bluuebunny/crossref_metadata_2025 huggingface dataset:
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

The 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

The 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

Rates 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

N‐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.
Selectivity of nitration reactions of aromatic compounds on zeolites H‐Y and H‐ZSM‐11
Authors: Nagy Sándor M. , Yarovoy Konstantin A. , Shubin Vyacheslav G. , Vostrikova Lidiya A.
Published: January 2005

Substrate and positional selectivity in the nitration reactions of benzene and its derivatives (halobenzenes, toluene and o‐xylene) by nitric acid and acyl nitrates supported on zeolites H‐Y and H‐ZSM‐11 were studied. The reaction mechanism and effects governing selectivity of the process are discussed.
Mechanism of reaction of azobenzene formation from aniline and nitrosobenzene in basic conditions. General base catalysis by hydroxyide ion
Authors: Dalmagro Jacir , Yunes Rosendo A. , Simionatto Edesio Luiz
Published: January 2005

The reaction of nitrosobenzene with aniline, to give azobenzene, in basic conditions was studied. It was shown that the reaction exhibits general base catalysis by different buffers giving a Brønsted coefficient β = 0·318. As in previous studies, a two‐step process with a first step of attack of aniline on nitrosobenzene to give an addition intermediate and a second step of dehydration of this intermediate is proposed to interpret the mechanism of the reaction. The analysis of the Brønsted relationship and of the intermediate of the reaction led to the suggestion that hydoxide ion catalyses the reaction by a mechanism of general base catalysis in the dehydration step.
σ–π and η–π Stabilization energies in vinyl and phenyl compounds
Authors: Luo Yu‐Ran , Holmes John L.
Published: January 2005

A means for determining the stabilization energies, Ec(X), resulting from σ–π and n–π interactions in vinyl and phenyl compounds is described. It is based on extensions of the additivity principle for thermochemical data for organic compounds. The results also provide a method for predicting heats of formation of unsaturated compounds from those of (known) saturated analogues. It is shown that there is a linear relationship between the stabilization energies Ec(X) and the Hammett substituent constants σ(X). The new results have been used to predict the heats of formation of a number of saturated and unsaturated Si‐, Ge‐, Sn‐, P‐, As‐, Se‐, Zn‐, Cd‐ and Hg‐containing species.
Molecular mechanics calculations (MM3) on alkyl iodides
Authors: Zhou Xuefeng , Allinger Norman L.
Published: January 2005

The MM3 force field has been extended to cover alkyl iodides. Structures, vibrational spectra and heats of formation have been well fitted to available experimental data. The experimental and ab initio conformational relationships are also satisfactorily reproduced.
Probing onium ions and carbocations by desorption/ionization mass spectrometry: Crossing the threshold between fragile salts and gaseous intact cations and subsequent guest–host and cation–molecule cluster chemistry
Authors: Laali Kenneth K.
Published: January 2005

The utility of desorption/ionization mass spectrometric (DI/MS) techniques in dealing with physical organic problems involving fragile onium salts and carbocation salts has been explored. The efficacy of these methods, especially when coupled to tandem mass spectrometry, goes much beyond the analytical aspects dealing with identity/purity determinations. For example, insight can be gained regarding the role of the counterion in subsequent cation decomposition pathways. Guest–host chemistry of onium ions and various hosts (crowns, calixarenes) can be probed. The decomposition chemistry of the resulting ion–molecule clusters can be studied. Examples of onium salts and carbocation salts that have been studied are few, probably because research in this area has been mainly a domain for analytical chemists. This paper calls attention to the possibilities for dealing with topical physical organic problems.
Acidity and basicity of thiocarboxamides in the gas phase
Authors: Decouzon Michèle , Exner Otto , Gal Jean‐françois , Maria Pierre‐Charles , Waisser Karel
Published: January 2005

The gas‐phase acidity and basicity of thioacetamide and the basicity of N,N‐dimethylthioformamide were measured by Fourier transform ion cyclotron resonance (FT‐ICR) mass sectrometry under conditions which minimized the extent of their decomposition. Thiocarboxamides are both much stronger acids and stronger bases than carboxamides. The relative stabilities of individual neutral and ionic species were assessed in terms of isodesmic reactions, using the published or estimated enthalpies of formation. The neutral molecules of carboxamides and thiocarboxamides are stabilized by interaction between the C = X and NH2 functional groups. This interaction is of a similar magnitude in the corresponding protonated forms but it is of greater strength in the deprotonated forms. With regard to the difference between thiocarboxamides and carboxamides, the most significant factor is probably the Ione pair–Ione pair repulsion operating in the anions.
Kinetic studies of the reactions of phthalimide with ammonia and pyrrolidine
Authors: Khan M. Niyaz , Ohayagha J. E.
Published: January 2005

The kinetics of the nucleophilic cleavage of phthalimide (PTH) in buffer solutions of ammonia and pyrrolidine were determined. The reaction rates for ammonolysis of PTH revealed a buffer‐catalysed second‐order term in the rate law, but the reaction rates for pyrrolidinolysis of PTH showed buffer‐catalysed second‐ and third‐order terms in the rate law. Both ammonia and pyrrolidine revealed nucleophilic reactivity towards ionized PTH (S−) only within the pH range of the present study. This is attributed to the occurrence of intramolecular general base–acid catalysis. General base catalysis is detected in the reactions of pyrrolidine with both non‐ionized PTH (SH) and S−. The general base‐catalysed third‐order rate constant for pyrrolidinolysis of SH is nearly 28 times larger than that of S−.
Synthesis and photophysics of diaza‐crown ether‐based bisporphyrins
Authors: Mårtensson Jerker , Sandros Kjell , Wennerström Olof
Published: January 2005

