bluuebunny/crossref_metadata_2025_split huggingface dataset:
Chiroptical Properties and Conformation of Dehydrophenylalanine Peptides
Authors: Pieroni Osvaldo , Fissi Adriano , Merlino Stefano , Ciardelli Francesco 
Published: November 2013

AbstractSome N‐acylated α, β‐unsaturated peptides (dehydropeptides) containing one, two or three dehydrophenylalanine (dehydro‐Phe) residues and a C‐terminal L‐amino acidic residue have been prepared and characterized by NMR, IR and UV absorption. For N‐actyl‐(dehydro‐Phe)2‐L‐Ala the crystal and molecular structure have been determined by X‐ray diffraction analysis. CD spectra of N‐acetyl‐(dehydro‐Phe)‐ϵ‐Cbzo‐L‐Lys and N‐acetyl‐(dehydro‐Phe)2‐ϵ‐Cbzo‐L‐Lys have been measured in different solvents and, of N‐acetyl‐(dehydro‐Phe)2‐L‐Val, in the presence of different amounts of urea. The results have been compared with those for N‐acetyl‐(dehydro‐Phe)‐(dehydro‐Tyr)‐L‐Glu, for the tetrapeptide N‐acetyl‐(dehydro‐Phe)3‐L‐Val, and for analogous compounds already reported [J. Am. Chem. Soc., 97, 5820 (1975)]. Crystalline molecular structure and chiroptical properties are consistent with the hypothesis that the dehydropeptides assume in solution a chiral conformation based on a 4 → 1 hydrogen‐bonded ten‐membered ring, also referred to as β‐turn. The importance of these systems for the understanding of the conformational properties of polypeptides containing D‐L sequences is discussed.
Direct Resolution of Amino Acid Enantiomers by High Pressure Liquid Chromatography
Authors: Lefebvre Bernard , Audebert Roland , Quivoron Claude 
Published: November 2013

AbstractWe have synthetized an optically active macromolecular chromatographic packing from cross‐linked polyacrylamide grafted with an optically active α‐amino acid. This packing, in the form of small beads, slightly swells in water. Its granularity, porosity and rigidity are suitable for high pressure liquid chromatography. Such packings, complexed by a metallic ion, are efficient for the resolution of many underivatized racemic α‐amino acids by elution with water. As an example, on one of these packings, containing Cu++ions, enantiomers of valine, threonine, isoleucine, serine, phenylalanine, tyrosine, tryptophan, and asparagine are completely resolved in less than one hour.
A New Case of Asymmetric Synthesis Using Circularly Polarized Light
Authors: Nicoud J. F. , Kagan H. B. 
Published: November 2013

AbstractAsymmetric photocyclization of N‐methyl N‐aryl enamines by circularly polarized light gives several optically active N‐methyl dihydroindoles. In the synthesis of N‐methyl 2‐phenyl dihydroindole, the optical yield was evaluated to be 0.20%, which is the same order of magnitude as in the asymmetric synthesis of helicenes. The mechanisms of these reactions are discussed.
A Chemical Mass Spectrometric Method for Mapping Short Range Order in Solid‐Solutions Undergoing Topochemical Dimerisation. Application to Some Racemic Mixtures Containing the Chiral Sec‐Butyl Group
Authors: Addadi L. , Gati E. , Lahav M. , Leiserowitz L. 
Published: November 2013

AbstractA new chemical mass‐spectrometric method for mapping short range order in crystalline solid‐solutions of enantiomers undergoing topochemical reactions is described. We have applied this method to assigning the microstructure of the racemic mixtures of the diacrylates 1 and 2, each one in two dimorphic modifications and which undergo topochemical (2π + 2π) photocyclodimerisation and photocyclopolymerisation. All crystalline phases are disordered with respect to the enantiomeric sec‐butyl group. While the two polymorphs of 1 and polymorph α of 2 form random solid‐solutions, polymorph β of 2 forms partially ordered crystals in which the degree of order depends on the method of crystallization. It could be established that in the latter system the degree of order across the two independent centres of inversion is measurably different. The scope and limitation of this new method are discussed.
Photochemical Asymmetric Synthesis. Irradiation of Ring and Open‐chain Derivatives of L‐Erythritol 1,4‐Dicinnamate
Authors: Green Bernard S. , Hagler Arnold T. , Rabinsohn Yechiel , Rejtõ Miriam 
Published: November 2013

