Ring-enlargement reactions of donor-acceptor-substituted cyclopropanes: which combinations are most efficient?

A detailed theoretical study of ring-enlargement reactions of 72 differently substituted donor-acceptor-substituted cyclopropanes is presented. Transition states, activation barriers, and, for representative examples, the behavior in solution were additionally determined using the B3LYP/6-311G(d) level of theory.     

Hydrogen electrocatalysis on overlayers of rhodium over gold and palladium substrates--more active than platinum?

We have investigated the stability and catalytic activity of epitaxial overlayers of rhodium on Au(111) and Pd(111). Both surfaces show a strong affinity for hydrogen. We have calculated the energy of adsorption both for a strongly and a more weakly adsorbed species; the latter is the intermediate in the hydrogen evolution reaction. Both the energy of activation for hydrogen adsorption (Volmer reaction) and hydrogen recombination (Tafel reaction) are very low, suggesting that these overlayers are excellent catalysts.     

The nature of active sites in Pt–ReO_X/TiO_2 catalysts for selective hydrogenation of carboxylic acids to alcohols

The highly dispersed Pt–Re OX(x ≤ 1) sites ca. 0.5 nm in size were formed via a successive and strong interaction of the Re precursor with titania and then of the Pt complex with deposited low-valent rhenium oxide clusters. The size, charge and chemical composition were characterized by means of HRTEM/STEM with EDX mapping, XPS, and FTIRS. These sites with Re/Pt = 2 were shown to be highly active and selective in the hydrogenation of carboxylic acid to alcohol under very mild conditions(T = 130 °C, P = 50 bar). The reaction rate constant for the hydrogenation of hexanoic acid increased linearly with the Pt content. As for the homogeneous pincer-type Ru-organic complexes, the active Pt–Re OXsites can dissociate heterolytically the molecular hydrogen with the formation of hydroxyl groups and Pt hydride for hydrogenation of the carboxylic group. Indeed, TOF of 20 h-1 and selectivity of 98%–99% are approaching the values typical of homogeneous catalysts. The first order kinetics described well the experimental data obtained in a wide range of reaction conditions.     

A highly active and selective palladium pincer catalyst for the formation of α-aryl ketones via cross-coupling

Several air and moisture stable Pd(II) pincer complexes were synthesized via oxidative addition of Pd(0) to novel PheBox pincer ligand precursors. Low loadings (1 mol%) of the Pd complex [t-BuPhebox-Me(2)]PdBr are capable of efficiently promoting the selective α-monoarylation of a variety of ketones with numerous aryl bromides in only 1 h at 70 °C with 82-99% yields.     

Oxidic or metallic palladium: which is the active phase in pd-catalyzed aerobic alcohol oxidation?

In situ X-ray absorption spectroscopy combined with on-line catalytic measurements using FT-IR spectroscopy unequivocally identified that metallic palladium is the more active phase in the aerobic oxidation of benzyl alcohol than palladium oxide. The aerobic oxidation of benzyl alcohol in cyclohexane at 50 degrees C was low over oxidized 0.5%Pd/Al2O3 and 5%Pd/Al2O3 catalysts. XANES and EXAFS showed that the catalysts in the as-received state were almost fully oxidized and no reduction of the palladium constituent was observed during time-on-stream. After in situ reduction by hydrogen-saturated cyclohexane, the catalysts were much more active (over 50 times) than before reduction. Both XANES and EXAFS uncovered that the palladium constituent was mainly in a reduced state under these conditions of high catalytic activity. This demonstrates that metallic palladium is the active phase for alcohol dehydrogenation.     

PEG-stabilized palladium nanoparticles： An efficient and recyclable catalyst for the selective hydrogenation of 1,5-cyclooctadiene in thermoregulated PEG biphase system

Polyethylene glycol （PEG）-stabilized palladium nanoparticles were prepared and applied to the selective hydrogenation of 1,5- cyclooctadiene （1,5-COD） in thermoregulated PEG biphase system, which allows a reaction in a single-phase at a higher temperature followed by a phase split at a lower temperature. Under the optimized reaction conditions, the conversion of 1,5-COD and the selectivity of cyclooctene （COE） were 100 and 98%, respectively. The catalyst could be easily separated from the product by phase separation and reused for 6 times without evident loss in activity and selectivity. 2007 Yan Hua Wang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.     

Pore size or geometry: which determines the shape and inverse-shape selective adsorption of alkane isomers?

The adsorption of pure pentane (C(5)) isomers and their ternary mixture is simulated in a series of carbon nanoslits. With decreasing nanoslit pore size, shape selective adsorption first occurs in the order of nC(5) > or = iC(5) > neoC(5) due to the configurational entropy effect, then inverse-shape selective adsorption occurs in the order of nC(5) < iC(5) < or = neoC(5) due to the area entropy effect, and finally no adsorption occurs. The entropy effects lead to a large adsorptive separation among the C(5) isomers from their mixture. Similar behavior has been observed from the simulation of C(5) adsorption in carbon nanotubes with variation in pore size. These results reveal that pore size rather than geometry determines the shape and inverse-shape selective adsorption of alkane isomers in nanopores.     

