Showing papers on "Reagent published in 2011"

Electrografting: a powerful method for surface modification

Abstract: Electrografting refers to the electrochemical reaction that permits organic layers to be attached to solid conducting substrates. This definition can be extended to reactions involving an electron transfer between the substrate to be modified and the reagent, but also to examples where a reducing or oxidizing reagent is added to produce the reactive species. These methods are interesting as they provide a real bond between the surface and the organic layer. Electrografting applies to a variety of substrates including carbon, metals and their oxides, but also dielectrics such as polymers. Since the 1980s several methods have been developed, either by reduction or oxidation, and some of them have reached an industrial stage. This critical review describes the methods that are used for electrografting, their mechanism, the formation and growth of the layers as well as their applications (742 references).

Photocatalytic reduction of carbon dioxide over Ag cocatalyst-loaded ALa4Ti4O15 (A = Ca, Sr, and Ba) using water as a reducing reagent.

Abstract: Ag cocatalyst-loaded ALa4Ti4O15 (A = Ca, Sr, and Ba) photocatalysts with 3.79–3.85 eV of band gaps and layered perovskite structures showed activities for CO2 reduction to form CO and HCOOH by bubbling CO2 gas into the aqueous suspension of the photocatalyst powder without any sacrificial reagents. Ag cocatalyst-loaded BaLa4Ti4O15 was the most active photocatalyst. A liquid-phase chemical reduction method was better than impregnation and in situ photodeposition methods for the loading of the Ag cocatalyst. The Ag cocatalyst prepared by the liquid-phase chemical reduction method was loaded as fine particles with the size smaller than 10 nm on the edge of the BaLa4Ti4O15 photocatalyst powder with a plate shape during the CO2 reduction. CO was the main reduction product rather than H2 even in an aqueous medium on the optimized Ag/BaLa4Ti4O15 photocatalyst. Evolution of O2 in a stoichiometric ratio (H2+CO:O2 = 2:1 in a molar ratio) indicated that water was consumed as a reducing reagent (an electron donor) fo...

Copper-catalyzed C(sp3)-C(sp3) bond formation using a hypervalent iodine reagent: an efficient allylic trifluoromethylation.

Abstract: An efficient copper-catalyzed allylic trifluoromethylation reaction has been developed. This reaction provides a general and straightforward way to synthesize allylic trifluoromethylated compounds under mild conditions.

Strong enhancement on fenton oxidation by addition of hydroxylamine to accelerate the ferric and ferrous iron cycles.

Abstract: The Fenton system generates reactive species with high oxidation potential such as hydroxyl radicals (HO•) or ferryl via the reaction between Fe (II) and H2O2. However, a number of drawbacks limit its widespread application including the accumulation of Fe (III) and the narrow pH range limits, etc. The aim of this study is to propose a much more efficient Fenton-HA system which is characterized by combining Fenton system with hydroxylamine (NH2OH), a common reducing agent, to relieve the aforementioned drawbacks, with benzoic acid (BA) as the probe reagent. The presence of NH2OH in Fenton’s reagent accelerated the Fe (III)/Fe (II) redox cycles, leading to relatively steady Fe (II) recovery, thus, increased the pseudo first-order reaction rates and expanded the effective pH range up to 5.7. The HO• mechanism was confirmed to be dominating in the Fenton-HA system, and the generation of HO• was much faster and the amount of HO• formed was higher than that in the classical Fenton system. Furthermore, the majo...

Pd(II)-catalyzed para-selective C-H arylation of monosubstituted arenes.

Abstract: Pd-catalyzed highly para-selective C–H arylation of monosubstituted arenes (including toluene) is developed for the first time using an F+ reagent as a bystanding oxidant. This finding provides a new retrosynthetic disconnection for para-substituted biaryl synthesis via C–H/C–H cross-coupling.

Accelerated bimolecular reactions in microdroplets studied by desorption electrospray ionization mass spectrometry

