Unlikeliness of Pd-Free Gold(I)-Catalyzed Sonogashira Coupling Reactions

Consolidation of the Oxidant-Free Au(I)/Au(III) Catalysis Enabled by the Hemilabile Ligand Strategy

In this minireview we survey the challenges and strategies in gold redox catalysis. Gold's reluctance to oxidative addition reactions due to its high redox potential limits its applicability. Initial attempts to overcome this problem focused on the use of sacrificial external oxidants in stoichiometric amounts to bring Au(I) compounds to Au(III) reactive species. Recently, innovative approaches focused on employing hemilabile ligands, which are capable of coordinating to Au(I) and stabilizing square-planar Au(III) intermediates, thus facilitating oxidative addition steps and enabling oxidant-free catalysis. Notable examples include the use of the (P^N) bidendate MeDalphos ligand to achieve various cross-coupling reactions via oxidative addition Au(I)/Au(III). Importantly, hemilabile ligand-enabled catalysis allows merging oxidative addition with π-activation, such as oxy- and aminoarylation of alkenols and alkenamines using organohalides, expanding gold's versatility in C-C and C-heteroatom bond formations and unprecedented cyclizations. Moreover, recent advancements in enantioselective catalysis using chiral hemilabile (P^N) ligands are also surveyed. Strikingly, versatile bidentate (C^N) hemilabile ligands as competitors of MeDalphos have appeared recently, by designing scaffolds where phosphine groups are substituted by N-heterocyclic or mesoionic carbenes. Overall, these approaches highlight the evolving landscape of gold redox catalysis and its tremendous potential in a broad scope of transformations.

Going for gold - the chemistry of structurally authenticated gold(I)-ethylene complexes

Gold coordination chemistry and catalysis involving unsaturated hydrocarbons such as olefins have experienced a remarkable growth during the last few decades. Despite the importance, isolable and well-characterized molecules with ethylene, the simplest and the most widely produced olefin, on gold are still limited. This review aims to cover features of, and strategies utilized to stabilize, gold-ethylene complexes and their diverse use in chemical transformations and homogeneous catalytic processes. Isolable and well-authenticated gold-ethylene complexes are important not only for structural, spectroscopic, and bonding studies but also as models for likely intermediates in gold mediated reactions of alkenes and gold-alkene species observed in the gas phase. There has also been development on AuI/III catalytic cycles. Nitrogen based ligands have been the most widely utilized ligand supports thus far for the successful stabilization of gold-ethylene adducts. Gold has a bright future in olefin chemistry and with ethylene.

Unraveling the Prominent Existence of Trace Metals in Photocatalysis: Exploring Iron Impurity Effects

Metal impurities can complicate the identification of active catalyst species in transition metal catalysis and electrocatalysis, potentially leading to misleading findings. This study investigates the influence of metal impurities on photocatalysis. Specifically, the photocatalytic reaction of inert alkanes using chlorides without the use of an external photocatalyst was studied, achieving successful C(sp3)-H functionalization. The observations reveal that Fe and Cu impurities are challenging to avoid in a typical laboratory environment and are prominently present in normal reaction systems, and iron impurities play a dominant role in the aforementioned apparent 'metal-free' reaction. Additionally, iron exhibits significantly higher catalytic activity compared to Cu, Ce, and Ni at low metal concentrations in the photocatalytic C(sp3)-H functionalization using chlorides. Considering the widespread presence of Fe and Cu impurities in typical laboratory environments, this study serves as a reminder of their involvement in reaction processes.

Design of a Mesoionic Janus-type Dicarbene

A versatile synthetic strategy for the preparation of homo- and heterobimetallic complexes bearing an unprecedented mesoionic Janus-type diNHC is presented. Moreover, its electronic property is evaluated, and a preliminary catalytic application in the direct diarylation of 1-methylpyrrole is demonstrated.

Noncovalent Synergy: Aurophilicity and Aryl Stacking in Bis(gold(I)aryl)-dmpm Complexes

Bis(dimethylphosphino)methane (dmpm) was used as a ligand to synthesize four semi-supported dinuclear gold(I) complexes, dmpm(AuR)₂ (R = Cl, C₆H₅, C₆Cl₅, and C₆F₅), which were studied concerning the synergistic effects of two weak noncovalent interactions: aurophilic and aryl-aryl stacking interactions. The chloro-substituted complex was synthesized by the ligand substitution of (tht)AuCl with dmpm and further functionalized by the reaction with PhMgBr or in situ-generated C₆Cl₅Li to afford the phenyl- and pentachlorophenyl-substituted compounds, respectively. The pentafluorophenyl-substituted gold complex was generated by the ligand substitution of (tht)Au(C₆F₅) with dmpm. All complexes were characterized by multinuclear NMR spectroscopy, CHN analyses, and X-ray diffraction experiments. Additionally, the basic photoluminescence properties of dmpm(AuCl)₂, dmpm(AuC₆Cl₅)₂, and dmpm(AuC₆F₅)₂ were examined. The aggregation behavior of dmpm(AuC₆F₅)₂ was further investigated by variable-temperature diffusion-ordered NMR spectroscopy experiments.