AbstractThe two analogs, (−)‐2,3‐di‐O‐methyl‐L‐erythritol 1,4‐di‐trans‐cinnamate, 1, and (−)‐2,3‐O‐isopropylidene‐L‐erythritol 1,4‐di‐trans‐cinnamate, 2, were irradiated in benzene solution. Intramolecular 2 + 2 photocycloaddition occurs in both derivatives and the stereochemistry of the cyclobutane products, after removal of the L‐erythritol, was established in both cases as β (symmetry m)‐ and δ (symmetry 2)‐truxinate. While the asymmetric induction was 85% in the synthesis of (+)‐dimethyl δ‐truxinate from the open‐chain dimethoxy derivative 1, the more rigid isopropylidene ring derivative led to an asymmetric induction of only 6% and afforded the opposite enantioner, (−)‐dimethyl δ‐truxinate, in excess. Computer simulation of the diastereomeric ground state conformations which are believed to lead to the observed products from 1 was carried out to enable better understanding of the factors which determine the asymmetric synthesis. The simulations afford geometries and relative energies of the product precursor conformations. The results are in qualitative agreement with the experimental observations in predicting an excess of one of the two possible δ‐diastereomers. Since the geometry is also obtained by the simulation, it allows one to predict the absolute configuration of the predominant chiral product.
Studies on Organodisulphides as Metal Ligands. II. The Crystal and Molecular Structure of Re2Br2(CO)6S2Me2, A Compound Containing a Dimethyldisulphide Bridge Across the Two Metals. Comparison Between Solid‐State and Solution Data for Re2Br2(CO)6S2R2(R = Me, Ph)
Authors: Bernal Ivan , Atwood Jerry L. , Calderazzo Fausto , Vitali Dario 
Published: November 2013

AbstractThe crystal and molecular structure of the compound having the composition Re2Br2(CO)6S2(CH3)2 was determined from three‐dimensional X‐ray diffraction data collected by automated counter methods. The substrate crystallises in the space group P21/c with cell constants a = 6.798(2), b = 14.493(5), c = 17.028(6) Å and β = 94.70(3)°, V = 1673.0 Å3 and Z = 4 molecules/unit cell for dcalc. 3.161g cm−3; dobe = 3.15(2)g cm−3. The dimeric molecules consist of two hexacoordinated Re(I) cations surrounded by three carbonyl groups in a fac arrangement with one occupying an axial position and two equatorial ones. The other two axial positions of the dimer are occupied by the two sulphur atoms of the bridging dimethyldisulphide ligand. The (OC)2ReBr2(OC)2 fragment is not planar, instead, there is a folding of the molecule along the Br–Br′ vector, such that the two ReBr2 fragments make an angle of 28.0°. The Re–CO fragments are normal and compare well in structural parameters with other studied elsewhere; the same is true of the Re–Br and of the Re–S bonds. The changes introduced in the dimethyldisulphide ligand are principally in the S–S distance, the C–S–S angle and the C–S–S–C torsional angle, which are noticeably affected by bonding to the Re(I) cation.Spectral data for the Re2Br2(CO)6S2R2 complexes (R=Me, Ph, C6H4–pMe) have been collected in solution. The equilibrium (R = Me, Ph)Re2Br2(CO)6S2R2 + 2 THF ⇋ Re2Br2(CO)6(THF)2 + S2R2has been found to be strongly affected by the nature of the group R. In spite of a larger variation of the torsional angle when dimethyldisulfide leaves the coordination sphere of the metal, the displacement of the S2Me2 ligand by tetrahydrofuran is a less favourable process than the corresponding reaction with Re2Br2(CO)6S2Ph2. If solvation effects by THF can be ignored, the conclusion is that the Re2S2Me2 system is inherently stronger than Re2S2Ph2. This conclusion could not easily be drawn from a consideration of solid state parameters alone.
X‐Ray Photoelectron Spectra of Inorganic Molecules. XIX. Thiocyanato and Isothiocyanato Complexes of the Transition Metals: Carbon 1s, Nitrogen 1s and Sulfur 2p Binding Energies and their Possible Correlation with the Mode of Thiocyanate Bonding
Authors: Best S. A. , Walton R. A. 
Published: November 2013

