bluuebunny/crossref_metadata_2025 huggingface dataset:
Stereochemical features of the (2+2) cycloaddition reactions of chiral allenes. V. Cycloaddition of 1‐tert‐butyl‐3‐methylallene with 1,1‐dichloro‐2,2‐difluoroethene
Authors: Pasto Daniel J. , Brophy John
Published: January 2005

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

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

1H, 13C, and 15N NMR studies, including the use of the 13C NMR deuterium perturbation method, showed that the interaction between naphthalene or its methylated derivatives and the nitrosonium cation result in the formation of π‐complexes. The chemical behaviour of these complexes was investigated; in particular, their ability to be converted into binaphthyl derivatives was established.
Stern–Volmer quenching kinetics for use of a dualistic quencher
Authors: Hasegawa Tadashi , Matsuhisa Ayumi , Yoshioka Michikazu
Published: January 2005

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

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

The stoichiometry of the reaction of TIIII with ascorbic acid (H2A) in the presence and absence of chloride ion conforms to
TIIII + H2A > TII + 2H+ + A.
where A is dehydroascorbic acid. The reaction exhibits complex kinetics and the reactivity of various chlorothallium (III) species decreases in the order of [TIOH]2+ > TI3+ > [TICI]2+ > [TICI2]+ > TICI3 > [TICI4]−. The mass balance relationship was employed to calculate the equilibrium concentration of the chloride ion for evaluation of the rate constants.
Ring‐opening reactions. 6. eliminative ring fission of aziridinium and azetidinium ions
Authors: Gionta Grazia , Lillocci Claudio
Published: January 2005

The reactivity ratio between the three‐ and four‐membered cyclic ammonium ions in ring‐opening elimination reactions was measured. The comparison with the analogous ratio obtained in a previous study on ring‐opening substitution reactions suggests a substantial difference in the effect of the stereochemical factors governing the two ring‐opening reactions and confirms the anomalous behaviour of small rings in the ring‐opening substitution reaction.
3‐Methylcarboxy‐1H‐indazole. Theoretical study of its formation via intramolecular aliphatic diazonium coupling and x‐ray crystal structure
Authors: Glaser Rainer , Mummert Caryn L. , Horan Christopher J. , Barnes Charles L.
Published: January 2005

The methyl ester of 1H‐indazole‐3‐carboxylic acid (2‐Me) is formed in the diazotization of o‐aminophenylacetic acid to o‐diazoniumphenylacetic acid (1) in an intramolecular aliphatic diazonium coupling. 2‐Me was identified and characterized by single‐crystal x‐ray diffraction and found to crystallize as hydrogen‐bonded trimers of crystallographically independent molecules. The methylcarboxy groups are rotated slightly out of the best plane of the trimer resulting in only a pseudo‐threefold axis. The crystal structure of 2‐Me is compared with other indazoles and pyrazole. Regioselective electrophilic diazonium ion addition to the enol tautomer of 1 and subsequent hyrazone–azo tautomerization are proposed as a possible mechanism for indazole formation under acidic conditions. The tautomerization equilibrium between acetic acid and its enol 1,2‐dihydroxyethene was studied and the effects of phenyl and o‐diazoniumphenyl substitution on this equilibrium were explored with semi‐empirical quantum mechanical methods. The performance of the semi‐empirical method was assessed by comparison with ab initio and/or experimental data. It was found that the enol of o‐diazoniumphenylacetic acid is stabilized greatly owing to extended conjugation and push–pull interactions in the enol form. These results suggest that the enol tautomer might be a viable reactive intermediate that warrants considerations in discussions of reaction mechanisms.
Radical reactions of epoxides. 2. intramolecular competition between cyclopropylmethyl and oxiranylmethyl radical ring‐opening rearrangements
Authors: Krosley Kevin W. , Gleicher Gerald Jay
Published: January 2005

