Elusive Dehydroalanine Derivatives with Enhanced Reactivity

For the first time, a simple methodology for the chemical synthesis and use of highly reactive 4-methylenoxazol-5(4H)-ones from serine is presented. These dehydroalanine derivatives, which resemble the natural 4-methylidenimidazole-5-one (MIO) cofactor present in lyases and aminomutases, undergo rapid reaction with carbon nucleophiles such as silyl enol ethers, as well as cycloaddition reactions with diazo compounds and reactive dienes, under very mild conditions and without any need for metal catalysts or ring-strain activation, offering potential for bioconjugation.     

Synthesis of α‐Diazo Carbonyl Compounds with the Shelf‐Stable Diazo Transfer Reagent Nonafluorobutanesulfonyl Azide

Nonafluorobutanesulfonyl azide is a shelf‐stable, cost‐effective and general diazo transfer reagent for the efficient synthesis of α‐diazo carbonyl compounds in excellent yields and in very short reaction times, under mild conditions. The diazo products can be readily isolated in pure form after a simple aqueous extractive work‐up that avoids chromatographic purification in most cases. Because of its high efficiency and wide substrate scope, shelf‐stability, relatively low cost, and ease of product purification, nonafluorobutanesulfonyl azide offers an advantageous alternative to other commonly used diazo transfer reagents.     

Transformations of molecules and secondary building units to materials: a bottom-up approach

A variety of complex inorganic solids with open-framework and other fascinating architectures, involving silicate, phosphate, and other anions, have been synthesized under hydrothermal conditions. The past few years have also seen the successful synthesis and characterization of several molecular compounds that can act as precursors to form open-framework and other materials, some of them resembling secondary building units (SBUs). Transformations of rationally synthesized molecular compounds to materials constitute an important new direction in both structural inorganic chemistry and materials chemistry and enable possible pathways for the rational design of materials. In this article, we indicate the potential of such a bottom-up approach, by briefly examining the transformations of molecular silicates and phosphates. We discuss stable organosilanols and silicate secondary building units, phosphorous acids and phosphate secondary building units, di- and triesters of phosphoric acids, and molecular phosphate clusters and polymers. We also examine the transformations of metal dialkyl phosphates and molecular metal phosphates.     

Application of modified clays in diazotization and azo coupling reactions in water

Diazotization and diazo coupling reaction of a series of aromatic amines and activated aromatic compounds over eco-friendly modified clay catalysts are described. The reaction was catalyzed by modified montmorillonite K-10 including Cu²⁺, Fe³⁺, and Zn²⁺ metal ions. The versatility of this method was checked by using various amines and phenols, which showed reasonable yields of products. These inexpensive, noncorrosive and reusable catalysts were found to exhibit bifunctional catalytic properties for diazotization and diazo coupling reactions. The catalysts could be reused several times without significant loss of their catalytic reactivity.     

Bicyclization of Diazomethanes: A Synthetic Strategy for Fused Pyrazoles

On the terminal diazo nitrogen atom, two C N bonds are formed in the bicyclization reaction of the readily available acyclic substrates with diazomethanes as 1,3‐dipoles. The tandem 1,3‐dipolar cycloaddition/intramolecular aza‐addition/oxidative aromatization reaction can tolerate a variety of functional groups and proceeds under mild metal‐free conditions to give substituted analogues of withasomnine alkaloids in good to high yields in a single step.     

CATALYST PRODUCT SEPARATION TECHNIQUES IN HECK REACTION

The Heck reaction finds several applications in industry because it is one of the effective tools for the formation of a new C─C bond. In addition to the catalytic activity and selectivity, catalyst-product separation strategies are very important for the industrial application. There are various methods of interest ranging from conventional heterogeneous catalysts to heterogenization of homogeneous catalysts. The heterogeneous catalysts are classified into supported metal catalysts, zeolite-encapsulated catalysts, colloids-nanoparticles, and intercalated metal compounds. The homogeneous metal complexes catalysts are heterogenized using modified silica catalysts, polymer-supported catalysts, biphasic catalysts, supported liquid-phase catalysts, nonionic liquids solvents, perfluorinated solvents, and reusable homogeneous complexes. In general, heterogeneous catalysts are effective and stable at higher temperatures, which may be important for the activation of less reactive but less expensive chloroaryls substrates. However, the heterogeneous catalysts have a major drawback of poor selectivity toward Heck coupling products. The heterogenized metal complexes catalysts operate under relatively mild conditions as compared with heterogeneous catalysts, and so they can be applied to the production of pharmaceuticals and fine chemicals. Catalysis using supercritical solvents with catalyst separation techniques is promising for the development of green chemistry processes. Although the concepts described in this article have been reviewed mainly for Heck reactions, they should be applicable to a wide range of other chemical transformations (hydrogenation, carbonylation, hydroformylation, and so on) that, currently, are homogeneously catalyzed reactions.     

