Gold-catalyzed Oxidation Research Articles

Gold-Catalyzed Oxidation Reactions of Thioalkynes with Quinoline N-Oxides: A DFT Study.

Mechanistic investigation of the gold-catalyzed oxidative reactions of thioalkynes with quinoline N-oxides was performed using density functional theory (DFT) calculations. For the oxidative rearrangement of thioalkynes with quinoline N-oxide to yield the same products, the Cβ-oxidation of thioalkynes was predicted to be competitive with Cα-oxidation, with the Cβ-oxidative process slightly more favorable. However, for the oxidative alkenylation of propargyl aryl thioethers with quinoline N-oxides, the Cβ-oxidation of thioether by quinoline N-oxide generated the product 3-hydroxy-1-alkylidene phenylthiopropan-2-one. Moreover, the ring opening of the four-membered sulfonium intermediate was achieved by the nucleophilic attack of quinoline N-oxide rather than a water molecule.

LOW-TEMPERATURE CATALYZED GROWTH AND PLASMONIC PROPERTIES OF COLUMNAR Au-SiOx NANOCOMPOSITE THIN FILMS

The optical properties of noble metal nanoparticles (NPs) can be efficiently controlled by their incorporation into host matrix films. Here, we report on the fabrication of composite films of gold NPs in a silicon suboxide matrix by a novel approach using a combination of pulsed laser deposition for NP production and gas-jet, electron-beam plasma chemical vapor deposition for low-temperature (300°C) synthesis of a SiO<sub>x</sub> (x = 0.38-1.55) thin film as a matrix for the NPs. The produced nanocomposite exhibits unexpected plasmonic properties, non-monotonically dependent on the matrix thickness, due to a porous columnar matrix structure grown from the NPs with variable oxygen content along the columns. This implies that low-temperature, gold-catalyzed oxidation of silicon occurs during the structure growth. Calculations based on Mie theory show that the refractive index of the obtained SiO<sub>x</sub> matrix can be as low as 1.2 at certain film thicknesses. Mechanisms of the columnar structure formation at different deposition stages are discussed. The synthesis approach can be used for the fabrication of optical thin-film materials with controllable low refractive index.

Gold-catalyzed oxidation of terminal alkynes to glyoxals and their reactions with 2-phenylimidazo[1,2-a]pyridines: one-pot synthesis of 1,2-diones.

A novel one-pot protocol for the convenient and efficient synthesis of (2-phenylimidazo[1,2-a]pyridin-3-yl)alkane-1,2-diones (3) in good yields (32-88%) from 2-phenylimidazo[1,2-a]pyridines (1) and terminal alkynes (2) has been established with a wide range of substrate scope. A tandem reaction sequence containing gold-catalyzed double oxidations of terminal alkynes to generate glyoxals, nucleophilic addition of 2-phenylimidazo[1,2-a]pyridines to glyoxals to yield α-hydroxyl ketones, and oxygenation of the α-hydroxyl ketones to afford the final products 3 under air atmosphere is involved in this method. Simple operation, mild reaction conditions, and widely available substrates make this strategy more affordable.

Gold-Catalyzed Oxidative Aminocyclizations of Propargyl Alcohols and Propargyl Amines to Form Two Distinct Azacyclic Products: Carbene Formation versus a 3,3-Sigmatropic Shift of an Initial Intermediate.

Gold-catalyzed oxidations of propargyl alcohols with nitrones by using a P(tBu)2 (o-biphenyl)Au+ catalyst, afforded bicyclic annulation products from the Mannich reactions of gold enolates. The same reactions of propargyl amines with nitrones by using the same gold catalyst gave distinct oxoarylation products. Our DFT calculations indicate that oxidation of propargyl alcohols with nitrones by using electron-rich gold catalysts lead only to gold carbenes, which can generate gold enolates or oxoarylation intermediates with enolate species having a barrier smaller than that of oxoarylation species.