The erythro‐thioimidazole precursor to the cyclopropyloxiranylmethyl radical has been prepared. Treatment with 7–19 equiv. of triphenyltin hydride at 70°C gave only 3‐cyclopropylprop‐2‐en‐1‐ol, the product of epoxide ring opening. No product in which the cyclopropyl ring had opened was observed. Kinetic analysis allowed the assignment of a lower limit for the rate of oxiranylcarbinyl radical rearrangements of 1 × 1010 s−1 at this temperature.
Photochemical and radiolytic cleavage of 10‐methylacridine dimer in solutions and cryogenic glasses
Authors: Adamus Jan , Rogowski Jacek , Michalak Jacek , Paneth Piotr , Gebicki Jerzy , Marcinek Andrzej , Platz Matthew S.
Published: January 2005

10‐Methylacridine dimer [(AcrH)2] dissociates into a pair of 10‐methylacridyl radicals (AcrH˙) on laser excitation (308 nm) in solvents of various polarities. Exposure of (AcrH)2 in methylcyclohexane glass at 77 K to γ‐rays of 60Co generates the radical cation (AcrH). Thermal annealing of the matrix to 90 K results in dissociation of (AcrH) into AcrH˙ and 10‐methylacridinium cation (AcrH+).
Substituent effects in carbon‐13 NMR spectra of aminopyridines
Authors: Rasała Danuta
Published: January 2005

13C NMR spectra of 45 vicinally substituted 2‐, 3‐ and 4‐aminopyridines were investigated. The substituent effects are evidently sensitive to the position of both the substituent and the fixed amino group. In the para position with respect to the substituent, long‐range electronic effects are expressed by the dual substituent parameter treatment (DSP), and the resonance contribution is more important than the inductive contribution. Correlation results for the ipso and ortho carbon atoms using the more sophisticated extended DSP equation suggest non‐electronic effects of unknown origin.
Stereochemistry of aminolysis of N‐pentafluorophenylcarbonimidoyl dichloride. Crystal and molecular structures of N2‐pentafluorophenyl‐N1,N1‐dibenzylchloroformamidine
Authors: Mikhailov V. A. , Yufit D. S. , Balabanov E. Yu. , Struchkov Yu. T. , Popov A. A. , Savelova V. A. , Kolesnikova I. V. , Petrova T. D. , Platonov V. E.
Published: January 2005

It has been established that the product of the reaction of N‐pentafluorophenylcarbonimidoyl dichloride (1) with dibenzylamine, viz. N2‐pentafluorophenyl‐N1,N1‐dibenzylchloroformamidine (4a) is a Z‐isomer, whereas according to the concept of stereoelectronic control one might have expected a product with E‐configuration. The heat of formation and the geometry of Z‐ and E isomer of N2‐pentafluorophenyl‐N1,N1‐diethylchloroformamidine (4b) (the latter is a product of the reaction of 1 with diethylamine) were calculated with the semi‐empirical MNDO method. The calculated geometry of (Z)‐4b was found to be in a good agreement with the experimentally determined structure of 4a. The formation of (Z)‐4b is more preferable energetically than that of (E)‐4b; the calculated difference in the heats of formation is 2·8 kcal mol−1. Thermodynamic control of the reaction stereochemistry is proposed.
Conformational analysis of substituted perhydro‐1,2‐oxazolo[3,2‐c] [1,4]oxazines by NMR spectroscopy
Authors: Wazeer Mohamed I. M. , Al‐Muallem Hasan A. , Ali Sk. Asrof
Published: January 2005

The 13C NMR spectra of most of the substituted perhydro‐1,2‐oxazolo[3,2‐c] [1,4]oxazines (3) at low temperature showed the presence of two isomers of unequal populations. The major isomer is shown to be the cis isomer {except in 2‐hydroxymethyl‐2‐methylperhydro‐1,2‐oxazolo[3,2‐c] [1,4] oxazine (3e)}, which is in equilibrium with the minor isomer (trans conformer) by a relatively slow nitrogen inversion. Intramolecular hydrogen bonding in oxazines, having 2‐hydroxymethyl substituents, is shown to be an important factor in determining the population ratio of the two isomers. The barrier to nitrogen inversion was determined by detailed band‐shape analysis of proton and carbon NMR spectra and were in the range 66·3–72·9 kJ mol−1. The chair inversion had been slowed down, in one case, trans‐dimethylperhydro‐1,2‐oxazol[3,2‐c] [1,4]oxazine‐2,3‐dicarboxylate (3j), to show the presence of the two forms of the cis isomers. The barrier to chair inversion is 41·5 kJ mol−1 as determined by proton NMR band‐shape analysis of 3j.
Intramolecular photoinduced electron transfer (PET) bond cleavage in some sulfonium salt derivatives. Effect of distance and thermodynamics on PET rate
Authors: Saeva Franklin D. , Martic Peter A. , Garcia Edwin
Published: January 2005