Abstract: Functional group derivatization reactions occur in the course of microdroplet/surface collisions in the ambient ionization process of desorption electrospray ionization (DESI). The unique environment in the microdroplet causes rate enhancements of as much as several orders of magnitude in typical bimolecular reactions that proceed through either cationic or anionic intermediates. The environment in the evaporating charged microdroplet differs from that of the bulk: (i) the pH of the solution moves towards the extremes, (ii) the concentrations of the reagents increase, (iii) the relative surface area increases and (iv) collision frequencies increase. The rates of acid-catalyzed reactions, such as the reaction of Girard T reagent with ketosteroids, increase with decreasing pH in positively-charged microdroplets compared to the bulk solution rates. Similarly, the increased pH in evaporating negatively-charged microdroplets contributes to an increase in the rates of base-catalyzed Michael reactions over those recorded under bulk solution conditions. The amount of product formed depends on the reaction time and the droplet size. Nanoelectrospray ionization generates larger droplets than the secondary droplets of DESI so it does not show significant product formation in the analysis period and can be used to analyze products of the DESI experiments. When secondary microdroplets (ca. 1 micron diameter) are generated either by spraying a homogeneous solution of both reagents against an inert surface (reactive DESI) or when a solution of Girard T reagent is sprayed against a solid surface bearing the ketosteroid significant amounts of product are generated. In the case of the Michael reaction with cinnamic acid an alternative dehydrogenated reaction product is formed under microdroplet conditions. Some parallels between the phenomenon reported here and the rate acceleration seen in sonochemistry are noted. The potential value of mass spectrometry in establishing conditions that enhance reaction rates is also indicated. It is possible that these observations will assist in the selection of reaction conditions involving the use of charged microdroplets to enhance the rates of ordinary bulk chemical reactions, especially those involving strong steric hindrance.

Differential behaviour of the dinitrosalicylic acid (DNS) reagent towards mono- and di-saccharide sugars

Abstract: 3,5-Dinitrosalicylic acid (DNS) reagent is widely used in the estimation of reducing sugars. The reagent shows a differential behaviour towards mono- and di-saccharides. This phenomenon has been misinterpreted in the literature. Contrary to the facts, it has been reported that the DNS test is less sensitive for the estimation of cellobiose than it is for the estimation of glucose. This communication clarifies the concept. In addition, the study also compares the reaction of different mono- and di-saccharides and discusses the difference in their reactivity.

Phenanthrene synthesis by iron-catalyzed [4+2] benzannulation between alkyne and biaryl or 2-alkenylphenyl Grignard reagent.

Abstract: The [4 + 2] benzannulation reaction of internal or terminal alkynes with 2-biaryl, 2-heteroarylphenyl, or 2-alkenylphenyl Grignard reagents in the presence of Fe(acac)3, 4,4′-di-tert-butyl-2,2′-bipyridyl, and 1,2-dichloro-2-methylpropane takes place at room temperature in 1 h to give 9-substituted or 9,10-disubstituted phenanthrenes and congeners in moderate to excellent yields. The reaction tolerates sensitive functional groups such as bromide and olefin. When applied to a 1,3-diyne, the annulation reaction takes place on both acetylenic moieties to give a bisphenanthrene derivative.

Influence of the Sequence of the Reagents Addition in the Citrate-Mediated Synthesis of Gold Nanoparticles

Abstract: The role of sodium citrate (SC) in the formation and growing mechanism of gold nanoparticles was systematically examined. Obtained results show that oxidation of SC can be induced either by the presence of HAuCl4 or by its thermal decomposition under air. By exchanging the order of reagent addition, it is possible to increase the oxidation rate of SC and hence control the size and morphology, thus allowing preparation of nanoparticles with a narrower size distribution than the standard Turkevich approach.

Palladium-Catalyzed Mono-α-arylation of Acetone with Aryl Halides and Tosylates

Abstract: We report the first example of selective Pd-catalyzed mono-α-arylation of acetone employing aryl chlorides, bromides, iodides, and tosylates. The use of appropriately designed P,N-ligands proved to be the key to controlling the reactivity and selectivity. The reaction affords good yields with substrates containing a range of functional groups at modest Pd loadings using Cs2CO3 as the base and employing acetone as both a reagent and the solvent.

Structure and Reactivity of a Preactivated sp2–sp3 Diboron Reagent: Catalytic Regioselective Boration of α,β-Unsaturated Conjugated Compounds

Abstract: A novel sp2–sp3 diboron reagent has been developed for the copper-catalyzed β-boration of α,β-unsaturated conjugated compounds. The reaction proceeds under mild conditions with various substrates, i.e., α,β-unsaturated esters, ketones, nitriles, ynones, amides, and aldehydes, in the absence of additives such as phosphine and sodium tert-butoxide to provide β-borylhomoenolates in good to excellent yields. The presence of an sp3-hybridized boron center, unambigously confirmed by X-ray crystallography, sufficiently activates the unsymmetrical pinacolato diisopropanolaminato diboron (PDIPA diboron, 2) to transfer the sp2-hybridized boron moiety chemoselectively. These observations suggest that the activation of one of the boron atoms is an essential step in the Cu-catalyzed β-boration catalytic cycle.

