bluuebunny/crossref_metadata_2025_split huggingface dataset:
Short communication. Structural studies of bromine adducts of 1,2‐ and 1,4‐bis(phenylseleno)benzenes. Evidence for the (TB, MC) structure of 1,2‐(PhSeBr2)2C6H4
Authors: Nakanishi W. , Okumura Y. , Hayashi S. 
Published: January 2005

Abstract1,2‐Bis(phenylseleno)benzene reacted with equimolar amount of bromine to yield a trigonal bipyramidal (TB) adduct, in contrast to the molecular complex (MC) structure of selenanthrene with bromine. Whereas the structure of 1,4‐(PhSeBr2)2C6H4 was (TB, TB) at the two selenium atoms, that of the 1,2‐derivative was demonstrated to be (TB, MC), the first example of an MC structure of PhSeC6H4Y with Br2 [Y = 2‐PhBr2Se(TB)]. Both electronic and steric effects of the group must play an important role in MC formation. Rapid structural exchange in this tetrabromide, (TB, MC) ⇄ (MC, TB), was also observed.
Review commentary. Solvation, the electrophilic driving force of ionic bromination of ethylenic compounds. The addition‐solvolysis analogy
Authors: Ruasse Marie‐Françloise , Motallebi Shahrokh 
Published: January 2005

AbstractSolvation is the main driving force of electrophilic bromination since it is impossible to obtain a bromonium ion from bromine and alkene in the gas phase, whereas it is a very fast reaction in solution. The role of a protic solvent in this addition was investigated experimentally by extended Winstein–Grunwald relationships, kinetic solvent isotope effects and R, the rate ratios in two solvents of similar ionizing powers but different nucleophilicities. It is shown that electrostatic medium effects and electrophilic assistance to bromide ion departure are the main rate‐determining factors of the reaction. These two contributions are roughly independent of the double bond substituents. Nucleophilic solvent assistance to positive charge development is also found; however, it provides only a small acceleration, the magnitude of which depends on alkene structure. This nucleophilic solvent involvement is annulled when crowded substituents inhibit approach of the solvent to the cationic part of the transition state or when positive charge is delocalized by conjugated electron‐donating groups. These several solvent roles are identical in nature and in magnitude with those observed in heterolytic solvolysis. In halogenated solvents, the driving force of bromination arises from catalysis by a second bromine molecule which assists heterolysis of the bromine‐bromine bond, leading to the bromonium‐tribromide ion pair. Similar halogen catalysis occurs also in some solvolyses. Finally, return is also found in both reactions; reversible formation of bromonium ions is observed when their nucleophilic attack, the productforming last step, is made energetically difficult either by steric inhibition or by poor nucleophilicity of the trapping nucleophiles. Similarities and differences between electrophilic bromine addition and limiting solvolysis are discussed in terms of respective intermediate stabilities and heats of formation.
Steric effects of alkyl groups: A ‘cone angle’ approach
Authors: Datta Dipankar , Majumdar Devashis 
Published: January 2005

AbstractA ‘cone angle’, θR, is defined for an alkyl group (R), which is proposed as a measure of the steric effect exerted by the group. The θR values for some 57 groups have been calculated mathematically by constructing the corresponding Corey—Pauling—Koltun molecular models. The variation in θR with the bulk of a group is satisfactory. However not all types of alkyl groups can be accommodated in this cone angle approach. In the case of the groups for which the cone angle can be measured, correspondences between θR and the two existing steric parameters (Taft's Es scale and Dubois' E′s scale) have been established. For some 23 alkyl groups θR has been used to explain the variation in the rate constant of the base‐catalysed hydrolysis of RCOOEt with the nature of R. Our studies independently substantiate the basic assumptions of the Taft—Ingold hypothesis that, while the base‐catalysed ester hydrolysis is mostly controlled by the steric and electronic effects of the R group, the acid‐catalysed one is controlled almost solely be the steric effect.
Relationships between electronic substituent parameters
Authors: Sæthre Leif J. , Thomas T. Darrah 
Published: January 2005

