Air-stable cationic gold(I) catalyst featuring a Z-type ligand: promoting enyne cyclizations

The Potential of Self-Activating Au(I) Complexes in Gold Catalysis

Gold catalysis has emerged as a groundbreaking field in synthetic chemistry, revolutionizing numerous organic transformations. Despite the significant achieved advancements, the mechanistic understanding behind many gold-catalyzed reactions remains elusive. This Concept article covers the so-called "self-activating" Au(I) complexes, sorting out their pivotal role in gold catalysis. We comment on how Au(I) complexes can undergo self-activation, triggering diverse catalytic transformations without the need for external additives. The most important examples reported so far that underlie the catalytic activity of these species are discussed. This intrinsic reactivity represents a paradigm shift in gold catalysis, offering new avenues for the design of efficient and sustainable catalytic systems. Furthermore, we explore the factors influencing the stability, reactivity, and selectivity of these Au(I) complexes, providing insights into their synthetic utility and potential applications. This area of research not only advances our fundamental understanding of gold catalysis but also paves the way for the development of novel catalytic strategies with broad implications in organic synthesis and the chemical industry.

Chemo-selective Stille-type coupling of acyl-chlorides upon phosphine-borane Au(i) catalysis

Phosphine-boranes do not promote oxidative addition of acyl chlorides to gold, but the phosphine-borane gold triflimide complex [iPr2P(o-C6H4)BCy2]AuNTf2 was found to catalyze the coupling of acyl chlorides and aryl stannanes. The reaction involves aryl/chloride-bridged dinuclear gold(i) complexes as key intermediates, as substantiated by spectroscopic and crystallographic analyses. Similar to Pd(0)/Pd(ii)-catalyzed Stille coupling with phosphine-borane ligands, the gold-catalyzed variant shows complete chemoselectivity for acyl chlorides over aryl iodides and bromides, enabling straightforward access to halogenated aryl ketones.

Sulfonium Cation in the Service of π-Acid Catalysis

While still rare, cationic ligands offer much promise as tunable electron-withdrawing ligands for π-acid catalysis. Recently, we introduced pincer-type sulfonium cations into the list of available strongly π-acidic ancillary ligands. However, the M-S bond in sulfonium complexes of these ligands was found highly labile, precluding their catalytic applications. Herein we demonstrate that this obstacle can be overcome by increasing the rigidity of the sulfonium pincer scaffold. X-ray analyses confirm that despite bearing a formal positive charge, the sulfur atom of this newly designed sulfonium ligand maintains its coordination to the Pt(II)-center, while DFT calculations indicate that by doing so it strongly enhances the electrophilic character of the metal. Kinetic studies carried out on three model cycloisomerization reactions prove that such a tris-cationic sulfonium-Pt(II) complex is highly reactive, compared to its thioether-based analogue. This proof-of-concept study presents the first example of employing sulfonium-based ligands in homogeneous catalysis.

Carbon-Bonding Metal Catalysis (CBMC): A Supramolecular Complex Directs Structural-Isomer Selection in Gold-Catalyzed Reactions

Carbon is a primary element to constitute organic molecules, while metal catalysis is a basic tool in organic synthesis. The establishment of a link between the ubiquitous carbon bonding and metal catalysis is thus a fundamentally important problem. However, there is yet no experimental example to introduce the role of carbon bonding in a metal catalysis process. Herein, we merged the topics of carbon bonding and metal catalysis together and demonstrated that a supramolecular carbon-bonding metal complex can not only give rise to catalytic activity but, more remarkably, direct structural-isomer selection events in gold-catalyzed reactions. The experimental results unveil the fact that the imposing of weak carbon-bonding interactions on a gold complex can alter the carbene as well as the Lewis acid property of these catalysts. These results illustrate a non-negligible role of weak carbon-bonding interactions in the modulation of metal catalysis. As such, carbon-bonding metal catalysis is suggested to be used as a routine tool not only in the development of reactions but more frequently in analyzing reaction processes in metal catalysis.

