Nam T.S. Phan

Metal–organic frameworks for catalysis: the Knoevenagel reaction using zeolite imidazolate framework ZIF-9 as an efficient heterogeneous catalyst

Crystalline zeolite imidazolate framework ZIF-9 was synthesized from the reaction of cobalt nitrate hexahydrate and benzimidazole by a solvothermal method, and was used as an efficient heterogeneous catalyst for the low temperature liquid phase Knoevenagel reaction. The solid catalyst was characterized by various techniques, including X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic laser light scattering (DLS), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), atomic absorption spectrophotometry (AAS), and nitrogen physisorption measurements. Excellent conversions were achieved under mild conditions. The ZIF-9 catalyst was easily isolated from the reaction mixture by simple filtration and reused several times without a significant degradation in activity. Moreover, there was no contribution from leached active species and conversion was only being possible in the presence of the solid catalyst.

Enhanced Cooperativity through Design: Pendant CoIIISalen Polymer Brush Catalysts for the Hydrolytic Kinetic Resolution of Epichlorohydrin (Salen=N,N′‐Bis(salicylidene)ethylenediamine Dianion)

The Co(III)--salen-catalyzed (salen=N,N-bis(salicylidene)ethylenediamine dianion) hydrolytic kinetic resolution (HKR) of racemic epoxides has emerged as a highly attractive and efficient method of synthesizing chiral C(3) building blocks for intermediates in larger, more complex molecules. HKR reaction rates have displayed a second order dependency on the concentration of active sites, and thus researchers have proposed a bimetallic transition state for the HKR mechanism. Here we report the utilization of pendant Co(III)--salen catalysts on silica supported polymer brushes as a catalyst for the HKR of epichlorohydrin. The novel polymer brush architecture provided a unique framework for promoting site-site interactions as required in the proposed bimetallic transition state of the HKR mechanism. Furthermore, the polymer brushes mimic the environment of soluble polymer-based catalysts, whereas the silica support permitted facile recovery and reuse of the catalyst. The polymer brush catalyst displayed increased activities over the soluble Jacobsen Co--salen catalyst and was observed to retain its high enantioselectivities (>99 %) after each of five reactions despite decreasing activities. Analysis indicated decomposition of the salen ligand as an underlying cause of catalyst deactivation.

A Zeolite Imidazolate Framework ZIF-8 Catalyst for Friedel-Crafts Acylation

Abstract A zeolite imidazolate framework, ZIF-8, was synthesized and characterized by dynamic laser light scattering, X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, Fourier transform infrared, atomic absorption spectrophotometry, and nitrogen adsorption measurements. The ZIF-8 was highly crystalline and porous with a surface area of over 1600 m2/g. Friedel-Crafts acylation of anisole and benzoyl chloride proceeded well in the presence of ZIF-8 (2-6 mol%) without the need for an inert atmosphere. The reaction afforded a selectivity of 93%-95% to the p-isomer. The solid catalyst can be separated from the reaction mixture by simple centrifugation and reused without significant degradation in catalytic activity. There was no leaching of active acid species into the reaction solution.

Towards applications of metal–organic frameworks in catalysis: C–H direct activation of benzoxazole with aryl boronic acids using Ni2(BDC)2(DABCO) as an efficient heterogeneous catalyst

A crystalline porous metal–organic framework Ni2(BDC)2(DABCO) was synthesized and characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR), atomic absorption spectrophotometry (AAS), and nitrogen physisorption measurements. The Ni2(BDC)2(DABCO) could be used as an efficient heterogeneous catalyst for the nickel-catalyzed direct heterocycle C–H arylation reactions between azoles and arylboronic acids forming 2-arylbenzoxazoles as the principal product. This approach avoids the use of hazardous aryl halides as well as the formation of halide byproducts in the synthesis of aryl-substituted benzoxazoles. The Ni2(BDC)2(DABCO) exhibited significantly higher catalytic activity than that of other Ni-MOFs such as Ni3(BTC)2, Ni(HBTC)(BPY), and that of some common nickel salts such as NiCl2, Ni(NO3)2, Ni2SO4, and Ni(OAc)2. To the best of our knowledge, application of Ni2(BDC)2(DABCO) in the field of catalysis as well as the C–C cross coupling reaction via direct C–H functionalization using a nickel heterogeneous catalyst have not previously been reported in the literature.

Efficient and recyclable Cu2(BDC)2(BPY)-catalyzed oxidative amidation of terminal alkynes: role of bipyridine ligand

We have described an efficient method for oxidative cross coupling reactions between activated N–H amines and terminal alkynes using heterogeneous Cu2(BDC)2(BPY) as recyclable catalyst (BDC = benzene-1,4-dicarboxylate; BPY = 4,4′-bipyridine). The optimal conditions involved the use of inexpensive NaHCO3 base and oxygen as terminal oxidant in toluene solvent at 80 °C. Reactions proceeds efficiently, and high selectivity with only trace amount of diynes as well as good yields were achieved in a short reaction time. The Cu2(BDC)2(BPY) exhibited excellent catalytic activity and selectivity as compared to other Cu-MOFs on a broad reaction scope. Interestingly, the presence of bipyridine ligand was shown to enhance the catalyst stability. Thus, the Cu2(BDC)2(BPY) could be facilely separated from the reaction mixture by simple centrifugation and could be reused several times with only a slight decrease in catalytic activity.

