Cationic Gold Species Research Articles

Enantioselective Gold(I) Catalysis with Chiral Monodentate Ligands

AbstractEnantioselective gold(I) catalysis is receiving a growing level of credit in the field of asymmetric synthesis. After a pioneering work in the mid‐1980s the area remained almost silent for several years, until densely substituted diphosphine chiral ligands were disclosed as competent chiral units for gold‐catalyzed stereoselective manipulation of unactivated unsaturated hydrocarbons. The chemistry of alkynes, allenes and most recently also alkenes received significant benefits from the latter developments. More recently, ad hoc designed chiral monodentate ligands have risen to prominence in producing robust, highly tunable (electronic and steric) and efficient chiral cationic mononuclear gold species for asymmetric transformations. In this scenario, electron‐deficient phosphorus‐based (i.e., phosphoramidites, phosphites) but also axially chiral electron‐rich carbene ligands deserve particular mention and the corresponding applications will be summarized herein.

Mechanistic Investigations of a Stable, Highly Active, Extremely Sterically Shielded Molecular Gold Catalyst

AbstractAn N‐heterocyclic carbene gold complex IPr**AuNTf2 has been synthesized, spectroscopically investigated, structurally characterized, and used as a highly active and stable catalyst in the Hashmi phenol synthesis (IPr**=1,3‐di‐p‐tolylimidazol‐2‐ylidene with four di‐tert‐butylbenzhydryl ortho substituents, Tf=trifluoromethansulfonyl). A side reaction comprises an irreversible arene oxide ring opening with subsequent 1,2 methyl shift. The advantage of the steric demand of the ancillary ligand was explained by higher equilibrium concentrations of the cationic gold species, the circumvention of inactive dinuclear intermediates, and the inhibition of catalyst decomposition pathways. The methanol addition–hydration of alkynes featured a turnover‐limiting proton transfer step of an alkenylgold catalyst resting state, indicated by a large primary kinetic isotope effect and an alkyne competition experiment.

Development of a Gold-Multifaceted Catalysis Approach to the Synthesis of Highly Substituted Pyrroles: Mechanistic Insights via Huisgen Cycloaddition Studies

A novel gold-catalyzed method for the regioselective synthesis of highly substituted pyrroles directly from oximes and alkynes was developed via independent optimization of two key steps of the process. Importantly, a cationic gold(I) species was shown to activate multiple steps along the reaction pathway and therefore act as a multifaceted catalyst. Initial gold-promoted addition of the oxime oxygen to the activated alkyne afforded the O-vinyloxime in situ. The O-vinyloxime was subsequently transformed into the pyrrole via a gold-catalyzed tautomerization, [3,3]-sigmatropic rearrangement, and cyclodehydration process. Notably, this method provides a functional group handle in the form of an ester at the 3/4-position for further exploitation. The proposed mechanistic pathway is supported by a novel application of the Huisgen cycloaddition click reaction, which was used to probe the relative stability of substituted O-vinyloximes. The intermediacy of N-alkenylhydroxylamine O-vinyl ethers and imino ketones or imino aldehydes along the reaction pathway were determined by high-temperature (1)H, (2)H{(1)H}, and (13)C{(1)H} NMR experiments. X-ray crystallographic evidence was used to further support the mechanistic hypothesis.

The effect of alkali metal on the surface properties of potassium doped Au-Beta zeolites

Beta zeolite was applied as support for gold introduced by gold-precipitation method and potassium added by impregnation or adsorption. The effect of zeolite composition and the amount of potassium introduced on the surface properties of the final materials was considered. Moreover, the interaction of gold and potassium species was found to be related to the adsorptive and catalytic behaviour of zeolites in NO reduction with propene and deodorization. K/Au-Beta(Impregnated) exhibits the best performance in the above mentioned processes because of the small gold particles (between 2 and 5nm) and interaction of gold with potassium species leading to the change of electronic properties of the surface (the appearance of cationic gold species). Potassium added as a promoter improves the catalytic properties of Au-zeolite in N2O decomposition and also in deodorization (increase of the ability to dibutyl sulphide oxidation). The catalysts prepared were characterized by XRD, XPS, UV–vis, TEM, pyridine adsorption combined with FTIR and test reaction (2-propanol transformation).