Triel Bonds with Au Atoms as Electron Donors

The novel triel bonds of BX₃ (X=H, F, Cl, Br, and I) and C₅ H₅ B as electron acceptors and AuR₂ (R=Cl and CH₃ ) as an electron donor were explored. The triel bond is a primary driving force for most complexes, while the contribution from a halogen-chlorine interaction in BX₃ -AuCl₂ (X=Cl, Br, and I) and an iodine-Au interaction in BI₃ -Au(CH₃ )₃ is also very important. Interestingly, the positively charged Au atom of AuCl₂ can attractively bind with the holes of BX₃ and C₅ H₅ B. The interaction energy lies in the range of 1 and 80 kcal/mol, in the order X=F<H<Cl<Br<I. In most cases, the triel bond of C₅ H₅ B is stronger than the triel bond of BX₃ . In the formation of B-Au triel bond, electrostatic energy is not dominant, while polarization energy including orbital interaction has the largest contribution for the strongly bonded complexes and dispersion energy for the weak triel bond.

Sulfonium cations as versatile strongly π-acidic ligands

More than a century old, sulfonium cations are still intriguing species in the landscape of organic chemistry. On one hand they have found broad applications in organic synthesis and materials science, but on the other hand, while isoelectronic to the ubiquitous tertiary phosphine ligands, their own coordination chemistry has been neglected for the last three decades. Here we report the synthesis and full characterization of the first Rh(i) and Pt(ii) complexes of sulfonium. Moreover, for the first time, coordination of an aromatic sulfonium has been established. A thorough computational analysis of the exceptionally short S-Rh bonds obtained attests to the strongly π-accepting nature of sulfonium cations and places them among the best π-acceptor ligands available today. Our calculations also show that embedding within a pincer framework enhances their π-acidity even further. Therefore, in addition to the stability and modularity that these frameworks offer, our pincer complexes might open the way for sulfonium cations to become powerful tools in π-acid catalysis.

Facile Synthesis and Utilization of Bis(o-phosphinophenyl)zinc as Isolable PZnP-pincer Ligands Enabled by Boron-Zinc Double Transmetallation

Bis(o-phosphinophenyl)zinc derivatives were successfully synthesized by the reaction of o-phosphinophenylboronates with dimethylzinc via boron-zinc double transmetallation. The transmetallation was significantly accelerated by the presence of the ortho PR2 substituent to...

Stannylenes based on pyrrole-phosphane and dipyrromethane-diphosphane scaffolds: syntheses and behavior as precursors to PSnP pincer palladium(II), palladium(0) and gold(I) complexes

2-Ditertbutylphosphanylmethylpyrrole (H₂pyrmP^tBu₂) and 2,2'-bis(diisopropylphosphanylmethyl)-5,5'-dimethyldipyrromethane ((HpyrmPⁱPr₂)₂CMe₂) have been used to synthesize new P-donor-stabilized stannylenes in which the Sn atom is attached to one, SnCl(HpyrmP^tBu₂) (1) and Sn{N(SiMe₃)₂}(HpyrmP^tBu₂) (2), or two pyrrolyl-phosphane scaffolds, Sn(HpyrmP^tBu₂)₂ (3), or to a dipyrromethane-diphosphane scaffold, Sn(pyrmPⁱPr₂)₂CMe₂ (4). It has been found that stannylenes 3 and 4 are excellent precursors to transition metal complexes containing PSnP pincer-type ligands. Their reactions with chlorido transition metal complexes have afforded [PdCl{κ³P,Sn,P-SnCl(HpyrmP^tBu₂)₂}] (6), [PdCl{κ³P,Sn,P-SnCl(pyrmPⁱPr₂)₂CMe₂}] (7) and [Au{κ³P,Sn,P-SnCl(HpyrmP^tBu₂)₂}] (8), which contain a PSnP pincer-type chloridostannyl ligand. While complexes 6 and 7 are square-planar palladium(II) complexes, compound 8 is an uncommon gold(I) complex having a T-shaped coordination geometry with a very long Sn-Au bond (3.120 Å). The T-shaped palladium(0) complex [Pd{κ³P,Sn,P-Sn(pyrmPⁱPr₂)₂CMe₂}] (9), which contains an unprecedented PSnP pincer-type stannylene that behaves as a Z-type (σ-acceptor) ligand, has been prepared from 4 and [Pd(η³-C₃H₅)(η⁵-C₅H₅)].

