bluuebunny/crossref_metadata_2025_split huggingface dataset:
Reactions with Aziridines, XXXII[1]. Mechanistic Aspects in Nucleophilic Opening of Three‐Membered Rings. Alcoholyses of Activated Aziridines
Authors: Bucholz B. , Stamm H. 
Published: November 2013

AbstractNucleophilic ring opening of three‐membered rings is discussed in terms of leaving group basicity. Reactions of activated (weakly basic leaving groups) 2,2‐dimethylaziridines 5 and 2‐phenylaziridines 6 with alcohols ROH are studied with reference to the site selectivity of ring opening and competing side‐reactions. Abnormal opening (at the substituted carbon) of 5 proceeds in neutral ROH without catalyst when the activation is strong (tosyl) whilst no reaction or mainly carbonyl attack is observed with weak activation (CONHAr). NaClO4 catalyzes the abnormal opening of 5. EtO− gives exclusively normal opening of 5 whilst, in contrast to the literature, MeO− gives also a substantial amount of abnormal ether. Attack on 6 always occurs at both positions with the predominance of the abnormal reaction being greater for MeO− than for EtO−. These reactions are considered SN2 with steric and benzylic differentiation between normal and abnormal attack depending on the size of ROH. Acid catalyzed openings of 5 and 6 proceed exclusively abnormally in the ether‐forming reaction as well as in side‐reactions. Competition experiments and increase of the side‐reactions with the size of ROH demonstrate a deceleration of the ether‐forming reaction with the size of ROH. Obviously, the side‐reactions originate from a carbenium ion whilst the ethers are mainly formed by a borderline mechanism. R(‐)6b and acidic MeOH give only 8% racemization in the abnormal ether. An order of activation is proposed for activated aziridines under acid catalysis (double activation).
The Synthesis of Vicinal Diacylamines via Bamberger Ring Cleavage of Substituted Imidazoles. 1,2,4‐Triacylaminobutanes Bearing Different Acyl Groups
Authors: Altman Janina , Shoef Nurit , Wilchek Meir , Warshawsky Abraham 
Published: November 2013

AbstractBamberger ring cleavage acylation of N‐p‐toluenesulfonyl (tosyl) histamine followed by hydrogenation yields 3,4‐diacylaminobutanetosylamine, a triamine containing groups with different chemical reactivities.
The Chichibabin Amination of 5‐Phenylpyrimidine and Phenylpyrazine [1,2]
Authors: Breuker K. , van Der Plas H.C. , van Veldhuizen A. 
Published: November 2013

Abstract5‐Phenylpyrimidine (1) on treatment by KNH2/NH3, affords a mixture of 2 amino‐ and 4‐amino‐5‐phenylpyrimidine. The presence of only the anionic C‐4 adduct. 4‐amino‐5‐phenyl‐1 (or 3), 4‐dihydropyrimidinide was shown by 1H‐ and 13C‐NMR spectroscopy of a solution of 1 in KNH2/NH3. Study of the amination by use of 15N‐labelled KNH2/NH3 convincingly shows that in the formation of the 2‐amino compound a ring opening‐ring closure sequence is involved. The amination of phenylpyrazine (11) into 5‐amino‐ and 3‐amino‐2‐phenylpyrazine has been found to occur without opening of the pyrazine ring. Examination by 1H‐ and 13C‐NMR spectroscopy of a solution of 11 in KNH2/NH3, reveals that at −60°C three adducts are present; i.e. 3‐amino‐, 5‐amino‐ and 6‐amino‐2‐phenyl‐dihydropyrazinides, at −40°C the C‐3 and C‐5 adducts are formed; and at room temperature only the thermodynamically most stable C‐5 adduct is present.
A New Route to Macrocycles Possessing the 2,6‐Pyridino Subunit via Bis‐1,3‐Dithianes [1]
Authors: Newkome George R. , Lee H.W. , Fronczek Frank R. 
Published: November 2013