Physicochemical characterization of Au/CeO2 solids. Part 2: The impregnation preparation method

Au/CeO2 solids with different gold contents were prepared using the impregnation method. Electron microscopy (SEM and TEM) studies indicated the formation of both nanoparticles and large gold particles on the surface of the ceria support. SEM and XRD analyses revealed that the number and size of large particles increases with the gold content in the solid. The XPS technique showed that 90% of the total gold is in the metallic form Au0 while the remaining 10% were cationic gold species Au+. These latter were formed following calcination under dry air at 400 °C and are located in the proximity of the O2− and/or Cl− present on the support. These Au+ species are present at the edge of gold particles and they were reduced into metallic gold when the solid was vacuum treated (5–7.10−4 mbar) at 400 °C for 1 h. When air was adsorbed at room temperature on the latter vacuum treated solids, two EPR signals were obtained. The first one was assigned to O2− species whereas attributing the second signal was difficult and required a more detailed investigation that will be presented in a forthcoming work.

ChemInform Abstract: Gold(I)‐Catalyzed Synthesis of Furans and Pyrroles via Alkyne Hydration.

AbstractA variety of symmetric and asymmetric diynes is converted to furans when treated according to A).

Library-friendly synthesis of fluorinated ketones through functionalized hydration of alkynes and investigation of the reaction mechanism

A library-friendly synthesis of α-substituted-α-fluoroketones through functionalized hydration of alkynes in one pot under mild conditions is reported. The ready availability of alkynes and organoboronic acids makes this reaction quite attractive. We also studied the key intermediate—a fluorinated cationic gold species—using in situ NMR spectroscopy and ESI-high resolution mass spectrometry.

Influence of Au particle size on Au/CeO 2 catalysts for CO oxidation

The Au particle size dependence of CO oxidation on Au/CeO 2 catalysts was extensively investigated. By keeping the size of CeO 2 particle to be about 12 nm, the increasing size of Au particle from 3.9 to 7.5 nm caused significant decrease in the intrinsic activity for CO oxidation. Comparative study on sodium cyanide leaching, which effectively removed the metallic gold particles but retained the cationic gold clusters in the matrix of ceria, revealed that the cationic gold species could catalyze CO oxidation only at higher temperatures, confirming the major contribution of the metallic gold particles in the Au/CeO 2 system. Therefore, it is evident that the Au–CeO 2 interface determined mainly by the size of Au particle plays an essential role in achieving the high reactivity.

Total CO oxidation over Fe-containing Au/HMS catalysts: Effects of gold loading and catalyst pretreatment

In this work we report the effects of catalyst pretreatment (oxidation versus reduction) and gold loading on the catalytic response of Au/HMS–Fe catalysts for the CO oxidation reaction. Hexagonal mesoporous silica (HMS) substrates modified with Fe3+ ions (Si/Fe molar ratio of 40) were prepared via a neutral S0I0 templating route employing one-pot synthesis, whereas the supported gold catalysts with different Au concentrations were prepared by the deposition–precipitation method. The activity was measured by temperature-programmed reaction tests in the 20–400°C range, and the catalysts were characterized by different techniques (SBET, XRD, H2-TPR, DRS UV–vis, XPS and DRIFTS). It was found that catalytic activity was a function of Au-loading being the most active the catalyst with the highest Au loading (5.1wt%). The light-off curves showed a positive effect on the catalytic activity by a reduction treatment prior to performing the catalytic reaction. From DRIFTS study of co-adsorption of CO and O2, it was concluded that the cationic gold species were the active phase in CO oxidation reaction. From the discussion of activity and characterization results it is proposed a reaction path where CO oxidation reaction takes place at the interface between gold cationic species and the iron sites.

ChemInform Abstract: Efficient Silver‐Free Gold(I)‐Catalyzed Hydration of Alkynes at Low Catalyst Loading.