On the nature of simultaneous formation of nano- and micron-size diamond fractions under pressure–temperature-induced transformations of binary mixtures of hydrocarbon and fluorocarbon compounds

Based on comparative studies of pressure–temperature-induced transformations of naphthalene, octafluoronaphthalene, and their binary mixtures, the nature of formation of nano- and micron-size diamond fractions in the products of transformations of hydrocarbons and fluorocarbons has been revealed. It was found that the main reason for the massive formation of nano-size diamonds is the specifics of carbonization of fluorocarbon compounds under pressure. In this particular process, micron-size particles of graphite are formed simultaneously with a significant amount of closed two- to five-layered carbon nanoparticles of 5–15-nm size, acting as precursors for the formation of nano-size diamond fractions. The obtained results open up a new avenue for the metal catalyst-free synthesis of nano/micron-size fractions of pure and doped diamonds.     

Olefin cyclopropanations via sequential atom transfer radical addition-dechlorination reactions

In organic synthesis, cyclopropanation reactions are often performed with Simmons-Smith-type reagents or by transition metal catalyzed reactions of olefins with diazo compounds. A novel method for the synthesis of substituted cyclopropanes is described that is based on a two-step reaction sequence. Olefins are reacted with 1,1'-dichlorides in a Ru-catalyzed atom transfer radical addition (ATRA) process and the resulting 1,3-dichlorides are directly converted into cyclopropanes by reductive coupling with magnesium. This one-pot procedure is applicable to a variety of substrates and can be performed in an inter- or intramolecular fashion.     

Catalytic activity of metals in petroleum and their removal

Experiments are described to elucidate the transformations of metals and metalloporphyrins during hydrosulphurization and to probe the catalytic implications of these transformations. It is shown that at least two reactions involving metal compounds are important, the catalytic hydrogenation reaction and a direct metal extraction by H₂S, both of which result in deposition of metal as sulphide on the catalyst. The former reaction predominates with metalloporphyrins but there is evidence that the latter may be significant with non-porphyrinic metals. The catalytic activity of the resultant sulphide deposits, in particular VS_x, although measurable, is perhaps an order of magnitude lower than that of CoMo.     

两种三芳胺类电荷传输材料的合成与光电导性能

以二苯溴甲烷和对硝基苯甲醛为原料,经过W ittig、还原和U llm ann 3步反应,合成了N,N-二苯基-4-(2,2-二苯基乙烯基)苯胺(Ⅴa)和N,N-二(4-甲基苯基)-4-(2,2-二苯基乙烯基)苯胺(Ⅴb)两个化合物,并且通过IR和MS对其进行了结构鉴定。以目标化合物Ⅴa和Ⅴb为电荷传输材料,分别以酞菁氧钛、偶氮类化合物为电荷产生材料复配制备的双层光电导体,在白光和红光曝光下测试了它们的光电导性能。结果表明,在白光曝光下,与偶氮类化合物复配时,其暗衰和残余电位低达7.5 V/s,5 V和7.5 V/s,3 V;与酞菁氧钛复配时,其光敏度仅为0.8 lux.s和1.0 lux.s。在红光曝光下,其电荷的保持能力高,暗衰率和残余电位都很低,光敏度低达0.024lux.s。这说明化合物Ⅴa和Ⅴb具有优良的电荷传输性能。     

One‐Step Synthesis of Strained Bicyclic Carboxylic and Boronic Amino Esters via Ruthenium‐Catalysed Tandem Carbene Addition/Cyclopropanation of Enynes

The reaction of 1,6‐ and 1,7‐enynes, derived from carboxylic and boronic amino acids, with diazo compounds in the presence of the (cyclooctadiene)(pentamethylcyclopentadiene)ruthenium chloride complex [RuCl(cod)(C₅Me₅)] catalyst leads to the formation of strained bicyclic proline or homoproline derivatives in good yields. This catalytic transformation proceeds under mild conditions, in one step from easily accessible enynes and was applied to various protecting groups. High stereoselectivities for the created alkenyl chain and excellent diastereoselectivities for proline derivatives were obtained.     