The bisporphyria N,N′‐bis[4″‐(meso‐triphenylporphyrinyl)benzyl]‐4,13‐diaza‐18‐crown‐6 and its mono‐ and dizinc derivatives were synthesized in 66%, 46% and 53% yields, respectively, from 5‐(4′‐bromomethylphenyl)‐10,15,20‐triphenylporphyrin or its zinc derivative and 4,13‐diaza‐18‐crown‐6. The zinc‐containing bisporphyrins form dimers in solution at low temperature or at high concentration. The unsymmetrical bisporphyrin; monozinc N,N′‐bis[4″‐(meso‐triphenylporphyrinyl)benzyl]‐4,13‐diaza‐18‐crown‐6 shows singlet–singlet energy transfer from the zinc porphyrin moiety to the free base moiety in both the monomeric and the dimeric form. The energy transfer rates were determined using time‐resolved fluorescence spectroscopy and were found to be 1·26 × 109 and 2·29 × 109 s−1 for the monomeric and dimeric form, respectively. The difference in energy transfer rates between the two forms can be rationalized by the difference in overlap between the donor fluorescence spectrum and acceptor absorption spectrum, donor–acceptor distance and donor–acceptor orientation.
Kinetics and mechanism of aminolysis of propargyl and 1‐methyl‐propargyl arenesulphonates
Authors: Oh Hyuck Keun , Cho In Ho , Jin Min Jeong , Lee Ikchoon
Published: January 2005

Kinetic studies were carried out on the aminolysis of propargly and 1‐methylpropargyl arenesulphonates in acetonitrile at 45·0°C. The cross‐interaction constants, ρxz and βxz, are similar to, but smaller than, those for the SN2 processes at other primary and secondary carbon centers. Compared with the allyl series, the smaller magnitude of ρxz and βxz reflects a looser transition state, which in turn leads to a lower rate despite the greater Taft's σ* value and the lower intrinsic (ΔE) and thermodynamic barriers (ΔE0).
Effect of cyclic acetal‐type cationic surfactants on the basic dehydrobromination reaction
Authors: Wilk Kazimiera A. , Bieniecki Albert , Matuszewska Barbara
Published: January 2005

The reaction of 2‐(p‐nitrophenyl)ethyl bromide with hydroxide ion was studied at 50 and 25°C in the presence of cationic chemodegradable surfactants, i.e. [2‐alkyl(1,3‐dioxolan‐4‐yl)methyl] trimethylammonium bromides Ia–c (alkyl: a = n‐C9H19, b = n‐C11H23, c = n‐C13H27). The kinetic data were interpreted with the pseudo‐phase ion‐exchange (PIE) formalizm at both temperatures. The results indicate that the major source of rate enhancement is the increased reactant concentration in the small micellar reaction volume. The surfactant stability in micellar conditions was probed by means of a hydrolysis reaction of the surfactant 1,3‐dioxolane ring. The kinetics of acid hydrolysis of Ia–c micelles, as a result of specific hydronium ion concentration, is one order of magnitude smaller than that of unaggregated systems.
Hydrogen bonding. 39. The partition of solutes between water and various alcohols
Authors: Abraham Michael H. , Chadha Harpreet S. , Dixon Julian P. , Leo Albert J.
Published: January 2005

The general solvation equation

was applied to the partition of solutes between water and isobutanol, pentanol, hexanol, octanol, decanol and oleyl alcohol. It is shown that the two main factors that influence partitioning are solute hydrogen‐bond basicity Σβ and solute volume Vx. The b coefficient becomes steadily more negative along the above series of alcohols, showing that the alcoholic phases, which are all less acidic than water, become less and less acidic as the chain length increases, and the water content of the alcoholic phase decreases. The v coefficient, on the other hand, becomes gradually more positive, indicating that as the chain length increases and the water content decreases, the alcoholic phase becomes more and more hydrophobic. Of great significance is that for all six alcohols, the a coefficient is effectively zero, so that all alcoholic phases have the same basicity as bulk water, no matter what their water content is. It is suggested that, contrary to results of solvatochromic measurements, the alcohols have similar hydrogen‐bond basicity to water.
Equilibrium acidities and homolytic bond dissociation enthalpies of the acidic CH bonds in dialkyl malonates and related compounds
Authors: Zhang Xian‐Man , Bordwell Frederick G.
Published: January 2005

