bluuebunny/crossref_metadata_2025_split huggingface dataset:
Alkali metal ion‐assisted cleavage of crown ether anisoles by toluenethiolate anion
Authors: Cacciapaglia Roberta , Mandolini Luigi , Romolo Francesco S. 
Published: January 2005

AbstractThe effect of alkali metal ions (Li+, Na+, K+, Cs+) on the reactivity of toluenethiolate anion in the demethylation of 2‐methoxy‐1,3‐xylyl‐18‐crown‐5, 2‐methoxy‐1,3‐xylyl‐24‐crown‐7 and the model compounds anisole and 2,6‐dimethylanisole was investigated in dimethylformamide (+3.3 M water) at 60°C. It was found that the metal ion effects are markedly influenced by the presence of the polyether chain in the reaction system. Whereas reactions of the model compounds are slightly inhibited by the presence of alkali metal ions, the latter strongly enhance rate of demethylation of the crown ether derivatives, the observed catalytic factors ranging over nearly three orders of magnitude. These remarkable rate‐enhancing effects are ascribed to a strong transition‐state stabilization by metal ions through cooperation of electrostatic binding with the negative charge developing on the oxygen atom of the methoxy group undergoing nucleophilic attack and coordinative interaction with the polyether chain.
Direct nucleophilic aromatic substitution reactions in the syntheses of anthraquinone derivatives: Chemistry and binding of podands, crown ethers, and a cryptand
Authors: Kim Hyunsook , Schall Otto F. , Fang Junping , Trafton John E. , Lu Tianbao , Atwood Jerry L. , Gokel George W. 
Published: January 2005

AbstractThe direct nucleophilic aromatic substitution reactions of anthraquinones have permitted the syntheses of more than 30 novel podands, crown ethers and lariat ethers. Anthraquinones having (ethyleneoxy)n sidearms were obtained by direct displacement of chloride by the anion of CH3(OCH2CH2)nOH. The ethyleneoxy‐substituted anthraquinones could, in turn, undergo direct replacement by nucleophiles that failed to displace chloride. This approach has been used for the preparation of two‐armed podand derivatives and several novel crown derivatives of anthraquinone. Binding comparisons are presented for several of these new anthraquinones. Direct substitution did not prove successful in the preparation of anthraquinone‐ [2 · 2] ‐cryptand which was obtained by alkylation. The crystal structure of the latter reveals an orientation of ring and anthraquinone appropriate for cation binding, a fact confirmed by cation binding constant measurements.
Supramolecular control of molecular electronic behaviour of TCNQ−. salts
Authors: Grossel Martin C. , Weston Simon C. 
Published: January 2005

AbstractCrown ether complexation of 1:1 alkali metal TCNQ salts, MTCNQ (M=K, Rb, Tl) leads to structures containing isolated TCNQ dimers. These materials provide valuable models for investigating the spectroscopic and electronic behaviour of TCNQ−. dimer components which are key structural fragments of, for example, organic metals and semiconductors.
Kinetics and mechanism of the dissociation of the Cu2+ and Ni2+ complexes with monooxo‐ and dioxomacrocyclic tetraamines
Authors: Siegfried Liselotte C. , Kaden Thomas A. 
Published: January 2005

AbstractThe kinetics of the acid‐induced dissociation of the Ni2+ and Cu2+ complexes of a monooxo and two dioxo tetraaza macrocycles were studied by stopped‐flow spectrophotometry at single wavelengths and using a photodiode array, For the monooxo derivative the dissociation rate follows the law vd = k1Ccom [H+]/(K2 + [H+]), whereas for the dioxo derivatives the law is vd = k2Ccom [H+]2/(K1 + K2 [H+] + [H+]2). The dissociation kinetics are interpreted by a mechanism in which protonated intermediates are formed in a rapid pre‐equibrium, the rate‐determining step being the dissociation of this species. Comparison of the spectral results from multi‐wavelength stopped‐flow measurements with those acquired in the equilibium study using spectrophotometric titrations allows the nature and structure of these intermediates to be discussed. The order of the dissociation rates in strongly acidic solution, where a plateau in the pH dependence is reached, was determined and it demonstrates the importance of the ring size on one hand and of the number of amide groups on the other.
Heats of formation of enols. The double‐bond stabilizing effect of the hydroxyl group
Authors: Keeffe J. R. , Kresge A. J. 
Published: January 2005

