Oxidation of benzoin catalyzed by oxovanadium (IV) schiff base complexes

BACKGROUND: The oxidative transformation of benzoin to benzil has been accomplished by the use of a wide variety of reagents or catalysts and different reaction procedures. The conventional oxidizing agents yielded mainly benzaldehyde or/and benzoic acid and only a trace amount of benzil. The limits of practical utilization of these reagents involves the use of stoichiometric amounts of corrosive acids or toxic metallic reagents, which in turn produce undesirable waste materials and required high reaction temperatures. In recent years, vanadium complexes have attracted much attention for their potential utility as catalysts for various types of reactions. RESULTS: Active and selective catalytic systems of new unsymmetrical oxovanadium(IV) Schiff base complexes for the oxidation of benzoin is reported. The Schiff base ligands are derived between 2-aminoethanol and 2-hydroxy-1- naphthaldehyde (H2L1) or 3-ethoxy salicylaldehyde (H2L3); and 2-aminophenol and 3-ethoxysalicylaldehyde (H2L2) or 2-hydroxy-1-naphthaldehyde (H2L4). The unsymmetrical Schiff bases behave as tridentate dibasic ONO donor ligands. Reaction of these Schiff base ligands with oxovanadyl sulphate afforded the mononuclear oxovanadium(IV) complexes (VIVOLx.H2O), which are characterized by various physico-chemical techniques. The catalytic oxidation activities of these complexes for benzoin were evaluated using H2O2 as an oxidant. The best reaction conditions are obtained by considering the effect of solvent, reaction time and temperature. Under the optimized reaction conditions, VOL4 catalyst showed high conversion (>99%) with excellent selectivity to benzil (~100%) in a shorter reaction time compared to the other catalysts considered. CONCLUSION: Four tridentate ONO type Schiff base ligands were synthesized. Complexation of these ligands with vanadyl(IV) sulphate leads to the formation of new oxovanadium(IV) complexes of type VIVOL.H2O. Elemental analyses and spectral data of the free ligands and their oxovanadium(IV) complexes were found to be in good agreement with their structures, indicating high purity of all the compounds. Oxovanadium complexes were screened for the oxidation of benzoin to benzil using H2O2 as oxidant. The effect of time, solvent and temperature were optimized to obtain maximum yield. The catalytic activity results demonstrate that these catalytic systems are both highly active and selective for the oxidation of benzoin under mild reaction conditions.Web of Scienc

Improving the extraction and purification of leaf and phloem sugars for oxygen isotope analyses

Rationale The oxygen isotopic composition (here shown as the δ 18O value) of soluble sugars in leaves and phloem tissue holds valuable information about plant functions in response to climatic changes. However, δ 18O analysis of sugars is prone to error, and thoroughly tested methods are lacking. Methods We performed three experiments to test if sample preparation modifies the δ 18O values of sugars. In experiment 1, we tested the effects of oven‐drying versus freeze‐drying, whereas in experiment 2 we focused on the extraction and purification of leaf sugars. In experiment 3, we investigated the exudation and purification of twig phloem sugars as a function of exudation time and different ethylenediaminetetraacetic acid (EDTA) exudation media. Results Freeze‐drying produced more consistent δ 18O values than oven‐drying for sucrose but not for phloem sugars. The extraction and purification of leaf sugars can be performed without a significant modification of their δ 18O values; yet the purified leaf and phloem sugars possessed higher δ 18O values than the fraction of water‐soluble compounds. Moreover, the exudation time significantly modulated the δ 18O values of phloem sugars, which is probably related to changes in the sugar composition. The addition of EDTA did not improve the determination of the δ 18O values of phloem sugars. Conclusions We show that the sample preparation of plant sugars for the reliable determination of δ 18O values requires a strict protocol, which is described in this paper. For phloem sugar, we recommend a maximum exudation time of 1 h to reduce the degradation of sucrose and minimise oxygen isotope exchange reactions between the resulting hexoses and water.ISSN:1097-0231ISSN:0951-419

Fluorescence enhancement through enzymatic cleavage of internally quenched dendritic peptides: a sensitive assay for the AspN endoproteinase

Directed precipitation of silicic acid during the formation of diatom cell walls is promoted by polyamines (see picture) and peptides. These species‐specific organic matrices, which direct the biomineralization process to amorphous SiO2, are accessible only after dissolution of the silica cell wall with hydrogen fluoride

Analysis and Experimental Inhibition of Distal Cholesterol Biosynthesis

Proteomic