The equilibrium acidities, pKHA, of 18 dialkyl malonates, five alkyl 2‐cyanoacetates and nine malononitriles and the oxidation potentials of their conjugate anions, EOX(A−), were measured in dimethyl sulfoxide solution. The homolytic bond dissociation enthalpies (BDEs) of their acidic C–H were estimated by combining their pKHA and EOX(A−) values. The pKHA values of the dialkyl malonates were found to increase from 15·9 to 16·4 to 18·4 as the dialkyl groups were changed from dimethyl to diethyl to di‐tert‐butyl, but the BDEs of the acidic C–H bonds remained constant [95·3±0·3 kcal mol−1] (1 kcal = 4·184 kJ). Introduction of methyl, ethyl, isopropyl and tert‐butyl groups into the 2‐position of diethyl malonate caused the equilibrium acidities to increase by 2·0, 2·4, 3·8 and 8·0 pKHA units, respectively, and the BDE values to decrease by 4·4, 3·7, 2·5 and 0·8 kcal mol−1, respectively. Introduction of a phenyl group into the 2‐position of diethyl malonate had no effect on the acidity, but weakened the acidic C–H bond by 10 kcal mol−1. The effects on acidity and BDE of introducing 3,4,5‐ (MeO)3C6H2CO, CF3, Me3N+, c‐C5H10N, p‐MeC6H4, p‐NO2C6H4, PhO, F and c‐C5H5N+ groups into the 2‐position of diethyl malonate were also examined.
Study of the hydrogen bond donor acidity of binary aqueous mixtures and their role in reversed‐phase liquid chromatography
Authors: Park Jung Hag , Dallas Andrew J. , Chau Phoebe , Carr Peter W.
Published: January 2005

The solvatochromic hydrogen bond donor (HBD) acidity parameter (αmix) of aqueous mixtures of methanol, acetonitrile, propan‐2‐ol and tetrahydrofuran were determined spectrophotometrically. The study was carried out at 25deg;C as a function of composition. The indicators used were 2,6‐diphenyl‐4‐(2,4,6‐triphenyl‐N‐pyridino)phenolate, 2,6‐dichloro‐4‐(2,4,6‐triphenyl‐N‐pyridino)phenolate and Fe(LL)2(CN)2 (LL = N‐(2‐pyridylbenzylidene)‐3,4‐dimethylaniline). The HBD acidity of the aqueous organic mixtures was related to retention in reversed‐phase liquid chromatography.
Relative thermodynamic stabilities of 2‐(methoxymethylene)tetrahydrofuran and 5‐methoxymethyl‐2,3‐dihydrofuran
Authors: Taskinen Esko
Published: January 2005

The relative thermodynamic stabilities of the title compounds were determined by iodine catalyzed chemical equilibration in cyclohexane solution. The main point of interest was a determination of the thermodynamic stability of the OCCO moiety found in the exocyclic isomers, i.e. the stabilizing effect of a MeO group on the olefinic linkage of 2‐methylenetetrahydrofuran. All three isomeric compounds have essentially similar enthalpy values, which, in comparison with some previous thermodynamic data, shows that the double bond stabilization energy of the MeO group in the exo isomers is only ca 3 kJ mol−1. The entropy difference between the geometrical isomers is negligible, whereas the endo isomer is favoured by an entropy contribution of ca 9 J K−1 mol−1.
Hyperbolic Model for the meta–para interrelationship in benzene derivatives
Authors: Reis JoãO Carlos R. , Segurado Manuel A. P. , De Oliveira Jaime D. Gomes
Published: January 2005

A new approach to the long‐standing problem of interrelating meta and para substituent constants is presented. An analysis of the unified σ0‐scale shows that the interrelation between σ and σ/σ can be modelled by a pair of conjugate rectangular hyperbolae, one for normal (n) and the other for special (s) substituents. The latter are characterized by a lone electron pair in the first atom. The equations σ (σ − γ0)/(σ − 2γ0) = λ0 σ and σ = γ0 + λ0 σ are derived and discussed in terms of Taft's separation of mesomeric and non‐mesomeric effects. Asymptotic values λ = 0.960 γ = −0.225 were obtained by non‐linear least rectangles fitting. A nonnegligible mesomeric contribution to σ0 constants for normal substituents is predicted by the hyperbolic model. The present results are at variance with Exner's analysis of the meta‐para interrelationship in benzene compounds with normal substituents. This divergence is ascribed to opposite views concerning the role of the π‐inductive effect.
Structural study on quasi‐phosphonium salts containing phosphorus–oxygen, phosphorus–oxygen, phosphorus–nitrogen and phosphorus‐sulphur bonds
Authors: Imrie Christopher , Modro Tomasz A. , Van Rooyen Petrus H. , Wagener Carl C. P. , Wallace Karen , Hudson Harry R. , McPartlin Mary , Nasirun Jusni B. , Powroznyk Luba
Published: January 2005

The crystal and molecular structues of six quasi‐phosphonium salts containing phosphorus‐heteroatom heteroatom=oxygen, sulphur or nitrogen0 bonds were determined. Comparison of the molecular parameters obtained with those available for reference structures demonstrated that the ‘double bond’ character for the P+  Y bond decreases in the order Y = N ≫ O > S.
Reaction of phenoxy radical with nitric oxide
Authors: Yu T. , Mebel A. M. , Lin M. C.
Published: January 2005

The associationof C6H5O with NO was studied with the cavity‐ring‐down method by directly monitoring the decay of C6H5O in the presence of varying, excess amounts of NO. The biomolecular rate constant determined in the temperature range 297‐373 K can be effectively rate constant determined in the temperature range 297–373 K can be effectively represented by k1 = 10 cm3 molecule−1 with a negative activation energy of 0.8 kcal mol−1 (1 kcal = 4.184 kJ). In order to understand better the mechanism of the reaction, ab initio molecular orbital calculations were also carried out at the MP4(SDQ)/6‐31G* level of theory using the HF optimized geometries. The molecular structues and energetics of five C6H5N1O2 isomers were calculated. Among them, the most likely and stable association product, phenyl nitrite (C4H5ONO), was found to be 17 kal mol−1 below the reactants, C6H5O + NO. Combining the measured rate constant and the calculated equilibrium constant for the association reaction, C6H5O + NO = C6H5ONO the rate constant for the unimolecular decomposition of C6H5ONO was obtained as k−1 = 4.6 × 1015E−8580/T s−1. The relatively large frequency factor suggests that a loose transition state was involved in the reaction, akin to those of its alkyl analogs (RONO, R  CH3, C2H5, etc.).
Isotherm of water sorption by human serum albumin in dioxane: Comparison with calorimetric data
Authors: Borisover Mikhail D. , Sirotkin Vladimir A. , Solomonov Boris N.
Published: January 2005

