes: Dual Phosphorescence from Heteroleptic Derivatives

A stereoselective synthetic route to homo- and heteroleptic facial tris-cyclometalated Pt^IV complexes is reported, involving the oxidative addition of 2-(2-pyridyl)- or 2-(1-isoquinolinyl)benzenediazonium salts to cis-[Pt(C^N)₂ ] precursors, with C^N=cyclometalated 2-(p-tolyl)pyridine (tpy), 2-phenylquinoline (pq), 2-(2-thienyl)pyridine or 1-phenylisoquinoline (piq), to produce labile diazenide intermediates that undergo photochemical or thermal elimination of N₂ . The method allows the preparation of derivatives bearing cyclometalated ligands of low π-π* transition energies. The new complexes exhibit phosphorescence in fluid solution at room temperature arising from triplet ligand-centered (³ LC) excited states, which, in the cases of the heteroleptic derivatives, involve the ligand with the lowest π-π* gap. The heteroleptic piq derivatives exhibit fluorescence and dual phosphorescence from different ligand-centered excited states in rigid media, demonstrating the potential of cyclometalated Pt^IV complexes as multi-emissive materials.

Mercury-Free Synthesis of Pincer [C^N^C]AuIII Complexes by an Oxidative Addition/CH Activation Cascade

Starting from the commercially available dimethyl sulfide-gold(I) chloride complex (DMSAuCl) and diazonium salts in the presence of 2,6-di-tert-butyl-4-methylpyridine as base, symmetric and unsymmetric [C^N^C]Au^III Cl complexes were synthesized in a selective, photosensitizer-free, photochemical reaction using blue LED light. This new protocol provides the first mercury-free synthesis of these types of pincer-complexes in moderate-to-excellent yields, starting from a readily available gold(I) precursor. Owing to the extraordinary properties of the target compounds, like excellent luminescence and high anticancer activities, the synthesis of such complexes is a highly active field of research, which might make its way to an industrial application. Owing to the disadvantages of the known protocols, especially the toxicity and the selectivity issues in the case of unsymmetric complexes, avoiding the use of mercury, should further accelerate this ongoing development.

Photoredox-Assisted Gold-Catalyzed Arylative Alkoxycyclization of 1,6-Enynes

The photoredox-assisted gold-catalyzed arylative cyclization of 1,6-enynes with aryldiazonium salts gives rise to cyclization products with the opposite configuration at the alkene than that obtained by gold(I)-catalyzed alkoxycyclization. The reaction occurs under mild conditions and shows high functional group tolerance.

Oxidative Addition of Alkenyl and Alkynyl Iodides to a AuI Complex

The first isolated examples of intermolecular oxidative addition of alkenyl and alkynyl iodides to Au^I are reported. Using a 5,5'-difluoro-2,2'-bipyridyl ligated complex, oxidative addition of geometrically defined alkenyl iodides occurs readily, reversibly and stereospecifically to give alkenyl-Au^III complexes. Conversely, reversible alkynyl iodide oxidative addition generates bimetallic complexes containing both Au^III and Au^I centers. Stoichiometric studies show that both new initiation modes can form the basis for the development of C-C bond forming cross-couplings.

Hydration of alkynes catalyzed by [Au(X)(L)(ppy)]X in the green solvent γ-valerolactone under acid-free conditions: the importance of the pre-equilibrium step

Stable and robust [Au(H₂O)(NHC)(ppy)](X)₂ successfully catalyses the hydration of alkynes in GVL, under acid-free conditions. DFT calculation and NMR measurements suggest that pre-equilibrium is the key step of the whole process.

Gold-Catalyzed Cross-Coupling Reactions: An Overview of Design Strategies, Mechanistic Studies, and Applications

Transition-metal-catalyzed cross-coupling reactions are central to many organic synthesis methodologies. Traditionally, Pd, Ni, Cu, and Fe catalysts are used to promote these reactions. Recently, many studies have showed that both homogeneous and heterogeneous Au catalysts can be used for activating selective cross-coupling reactions. Here, an overview of the past studies, current trends, and future directions in the field of gold-catalyzed coupling reactions is presented. Design strategies to accomplish selective homocoupling and cross-coupling reactions under both homogeneous and heterogeneous conditions, computational and experimental mechanistic studies, and their applications in diverse fields are critically reviewed. Specific topics covered are: oxidant-assisted and oxidant-free reactions; strain-assisted reactions; dual Au and photoredox catalysis; bimetallic synergistic reactions; mechanisms of reductive elimination processes; enzyme-mimicking Au chemistry; cluster and surface reactions; and plasmonic catalysis. In the relevant sections, theoretical and computational studies of Au^I /Au^III chemistry are discussed and the predictions from the calculations are compared with the experimental observations to derive useful design strategies.

Light-Induced Mechanistic Divergence in Gold(I) Catalysis: Revisiting the Reactivity of Diazonium Salts

In a systematic study of the Au-catalyzed reaction of o-alkynylphenols with aryldiazonium salts, we find that essentially the same reaction conditions lead to a change in mechanism when a light source is applied. If the reaction is carried out at room temperature using a AuI catalyst, the diazonium salt undergoes electrophilic deauration of a vinyl AuI intermediate and provides access to substituted azobenzofurans. If the reaction mixture is irradiated with blue LED light, C-C bond formation due to N2 -extrusion from the diazonium salt is realized selectively, using the same starting materials without the need for an additional photo(redox) catalyst under aerobic conditions. We report a series of experiments demonstrating that the same vinyl AuI intermediate is capable of producing the observed products under photolytic and thermal conditions. The finding that a vinyl AuI complex can directly, without the need for an additional photo(redox) catalyst, result in C-C bond formation under photolytic conditions is contrary to the proposed mechanistic pathways suggested in the literature till date and highlights that the role of oxidation state changes in photoredox catalysis involving Au is thus far only poorly understood and may hold surprises for the future. Computational results indicate that photochemical activation can occur directly from a donor-acceptor complex formed between the vinyl AuI intermediate and the diazonium salt.