AbstractAqueous solution free energies of ketonization have been combined with enthalpies of reduction of the keto forms and other thermochemical data to yield estimates of the heats of formation of the corresponding enols. A double‐bond stabilization term, DOH = 5.4 ± 0.5 kcal mol−1 (1 kcal = 4.184 kJ), is obtained for the OH group. This quantity is close to Hine's double‐bond stabilization parameter for the methoxyl group (4.9 ± 0.2 kcal mol−1). Comparison with available experimental gas‐phase enthalpies of formation suggest that, relative to the keto form, enols are more stable in the gas phase than in water. The differences are unlikely to be due entirely to entropic effects. A wholly enthalpic, gas‐phase double‐bond stabilization term, DOH = 8.1 ± 0.6 kcal mol−1, may be calculated from the available experimental data. This result indicates that the hydroxyl group more effectively stabilizes the CC bond in the gas phase than in aqueous solution. An important limitation is that the results reported bere apply only to simple enols, that is, monofunctional enols not stabilized by extra resonance or hydrogen‐bonding effects.
Partial rate factors in photochemistry: The meta photocycloaddition of methylbenzenes to cyclopentene
Authors: de vaal P. , Lodder G. , Cornelisse J. 
Published: January 2005

AbstractThe partial rate factors of the four modes of meta photocycloaddition of methyl‐substituted benzenes to cyclopentene have been determined by measuring quantum yields of formation of the most important meta photocycloadducts of benzene, toluene and the three xylenes. It is demonstrated that the results can be applied to predict quantum yields of any meta photocycloadduct of methylbenzenes to cyclopentene. The predicted yields agree very well with those determined experimentally, and it is shown that the effects of two or more methyl groups are fully additive. The method is promising for the investigation of mechanisms of photochemical reactions and can be used to predict regioselectivities.
Ionization kinetics of rhodamine b leucocyanide in the presence of poly(vinylbenzyltrimethylammonium chloride)
Authors: Zimerman O. E. , Cosa J. J. , Gsponer H. E. 
Published: January 2005

AbstractThe participation of the carboxylate group in the ionization of Rhodamine B leucocyanide was studied in an aqueous solution of poly(vinylbenzyltrimethylammonium chloride) (PVBTA) or cetyltrimethylammonium bromide (CTAB) at constant temperature and pH 8. The experimental rate constant (kexp) decreases with increasing PVBTA concentration. The results are interpreted in terms of the binding of the ionized form of Rhodamine B leucocyanide (RBCN−) to a single class of completely independent binding sites on the PVBTA. From this model, a value of 1.09 × 106 l mol−1 was obtained for the binding constant. The effect of CTAB is different from that obtained with PVBTA. After a region in which there are large changes in kexp with the CTAB concentration, this constant reaches a zone where its value is independent of the surfactant concentration. The decrease in kexp in the first zone was explained in terms of an ionic association between RBCN− and the cationic heads. In the second zone, it was assumed that a micellar effect operates on the ionization kinetics. In a similar form, the esterification of the carboxylate group leads to a slower ionization rate.
Degradation of atrazine by hydrolysis and by hydroxyl radicals
Authors: Chan Gabriel Y. S. , Hudson Michael J. , Isaacs Neil S. 
Published: January 2005

