Current Organic Chemistry - Volume 16, Issue 23, 2012

Metal nanoparticles (NPs) have been reduced with NaBH4 from Pt, Rh and Ir precursors salts in presence of (2R,4R)-2,4- bis(diphenylphosphino pentane), ((R,R)-BDPP), and SiO2. This synthesis procedure with a chiral ligand generates chiral stabilized NPs of small and narrowly distributed core sizes depending of metal precursor, metallic loading and ligand stabilizer concentration. These novel catalysts with chiral surface have been used in enantioselective hydrogenation of ethyl pyruvate as a reference substrate and 1-phenylpropane- 1,2-dione, acetophenone, 2,3-butanedione and 3,4-hexanedione, all scarcely studied ketones. Catalysts were characterized by TEM, EDS, electron diffraction, N2 adsorption-desorption isotherms and XPS. Catalytic results indicate high conversion levels and enantioselectivities up to 78% in the hydrogenation of some substrates, due to the presence of chiral ligand on metal surface. All results are promising because is the first report of high enantiomeric excess (ee) in hydrogenations using these kind of heterogeneous catalytic systems.

We report the use of V, Bi and Bi-V Keggin structure where Mo is partially replaced by V, Bi, and Bi and V, respectively, in the solvent-free multicomponent synthesis of 3,4-dihydropyrimidin-2-(1H)-ones by the Biginelli method. The incorporation of V, Bi and Bi-V in the structure of PMo notably increases the catalytic activity. The following correlation between the yields of the 3,4- dihydropyrimidin-2-(1H)-ones and the number of acidic sites of the catalysts was observed: PMoBiV > PMoV > PMoBi > PMo. The reaction experiments were performed in the absence of solvent, at 80ºC (1 h). Under these conditions and using the most active catalyst (PMoBiV), twelve examples were obtained with very good yields (80% - 98%) and high selectivity. The catalyst was easily recycled and reused without appreciable loss of its catalytic activity. The synthetic method presented is a simple, clean and environmentally friendly alternative for the obtention of substituted 3,4-dihydropyrimidin-2-(1H)-ones.

The hydrogenation of m-dinitrobenzene to m-phenylenediamine in liquid phase was studied with the platinum catalysts supported on TiO2, Al2O3 and TiO2-Al2O3 carriers. These materials were successfully activated either by simple in situ reduction technique that makes use of the same reaction conditions or conventional reduction with H2 flowing at 723K. Among the catalysts tested, the Pt/TiO2-Al2O3 catalyst showed the highest activity and selectivity towards m-phenylenediamine, over which 99% m-dinitrobenzene conversion and 97% m-phenylenediamine selectivity were obtained at 363K under hydrogen pressure of 8,1 MPa after reaction for 22 h when using ethanol as solvent and the catalyst was previously activated by the in situ activation method.

The oligomerization of glycerol to di- and triglycerol is an attractive reaction because these products find many applications in cosmetics, food industry and polymer industry. To control the oligomerization degree, it is necessary to develop a selective catalyst. In this work, two cesium mesoporous silica (MCM-41 and SBA-15) are compared in the selective oligomerization of glycerol into di- and triglycerol. The catalysts were prepared by incipient-wetness impregnation. The results showed that the stability and the activity of catalysts depend on the support nature. The use of SBA-15 support instead of MCM-41 leads to a more stable catalyst without changing polyglycerols selectivity. Different characterizations showed that it is due to the larger wall thickness of SBA-15. This study also showed that the nature of catalyst could influence the diglycerol isomers distribution. In fact the glycerol activation inside or outside the pores can strongly modify the selectivity to linear or branched diglycerol isomers.

Palladium and platinum nanoparticles on an activated carbon were used as catalysts in the citral hydrogenation reaction in liquid phase at 363 K and 620 kPa of hydrogen pressure. The effect of metallic precursor salt to prepare mono and bimetallic catalysts was evaluated using PdCl2, Pd(NO3)2 and H2PtCl6. X-ray Photoelectron Spectroscopy suggests that Pdº, Pdδ+ (with δ close to 0) and Ptº are present on the monometallic catalysts surface; while Pdδ+, Pdn+, Ptδ-and Ptδ+ (with δ ≈ 0 and n ≈ 2) are detected on the bimetallic Pd-Pt/C. Monometallic PtCl/C displayed a very low total conversion of citral. All Pd based catalysts (PdN/C, PdCl/C and Pd-Pt/C) were highly selective to citronellal and isopulegol (desired products). The bimetallic Pd-Pt/C was the most active and selective catalyst. Electrondeficient and electron-rich species, chlorine amount on carbon surface and the modification of surface groups produced during the catalysts synthesis played an important role on the selectivity values to obtained products.