AbstractTwo rigid crown ethers 12, which possess a terpyridine subunit, are herein prepared by the construction of the middle pyridine moiety via a bis‐dithiane precursor. The intermediate diketone and derivatives are sufficiently mobile to permit macrocyclization; then their irreversible conversion to a pyridine nucleus circumvents the inherent structural constraints associated with the synthesis of these macromolecules.
Design of New Calcium Antagonists. 4‐Aryl‐1,4‐Dihydropyrimidine‐5‐Carboxylates from Olefinic Acetoacetic Esters and Amidine via Regiospecific Ring Closure [1]
Authors: Weis Alexander L. , Vishkautsan Rozita 
Published: November 2013

Abstract5‐Functionalized‐l,4‐dihydropyrimidines (5) were prepared via a twostep reaction between arylidene acetoacetic esters (1) and amidines (2). Intermediate 6‐hydroxytetrahydropyrimidines (4) were isolated under mild conditions, and subsequently dehydrated by heating in glacial acetic acid. Warming a solution of 1a and 2a, without isolation of the intermediate product, afforded 5‐acetyl‐6‐oxo‐1,4,5,6‐tetrahydro‐pyrimidine (3), indicating ring closure via the α,β‐unsaturated ester fragment of 1. The mechanism of formation and structure of cyclic adduct intermediates 4, and final dihydropyrimidines 5 are discussed.
Molecular Dynamics Simulations of DNA Double Helices: Studies of Sequence Dependence and the Role of Mismatch Pairs in the DNA Helix
Authors: Rao Shashidhar N. , Chandra Singh U. , Kollman Peter A. 
Published: November 2013

AbstractWe present the results of molecular dynamics simulations on base paired deoxyribonucleotides dA10·dT10, d(GCGCGCGCGC)2 d(ATATATATAT)2 d(CGCGA)·d(TCACG) (AC mismatch) and d(CGAGA)·d(TCGCG) (GA mismatch). The average structural properties — helix parameters, H‐bond parameters, sugar puckers, torsional angles, root mean square atomic fluctuations — are compared to those found in earlier studies on the sequence d(CGCGA)·d(TCGCG).1 The properties of the decamer helices remain B DNA like throughout the simulation, confirming our inference1 that a model which treats solvent/counterions in a simple fashion (full charge on phosphates, but use of large hydrated counterions and a distance dependent dielectric constant) is a reasonable “first order” model to simulate the properties of DNA helices over periods of at least 100 psec. There are a number of sequence dependent effects observed in the three decanucleotides studied here, among them differences in H‐bond properties, sugar pucker and helix repeat/base tilts. In d(GCGCGCGCGC)2 and d(ATATATATAT)2 the H‐bonds all stay near the classic Watson‐Crick distance and the base tilts and twists are small (10–20°). On the other hand, in dA10·dT10 there are much larger deviations of H‐bonding from classic Watson‐Crick and H‐bond exchange during the simulation. There are many more examples of sugar repuckering in the AT decamers, and the relative frequency of such repuckering is qualitatively consistent with what one would expect based on molecular mechanics studies. A comparison of the torsional angles, sugar puckers, helical parameters, root mean square atom amplitudes and hydrogen bond parameters of the sequence d(CGCGA)·d(TCGCG) with the two mismatched analogues demonstrates that the effect of the poorer hydrogen bonding at the central base pair is transmitted throughout the sequence, leading to significantly greater root mean square fluctuations in all dihedral angles, significantly higher % of non‐“standard C2′ endo” sugar puckers, generally longer H‐bond distances, with greater % of “long” H bonds during the simulation, larger RMS amplitude of atomic fluctuations and greater helix twist and tilt of the base pairs. The average helix repeat is 9.0 for the AC mismatch, and 10.1 for the GA mismatch, compared to 10.0 for the normal sequence.
Computer Simulation and the Design of New Biological Molecules
Authors: Wong Chung F. , Andrew McCammon J. 
Published: November 2013

