Cationic Bis(Gold) Indenyl Complexes

Reaction of (P)AuOTf [P=P(t-Bu)2o-biphenyl] with indenyl- or 3-methylindenyl lithium led to isolation of gold η1-indenyl complexes (P)Au(η1-inden-1-yl) (1 a) and (P)Au(η1-3-methylinden-1-yl) (1 b), respectively, in >65 % yield. Whereas complex 1 b is static, complex 1 a undergoes facile, degenerate 1,3-migration of gold about the indenyl ligand (ΔG≠ 153K=9.1±1.1 kcal/mol). Treatment of complexes 1 a and 1 b with (P)AuNTf2 led to formation of the corresponding cationic bis(gold) indenyl complexes trans-[(P)Au]2(η1,η1-inden-1,3-yl) (2 a) and trans-[(P)Au]2(η1,η2-3-methylinden-1-yl) (2 b), respectively, which were characterized spectroscopically and modeled computationally. Despite the absence of aurophilic stabilization in complexes 2 a and 2 b, the binding affinity of mono(gold) complex 1 a toward exogenous (P)Au+ exceed that of free indene by ~350-fold and similarly the binding affinity of 1 b toward exogenous (P)Au+ exceed that of 3-methylindene by ~50-fold. The energy barrier for protodeauration of bis(gold) indenyl complex 2 a with HOAc was ≥8 kcal/mol higher than for protodeauration of mono(gold) complex 1 a.

Energetics of key Au(III)-substrate adducts relevant to catalytic hydroarylation of alkynes

(P,C)-cyclometalated Au(III) complexes have shown remarkable ability to catalyze the intermolecular hydroarylation of alkynes. Evidence of an outer-sphere mechanism has been provided in a previous study and is confirmed here by analysing the experimental data and DFT calculations. In this work, we propose evaluation of critical energies of dissociation of Au(III) complexes with different substrates via energy-resolved mass spectrometry (ERMS) experiments and kinetic modelling. The kinetic model is based on a multi-collisional approach. On the one hand, the classification confirms the mechanism previously proposed; on the other hand, it supports the collisional model and its application to particularly fragile adducts.

Isolation and reactivity of a gold(I) hydroxytrifluoroborate complex stabilized by anion-π+ interactions

A 9,9-dimethylxanthene-based ligand substituted at the 4- and 5-positions by a phosphine and a xanthylium unit, respectively, has been prepared and converted into an AuCl complex, the structure of which reveals an intramolecular Au-Cl⋯π⁺ interaction. This new ligand platform was also found to support the formation of an unprecedented hydroxytrifluoroborate derivative featuring a "hard/soft" mismatched Au-μ(OH)-BF₃ motif. Despite its surprising stability, this gold hydroxytrifluoroborate complex is a remarkably potent carbophilic catalyst which readily activates alkynes, without activator.

[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.

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.

Synthesis, Structure, Reactivity and Catalytic Implications of a Cationic, Acetylide-Bridged Trigold-JohnPhos Species

The cationic complex [(JohnPhos-Au)₃ (acetylide)][SbF₆ ] (JohnPhos=(2-biphenyl)di-tert-butylphosphine, L1) has been characterised structurally and features an acetylide-trigold(I)-JohnPhos system; the trinuclear-acetylide unit, coordinated to the monodentate bulk phosphines, adopts an unprecedented μ,η¹ ,η² ,η¹ coordination mode with an additional interaction between distal phenyl rings and gold centres. Other cationic σ,π-[(gold(I)L1)₂ ] complexes have also been isolated. The reaction of trimethylsilylacetylene with various alcohols (iPrOH, nBuOH, n-HexOH) catalysed by cationic [Au^I L1][SbF₆ ] complexes in CH₂ Cl₂ at 50 °C led to the formation of acetaldehyde acetals with a high degree of chemo- and regioselectivity. The reaction mechanism was studied, and several organic and inorganic intermediates have been characterised. A comparative study with the analogous cationic [Cu^I L1][PF₆ ] complex revealed different behaviour; the copper metal is lost from the coordination sphere leading to the formation of cationic vinylphosphonium and copper nanoparticles. Additionally, a new catalytic approach for the formation of this high-value cationic vinylphosphonium has been established.

Dinuclear gold(i) complexes: from bonding to applications

The last two decades have seen a veritable explosion in the use of gold(i) complexes bearing N-heterocyclic carbene (NHC) and phosphine (PR₃) ligands.