The dependence of the amount of water bound to human serum albumin (HSA) suspended in water–dioxane mixtures vs the equilibrium water concentration in the liquid phase was determined by the Fisher method at 298 K. The Langmuir model was used in order to describe the isotherm of the sorption of water by HSA at low water concentrations in the solvent. The calculated equilibrium constant of water adsorption (3.8 ± 0.6 l mol−1) is in good agreement with the adsorption constant obtained earlier from calorimetric data. The comparison of the determined isotherm of water sorption by HSA with the reported enthalpies of suspension formation showed that at low water concentrations in the solvent, water sorption is the only process contributing to the heat effects of the formation of the ‘protein + liquid’ heterogeneous system. From this comparison, the enthalpy of water adsorption by HSA was evaluated as −11.9 ± 1.7 kJ mol−1. At higher water concentrations in the solvent the amount of water adsorbed by HSA increased considerably. This increase in the amount of water on HSA at water activities above 0.5 is assumed to be due to the enlargement of the protein surface area.
Amines as leaving groups in nucleophilic aromatic substitution reactions. III. Hydrolysis of 1‐amino‐2,4‐dinitrobenzenes
Authors: de Vargas Elba Buján , Remedi M. Virginia , de Rossi Rita H.
Published: January 2005

The kinetic study of the reaction of 1‐pyrrolidino‐2,4‐dinitrobenzene, 1‐piperidino‐2,4‐dinitrobenzene and 1‐morpholino‐2,4‐dinitrobenzene with NaOH in the presence and absence of the amine leaving group was carried out in aqueous solutions at 25°C, giving 2,4‐dinitrophenol as the only product. A mechanism involving the formation of σ complexes by addition of HO− or the amine to the unsubstituted positions of the aromatic ring is proposed. These complexes were found to react faster than the original substrates.
Theoretical studies on the acid‐catalysed aromatization of benzene‐cis‐1,2‐dihydrodiols
Authors: Kim Chang Kon , Chung Dong Soo , Chung Kyoo Hyun , Lee Bon‐Su , Lee Ikchoon
Published: January 2005

Theoretical studies on the acid‐catalysed dehydration of 15‐substituted dihydrodiols were carried out using the AM1 method. In agreement with the experimental results, the rate‐limiting step is dehydration of the protonated diols, R+, and the o‐phenol‐forming pathway (path 1) is favoured both kinetically and thermodynamically over m‐phenol product formation. The transition state for path 1 (TS 1) is found to be at an early position (ca 23% progress) along the reaction coordinate and a better correlation of the rate constant is obtained with σ+. Three resonance forms can exist for the benzenonium intermediate but the contribution of one form is dominant in the later TS, which resembles the cationic intermediate. The contribution of the other two, however, weakens the σ+ effect of +M subsituents. The σP correlation of the rate constant in acid solution is therefore suggestive of a later TS resembling the cationic intermediate which is stabilized by solution.
First successful correlation of the UV spectra of three kinds of styrenes with spin‐delocalization substituent constants σ.
Authors: Jiang Xi‐Kui , Ji Guo‐Zhen , Wang Daniel Ze‐Rong
Published: January 2005

UV spectra of 17 para‐Y‐substituted styrenes, 8 para‐Y‐substituted α‐methylstyrenes and 20 para‐Y‐substituted α,β,β‐trifluorostyrenes were measured and correlated against spin‐delocalization substituent constants (σ.) with confidence levels above 99·9%.
The question of pentaphenylethyl: An AM1 study
Authors: Smith William B.
Published: January 2005

Semi‐empirical AM1 theoretical calculations were carried out on the pentaphenylethyl and 9‐trityfluorenyl free radicals, related radicals and their precursor hydrocarbons. Previous reports of the preparation of the former radical have been questioned, although the existence of the second is on secure ground. Based on simple energy considerations, it is concluded that pentaphenylethyl should be as readily prepared. However, the acidity of the precursor hydrocarbon is considerably less than that of 9‐tritylfluorene, and possible radical decomposition via dissociation to triphenylmethyl and diphenylcarbene is calculated to be more facile than the corresponding decomposition of 9‐tritylfluorenyl. A previous postulation of a rapid phenyl equilibration of the pentaphenylethyl radical is shown to be untenable.
The search for the ethynylcation: Nitrosation of N,N‐bis(trimethylsilyl)ynamines
Authors: Alvarez Roberto Martínez , Hanack Michael , Schmid Thomas , Subramanian L. R.
Published: January 2005

The reaction of bis‐silylated ynamines with different nitrosyl reagents affords products derived from an electrophilic attack at the β‐carbon atom and not the expected alkynyldiazonium salts.
Stereochemical control in anionic polymerization of 2‐vinylpyridine and related vinyl monomers
Authors: Hogen‐Esch Thieo E. , Jin Qiao , Dimov Dimo
Published: January 2005