Gold-Catalyzed Oxidations of 1,3-Diynamides with C(1) versus C(3) Regioselectivity: Catalyst-Dependent Oxidative Cyclizations in the C(3) Oxidation.

This work reports two distinct paths in catalytic oxidations of 1,3-diynamides with 8-methylquinoline oxide. A typical C(1) regioselectivity was observed for aryl-substituted 1,3-diyn-1-amides, whereas an unexpected C(3) regioselectivty occurred for 5-hydroxy1,3-diyn-1-amides. We focused on the C(3) oxidations of 5-hydroxy1,3-diyn-1-amides because we observed two oxidative cyclizations of the same substrates to yield 2-aminomethylenefuran-3(2H)-ones and 2-amino-4H-pyran-4-ones using AuCl3 and a cationic gold catalyst, respectively. Density functional theory calculations were performed to rationalize the C(3) regioselectivity on 5-hydroxy1,3-diyn-1-amides.

Gold-Catalyzed Oxidation of Thioalkynes To Form Phenylthio Ketene Derivatives via a Noncarbene Route.

Gold-catalyzed oxidations of thioalkynes with 8-methylquinoline oxides afford 2-phenylthioketenes that can be trapped efficiently with alcohols. The synthetic utility is manifested by terminal and internal thioalkynes over a wide scope, bearing esters, ketones, alkyl, and oxime substituents. Our density functional theory calculations suggest that gold-catalyzed oxidations of terminal and internal thioalkynes with 8-methylquinoline oxides generate gold-bound ketene intermediates without the intermediacy of α-oxo gold carbene.

Wolff Rearrangement of Oxidatively Generated α-Oxo Gold Carbenes: An Effective Approach to Silylketenes.

Gold-catalyzed oxidations of alkynes by N-oxides offer direct access to reactive α-oxo gold carbene intermediates from benign and readily available alkynes instead of hazardous diazo carbonyl compounds. Despite various versatile synthetic methods developed based on this strategy, one of the hallmarks of α-oxo carbene/carbenoid chemistry, that is, the Wolff rearrangement, has not been realized in this context. This study discloses the first examples that show the Wolff rearrangement can be readily realized by α-oxo gold carbenes oxidatively generated from TBS-terminated alkynes (TBS=tert-butyldimethylsilyl). The thus-generated silylketenes can be either isolated pure or subsequently trapped by various internal or external nucleophiles in one pot to afford α-silylated carboxylic acids, their derivatives, or TBS-substituted allenes.

Au(I)-Catalyzed Access to 1 H-Pyrrolo[1,2- a]indol-2(3- H)-ones via Oxidation of Terminal Alkynes.

A facile gold-catalyzed oxidation of terminal alkynes for the construction of 1 H-pyrrolo[1,2- a]indol-2(3- H)-ones is reported. Under relatively simple procedures, various 1 H-pyrrolo[1,2- a]indol-2(3- H)-ones derivatives were obtained in good to excellent yields, thus providing an efficient route for the construction of 1 H-pyrrolo[1,2- a]indol-2(3- H)-ones derivatives.

Gold-catalyzed intermolecular oxidation of phenylacetylene and allylic sulfides: an efficient and practical synthesis of α-phenylthio ketone

An efficient and practical synthesis of α-phenylthio ketone through gold-catalyzed intermolecular oxidation of phenylacetylene and substituted aryl(benzyl) allylic sulfides was developed. The reaction scope is fairly good with substituted aryl(benzyl) allylic sulfides, tolerating various functional groups, and the reaction affords the yields of 63%—85%.

Parametric Studies of Titania-Supported Gold-Catalyzed Oxidation of Carbon Monoxide.