AbstractCore‐electron energies for sulphur in a series of 2‐substituted thiophenes have been used to investigate the relationship between electronic substituent parameters as developed by Taft and Topsom and by Charton. Initial‐ and final‐state energies are analysed in terms of familiar substituent effects, such as field, delocalization and polarizability. The results show that the data correlate equally well with both sets of parameters and that there are many common features between the two methods. Both agree that resonance delocalization contributes significantly to the initial state, but has virtually no effect on the final state. In connection with this investigation we have also correlated the parameters for 31 substituents that are parameterized in the two approaches. The results of this more general study indicate some real differences between these and suggest that neither of the parameterizations incorporates all the electronic effects.
Effects of structures and concentrations of mono and BIS‐aryliminodimagnesium on the reactions with nitrobenzenes. Formation of azoxy and azo products assisted by aggregated excess of magnesium reagent
Authors: Ōkubo Masao , Tanaka Mikio , Shiku Haruko , Yamauchi Akira , Matsuo Koji 
Published: January 2005

AbstractThe relative yields of azoxy‐ and azo‐type products in the reactions of aryliminodimagnesium [ArN(MgBr)2 (IDMg)] with nitrobenzene were greatly affected by changes in the molar concentration in tetrahydrofuran, addition of hexamethylphosphoramide to the reagent solution and the structures of mono‐ and bis‐IDMg reagents. All the modes of variation of the product distribution, including extreme cases, were roughly but systematically related to a series of qualitative degrees of aggregation of IDMg. The role of aggregation is ascribed to the assistance of excess reagent molecules in the product formation process in the reaction. From the present and previous results and discussion on the reactions of IDMg and RMgBr, the IDMg aggregation is concluded to be an essential modification factor for useful reactions of the NMg (and also CMg) reagents having a suitable efficiency of single electron transfer.
Towards a general approach to the deprotonation of carbon acids, including nitroalkanes
Authors: Arnaut Luís G. 
Published: January 2005

AbstractThe deprotonation of nitroalkanes and ketones was studied through the intersecting state model of Formosinho. The anomalous Brønsted coefficients observed when substituted nitroalkanes react with a common base are explained in terms of variable transition‐state electronic structures. These are very sensitive to substitution in the nitroalkane, owing to the high electron affinity of the nitro group. In ketones, such electronic effects are much attenuated. The transition states are characterized by the bond order, n,*, which is quantitatively correlated to the field/inductive and resonance effects of the substituents. The deprotonation of fluorence and other carbon acids was also studied. The theoretical model applied shows that all these reactions follow a mechanistic continuum, dictated by the extent into which the group directly bonded to the acidic carbon is mixed in the reaction coordinate.
Kinetic model for acid‐catalysed hydrolysis of benzohydroxamic acid
Authors: Ghosh Kallol K. , Rajput Surendra K. , Krishnani Kishore K. 
Published: January 2005

AbstractThe kinetics of the hydrolysis of benzohydroxamic acid have been investigated in hydrochloric, sulphuric and perchloric acids in 10% (v/v) dimethyl sulphoxide—water at 55 °C. Activation parameters were also determined. Rate correlation by the Cox—Yates excess acidity method shows an A‐2 mechanism involving rapid pre‐equilibrium protonation of the substrate followed by rate‐limiting attack of water at the carbonyl carbon atom to form a tetrahedral intermediate which collapses, in a fast step, to the products.
Transimination reaction between pyridoxal‐5′‐phosphate schiff bases with dodecylamine and amino acids
Authors: Vázquez M. A. , Muñtoz F. , Donoso J. , García Blanco F. 
Published: January 2005

AbstractSome transimination processes were studied using an intermolecular model formed by pyridoxal‐5′ ‐phosphate (PLP) an amino acid and dodecylamine (DOD) in an aqueous medium. All the kinetic constants for the reversible reaction were determined. The results show that in these cases transimination proceeds through an addition‐elimination by forming a diamine geminal intermediate. Equilibria are always shifted to dodecylamine‐PLP Schiff base formation. Differences between the stability of this Schiff base and the ϵ‐aminocaproic Schiff base cannot be explained only on the basis of the different nucleophicities of amine groups and therefore differences in the imine double bond environment must be taken into account to explain this behaviour.
Source of the intraannular hydrogens in the dehydroxylation of calix [4] arene diethyl phosphate ester derivatives
Authors: Grynszpan Flavio , Bialio Silvio E. 
Published: January 2005