Catalytic, asymmetric sulfur ylide-mediated epoxidation of carbonyl compounds: scope, selectivity, and applications in synthesis

The reaction of sulfur ylides with carbonyl compounds to give epoxides is an important synthetic method. This Account charts the recent advances in rendering this process both asymmetric and catalytic. Two catalytic methods have been developed: the first involving the reaction of a sulfide with an alkyl halide in the presence of a base and aldehyde and the second involving the reaction of a sulfide with a diazo compound or diazo precursor in the presence of a metal catalyst and aldehyde. These catalytic methods coupled with suitable chiral sulfides provide a new catalytic asymmetric epoxidation process for the preparation of epoxides. The scope of the two catalytic processes is discussed together with the factors that influence both relative and absolute stereochemistry. The application of these methods in target-orientated synthesis is also reviewed.     

Plant Products for Pharmacology: Application of Enzymes in Their Transformations

Different plant products have been subjected to detailed investigations due to their increasing importance for improving human health. Plants are sources of many groups of natural products, of which large number of new compounds has already displayed their high impact in human medicine. This review deals with the natural products which may be found dissolved in lipid phase (phytosterols, vitamins etc.). Often subsequent convenient transformation of natural products may further improve the pharmacological properties of new potential medicaments based on natural products. To respect basic principles of sustainable and green procedures, enzymes are often employed as efficient natural catalysts in such plant product transformations. Transformations of lipids and other natural products under the conditions of enzyme catalysis show increasing importance in environmentally safe and sustainable production of pharmacologically important compounds. In this review, attention is focused on lipases, efficient and convenient biocatalysts for the enantio- and regioselective formation / hydrolysis of ester bond in a wide variety of both natural and unnatural substrates, including plant products, eg. plant oils and other natural lipid phase compounds. The application of enzymes for preparation of acylglycerols and transformation of other natural products provides big advantage in comparison with employing of conventional chemical methods: Increased selectivity, higher product purity and quality, energy conservation, elimination of heavy metal catalysts, and sustainability of the employed processes, which are catalyzed by enzymes. Two general procedures are used in the transformation of lipid-like natural products: (a) Hydrolysis/alcoholysis of triacylglycerols and (b) esterification of glycerol. The reactions can be performed under conventional conditions or in supercritical fluids/ionic liquids. Enzyme-catalyzed reactions in supercritical fluids combine the advantages of biocatalysts (substrate specificity under mild reaction conditions) and supercritical fluids (high mass-transfer rate, easy separation of reaction products from the solvent, environmental benefits based on excluding organic solvents from the production process).     

In situ Raman spectroscopy studies of catalysts

In situ Raman spectroscopy is rapidly becoming a very popular catalyst characterization method because Raman cells are being designed that can combine in situ molecular characterization studies with simultaneous fundamental quantitative kinetic studies. The dynamic nature of catalyst surfaces requires that both sets of information be obtained for a complete fundamental understanding of catalytic phenomena under practical reaction conditions. Several examples are chosen to highlight the capabilities of in situ Raman spectroscopy to problems in heterogeneous catalysis: the structural determination of the number of terminal M=O bonds in surface metal oxide species that are present in supported metal oxide catalysts; structural transformations of the MoO₃/SiO₂ and MoO₃/TiO₂ supported metal oxide catalysts under various environmental conditions, which contrast the markedly different oxide–oxide interactions in these two catalytic systems; the location and relative reactivity of the different surface M–OCH₃ intermediates present during CH₃OH oxidation over V₂O₅/SiO₂ catalysts; the different types of atomic oxygen species present in metallic silver catalysts and their role during CH₃OH oxidation to H₂CO and C₂H₄ epoxidation to C₂H₄O; and information about the oxidized and reduced surface metal oxide species, isolated as well as polymerized species, present in supported metal oxide catalysts during reaction conditions. In summary, in situ Raman spectroscopy is a very powerful catalyst characterization technique because it can provide fundamental molecular‐level information about catalyst surface structure and reactive surface intermediates under practical reaction conditions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Degradation of rubber to metals bonds during its cathodic delamination, validation of an artificial ageing test