Alternative Conceptual Approach to the Design of Bifunctional Catalysts: An Osmium Germylene System for the Dehydrogenation of Formic Acid

The reaction of the hexahydride OsH₆(PⁱPr₃)₂ with a P,Ge,P-germylene-diphosphine affords an osmium tetrahydride derivative bearing a Ge,P-chelate, which arises from the hydrogenolysis of a P–C(sp³) bond. This Os(IV)–Ge(II) compound is a pioneering example of a bifunctional catalyst based on the coordination of a σ-donor acid, which is active in the dehydrogenation of formic acid to H₂ and CO₂. The kinetics of the dehydrogenation, the characterization of the resting state of the catalysis, and DFT calculations point out that the hydrogen formation (the fast stage) exclusively occurs on the coordination sphere of the basic metal center, whereas both the metal center and the σ-donor Lewis acid cooperatively participate in the CO₂ release (the rate-determining step). During the process, the formate group pivots around the germanium to approach its hydrogen atom to the osmium center, which allows its transfer to the metal and the CO₂ release.

An alternative class of bifunctional catalysts can be assembled by coordination of σ-donor Lewis acids to platinum-group-metal basic fragments. In contrast to what happens with the previously reported bifunctional catalysts, this design allows enhancing the basicity of the base and the acidity of the acid. According to this, a bifunctional catalyst for the dehydrogenation of formic acid, based on an osmium(IV)-germylene cooperative system, has been prepared and the mechanism of the catalysis established.

The Elusive Au(I)···H-O Hydrogen Bond: Experimental Verification

Our long-standing interest in atypical bonding situations has recently led us to target complexes in which a metallobasic gold(I) center is hydrogen-bonded to a nearby OH functionality. Here, we report on the synthesis and characterization of two neutral gold(I) indazol-3-ylidene complexes bearing a carbinol or silanol group at the 4-position. As indicated by X-ray diffraction, ¹H NMR spectroscopy, IR spectroscopy, and extensive computational modeling, the OH group of these derivatives is engaged in a bona fide Au···H-O interaction. In addition to shedding light on an elusive bonding situation, these results also indicate that increasing the acidity of the OH functionality is not necessarily beneficial to the stability of the Au(I)···H-O interaction.

Optimizing Catalyst and Reaction Conditions in Gold(I) Catalysis-Ligand Development

This review considers phosphine and N-heterocyclic carbene complexes of gold(I) that are used as (pre)catalysts for a range of reactions in organic synthesis. These are divided according to the structure of the ligand, with the narrative focusing on studies that offer a quantitative comparison between the ligands and readily available or widely used existing systems.

Synthetic Strategy for Construction of Highly Congested Tetracyclic Core (6-5-7-4) of Harziane Diterpenoids

The structurally intriguing tetracyclic core of complex harziane diterpenoid was constructed in 14 steps from commercially available 3-ethoxycyclohex-2-en-1-one. The key steps were a Mn/Cu-mediated oxidative 1,3-dicarbonyl radical cascade cyclization reaction, which diastereoselectively formed the core of dimethylbicyclo[3.2.1]octane structure, and a Au-catalyzed diastereoselective formal [2 + 2] cycloaddition for construction of the harziane diterpenoid tetracyclic framework. The developed method paves the way for achieving total synthesis of this type of complex natural product.