Structures and bonding properties of CPt2 -/0 and CPt2H-/0: Anion photoelectron spectroscopy and quantum chemical calculations

We present a combined anion photoelectron spectroscopic and quantum chemical investigation on the structures and bonding properties of CPt₂ ^-/0 and CPt₂H^-/0. The experimental vertical detachment energies of CPt₂ ^- and CPt₂H^- are measured to be 1.91 ± 0.08 and 3.54 ± 0.08 eV, respectively. CPt₂ ^- is identified as a C_2v symmetric Pt-C-Pt bent structure, and CPt₂ has a D_∞h symmetric Pt-C-Pt linear structure. Both anionic and neutral CPt₂H adopt a Pt-C-Pt-H chain-shaped structure, in which the ∠PtCPt and ∠CPtH bond angles of CPt₂H^- are larger than those of CPt₂H. The Pt-C bonds in CPt₂ ^-/0 and CPt₂H^-/0 exhibit covalent double bonding characters. The Pt=C bonds are much stronger than the C-H bond that may explain why the C atom CPt₂H^-/0 prefers to form Pt=C bonds rather than C-H bonds. It may also explain why platinum can insert into the C-H bond to activate the C-H bond as reported in the literature.

Gold-Catalyzed Aromatizations of 3-Ene-5-siloxy-1,6-diynes with Nitrosoarenes To Enable 1,4-N,O-Functionalizations: One-Pot Construction of 4-Hydroxy-3-aminobenzaldehyde Cores

This work describes gold-catalyzed aromatizations of 3-ene-5-siloxy-1,6-diynes with nitrosoarenes to form 4-hydroxy-3-aminobenzaldehyde derivatives, manifesting the use of nitrosoarenes as 1,4-N,O-functionalization sources. Various 3-ene-5-siloxy-1,6-diynes in benzoid and nonbenzoid types are applicable substrates. A series of ¹⁸O- and ²H-labeling experiments have been conducted to exclude gold-π-alkyne intermediates. We postulate a mechanism of dual gold catalysis involving initial formation of gold-π-alkynylgold species that activates a 1,5-hydrogen shift to form reactive 1,6-dipoles, thus furnishing intramolecular Michael-type reactions with nitrosonium electrophiles.

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. 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.

Direct Evidence for the Origin of Bis-Gold Intermediates: Probing Gold Catalysis with Mass Spectrometry

Gold-catalyzed alkyne hydration was studied by using in situ reacting mass spectrometry (MS) technology. By monitoring the reaction process in solution under different conditions (regular and very diluted catalyst concentrations, different pH values) and examining the reaction occurrence in the early reaction stage (1-2 ms after mixing) with MS, we collected a series of experimental evidence to support that the bis-gold complex is a potential key reaction intermediate. Furthermore, both experimental and computational studies confirmed that the σ,π-bis-gold complexes are not active intermediates toward nucleophilic addition. Instead, formation of geminally diaurated complex C is crucial for this catalytic process.

Bimetallic gold(i) complexes of photoswitchable phosphines: synthesis and uses in cooperative catalysis

The first photoswitchable bimetallic gold catalysts based on an azobenzene backbone have been synthesized and their catalytic properties have been investigated.

Role of σ,π-Digold(I) Alkyne Complexes in Reactions of Enynes

Gold(I) acetylide and σ,π-digold(I) alkyne complexes derived from one prototypical 1,6-enyne and from 7-ethynyl-1,3,5-cycloheptatriene have been prepared and structurally characterized. Their possible role in gold(I)-catalyzed cycloisomerizations has been studied by experiment and by DFT calculations. Gold(I) acetylides are totally unproductive complexes in the absence of Brønsted acids. Similarly, no cyclizations were observed by heating σ,π-digold(I) alkyne digold(I) at least up to 130 °C. Theoretical studies provide a rationale for the much lower reactivity of digold species in reactions of enynes.

Gas phase anion photoelectron spectroscopy and theoretical investigation of gold acetylide species

We conducted gas phase anion photoelectron spectroscopy and density functional theory studies on a number of gold acetylide species, such as AuC₂H, AuC₂Au, and Au₂C₂H. Based on the photoelectron spectra, the electron affinities of AuC₂H, AuC₂Au, and Au₂C₂H are measured to be 1.54(±0.04), 1.60(±0.08), and 4.23(±0.08) eV, respectively. The highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gaps of AuC₂H and AuC₂Au are measured to be about 2.62 and 2.48 eV, respectively. It is interesting that photoelectron spectra of AuC₂H^- and AuC₂Au^- display similar spectral features. The comparison of experimental and theoretical results confirms that the ground-state structures of AuC₂H^-, AuC₂Au^-, and their neutrals are all linear with Au-C≡C-H and Au-C≡C-Au configurations. The similar geometric structures, spectral features, HOMO-LUMO gaps, and chemical bonding between AuC₂H^-/0 and AuC₂Au^-/0 demonstrate that Au atom behaves like H atom in these species. The photoelectron spectrum of Au₂C₂H^- shows that Au₂C₂H has a high electron affinity of 4.23(±0.08) eV, indicating Au₂C₂H is a superhalogen. Further, we found an unusual similarity between the terminal Au atom of Au₂C₂H^- and the iodine atom of IAuC₂H^-.

