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A 1:1 mixture of Au[P(t-Bu)2(o-biphenyl)]Cl and AgX (X = OTf, OTs) is found to be an exceptionally active catalyst system for the exo-hydroamination of N-allenyl carbamates and also for the exo-hydroalkoxylation of allenyl alcohols and the exo-hydroarylation of 2-allenyl indoles. Au-catalyzed cyclization of N-allenyl carbamates, allenyl alcohols and 2-allenyl indoles that possessed an axial chiral allenyl moiety occurred with transfer of chirality from the allenyl moiety to the newly formed stereogenic tetrahedral carbon atom.

The use of Pd(II) complexes to catalyze the cyclization/carbonylation of 2-alkenyl indoles is reported. This is the first example of a selective process to synthesize functionalized polycyclic indole derivatives from simple precursors.

Polychlorinated biphenyls (PCBs) have been found to be persistent in the environment and possibly harmful. Many buildings are characterized with high PCB concentrations. Knowledge about partitioning between primary sources and building materials is critical for exposure assessment and practical remediation of PCB contamination. This study develops a C-depth method to determine diffusion coefficient (D) and partition coefficient (K), two key parameters governing the partitioning process. For concrete, a primary material studied here, relative standard deviations of results among five data sets are 5%-22% for K and 42-66% for D. Compared with existing methods, C-depth method overcomes the inability to obtain unique estimation for nonlinear regression and does not require assumed correlations for D and K among congeners. Comparison with a more sophisticated two-term approach implies significant uncertainty for D, and smaller uncertainty for K. However, considering uncertainties associated with sampling and chemical analysis, and impact of environmental factors, the results are acceptable for engineering applications. This was supported by good agreement between model prediction and measurement. Sensitivity analysis indicated that effective diffusion distance, contacting time of materials with primary sources, and depth of measured concentrations are critical for determining D, and PCB concentration in primary sources is critical for K. [ABSTRACT FROM AUTHOR]

The activation of C02 and its hydrogenation to methanol are of much interest as a way to utilize captured C02. Here, we investigate the use of sizeselected Cu4 clusters supported on Al203 thin films for C02 reduction in the presence of hydrogen. The catalytic activity was measured under near-atmospheric reaction conditions with a low C02 partial pressure, and the oxidation state of the clusters was investigated by in situ grazing incidence X-ray absorption spectroscopy. The results indicate that size-selected Cu4 clusters are the most active low-pressure catalyst for catalytic C02 conversion to CH3OH. Density functional theory calculations reveal that Cu4 clusters have a low activation barrier for conversion of C02 to CHjOH. This study suggests that small Cu clusters may be excellent and efficient catalysts for the recycling of released C02. [ABSTRACT FROM AUTHOR]

Upgrading furan and small oxygenatesobtained from the decompositionof cellulosic materials via formation of carbon–carbon bondsis critical to effective conversion of biomass to liquid transportationfuels. Simulation-driven molecular level understanding of carbon–carbonbond formation is required to design efficient catalysts and processes.Accurate quantum chemical methods are utilized here to predict thereaction energetics for conversion of furan (C4H4O) to C5–C8ethers and the transformationof furfural (C5H6O2) to C13–C26alkanes. Furan can be coupled with variousC1to C4low molecular weight carbohydratesobtained from the pyrolysis via Diels–Alder type reactionsin the gas phase to produce C5–C8cyclicethers. The computed reaction barriers for these reactions (∼25kcal/mol) are lower than the cellulose activation or decompositionreactions (∼50 kcal/mol). Cycloaddition of C5–C8cyclo ethers with furans can also occur in the gas phase,and the computed activation energy is similar to that of the firstDiels–Alder reaction. Furfural, obtained from biomass, canbe coupled with aldehydes or ketones with α-hydrogen atoms toform longer chain aldol products, and these aldol products can undergovapor phase hydrocycloaddition (activation barrier of ∼20 kcal/mol)to form the precursors of C26cyclic hydrocarbons. Thesethermochemical studies provide the basis for further vapor phase catalyticstudies required for upgrading of furans/furfurals to longer chainhydrocarbons. [ABSTRACT FROM AUTHOR]

The mechanism of the reverse water-gas shift reaction (CO2 + H2 → CO + H2O) was investigated using the 3d transition metal complexes L′M (M = Fe, Mn, and Co, L′ = parent β-diketiminate). The thermodynamics and reaction barriers of the elementary reaction pathways were studied with the B3LYP density functional and two different basis sets: 6-311+G(d) and aug-cc-pVTZ. Plausible reactants, intermediates, transition states, and products were modeled, with different conformers and multiplicities for each identified. Different reaction pathways and side reactions were also considered. Reaction Gibbs free energies and activation energies for all steps were determined for each transition metal. Calculations indicate that the most desirable mechanism involves mostly monometallic complexes. Among the three catalysts modeled, the Mn complex shows the most favorable catalytic properties. Considering the individual reaction barriers, the Fe complex shows the lowest barrier for activation of CO2. [ABSTRACT FROM AUTHOR]

A highly diastereo- and enantioselective asymmetric allylic alkylation reaction with respect to prochiral 3-substituted benzofuran-2(3H)-ones and MBH carbonate by a chiral biscinchona alkaloid catalyst was investigated. The corresponding adducts, containing a quaternary center at the C3-position of the benzofuran-2(3H)-one as well as a vicinal tertiary center, were generally obtained in high yields (up to 97%) with very good diastereo- (up to 98:2 dr) and enantioselectivities (up to 95% ee). [ABSTRACT FROM AUTHOR]