AbstractMolecular dynamics simulations have been carried out for the enzyme bovine trypsin, for the small inhibitor benzamidine, and for the complex formed by these two molecules, all in the presence of enough water to approximate dilute solution conditions. The simulations have been analyzed to characterize the structure and dynamics of the enzyme and its solvent surroundings. Using the recently developed thermodynamic cycle‐perturbation method, the simulations have also been analyzed to determine how changes in the structure of the enzyme and the inhibitor alter the free energy of complex formation. The results suggest that such simulations may be useful in the design of new enzymes and enzyme inhibitors.
The Free Energy of Xenon Binding to Myoglobin from Molecular Dynamics Simulation
Authors: Hermans J. , Shankar S. 
Published: November 2013

AbstractThe interaction of xenon atoms with the interior of the myoglobin molecule has been simulated by molecular dynamics calculations. The standard free energy of binding has been computed from a simulation in which a forcing potential was applied; computed and experimental values differ by only 2 kJ/mole. In agreement with experimental evidence, the simulations indicate that insertion of the first xenon atom causes a perturbation of the protein conformation that facilitates insertion of one or more additional xenona.
Ligand Substitution Reactions of Transition Metal Organometallic Complexes and Homogeneous Catalysis
Authors: Basolo Fred 
Published: November 2013

AbstractTransition metal organometallic homogeneous catalysts depend on the ease of ligand substitution in one or more of the catalytic steps. Thus, it follows that the more we learn about the factors which affect rates of ligand substitution, the better we will be able to design useful catalysts. Three different types of approaches to the enhancement of ligand substitution are discussed. These are (1) through the use of spectator ligands which permit an 18‐electron system to react by an associative process, (2) the relative rates of substitution of a 17‐electron and a corresponding 18‐electron system, and (3) the reactivity order 1st row < 2nd row > 3rd row transition metals for a given metal triad. Specific examples are given for each of these approaches to generate more substitution labile organometallic compounds.
Homogeneous Catalysis. Mechanisms of Metal Catalysed Claisen Rearrangements of O‐allylic‐S‐methyl dithiocarbonates
Authors: Auburn P.R. , Whelan J. , Bosnich B. 
Published: November 2013

AbstractA number of catalysts of Pd(II), Pt (II), Rh (I) and Ir(I) induce rearrangements of O‐allylic‐S‐methyl dithiocarbonates at 25° C. For most substrates, the Pd(II) and Pt(II) catalysts cause [3,3] sigmatropic (Claisen) rearrangements but the Rh(I) and Ir(I) catalysts give, in addition, other products depending on the catalyst and the substrate. The Claisen rearrangements observed with the Pd(II) and Pt(II) catalysts are believed to occur by a cyclization induced mechanism, whereas those rearrangements associated with the Rh(I) and Ir(I) catalysts, as well as in one case with the Pd(II) catalyst, appear to involve metal stabilized carbocation intermediates. When cyclic substrates are used retention of configuration is observed predominantly for all catalysts tried.
The Reaction of Allylbenzene and CoH(CO)4
Authors: Ungvary F. , Marko L. , Bockman T.M. , Garst J.F. , King R.B. 
Published: November 2013

AbstractThe carbon monoxide uptake in the reaction of allylbenzene with CoH(CO)4 in n‐octane leading to acylcobalt tetracarbonyls and aldehydes is catalyzed by Co2(CO)8. The rate is half order in Co2(CO)8, first order in allylbenzene and shows a complex dependence in CoH(CO)4 and CO. From kinetic experiments between 15 and 45° C the temperature dependence of the observed rate constant was determined. The complete analogy of this kinetics with that obtained for heptene‐1 shows that the main aldehydic product 2‐phenylbutyraldehyde is formed by a mechanism characteristic for aliphatic olefins and not by one operative for styrenes.
Oxidation of para‐ and meta‐ Cresols by C‐ Meso‐5,7,7,12,14,14‐hexamethyl‐1,4,8,11‐tetraazacyclotetradecane‐Nickel (III), to the Corresponding Aldehydes in Aqueous Media
Authors: Uzan Y. , Meyerstein D. 
Published: November 2013