AbstractThe X‐ray photoelectron spectra of a series of thiocyanato and isothiocyanato complexes of the transition metals have been recorded. Carbon 1st, nitrogen 1s and sulfur 2p binding energies were measured in order to establish whether there was any clear correlation with the mode of thiocyanate bonding. Complexes which were studied included those possessing M–NCS, M–SCN and M–NCS–M units. In addition, several species were investigated which contained two different thiocyanate environments within the same molecule. These were the ionic complexes [Cu(tren)(NCS)]SCN and [Cu(trien)(SCN)]SCN, where tren = tris(2‐aminoethyl)amine and trien = triethylenetetramine, and Pd(dppe)(NCS)(SCN), where dppe = 1,2‐bis(diphenylphosphino)ethane. The latter complex contains both N‐ and S‐bound thiocyanate. Within the range of complexes studied, the S 2p energies of M–NCS, M–SCN and SCN− differ at most by a few tenths of an electron volt. This indicates that the XPS technique will not usually permit a ready distinction between these different bonding modes in most transition metal complexes.
Electron Transfer Reactions Induced by Excited State Quenching
Authors: Meyer Thomas J. 
Published: November 2013

AbstractElectron transfer quenching of excited states can lead to separated redox products for which back electron transfer can be highly favored. Application of flash photolysis leads to a relaxation technique in which a system at equilibrium is perturbed by separate excitation‐quenching steps. The rates of several electron transfer reactions have been studied using the technique based on redox quenching of the CT excited state Ru(bpy)2+−3. Quenching and back electron transfer has also been observed for a series of excited states including examples of CT(Ru(phen)2+3, etc), π ‐ π * (Pd(OEP)), and f‐f(Eu(phen)3+3) states. In recent work, even non‐emitting excited states have been shown to be quenched by electron transfer which suggests that the technique may have broad applicability. In properly designed systems containing more than one redox reagent, the quenching and back electron transfer steps can be separated and reactions have been studied which are driven photocatalytically by excited states like Ru(bpy)2+−3. A modification of the technique has been used to estimate the self‐exchange rate for the Ru(bpy)3+/2+3 couple and to measure rates of electron transfer for a series of systems where the rates measured do not involve a Ru‐couple.
Catalytic Asymmetric Hydrogenation Using Hydridorhodium(I) Diop Complexes [Diop = 2,3‐0‐Isopropylidene‐2,3‐Dihydroxy‐1,4‐Bis (Diphenylphosphino)Butane]
Authors: James Brian R. , Mahajan Devinder 
Published: November 2013

AbstractThe hydridorhodium(I) phosphine complex HRh[(+)‐diop]2, [(+)‐diop = (+)‐2,3,‐0‐isopropylidene‐2,3‐dihydroxy‐1,4‐bis(diphenylphosphino)butane] has been studied for catalytic hydrogenation of a number of olefinic substrates. Certain prochiral olefinic carboxylic acids have been reduced to products with up to about 60% enantiomeric excess. Rate data show the importance of the degree of substitution at the olefinic bond in terms of steric effects, and the ‐COOH function of the substrate likely plays a role. Kinetic and spectrophotometric studies on the hydrogenation of itaconic acid in n‐butanol‐toluene (2:1) solutions under mild conditions are best interpreted in terms of a mechanism involving formation of a metal alkyl via coordination of the olefinic substrate, followed by reaction with H2 to give saturated product and the starting catalyst complex. The actual catalyst is thought to be HRh(diop)(diop*), where diop* represents a monodentate diop with one dangling ‐CH2PPh2 moiety.The catalyst systems are somewhat complicated by the occurrence of slower side‐reactions, the extent of these varying from negligible for the more rapidly reducing substrates, to possibly of prime importance in the slow hydrogenations. The initial hydride catalyst, for example, is slowly decomposed to the cationic [Rh(diop)2]+ complex by protons from the olefinic substrate. Under H2, the cation reversibly forms a dihydride. The cation is also formed from a reaction that appears to involve an intermediate alkyl complex and the butanol.
Simultaneous Asymmetric Hydrogenation and Hydroformylation by Chiral Platinum Complexes
Authors: Consiglio Giambattista , Pino Piero 
Published: November 2013