Protein amyloid oligomers have been strongly linked to amyloid diseases and can be intermediates to amyloid fibers. β-Sheets have been identified in amyloid oligomers. However, because of their transient and highly polymorphic properties, the details of their self-association remain elusive. Here we explore oligomer structure using a model system: macrocyclic peptides. Key amyloidogenic sequences from Aβ and tau were incorporated into macrocycles, thereby restraining them to β-strands, but limiting the growth of the oligomers so they may crystallize and cannot fibrillate. We determined the atomic structures for four such oligomers, and all four reveal tetrameric interfaces in which β-sheet dimers pair together by highly complementary, dry interfaces, analogous to steric zippers found in fibers, suggesting a common structure for amyloid oligomers and fibers. In amyloid fibers, the axes of the paired sheets are either parallel or antiparallel, whereas the oligomeric interfaces display a variety of sheet-to-sheet pairing angles, offering a structural explanation for the heterogeneity of amyloid oligomers. [ABSTRACT FROM AUTHOR]

This paper describes studies of a series of macrocyclic β-sheet peptides 1 that inhibit the aggregation of a tau-protein-derived peptide. The macrocyclic β-sheet peptides comprise a pentapeptide "upper" strand, two δ-linked ornithine turn units, and a "lower" strand comprising two additional residues and the β-sheet peptidomimetic template "Hao". The tau-derived peptide Ac-VQIVYK-NH2 (AcPHF6) aggregates in solution through β-sheet interactions to form straight and twisted filaments similar to those formed by tau protein in Alzheimer's neurofibrillary tangles. Macrocycles 1 containing the pentapeptide VQIVY in the "upper" strand delay and suppress the onset of aggregation of the AcPHF6 peptide. Inhibition is particularly pronounced in macrocycles 1a, 1d, and 1f, in which the two residues in the "lower" strand provide a pattern of hydrophobicity and hydrophilicity that matches that of the pentapeptide "upper" strand. Inhibition varies strongly with the concentration of these macrocycles, suggesting that it is cooperative. Macrocycle 1b containing the pentapeptide QIVYK shows little inhibition, suggesting the possibility of a preferred direction of growth of AcPHF6 β-sheets. On the basis of these studies, a model is proposed in which the AcPHF6 amyloid grows as a layered pair of β-sheets and in which growth is blocked by a pair of macrocycles that cap the growing paired hydrogen-bonding edges. This model provides a provocative and appealing target for future inhibitor design. [ABSTRACT FROM AUTHOR]

The reverse water−gas shift reaction CO2+ H2→ H2O + CO has been investigated using a set of homogeneous catalyst models L′MI(L′ = β-diketiminate, C3N2H5−; M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn). The thermodynamics of prototypical reaction pathways were simulated at two levels of theory: B3LYP/6-311+G(d) and B3LYP/aug-cc-pVTZ. The modeled catalytic reaction has been considered in the following steps: coordination of CO2by the catalyst to generate a carbon dioxide complex, L′M(CO2); scission of L′M(CO2) to yield L′M(CO) and L′M(O); L′M(O) hydrogenation to form L′M(H2O). The final products, H2O and CO, were obtained from the dissociation of L′M(H2O) and L′M(CO). All of the reactants, intermediates, and products were modeled, where different possible conformers and multiplicities were identified and considered as potential minima. The reaction enthalpy ΔH, of all steps for each catalyst as a function of transition metal have been determined. The Mn and Fe catalysts show more thermodynamically accessible pathways than the other catalyst models studied. The overall reaction enthalpy has been determined not only by B3LYP/6-311+G(d) and B3LYP/aug-cc-pVTZ but also via a more rigorous ab initio electron-correlation-based approach, the correlation consistent Composite Approach (ccCA). [ABSTRACT FROM AUTHOR]

Treatment of 2-allenyl indole 4with a catalytic 1:2 mixture of (S)-2Au2Cl2(S)-2 (S)-3,5-tBu-4-MeO-MeOBIPHEP and AgBF4in toluene at −10 °C for 17 h led to isolation of tetrahydrocarbazole 5 in 88% yield with 92% ee. The protocol was effective for the cyclization of terminally disubstituted allenes and for the formation of seven-membered rings. [ABSTRACT FROM AUTHOR]

Fused polycyclic indoles including the carbazoles, carbolines, and their partially saturated counterparts represent a prominent class of heterocyclic compounds with varied and often potent biological activity. As a result, considerable effort has been directed in this article to the development of new and efficient methods for the synthesis of polycyclic indoles. The article finally, develops a mild and effective platinum-catalyzed procedure for the intramolecular alkylation of indoles with unactivated olefins and establishes a mechanism for indole alkylation involving nucleophilic attack on a platinum-complexed olefin.

Understanding ripening and senescence processes in postharvest stored fruit is key to the identification and implementation of effective treatment methods. Here, we explored the effects of exogenous applications of melatonin (MT) and nitric oxide (NO) on ripening and softening processes in three cultivars of European pear ( Pyrus communis L.). The results showed that MT and NO played important roles in the two processes: they decreased the rate of upregulation of PcCel and PcPG, inhibited expression of ethylene synthetase genes ( PcACS and PcACO), and reduced rates of respiration and ethylene production. MT increased activity of NO synthase through upregulation of expression of PcNOS that subsequently led to an increase in NO content. However, when NO synthesis was inhibited, the delaying effect of MT on fruit senescence was almost eliminated. These findings indicate that MT acted on the upstream process of NO synthesis that then delayed senescence in pear fruit.