The alkylation, silylation and other electrophilic reactions of 1‐lithio‐1,3‐bis(2‐pyridyl)butane (2a) in THF at −78°C leads to meso‐like products that may be accounted for by intramolecular coordination of Li ion with the penultimate 2‐pyridyl group. This is also demonstrated by proton‐transfer equilibria between 1‐lithio‐2‐ethylpyridine and 2a, allowing the determination of the ΔH and ΔS of intramolecular chelation in 2a. The ΔS of chelation is nearly zero, probably as a result of chelation‐induced desolvation of Li ion. Surprisingly, however, the corresponding addition of 2‐vinylpyridine to 2a and higher P2VPLi homologs in THF is not stereoselective. In toluene at −78°C the t‐BuLi‐initiated polymerization of 2‐VP is actually somewhat syndiotactic (rr = 0·44). The presence in this polymerization of at least one equivalent of t‐BuOLi or other tertiary ROLi surprisingly leads to isotactic P2VP. SEC viscometric and other studies indicate that both the P2VPLi– and P2VPLi–ROLi complexes are monomeric in toluene. The stereochemistry is apparently due to a 1 : 1 P2VPLi–ROLi complex that discriminates between the two pro‐chiral faces of the anion by preferntial coordination of the (ROLi2) moiety to the meso‐prochiral face. Isotactic‐like addition now occurs by electrophilic attack of 2‐VP that is syn with respect to the (ROLi2) ‘counter ion.’
Mechanistic aspects of silicon‐mediated polymerization (group transfer polymerization) of methyl methacrylate with ester enolate anions as nucleophilic catalysts
Authors: Quirk Roderic P. , Kim Jung‐Soo
Published: January 2005

The role of ester enolate anions in silicon‐mediated polymerization (group transfer polymerization) of methyl methacrylate (MMA) was investigated using cesium, tetrabutylammonium and lithium salts of 9‐methylfluorenide as the nucleophilic catalysts. The results of normal anionic polymerization of MMA using these carbanion salts were compared directly with those of analogous GTP procedures with these carbanions in the presence of the silyl ketene acetal initiator 1‐methoxy‐1‐(trimethylsiloxy)‐2‐methylprop‐1‐ene (MTS). In the absence of MTS these reactions are characterized by low conversions (14–52%), no control of Mn and broad molecular weight distributions. In the presence of MTS, controlled MMA polymerization is observed with molecular weights calculated based on the moles of MTS and not fluorenide anion. The MMA stereochemistry was also determined for both anionic polymerization and GTP for the same counter ions (tetrabutylammonium, cesium, lithium); within experimental error, the triad tacticities are the same for anionic polymerization and GTP. It is concluded that these results are consistent with the ester enolate anions as propagating intermediates in nucleophile‐catalyzed GTP. These results are consistent with an associative GTP mechanism only if it is postulated that the chemistry and stereochemistry of a pentacovalent siliconate anion are the same as those of an ester enolate anion intermediate.
Living carbocationic copolymerizations. II. Reactivity ratios and microstructure of isobutylene/p‐methylstyrene copolymers
Authors: Nagy A. , Országh I. , Kennedy J. P.
Published: January 2005

A novel method was developed for the quantitative description of copolymerization behavior, specifically for the derivation of reactivity ratios and micro‐ and macro‐compositions of monomer pairs with significantly different reactivities. The novelty of the method lies in the manner of generating instantaneous feed and copolymer composition data from experimentally readily obtainable cumulative copolymer compositions, although the latter drifts with progressing copolymerization. It is demonstrated that such copolymer systems can be described by two reactivity ratios, albeit the values are valid only over the narrow feed composition range within which they were determined. The great advantage of this method is that whereas the differential copolymer composition equation is strictly valid only at 0% conversion, the method produces corresponding instantaneous feed and copolymer compositions at any conversion. Assumptions or approximations are unnecessary to calculate reactivity ratios. Subsequently the procedure was applied to obtain reliable monomer reactivity ratios of isobutylene (IB)–p‐methylstyrene (pMeSt) copolymers, rIB and rpMest, prepared by living carbocationic copolymerizations with [IB]/[pMeSt] feeds compositions of 90:10 and 97:3 mol/mol, i.e., with feed compositions of industrial importance. According to our procedure, rIB = 0·74±0·11 and rpMeSt = 7·99±3·34. These reactivity ratios were then used to calculate overall IB–pMeSt copolymer compositions over the entire comonomer conversion range and to compare calculated with experimental compositions. The good quantitative agreement between calculated and experimental compositions indicates that the description of both macro‐ and micro‐compositions of IB–pMeSt copolymers obtained in the 90:10 and to 97:3 mol/mol feed composition range is satisfactory.
Theoretical studies on the reactions of substituted phenolate anions with formate esters
Authors: Park Young Sook , Kim Chang Kon , Lee Bon‐Su , Lee Ikchoon , Lim Woong Mook , Kim Wang Ki
Published: January 2005

