bluuebunny/crossref_metadata_2025_split huggingface dataset:
Metal ions that promote the hydrolysis of nucleoside phosphoesters do not enhance intramolecular phosphate migration
Authors: Kuusela Satu , Lönnberg Harri 
Published: January 2005

AbstractThe effects of several metal ions and metal ion complexes on the hydrolysis and interconversion of uridylyl(2′,5′)uridine and its 3′,5′‐isomer were studied as a function of pH and metal ion concentration. The hydrolysis was shown to be markedly accelerated by Zn2+, Cd2+ and trivalent lanthanide ions and by tri‐ and tetraaza complexes of Zn2+. In contrast, none of these species appreciably promotes the interconversion of the 2′,5′‐ and 3′,5′‐isomers, in spite of the fact that this reaction proceeds through the same pentacoordinated intermediate as the hydrolysis. Lanthanide ions also promote the hydrolytic dephosphorylation of uridine 2′‐ and 3′‐monophosphates, but have a barely noticeable effect on their interconversion. The mechanisms of the metal ion‐promoted reactions are discussed.
Kinetic solvent isotope effect studies on the methanolysis of 1‐phenylethyl chlorides
Authors: Lee Ikchoon , Lee Won Heui , Lee Hai Whang 
Published: January 2005

AbstractThe negative slope (ΔρY+ < 0) of the Hammett‐type plot using kinetic solvent isotope effect, log kSOH/kSOD versus σ+, for methanolysis of 1‐(Y‐phenyl)ethyl chlorides is rationalized by an ion‐pair mechanism in which a solvent molecule attacks the relatively stable carbocation formed in the pre‐equilibrium.
Tuning intramolecular carbenic insertions
Authors: Moss Robert A. , Liu Weiguo , Ge Chuan‐Sheng 
Published: January 2005

AbstractThe 1,2‐carbenic hydride shift of neopentylmethoxycarbene was suppressed by the methoxy substituent. Thermally activated hydride shifts were observed with phenoxymethylmethoxycarbene and phenoxymethyltrifluoro‐ethoxycarbene, where appropriate potentiating substituents were introduced at both the migration origin and the migration terminus. Similarly, the 1,3‐CH insertion reaction of tert‐butylfluorocarbene could be induced by thermal activation.
Kinetic studies of solvent and pressure effects on thermal isomerizations ofN‐phenyl‐ andN‐methylrhodaminelactams
Authors: Sueishi Yoshimi , Sugiyama Yukie , Yamamoto Shunzo , Nishimura Norio 
Published: January 2005

AbstractSolvent and pressure effects were examined for the isomerization rates ofN‐phenyl‐ andN‐methylrhodaminelactams (RL‐Ph and RL‐CH3) from their zwitterionic to the spiro forms. From the pressure dependence of the reaction rate, the activation volumes were estimated to be about 5 cm3mol−1for RL‐Ph and 3–10 cm3mol−1for RL‐CH3. It is proposed that there are two rotameric isomers for the coloured zwitterionic forms, and the role of these rotamers in the kinetic effects of solvent and pressure is discussed. The results are consistent with a reaction mechanism involving heterolytic ring closure on activation.
Dipole moments and electronic structure of 2,6‐bis(dialkylamino) derivatives of 4H‐pyran‐4‐one(thione) and 4H‐thiopyran‐4‐one(thione)
Authors: Schroth Werner , Spitzner Roland , Minkin Vladimir 
Published: January 2005