The rate of intramolecular photoinduced electron transfer (PET) between a naphthacene electron donor (D) and a sulfonium electron acceptor (A) was found to be dependent on the thermodynamics for PET and distance between D and A. The slopes of In kET vs through‐bond and through‐space distance plots was found to be −0·60 and −0·91 Å−1, respectively. The product distribution obtained from the photolysis of regioisomeric naphthacenyl sulfonium salt derivatives was found to be independent of distance. Photolysis of three regioisomeric naphthacenyl phenylmethyl‐p‐cyanobenzylsulfonium salts produced unsubstituted thiomethylphenylnaphthacene and mono‐, di‐ and tri‐p‐cyanobenzyl‐substituted thiomethylphenylnaphthacene. The appearance of the out‐of‐cage di‐ and trisubstituted photoproducts suggests that secondary photochemistry and intermolecular electron transfer bond‐cleavage reactions are occurring when the concentration of the sulfonium salt in acetonitrile is 10−2 and 10−3 M and the degree of conversion is high. The primary photoproduct is the mono‐substituted naphthacene derivative.
Simultaneous determination of primary and secondary thermodynamic isotope effects in tautomeric equilibria
Authors: Zhang Ben Li , Mabon Françoise , Martin Maryvonne L.
Published: January 2005

NMR determination of site‐specific hydrogen isotope ratios at natural deuterium abundance (SNIF‐NMR) provides the basis for simultaneous access to primary and secondary thermodynamic fractionation factors in exchange reactions and avoids the need for selective isotope labelling of the reagents. The method was applied to the measurement of fractionation parameters involving OH, CH2, CH3 and =CH groups in keto–enol tautomeric equilibria. The fractionation factors relating the =CH and OH sites of the enol species are simply derived from 2H NMR spectra whereas the determination of isotope parameters which relate keto and enol positions exploits a combination of 2H and 1H NMR experiments. Since only monolabelled isotopomers have to be considered at natural abundance, the method also offers the advantage of avoiding the occurrence of complex equilibria associated with multi‐labelled species possibly introduced by deuterium enrichment. The primary equilibrium isotope effects illustrate a preference of deuterium for the methylene fragment of the keto form with respect to the ethylenic position of the enol tautomer. Since the enol species is itself engaged in a fast tautomeric equilibrium associated with a symmetric or unsymmetric double minimum potential, the thermodynamic parameters are averaged over the exchanging partners. It is shown that the average thermodynamic fractionation factor relating the OH and =CH hydrogens of the enol are significantly influenced by the nature of the substituents at both carbonyl positions of the β‐diketones. Moreover, methyl and chlorine substitution increases by a factor of about 1·1 the thermodynamic isotopic fractionation factor relating the –COCHCO– position of the keto form to the hydroxyl position of the enol.
Ion–molecule pairs as intermediates in solvolysis. Catalysis by the pyridine leaving group on the elimination reaction
Authors: Thibblin Alf , Sidhu Harvinder
Published: January 2005

Solvolysis of the 1‐(1‐methyl‐1‐phenylethyl)pyridinium cation (1‐P+) in 25% (v/v) acetonitrile in water at 60°C provides the alcohol 2‐hydroxy‐2‐phenylpropane (1‐OH) as the main product along with the alkene 2‐phenylpropene (3). The formation of the elimination product 3 is promoted by the leaving pyridine. Thus, eight times more 3 is obtained from 1‐P+ than from the protonated ether 1‐OMeH+. Hydron abstraction by the leaving pyridine is only two times less efficient than with AcO− as leaving group. The results indicate that the ion–molecule pair 1+ –P has a significant lifetime. The elimination product is formed mainly from the ion–molecule pair. The free carbocation yields almost exclusively the substitution product.
Reactions of cycloalkyl chlorides and bromides with diphenylphosphide ions in liquid ammonia
Authors: Nazareno Mónica A. , Palacios Sara M. , Rossi Roberto A.
Published: January 2005