AbstractAn asymmetric induction of 37.6% has been obtained in the asymmetric hydrogenation of α‐ethylstyrene with the [(−)‐DIOP]‐PtCl2/SnCl2 catalytic system. When carbon monoxide is present, both hydrogenation and hydroformylation of the substrate occur. The prevailing antipode arises from the opposite prochiral face of the olefin in hydrogenation and hydroformylation respectively.
Iridium and Rhodium Nitrosyl o‐Catecholato Metal Complexes
Authors: Ghedini Mauro , Denti Gianfranco , Dolcetti Giuliano 
Published: November 2013

AbstractThe synthesis and characterization of several novel tetra and pentacoordinate catecholato metal nitrosyl complexes of the type Ir(NO)(1,2‐O2C6H3‐3 or 4‐R)‐P(C6H5)3): 3‐R = iso C3H7; 4‐R = COOH, H, CH3, terC4H9; and M(NO)(1,2‐O2C6H3‐4‐R)(P(C6H5)3): M = Ir and Rh; R = NO2, CHO, COOCH3; M = Rh; R = H; are reported together with their IR spectra. Two different classes of cathecolato complexes are obtained, tetracoordinate and pentacoordinate. The pentacoordinate complexes achieve tetracoordination, in solution, by spontaneous and reversible release of a phosphine molecule. The separation of the crystalline species seems to be determined either by the electron withdrawing or releasing power of the cathecolato ring substituents rather than by the solubility of the single species in our preparative conditions. The possibility for the pentacoordinate complexes to have a packed trigonal bipyramid geometry with axial bent nitrosyl NO− and axial equatorial catechol ligand is supported by the easy phosphine dissociation to form the tetracoordinate, which has been observed both in solution and in field desorption mass spectra measurements. For the tetracoordinate complexes a square planar geometry with linear nitrosyl, NO+, is proposed.
Chemical Analysis of Hybrids of Withania somnifera L. (Dun.). 1. Chemotypes III (Israel) by Indian I (Delhi)
Authors: Kirson Isaac , Abraham Arieh , Lavie David 
Published: November 2013

AbstractReciprocal crossbreeding between Withania somnifera chemotype III (Israel) and chemotype Indian I (Delhi) were performed. The F1 offspring of this cross was characterized by significant changes in the main steroidal lactone constituents of the parent plants. From the withaferin A (9) and the withanone (2) both present as major constituents in the parent Indian I, the former had disappeared in the hybrid and was replaced by withanolide D (10), whereas the latter was still present, though in about one third of the original quantity. It was accompanied by 17‐deoxy‐20‐hydroxy withanone (2) and by withanolide T (1); the enzymatic system leading to hydroxylation at C20 (20αF‐OH) has a dominant character. Two new withanolides have been isolated from this F1 offspring, namely withanolide T (1) (20αF‐hydroxy withanone) and withanolide U (5) (4β‐hydroxy‐withanolide G). The structures assigned to these two new compounds are based on spectral evidence (NMR, IR and UV) and analysis of their fragmentation under electron impact.
The Flavonoids: Indispensable Additions to a Recent Coverage
Authors: Gottlieb Otto R. 
Published: November 2013

