Effective way of modeling chemical catalysis: Empirical valence bond picture of role of solvent and catalyst in alkylation reactions

A general methodology for the study of chemical catalysis is presented and demonstrated in a study of Friedel–Crafts‐type alkylation reactions that are constrained to collinear configurations. Ab initio potential energy surfaces in solution and relevant experimental results are used to calibrate general empirical valence bond (EVB) potential surfaces for studies of such reactions. The EVB surfaces allow one to interpolate the ab initio results to studies of the effect of different solvents, substituents, and catalysts on the alkylation reactions. This implicit approach introduces such effects by shifting the diagonal energies of the corresponding resonance structures. Such an EVB/shift approach appears valuable for assessing the effects of different catalysts and solvents on complex chemical reactions. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 607–625, 2000     

Synthesis of amphiphilic phenylazophenyl glycosides and a study of their liquid crystal properties

Several 4-(4'-N,N-didodecylaminophenylazo)phenyl 1,2-trans glycosides 5a-e with various carbohydrate heads (beta-D-gluco, beta-D-galacto, beta-lacto, beta-D-xylo, and alpha-D-manno) have been synthesized. The key step was the formation of phenyldiazonium tetrafluoroborates 2a-e from the per-O-acetylated 4-aminophenyl glycosides 1a-e. These salts were condensed with N,N-didodecylaniline under phase transfer conditions and the per-O-acetylated 4-(4'-N,N-didodecylaminophenylazo)phenyl 1,2-trans glycosides 4a-e were fully de-O-acetylated by the Zemplén method. The self-organizing liquid crystal properties of the compounds were investigated by a variety of techniques, including polarized light microscopy, differential scanning calorimetry, and X-ray diffraction. All but one of the materials exhibited smectic A, lamellar phases. Remarkably, the glucose derivative exhibited a rectangular disordered columnar phase. This result has implications with respect to the induced curvature created by the recognition processes of the glucose headgroup relative to the other sugar moieties and to the prevalence of various glycolipids in cell membranes     

Formation pathways of DMSO2 in the addition channel of the OH‐initiated DMS oxidation: A theoretical study

The production of dimethyl sulfoxide (DMSO) and dimethyl sulfone (DMSO₂) in the dimethyl sulfide (DMS) degradation scheme initiated by the hydroxyl (OH) radical has been shown to be very sensitive to nitrogen oxides (NO_x) levels. In the present work we have explored the potential energy surfaces corresponding to several reaction pathways which yield DMSO₂ from the CH₃S(O)(OH)CH₃ adduct [including the formation of CH₃S(O)(OH)CH₃ from the reaction of DMSO with OH] and the reaction channels that yield DMSO or/and DMSO₂ from the CH₃S(O₂)(OH)CH₃ adduct are also studied. The formation of the CH₃S(O₂)(OH)CH₃ adduct from CH₃S(OH)CH₃ (DMS‐OH) and O₂ was analyzed in our previous work. All these pathways due to the presence of NO_x (NO and NO₂) and also due to the reactions with O₂, OH and HO₂ are compared with the objective of inferring their kinetic relevance in the laboratory experiments that measure DMSO₂ (and DMSO) formation yields. In particular, our theoretical results clearly show the existence of NO_x‐dependent pathways leading to the formation of DMSO₂, which could explain some of these experimental results in comparison with experimental measurements carried out in NO_x‐free conditions. Our results indicate that the relative importance of the addition channel in the DMS oxidation process can be dependent on the NO_x content of chamber experiments and of atmospheric conditions. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2009     

Solvent exchanges among molecular compounds

Solid-solid transformations between solvates of pharmaceutical compounds are investigated under various conditions. In the case of Roxithromycin, it is shown that starting from single crystals of the acetonitrile solvate, a transformation towards the monohydrate occurs according to a cooperative mechanism. This smooth exchange of solvent probably involves a transport of matter within channels, and the comparison of crystal structures is consistent with the persistence of the main features of the 3D lattice. By contrast, starting from the DMSO solvate of Dexamethasone acetate, the transformation towards the sesquihydrated form, induced by the immersion of the DMSO solvate in water, is fully destructive and reconstructive. This occurs far-from-equilibrium and is therefore controlled by kinetic factors. The existence of an intermediate liquid phase within the particle is postulated to account for the appearance of whisker-like crystals growing first on high-energy sites of the former particle. An extended analysis of these transformations between solvates shows that they could be classified according to rules previously proposed in the case of desolvation mechanisms. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthesis and biological testing of 3‐phenyloctahydro‐pyrimido[1,2‐a]‐s‐triazine derivatives

A series of 46 3‐phenyloctahydropyrimido[1,2‐a]‐s–triazine derivatives were synthesized. This synthesis was performed via iminodimethylation of dialkylated 2‐aminopyrimidinedione synthons by substituted primary arylamines. In vitro pharmacological evaluation of these compounds is reported. One of them exhibited antifungal activity against Microsporum canis (10^?6₅₀10^?5 mol/L), and another showed affinity for serotoninergic 5‐HT_1A and 5‐HT_2b receptors (10^?8₅₀10^?7 mol/L).     