Rubber-metal assemblies used for submarine applications are often associated to cathodic protection which can lead to cathodic disbonding of the polymer/metal interface.The degradation mechanisms of polychloroprene/epoxy adhesive/copper-aluminium substrate systems associated to an accelerated ageing test are compared to the degradations resulting from natural ageing conditions in the Mediterranean sea. From destructives tests, a kinetic in four steps is identified for both ageing conditions. This kinetic is confirmed by Electrochemical Impedance Spectroscopy data during ageing. In order to further identify the failure mode, the disbonded surfaces on polymer and steel sides are analyzed using SEM and FTIR. Both ageing conditions present the same type of degradation which validates the fact that the accelerated ageing protocol reproduces the natural conditions. The chemical transformations in delaminated epoxy are very similar to the chemical transformation in epoxy coatings under high energy electron-beam irradiation. However, bidendate metal carboxylates rather than carboxylic acid chain ends are the main oxidation products due to the alkaline environment at the coating-metal interface. It was shown that the metal/polymer interface is not destroyed by an alkaline attack but by a strongly oxidative attack caused by the intermediates of the oxygen reduction.     

Improved procedure for titrating cyclopropene esters with hydrogen bromide

The titration of cyclopropenes in cottonseed and other oils with hydrogen bromide was improved by changes in the preparation of the sample and the titration. Methanolysis of the oil under selected conditions with a large excess of alkaline catalyst (sodium methoxide or tetramethylammonium hydroxide) yielded methyl esters free of oxidation products and partial glycerides and eliminated the need for treatment with a large amount of activated alumina, which disproportionates methyl esters. The color indicator, 4-phenylazodi-phenylamine, exhibited great sensitivity in toluene at 25 C. The best procedure consisted of titrating methyl esters with hydrogen bromide at 60–65 C to just past the end-point, cooling to about 25 C, and back-titrating with aniline in toluene. With 10- to 2-g samples containing less than 1% cyclopropenes, reproducibility usually was within ±0.003% of the average value.     

Sulphur poisoning and regeneration of precious metal catalysed methane combustion

In many of the applications of catalytic combustion small amounts of sulphur compounds are present in the feed gas. In the case of natural gas combustion, and solid fuel syngas, the sulphur compounds are in reduced forms. The present work investigates the influence of small quantities of reduced sulphur compounds on the combustion of methane over alumina-supported precious metal catalysts. The kinetics of the methane combustion in the presence of low concentrations of a mixture of sulphur compounds (ethyl mercaptan, methyl mercaptan, carbonyl sulphide and hydrogen sulphide) are compared with those in the absence of sulphur compounds. The ease of regeneration of the poisoned catalysts, via low temperature reduction with hydrogen, is also examined. In the conditions studied all catalysts have reduced activity in the presence of the sulphur-based gas mixture, but Pt/Al₂O₃ and Rh/Al₂O₃ are more strongly poisoned than Pd/Al₂O₃. Qualitative studies using gas chromatography with atomic emission detection of the exhaust gases, and FTIR spectroscopy of the spent Rh/Al₂O₃ catalyst, suggest that the catalysts experience a mixture of reduced and oxidised species under reaction conditions, and that sulphating of the support occurs. The regeneration step facilitates metal mobility and meets with varying success depending upon the metal. Pt/Al₂O₃ in particular is difficult to regenerate by reduction in hydrogen (400 °C for 0.5 h), and agglomeration is observed by TEM. Rh/Al₂O₃ regenerates well, and low metal particle size is maintained. A non-linear deactivation model is tested to separate sulphur-induced deactivation from ‘natural’ deactivation in the reaction mixture and preliminary results are presented.     