AbstractWhile the flavonoids are biosynthetically and constitutionally a homogeneous group of compounds, the neoflavonoids, as presently defined, may be biosynthetically and are constitutionally a heterogeneous group. Biogenetic considerations which lead to these conclusions and reviews on occurrence, structural elucidation and synthesis are presented for 1,3‐diarylpropanes and flavans (flavonoids), as well as for cinnamylphenols and 2‐aryl‐3‐methylbenzofurans (neoflavonoids), with the purpose of providing addenda to the 1975 monograph, The Flavonoids, edited by Harborne, Mabry and Mabry for Chapman and Hall, London.
Techniques for Signal Assignment in 13C‐NMR Spectroscopy of Naturally Occurring Aromatic Compounds
Authors: Gottlieb Hugo E. 
Published: November 2013

AbstractTechniques of signal assignment of 13C‐NMR spectra of organic compounds, and in particular those of natural origin, are reviewed. They include symmetry, chemical shift considerations, spin‐spin coupling (both one‐bond and long‐range), spin relaxation (T1, NOE), isotopic enrichment and complexing agents. Special emphasis is given to the effects of oxygenated substituents on aromatic rings.
XIII. Charge Deformation and Vibrational Smearing
Authors: Hirshfeld F. L. 
Published: November 2013

AbstractA tractable model for treating the vibrational smearing is provided by the convolution approximation, which divides the molecular charge into fragments that are assumed to move rigidly with the atomic nuclei. This model is of questionable validity for internal vibrations, the required charge decomposition is largely arbitrary, the atomic vibration amplitudes may not be well determined and rotational motion introduces special difficulties. These deficiencies are especially severe if we attempt, by means of a charge deformation model and least‐squares refinement, to derive static charge density maps from the X‐ray data. The greatest uncertainty occurs at the nuclear positions, around hydrogen atoms, or in groups undergoing large librational motion. Yet with suitable caution and a well tested deformation model, least‐squares refinement can yield more detailed and dependable charge‐density information than can Fourier methods.
XIV. Static Versus Dynamic Densities
Authors: Coppens Philip , Stevens Edwin D. 
Published: November 2013

AbstractExperimental density maps are affected by both thermal smearing and limited resolution. The two effects commute and have to be introduced in theoretical density maps for a quantitative comparison between theory and experiment. An appropriate procedure is discussed and some applications are given. The error expression for the thermally smeared multipolar density is given. No error analysis exists for the thermally deconvoluted “at rest” density at infinite resolution, which corresponds to an extrapolation beyond the limit of experimental information.
XVII. Spatial Partitioning of Charge Density
Authors: Hirshfeld F. L. 
Published: November 2013

AbstractQuantitative indices of a molecular charge distribution are provided by the values of volume integrals of the deformation density times a set of appropriate sampling functions. These integrals may, for example, represent the net charges in particular atomic or bond regions. A sharing function derived from the promolecule density defines a general partitioning of any molecule into bonded‐atom fragments, whose net charges and multipole moments determine the electrostatic properties of the molecule. Expressions are available for estimating the experimental accuracy of such indices but they are not easy to evaluate nor very reliable.
Acceleration and Retardation by Mercaptans of Photoreduction by Amines
Authors: Cohen Saul G. , Rose Anita W. , Stone Paul G. 
Published: November 2013