Adsorption of gases in carbon nanotubes: are defect interstitial sites important?

Molecular simulations are used to shed light on an ongoing controversy over where gases adsorb on single walled carbon nanotube bundles. We have performed simulations using models of carbon nanotube bundles composed of tubes of all the same diameter (homogeneous) and tubes of different diameters (heterogeneous). Simulation data are compared with experimental data in an effort to identify the best model for describing experimental data. Adsorption isotherms, isosteric heats of adsorption, and specific surface areas have been computed for Ar, CH 4, and Xe on closed, open, and partially opened homogeneous and heterogeneous nanotube bundles. Experimental data from nanotubes prepared from two different methods, electric arc and HiPco, were examined. Experimental adsorption isotherms and isosteric heats for nanotubes prepared by the electric arc method are in best agreement with simulations for heterogeneous bundles of closed nanotubes. Models including adsorption in defect interstitial channels are required to achieve good agreement with experiments. Experimental isosteric heats and specific surface areas on HiPco nanotubes are best described by a model consisting of heterogeneous bundles with approximately 11% of the nanotubes opened.     

Heat-treated Fe/N/C catalysts for O2 electroreduction: are active sites hosted in micropores?

Limited availability of platinum is a potential threat to fuel cell commercialization. Since the 1970s, alternative catalysts to the electrochemical reduction of oxygen have been obtained from heat treatment at T > 600 degrees C of carbon with a non-noble metal and a source of nitrogen atoms. However, the process by which the heat treatment activates these materials remains an open question. Here, we report that the activation process of carbon black and iron acetate heat-treated in NH(3) comprises three consecutive steps: (i) incorporation of nitrogen atoms in the carbon, (ii) micropore formation through reaction between carbon and ammonia, and (iii) completion of active sites in the micropores by reaction of iron with ammonia. Step (ii) is the slowest. Moreover, the microporous surface per mass of catalyst controls the macroscopic activity when enough nitrogen atoms are incorporated in the structure of the carbon support. These facts should help in determining the structure of the active sites and in identifying methods to increase the site density of such catalysts.     

What causes iron-sulphur bonds in active sites of one-iron superoxide reductase and two-iron superoxide reductase to differ?

The electronic and vibrational properties of [Fe(NHis)₄(SCys)] sites responsible for the catalysis of superoxide reduction in two types of superoxide reductase (SOR), one-iron superoxide reductase (1Fe-SOR) and two-iron superoxide reductase (2Fe-SOR), were compared previously (Clay et al., 2003); the differences between these two classes of SOR, examined by UV-VIS and NIR absorption, VTMCD, and vibrational spectroscopy techniques, were interpreted as being indicative of weaker Fe-S bonds in 2Fe-SOR in comparison with 1Fe-SOR. Here, we report on density functional (DFT) and semi-empirical (ZINDO/S-CI) calculations exploring the extent of this difference in bonding between the two classes of SOR. The differences observed experimentally between the electronic spectra of the two SORs are shown to probably arise either from different degrees of torsion between the Fe—ligand bonds or from differences in length of the Fe—carboxylate bond, but are shown to be incompatible with any significant differences in Fe—S bond lengths. The differences observed in the vibrational spectra between the two SORs are shown to correlate with differences in the Fe-S bond length of no more than 0.01 ?, which in turn arise from slight differences in the polarity of the medium surrounding the iron active site in the two proteins.     

Structure and stability of short beta-peptide nanotubes: a non-natural representative of collagen?

Since secondary structure elements are known to play a key role in stabilizing the 3D-fold of proteins for the design of non-natural proteins composed of beta-amino acid residues, the construction of suitable secondary structural elements is mandatory. Folding analogues of alpha-helices and beta-strands of beta-polypeptides were already described (Chem. Biodiversity 2004, 1, 1111 (1)). Here, we present several collagen-like folds composed exclusively of beta-Ala(s). Unlike their natural counterpart, these tubular nanostructures can be composed of more than three polypeptide chains aligned parallel and/or antiparallel. By using ab initio and DFT calculations we have optimized a large number of versatile collagen-like antiparallel nanostructures. In these tubular systems, oligopeptide strands are interconnected by i --> (i) type H-bonds, except for the "closing" set. This latter is called "the H-bond zipper" and is either (i) --> i, ( i + 1) --> i, or ( i + 2) --> i type. Antiparallel, tubular foldamers composed of l number of strands, each of k number of beta-amino acid residues (e.g., apbeta-T(l) i+l ) k , ap(beta-T(l) i+1 ) k , or ap(beta-T(l) i+2 ) k ), are unexpectedly stable supramolecular complexes. Independent of k and l, the local backbone fold of the amino acid residues is usually spiral, abbreviated as "S(P)" or "S*(P)". Nevertheless, in contrast to parallel, in antiparallel nanotubes the backbone fold can occasionally twist out from S(P) or S*(P) type into an alternative local structure. However, the more the local geometry of the strands resembles to S(P) or S*(P), the higher the stability is. Besides the backbone twisting, the overall stability is determined by the type and the geometrical properties of the constituent H-bonds. Interestingly, higher number of total H-bonds can provide a lower overall stability, when H-bond parameters are inferior. In general, the increase of both the number of strands and their length stabilize the supramolecular complex. Now that, for beta-peptides, collagen-like overall folds with their stability were determined, their POG- or PPG-like sequence specificity has to be revealed.     