AbstractThe tervalent nickel complex with C‐meso‐ 5,7,7,12,14,14‐hexamethyl‐1,4,8,11‐tetraazacyclotetradecane, Ni‐(III)L, as its disulphate form, Ni(III)L(SO4)2, oxidizes cresols in aqueous solutions. The major products after two electron equivalents are consumed are the corresponding hydroxybenzyl alcohols. The products after four electron equivalents are consumed are the corresponding hydroxybenzaldehydes. Excess tervalent nickel does not lead to the formation of the corresponding hydroxybenzoic acids. The results indicate that the oxidation reactions observed occur via the outer sphere mechanism. The use of NiL2+ as a catalyst for the oxidation of cresols is discussed.
Origin of the Multi‐dimensional Properties and the Charge Distribution Rule of the Molecular Conductors Based on π‐Donor and π‐Acceptor Molecules
Authors: Kobayashi Hayao , Kato Reizo , Kobayashi Akiko 
Published: November 2013

AbstractThe new aspects of molecular conductors are reviewed. The origin of the two‐dimensionality of the system is interpreted on the basis of the simple tight‐binding band examinations. The charge distribution rule which emerges in these systems is described in relation to the multi‐dimensionality of these complexes.
Conducting and Superconducting BEDT‐TTF Based Synmetals
Authors: Wang Hau H. , Beno Mark A. , Whangbo M.‐H. , Emge Thomas J. , Geiser Urs , Douglas Carlson K. , Venturini Eugene L. , Williams Jack M. 
Published: November 2013

AbstractThe synthesis, crystal structures and band electronic structures of BEDT‐TTF based synmetals are discussed, where BEDT‐TTF is bis(ethylenedithio)‐tetrathiofulvalene here abbreviated to “ET”. The electrical properties of ET:X salts above their phase transitions are either 1D [for (ET)2X, where X− = ICl2−, AuBr2−, Au(CN)2− and Ag(CN)2−], 2D metallic [for (ET)2X, where X− = I3−, AuI2−, IBr2− and I2Br−] or 3D metallic [for (ET)Ag4(CN)5]. The β‐(ET)2X (X− = I3−, AuI2− and IBr2−) salts become ambient pressure organic superconductors at 1,4 K (or ∼ 8 K under pressure), 5.0 K, and 2.8 K, respectively. The variation of critical temperatures of this series of β‐(ET)2X salts are correlated to the respective changes in unit cell volume or in the density of electronic states at the Fermi level.
Electrical and Spectroscopic Properties of TTF and TSF Salts with Tetrahalogenozinc(II) and ‐cadmium(II) Anions
Authors: Tanaka Atsushi , Matsubayashi Gen‐Etsu , Tanaka Toshio 
Published: November 2013

AbstractSeveral tetrathiafulvalene (TTF) and tetraselenafulvalene (TSF) salts of tetrahalogenozinc(II) and ‐cadmium(II) anions Dx [MX4] (D = TTF or TSF; M = Zn and Cd; X = Cl, Br, and I; x = 2.0‐6.6) have been prepared by the reaction of [TTF]3[BF4]2 or [TSF]3[BF4]2 with [NEt4]2[MX4] in acetonitrile and by the electrolysis of TTF or TSF in acetonitrile containing [NEt4]2[MBr4]. Electrical resistivities of these salts measured for compressed pellets fall in the range 1 × (101–107) Ω cm at 25 °C; the value decreases with increasing bulkiness of the anions: [MCl4]2– > [MBr4]2− > [MI4]2−. Stackings and electronic states of TTF and TSF molecules are discussed on the basis of electronic reflectance, ESR, IR, and Raman spectra.
Use of Electrochemical Techniques in the Synthesis, Characterization and Application of Molecular Conductors: Cation Effects in the Oxidation and Reduction of TTF[Ni(dmit)2]2 Electrodes
Authors: Valade L. , Legros J‐P. , De Montauzon D. , Cassoux P. , Interrante L. V. 
Published: November 2013