The reactions of cycloalkyl (butyl, pentyl, hexyl and heptyl) chlorides and bromides with diphenylphosphide ions were studied in liquid ammonia. Cyclobutyl chloride was unreactive, whereas the bromide reacted giving the substitution product cyclobutyldiphenylphosphine (isolated as the oxide) in good yields; this reaction was catalysed by light and partially inhibited by p‐dinitrobenzene (p‐DNB). Cyclopentyl, cyclohexyl and cycloheptyl chlorides did not react in the dark, but the substitution products were formed under irradiation, and these reactions were inhibited by p‐DNB. The bromides reacted in the dark or under irradiation, and these reactions were partially inhibited by p‐DNB. It can be then concluded that the reactivity of cycloalkyl halides depends on the ring size and the nucleofugal group. In addition, as the overall reactivity decreases, there is an increase in electron transfer reactions.
Structure, stabilization energies and chemical shifts of the cyclobutenyl cation. Does it have ‘aromatic’ homocyclopropenium ion character? An ab initio study
Authors: Sieber Stefan , Schleyer Paul v. R. , Otto André Henry , Gauss Jürgen , Reichel Felix , Cremer Dieter
Published: January 2005

The relatively high stability and the structure of the simplest homoaromatic carbocation, the cyclobutenyl cation 1, was established ab initio by using correlated wave functions at HF 6–31G*, MP2(full)/6–31G*, MP2(full)/6–311G** and MP4(SDQ,full)/6–31G* geometries. The stability of 1 was estimated using homodesmotic and isodesmic reactions. The heat of formation of 1 was estimated to be 244 kcal mol−1 (1 kcal = 4·184 kJ). Chemical shift calculations were carried out at correlated levels and are in good agreement with the experimental values. At all levels of theory chemical shift calculations confirm the bent structure of 1. The MP4(SDTQ,fc)/6–311G**//MP2(full)/6–311G** + ZPE(MP2/(full)6–31G*), QCISD(T,fc)/6–31 + G*//MP2(full)/6–31G* + ZPE(MP2(full)/6–31G*) and MP4(SDQ,full)/6–31G*//MP4(SDQ,full)/6–31G* + ZPE(MP2(full)/6–31G*) ring inversion energies of 9·1, 9·3 and 9·0 kcal mol−1, respectively, agree with experiment (8·4 ± 0·5 kcal mol−1). Triplet electronic states are not competitive energetically. The homoaromatic character of the cyclobutenyl cation is shown by the nearly equal charges on C‐1, C‐2 and C‐3, the considerable 1,3‐bond order, the short 1·74 Å C‐1C‐3 distance and the large stabilization energy relative to the allyl cation.
Multinuclear NMR study of a five‐membered ring ditellurium derivative
Authors: Stefaniak Lech , Kamienski Bohdan , Schilf Wojciech , Amosova Svetlana V. , Webb Graham A.
Published: January 2005

125Te, 1H and 13C NMR data are presented for a five‐membered ring ditellurium derivative. The signals are assigned by means of integration and correlation spectroscopy. The NMR data are most consistent with the presence of one of the two possible structures.
Photochlorination of tetrachloroethene in carbon tetrachloride solutions
Authors: Miller J. S. , Ledakowicz S.
Published: January 2005

Validation of the mechanism of tetrachloroethene photochlorination at low C2Cl4 concentrations was investigated. The reaction was examined in carbon tetrachloride solution. For initial tetrachloroethene concentrations below 2 mol I−1, the solvent participates in a free‐radical chain photochlorination. The presence of CCl4 disturbs the course of this particular chain reaction and influences the duration of the induction period.
Heats of formation of organic molecules by ab initio calculations: Alkyl radicals
Authors: Liu Ruifeng , Allinger Norman L.
Published: January 2005

A bond and group equivalent scheme that allows the calculation of heats of formation for alkyl radicals from the ab initio restricted open‐shell Hartree–Fock 6–31G* energies was developed. For a group of 12 radicals, the rms error for the calculated heats of formation was 0·65 kcal mol−1 (1 kcal = 4·184 kJ). Heats of formation were predicted for the pentyl radical and six of its isomers for which the experimental values are unknown.
Solvent effects on initial and transition states in the reaction of m‐fluorophenyltriethyltin with mercuric chloride in methanol and methanol–water mixtures
Authors: Sedaghat‐Herati M. Reza , Enkvetchakul Bobby
Published: January 2005