[Development of Gold-catalyzed Reaction Utilizing Electron Acceptability of Z-type Ligand]

The interaction between transition metals and ligands is important for catalytic reactions. The ligands are largely dominated by the covalent X-type (hydride, alkyl and halogen) and/or dative L-type ligands (e.g., P, N, CO, olefin, etc.). Therefore, the interaction of the Z-type ligands (B, Al and Si, etc.) with transition metals is emerging as a new concept for the reactivity of the metal center. Recently, we developed the synthesis of the gold complex Au(DPB)X (DPB=diphosphine-borane) featuring the Z-type ligand, and their catalytic reaction. The gold catalysts showed a high activity compared to the general catalysts (without Z-ligand) for the various cyclization reactions due to the electron-withdrawing effect of the Z-ligand on the coordinating gold center. In this review, first the structure analysis of the synthesized Au→Z complex is introduced in detail, and second, the catalytic reactions based on the alkyne activation are described.

Metal-metal multiple bond formation induced by σ-acceptor Lewis acid ligands

The reaction of [Cp*Ru(μ-NHPh)]2 (Cp* = η5-C5Me5) with Lewis acids of the type MX2 (M = Zn, Sn, Pb; X = Cl, OTf) affords Ru2 → M donor-acceptor adducts characterized as π complexes of a Ru[double bond, length as m-dash]Ru double bond with M(ii) Lewis acids. The results illustrate for the first time the ability of σ-acceptor Lewis acid ligands to induce the formation of a metal-metal multiple bond via stabilizing dative interactions.

Ligand-enforced intimacy between a gold cation and a carbenium ion: impact on stability and reactivity

Controlling the reactivity of transition metal complexes by positioning non-innocent functionalities around the catalytic pocket is a concept that has led to significant advances in catalysis. Here we describe our efforts toward the synthesis of dicationic phosphine gold complexes of general formula [(o-Ph₂P(C₆H₄)Carb)Au(tht)]²⁺ decorated by a carbenium moiety (Carb) positioned in the immediate vicinity of the gold center. While the most acidic examples of such compounds have limited stability, the dicationic complexes with Carb⁺ = 9-N-methylacridinium and Carb⁺ = [C(Ar^N)₂]⁺ (Ar^N = p-(C₆H₄)NMe₂) are active as catalysts for the cycloisomerization of N-propargyl-4-fluorobenzamide, a substrate chosen to benchmark reactivity. The dicationic complex [(o-Ph₂P(C₆H₄)C(Ar^N)₂)Au(tht)]²⁺, which also promotes hydroarylation and enyne cyclization reactions, displays a higher catalytic activity than its acridinium analog, indicating that the electrophilic reactivity of these complexes scales with the Lewis acidity of the carbenium moiety. These results support the role of the carbenium unit as a non-innocent functionality which can readily enhance the activity of the adjacent metal center. Finally, we also describe our efforts toward the generation and isolation of free γ-cationic phosphines of general formula [(o-Ph₂P(C₆H₄)Carb)]⁺. While cyclization into phosphonium species is observed for Carb⁺ = [C(Ar^N)₂]⁺, [C(Ph)(Ar^N)]⁺, and 9-xanthylium, [(o-Ph₂P(C₆H₄)-9-N-methylacridinium)]⁺ can be isolated as an air stable, biphilic derivative with uncompromised Lewis acidic and basic properties.

This work describes the synthesis of carbenium-based, γ-cationic phosphines and their coordination to Au(i) cations , leading to carbophilic catalysts whose activity is enhanced by the ligand-enforced convergence of the positively charged moieties.

Tunable carbocation-based redox active ambiphilic ligands: synthesis, coordination and characterization

The synthesis of novel redox active ambiphilic ligands L1-L3 and their coordination chemistry to first-row late transition metal halides (M = Co and Ni) is reported. The heterocyclic carbocation scaffolds act as Lewis acid moieties while the pyridine anchor acts as the coordinating Lewis base. The high synthetic tunability of this ligand scaffold allows for control of its rigidity and electronic properties. Anion exchange and coordination of a chloride anion to the metal center was observed resulting in the formation of [MCl3]- metallate. Upon coordination to the pyridine anchor, the metallate centers adopt a canonical tetrahedral geometry, resulting in an overall neutral complex best described as a zwitterionic metallate trichloride bound to a cationic ligand. Characterization techniques including single crystal X-ray diffraction, cyclic voltammetry, and UV-Vis absorption spectroscopy were employed to better understand the structural and chemical properties of the ligands and metal complexes. A possible weak interaction between one of the chlorides and the carbenium moiety in the ligand is observed in crystals of both of the Co(ii) and Ni(ii) complexes with ligand L1. Density functional theory (DFT) calculations support that this electrostatic interaction for complexes 2a and 2b exists only in the solid state.

