Gexin Chen

Heterogeneous biomimetic aerobic synthesis of 3-iodoimidazo[1,2-a]pyridines via CuOx/OMS-2-catalyzed tandem cyclization/iodination and their late-stage functionalization

In the presence of copper supported on manganese oxide-based octahedral molecular sieves OMS-2 (CuOx/OMS-2), the heterogeneously catalytic, aerobic synthesis of 3-iodoimidazo[1,2-a]pyridines from acetophenones, 2-aminopyridines and I2via tandem cyclization/iodination in a one-pot manner is achieved. As a heterogeneous catalyst, OMS-2 acts not only as a support for catalytic Cu species but also as an electron-transfer mediator (ETM), which combines with Cu to generate a low-energy pathway for rapid electron transfer. In this way, the biomimetic, catalytic oxidation could directly employ air as a green terminal oxidant under mild conditions, and provide corresponding products with broad substrates in moderate to excellent yields using very low catalyst loading (0.2 mol% Cu). In this process, I2 not only plays the role of catalyst for the initial cyclization, with assistance from CuOx/OMS-2, but also acts as a reactant for the next electrophilic oxidative iodination, which makes the reaction highly atom economic. Besides, the late-stage functionalization of the I-substituted imidazo[1,2-a]pyridines is also demonstrated by various coupling reactions, which show its potential applications in synthetic and pharmaceutical chemistry. Moreover, the catalyst is truly heterogeneous and reusable.

OMS-2-Supported Cu Hydroxide-Catalyzed Benzoxazoles Synthesis from Catechols and Amines via Domino Oxidation Process at Room Temperature

In the presence of manganese oxide octahedral molecular sieve (OMS-2) supported copper hydroxide Cu(OH)x/OMS-2, aerobic synthesis of benzoxazoles from catechols and amines via domino oxidation/cyclization at room temperature is achieved. This heterogeneous benzoxazoles synthesis initiated by the efficient oxidation of catechols over Cu(OH)x/OMS-2 tolerates a variety of substrates, especially amines containing sensitive groups (hydroxyl, cyano, amino, vinyl, ethynyl, ester, and even acetyl groups) and heterocycles, which affords functionalized benzoxazoles in good to excellent yields by employing low catalyst loading (2 mol % Cu). The characterization and plausible catalytic mechanism of Cu(OH)x/OMS-2 are described. The notable features of our catalytic protocol such as the use of air as the benign oxidant and EtOH as the solvent, mild conditions, ease of product separation, being scalable up to the gram level, and superior reusability of catalyst (up to 10 cycles) make it more practical and environmentally friendly for organic synthesis.

Catalytic wet air oxidation of high-concentration organic pollutants by upflow packed-bed reactor using a Ru–Ce catalyst derived from a Ru3(CO)12 precursor

Low loading catalysts Ru/γ-Al2O3 and Ru–Ce/γ-Al2O3 (Ru: 0.3 wt%) were prepared by thermolysis of Ru3(CO)12, which exhibited high activity in Catalytic Wet Air Oxidation (CWAO) of high concentration organic pollutants by upflow packed-bed reactor with initial COD 200 000 mg L−1. XPS results revealed that two new Ru species (RuA and RuB) were detected due to the chemical interaction between Ru3(CO)12 and the active OH groups on the surface of Al2O3 during the process of Ru3(CO)12 incomplete decomposition, and the reduction of them can lead to more dispersed metallic phases. The TPR spectrum reveals that the catalyst with addition of Ce produced a strong interaction between Ru and the CeO2–Al2O3 support, which was helpful for the catalytic wet air oxidation. Low loading catalyst Ru–Ce/γ-Al2O3 reduced in H2 had the highest activity and the COD removal reached 99.5% above 245 °C. The operating parameters investigated included temperature, reactor pressure, gas flow rate and liquid hourly space velocity (LHSV). The results showed that the COD removal is considerably affected by the temperature; when temperature increased around 2–3 °C, COD removal was improved about 40% due to the obvious exothermic reaction. The experimental results indicated that the 0.3 wt% Ru–Ce/γ-Al2O3 catalyst has excellent activity and stability in the CWAO of high concentration organic pollutants in as packed-bed reactor during 100 hours.