Semiempirical MO (PM3) studies are reported on the reactions of 18 substituted phenolate anions with m‐(MNPF), p‐(PNPF) and 3,4‐dinitrophenyl formates (DNPF). The mechanistic change from rate‐limiting formation to breakdown of the anionic intermediate (T−) can occur in the gas phase, especially for MNPF, at approximately an equal proton affinity (PA) of the nucleophile and leaving group phenolates, i.e. ΔPA = 0 as the PA of the phenolate anion is decreased. In solution, however, owing to the low stability of T and a greater stabilization by solvation of the transition state for the breakdown step (TS2), all three formate esters are predicted to proceed by a concerted mechanism, which is in agreement with the experimental results. The low stability of T can be ascribed to the relatively high electron affinity of the phenolate anions.
Mechanism of bimolecular substitution reactions of hydroximoyl halides with amines in actonitrile solution
Authors: Johnson James E. , Dutson Susan M. , Dolliver Debra D. , Todd Susan L. , Hotema Martha
Published: January 2005

The reaction in acetonitrile solution of (Z)‐O‐methylbenzohydroximol chlorides [ArC (Cl)NOCH3] with morpholine, pyrrolidine and azetidine gives the corresponding (Z)‐amidoximes [ArC(NR1R2)NOCH3] in acetonitrile. The rates of these reactions were measured under pseudo‐first‐order conditions (excess amine). The reactions were found to follow overall second‐order kinetics (first order in amine). The Hammett ρ‐value (with σ) for the reaction with pyrrolidine is +0·92. The reaction of the p‐nitro compound with pyrrolidine gives a significant element effect [k(p‐nitrobenzohydroximoyl bromide)/k(p)‐nitrobenzohydroximoyl chloride = 10]. A Brϕonsted β‐value of 0·38 was estimated from the reactions of morpholine and piperidine with the p‐nitrohydroximoyl chloride. The slower reaction of (E)‐O‐methyl‐p‐nitrobenzohydroximoyl chloride with azetidine gives primarily the (E)‐amidoxime (E:Z ≈ 98:2) This reaction also follows second‐order kinetics. The kinetic observations made in this study are compared with the corresponding results obtained in earlier work in benzene solution. It is suggested that in acetonitrile solution the reactions proceed by an addition–elimination mechanism with rate‐determining loss of chloride ion (AN + DN#). It is further suggested that acetonitrile is assisting in the breakdown of the tetrahedral intermediates formed in these reactions.
Nucleophilic substitution at nitrogen‐centered radicals: Reactions of diphenylphosphide ions with N,N‐dibutyl‐p‐toluenesulfonamide by the SRN1 mechanism
Authors: Foray Gabriela S. , Peñéñory Alicia B. , Rossi Roberto A.
Published: January 2005

The photostimulated reaction of N,N‐dibutyl‐p‐toluenesulfonamide with diphenylphosphide ions in liquid ammonia leads to the corresponding phosphinic amides after oxidation Inhibition experiments with p‐dinitrobenzene and 2,2,6,6‐tetramethyl‐1‐piperidinyloxy free radical (TEMPO) and the fact that the reaction is retarded in the dark suggest that it proceeds by the SRN1 mechanism.
Dioxetane decomposition revisited: A semi‐empirical study of the potential energy surface
Authors: Wilson Thérèse , Halpern Arthur M.
Published: January 2005

The potential energy surface describing the unimolecular thermolysis of 1,2‐dioxetane into two formaldehydes is explored via semi‐empirical calculations using the PM3 Hamiltonian with multi‐electron configuration interaction. An active space of the four highest occupied orbitals and the two lowest virtual orbitals is used. Several reaction coordinates were examined, and the in‐plane OO distance was used most extensively. The results indicate that the activation barrier to fission is on the ground state surface and is about 18 kcal mol−1, in good agreement with experimental results. Furthermore, calculations of the vertical triplet‐state energies show that the ground and triplet states are nearly degenerate for OO distances beyond that corresponding to the 18 kcal mol−1 barrier, until the molecule abruptly dissociates into two formaldehydes (at ca 2·55 A). This picture is indicative of a diradical pathway, where the activation energy is associated with motion of the system on the ground‐state surface, not the triplet surface, but where generation of a triplet product can be expected. Similar results were obtained for dimethyl‐ and tetramethyldioxetanes, which require higher Ea for dissociation, and for dimethyldioxetanone. Thermochemical calculations pertaining to these reactions and to the formation of triplet products are also presented.
Solution electron affinity changes via the deuteriation of the methyl groups of p‐xylene
Authors: Stevenson Cheryl D. , Wagner Eugene P. , Reiter Richard C.
Published: January 2005

EPR techniques, including spectral double integration, were utilized to measure the equilibrium contants in dimethoxyethane at 195 K for electron transfer between perdeuteriated p‐xylene and xylene, α‐d3‐p‐xylene and α,α′‐d6‐p‐xylene. The results, coupled with the law of Hess, yield all the relative solution electron affinities (EA). It was found that methyl deuteriation lowers the solution EA by 160 J l mol−1 per deuterium, whereas ring deuteriation in these systems lowers the solution EA by about 370 J l mol−1 per deuterium, which is about the same effect as is observed in the benzene system. The results are discussed in terms of hyperconjugative, inductive and solvation effects.
Kinetic study of proton/deuteron transfer reactions of 2,4,6‐trinitrotoluene and its derivatives with tertiary amines
Authors: Sueishi Yoshimi , Fujimoto Takanori , Yoshida Tohru , Kurita Takeharu , Yamamoto Shunzo , Nishimura Norio
Published: January 2005