AbstractDipole moments of a series of 2,6‐bis(dimethylamino) and 2,6‐bis(pyrrolidino) derivatives of 4H‐pyran‐4‐one (2), 4H‐thiopyran‐4‐thione (3), 4H‐pyran‐4‐thione (4) and 4H‐thiopyran‐4‐thione (5) were determined in benzene and dioxane solutions at 25°C. High values of the dipole moments (6·44–8·44 D) and mesomeric moments originated from delocalization of the nitrogen lone pairs into the π‐system of the heterocyclic moieties (1·11–3·08 D) indicate a strong polarization of the molecules 2–5 due to a tendency to stabilize pentamethinium and pyrylium (thiopyrylium) ionic structures. The MNDO‐calculated electronic distribution in 2–5 compared with that for the parent 4H‐pyran(thiopyran)‐4‐one(thione)s is in line with this conclusion.
Theoretical studies of the effects of ortho‐methyl substitution on SN1 and SN2 processes at primary and secondary benzylic carbons
Authors: Lee Ikchoon , Kim Nam Doo , Kim Chang Kon 
Published: January 2005

AbstractThe effects of o‐methyl substitution on the transition‐state (TS) structure and reactivity of SN1 and SN2 processes were investigated for primary and secondary benzyl (α‐tert‐butyl) derivatives MO theoretically using the AM1 method. The o‐methyl groups have both stabilizing (by electron donation) and destabilizing (steric) effects on the SN2 TS; the former prevails for the primary whereas the latter effect becomes dominant for the secondary compounds. In contrast, complex interplay of the two effects are operative in the SN1 TS of both primary and secondary derivatives leading to the reactivity order which is consistent with that observed by solvolytic experiments, o,o′‐dimethyl < H < o‐methyl. The bond length–reactivity rule applies only for the primary benzyl SN2 series for which steric effects are insignificant. For reactions of benzyl derivatives is general the coplanarity between the carbenium sp2 centre and the benzene ring, π–σ, is of the utmost importance in stabilizing TS structure for which other structural requirements may be relaxed.
Efficient formation of some amidines by the reactions of aryliminodimagnesium with weakly electron‐accepting benzonitrile, N,N‐dimethylformamide and related compounds. Features of reactions governed by σ‐complexation
Authors: Okubo Masao , Matsuo Koji , Tsurusaki Nobuyuki , Niwaki Keiji , Tanaka Mikio 
Published: January 2005

AbstractIn many reactions of magnesium reagents, including Grignard and aryliminodimagnesium [ArN(MgBr)2, IDMg], the formation of abnormal products via reactions such as radical dimerization, hydrogen abstraction and conjugate 1,4‐addition orginating from efficient single electron transfer (SET) is not excluded. In IDMg reactions with benzonitrile and N,N‐dimethylformamide in tetrahydrofuran, however, clean reactions took place affording N‐arylbenzamidine and N,N′‐diarylformamidine via exclusive 1,2‐addition and condensation plus replacement, respectively. Some alkyl and arylvinyl cyanides gave the corresponding amidines without 1,4‐addition or hydrogen abstraction. These results are the first observation of such reactions of magnesium reagents, and are attributed to a combination of the weak electron‐donating ability (EDA) of IDMg with the weak electron‐accepting ability (EAA) of nitriles. Although ESR is therefore inapplicable, factors governing the product yields were studied on the basis of effects caused by the addition of strongly coordinating hexamethylphosphoramide and pyridine to the reagent solution, and by addition of nitrobenzenes having a strong EAA. From the results, a large participation of σ‐complexation followed by implicit SET in the inner sphere of the complex is proposed.
Calculation of non‐linear optical properties of pyridinium cyclopentadienylides
Authors: Pranata Julianto , Murray Christopher J. 
Published: January 2005

AbstractFinite field/PM3 calculations of the dipole moments, polarizabilities, hyperpolarizabilities, and second‐order hyperpolarizabilities of pyridinium cyclopentadienylide and some nitro and amino derivatives suggest that these compounds are good candidates for non‐linear optical materials.
A comparison of the relative magnitudes of the interaction energies between functional groups with zero, one and two intervening 2p AOs on Carbon, and substituent effects on the relative ground‐state energies of E‐1,2‐disubstituted ethenes
Authors: Pasto Daniel J. 
Published: January 2005