AbstractThe different possible sources of the intraannular hydrogens in the dehydroxylated calixarenes obtained by reductive cleavage of the calix [4] arene diethyl phosphate esters 2 and 6 are analysed. Two calixarene diethyl phosphate esters (4 and 5) full deuterated in the ethyl groups were synthesized. Reductive cleavage of 4 and 5 (potassium‐ammonia) resulted in the formation of the OH‐depleted calixarenes 3 and 7, respectively, which did not incorporate any deuterium at the intraannular positions, as judged by integration of the NMR signals. Quenching with D2O of the reaction mixture of either 2 or 6 and potassium‐ammonia did not result in any deuterium incorporation in the products. The labelling experiments rule out the possibility that the source of the intraannular hydrogens is the diethyl phosphate ester groups or the quencher. It is concluded that the most probable sources of the hydrogen atoms in the OH‐depleted calixarenes is the ammonia molecule.
Micellar effects upon oxidation of organic sulfides by anionic oxidants
Authors: Bacaloglu Radu , Blaskó Andrei , Bunton Clifford A. , Foroudian Houshang 
Published: January 2005

AbstractPeroxymonosulfate ion (HSO5−) rapidly oxidizes 2‐chloroethyl phnyl sulfide (1). Dilute micelle‐forming cationic surfactants slightly speed the reaction but the reaction rate decreases sharply with increasing surfactant concentration. Rate‐surfactant concentration profiles in solutions of cetyltrimethylammonium hydrogensulfate can be treated quantitatively by estimating concentrations of 1 and HSO5− at the micellar surface with allowance for competition between HSO5− and HSO4−. The second‐Order rate constant in the micellar pseudo‐phase is lower that in water by a factor of ca 400. Similar rate–surfactant concentration profiles were observed for the oxidation of di‐n‐propyl sulfide by periodate ion in solutions of cetyltrimethylammonium bromide, chloride and mesylate, and second‐order rate constants in the micellar pseudo‐phases are lower than in water by factors of ca 400. These oxidations are slowed by a decrease in the water content of aqueous acetonitrile and solvent, and micellar rate effects are consistent with transition states in which positive charge builds up on sulfur.
Multi‐step processes in gas‐phase reactions of halomethyl anions XCH2− (X = Cl,Br) with CH3X and NH3
Authors: Matthias Bickelhaupt F. , De Koning Leo J. , Nibbering Nico M. M. , Jan Baerends Evert 
Published: January 2005

AbstractA mechanistic investigation of the gas‐phase reactivity of the halomethyl anions XCH2− (X = Cl, Br) towards a mixture of the corresponding halomethane and ammonia was performed using Fourier transform ion cyclotron resonance mass spectrometry. The interpretation of the experimental data is supported by high‐level density functional theoretical (DFT) calculations for the chlorine‐containing systems (X = Cl). When the specific isotopomer AXCH2− (AX = 35 Cl, 79Br) is allowed to react in an atmosphere of CH3X and NH3, the exclusive formation of the isotope cluster of the halide anions AX− and BX− (BX = 37Cl, 81Br) is observed. However, the intensity ratio I(AX−)/I(BX−) exceeds significantly the value expected from the natural relative isotope abundances and depends linearly on the pressure ratio p(NH3)/P(CH3X). The experimental results are interpreted in terms of three competing reaction mechanisms: (i) The by far dominating process is the more than 70 kcal mol−1 exothermic one‐step SN2 substitution of AXCH2− on CH3X, generating haloethane AXCH2CH3 and X− isotopomers, the latter in the proportion of their natural abundances (direct SN2). The experimentally observed excess of AX− stems from two minor reaction pathways: (ii) in a secondary reaction, the halide X− in the primary product anion‐molecule complex [AXCH2CH3 … X−] * of the SN2 substitution induces a 1,2‐elimination, leading to the formation of the AX− isotopomer (two‐step SN2/E2). (iii) Finally, ACH2− can react with ammonia by consecutive endothermic proton transfer (PT) from NH3 to AXCH2− and a very exothermic SN2 substitution of the resulting amide on AXCH3 leading to CH3NH2 and an excess of AX− which depends linearly on p(NH3)/p(CH3X) (PT/SN2). Theoretical calculations show that in the case of [ClCH2−…NH3] *, the PT/SN2 reaction has no stable intermediate. Therefore, it is concluded that this reaction is not a two‐step but a one‐step process.
ESR study of solvent effects on the electronic structure of 2‐(4′ ‐dialkylaminophenyl)indan‐1,3‐dionyl radicals
Authors: Sueishi Yoshimi , Isozaki Takanori , Yamamoto Shunzo , Nishimura Norio 
Published: January 2005