Catalytic Au(i)/Au(iii) arylation with the hemilabile MeDalphos ligand: unusual selectivity for electron-rich iodoarenes and efficient application to indoles

The ability of the hemilabile (P,N) MeDalphos ligand to trigger oxidative addition of iodoarenes to gold has been thoroughly studied. Competition experiments and Hammett correlations substantiate a clear preference of gold for electron-enriched substrates both in stoichiometric oxidative addition reactions and in catalytic C-C cross-coupling with 1,3,5-trimethoxybenzene. This feature markedly contrasts with the higher reactivity of electron-deprived substrates typically encountered with palladium. Based on DFT calculations and detailed analysis of the key transition states (using NBO, CDA and ETS-NOCV methods in particular), the different behavior of the two metals is proposed to result from inverse electron flow between the substrate and metal. Indeed, oxidative addition of iodobenzene is associated with a charge transfer from the substrate to the metal at the transition state for gold, but opposite for palladium. The higher electrophilicity of the gold center favors electron-rich substrates while important back-donation from palladium favors electron-poor substrates. Facile oxidative addition of iodoarenes combined with the propensity of gold(iii) complexes to readily react with electron-rich (hetero)arenes prompted us to apply the (MeDalphos)AuCl complex in the catalytic arylation of indoles, a challenging but very important transformation. The gold complex proved to be very efficient, general and robust. It displays complete regioselectivity for C3 arylation, it tolerates a variety of functional groups at both the iodoarene and indole partners (NO2, CO2Me, Br, OTf, Bpin, OMe…) and it proceeds under mild conditions (75 °C, 2 h).

Mechanism of Photoredox-Initiated C-C and C-N Bond Formation by Arylation of IPrAu(I)-CF3 and IPrAu(I)-Succinimide

Herein, we report on the photoredox-initiated gold-mediated C(sp²)-CF₃ and C(sp²)-N coupling reactions. By adopting gold as a platform for probing metallaphotoredox catalysis, we demonstrate that cationic gold(III) complexes are the key intermediates of the C-C and C-N coupling reactions. The high-valent gold(III) intermediates are accessed by virtue of photoredox catalysis through a radical chain process. In addition, the bond-forming step of the coupling reactions is the reductive elimination from cationic gold(III) intermediates, which is supported by isolation and crystallographic characterization of key Au(III) intermediates.

Oxidative Addition, Transmetalation, and Reductive Elimination at a 2,2'-Bipyridyl-Ligated Gold Center

Three-coordinate bipyridyl complexes of gold, [(κ²-bipy)Au(η²-C₂H₄)][NTf₂], are readily accessed by direct reaction of 2,2'-bipyridine (bipy), or its derivatives, with the homoleptic gold ethylene complex [Au(C₂H₄)₃][NTf₂]. The cheap and readily available bipyridyl ligands facilitate oxidative addition of aryl iodides to the Au(I) center to give [(κ²-bipy)Au(Ar)I][NTf₂], which undergo first aryl-zinc transmetalation and second C-C reductive elimination to produce biaryl products. The products of each distinct step have been characterized. Computational techniques are used to probe the mechanism of the oxidative addition step, offering insight into both the origin of the reversibility of this process and the observation that electron-rich aryl iodides add faster than electron-poor substrates. Thus, for the first time, all steps that are characteristic of a conventional intermolecular Pd(0)-catalyzed biaryl synthesis are demonstrated from a common monometallic Au complex and in the absence of directing groups.

Quantifying and understanding the steric properties of N-heterocyclic carbenes

This Feature Article presents and discusses the use of different methods to quantify and explore the steric impact of N-heterocyclic carbene (NHC) ligands. These include (a) the percent buried volume (%V_bur), which provides a convenient single number to measure steric impact and (b) steric maps, which provide a graphical representation of the steric profile of a ligand using colour-coded contour maps. A critical discussion of the scope and limitations of these tools is presented, along with some examples of their use in organometallic chemistry and catalysis. This Article should provide all users of NHCs, from organic, organometallic, and inorganic chemistry backgrounds, with an appreciation of how these tools can be used to quantify and compare their steric properties.

Gold(i)-catalyzed cross-coupling reactions of aryldiazonium salts with organostannanes

Gold(i)-catalyzed cross-coupling reactions of aryldiazonium salts with organostannanes are described.

Well defined difluorogold(iii) complexes supported by N-ligands

Rare difluorogold(iii) complexes supported by N-ligands can be synthesized simply either from Au(i) precursors using XeF₂ or Au(iii) using KF.

Visible light mediated desilylative C(sp2)–C(sp2) cross-coupling reactions of arylsilanes with aryldiazonium salts under Au(i)/Au(iii) catalysis

Desilylative C(sp²)–C(sp²) cross-coupling reactions of arylsilanes with aryldiazonium salts under Au(i)/photoredox catalysis have been reported.