Pilot Study to Quantify Palladium Impurities in Lead-like Compounds Following Commonly Used Purification Techniques

Palladium-catalyzed reactions are among the most commonly used procedures in organic synthesis. The products have a range of uses, including as intermediates in total synthesis and as screening compounds for drug discovery or agrochemical projects. Despite the known and potentially deleterious effects of low-level metal impurities in biological assays, the quantification of metal remaining in reaction products to verify the effective removal of the transition element is rarely reported. Using palladium as an exemplar, we describe a pilot study that for the first time quantifies residual metal levels in reaction products following increasingly rigorous purification protocols. Our results demonstrate that significant levels of residual palladium can remain in isolated reaction products following chromatographic purification, and only by using a subsequent metal scavenging step are they reliably reduced to a low level. Finally, we provide a set of simple guidelines that should minimize the potential for issues associated with residual palladium in reaction products.

An Efficient and Sustainable Approach to Decarboxylative Cross-Coupling Using Silica Coated Magnetic Copper Nanocatalyst for the Synthesis of Internal Alkynes

A highly efficient magnetically separable copper nanocatalyst has been developed for decarboxylative cross-coupling reaction for the alkynylation of haloarenes using alkynoic acid as a reaction partner. The chemical nature, morphology, size, and magnetic properties of the prepared nanocatalyst were studied by SEM, TEM, EDS, FT-IR, VSM, and ICP techniques. Remarkably, this catalyst represents the first successful copper based heterogeneous system for this type of coupling that provides a low-cost, stable, and environmentally friendly magnetically recoverable entity that can be re-used for seven consecutive runs without appreciable loss in its catalytic performance.

Hemilabile MIC^N ligands allow oxidant-free Au(I)/Au(III) arylation-lactonization of γ-alkenoic acids

Oxidant-free Au-catalyzed reactions are emerging as a new synthetic tool for innovative organic transformations. Oxidant-free Au-catalyzed reactions are emerging as a new synthetic tool for innovative organic transformations. Still, a deeper mechanistic understanding is needed for a rational design of these processes. Here we describe the synthesis of two Au(i) complexes bearing bidentated hemilabile MIC^N ligands, [Au^I(MIC^N)Cl], and their ability to stabilize square-planar Au(iii) species (MIC = mesoionic carbene). The presence of the hemilabile N-ligand contributed to stabilize the ensuing Au(iii) species acting as a five-membered ring chelate upon its coordination to the metal center. The Au(iii) complexes can be obtained either by using external oxidants or, alternatively, by means of feasible oxidative addition with strained biphenylene C_sp²–C_sp² bonds as well as with aryl iodides. Based on the fundamental knowledge gained on the redox properties on these Au(i)/Au(iii) systems, we successfully develop a novel Au(i)-catalytic procedure for the synthesis of γ-substituted γ-butyrolactones through the arylation-lactonization reaction of the corresponding γ-alkenoic acid. The oxidative addition of the aryl iodide, which in turn is allowed by the hemilabile nature of the MIC^N ligand, is an essential step for this transformation.

A novel hemilabile MIC^N ligand-based Au(i)-catalytic procedure for the synthesis of γ-substituted γ-butyrolactones through the arylation-lactonization reaction of the corresponding γ-alkenoic acid is presented.

Alkynyl transmetalation triggered by a nucleophilic attack

We report an interesting alkynyl transmetalation process in the cyclization of π-Au,σ-M acetylides (M = Pd, Ni, Pt) bearing a N-propiolic formamidine moiety. The cyclization was triggered by nucleophilic attack,...

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(C₂H₄)]⁺ 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.

