Asymmetric dehydration of β-hydroxy esters and application to the syntheses of flavane derivatives

Catalytic asymmetric dehydration of β-aryl or alkyl substituted β-hydroxy esters via kinetic resolution has been investigated. A brief survey of 10 different chiral ligands is conducted to examine the effects of chiral ligand structure on selectivity of the dehydration. The kinetic resolution of a variety of rac-β-hydroxy tert-butyl esters in the presence of prolinol chiral ligand 2 and BrZnCH₂CO₂-t-Bu can provide highly enantioenriched β-hydroxy esters 14-21 with selectivity factors ranging from 11 to 66. Also, application of this asymmetric synthetic methodology to the preparation of enantioenriched flavane derivatives 25-29 is demonstrated.     

To the theory of homogeneous nucleation: Cluster energy

An attempt is made to critically analyze the modern state of the theory of homogeneous nucleation as concerns its ability to describe experiments with high accuracy. An analysis of the experimental data led us to conclude that the dependence of the nucleation rate on supersaturation and temperature T was not described by the theory, which underestimates the critical cluster size compared with the Gibbs-Thomson equation. The possibility of applying density functional theory (one of the latest achievements in the theory of homogeneous nucleation) was questioned. Within this theory, the Gibbs-Thomson equation remains valid even outside the classic capillary approximation. It is suggested that, to bring theory in consistency with experiment, certain fundamental propositions of the theory of nucleation should be revised. The inclusion of an additional contribution to the Gibbs energy of a cluster caused by the size dependence of the specific heat capacity of the cluster decreases the critical cluster size compared with the value calculated by the Gibbs-Thomson equation. The calculated dependence of nucleation rate on supersaturation was in agreement with the experimental results.     

Preparation of two new liquid chromatographic chiral stationary phases based on diastereomeric chiral crown ethers incorporating two different chiral units and their applications

Two new liquid chromatographic chiral stationary phases based on diastereomeric chiral crown ethers incorporating two different chiral units such as optically active 3,3'-diphenyl-1,1'-binaphthyl and tartaric acid unit were prepared. Between the two CSPs, one was much superior to the other especially in the resolution of tocainide and its analogues (for example, in the resolution of tocainide the separation factor, alpha, was 4.26 vs. 1.00 on the two CSPs). From these results, the two chiral units composing the two diastereomeric chiral crown ether moieties of the stationary phases were expected to show "matched" or "mismatched" effect on the chiral recognition according to their stereochemistry. The different chiral recognition abilities of the two CSPs were rationalized by the different three-dimensional structures of the two diastereomeric chiral crown ethers.     

Easily Scalable Shell-Structured Copper Catalyst with High Activity and Durability for Carbon Dioxide Hydrogenation

Inducing strong metal-support interaction (SMSI) has been a useful way to control the structure of surface active sites. The SMSI often causes the encapsulation of metal particles with an oxide layer. Herein, an amorphous ceria shell was formed on Cu nanoparticles under a mild gas condition with high activity and durability for surface reaction. Cu−Ce solid solution promoted the transfer of surface oxygen species, which induced the ceria shell formation on Cu nanoparticles. This catalyst was used for CO₂ hydrogenation, selectively producing CO with high low-temperature activity and good durability for operation at high temperature. CO₂ activation and H₂ spillover could occur at low temperatures, enhancing the activity. The shell prevented the sintering, assuring durability. This catalyst was applied to a bench-scale reactor without loss in performance, resulting in high CO productivity in all temperature ranges.     

Highly Cooperative CO2 Adsorption via a Cation Crowding Mechanism on a Cesium-Exchanged Phillipsite Zeolite

An understanding of the CO₂ adsorption mechanisms on small-pore zeolites is of practical importance in the development of more efficient adsorbents for the separation of CO₂ from N₂ or CH₄. Here we report that the CO₂ isotherms at 25–75 °C on cesium-exchanged phillipsite zeolite with a Si/Al ratio of 2.5 (Cs-PHI-2.5) are characterized by a rectilinear step shape: limited uptake at low CO₂ pressure (P_CO2) is followed by highly cooperative uptake at a critical pressure, above which adsorption rapidly approaches capacity (2.0 mmol g⁻¹). Structural analysis reveals that this isotherm behavior is attributed to the high concentration and large size of Cs⁺ ions in dehydrated Cs-PHI-2.5. This results in Cs⁺ cation crowding and subsequent dispersal at a critical loading of CO₂, which allows the PHI framework to relax to its wide pore form and enables its pores to fill with CO₂ over a very narrow range of P_CO2. Such a highly cooperative phenomenon has not been observed for other zeolites.     