AbstractAromatic mercaptans and disulfides retard photoreduction of benzophenone by amines and are equilibrated to ∼62% thiyl as mercaptan in the process. Racemization of sec‐butylamine and introduction of deuterium into triethylamine indicate that ∼55% of the retardation arises from a hydrogen‐transfer repair mechanism, after correction for light absorption and quenching. The limiting step in the cyclic repair mechanism is reduction by mercaptan of the amine derived radical k10, in competition with oxidation of it by ground state ketone, k9. Ratios of rate constants for these reactions are, with 2‐mercaptomesitylene as retarder, for sec‐butylamine, k10a/k9a = 30 and for triethylamine, k10b/k9b = 310. The latter high value is attributed to low k9b, slow oxidation of this amine‐derived radical by ketone. The similar high value in reduction by 2‐propanol is attributed to rapid reduction of alcohol‐derived radical by the mercaptan. Aliphatic mercaptan, 0.02 M, accelerates photoreduction of 0.4 M benzophenone by 2 M sec‐butylamine in benzene by 50%, Φ = 1.82, and is not oxidized or equilibrated in the process. Acceleration is less efficient in acetonitrile and tert‐butyl alcohol and is not observed in water. Rate of the less efficient reduction by secondary amine is more than doubled by mercaptan. Inefficient retardation is observed in photoreduction by triethylamine in acetonitrile and in all cases at high concentration of aliphatic mercaptan. In retardation by n‐pentyl mercaptan, k10c/k9a = 0.24 in reduction by sec‐butylamine, and k10d/k9b = 0.88 in reduction by triethylamine. Ratios of reactivity toward aromatic and aliphatic mercaptans are k10a/k10c = 125 with sec‐butylamine, and k10b/k10d = 350 with triethylamine, both much higher than with 2‐propanol, suggesting lower reactivity of α‐aminoalkyl than α‐hydroxyalkyl radical toward reduction by mercaptan. Acceleration by aliphatic mercaptan arises from decrease in the quenching component normal in photoreduction by amines, and is attributed to catalysis by mercaptan of proton transfer from amine‐derived radical‐cation to ketyl radical‐anion in a ternary complex. Similar ternary complexes with aromatic mercaptan or thioether may lead to increased quenching and to retardation not accounted for by hydrogen‐transfer repair.
Photochemical Reactions of 2,2,5,5‐Tetramethyltetrahydrofuran‐3,4‐dione, an Unusual α‐Diketone
Authors: Rubin Mordecai B. , Ben‐Bassat Joseph M. , Weiner Moshe 
Published: November 2013

AbstractPhotochemical reactions of the red title compound (TTFD) in the absence of oxygen have been examined. With benzhydrol the α‐diketone was reduced to α‐hydroxyketone. Addition reactions with toluene and xylene afforded α‐hydroxy‐α‐benzyl‐ketones. Cyclopentene and cyclohexene gave α‐hydroxy‐α‐(3‐cycloalkenyl)‐ketones derived from abstraction of allylic H‐atom as major products; in addition [2 + 2] and [2 + 4] cycloadducts were obtained. In spite of the relatively low triplet energy of TTFD, its photochemistry parallels that of other α‐diketones.
Tautomerism in Xenon Hexafluoride: An Investigation of Xenon Hexafluoride and its Complexes by Raman Spectroscopy
Authors: Adams Chris J. , Bartlett Neil 
Published: November 2013

AbstractRaman spectra are reported for (i) complexes of xenon hexafluoride with the fluoride‐ion acceptors BF3, PdF4, PF5, AsF5, RuF5, PtF5, and AuF5, and (ii) for solutions of xenon hexafluoride in HF and in WF6. Vibrational assignments are made for the cations XeF+5and Xe2F+11, and evoke a normal coordinate analysis for XeF+5. Xenon hexafluoride, present in concentrated solution in HF as [XeF+5F−]4, progressively ionizes on dilution to give Xe2F+11; and XeF+5; on the other hand, in WF6solution [XeF+5F−]4exists in equilibrium only with molecular XeF6.
Organotin Derivatives of Hexafluorobenzene
Authors: Roesky Herbert W. , Ambrosius Klaus 
Published: November 2013

AbstractHexafluorobenzene reacts with N[Sn(CH3)3]3 to give C6F5N[Sn(CH3)3]2. Better yields of this compound were obtained either starting from C6F5NH2 and treating this with Li‐butyl and (CH3)3SnCl or reacting C6F5NSO with N[Sn(CH3)3]3. A byproduct of the reaction with C6F5NH2 is C6F5NHSn(CH3)3. The cleavage of the tin‐nitrogen bond in C6F5N[Sn(CH3)3]2 was achieved with the anhydrides (CF3C(O))2O and (CF3SO2)2O resulting in the following products: C6F5NC(O)CF3Sn(CH3)3, C6F5NSO2CF3Sn(CH3)3 and C6F5N(SO2CF3)2. When C6F5N[Sn(CH3)3]2 is treated with S3N2Cl2 a chain‐like compound of sulfur and nitrogen atoms is obtained (C6F5NSN)2S. The mass and NMR spectra of the new compounds are reported.