AbstractThe ESR spectra of 2‐arylindan‐1,3‐dionyl radicals in various solvents at 343 K were obtained and assignments of the hyperfine structures were made. The hyperfine coupling constant (hfcc) of the dimethylamino nitrogen increases whereas the 2',6'‐hydrogen hfcc decreases as the ET value, a solvent polarity parameter, increases. MO calculations were made according to the semi‐empirical procedure of Rieger and Fraenkel; the nitrogen and oxygen Coulomb integrals were adjusted so that the calculated spin density on the nitrogen atom agrees with that estimated from its hfcc. It is found that the π‐moment of the radicals increases from 9·13 D in toluene to 9·85 D in acetic acid. The solvent dependence of the equilibrium between the radicals and its dimer has been explained in terms of the solventinduced π‐moment in polar solvents.
Solvolysis of 1,1‐diphenylethyl derivatives in highly aqueous media. Reaction via ion pairs
Authors: Thibblin Alf 
Published: January 2005

AbstractSolvolysis of 1,1‐diphenyl‐1‐X‐ethane (1‐X) [X = 4‐nitrobenzoate (PNB), 3,5‐dinitrobenzoate (DNB), AcO, MeOH+, EtOH+ or H2O+] was studied in 20 vol% dimethyl sulphoxide in water, in which specific salt effects are very small, and in 25 vol% acetonitrile in water. Substrates with negatively charged leaving groups yield more elimination product 1,1‐diphenylethene (3) than those with uncharged groups, indicating the intermedicacy of ion pairs. Thus, three times more alkene is formed in the reactions of the substrates 1‐OAc and 1‐PNB than in the acid‐catalysed hydrolysis of 1‐OMe and 1‐OAc. The substitution products may be formed via the solvent‐equilibrated carbocation since the measured nucleophilic selectivities are very similar with different leaving groups.
Complexes of TRI‐ and tetraprotonated forms of 1,4,7,10,13‐pentaazacyclopentadecane with various mono‐ and divalent anions in aqueous media
Authors: Gelb Robert I. , Alper Joseph S. , Schwartz Marietta H. 
Published: January 2005

AbstractAcid dissociation and anion complexation properties of 3+ and 4+ forms of 1,4,7,10,13‐pentaazacyclopentadecane were investigated by pH potentiometry, conductometric titration and13C NMR experiments. The 3+ and 4+ species form complexes with a variety of mono‐ and divalent anions, and their complexation constants can be determined by pH potentiometric methods. However, no iodide complexes are detected and neither is a pentaprotonated species. Enthalpy and entropy changes for the acid dissociation of the 3+ ligand and for its complexation with chloride are estimated. The complexation and acid dissociation properties of the ligand are discussed in terms of internal hydrogen bonding interactions and solvation effects.
Kinetic evidence for interdigitation in model lipid bilayers
Authors: Moss Robert A. , Bhattacharya Soumendu 
Published: January 2005

AbstractPseudoglyceryl ammonium ion coliposomes constructed from an arachidoyl probe lipid and either stearoyl or palmitoyl host lipids display unusually slow transverse bilayer migration (‘flip‐flop’) of the arachidoyl probe molecule compared with the behavior of shorter probe lipids in similar colipsomes. This phenomenon is probably associated with interdigitation of the arachidoyl lipid across the bilayer's midplane.
Metal ion complexation by tetraester derivatives of bridged calix [4] arenes
Authors: Arnaud‐neu Francoise , Böhmer Volker , Guerra Lourdes , McKervey M. Anthony , Paulus Erich F. , Rodriguez Amado , Schwing‐weill Marie‐José , Tabatabai Moniralsadat , Vogt Walter 
Published: January 2005

AbstractA series of bridged calix [4] arenes (5–10 methylene groups in the bridge) were synthesized and transformed into their tetraester derivatives by reaction with ethyl bromoacetate. The stability constants for complexes of the tetraester derivatives with sodium, potassium and silver cations, determined in methanol by spectroscopic or potentiometric techniques, show a drastic decrease (more than 105 for Na +) for the shorter methylene chains. 1H NMR studies demonstrate a conformational rearrangement of the calixarene part to fourfold symmetry to be necessary for the complexation of a cation, which is prevented by the shorter chains. This is further confirmed by the X‐ray structure of a tetraester derivative with a CH2CH2COCH2CH2 bridge; crystals were monoclinic, space group P21/c,a = 11.847(2), b = 39.773(5), c = 12 · 127(2) Å, β = 109.24(1), V = 5395.1 Å3, Z = 4, Dx = 1.186 Mg m−3.
Host–guest chemistry. Methanol, ethanol and propan‐1‐ol inclusion compounds of 2‐[o‐(triphenylphosphoranylidenamino) benzyliden] amino‐1H‐2, 3‐dihydroindazol‐3‐one. X‐ray structural characterization of the free host and its ethanol inclusion compound
Authors: Molina Pedro , Arques Antonio , Obón Rosario , Llamas‐saiz Antonio L. , Foces‐Foces Concepcion , Claramunt Rosa Maria , López Concepcion , Elguero Jose 
Published: January 2005