Heterogeneous Reactions of Alkylamines with Ammonium Sulfate and Ammonium Bisulfate

Abstract: The heterogeneous reactions between alkylamines and ammonium salts (ammonium sulfate and ammonium bisulfate) have been studied using a low-pressure fast flow reactor coupled to an ion drift-chemical ionization mass spectrometer (ID-CIMS) at 293 ± 2 K. The uptake of three alkylamines, i.e., monomethylamine, dimethylamine, and trimethylamine, on ammonium sulfate shows a displacement reaction of ammonium by aminium, evidenced by the release of ammonia monitored using protonated acetone dimer as the reagent ion. For the three alkylamines, the initial uptake coefficients (γ0) range from 2.6 × 10–2 to 3.4 × 10–2 and the steady-state uptake coefficients (γss) range from 6.0 × 10–3 to 2.3 × 10–4 and decrease as the number of methyl groups on the alkylamine increases. A different reaction mechanism is observed for the uptake of the three alkylamines on ammonium bisulfate, which is featured by an acid–base reaction (neutralization) with irreversible alkylamine loss and no ammonia generation and occurs at a rate lim...

High-temperature stable, iron-based core-shell catalysts for ammonia decomposition.

Abstract: High-temperature, stable core-shell catalysts for ammonia de- composition have been synthesized. The highly active catalysts, which were found to be also excellent model sys- tems for fundamental studies, are based on a-Fe2O3 nanoparticles coated by porous silica shells. In a bottom-up approach, hematite nanoparticles were firstly obtained from the hydrothermal reaction of ferric chlorides, l-lysine, and water with adjustable average sizes of 35, 47, and 75 nm. Secondly, parti- cles of each size could be coated by a porous silica shell by means of the base-catalyzed hydrolysis of tetraethyl- A (TEOS) with cetyltetra- methylammonium bromide (CTABr) as porogen. After calcination, TEM, high-resolution scanning electron mi- croscopy (HR-SEM), energy-dispersive X-ray (EDX), XRD, and nitrogen sorption studies confirmed the success- ful encapsulation of hematite nanopar- ticles inside porous silica shells with a thickness of 20 nm, thereby leading to composites with surface areas of ap- proximately 380 m 2 g 1 and iron con- tents between 10.5 and 12.2 wt %. The obtained catalysts were tested in am- monia decomposition. The influence of temperature, iron oxide core size, pos- sible diffusion limitations, and dilution effects of the reagent gas stream with noble gases were studied. The catalysts are highly stable at 7508C with a space velocity of 120 000 cm 3 gcat 1 h 1 and maintained conversions of around 80 % for the testing period time of 33 h. On the basis of the excellent stability under reaction conditions up to 8008C, the system was investigated by in situ XRD, in which body-centered iron was determined, in addition to FeNx, as the crystalline phase under reaction condi- tions above 6508C.

Oxidative Modification of Native Protein Residues Using Cerium(IV) Ammonium Nitrate

Abstract: A new protein modification strategy has been developed that is based on an oxidative coupling reaction that targets electron-rich amino acids. This strategy relies on cerium(IV) ammonium nitrate (CAN) as an oxidation reagent and results in the coupling of tyrosine and tryptophan residues to phenylene diamine and anisidine derivatives. The methodology was first identified and characterized on peptides and small molecules, and was subsequently adapted for protein modification by determining appropriate buffer conditions. Using the optimized procedure, native and introduced solvent-accessible residues on proteins were selectively modified with polyethylene glycol (PEG) and small peptides. This unprecedented bioconjugation strategy targets these under-utilized amino acids with excellent chemoselectivity and affords good-to-high yields using low concentrations of the oxidant and coupling partners, short reaction times, and mild conditions.

Hydride as a Leaving Group in the Reaction of Pinacolborane with Halides under Ambient Grignard and Barbier Conditions. One-Pot Synthesis of Alkyl, Aryl, Heteroaryl, Vinyl, and Allyl Pinacolboronic Esters

Abstract: Grignard reagents (aliphatic, aromatic, heteroaromatic, vinyl, or allylic) react with 1 equiv of 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (pinacolborane, PinBH) at ambient temperature in tetrahydrofuran (THF) to afford the corresponding pinacolboronates. The initially formed dialkoxy alkylborohydride intermediate quickly eliminates hydridomagnesium bromide (HMgBr) and affords the product boronic ester in very good yield. Hydridomagnesium bromide (HMgBr) in turn disproportionates to a 1:1 mixture of magnesium hydride (MgH2) and magnesium bromide (MgBr2) on addition of pentane to the reaction mixture. DFT calculations (Gaussian09) at the B3LYP/6-31G(d) level of theory show that disproportionation of HMgBr to MgH2 and MgBr2 is viable in the coordinating ethereal solvents. This reaction also can be carried out under Barbier conditions, where the neat PinBH is added to the flask prior to the in situ formation of Grignard reagent from the corresponding organic halide and magnesium metal. Pinacolboronic ester syn...