This paper remarks the general correlations of the shape and crystallinity of titanium dioxide (TiO2) support on gold deposition and carbon monoxide (CO) oxidation. It was found that due to the larger rutile TiO2 particles and thus the pore volume, the deposited gold particles tended to agglomerate, resulting in smaller catalyst surface area and limited gold loading, whilst anatase TiO2 enabled better gold deposition. Those properties directly related to gold particle size and thus the number of low coordinated atoms play dominant roles in enhancing CO oxidation activity. Gold deposited on anatase spheroidal TiO2 at photo-deposition wavelength of 410 nm for 5 min resulted in the highest CO oxidation activity of 0.0617 mmol CO/s.gAu (89.5% conversion) due to the comparatively highest catalyst surface area (114.4 m2/g), smallest gold particle size (2.8 nm), highest gold loading (7.2%), and highest Au0 content (68 mg/g catalyst). CO oxidation activity was also found to be directly proportional to the Au0 content. Based on diffuse reflectance infrared Fourier transform spectroscopy, we postulate that anatase TiO2-supported Au undergoes rapid direct oxidation whilst CO oxidation on rutile TiO2-supported Au could be inhibited by co-adsorption of oxygen.

Gold-Catalyzed Oxidation Terminal Alkyne: An Approach to Synthesize Substituted Dihydronaphthalen-2(1H)-ones and Phenanthrenols.

A facile gold-catalyzed oxidation terminal alkynes to synthesize substituted dihydronaphthalen-2(1H)-ones 3 and phenanthrenols 5 was realized. Various useful structures and drug precursors were generated in up to 99% yield under mild condition and low catalyst loading.

Carbon Support Surface Effects in the Gold-Catalyzed Oxidation of 5-Hydroxymethylfurfural.

Oxidation of 5-hydroxymethylfurfural into 2,5-furandicarboxylic acid is an important transformation for the production of bio-based polymers. Carbon-supported gold catalysts hold great promise for this transformation. Here we demonstrate that the activity, selectivity, and stability of the carbon-supported gold nanoparticles in the oxidation of 5-hydroxymethylfurfural strongly depend on the surface properties of the carbon support. Gold nanoparticles supported on basic carbon materials with a low density of functional groups demonstrate higher activity in 5-hydroxymethylfurfural oxidation (TOFAu up to 1195 h–1), higher selectivity to 2,5-furandicarboxylic acid, and better stability in comparison to gold nanoparticles supported on carbon materials with acidic surface groups. Surface groups of basic carbon supports that are positively charged under the reaction conditions result in a higher adsorption and local concentration of hydroxyl ions, which act as cocatalysts for gold and enhance gold-catalyzed dehydrogenation. Negatively charged surface groups of acidic carbons repel hydroxyls and the intermediate monoacid anions, which leads to lower reaction rates and a high selectivity toward 2,5-hydroxymethylfurancarboxylic acid. Understanding the role of support surface charge and local hydroxyl anion concentration provides a basis for the rational design of the optimal carbon support surface chemistry for highly active, selective, and stable catalysts for the oxidation of 5-hydroxymethylfurfural and related reactions.

ChemInform Abstract: Gold‐Catalyzed Oxidation/C—H Functionalization of Ynones: Efficient and Rapid Access to Functionalized Polycyclic Salicyl Ketones.

AbstractA variety of salicylic ketones is available by utilizing this non‐diazo approach.

Highly Efficient One-Pot Synthesis of 2,4-Disubstituted Thiazoles Using Au(I) Catalyzed Oxidation System at Room Temperature

In the present work, gold complex catalysts with Mor-DalPhos ligands were successfully prepared using mesylates as counter ions. Seven ammonium sulfonates were synthesized to promote the production of intermediate sulfonyloxymethyl ketone. It was found that low-acidity N,N-dimethylbenzenaminium methanesulfonate showed excellent activity in the reaction. Furthermore, the catalysts effectively avoided the loss of activity due to the low acidity. Various thioamides were directly added to the resulting reaction mixture without the separation of intermediate product. Then, twenty kinds of 2,4-disubstituted thiazoles were efficiently synthesized at room temperature with the highest yield of 91%. This work provides an efficiency and mild gold-catalyzed oxidation system for the one-pot synthesis of thiazole and its derivatives.

