bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

AbstractThe 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

AbstractA 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

AbstractSemiempirical 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractEPR 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

AbstractThe 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

AbstractFree 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

AbstractRing 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

AbstractNovel 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

Abstract1H, 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

AbstractThe 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.
Structure and conformation of (3S*, 4S*, 6R*)‐ and (3R*, 4R*, 6R*)‐3,6‐dimethyl‐4‐phenylsulphonyltetrahydropyran‐2‐ones
Authors: Tinant B. , Touillaux R. , Declercq J. P. , Miserque O. 
Published: January 2005

AbstractThe nucleophilic ring opening of propylene oxide by the lithio derivative of the trioxabicyclic orthoester 1 leads, after acidic treatment, to the title lactones 3a and 3b with no evidence of δ‐lactones 3c and 3d. The stereochemistry of these two lactones has been established by X‐ray diffraction analysis. The ring conformations observed in the solid phase are similar in both lactones and are compared with those observed in related molecules. However, the NMR spectroscopic analysis shows that this similarity does not exist in solution; lactone 3a adopts a different conformation.
Photochemical formal [4π + 2π + 2π] cycloreversion of 7,8‐diaza‐3‐oxatricyclo[4.2.102,5]non‐7‐enes
Authors: Kohmoto Shigeo , Kishikawa Keiki , Yamamoto Makoto , Yamada Kazutoshi 
Published: January 2005

AbstractThe photolysis and thermolysis of 7,8‐diaza‐3‐oxatricyclo[4.2.1.02,5]non‐7‐enes were studied. Photolysis showed a novel [4π + 2π + 2π] cycloreversion in a formal way, the first example in azoalkane chemistry. This cycloreversion was found to be sensitized by the benzophenone produced. In contrast, thermolysis showed the typical decomposition pattern of azoalkanes, extrusion of nitrogen and subsequent σ‐bond formation of the resulting biradical to afford the tricyclic oxetanes