Density functional theory calculations of the lowest energy quintet and triplet states of model hemes: role of functional, basis set, and zero-point energy corrections

We investigated the effect of several computational variables, including the choice of the basis set, application of symmetry constraints, and zero-point energy (ZPE) corrections, on the structural parameters and predicted ground electronic state of model 5-coordinate hemes (iron(II) porphines axially coordinated by a single imidazole or 2-methylimidazole). We studied the performance of B3LYP and B3PW91 with eight Pople-style basis sets (up to 6-311+G*) and B97-1, OLYP, and TPSS functionals with 6-31G and 6-31G* basis sets. Only hybrid functionals B3LYP, B3PW91, and B97-1 reproduced the quintet ground state of the model hemes. With a given functional, the choice of the basis set caused up to 2.7 kcal/mol variation of the quintet-triplet electronic energy gap (DeltaEel), in several cases, resulting in the inversion of the sign of DeltaEel. Single-point energy calculations with triple-zeta basis sets of the Pople (up to 6-311G++(2d,2p)), Ahlrichs (TZVP and TZVPP), and Dunning (cc-pVTZ) families showed the same trend. The zero-point energy of the quintet state was approximately 1 kcal/mol lower than that of the triplet, and accounting for ZPE corrections was crucial for establishing the ground state if the electronic energy of the triplet state was approximately 1 kcal/mol less than that of the quintet. Within a given model chemistry, effects of symmetry constraints and of a "tense" structure of the iron porphine fragment coordinated to 2-methylimidazole on DeltaEel were limited to 0.3 kcal/mol. For both model hemes the best agreement with crystallographic structural data was achieved with small 6-31G and 6-31G* basis sets. Deviation of the computed frequency of the Fe-Im stretching mode from the experimental value with the basis set decreased in the order: nonaugmented basis sets, basis sets with polarization functions, and basis sets with polarization and diffuse functions. Contraction of Pople-style basis sets (double-zeta or triple-zeta) affected the results insignificantly for iron(II) porphyrin coordinated with imidazole. Poor performance of a "locally dense" basis set with a large number of basis functions on the Fe center was observed in calculation of quintet-triplet gaps. Our results lead to a series of suggestions for density functional theory calculations of quintet-triplet energy gaps in ferrohemes with a single axial imidazole; these suggestions are potentially applicable for other transition-metal complexes.     

Nanoparticle assisted magnetic resonance imaging of the early reversible stages of amyloid beta self-assembly

Co@Pt-Au nanoparticles, which have enhanced magnetism and high stability in aqueous media, are utilized in conjunction with MRI to monitor the structural evolution of Abeta assemblies, especially Abeta protofibrils in the early reversible stages.     

Synthesis of nanoparticle-cored dendrimers by convergent dendritic functionalization of monolayer-protected nanoparticles

This article presents a synthesis method for nanoparticle-cored dendrimers (NCDs), which have dendritic architectures around a monolayer-protected gold nanoparticle. The synthesis method is based on a strategy in which the synthesis of monolayer-protected nanoparticles is followed by adding dendrons on functionalized nanoparticles by a single coupling reaction. NMR spectroscopy, IR spectroscopy, and thermogravimetric analysis (TGA) characterizations confirmed the successful coupling reaction between dendrons with different generations ([G1], [G2], and [G3]) and COOH-functionalized nanoparticles ( approximately Au201L71). The dendrimer wedge density also could be controlled by reacting nanoparticles having different loading of COOH groups ( approximately 60 and approximately 10% COOH of the 71 ligands per gold nanoparticle) with functionalized dendrons. Transmission electron microscope results showed that this synthesis strategy maintains the average size of the nanoparticle core during dendron coupling reactions. This control over the composition and core size makes the systematic study of NCDs with different generations possible. The chemical stability of NCDs was found to be affected by dendron generation around the nanoparticle core. The current-potential response of NCD films on microelectrode arrays exhibited better electrical conductivity for NCDs with lower dendron generation.     

Kinetic parameter estimation for 1,1,3,3-tetramethylguanidinum trifluoroacetate ionic liquid in the reaction of glycidyl methacrylate with carbon dioxide

The ionic liquid (IL) of 1,1,3,3-tetramethyl guanidinum trifluoroacetate, which was synthesized by reacting 1,1,3,3-tetramethylguanidine (TMG) and trifluoroacetic acid, was used as a catalyst of the reaction between carbon dioxide and glycidyl methacrylate (GMA)., The initial absorption rate of carbon dioxide into GMA solutions containing IL was measured in a semi-batch stirred tank with a plane gas-liquid interface at 101.3 kPa to obtain the reaction kinetics. The reaction rate constants of the reaction between carbon dioxide and GMA were evaluated from analysis of the mass transfer mechanism accompanied by the elementary reactions based on the film theory. Solvents such as toluene,N-methyl-2-pirrolidinone and dimethyl sulfoxide affected the reaction rate constants.     

Structural transformation due to Co-host inclusion in ionic clathrate hydrates

The present findings on the co-host role in restructuring the host water framework might provide important information on tuning the cage dimensions via lattice distortion and promoting the total number of cages via structural transformation. This co-host-induced structural modification can improve the physicochemical properties of ionized clathrate hydrates, particularly given that the host framework is able to function as a pathway to deliver protons or electrons.