AbstractRates of degradation of the triazine herbicide atrazine are reported, together with an analysis of the sequence of stages involved, using hydroxyl radicals generated by hydrogen peroxide in the presence of UV light and by the pure photochemical reaction. These conditions bring about the displacement of chlorine by hydroxyl, dealkylation of the alkylamino groups and finally deamination of the triazine ring. The significance of these results to water purification is discussed.
Structure of, and internal rotation in, hexakis [4‐(2‐phenylpropan‐2‐yl) phenoxy] benzene, and structural comparison with its precursor phenol 4‐(2‐phenylpropan‐2‐yl)Phenol
Authors: MaCnicol D. D. , Mallinson P. R. , Vallance I. 
Published: January 2005

AbstractThe title compounds (1 and 3, respectively) were studied by X‐ray crystal structure analysis. Both are trigonal, space group R3, with lattice parameters a = 27.718(6), c = 8.161(2) Å, Z = 3 and a = 31.267(7), c = 6.560(2), Z = 18, respectively. The dodecamethyl compound 1 is isomorphous with its parent hexakis(4‐benzylphenoxy)benzene (2), a known host. However, ‘collapse’ along the c‐axial direction has occurred such that the appreciable clathrate cavity of 2 has been replaced by only a very small residual void for 1. Phenol 3 does not pack in discrete [OH]6 hydrogenbonded hexamers; instead, molecules are assembled in infinite chains, linked by OH…︁O hydrogen bonds of length 2.735(3) Å, which are propagated along threefold screw axes. A variable‐temperature CPMAS NMR study of sidechain aryl group rotation in 1 gives a value ΔG≠ of 14.0 ± 0.7 kcal mol−1 (at 42 °C) for the (crystallographically unique) para ‐disubstituted ring, a similar 14 kcal mol−1 free energy barrier being found hindering rotation of this ring, and of the outer phenyl ring of 1, at 9°C.
Stereoelectronic effects at carboxyl oxygen
Authors: Tadayoni B. Mitra , Rebek Julius 
Published: January 2005

AbstractThe recent attempts to evaluate the stereoelectronic effects at carboxyl oxygen using Kemp's triacid derivatives are reviewed. Contexts include metal‐ion chelation, acidity measurements, intramolecular general base catalysis and nucleophilic substitution. These give a range of 0–8 kcal/mol for the effects, and leave the issue open for more incisive experiments.
Biphasic kinetics and flip‐flop behaviour of vesicles of fluorocarbon amphiphiles with a 1,3‐disubstituted glycerol structure
Authors: Liang Kangning , Hui Yongzheng 
Published: January 2005

AbstractFluorocarbon amphiphiles with a 1,3‐disubstituted thioglycerol structure form stable unilamellar vesicles. The fluorocarbon bilayer of an amphiphile which has an ammonium salt as head group is an ‘insulator’ for the permeation of hydroxide anions. In the presence or absence of a fluorocarbon nucleophile, hydrolysis of a probe amphiphile carrying a cleavable p‐nitrophenyl ester group in the vesicle under alkaline conditions shows a pattern of biphasic kinetics, in which the fast and slow reaction can be attributed to the hydrolysis of probe molecules at outer and inner surfaces of vesicles, respectively. The fact that the slow rate constant always remains constant at 0.25 min−1 at 25°C, independent of the pH of the system and of concentration of the nucleophile, indicates that the slow process is an outward flip‐flop process of probe molecules within the fluorocarbon domain.
Nucleophilic substitution reactions of methallyl arenesulphonates with anilines and N, N‐dimethylanilines
Authors: Oh Hyuck Keun , Shin Chul Ho , Lee Ikchoon 
Published: January 2005