Abstract The use of [(IPr)AuOH] as versatile, air- and moisture-stable pre-catalyst permits the in situ generation of the cationic gold(I) species [(IPr)Au]X after reaction with a Bronsted acid. This catalytic system presents as a main advantage the lack of use of a silver salt activator or co-catalyst which is often air-, light- and moisture-sensitive. A general gold(I)-catalyzed procedure using this in situ activation at very low catalyst loading is reported for the hydration of a broad range of internal and terminal alkynes.

Gold(i)-catalyzed synthesis of furans and pyrroles via alkyne hydration

Furans and pyrroles were prepared via the gold(I)-catalyzed alkyne hydration of diynes. The use of [Au(IPr)OH] as precatalyst in a silver-free protocol permits low catalyst loadings and in situ generation of the active cationic gold species. A detailed computational study confirmed the experimental results and supports the proposed mechanism.

Gold(I)-Catalyzed Intramolecular Tandem Addition/Friedel−Crafts Reactions between Acyclic Enamides and 1-Arylalkynes

Electron-deficient acyclic enamine derivatives react with electron-rich 1-arylalkynes using cationic gold(I) species as catalysts in an intramolecular process to form annulated 1-amido-substituted indene derivatives as the major products. Yields for this process range between 21% and 98%. In some cases, a two-step process that includes a subsequent alkene isomerization is needed.

Efficient silver-free gold(I)-catalyzed hydration of alkynes at low catalyst loading

The use of [(IPr)AuOH] as versatile, air- and moisture-stable pre-catalyst permits the in situ generation of the cationic gold(I) species [(IPr)Au]X after reaction with a Brønsted acid. This catalytic system presents as a main advantage the lack of use of a silver salt activator or co-catalyst which is often air-, light- and moisture-sensitive. A general gold(I)-catalyzed procedure using this in situ activation at very low catalyst loading is reported for the hydration of a broad range of internal and terminal alkynes.

Gold-Catalyzed Synthesis of Pyrroles and Furans

Reported is an efficient and rapid synthesis of furans 2a and pyrroles 2b from readily available 3-alkyne-1,2-diols 1a and amino-3-alkyn-2-ols 1b under combined gold and silver catalysis. Interestingly, the intramolecular cyclization fails when gold and silver compounds alone are used. The optimized conditions were applied to various alkynyldiols 1a using [(PPh3)AuCl-AgNTf2] for the synthesis of furans 2a and amino-3-alkyn-2-ols 1b using [(PPh3)AuCl-AgNOTf] for the preparation of di- and monosubstituted pyrroles 2b. The reaction is suggested to proceed via a coordination of the acetylene bond to cationic gold species (enhancing the electrophilicity) followed by a 5-endo cyclization and finally dehydration. It was observed that the reactivity of the diastereoisomers (syn- and anti-3) is slightly different.

CO oxidation on Au/FePO 4 catalyst: Reaction pathways and nature of Au sites

In situ FTIR spectroscopy coupled with downstream mass spectrometry has been used to clarify the pathways for room temperature (rt) CO oxidation over iron phosphate-supported Au catalyst. The charge state of Au on Au/FePO 4 after calcination, reduction, or under reaction conditions was assessed by both FTIR spectroscopy (CO probing) and X-ray absorption near edge spectroscopy (XANES). Results from both approaches show that cationic gold species dominate the surface after pretreatment in O 2 at 200 °C. A portion of the cationic gold on Au/FePO 4 can be reduced by the initial CO adsorption at rt, and subsequently repeated CO exposures do not reduce the remaining cationic Au. FTIR and Raman results from cycled CO reduction and O 2 reoxidation of Au/FePO 4 indicate that there are active structural oxygen species on the surface of Au/FePO 4that can be consumed by CO and then replenished by gaseous O 2 at rt. Au activates both CO and O 2 so that the FePO 4 support can undergo reduction (by CO) and reoxidation (by O 2) cycles. The results of CO oxidation with labeled 18O 2 suggest the operation of two parallel reaction pathways at rt: (1) a redox pathway in which FePO 4 supplies active oxygen and (2) a direct pathway on metallic Au, via either Langmuir–Hinshelwood or Eley–Rideal mechanism, in which gas phase O 2 provides the active oxygen.