The rate of proton/deuteron transfer reactions of 2,4,6‐trinitrotoluene (TNT), 2,2′4,4′6,6′‐hexanitrobibenzyl (HNBB) and 2,4,6‐trinitrobenzyl chlorides (TNB) with tertiary amines was followed spectrophotometrically. The rate constants obtained under various conditions were interpreted in terms of isotope, solvent and steric effects. The Arrhenius plots curved upward at low temperatures for TNT and TNB, indicating clearly the intervention of tunnelling. The tunnelling parameteres were calculated semi‐empirically on the basis of Bell's theory. The observed rate constants at low temperatures agreed well with the predicted values. In the present systems, some side and consecutive reactions involving the formation of 2,2′,4,4′6,6′‐hexanitrostilbene took place, and in some cases interrupted the rate measurements in the proton/deuteron transfer steps.
Aqueous solvation effect on the prototropic tautomerism of 2‐thiocytosine
Authors: Contreras J. Guillermo , Alderete Joel B.
Published: January 2005

Free energies of solvation were calculated for the four most stable gas‐phase tautomeric forms of 2‐thiocytosine (TC) using a local field SCF procedure with the solvation model SM2. The calculated changes in the free energies for each pair of tautomers reveal that all six possible equilibria tend to produce the thione‐amino TC(1, 8, 8) and TC(3, 8, 8) species. This is in agreement with UV and ionization constant data.
Additivity of Proton affinities: Theoretical studies of fluorine‐ and methyl‐substituted benzenes
Authors: Eckert‐maksić M. , Klessinger M. , Maksić Z. B.
Published: January 2005

Ring proton affinities (PAs) in fluorobenzene and toluene were examined by the MP2(fc)/6–31G**//HF/6–31G* + ZPE(HF/6–31G*) model. The calculated PAs are in good accordance with the available experimental evidence, their order being PA(p) > PA(o) > PA(m) > PA(i), where p, o, m and i stand for para, ortho, meta and ipso positions, respectively. The relative values of the proton affinities can be interpreted in terms of the ground‐state charge distribution (initial state effect) and the characteristic π‐bond fixation produced by protonation (final state effect). The influence of the latter is either concerted with the initial charge distribution leading to higher PAs (ortho and para positions) or disconcerted as in meta protonation, which has a lower PA value. Finally, it is shown that PAs in difluorobenzenes and fluorotoluenes are additive and can be reduced to the characteristic PAs of fluorobenzene and toluene with good accuracy.
Multiple structure–reactivity relationships for nucleophilic substitution reaction
Authors: Yoh Soo‐Dong , Cheong Duk‐Young
Published: January 2005

Novel mechanistic criteria for SN reactions were proposed on the relative magnitudes of ρZ and interaction terms, ρij or Pij values. The mechanistic change by the sign of the product of ρijs or Pijs was also proposed. These criteria were applied to the reaction between substituted phenyl benzoates and amines and indicated that the sign of ρZ is positive and the interaction term PXZ is larger than PYZ, hence these reactions proceed via an SAN mechanism and a change in the substituent of the substrate leads to a change in the substituent of the substrate leads to a change in the transition state towards the Hammond effect.
Effects of the ring size and solvent polarity on the stability of the cyclic intramolecular excimer of saturated diamines
Authors: Ahmed Omar , Yamamoto Shunzo
Published: January 2005

The fluorescence of some aliphatic diamines was studied in the gas phase and in some solvents at several temperatures. It was found that N,N,N′,N′‐tetramethylpropane‐1,3‐diamine (TMPD) and N,N,N′,N′‐tetramethylbutane‐1,4‐diamine (TMBD) have two emission bands in the gas phase. TMBD also shows two bands in solution. These bands were assigned previously to an excited monomer and an intramolecular excimer. From the temperature dependence of the intensity ratio of the two bands, the enthalpy changes for the excimer formation were evaluated. The stability of the cyclic excimer is discussed in terms of the ring size effect and the solvent effect on the band positions.
Non‐steric stereochemistry solely controlled by orientation of dipolar function
Authors: Ohno Atsuyoshi
Published: January 2005

The mechanism of biologically important (net) hydride transfer from NADH or NADPH to a substrate has been discussed from the viewpoint of physical organic chemistry. It is emphasized that the (net) hydride transfer involves an electron‐transfer complex as an intermediate of the reaction. The intermediate plays a crucial role in determining the stereochemistry of the reaction. Orientation of a dipolar functional group such as carbonyl or sulphinyl determines the molecular face for complexation, and consequently defines the face of reaction. Steric bulkiness is of secondary importance in defining the face of reaction. The mechanism observed is called non‐steric stereochemistry. Molecular interactions observed in organic reactions and discussed herein are found to be closely related to those observed in biological reactions.
Kinetics and mechanism of the hydrolysis of substituted phenyl benzoates catalyzed by the o‐iodosobenzoate anion
Authors: El Seoud Omar A. , Martins Monica F.
Published: January 2005