We have investigated solvent effects on the aromatic electrophilic substitution reaction of m‐fluorophenyltriethyltin, 1, with mercuric chloride in order to obtain information about the polarity of the transition state. Second‐order rate constants have been determined for the reaction of 1 with mercuric chloride in methanol and methanol–water mixtures at 25·0°C, allowing determination of δΔG‡. Molar standard free energies of transfer (ΔG) for the same solvent mixtures have also been determined for 1 at 25·0°C. Combination of our δΔG‡ and δΔG values with literature data for ΔG of HgCl2 yield values of ΔG(TS), the standard free energy of transfer of the corresponding transition state (TS) in the reaction from methanol to aqueous methanol. It is shown that the reduction in activation energy accompanying replacement of methanol by water is due to initial state destabilization and not to TS stabilization. In fact, the TS is destabilized as methanol is replaced by water. Further analysis permits dissection of the free energies of transfer of the TS into electrostatic and non‐electrostatic components. Comparison of these electrostatic components for the 1/HgCl2 system with some model reactions shows that charge development in TS of 1/HgCl2 is approximately 0·5 units of electronic charge.
Studies on the thermal decarboxylation of 1‐alkoxycarbonylbenzotriazoles
Authors: Katritzky Alan R. , Zhang Gui‐Fen , Fan Wei‐Qiang , Wu Jing , Pernak Juliusz
Published: January 2005

1‐Alkoxycarbonylbenzotriazoles on thermolysis lose carbon dioxide; the decarboxylation is accompanied by the formation of a mixture of 1‐ and 2‐alkylbenzotriazoles, with the N‐1 isomer predominating over the N‐2 isomer in all cases. A cross‐over experiment, in which heating equimolar amounts of 1‐benzyloxycarbonylbenzotriazole and 1‐(4‐methylbenzyloxycarbonyl)‐5,6‐dimethylbenzotriazole gave almost all the cross‐over products, supports the proposed intermolecular mechanism for this decarboxylation and the formation of 1‐ and 2‐alkylbenzotriazoles. No decarboxylation was observed for 1‐phenoxycarbonylbenzotriazole.
Use of the multifunctional autocorrelation method to estimate molar volumes of alkanes and oxygenated compounds. Comparison between components of autocorrelation vectors and topological indices
Authors: Zakarya Driss , Tiyal Fathallah , Chastrette Maurice
Published: January 2005

The Multifunctional Autocorrelation Method (MAM) derived from the autocorrelation method of Moreau et al. was used to calculate molar volumes for 300 linear or branched alkanes from C6 to C11 and 104 oxygenated compounds including 40 alcohols, 30 ethers and 34 ketones. Components Ci of a MAM connectivity vector were compared with several topological descriptors. MAM descriptors were generally more efficient than topological descriptors but good regressions were obtained for alkanes only when more than two components Ci were used. An excellent regression was obtained with four components (C0 to C4):

Models obtained using descriptors based on connectivity were good enough for many practical purposes. However, to derive models adapted to estimate molar volumes for alkanes, an autocorrelation vector V based on Van der Waals volumes (as given by Bondi) was considered. The best equation obtained again includes three components:

To extend the use of MAM to compounds containing heteroatoms, a set of alcohols, ketones and ethers, were each functional group represents a different interaction type, was considered. Chemical families were first studied individually, then the total set was considered. The components of an autocorrelation vector V based on Van der Waals volumes were used as molecular descriptors. Oxygenated compounds are correctly described by only one component V0. The correlation coefficient is 0·993 for all the chemical families studied, and the standard error of the estimate is low (s = 2·56). Comparison of standard errors of the estimate for Wiener's indices (s = 17·38) and autocorrelation components (s = 3·95), for all chemical families, shows that components of MAM are more efficient as molecular descriptors. The quality of regression equations was not significantly changed when 104 oxygenated compounds and 141 alkanes were studied in a global set.
Study of initial state and transition state solvation in the Menschutkin reaction of triethylamine with ethyl iodide in alcohols from infinite dilution activity coefficients
Authors: Gonçlalves Raquel M. C. , Calado António R. T. , Pinheiro Lídia M. V. , Albuquerque Lídia M. P. C. , Macedo Eugénia A.
Published: January 2005