The effect of FeOx on the activity and selectivity in the cinnamaldehyde hydrogenation over Ir/SiO2 catalyst was studied. FeOx/SiO2, Ir/FeOx/SiO2 and Ir/SiO2 catalysts were prepared by impregnation. Fe and Ir content by weight% was 5%Fe and 1%Ir respectively. Samples were calcined at 673 K and reduced in H2 at 573 K. All catalysts were characterized by N2 adsorption, TEM, H2-TPR and DRIFTS of CO at room temperature. Cinnamaldehyde hydrogenation was carried out at 363 K in a batch reactor. The presence of FeOx has practically no effect in the conversion, but the selectivity to the desired unsaturated alcohol changes drastically when FeOx/SiO2 is the support. In addition, selectivity toward unsaturated alcohol increased with the conversion for Ir/SiO2 catalyst while for Ir/FeOx/SiO2 catalyst remained constant. A combination of particle size effect and the presence of Fen+ sites in proximity of iridium sites are responsible for the preferential hydrogenation of the CO bond over Ir/FeOx/SiO2 catalyst.

Catalytic application of niobium catalysts is presented in this work. The regioselective oxidation of geraniol was studied over niobium catalysts using hydrogen peroxide (H2O2) as oxidant in the absence of any organic solvent. The catalysts studied were crystalline Nb2O5 bulk, a mixed oxide of Nb2O5/SiO2 (N-S) and Nb2O5/SiO2 mesoporous (N-M). The catalysts were characterized by nitrogen adsorption- desorption isotherms at 77 K, X-ray diffraction (XRD) and UV-Vis. High selectivity to the allylic C=C double bond was observed in all catalysts studied. However, for Nb-M and N-S the selectivity to epoxide decreases with the conversion, contrary to observed for Nb2O5 bulk. The differences in selectivity may be due to different routes for the activation of oxidant over the metallic center. This work proposes a useful application of Nb2O5 commercially available in selective organic reactions.

Chiral 2-imidazolidinones have been exploited with respect to asymmetric alkylation reactions, aldol reactions, Michael addition reactions, etc, and their convenient preparation from many kinds of available materials, greater stability to ring-opening reaction than chiral oxazolidinones and high levels of asymmetric induction make them excellent and show more widely considerable promise as chiral auxiliaries in organic synthesis. In this context, the general construction of chiral 2-imidazolidinones will be presented and the most important applications in the field of organic synthesis will be highlighted.

Xanthones belong to a class of O-heterocycles which are known for their great variety of biological/pharmacological activities. The xanthone scaffold can be considered as a privileged structure and for that reason it is an excellent starting point for the search of new bioactive molecules. This review intends not only to make a literature survey about the synthesis of xanthones, but also to organize the most important and common syntheses that have been published from 2005 to 2012 in an integrative way. Among the new developed approaches, we highlight the synthesis of xanthones via benzyne, palladium-catalyzed, domino type reactions and imidazolidenyl carbene-catalyzed reactions. In the first three sections it is given an overview about the classic synthetic approaches, focusing on recent developments; the fourth section orders different approaches described for the synthesis of xanthones using chromen-4-ones as building blocks. As far as our knowledge, this is the first time that a review systematizes the synthesis of xanthones based on chromen-4-ones. The fifth section refers to a miscellaneous of synthetic approaches that do not fit in any of the previous but nevertheless are useful. The last section is devoted to molecular modifications that can be done directly from the xanthone scaffold.

A novel Schiff base ligand (H2L) from the reaction of 1-phenyl-3-methyl-4-(2-furoyl)-5-pyrazolone with β-alanine and its copper (II) complex [CuLCH3OH]n (1) was synthesized and characterized by elemental analysis, UV, IR, 1H NMR, 13C NMR, MS, EPR, X-ray single crystal diffraction, variable temperature susceptibility measurement and thermogravimetric analysis. The crystal of the complex is a orthorhombic system with crystal parameters: P212121, a= 7.5169(6) Å, b= 13.2788(10) Å, c= 18.5631 (14) Å, α=90.00°, β= 90.00°, γ= 90.00°, V= 1852.9(2) Å3, Z= 4. It was found that the copper (II) complex had a distorted square pyramid coordination environment with two O, one N atoms of H2L ligand and one O atom at carboxyl group of another molecular in the square plane, and with CH3OH at the apex of polyhedron. The complex molecular is a coordination polymer. DNA-Binding properties of the Cu(II) complex and Schiff base ligand have been carried out by UV-vis, fluorescence spectra, circular dichroism spectra and in cleavage of plasmid pGreen DNA. The experimental results indicated that complex 1 and H2L both bind through electrostatic interaction to DNA. A competitive binding study showed that the enhanced emission intensity of ethidium bromide (EB) in the presence of DNA was quenched by the addition of complex 1 and the ligand, indicating that they displace EB from its binding site in DNA. Further, Circular dichroism (CD) spectra of DNA with the addition of the H2L and [CuLCH3OH]n exhibit a decrease in both the positive and negative peaks. Agarose gel electrophoresis shows that they can cleave plasmid pGreen DNA.

The tremendous advances in N-Heterocyclic carbenes (NHCs) have made them the ligand of choice for many transition-metalcatalyzed transformations. Due to the stability of the transition metal complexes of NHCs as well as their stability to air and moisture, NHCs are regarded as an attractive alternative to their phosphine counterparts. To date, research has largely focused on five-membered ring carbenes. However, several reports have demonstrated that changes to the size of the heterocyclic ring (six-, seven-, and eightmembered derivatives) can affect the properties including increased basicity, greater steric demand, and larger N-CNHC-N angle. In this review, we survey recent developments in the design and synthesis of some expanded ring NHCs with a particularly strong influence on the metal's coordination.