The o‐iodosobenzoate (IBA−)‐catalyzed hydrolysis of the following esters was studied spectrophotometrically: 4‐nitrophenyl 4‐X‐benzoate (series I, a–e, X = CH3, H, Cl, CN, NO2), 4‐Y‐phenyl 4‐nitrobenzoate (series II, a–d, Y = CH3, H, Cl, CN), 2,4‐dinitrophenyl 4‐X‐benzoate (series III, b–e, X = H, Cl, CN, NO2) and 4‐Y‐phenyl 2,4‐dinitrobenzoate (series IV, a–e, Y = CH3, H, Cl, CN, NO2). Direct detection of a reaction intermediate, viz 1‐(4‐nitrobenzoyloxy)‐1,2‐benzodoxol‐3(1H)‐one, the absence of (general base) catalysis by the leaving group, the small kinetic solvent isotope effect and the fast catalyst ‘turnover’ show that the mechanism of catalysis by IBA− is nucleophilic. Catalytic rate constants, activation parameters and application of the Hammett equation show the following mechanistic features: the first step of the reaction, i.e. that leading to the formation of the above‐mentioned intermediate is irreversible, the leaving group is the phenol and the rate‐limiting step is the collapse of the tetrahedral intermediate that is formed by attack of IBA− on the ester. A comparison of the results of hydrolysis by IBA− and by imidazole (Imz) revealed that IBA− and Imz differ in their sensitivity toward an ester structure and, for the same ester series, IBA− is a much more efficient catalyst. The reasons for the higher catalytic activity of IBA− include nucleophilicity enhancement due to the α‐effect, a smaller influence of steric effects and a more favorable entropy of activation.
Plurilinear improvement of the Hammett equation
Authors: Reis João Carlos R. , Segurado Manuel A. P. , De Oliveira Jaime D. Gomes
Published: January 2005

The original Hammett equation, Δ = ρσ, is transformed in a constrained tetralinear relationship where each straight line with variable intercept term correlates one of the following four groups or subsets of dipolar substituents: normal and special substituents (depending on the absence or the presence of a lone electron pair in their atom next to the aromatic ring) and, in each of these classes, separating meta and para derivatives. There are a total of four fitting parameters in the resulting plurilinear Hammettian transformation (PHT) from which the statistically corrected parameters λ and γ are derived; λ and γ are the asymptotic values in a hyperbolic model for the representation of Δ4 vs Δ4/Δ3. This meta–para interrelationship is assumed to hold in the absence of through‐resonance effects which, in turn, are allowed for by the use of alternative sigma scales of substituent constants. By applying the PHT to a large number of selected literature data, parameters λ and γ are determined for the ionization equilibria of 3‐ and 4‐monosubstituted benzoic acids, anilinium ions, phenols and pyridinium ions. In these reactions series, parameter λ, which measures the para/meta ratio of field/inductive effects, is lower than unity and shows a marked dependence on the basic molecular framework. It is best modelled in terms of a through‐space field effect approach. The ratio γ/γ0, where γ0 is referred to the unified sigma‐zero scale, is shown to correspond to the original Hammett's reaction constant ρ. It is concluded that the PHT constitutes an improved Hammett equation for the analysis of substituent effects in benzene derivatives taking into account statistical errors and making allowance for different transmission coefficients for the field/inductive effect from meta and para positions in different reaction series.
The tautomerism of nitraminopyridines
Authors: Gawinecki Ryszard , Kolehmainen Erkki , Rasała Danuta , Suontamo Reijo
Published: January 2005

1H, 13C and 15N NMR data for nitraminopyridines are discussed in terms of tautomeric equilibria in these compounds. The favoured tautomer is determined mainly from 15N NMR spectroscopy. The chemical shift of the nitrogen atom of the nitro group in nitraminopyridines and N‐nitroanilines which cannot tautomerize vary from 28·0 to 35·4 ppm in DMSO solution. 3‐Nitraminopyridine and 2‐nitramino‐3‐ and ‐5‐nitropyridines behave similarly. In the 15N NMR spectra of nitrimino‐1‐methyldihydropyridines, used as models, an upfield shift of that atom, different from that observed for 2‐nitraminopyridine, indicates the significance of the nitrimino tautomer. In contrast, a downfield shift of the ring nitrogen atom of some other compounds shows an increased weight of the nitramino tautomer. 3‐Nitraminopyridine, when dissolved in DMSO, does not have the zwitterionic structure. Changes in the chemical shift of the amino nitrogen atom are not readily interpreted. Some proton and carbon chemical shifts and hydrogen–hydrogen and carbon–hydrogen spin–spin coupling constants can also be used to determine the predominant tautomer of 2‐ and 4‐nitraminopyridines.
Structure of the intermediate formed in the synthesis of symmetrical pyrylium ions
Authors: Gazeau M.‐C. , Simalty M. , Valat P. , Wintgens V. , Kossanyi J. , Doizi D. , Salvetat G. , Jaraudias J. , Perly B. , Vian J. , Peglion J.‐L.
Published: January 2005

The synthesis of symmetrical blocked pyrylium derivatives by reaction of a cyclic aromatic ketone with ethyl orthoformate goes through an intermediate which can be isolated under certain conditions. This intermediate is unstable when starting from 1‐tetralone derivatives but, surprisingly, it can be isolated easily when the synthesis is performed with 1‐indanone derivatives. Further, in the latter case, this intermediate cyclizes hardly into the expected blocked pyrylium derivative. Its ionic structure was demonstrated by mass spectrometry and by 500 MHz 1H and 13C NMR techniques including heteronuclear H–C COSY and DEPT polarization transfer experiments. Hydrolysis of this intermediate in aqueous ethanol solution forms, in acidic or basic media, a pseudo‐base, which in the latter case presents strong similarities with that obtained by the base‐catalysed ring‐opening of a blocked pyrylium ion. In both acidic and basic solutions, the pseudo‐base obtained by the two sources shows identical UV–visible absorption spectra. The difficulty of the intermediate (g) originating from 1‐indanone derivatives in cyclizing into the blocked pyrylium structure, in contrast to the intermediate formed from 1‐tetralone derivatives, is tentatively explained.