Where are ionic liquid strategies most suited in the pursuit of chemicals and energy from lignocellulosic biomass?

Certain ionic liquids have been shown to dissolve cellulose, other biopolymers, and even raw biomass under relatively mild conditions. This particular ability of some ionic liquids, accompanied by a series of concurrent advantages, enables the development of improved processing strategies for the manufacturing of a plethora of biopolymer-based advanced materials. The more recent discoveries of dissolution of lignocellulosic materials (e.g., wood) in ionic liquids, with at least partial separation of the major constituent biopolymers, suggest further paths towards the achievement of a truly sustainable chemical and energy economy based on the concept of a biorefinery which provides chemicals, materials, and energy. Nonetheless, questions remain about the use of ionic liquids and the advisability of introducing any new process which utilizes bulk synthetic chemicals which have to be made, disposed of, and prevented from entering the environment. In this article, we discuss our own journey from the discovery of the dissolution of cellulose in ionic liquids to the cusp of an enabling technology for a true biorefinery and consider some of the key questions which remain.     

Cationic–anionic palladium complexes: effect of hydrogen bond character on their stability and biological activity

The solution state of palladium cationic–anionic complexes (AmH _n ) _k [PdCl₄] prepared for the first time, where Am is morpholine, methylmorpholine, aminoethylmorpholine, 5-aminovaleric acid, L-1-phenyl-2-methylaminopropanol, and m-xylilenediamine, has been studied by electronic absorption spectroscopy, NMR, and pH measurements. The agreement of obtained results for the state of the complexes in water and NaCl solutions with IR and X-ray diffraction data for these complexes has allowed us to substantiate the principle for designing patent formulation (C₅H₁₂NO)₂[PdCl₄], a new type of palladium complexes, palladium(II) cationic–anionic complexes showing high antitumor and antimetastatic activity. Crystallographic data for six obtained complexes have been presented.     

Structure and reaction pathway of TMP-Zincate: amido base or alkyl base?

The novel directed ortho metalation (DoM) reagents for functionalized aromatic rings, TMP-Zn-ates (R2Zn(TMP)Li (R = Me, 1; tBu, 2)), have been reported to be synthetically useful for the chemo- and regioselective construction of multi-functionalized aromatic compounds. Here, we present the first comprehensive structural and mechanistic investigation by means of X-ray, NMR, and DFT studies on the DoM reaction employing our original TMP-Zn-ate base. The structures of TMP-Zn-ates in solution and in the solid state were determined. The DFT study strongly suggested that the deprotonation involving the TMP ligand on the TMP-Zn-ate is kinetically more favorable than that involving the alkyl ligand, and this view was supported by monitoring of the 13C NMR spectrum of the reaction mixture.     

Production and crystallization of protein domains: how useful are disorder predictions ?

The failure to produce and/or crystallize proteins is often due to their modular structure. There exists therefore considerable interest to develop strategies for tailoring proteins into crystallizable domains. In the framework of a Structural Genomics Project on soluble yeast proteins, we have tested the expression of numerous genetic constructs of our targets in order to produce and crystallize proteins and protein domains and solve their three-dimensional structure. In some cases, the choice of the domain boundaries was guided by prediction from sequence using various software packages, including Prelink, a home-made prediction method for detecting unfolded regions. In other cases, large numbers of constructs were generated using molecular biology or biochemical methods. In this paper, we analyze the results of the over-expression in E. coli and crystallization of these constructs, and compare these with the predictions that can be obtained from our software and from others.     

What are the most efficient basis set strategies for correlated wave function calculations of reaction energies and barrier heights?

As electronic structure methods are being used to obtain quantitatively accurate reaction energies and barrier heights for increasingly larger systems, the choice of an efficient basis set is becoming more critical. The optimum strategy for achieving basis set convergence can depend on the way that electron correlation is treated and can take advantage of flexibility in the order in which basis functions are added. Here we study several approaches for estimating accurate reaction energies and barrier heights from post-Hartree-Fock electronic structure calculations. First and second, we evaluate methods of estimating the basis set limit of second order Mo?ller-Plesset perturbation theory and of coupled cluster theory with single and double excitations and a quasiperturbative treatment of connected triple excitations by using explicitly correlated basis functions (in the F12a implementation) along with valence, polarization, and diffuse one-electron basis functions. Third, we test the scheme of adding a higher-order correction to MP2 results (sometimes called MP2∕CBS + ΔCCSD(T)). Finally, we evaluate the basis set requirements of these methods in light of comparisons to Weizmann-3.2, Weizmann-4, and CCSDT(2)(Q)∕CBS+CV+R results.