AbstractThe use of various electrochemical techniques in the synthesis, characterization and applications of molecular conductors derived from transition metal complexes of the dmit2− ligand, is reviewed (H2dmit = 4,5‐dimercapto‐1,3‐dithia‐2‐thione). Cyclic voltammetry can be used for the detection of the formation of conductive species derived from the concerned complexes. The use of the TTF[Ni(dmit)2]2 compound1 as electrode material and its electrochemical behaviour with various supporting electrolytes, is extensively studied.
The 1:1 Mixed Stack Complex of Tetrathiafulvalene and 2,5‐Dibenzyltetracyanoquinodimethane (TTF:DBTCNQ) at 115 K
Authors: Aharon‐Shalom E. , Becker J. Y. , Bernstein J. , Bittner S. , Shaik S. S. 
Published: November 2013

AbstractCrystallographic constants for the title compound are as follows: Mr = 384.5 + 204.3, triclinic, PĪ, a = 6.878 (2), b = 10.309 (4), c = 11.159 (7) Å, α = 109.94 (4), β = 100.91 (4), γ = 105.64 (3)°, Z = 1, V = 680.8 Å3, Dc = 1.46 g cm−3, Dm= 1.40 g cm−3 (flotation, aqueous KI), λ(MoKα) = 0.71069 Å, μ = 3.27 cm−1, F(000) = 304, RF = 0.085 for 1948 data with |F0| > 2.5 σ(F0). The complex crystallizes in mixed stacks of alternating donors and acceptors with a maximum of intrastack overlap and a minimum of interstack interactions. The degree of charge transfer is shown by a number of criteria to be essentially zero, in stark contrast to the complex of TTF with unsubstituted TCNQ. Comparison is made with the relatively few other complexes of derivatives of TTF and TCNQ which form mixed stack complexes.
Charge Transfer Interactions in Micelles and Vesicles. Inter‐ and Intramolecular Probes of Solubilization Site Polarity
Authors: Shin Dong M. , Schanze Kirk S. , Otruba Joseph P. , Brown Patti E. , Whitten David G. 
Published: November 2013

AbstractA study of intra‐ and intermolecular charge transfer interactions with organic reagents which can be solubilized in micelles and vesicles is reported. The first part of the paper details the study on the formation of intermolecular charge transfer complexes between aromatic hydrocarbons (“enes”) and the methyl viologen dication; these complexes show a weak charge transfer transition which is dependent on solvent polarity. A study of the charge transfer transition in homogeneous solutions and microheterogeneous media shows that the complexes occur for micelles at a reasonably polar “interfacial” site. A quantitative study of complex formation for several different substrates and different surfactant media reveals that the precise micropolarity for various combinations varies and hence indicates that the solubilization site provided for a given set of reagents in a given detergent is variable and cannot be too precisely defined. Studies of two polyenes containing electron donor and electron acceptor substituents within the same molecule have also been carried out. In the case of 4‐nitro‐4′‐methoxystilbene (NMS) it has been found that the triplet‐triplet absorption spectrum observed following laser flash excitation is sensitive to both solvent polarity and viscosity; in micelles, NMS exhibits a single moderately polar solubilization site in good agreement with that reported for the aforementioned charge transfer complexes. In contrast, it has been found that vesicles solubilize NMS in two distinctively different solubilization sites. One of the solubilization sites is attributed to an “interfacial” location while the other appears less polar but more viscous. Studies of the analogous diene, NND, have focused on its fluorescence and indicate the presence once again of a site which is sensitive to the hydrocarbon chains in bilayer vesicles. The hydrocarbon‐sensitive site in vesicles for both NMS and NND could be associated with a “wedge”‐shaped or cleft site in the vesicles which exists most prominently below the phase transition temperature of the vesicle.
NAD(P)+‐NADPH Models. 60. Induction of Double Asymmetry in the Reduction of α‐Keto Esters
Authors: Ohno Atsuyoshi , Yasuma Tsuneo , Nakamura Kaoru , Oka Shinzaburo 
Published: November 2013