AbstractAb initio theoretical calculations have been carried out on directly bonded donor–acceptor (Ẍ–Y) species and on variously substituted E‐1,2‐disubstituted ethanes, and π interaction energies have been estimated. In conjunction with the previously calculated radical stabilization energies of mono‐ and disubstituted methyl radicals, a comparison is made of the relative magnitudes of the π interaction energies between groups with zero, one and two intervening 2p AOs on carbon atoms. It is concluded that the stabilizing effect of the π interaction between directly bonded donor Ẍ and acceptor Y groups (zero intervening 2p AOs) is approximately 2·1 times that in the disubstituted methyl radicals (one intervening 2p AO), while in the E‐1,2‐disubstituted ethenes (two intervening 2p AOs) the stabilizing effect is ˜ 0·7 of that calculated for the disubstituted methyl radicals for both acceptor–donor and acceptor–acceptor disubstituted systems. No correlation is observed with the π stabilization energies of the ‘cross‐conjugated’ 1,1‐disubstituted ethenes.
Theoretical solute parameters and correlations of log P and pKa. A comment on a recent development and statistical protocol
Authors: Herndon William C. 
Published: January 2005

AbstractPhysical–chemical properties can be correlated and predicted using linear free energy or linear solvation energy relationships (LSER). Procedures to obtain theoretical parameters from calculated molecular structures that can be used in LSER have recently been evaluated [G R. Famini, C. A. Penski and L. Y. Wilson, J. Phys. Org. Chem. 5, 395–408 (1992)]. Among other applications, these procedures were applied to correlate octanol–water partition coefficients and pKa values for two groups of structurally diverse solutes with very good reported results. In this paper, the statistical results of those studies are re‐examined. The corrected statistical parameters do not provide tenability for the appropriateness of the methodology in these applications. However, valid multilinear relationships of the theoretical LSER parameters with the experimental properties are found which do substantiate the original conclusions of Famini et al.
σ–π Energy diagrams for substituted rings
Authors: Jug Karl , Matuschewski Marta , Köster Andreas M. 
Published: January 2005

AbstractThe recently developed σ–π energy separation technique was applied to substituted monocyclic ring systems. Modified correlation diagrams are presented which allow the influence of the substituent on the σ and π energies to be assessed. The slopes of the σ and π electron energy curves can be used to classify aromatic, non‐aromatic and anti‐aromatic compounds. A two‐dimensional resonance coordinate representation shows a clustering of compounds of the three classes in distinctly different regions of this plane.
AM1 and polarized‐π frontier molecular orbital (PPFMO) studies of facial selectivity in hydrogen transfer to substituted adamantyl radicals
Authors: Huang Xiao Ling , Dannenberg J. J. 
Published: January 2005

AbstractFully optimized transition states for hydrogen transfer from methane and propene to substituted 2‐adamantyl radicals were calculated using the AM1 molecular orbital method. Methane and propene were chosen to provide an early (propene) and a late (methane) transition state. For 5‐substituted radicals (F, Cl, Ph, CH3, CF3) the enthalpic differences between syn and anti reactions was found to be small [<0·1 kcal mol−1 (1 kcal = 4·184 kJ)]. Other radicals with nitrogen or boron included directly in the adamantyl fragment showed greater selectivities, with aza substitution favoring syn and bora substitution favouring anti attack, as did the 4,9‐difluoro derivative. The calculated selectivities are all in qualitative agreement with experimental results (where available) on bromine atom transfer. PPFMO calculations showed the polarization of the SOMO to be generally in accord with the AM1 results, whereas the polarization of the LUMO's was less indicative. The bond orders for the bonds syn and anti to the incipient CH bond in the adamantyl fragments indicated that the anti bonds were always weaker than those syn, in agreement with the suggestion by Cieplak that has been used to explain the experimental selectivities. However, the bond lengths of the incipient CH bonds are always shorter for the side of preferred attack, in apparent agreement with the suggestion by Anh.
Semi‐empirical calculations on heterocumulene–heterodiene cycloaddition reactions: Ketene + 1‐azabutadiene
Authors: Fabian Walter M. F. , Kollenz Gert 
Published: January 2005