AbstractKinetic studies of the reactions of methallyl arenesulphonates (II) with anilines and N,N‐dimethylanilines in acetonitrile at 45.0°C are reported. The sign and magnitude of the cross‐interaction constants pxz (and βxz) between substituents in the nucleophile (X) and leaving group (Z) suggest that the transition state (TS) is slightly tighter than that for the corresponding reactions of allyl arenesulphonates (I). This is also supported by the observation that the magnitudes of ρx and ρz for II are uniformly greater than those for the reactions of I. These results are in line with the simple MO theory that the 2‐position of the allyl system is inactive electronically. The steric effect of the 2‐methyl group in II causes a rate retardation and a shift of the TS toward a later position along the reaction coordinate with a slight increase in the overall tightness of the TS structure. The large |ρxz| value obtained eliminates the possibility of an SN2′ mechanism.
Gas‐phase ambident reactivity of cyclic enolate anions
Authors: Freriks Ivo L. , de Koning Leo J. , Nibbering Nico M. M. 
Published: January 2005

AbstractThe gas‐phase reactions between cyclic enolate anions and unsaturated perfluorocarbon compounds were studied by Fourier transform ion cyclotron resonance. A correlation is observed between the experimental electron detachment threshold energies, the reaction selectivity and the ring size of the cyclic enolate anions. The results suggest that frontier orbital interactions play an important role in the course of the reactions of cyclic enolate anions, as has been shown previously for the reactions of acyclic enolate anions. In addition, the reaction selectivity of cyclic enolate anions may also be influenced by the charge distribution and by transannular interactions. In comparison with the acyclic enolate anions, the HOMO energy of the small cyclic enolate anions (C4–C5) has decreased and the carbanion character has increased, both of which favour reaction via carbon. The medium ring‐sized cycloketone enolate anions behave very similarly to their acyclic analogues, whereas for the larger ring‐sized cycloketone enolate anions (C8–C12) the reaction via the carbon nucleophilic centre gains importance as a result of the stabilization of the HOMO due to transannular interactions.
Z–E Isomerism of 5‐cyclohexylmethylenehydantoins
Authors: Tan Sau‐Fun , Au‐Yong Po‐Lung , Lee Lay‐Ting 
Published: January 2005

Abstract5‐Cyclohexylmethylenehydantoin and its 1‐ and 3‐methyl derivatives were studied as the aliphatic analogues of the 5‐benzylidenehydantoins to examine the effects of replacing a benzene by a cyclohexane ring on Z–E isomerism. The relative stabilities of the Z‐ and E‐isomers were compared by estimating the free energy differences from thermal equilibration experiments and the heats of formation by AM1 calculation. In the Z‐isomers, the possible existence of weakly attractive interaction between the C(4) = 0 and C(6)–H groups, which form parts of a stereochemically favourable, conjugated α,β‐unsaturated system, is suggested as a contributory factor to the observed configurational preference. Assignment of configurations was based mainly on NMR and is supported by IR and UV spectra. NMR spectra also revealed the conformation of the cyclohexane ring and the cyclohexyl proton signals were assigned by double resonance and by high‐field NMR in two solvents where solubility allows.
Diastereofacial selectivity in 1,3‐dipolar cycloadditions to cyclobutens. 8. HF/3–21G transition structures of the reactions of formonitrile oxide with CIS‐3,4‐dichlorocyclobutene and norbornene
Authors: Bagatti Marisa , Rastelli Augusto , Burdisso Marina , Gandolfi Remo 
Published: January 2005

AbstractA transition structure for the syn and anti attack of formonitrile oxide on both cis‐3,4‐dichlorocyclobutene and norbornene was obtained using HF/3–21G method. These calculations correctly predict dominance of the syn attack in the reaction of dichlorocyclobutene and 100% syn selectivity, with respect to the methano bridge, in the reaction of norbornene. Analysis of the activation energy shows that the inherent facial bias of dichlorocyclobutene is reflected, at the transition state, in a complex way in the deformation energy of both dichlorocyclobutene and formonitrile oxide and in the interaction energy between them. With norbornene the out‐of‐plane deformation energy of the olefinic hydrogens clearly emerges as the major factor in controlling facial selectivity.
Kinetics and mechanism of the oxidation of substituted benzaldehydes by ethyl N‐chlorocarbamate
Authors: Varshney Seema , Kothari Seema , Banerji Kalyan K. 
Published: January 2005