AbstractUsing a chiral 1,4‐dihydronicotinamide derivative, N‐α‐methylbenzyl‐1‐propyl‐2,4‐dimethyl‐1,4‐dihydronicotinamide (Me2PNPH), we have studied the reduction of various α‐keto esters that have a chiral center in the acid‐moiety. All of the four possible diastereomers of the corresponding α‐hydroxy esters were afforded. A detailed investigation into the diastereomeric composition of the product has revealed that the molecular arrangement and the conformations of the substrate at the transition state of the reduction are controlled by the polarity and/or the polarizability of the substituents. The 2R,3R‐isomer is the most predominant product when 4R,9R‐Me2PNPH is employed as the reductant.
Photoaccumulation in Photosystem I Does Not Produce a Phylloquinone Radical
Authors: Barry Bridgette A. , Bender Christopher J. , Mcintosh Lee , Ferguson‐Miller Shelagh , Babcock Gerald T. 
Published: November 2013

AbstractA characteristic EPR spectrum is observed from Photosystem I (PS I) particles that are illuminated under reducing conditions. This signal is thought to arise from the secondary electron acceptor of PS I, A1. This EPR spectrum, along with data from other spectroscopic methods, has been taken as evidence that A1 is phylloquinone. In the work reported here, we have used a methionine auxotroph of Anabaena to deuterate specifically the 2‐methyl group on the phylloquinone ring. Mass spectrometry of the isolated quinone shows 65% yield in the labeling procedure. The photoaccumulated A1 EPR spectra in protonated and deuterium‐labeled cells are indistinguishable, from which we conclude that the photoaccumulated radical is not phylloquinone.
Towards a New Taxonomy of Photosynthetic Bacteria: ADMR‐Monitored Triplet Difference Spectroscopy of Reaction Center Pigment–Protein Complexes
Authors: Antonie Dijkman J. , Den Blanken Hubert J. , Hoff Arnold J. 
Published: November 2013

AbstractLow‐temperature (1.2 K) triplet‐minus‐singlet absorbance difference spectra of a number of photosynthetic bacteria, including members of almost all genera with the exception of Heliobacteriaceae, have been recorded by absorbance‐detected magnetic resonance (ADMR). The spectra of the purple bacteria fall into two distinct classes. It is argued that these classes reflect the extent of delocalization of the primary donor triplet state over its two bacteriochlorophylls. The spectra from the green sulfur bacteria form a third class. In some cases, the bleaching of the long‐wavelength primary donor absorption is split, or shows a strong shoulder at lower energy. This phenomenon is attributed to an intrinsic heterogeneity of the reaction center with regard to the precise configuration of the primary donor dimer. It is argued that the zero‐field splitting parameters and the triplet‐minus‐singlet difference spectra may be used as a taxonomic tool.
Photocleavage of Water in Organized Molecular Systems
Authors: YA. Shafirovich Vladimir , Shilov Alexander E. 
Published: November 2013

AbstractPhotocleavage of water by solar light in molecular systems is discussed. The advantages of organized molecular photosystems are demonstrated for the main steps of the photocleavage process: charge photoseparation, stabilization of the separated charges, and catalytic production of hydrogen and oxygen under the action of one‐electron reductants and oxidants, respectively.
Electronic Energy Transfer in Photosynthetic Bacterial Reaction Centers
Authors: Jean John M. , Chan Chi‐Kin , Fleming Graham R. 
Published: November 2013

AbstractWe present simulations of energy transfer in reaction centers of the purple bacteria Rhodopseudomonas viridis and Rhodobacter sphaeroides. Calculations using a weak coupling model based on Förster's theory give transfer rates that are at least an order of magnitude slower than the results from recent femtosecond transient absorption studies. The discrepancy between experimental and theoretical results is discussed in terms of the various assumptions upon which the model is based.
Mechanisms of the Early Steps in Bacterial Photosynthesis and Their Implications for Experiment
Authors: Marcus Rudolph A. 
Published: November 2013

AbstractThe current situation on experiment and theory as regards the early electron transfer steps in bacterial photosynthesis is discussed. Recent experiments have limited the mechanistic possibilities, while an internal consistency test has limited the values of some parameters in the superexchange mechanism. The partitioning method has provided a useful and unified way of treating superexchange and other properties in these systems. The alternative reaction mechanisms have a number of consequences, and various experimental tests are considered or suggested.