Nickel-Catalyzed Cross-Coupling of Aryl Bromides with Tertiary Grignard Reagents Utilizing Donor-Functionalized N-Heterocyclic Carbenes (NHCs)

Abstract: Metal-catalyzed cross-coupling reactions are among the most important transformations in organic synthesis, allowing the efficient construction of complex structures from simpler, readily available building blocks. Many applications in large and small-scale synthesis can be found in different areas such as agrochemicals, pharmaceuticals and supramolecular chemistry. Whereas the coupling of sp-hybridized carbon atoms in either reaction partner is well established, the use of C ACHTUNGTRENNUNG(sp3)-hybridized substrates presents some challenges. Catalytic cross-coupling of sterically hindered tertiary alkyl substrates is especially difficult, generally resulting in low yields, and thus, only few reports exist. A big challenge in this field is not only to get the required level of reactivity, but also to overcome competing pathways like b-hydride elimination, hydrodehalogenation or isomerization. In an early report, Kumada et al. have shown a nickelcatalyzed cross-coupling of tertiary alkyl Grignard reagents with b-bromostyrene. Cahiez et al. could obtain the same product in similar yield using an Fe catalyst, however, b-hydride elimination became competitive. Furthermore, the group of Bell used a stoichiometric amount of a copper(I) salt to alkylate pyridines and quinolines. Recently, Hintermann et al. reported an impressive selective copper-catalyzed cross-coupling of tertiary Grignard reagents of certain azacyclic electrophiles, but this protocol is limited to chloroazacycles such as pyridines, pyrimidines, quinazolines and quinoxalines (Scheme 1). A general method for the coupling of tertiary alkyl metal reagents with aryl halides would be very desirable. Due to their attractive properties N-heterocyclic carbene (NHC) ligands have found widespread applications in organometallic chemistry and transition-metal catalysis. NHCs have already been successfully applied in Kumada–Corriu– Tamao reactions. Herein we report the Kumada– Corriu–Tamao-type nickel-catalyzed cross-coupling of aryl halides with tertiary Grignard reagents. This practical advance provides selective access to highly substituted tertiary alkyl benzene derivatives, an interesting class of chemical compounds. The coupling of 4-bromobiphenyl with tBuMgCl was used as the initial model system. The high reactivity of Grignard reagents is desirable in this coupling, as is their easy accessibility. Functional group compatibility, often a problem in reactions of Grignard reagents, can be expanded, if sufficiently mild reaction conditions can be realized. Using NHC ligands, we tried to prevent the formation of undesired side products. Thus, the sterically demanding easily synthesized ligand precursor L1, IAd·HBF4, was selected for the first screening reactions. Screening of L1 together with various commercial Ni, Pd and Cu sources showed [Ni ACHTUNGTRENNUNG(acac)2] to be a uniquely suited metal precursor, with all other complexes giving lower or no catalytic activity (Table 1, entries 1 and 3). Naturally, no product was obtained in the absence of any metal source (entry 2). In addition, different solvents, bases and reaction temperatures were evaluated. The reaction can be successfully run at 0 8C, and changing the temperature to +40 or 78 8C did lower the yield significantly (entries 4 and 5). The solvent played an important role, non-coordinating solvents only afforded poor or no conversion. On the contrary, ethers, especially THF, were suitable (entries 7, 14, 16). Without an NHC ligand, the desired product could be obtained in decreased yield and selectivity after rather long reaction time (entry 8). For many other substrates this effect was even more pronounced, as became obvious upon studying the substrate scope of this reaction (see below). [a] C. Lohre, T. Drcge, Dr. C. Wang, Prof. Dr. F. Glorius Westf lische Wilhelms-Universit t M nster Organisch-Chemisches Institut, Corrensstrasse 40 48149 M nster (Germany) Fax: (+49)251-833-3202 E-mail : glorius@uni-muenster.de Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201100909. Scheme 1. Challenging cross-couplings of tertiary alkyl Grignard reagents.