AbstractThe kinetics of the oxidation of a number of para‐ and meta‐monosubstituted benzaldehydes by ethyl N‐chlorocarbamate (ECC) were studied in aqueous acetic acid solution in the presence of perchloric acid. The main product of the oxidation is the corresponding benzoic acid. The reaction is first order with respect to the aldehyde, ECC and hydrogen ions. The oxidation of benzaldehyde exhibits a substantial primary kinetic isotope effect (kH/kD = 5·20 ± 0·10). Addition of ethyl carbamate has no effect on the rate. [EtOC(OH)NHCI]+ is postulated as the reactive oxidizing species. The rates of oxidation of para‐substituted benzaldehydes exhibit an excellent correlation with Taft's σ1 and σ values whereas those of the meta‐substituted compounds compounds correlate best with σ1 and σ values. The reaction constants are large and negative. A mechanism involving transfer of a hydride ion from the aldehyde to the oxidant, in the rate‐determining step, is proposed.
Interaction of dichlorocarbene with imines
Authors: Kostikov R. R. , Khlebnikov A. F. , Bespalov V. Ya. 
Published: January 2005

AbstractThe interaction of dichlorocarbene with imines and ketene imines was studied by means of the MNDO method. The results indicate that the lack of ylide cyclization products in the reactions of dichlorocarbene with ketene imines is due to a high reactivity of ketene imine ylides in intermolecular reactions, rather than stereoelectronic factors preventing cyclization. It was shown that the dichlorocarbene reaction with N‐benzylideneaniline resulting in gem‐dichloraziridine proceeds through transient formation of ylide.
Stereochemical features of the (2+2) cycloaddition reactions of chiral allenes. V. Cycloaddition of 1‐tert‐butyl‐3‐methylallene with 1,1‐dichloro‐2,2‐difluoroethene
Authors: Pasto Daniel J. , Brophy John 
Published: January 2005

AbstractThe (2 + 2) cycloaddtion reactions of 1‐tert‐butyl‐3‐methylallene (tBMA) with radicophiles were investigated. The attempted cycloaddition reactions with N‐phenylmaleimide, acrylonitrile and methyl acrylate produce only (4 + 2) cycloadducts of 1‐tert‐butyl‐1,3‐butadiene which is formed by the more rapid [1.3] hydrogen sigmatropic rearrangement of the tBMA. The (2 × 2) cycloaddition of tBMA with 1,1‐dichloro‐2,2‐difluoroethene (1122) occurs more rapidly than does the sigmatropic rearrangement, and produces a mixture of the four cycloadducts 1–4. The cycloaddition of 1122 with enantioenriched tBMA produces one cycloadduct (3) in which ca 91% of the enantiomeric excess (ee) of the tBMA is transferred to the cycloadduct. The other three cycloadducts are formed retaining much less of the ee of the starting tBMA. The results are interpreted on the basis of molecular modeling calculations carried out on the 1122–1,3‐dimethylallene system reported previously. It is suggested that cycloadduct 3 is formed by essentially only one continuous minimum‐energy reaction pathway, while cycloadducts 2 and 4 are formed by two competitive minimum‐energy reaction pathways which result in the formation of cycloadducts possessing opposite absolute configurations. The combined contributions of the two competitive pathways result in much lower overall degrees of transfer of the ee of the tBMA to the diradical intermediates and cycloadducts.
Acid–base equilibria and kinetics between triarylmethane dye carbinols and organic acids in apolar aprotic solvents: Relative basicities of dye carbinols in toluene
Authors: Sen Gupta Susanta K. , Arvind Udai 
Published: January 2005