C-C Cross-Couplings from a Cyclometalated Au(III) C ∧ N Complex: Mechanistic Insights and Synthetic Developments

In recent years, the reactivity of gold complexes was shown to extend well beyond π-activation and to hold promises to achieve selective cross-couplings in several C-C and C-E (E=heteroatom) bond forming reactions. Here, with the aim of exploiting new organometallic species for cross-coupling reactions, we report on the Au(III)-mediated C(sp2 )-C(sp) occurring upon reaction of the cyclometalated complex [Au(CCH2 N)Cl2 ] (1, CCH2 N=2-benzylpyridine) with AgPhCC. The reaction progress has been monitored by NMR spectroscopy, demonstrating the involvement of a number of key intermediates, whose structures have been unambiguously ascertained through 1D and 2D NMR analyses (1 H, 13 C, 1 H-1 H COSY, 1 H-13 C HSQC and 1 H-13 C HMBC) as well as by HR-ESI-MS and X-ray diffraction studies. Furthermore, crystallographic studies have serendipitously resulted in the authentication of zwitterionic Au(I) complexes as side-products arising from cyclization of the coupling product in the coordination sphere of gold. The experimental work has been paralleled and complemented by DFT calculations of the reaction profiles, providing valuable insight into the structure and energetics of the key intermediates and transition states, as well as on the coordination sphere of gold along the whole process. Of note, the broader scope of the cross-coupling at the Au(III) CCH2 N centre has also been demonstrated studying the reaction of 1 with C(sp2 )-based nucleophiles, namely vinyl and heteroaryl tin and zinc reagents. These reactions stand as rare examples of C(sp2 )-C(sp2 ) cross-couplings at Au(III).

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.

Fluorinated Iron(ii) clathrochelate units in metalorganic based copolymers: improved porosity, iodine uptake, and dye adsorption properties

We report the synthesis of metalorganic copolymers made from the palladium catalyzed Sonogashira cross-coupling reaction between various iron(ii) clathrochelate building blocks with diethynyl-triptycene and fluorene derivatives. The target copolymers CCP1-5 were isolated in excellent yield and characterized by various instrumental analysis techniques. Interestingly, investigation of the copolymers' porosity properties discloses BET surface areas up to 337 m2 g-1 for the target compounds bearing fluorinated iron(ii) clathrochelate units CCP2,5. Moreover, the fluorinated copolymers display an outstanding uptake capacity of iodine with a maximum adsorption of 200 wt%. The target metalorganic copolymers CCP1-5 reveal very good adsorption of organic dyes, namely, methyl blue and methylene blue, from aqueous media.

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.

Photocontrolled Synthesis of n-Type Conjugated Polymers

Current approaches to synthesize π-conjugated polymers (CPs) are dominated by thermally driven, transition-metal-mediated reactions. Herein we show that electron-deficient Grignard monomers readily polymerize under visible-light irradiation at room temperature in the absence of a catalyst. The product distribution can be tuned by the wavelength of irradiation based on the absorption of the polymer. Conversion studies are consistent with an uncontrolled chain-growth process; correspondingly, chain extension produces all-conjugated n-type block copolymers. Preliminary results demonstrate that the polymerization can be expanded to donor-acceptor alternating copolymers. We anticipate that this method can serve as a platform to access new architectures of n-type CPs without the need for transition-metal catalysis.

Highly Crystallized Pd/Cu Nanoparticles on Activated Carbon: An Efficient Heterogeneous Catalyst for Sonogashira Cross-Coupling Reaction

In the quest for efficient and recyclable heterogeneous catalysts for Sonogashira coupling reactions, a PEG-OMe (Methoxy Polyene Glycol) - mediated method was developed to immobilize Pd/Cu bimetallic nanoparticles on activated carbon (AC). Catalytic experiments showed that Pd/Cu@AC prepared in a PEG-OMe 500 had the highest activity. The morphology and composition of the catalyst were determined, and the identified crystallized and heterometallic nanoparticles of Pd/Cu were essential for an efficient catalytic cycle. The Pd/Cu@AC catalyst was successfully used in Sonogashira reactions (21 examples) including with aryl bromides as the coupling partner in EtOH at 80–100 °C. The catalyst was recycled at least nine times.

Ligand-enabled gold-catalyzed 1,2-heteroarylation of alkenes

By adopting the interplay between ligand-enabled Au(i)/Au(iii) catalysis and the unique π-activation mode of gold complexes, a highly coveted 1,2-heteroarylation of alkenes has been accomplished.

Pd-Promoted cross coupling of iodobenzene with vinylgold via an unprecedented phenyl transmetalation from Pd to Au

Oxidative addition of Pd/Au vinylene species with iodobenzene reveals the effect of a strong metallophilic Pd⋯Au interaction on phenyl transmetalation from Pd^IV to Au^I, which allows subsequent reductive elimination of LAuI at Pd and Au.

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.

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).