AbstractThe title compound (3) which is a 2‐aminoindazol‐3‐one derivative linked through a methinephenyl bridge to an iminophosphorane, and unrelated to any previous host, is described. The bulky triphenylphosphino group provides the cavity and the indazol‐3‐one the hydrogen bonds (both donor NH and acceptor CO). X‐ray structural analyses of the free host 3 and that of its ethanol inclusion compound 3 · EtOH are reported. Hydrogen bonds resulting in infinite chains dominate the molecular packing. The use of thermal analysis and 13C CP/MAS NMR shed light in the structures of two other inclusion compounds, 3 · MeOH and 3 PrOH. The conclusion is that all three compounds are similar. An analysis of the cavities, using a model of interpenetrating spheres of van der Waals radii, demonstrates that in the case of the ethanol derivative, the cavity has the space and form requirements necessary to include the propanol guest.
Supramolecular metallocatalysts for the cleavage of amino acid esters
Authors: Scrimin Paolo , Tecilla Paolo , Tonellato Umberto 
Published: January 2005

AbstractTransition metal ions are effective catalysts of the hydrolytic cleavage of amino acid esters and their effects can be enhanced and properly directed when they are chelated to functionalized ligands. The resulting metallocatalysts are attracting increasing attention and the systems so far investigated are briefly reviewed. Particular emphasis is given to supramolecular systems which may add to the metallocatalysts the benefits of the cooperativity, set upon convergent non‐covalent interactions of their components, needed for substrate recognition. The results obtained with metallomicellar aggregates and molecular metalloreceptors, with particular reference to those studied in the authors' laboratory, are reported in more detail. In the case of loosely structured metallomicelles, remarkable accelerations and, generally, modest selectivities have been observed; less spectacular kinetic effects, but promising substrate selectivity, have been obtained with structurally well defined metalloreceptors.
Molecular recognition by macrocyclic receptors having multiple hydrophobic branches in a synthetic bilayer membrane
Authors: Kikuchi Jun‐Ichi , Matsushima Chiemi , Tanaka Yumi , Hie Ken‐Ichi , Suehiro Kazuaki , Hayashida Osamu , Murakami Yukito 
Published: January 2005

AbstractHybrid molecular assemblies were prepared in combinations of a synthetic peptide lipid, involving an L‐alanine residue interposed between an anionic head group and a hydrophobic double‐chain segment, with cationic macrocyclic hosts, a steroid cyclophane bearing four rigid steroid moieties and octopus cyclophanes having eight flexible hydrocarbon branches. On addition of the cyclophanes to multi‐walled bilayer membranes composed of the anionic lipid, thermodynamic parameters (ΔHand ΔS) associated with the phase transition between the gel and liquidcrystalline states were subjected to changes that are consistent with the formation of the hybrid assemblies. Anionic fluorescent guests, 8‐anilinonaphthalene‐1‐sulphonate and 6‐p‐toluidinonaphthalene‐2‐sulphonate, were effectively incorporated into the hydrophobic cavities provided by the cationic cyclophanes embedded in the bilayer membrane through hydrophobic and electrostatic interactions. The guest‐binding modes of the hybrid assemblies are classified into two types; a guest is included in the proximity of the hydrogen‐belt domain of the bilayer membrane in one mode, and a guest is incorporated into the hydrophobic bilayer domain composed of double hydrocarbon chains of the lipid in another.
The photochemistry of pyrene–cytosine conjugates: Modelling the carcinogenic action of aromatic hydrocarbons
Authors: Sessler Jonathan L. , Kubo Yuji , Harriman Anthony 
Published: January 2005

AbstractPhotoreduction, but not the corresponding photooxidation, of cytosine can be sensitized by a covalently appended pyrene molecule in a process that may have some importance for understanding the known carcinogenic activity of polynuclear aromatic hydrocarbons.