AbstractSemi‐empirical (AM1) calculations on the 12 isomeric products and the corresponding transition states of ketene–1‐azabutadiene [4 + 2] and [2 + 2] cycloaddition reactions as a model system for the reaction of ketenes with 4‐iminobenzylfuran‐2,3‐diones are presented. A [4 + 2] type of reaction of the ketene CC double bond leading to a six‐membered lactam compound is found to be highly favoured both thermodynamically and kinetically. [2 + 2] Cycloadducts generally have significantly higher activation energies. Reactions involving the formation of a carbon–nitrogen bond proceed in most cases via attack of the nitrogen lone pair leading to a zwitterionic intermediate. Depending on the respective cycloadduct, both two‐step and concerted, albeit asynchronous, processes were obtained. Based on the structures of the various transition states, some predictions with respect to substituent effects are made. Similarities to and differences from the analogous reaction of ketenimines with oxa‐1,3‐dienes are discussed.
N‐alkylation and N‐arylation of anilines starting from a mild NMg reagent: Its activation causing the ‘NC’ coupling to extend the unified structure–reactivity relationship
Authors: Matsuo Koji , Shichida Yoshiaki , Nishida Hiroshi , Nakata Satsuki , Okubo Masao 
Published: January 2005

AbstractNew N‐alkylation and N‐arylation procedures starting from anilinomagnesium (ArNHMgBr) are reported. For N‐alkylation with alkyl bromides, addition of hexamethylphosphoramide to an ArNHMgBr solution in tetrahydrofuran (THF) is effective. After heating at 55°C, N‐monoalkylation product was obtained in 60–90% yield, slight dialkylation taking place. The combined use of aryliminodimagnesium [ArN(MgBr)2] with α,ω‐dibromoalkanes led to N‐arylazacycloalkanes. For N‐arylation with iodobenzene, replacement of THF with pyridine and additional use of copper(I) iodide are effective. After heating at 115°C with iodobenzene, mono‐ and diarylation products were obtained, the former being predominant. The combined use of ArNHMgBr and N,N,N′,N′‐tetramethylethylenediamine as the ligand of copper species is effective for elimination of diarylation and other undesired products, and leads to diarylamines in excellent yield. The method is of advantage over the conventional Ullmann and Chapman methods. The polar solvents and copper salt are effective additives for inducing ‘inert combinations’ of ArNHMgBr or ArN(MgBr)2 with alkyl and aryl halides into NC coupling, to extend the unified view proposed for the reactivity of magnesium reagents. The difference in the roles of NMg and NCu species is discussed.
Variation of ρx(ρnuc) with the extent of bond making in SN2 transition states
Authors: Lee Ikchoon , Koh Han Joong , Lee Byung Choon 
Published: January 2005

AbstractThe magnitude ρx(ρnuc) is shown to decrease with increase in the extent of bond making estimated by the kinetic isotope effect for the reactions of Y‐benzoyl chlorides with anilines, XC6H4NH2, in acetonitrile at 25·0°C.
The influence of solvents on the basicity of dipolar amines
Authors: Headley Allan D. , McMurry Mike E. 
Published: January 2005