Gold-Catalyzed Synthesis of Small Rings

Three- and four-membered rings, widespread motifs in nature and medicinal chemistry, have fascinated chemists ever since their discovery. However, due to energetic considerations, small rings are often difficult to assemble. In this regard, homogeneous gold catalysis has emerged as a powerful tool to construct these highly strained carbocycles. This review aims to provide a comprehensive summary of all the major advances and discoveries made in the gold-catalyzed synthesis of cyclopropanes, cyclopropenes, cyclobutanes, cyclobutenes, and their corresponding heterocyclic or heterosubstituted analogs.

Evaluation of Pd→B Interactions in Diphosphinoborane Complexes and Impact on Inner-Sphere Reductive Elimination

The dative Pd→B interaction in a series of R DPBR' Pd0 and PdII complexes (R DPBR' =(o-PR2 C6 H4 )2 BR', diphosphinoborane) was analyzed using XRD, 11 B NMR spectroscopy and NBO/NLMO calculations. The borane acceptor discriminates between the oxidation state PdII and Pd0 , stabilizing the latter. Reaction of lithium amides with [(R DPBR' )PdII (4-NO2 C6 H4 )I] chemoselectively yields the C-N coupling product. DFT modelling indicates no significant impact of PdII →B coordination on the inner-sphere reductive elimination rate.

Gold-Catalyzed Synthetic Strategies towards Four-Carbon Ring Systems

Four carbon ring systems are frequently present in natural products with remarkable biological activities such as terpenoids, alkaloids, and steroids. The development of new strategies for the assembly of these structures in a rapid and efficient manner has attracted the interest of synthetic chemists for a long time. The current research is focused mainly on the development of synthetic methods that can be performed under mild reaction conditions with a high tolerance to functional groups. In recent years, gold complexes have turned into excellent candidates for this aim, owing to their high reactivity, and are thus capable of promoting a wide range of transformations under mild conditions. Their remarkable efficiency has been thoroughly demonstrated in the synthesis of complex organic molecules from simple starting materials. This review summarizes the main synthetic strategies described for gold-catalyzed four-carbon ring formation, as well as their application in the synthesis of natural products.

Catalysis using transition metal complexes featuring main group metal and metalloid compounds as supporting ligands

Recent development in catalytic application of transition metal complexes having an M-E bond (E = main group metal or metalloid element), which is stabilized by a multidentate ligand, is summarized. Main group metal and metalloid supporting ligands furnish unusual electronic and steric environments and molecular functions to transition metals, which are not easily available with standard organic supporting ligands such as phosphines and amines. These characteristics often realize remarkable catalytic activity, unique product selectivity, and new molecular transformations. This perspective demonstrates the promising utility of main group metal and metalloid compounds as a new class of supporting ligands for transition metal catalysts in synthetic chemistry.

Fluorosilane Activation by Pd/Ni→Si-F→Lewis Acid Interaction: An Entry to Catalytic Sila-Negishi Coupling

A new mode of bond activation involving M→Z interactions is disclosed. Coordination to transition metals as σ-acceptor ligands was found to enable the activation of fluorosilanes, opening the way to the first transition-metal-catalyzed Si-F bond activation. Using phosphines as directing groups, sila-Negishi couplings were developed by combining Pd and Ni complexes with external Lewis acids such as MgBr₂. Several key catalytic intermediates have been authenticated spectroscopically and crystallographically. Combined with DFT calculations, all data support cooperative activation of the fluorosilane via Pd/Ni→Si-F→Lewis acid interaction with conversion of the Z-type fluorosilane ligand into an X-type silyl moiety.