Heterogeneous Cu/OMS-2 as an efficient catalyst for the synthesis of tetrasubstituted 1,4-enediones and 4H-pyrido[1,2-a]-pyrimidin-4-ones

An efficient Cu/OMS-2-catalyzed oxidative heterogeneous protocol has been developed for the synthesis of tetrasubstituted unsymmetrical 1,4-enediones and 4H-pyrido[1,2-a]-pyrimidin-4-ones from 1,4-enediones and amines or 2-aminopyridines. The present catalytic system employs air as the terminal oxidant and tolerates a broad range of substrates using low loading copper (1.3 wt%). The Cu/OMS-2 was characterized by various methods, including XRD, XPS, BET as well as TEM/SEM, and the results of H2-TPR shows that the redox ability of the catalyst was improved by the combination of Cu and OMS-2. Moreover, the catalyst is recyclable.

Influence of Ru precursors on the activity of Ru/Al2O3–TiO2 catalysts for catalytic wet air oxidation of high concentration organic compounds

RuCl3, Ru3(CO)12 and Ru(NO)(NO3)3 were used as Ru precursors to prepare supported Ru catalysts by an impregnation method. The effect of the precursors on the catalysts was investigated in the catalytic wet air oxidation (CWAO) of high concentration organic compounds. The surface properties of the catalysts were characterized by BET, XRD, XPS, TEM and H2-TPR. The different Ru precursors led to different particle sizes and distributions, reducibilities, and content of chemisorbed as-synthesized catalysts, which resulted in discrepancies in their catalytic performances. The order of the catalyst activity was as follows: Ru(NO)/Al2O3–TiO2 > Ru(CO)/Al2O3–TiO2 > Ru(Cl)/Al2O3–TiO2. The surface structure and the concentration of chemisorbed oxygen on the surface of the catalysts played important roles in the activity of the Ru catalysts in the CWAO of organic compounds.

Ball-milling synthesized hydrotalcite supported Cu-Mn mixed oxide under solvent-free conditions: an active catalyst for aerobic oxidative synthesis of 2-acylbenzothiazoles and quinoxalines

A rapid solvent-free ball-milling method was developed to prepare a hydrophobic hydrotalcite supported Cu–Mn mixed oxide catalyst (Cu–Mn/HT). The mechanochemically prepared catalyst exhibited high catalytic activity and recyclability towards the aerobic synthesis of 2-acylbenzothiazoles and quinoxalines in green medium ethanol compared with the ones synthesized via grinding and wet-impregnation. Moreover, control experiments showed that the catalyst was successfully used in green oxidative esterification and coupling as well. Cu–Mn/HT was characterized by BET, ICP, XRD, XPS, SEM and TEM, which indicated that more surface oxygen vacancies and formed CuMn2O4 species on HT might contribute to the catalytic activity.

Degradation of highly concentrated organic compounds over a supported Ru-Cu bimetallic catalyst

Ru catalysts and Ru–Cu bimetallic catalysts were prepared via an impregnation method and investigated for the catalytic degradation of high-concentration organics. A series of Al2O3–TiO2-1, Al2O3–TiO2-2, and Al2O3 and TiO2-supported Ru and Ru–Cu catalysts were fabricated and characterized by BET, XRD, XPS, H2-TPR, and TEM to analyze the physicochemical properties of the catalysts. There was a strong interaction between Ru and Cu over Ru–Cu catalysts; moreover, the coexistence of Ru and Cu improved the dispersion of Ru. The addition of Cu increased the performance of Ru catalysts because of the synergistic effects of Ru and Cu for Ru–Cu catalysts; however, the support was also crucial for the catalytic activity. Among the as-prepared Ru and Ru–Cu catalysts, the Ru–Cu/Al2O3–TiO2-1 catalyst, with an Al2O3/TiO2 ratio of 1 : 1, exhibited the highest activity, with 91.5% chemical oxygen demand removal after a 2 h reaction at 200 °C and 1.6 MPa oxygen pressure. Furthermore, the pseudo-first-order reaction kinetic model conformed well to the experimental data, with the coefficient correlation greater than 0.98. This catalyst could be reused five times without significant loss of activity.