Current Catalysis - Volume 2, Issue 3, 2013

This article highlights the cascade synthesis of various N-heterocycles and C–N bonding organic transformations catalyzed by a simple and efficient heterogeneous CuII–hydrotalcite (CuII–HT) catalyst. The CuII–HT was found to be a dynamic catalyst and exhibited promising catalytic activity towards one-pot MCRs, namely, synthesis of 1,4- disubstituted 1,2,3-triazoles, synthesis of 2H-indazoles, and C–N bond forming reactions. These protocols are credited with several advantages such as readily available starting materials, simple workup procedure, better yields, shorter reaction times, and broader substrate scope. The in-situ generated organic azide (R–N3) and heterocyclization (C–N and N–N bond formation) are the key steps to deliver the desired 1,4-disubstituted 1,2,3-triazoles and 2H-indazoles, respectively.

The tendency in recent decades is to develop environmentally friendly heterogeneous catalytic systems for organic transformations. In particular, chemical reactions cooperatively promoted by solid acid-base bifunctional catalysts with good dispersion of active sites and high catalytic performances have aroused much attention. In this review, recent advances in preparation and application of solid acid-base bifunctional catalysts for organic transformations are described, and special emphasis is paid to understand the catalytic mechanisms of the acid-base bifunctionality. Furthermore, strategies for well positioning acidic sites and basic sites to cohabitation and cooperation are briefly discussed.

Nano-sized silver doped Mn2O3 was synthesized by starch assisted co-precipitation technique. The prepared samples were tested for CO oxidation reaction. Ag doped Mn2O3 sample was found to show higher activity then pristine Mn2O3. XRD patterns authenticate the formation of Mn2O3 phase. Electron microscopy images show that the particles are in nano-sized, agglomerated and 40 - 70 nm in range. Thermal analysis data indicates the phase change from Mn2O3 to Mn3O4 beyond 950°C. Magnetic susceptibility data indicates the paramagnetic nature of these samples. Electrical resistivity measurement shows that these samples are semiconductors.

A Brönsted acidic ionic liquid catalyst with a sulfonic acid group tethered to imidazolium cation, and immobilized on polystyrene was prepared by a two step method in 83% overall yield. This immobilized Bronsted acidic ionic liquid catalyst was shown to be a significantly better catalyst than H+ form of Dowex-50X8 of similar SO3H group loading for hydrolysis of untreated Sigmacell cellulose (DP ~ 450) in water under hydrothermal conditions. For example cellulose hydrolyzed with immobilized Bronsted acidic ionic liquid catalyst produced 50.1% yield of total reducing sugars (TRS) after 3 hr, at 160° C, whereas sample heated with Dowex-50X8 produced only 9.2% TRS yield under identical conditions. Similarly, cellulose hydrolysis using immobilized ionic liquid catalyst produced a maximum glucose yield of 21.7% after 3 hr, at 160 °C, while the maximum glucose yield with Dowex-50X8 catalyst was 4.5%, which was achieved after 3 hr, at 170° C. This enhancement in catalytic activity is explained as a result of an interaction between cellulose, and immobilized imidazolium ionic liquid structure with chloride anion.

Immobilization of enzymes on suitable supports is an interesting area of investigation to increase their application spectrum in the synthetic, therapeutic and biosensor fields. In the present study, a new series of nanogels was used for the immobilization of glucose oxidase (GOx). Nanogels based on poly(acrylic acid) cross-linked with N,N-methylene bisacrylamide were synthesized using a surfactant free emulsification method. The carboxylic groups of the assynthesized nanogels were further functionalized to azide and isocyanate forms. The activity assay of the free GOx and the GOx-immobilized-nanogels was carried out using a standard protocol. The GOx-immobilized nanogels were investigated as devices for the loading and release of insulin. Nanogels were characterized by various techniques to obtain evidence of nanogel formation and GOx immobilization. The activity of various GOx-immobilized nanogels was observed to be dependent on the structure of the nanogels. The activity, thermal stability and substrate affinity of GOx were enhanced after immobilization. Excellent reusability of nanogels was observed up to ten repeat cycles. The GOx immobilized nanogels were loaded with insulin and the release study of the latter was carried out in the presence of glucose, the GOx substrate,under the physiological conditions.

The catalytic activity of the dichlorodioxomolybdenum (MoO2Cl2·2DMSO) complex was investigated for oxidation of aromatic alcohols to corresponding aldehydes using dimethyl sulfoxide (DMSO) as an oxidant under solvent free conditions. The catalyst showed very high conversion and selectivity for aldehyde with high turnover number up to 1000 and high turnover frequency. For substituted benzyl alcohols there was increase in the conversion with electron donating substituents whereas there was decrease in conversion with electron withdrawing substituents. Interestingly though, the catalyst is homogeneous in nature, it was efficiently recycled even up to five recycles without appreciable loss in the activity. Based on the products obtained during reaction the mechanism for oxidation has been proposed.

A protocol describes the synthesis of naphtha-1,3-oxaizine derivatives via one-pot multi-component reaction of α-naphthol, formaldehyde and various substituted anilines catalyzed by L-proline as an efficient catalyst. A method offers delightful advantages such as mild reaction conditions, environmentally benign, easy experimental workup and good to excellent yields.