Supercritical water assisted preparation of recyclable gold nanoparticles and their catalytic utility in cross-coupling reactions under sustainable conditions

Preparation of gold nanoparticles (AuNPs) in environmentally friendly water without using any reducing agents under supercritical conditions is demonstrated. PXRD, XPS, FE-SEM and HR-TEM analysis confirmed the formation of phase-pure and crystalline AuNPs of the size of ∼10-30 nm. The catalytic potential of AuNPs was manifested through a generalized green procedure that could accommodate both Sonogashira as well as Suzuki coupling under aqueous conditions at low catalytic loading (0.1 mol%). The AuNP catalyst was found to be recuperated after the reaction and reused for up to six catalytic cycles with no leaching out of gold species as confirmed through ICP-OES analysis. With no confinement of AuNP catalysis to cross-coupling reaction, synthetic extension to one-flask preparation of π-conjugated semiconductors (4 examples) and their optoelectronic properties were also investigated. Other significant features of the present work include short reaction time, site-selectivity, wide substrate scope, high conversion, good chemical yields and applicability in gram-scale synthesis. Overall, the results of this paper signify an operationally sustainable supercritical fluid processing method for the synthesis of AuNPs and their catalytic application towards cross-coupling reactions in green media.

Oxidant speciation and anionic ligand effects in the gold-catalyzed oxidative coupling of arenes and alkynes

The mechanism of the gold-catalyzed oxidative cross-coupling of arenes and alkynes has been studied in detail combining stoichiometric experiments with putative reaction intermediates and DFT calculations.

The mechanism of the gold-catalyzed oxidative cross-coupling of arenes and alkynes has been studied in detail combining stoichiometric experiments with putative reaction intermediates and DFT calculations. Our data suggest that ligand exchange between the alkyne, the Au(i)-catalyst and the hypervalent iodine reagent is responsible for the formation of both an Au(i)-acetylide complex and a more reactive “non-symmetric” I(iii) oxidant responsible for the crucial Au(i)/Au(iii) turnover. Further, the reactivity of the in situ generated Au(iii)-acetylide complex is governed by the nature of the anionic ligands transferred by the I(iii) oxidant: while halogen ligands remain unreactive, acetato ligands are efficiently displaced by the arene to yield the observed Csp²–Csp cross-coupling products through an irreversible reductive elimination step. Finally, the nature of competitive processes and catalyst deactivation pathways has also been unraveled. This detailed investigation provides insights not only on the specific features of the species involved in oxidative gold-catalyzed cross couplings but also highlights the importance of both ancillary and anionic ligands in the reactivity of the key Au(iii) intermediates.

The CuFe2O4@SiO2@ZrO2/SO42−/Cu nanoparticles: an efficient magnetically recyclable multifunctional Lewis/Brønsted acid nanocatalyst for the ligand- and Pd-free Sonogashira cross-coupling reaction in water

Herein, the synthesis and application of copper-incorporated sulfated zirconium oxide supported on CuFe₂O₄ NPs (CuFe₂O₄@SiO₂@ZrO₂/SO₄²⁻/Cu NPs) as a novel Lewis/Brønsted acid nanocatalyst were studied for the Sonogashira C–C cross-coupling reaction. The fabricated CuFe₂O₄@SiO₂@ZrO₂/SO₄²⁻/Cu catalyst exhibited efficient activity for a large variety of aryl iodides/bromides and, most importantly, aryl chlorides in water and in the presence of NaOH as a base in short reaction times. The catalyst was fully characterized by FTIR, TG-DTG, VSM, XRD, EDX, FE-SEM and TEM analyses. A synergetic effect could be considered to have arisen from the various Lewis acid and Brønsted acid sites present in the catalyst. The efficient incorporation of copper into zirconia provided a robust highly stable hybrid, which prevented any metal leaching, whether from the magnetite moiety and/or Cu sites in the reaction mixture. Moreover, the catalyst was successfully recovered from the mixture by a simple external magnet and reused for at least 9 consecutive runs. Zero metal leaching, stability, consistency with a variety of substrates, fast performance, cost-effectiveness, environmental friendliness, and preparation with accessible and cheap materials are some of the advantages and highlights of the current protocol.

A mild and green protocol was developed by immobilizing copper-incorporated sulfated zirconium oxide on CuFe₂O₄ as an efficient inorgano-nanocatalyst for the Sonogashira reaction.

Recent Developments in C-H Activation for Materials Science in the Center for Selective C-H Activation

Abstract: Organic electronics is a rapidly growing field driven in large part by the synthesis of ∏-conjugated molecules and polymers. Traditional aryl cross-coupling reactions such as the Stille and Suzuki have been used extensively in the synthesis of ∏-conjugated molecules and polymers, but the synthesis of intermediates necessary for traditional cross-couplings can include multiple steps with toxic and hazardous reagents. Direct arylation through C-H bond activation has the potential to reduce the number of steps and hazards while being more atom-economical. Within the Center for Selective C-H Functionalization (CCHF), we have been developing C-H activation methodology for the synthesis of ∏-conjugated materials of interest, including direct arylation of difficult-to-functionalize electron acceptor intermediates and living polymerization of ∏-conjugated polymers through C-H activation.