Gold-catalyzed Intermolecular Oxidations of 2-Ketonyl-1-ethynyl Benzenes with N-Hydoxyanilines to Yield 2-Aminoindenones via Gold Carbene Intermediates.

Gold-catalyzed oxidations of 2-ketonyl-1-ethynyl benzenes with N-hydroxyanilines yield 2-aminoindenone derivatives efficiently. Experimental data suggests that this process involves an α-oxo gold carbene intermediate, generated from the attack of N-hydroxyaniline on furylgold carbene intermediate, rather than the typical attack of oxidants on π-alkynes.

Gold-Catalyzed Oxidation of Anilines to Nitrosobenzenes by Hydrogen Peroxide

Gold-Catalyzed Oxidation of Anilines to Nitrosobenzenes by Hydrogen Peroxide

Gold-Catalyzed Oxidation/C-H Functionalization of Ynones: Efficient and Rapid Access to Functionalized Polycyclic Salicyl Ketones.

An efficient strategy to construct salicyl ketones through gold-catalyzed oxidation/C-H functionalization of ynones is reported. A variety of functionalized salicyl ketones are readily accessed by utilizing this non-diazo approach, thus providing a viable alternative to synthetically useful salicyl ketones with a yield up to 98 %. The α-oxo gold carbenes generated in situ through gold-catalyzed oxidation of ynones can be trapped effectively by internal aryl and heteroaromatic groups. Electronic and steric effects were also investigated in this reaction. The anticancer activity of one salicyl ketone analogue was investigated and its cytotoxicity assays against the PC-3 prostate cancer cell line and SKOV-3 human ovarian carcinoma cell line yield IC50 were 0.81±0.05 and 0.87±0.15 μm, respectively, demonstrating that salicyl ketone analogues showed good anticancer activity.

Multiple CO Oxidation Promoted by Au2 Dimers in Au2TiO4− Cluster Anions

Time-of-flight mass spectrometry experiments demonstrated that laser ablation generated and mass selected Au2 TiO4 (-) cluster anions can unexpectedly oxidize three CO molecules in an ion trap reactor. This is an improvement on the more commonly observed oxidation of at most two CO molecules by a doped cluster. Quantum chemistry calculations were performed to rationalize the reactions. The lowest energy isomer of Au2 TiO4 (-) contains a superoxide unit, the participation of which in CO oxidation can be promoted by the Au2 dimer. The Au2 dimer functions as a rather flexible electron reservoir in each CO oxidation step in terms of the release and storage of electrons to drive the dissociation of superoxide to peroxide and then to lattice oxygen atoms, which can be removed by reaction with CO molecules. This gas-phase study enriches our understanding toward the nature of reactive oxygen species involved in gold-catalyzed oxidation reactions.

ChemInform Abstract: Evolution in the Chemical Making of Gold Oxidation Catalysts

AbstractReview: 98 refs.

Improvement of Gold-Catalyzed Oxidation of Free Carbohydrates to Corresponding Aldonates Using Microwaves

An efficient microwave-assisted methodology for the oxidation of free carbohydrates to corresponding aldonates is described. Supported gold catalyst, hydrogen peroxide, and a mineral base were used to perform the oxidation reaction in water under microwave irradiation. These conditions combined with Au/Al2O3 catalyst allowed to reach in 10 min a quantitative conversion of glucose. The catalyst was highly selective and reusable, and only 0.004 mol % of gold compared to sugar was used. The reaction was performed using a variety of substrates (mono- or oligosaccharides, neutral or acidic sugars), and good to excellent conversion yields and selectivity to corresponding aldonates were obtained. After filtration and freeze-drying, sodium or potassium aldonates were obtained without any purification. Thanks to very high turnover frequencies (up to 438 000 h–1), this methodology significantly improves the previously described methods for the gold-catalyzed oxidation of carbohydrates.