Direct arylation of heterocycles through C–H bond cleavage using metal–organic-framework Cu2(OBA)2(BPY) as an efficient heterogeneous catalyst

A crystalline porous metal–organic-framework Cu2(OBA)2(BPY) was synthesized and characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR), atomic absorption spectrophotometry (AAS), and nitrogen physisorption measurements. The Cu-MOF was shown to be an efficient heterogeneous catalyst for direct C-arylation of a variety of heterocycles by iodoarenes. The optimal conditions employed tBuOLi in dioxane at elevated temperature. In addition, a leaching test was also conducted to investigate the heterogeneity. Gratifyingly, the MOF catalyst can be facilely recycled several times under identical conditions without remarkable loss in catalytic reactivity.

Nickel-catalyzed oxidative coupling of alkynes and arylboronic acids using the metal–organic framework Ni2(BDC)2(DABCO) as an efficient heterogeneous catalyst

A crystalline porous metal–organic framework Ni2(BDC)2(DABCO) was synthesized and characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR), atomic absorption spectrophotometry (AAS), and nitrogen physisorption measurements. Ni2(BDC)2(DABCO) could be used as an efficient and recyclable heterogeneous catalyst for the cross-coupling reaction between phenylboronic acids and phenylacetylenes in the presence of molecular oxygen in air as the stoichiometric oxidant. High conversions of the cross-coupling products were achieved with no homo-coupling products being detected by GC. Ni2(BDC)2(DABCO) exhibited significantly higher catalytic activity in the coupling reaction than that of other Ni-MOFs such as Ni3(BTC)2 and Ni(HBTC)(BPY). Zn2(BDC)2(DABCO) and Co2(BDC)2(DABCO) were found to be completely inactive for the transformation, while the reaction using Cu2(BDC)2(DABCO) as the catalyst led to the formation of a 10% homo-coupling product. To the best of our knowledge, a nickel-based catalyst was not previously used for the oxidative coupling reactions between phenylacetylenes and phenylboronic acids.

Efficient and robust superparamagnetic copper ferrite nanoparticle-catalyzed sequential methylation and C–H activation: aldehyde-free propargylamine synthesis

We have described a method for oxidative cross-coupling reactions between N-alkyl anilines and terminal alkynes forming N-aryl-N-methylpropargylamines. Superparamagnetic CuFe2O4 nanoparticles were employed as efficient and robust catalyst. The optimum conditions involved the use of tert-butyl hydroperoxide (TBHP) as oxidant and methylating reagent in dimethylacetamide (DMA) solvent at 140 °C. The two-step reaction, methylation and C–C cross-coupling, proceeds efficiently and has high selectivity, and good conversions were achieved in short reaction times. Preliminary mechanistic investigation was conducted. The CuFe2O4 nanoparticles could be facilely separated from the reaction mixture by magnetic decantation and could be reused several times with only a slight decrease in catalytic activity.

Expanding applications of zeolite imidazolate frameworks in catalysis: synthesis of quinazolines using ZIF-67 as an efficient heterogeneous catalyst

A cobalt zeolite imidazolate framework (ZIF-67) was successfully synthesized and characterized by several techniques. The ZIF-67 was used as an efficient heterogeneous catalyst for the cyclization reaction of 2-aminobenzoketones and benzylamine derivatives to form quinazoline products. The optimal conditions involved the use of TBHP oxidant in toluene solvent at 80 °C. Remarkably, the ZIF-67 catalyst exhibited better performance in the cyclization reaction than common cobalt salts such as Co(NO3)2, CoCl2, and Co(OAc)2 and other Co-MOFs such as ZIF-9, Co-MOF-74, and Co2(BDC)2(DABCO). In addition, the cyclization reaction could only proceed in the presence of the solid Co-ZIF catalyst and there was no contribution from leached active sites present in the solution. The catalyst could be recovered and reused several times without a significant degradation in catalytic activity.

Ligand-free N-arylation of heterocycles using metal–organic framework [Cu(INA)2] as an efficient heterogeneous catalyst

A metal–organic framework [Cu(INA)2] was synthesized and used as a heterogeneous catalyst for arylation of a wide range of N–H heterocycles and aryl halides under ligand-free conditions. The N-arylation reaction involved the use of 5 mol% Cu-MOF catalyst with K3PO4 or tBuOLi as the base in dimethylacetamide (DMA) solvent at 100 °C in 6 h. [Cu(INA)2] exhibited higher catalytic activity for the N-arylation transformation than that of common homogeneous copper catalysts and other Cu-MOFs with unsaturated open metal sites such as Cu2(BDC)2(BPY), Cu3(BTC)2, and Cu2(BDC)2(DABCO). Interestingly, reaction conditions are compatible with a wide range of N–H heterocycles, functional groups, and aryl chlorides. A leaching test indicated no contribution of leached active species in the reaction filtrate. Furthermore, the [Cu(INA)2] catalyst could be facilely separated from the reaction mixture and recovered and reused several times without a significant degradation in catalytic activity.