Gold(I)-catalyzed Tandem Cyclization of 3-(tert-Butoxycarbonyloxy)-1,6-enynes

The capability of cationic gold(I) species that activates alkynes towards heteroatom and carbon nucleophiles led to the dis-covery of numerous novel catalytic transformations. Diverse structures generated by these reactions make this process a very powerful strategy for organic synthesis. Tandem reactions of 1,n-enyne substrates are particularly interesting in this regard.

Gold nanoparticles supported on manganese oxides for low-temperature CO oxidation

A series of gold catalysts deposited on various single-phase manganese oxides (MnOx, including MnO2, Mn2O3 and Mn3O4) were studied in relation to their performance in the low-temperature oxidation of CO. Gold was deposited onto the MnOx support by deposition–precipitation with urea (DP urea). The activity of the Au/MnOx catalysts for CO oxidation strongly depends on the nature of the manganese oxide support, ranking in the order Au/Mn2O3>Au/MnO2>Au/Mn3O4. Transmission electron microscopy (TEM) results revealed different particle size distributions of gold for these Au-based catalysts, among which the Au/Mn2O3 catalyst showed the highest gold dispersion. Temperature programmed reduction (TPR) experiments indicated that the reducibilities of Au/MnOx catalysts follow the same sequence with the activity. X-ray photoelectron spectroscopy (XPS) results suggested that both metallic and cationic gold species existed in the Au/Mn2O3 catalyst, while only metallic gold species were present in the other catalysts. The unique surface redox properties of Mn2O3 show great superiority for anchoring and dispersing of gold nanoparticles, which in turn leads to a higher reducibility of Mn2O3 and activity of the Au/Mn2O3 material for CO oxidation.

Influence of Charge State on the Mechanism of CO Oxidation on Gold Clusters

We present results from our joint experimental and theoretical study of the reactivity of anionic and cationic gold oxide clusters toward CO, focusing on the role of atomic oxygen, different charge states, and mechanisms for oxidation. We show that anionic clusters react by an Eley-Rideal-like mechanism involving the preferential attack of CO on oxygen rather than gold. In contrast, the oxidation of CO on cationic gold oxide clusters can occur by both an Eley-Rideal-like and a Langmuir-Hinshelwood-like mechanism at multiple collision conditions as a result of the high adsorption energy of two CO molecules. This large energy of CO adsorption on cationic gold oxide clusters is the driving force for the CO oxidation. Therefore, in the presence of cationic gold species at high pressures of CO, the oxidation reaction is self-promoting (i.e., the oxidation of one CO molecule is promoted by the binding of a second CO). Our findings provide new insight into the role of charge state in gold-cluster-based nanocatalysis.

Catalysis by gold dispersed on supports: the importance of cationic gold

There are many examples of catalysis in solution by cationic complexes of gold, and recent results, reviewed here in this critical review, demonstrate that cationic gold species on oxide and zeolite supports are also catalytically active, for reactions including ethylene hydrogenation and CO oxidation. The catalytically active gold species on supports are evidently not restricted to isolated mononuclear gold complexes, but include gold clusters, which for at least some reactions are more active than the mononuclear complexes and for some reactions less active. Fundamental questions remain about the nature of cationic gold in supported catalysts, such as the nature of the cationic gold clusters and the nature of gold atoms at metal-support interfaces (88 references).

Heterogeneous Gold‐Catalysed Synthesis of Phenols

AbstractNanoparticles of gold supported on nanocrystalline CeO2 catalyse the isomerisation of ω‐alkynylfurans to phenols. Initial leaching of gold was observed, which could be minimised by calcining. Subsequent runs showed that once all soluble species had leached, the surface‐bound, cationic gold species is still active and can reach turnover numbers of up to 391. This is the first time that a heterogeneous gold catalyst showed activity in the gold‐catalysed phenol synthesis.