Activation of a Metal-Halogen Bond by Halogen Bonding

In recent years, the non-covalent interaction of halogen bonding (XB) has found increasing application in organocatalysis. However, reports of the activation of metal-ligand bonds by XB have so far been limited to a few reactions with elemental iodine or bromine. Herein, we present the activation of metal-halogen bonds by two classes of inert halogen bond donors and the use of the resulting activated complexes in homogenous gold catalysis. The only recently explored class of iodolium derivatives were shown to be effective activators in two test reactions and their activity could be modulated by blocking of the Lewis acidic sites. Bis(benzimidazolium)-based halogen bonding activators provided even more rapid conversion, while the non-iodinated reference compound showed little activity. The role of halogen bonding in the activation of metal-halogen bonds was further investigated by NMR experiments and DFT calculations, which support the mode of activation occurring via halogen bonding.

Synthesis and Photophysical Properties of T-Shaped Coinage-Metal Complexes

The photophysical properties of a series of T‐shaped coinage d¹⁰ metal complexes, supported by a bis(mesoionic carbene)carbazolide (CNC) pincer ligand, are explored. The series includes a rare new example of a tridentate T‐shaped Ag^I complex. Post‐complexation modification of the Au^I complex provides access to a linear cationic Au^I complex following ligand alkylation, or the first example of a cationic square planar Au^III−F complex from electrophilic attack on the metal centre. Emissions ranging from blue (Cu^I) to orange (Ag^I) are obtained, with variable contributions of thermally‐dependent fluorescence and phosphorescence to the observed photoluminescence. Green emissions are observed for all three gold complexes (neutral T‐shaped Au^I, cationic linear Au^I and square planar cationic Au^III). The higher quantum yield and longer decay lifetime of the linear gold(I) complex are indicative of increased phosphorescence contribution.

Rhodium-Catalyzed C-H Activation Enabled by an Indium Metalloligand

Rhodium complexes with an indium metalloligand were successfully synthesized by utilizing a pyridine-tethered cyclopentadienyl ligand as a support for an In-Rh bond. The indium metalloligand dramatically changes the electronic and redox properties of the rhodium metal, thereby enabling catalysis of sp² C-H bond activation.

Lewis Acid Transition-Metal-Catalyzed Hydrogen Activation: Structures, Mechanisms, and Reactivities

As a new type of bifunctional catalyst, the Lewis acid transition-metal (LA-TM) catalysts have been widely applied for hydrogen activation. This study presents a mechanistic framework to understand the LA-TM-catalyzed H₂ activation through DFT studies. The mer(trans)-homolytic cleavage, the fac(cis)-homolytic cleavage, the synergetic heterolytic cleavage, and the dissociative heterolytic cleavage should be taken as general mechanisms for the field of LA-TM catalysis. Four typical LA-TM catalysts, the Z-type κ⁴ -L₃ B-Rh complex tri(azaindolyl)borane-Rh, the X-type κ³ -L₂ B-Co complex bis-phosphino-boryl (PBP)-Co, the η² -BC-type κ³ -L₂ B-Pd complex diphosphine-borane (DPB)-Pd, and the Z-type κ² -LB-Pt complex (boryl)iminomethane (BIM)-Pt are selected as representative models to systematically illustrate their mechanistic features and explore the influencing factors on mechanistic variations. Our results indicate that the tri(azaindolyl)borane-Rh catalyst favors the synergetic heterolytic mechanism; the PBP-Co catalyst prefers the mer(trans)-homolytic mechanism; the DPB-Pd catalyst operates through the fac(cis)-homolytic mechanism, whereas the BIM-Pt catalyst tends to undergo the dissociative heterolytic mechanism. The mechanistic variations are determined by the coordination geometry, the LA-TM bonding nature, the electronic structure of the TM center, and the flexibility or steric effect of the LA ligands. The presented mechanistic framework should provide helpful guidelines for LA-TM catalyst design and reaction developments.