An Overview on the Performance of 1,2,3-Triazole Derivatives as Corrosion Inhibitors for Metal Surfaces

This review accounts for the most recent and significant research results from the literature on the design and synthesis of 1,2,3-triazole compounds and their usefulness as molecular well-defined corrosion inhibitors for steels, copper, iron, aluminum, and their alloys in several aggressive media. Of particular interest are the 1,4-disubstituted 1,2,3-triazole derivatives prepared in a regioselective manner under copper-catalyzed azide-alkyne cycloaddition (CuAAC) click reactions. They are easily and straightforwardly prepared compounds, non-toxic, environmentally friendly, and stable products to the hydrolysis under acidic conditions. Moreover, they have shown a good efficiency as corrosion inhibitors for metals and their alloys in different acidic media. The inhibition efficiencies (IEs) are evaluated from electrochemical impedance spectroscopy (EIS) parameters with different concentrations and environmental conditions. Mechanistic aspects of the 1,2,3-triazoles mediated corrosion inhibition in metals and metal alloy materials are also overviewed.     

Group 14 hydrides with low valent elements for activation of small molecules

Transition metal compounds are well known as activators of small molecules, and they serve as efficient catalysts for a variety of homogeneous and heterogeneous transformations. In contrast, there is a general feeling that main group compounds cannot act as efficient catalysts because of their inability to activate small molecules. Traditionally, the activation of small molecules is considered one of the key steps during a catalytic cycle with transition metals. As a consequence, researchers have long neglected the full range of possibilities in harnessing main group elements for the design of efficient catalysts. Recent developments, however, have made it possible to synthesize main group compounds with low-valent elements capable of activating small molecules. In particular, the judicious use of sterically appropriate ligands has been successful in preparing and stabilizing a variety of Group 14 hydrides with low-valent elements. In this Account, we discuss recent advances in the synthesis of Group 14 hydrides with low-valent elements and assess their potential as small-molecule activators. Group 14, which comprises the nonmetal C, the semimetals Si and Ge, and the metals Sn and Pb, was for years a source of hydrides with the Group 14 element almost exclusively in tetravalent form. Synthetic difficulties and the low stability of Group 14 hydrides in lower oxidation states were difficult to overcome. But in 2000, a divalent Sn(II) hydride was prepared as a stable compound through the incorporation of sterically encumbered aromatic ligands. More recently, the stabilization of GeH(2) and SnH(2) complexes using an N-heterocyclic carbene (NHC) as a donor and BH(3) or a metal carbonyl complex as an acceptor was reported. A similar strategy was also employed to synthesize the Si(II) hydride. This class of hydrides may be considered coordinatively saturated, with the lone pair of electrons on the Group 14 elements taking part in coordination. We discuss the large-scale synthesis of hydrides of the form LMH (where M is Ge or Sn, L is CH(N(Ar)(CMe))(2), and Ar is 2,6-iPr(2)C(6)H(3)), which has made it possible to test their reactivity in the activation of small molecules. Unlike the tetravalent Group 14 hydrides, the Ge(II) and Sn(II) hydrides were found to be able to activate a number of small molecules in the absence of any externally added catalyst. For example, the Ge(II) hydride and Sn(II) hydride can activate CO(2), and the reaction results in the formation of Ge(II) and Sn(II) esters of formic acid. This product represents a prototype of a new class of compounds of Group 14 elements. Moreover, we examined the activation of carbonyl compounds, alkynes, diazo and azo compounds, azides, and compounds containing the C═N bond. These Group 14 hydrides with low-valent elements are shown to be able to activate a number of important small molecules with C≡C, C═O, N═N, and C═N bonds. The activation of small molecules is an important step forward in the realization of main group catalyst development. Although it is not yet customary to assay the potential of newly synthesized main group compounds for small-molecule activation, our results offer good reason to do so.     

Borrowing Hydrogen in the Activation of Alcohols

Alcohols can be temporarily converted into carbonyl compounds by the metal‐catalysed removal of hydrogen. The carbonyl compounds are reactive in a wider range of transformations than the precursor alcohols and can react in situ to give imines, alkenes, and α‐functionalised carbonyl compounds. The metal catalyst, which had borrowed the hydrogen, then returns it to the transformed carbonyl compound, leading to an overall process in which alcohols can be converted into amines, compounds containing C C bonds and β‐functionalised alcohols.