Dynamic solubility limits in nanosized olivine LiFePO4

Because of its stability, nanosized olivine LiFePO(4) opens the door toward high-power Li-ion battery technology for large-scale applications as required for plug-in hybrid vehicles. Here, we reveal that the thermodynamics of first-order phase transitions in nanoinsertion materials is distinctly different from bulk materials as demonstrated by the decreasing miscibility gap that appears to be strongly dependent on the overall composition in LiFePO(4). In contrast to our common thermodynamic knowledge, that dictates solubility limits to be independent of the overall composition, combined neutron and X-ray diffraction reveals strongly varying solubility limits below particle sizes of 35 nm. A rationale is found based on modeling of the diffuse interface. Size confinement of the lithium concentration gradient, which exists at the phase boundary, competes with the in bulk energetically favorable compositions. Consequently, temperature and size diagrams of nanomaterials require complete reconsideration, being strongly dependent on the overall composition. This is vital knowledge for the future nanoarchitecturing of superior energy storage devices as the performance will heavily depend on the disclosed nanoionic properties.     

Preparation of Primary and Secondary Azidosugars from Diols using the Dioxaphosphorane Methodology

ABSTRACT

Treatment of methyl 2,3-di-O-benzyl-α-D-glucopyranoside (1), methyl 2,3-di-O-acetyl-α-D-glucopyranoside (4), 3-O-benzyl-1,2-O-(1-methylethylidene)-α-D-glucofuranose (6), 3-O-acetyl-1,2-O-(1-methylethylidene)-α-D-glucofuranose (9), 1,2-O-(1-methylethylidene)-α-D-xylofuranose (11) and methyl 2,3-di-O-acetyl-α-D-galactopyranoside (15) with diisopropylazodicarboxylate-triphenylphosphine in tetrahydrofuran led to the corresponding dioxaphosphoranes, which were opened by trimethylsilyl azide affording the silylated primary azidodeoxysugars. When the same reaction was performed on methyl 2,3-di-O-benzyl-α-D-galactopyranoside (20), an inversion of the regioselectivity of the dioxaphosphorane opening was observed, leading mainly to the 4-azido-4-deoxy-α-D-glucopyranoside derivative 27.     

The mechanism of neutral amino acid decomposition in the gas phase. The elimination kinetics of N,N‐dimethylglycine ethyl ester,ethyl 1‐piperidineacetate,and N,N‐dimethylglycine

The gas‐phase elimination kinetics of the ethyl ester of two α‐amino acid type of molecules have been determined over the temperature range of 360–430°C and pressure range of 26–86 Torr. The reactions, in a static reaction system, are homogeneous and unimolecular and obey a first‐order rate law. The rate coefficients are given by the following equations. For N,N‐dimethylglycine ethyl ester: log k₁(s^?1) = (13.01 ± 3.70) ? (202.3 ± 0.3)kJ mol^?1 (2.303 RT)^?1 For ethyl 1‐piperidineacetate: log k₁(s^?1) = (12.91 ± 0.31) ? (204.4 ± 0.1)kJ mol^?1 (2.303 RT)^?1 The decompositon of these esters leads to the formation of the corresponding α‐amino acid type of compound and ethylene. However, the amino acid intermediate, under the condition of the experiments, undergoes an extremely rapid decarboxylation process. Attempts to pyrolyze pure N,N‐dimethylglycine, which is the intermediate of dimethylglycine ethyl ester pyrolysis, was possible at only two temperatures, 300 and 310°C. The products are trimethylamine and CO₂. Assuming log A = 13.0 for a five‐centered cyclic transition‐state type of mechanism in gas‐phase reactions, it gives the following expression: log k₁(s^?1) = (13.0) ? (176.6)kJ mol^?1 (2.303 RT)^?1. The mechanism of these α‐amino acids differs from the decarbonylation elimination of 2‐substituted halo, hydroxy, alkoxy, phenoxy, and acetoxy carboxylic acids in the gas phase. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33:465–471, 2001     

Ecdysteroids from Silene claviformis

The ecdysteroid content ofSilene claviformishas been analyzed and the four major compounds have been identified as 20-hydroxyecdysone, 2-deoxy-20-hydroxyecdysone, 2-deoxyecdysone, and integristerone A. Two new minor compounds have also been identified An erratum to this article can be found at