Fluorescence resonance energy transfer (FRET) for the verification of dual gold catalysis

Two gold complexes with different bodipy-tagged N-heterocyclic carbene ligands, which are a potential FRET pair, were synthesized. It was shown, that the formation of dinuclear intermediates in alkyne transformations (“dual gold catalysis”) are characterized by a FRET signal.

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.

Bis-phosphine allene ligand: coordination chemistry and preliminary applications in catalysis

A 1,3-bis-diphenylphosphine allene provided new coordination complexes with palladium, platinum and gold salts. The dinuclear gold(i) complex gave promising results in asymmetric catalysis.

Reaction Intermediates Kinetics in Solution Investigated by Electrospray Ionization Mass Spectrometry: Diaurated Complexes

A new method to investigate the reaction kinetics of intermediates in solution by electrospray ionization mass spectrometry is presented. The method, referred to as delayed reactant labeling, allows investigation of a reaction mixture containing isotopically labeled and unlabeled reactants with different reaction times. It is shown that we can extract rate constants for the degradation of reaction intermediates and investigate the effects of various reaction conditions on their half-life. This method directly addresses the problem of the relevance of detected gaseous ions toward the investigated reaction solution. It is demonstrated for geminally diaurated intermediates formed in the gold mediated addition of methanol to alkynes. Delayed reactant labeling allows us to directly link the kinetics of the diaurated intermediates with the overall reaction kinetics determined by NMR spectroscopy. It is shown that the kinetics of protodeauration of these intermediates mirrors the kinetics of the addition of methanol demonstrating they are directly involved in the catalytic cycle. Formation as well as decomposition of diaurated intermediates can be drastically slowed down by employing bulky ancillary ligands at the gold catalyst; the catalytic cycle then proceeds via monoaurated intermediates. The reaction is investigated for 1-phenylpropyne (Ph-CC-CH3) using [AuCl(PPh3)]/AgSbF6 and [AuCl(IPr)]/AgSbF6 as model catalysts. Delayed reactant labeling is achieved by using a combination of CH3OH and CD3OH or Ph-CC-CH3 and Ph-CC-CD3.

Divergent Gold(I)-Catalyzed Skeletal Rearrangements of 1,7-Enynes

The gold(I) complex catalyzed cycloisomerization and skeletal rearrangement of 1,n-enynes (n=5-7) is a powerful methodology for the efficient synthesis of complex molecular architectures. In contrast to 1,6-enynes, readily accessible homologous 1,7-enynes are largely unexplored in such transformations. Here, the divergent skeletal rearrangement of all-carbon 1,7-enynes by catalysis with a cationic gold(I) complex is reported. Depending on electronic and steric factors, differently substituted 1,7-enynes react via different carbocations formed from a common gold carbene intermediate to yield on the one hand novel exocyclic allenes and on the other hand tricyclic hexahydro-anthracenes through a novel dehydrogenative Diels-Alder reaction.

Anatomy of gold catalysts: facts and myths

This review article covers the main types of gold(i) complexes used as precatalysts under homogeneous conditions in organic synthesis and discusses the different ways of catalyst activation as well as ligand, silver, and anion effects.

Catalytic intramolecular carbonyl–ene reaction with ketones: evidence for a retro–ene process

The ene and reverse ene-processes occur with ketones, as shown by control experiments and by ESI-MS analysis.

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

An air-stable cationic Au(I) complex featuring a Z-type ligand (boron atom) as a σ-acceptor was developed for elucidating the effect of B on catalytic reactions. An enyne cyclization in the presence of either [Au→B](+) or [Au](+) showed that [Au→B](+) promotes the reactivity, which enabled the effective construction of not only five- and six-membered rings, but also seven-membered rings.

Meeting the challenge of intermolecular gold(I)-catalyzed cycloadditions of alkynes and allenes

The development of gold(I)-catalyzed intermolecular carbo- and hetero-cycloadditions of alkynes and allenes has been more challenging than their intramolecular counterparts. Here we review, with a mechanistic perspective, the most fundamental intermolecular cycloadditions of alkynes and allenes with alkenes.