AbstractEquilibria and kinetics of acid–base reactions in toluene between a carboxylic acid (m‐toluic, salicylic acid) and a set of carbinol bases (derived from basic triarylmethane dyes) were studied spectrophotometrically. The results for the equilibrium and kinetic parameters were critically analysed. The rate constant of the forward step of the acid–carbinol base equilbrium was found to provide a suitable scale of basicities in toluene. The basicities of the dye carbinols chosen decrease in the order Ethyl Violet‐ >Crystal Violet‐ >Methyl Violet‐ ≈ Victoria Pure Blue BO‐ ≫ Brilliant Green‐ >Malachite Green–carbinol. This is fairly consistent with the order observed in water. A fairly good correlation between base strengths of the carbinols and σ parameters of their para‐alkylamino substituents was obtained.
π‐Complexes of naphthalene and its derivatives with nitrosonium cation
Authors: Borodkin Gennady I. , Elanov Innokenty R. , Shakirov Makhmut M. , Shubin Vyacheslav G. 
Published: January 2005

Abstract1H, 13C, and 15N NMR studies, including the use of the 13C NMR deuterium perturbation method, showed that the interaction between naphthalene or its methylated derivatives and the nitrosonium cation result in the formation of π‐complexes. The chemical behaviour of these complexes was investigated; in particular, their ability to be converted into binaphthyl derivatives was established.
Stern–Volmer quenching kinetics for use of a dualistic quencher
Authors: Hasegawa Tadashi , Matsuhisa Ayumi , Yoshioka Michikazu 
Published: January 2005

AbstractThe Stern–Volmer quenching equation for use of a dualistic quencher, which serves not only as the quencher for an excited starting molecule but also as the promoting agent for product formation, was obtained by applying the steady‐state approximation to the photoreaction system. The equation was applied to the kinetic analysis of the Norrish type II photoreaction of valerophenone using triethylamine, which serves as the excited‐state quencher and as the promoting agent for product formation.
[4] Metacyclophane: STO‐3G molecular structure, strain energy and CNDO/SCl electronic transition energies
Authors: Jenneskens Leonardus W. , Louwen Jaap N. 
Published: January 2005

AbstractThe molecular structure of [4] metacyclophane is optimized at the ab initio STO‐3G SCF level and a genuine minimum is located. In the STO‐3G structure the benzene ring is less bent and geometrically distorted than in the previously reported MNDO structure. Although some bond alternation is calculated for the carbon–carbon bond lengths of the bent benzene ring, their values still fall in the range of highly delocalized compounds. Despite the fact that the STO‐3G strain energy [SE(tot.)] is larger than the corresponding MNDO strain energy, the distribution of SE(tot.) over the bent benzene ring [SE(bb.)] and the oligomethylene bridge [SE(br.)] is similar. To facilitate the hitherto unsuccesful identification of [4] metacyclophane, its electronic transition energies were calculated with CNDO/S–CI using the STO‐3G geometry as input geometry. The reliability of the CNDO/S–CI results was assessed by a comparison of calculated and experimental electronic transition energies for p‐ and, m‐xylene, [5] para‐ and [5] metacyclophane and [4] paracyclophane following the same approach.
Kinetics and mechanism of electron transfer reactions of aquo‐thallium(III) and coordinated thallium(III). Reduction of thallium(III) and chlorothallium(III) complexes by L‐ascorbic acid in aqueous acid medium
Authors: Agrawal A. , Nahar S. , Mishra S. K. , Sharma P. D. 
Published: January 2005

AbstractThe stoichiometry of the reaction of TIIII with ascorbic acid (H2A) in the presence and absence of chloride ion conforms to
TIIII + H2A > TII + 2H+ + A.
where A is dehydroascorbic acid. The reaction exhibits complex kinetics and the reactivity of various chlorothallium (III) species decreases in the order of [TIOH]2+ > TI3+ > [TICI]2+ > [TICI2]+ > TICI3 > [TICI4]−. The mass balance relationship was employed to calculate the equilibrium concentration of the chloride ion for evaluation of the rate constants.