AbstractThe relative basicities of six monosubstituted dipolar trimethylamines (XCH2NMe2) were determined in nine different solvents from potentiometric titration. Solvation effects on basicity variations were analyzed by the solvatochromic parameters (π*, α and β). Non‐specific solvation of the substituted dimethylammonium ions is not reflected adequately by the solvent's dipolarity–polarizability constant, π*. A better representation of non‐specific interaction between solvents and the monosubstituted dipolar trimethylammonium ions is gained from the product of π* and the solvent dipole moment (μ). For these ions where the dipolar substituents are separated from the reaction center by one methylene unit, there is an interaction between the charge of the ammonium ion and the dipolar substituent. As a result, solvent molecules must not only solvate the dipolar substituents, but must also gain access between the lines of force of the dipolar substituent and the charge. Bulky solvents, such as nitrobenzene, cannot access adequately the region between the dipolar substituents and the reaction center to accomplish similar solvation as less bulky solvents.
Kinetics and mechanism of electron‐transfer reaction of aquothallium(III) and coordinated thallium(III). Oxidation of metol with thallium(III) in acid perchlorate medium
Authors: Nahar Sharmila , Sharma P. D. 
Published: January 2005

AbstractThe kinetics of oxidation of metol (N‐methyl‐p‐aminophenol) by thallium(III) have been studied in a perchloric acid medium. This organic compound is employed extensively in photographic work. The stoichiometry of the reaction is represented by: C6H4(OH)NHCH3 + TIIII → C6H4(O)NCH3 + TII + 2H+The kinetics indicate the rate law:

where k′ is the pseudo‐first‐order rate constant. The rate constants and thermodynamic parameters for the rate‐limiting step have been evaluated.
Calibration of a semi‐empirical procedure for predicting the ground‐state spin multiplicities of open‐shell molecules. Applications to new systems
Authors: Ichimura Andrew S. , Koga Noboru , Iwamura Hiizu 
Published: January 2005

AbstractA semi‐empirical configuration interaction (CI) algorithm is presented for predicting the ground‐state spin multiplicity of open‐shell systems. The computed energy gaps ΔE(LSHS) of model systems, Y‐phenyl‐X‐phenyl‐Y, where Y is a nitrene or nitroxide radical and X is an exchange coupling unit, are critically compared with the available experimental data. The current method has attenuated the deficiencies of the earlier algorithm proposed by Lahti and Ichimura and the predictions are qualitatively clearer, facilitating the classification of high‐ and low‐spin topologies for isomeric species. The relative effectiveness of different exchange couplers can be discerned semi‐quantitatively, and several new exchange couplers are described. The reasons for the qualitative failure of the earlier algorithm are discussed.
Linear free energy relationships in radical reactions. II. hydrogen abstraction from substituted toluenes by Tert‐Butyl, Tert‐Butoxyl and Tert‐Butylperoxyl radicals
Authors: HÉBerger KÁRoly 
Published: January 2005

AbstractCalculations were carried out to study the validity of σ‐scales for hydrogen abstraction from substituted toluenes by tert‐butyl, tert‐butoxyl and tert‐butylperoxyl radicals. Rate constants were compiled and evaluated from the literature for meta‐ and/or para‐substituted toluenes. The substituents were characterized by factored ionic sigmas (σI, σR, σ), ionic scales (σ, σ+) and various radical sigmas (σσ). The dependence of log k values on these substituent descriptors was investigated using ‘stepwise linear regression’ and ‘all possible regression’ methods. The following predictive equations can be recommended:
for tert‐butyl radicals, at 321 K:

for tert‐butoxyl radicals, at 313 K:

and for tert‐butylperoxyl radicals, at 303 K:

The results suggest that there is no unversal radical scale for hydrogen abstraction reactions, that the rate is primarily influenced by polar factors (inductive, resonance) and that only two radical scales (σĊ and σ) are appropriate, showing a small, yet significant, role of radical stabilization.
Spectrometry and reactivity of phenalenyl anions
Authors: Hempenius Mark A. , Heinen Wouter , Mulder Patrick P. J. , Erkelens Cees , Zuilhof Han , Lugtenburg Johan , Cornelisse Jan 
Published: January 2005

AbstractThe aromatic odd‐alternant phenalenyl anion and a number of its derivatives were prepared in order to study the perturbation of this conjugated anion by methyl and methoxy groups. The conjugated anions were studied by means of 1H and 13C NMR spectrometry, alkylation experiments and semi‐empirical calculations. It was found that a substituent at a charged carbon atom perturbs the entire conjugated system, whereas substituents at inactive (uncharged) carbon atoms have a large effect on the positions ortho to the substituent.
Hydride transfer reactions of Michler's hydride with different π‐accetors
Authors: Zaman Khan. M. , Nishimura Norio , Yamamoto Shunzo , Sueishi Yoshimi 
Published: January 2005

AbstractThe hydride transfer reactions of 4,4′‐bis(dimethylaminophenyl)methane (Michler's hydride) with p‐benzoquinones were studied. The rate of formation of Michler's Hydrol Blue was followed spectrophotometrically. The second‐order rate constants and the activation parameters were estimated. The formation of a charge‐transfer complex was observed at low temperatures. Stable and unstable paramagnetic species formed during the reactions were assigned by ESR spectroscopy. The observed kinetic behaviour and the stoichiometry were in line with those previously obtained for the systems involving Leuco Crystal Violet, Leuco Malachite Green and Leuco Bindschedler's Green. Hence the reaction is considered to proceed according to the so‐far accepted multi‐step mechanism. However, when tetracyano‐p‐quinodimethane or tetracyanoethylene was used as a π‐acceptor, a comparatively stable radical ion pair was formed as a result of a one‐electron transfer, followed by the gradual formation of Michler's Hydrol Blue. The kinetic behaviour and the stoichiometry of the reaction were examined, together with the enhanced kinetic effects of added triethylamine. A modified mechanism for these systems is proposed. The role of cyanomethylenes was found to be essentially different from that of p‐benzoquinones, and therefore π‐acceptors are divided into two groups of cyanomethylenes and p‐benzoquinones.
Vinyl ether hydrolysis. 30. Effect of β‐carboxy and β‐carbomethoxy substitution
Authors: Kresge A. J. , Ubysz D. 
Published: January 2005

AbstractRates of hydrolysis of the vinyl ether functional groups of (Z)‐ and (E)‐β‐methoxyacrylic acid and (Z)‐and (E)‐β‐methoxymethacrylic acid and their methyl esters were measured in aqueous perchloric acid solution. Additional rate measurements were also made for one substrate, (Z)‐β‐methoxymethacrylic acid, in buffer solutions down to pH 7, and a rate profile was constructed. The results show that the β‐carboxy and β‐carbomethoxy substituents produce strong rate retardations, ranging from 2000‐ to 25 000‐fold, for both Z‐ and E‐isomers in both the acrylic and methacrylic acid series. The rate profile for (Z)‐β‐methoxymethacrylic acid indicates that ionization of this substrate to the carboxylate ion form rises the rate of hydrolysis by a factor of 240. It is argued that this difference in reactivity of ionized and non‐ionized forms of the substrate is due to conjugative and inductive effects of the substituents, rather than β‐lactone formation as suggested in an earlier observation of the same phenomenon in a different system.
Catalytic and computer simulation studies of carbon–sulphur bond cleavage over zeolite‐Y
Authors: Bhawal B. M. , Vetrivel R. , Reddy T. Indrasena , Deshmukh A. R. A. S. , Rajappa S. 
Published: January 2005

AbstractN‐Substituted carbonimidodithioic acid dimethyl esters, when reacted with an active methylene compound such as nitromethane, undergo carbon–carbon bond formation followed by carbon–sulphur bond cleavage in the presence of zeolite catalysts to give 1‐substituted amino‐1‐methylthio‐2‐nitroethenes. This carbon‐sulphur bond cleavage is facilitated by the presence of rare earth cations in the zeolite‐Y framework. The shape selectivity of the zeolite also plays an important role in this cleavage reaction. Force field calculations adopted for the molecules involved in the reaction indicate the geometry and conformational flexibility of these molecules. The computer‐simulated model for the zeolite‐Y is correlated with the conformation and shape of the reactant and product molecules to explain the variation in yields obtained in the conversion of substituted dimethyl esters.