The Gibbs energies of activation of the Menschutkin reactions of trethylamine with ethyl iodide in 10 monoalcohols and nine dialcohols, were dissected into contributions from the initial state and transition state. To perform this study, the infinite dilution activity coefficient values of the solutes in the solvents, γ∞, were determined by the UNIFAC group‐contribution method, using the modified Flory–Huggins equation in the combinatorial term. For triethylamine, the γ∞ values were calculated using group‐interaction parameters from the VLE Parameter Table, due to the nonavailability of specific γ∞ UNIFAC interaction parameters for the relevant groups. For ethyl iodide, the γ∞ values were calculated using the group‐interaction parameters from the specific γ∞ UNIFAC Parameter Table, except for the solvents diethylene glycol and triethlene glycol, where VLE parameters were used for the same reasons as explained above for the triethylamine. The results were compared with those obtained for the unimolecular decomposition of tert‐butyl halides. For the Menschutkin reaction, we conclude that solvent disruption and reorganization to a state appropriate to solvating the molecular species is a dominant interaction mechanisim.
A reinvestigation of the reactions between diazomethane and 1‐phenylurazole
Authors: Bausch M. J. , Wang L.‐H.
Published: January 2005

Exhaustive methylation of 1‐phenylurazole (1), using ethereal diazomethane as the methylating agent, results in the formation of three products: 1‐phenyl‐3,5‐dimethoxy‐1,2,4‐triazole (2), 1‐phenyl‐3‐methoxy‐4‐methyl‐Δ2‐1,2,4‐triazolinev5‐one (3) and 1‐phenyl‐2,4‐dimethylurazole (4). Based on the amounts of 2–4 isolated, the overall yields in these reactions are typically >90%. When 1 and diazomethane were allowed to react in a fashion that resulted in the formation of monomethylated analogues of 1, 1‐phenyl‐3‐methoxy‐Δ2‐1,2,4‐triazoline‐5‐one (5) and 1‐phenylv2‐methylurazole (6) were formed. In separate experiments, the monomethylated species 5 and 6 were allowed to react with diazomethane in efforts to develop a sequence of reactions that reasonably accounts for the formation of 2–4. Whereas N‐ and O‐methylated products were obtained when 5 was treated with ethereal diazomethane, the reaction of 6 and diazomethane produced only the N‐alkylated product 1‐phenyl–2,4‐dimethylurazole (in quantitative yield). The outcomes of these experiments are consistent with a sequence of reactions in which the treatment of 1 with diazomethane results, initially, in the formation of the monomethylated species 5 and 6. In the presence of sufficient diazomethane, 5 and 6 then undergo further mothylation, forming the dimethylated species 2–4.
Oxidation kinetics of p‐aminodiphenylamine with peroxydisulphate
Authors: El‐Sheikh M. Y. , Salem M. A. , Ismail A. A. , Zaki A. B.
Published: January 2005

The kinetics of oxidation of p‐aminodiphenylamine with the peroxydisulphate ion were investigated spectrophotometrically. The oxidation was accompanied by the appearance of a wine‐red colour, which changed to become colourless. The reaction rate was measured by the stopped‐flow technique with a UV spectrophotometer. The reactions proceeded with second‐order kinetics, first‐order for each individual reactant. Cationic micelles (cetyltrimethyl ammonium bromide) caused an enhancement in the reaction rate followed by inhibition at higher concentrations. Anionic micelles (sodium dodecyl sulphate), on the other hand, had a slightly retarding effect. The effects of radical scavenger, pH and ionic strength on the fast oxidation rate were examined. A mechanism consistent with the observations is discussed.
Cooperation of aryliminodimagnesium molecules: Deoxygenation of azoxyarenes by mono‐ and bifunctional reagents in comparison with that by phosphorus(III) reagents
Authors: Okubo Masao , Inatomi Yoshito , Eguchi Izumi , Nishida Hiroshi , Gotoh Seiji , Matsuo Koji
Published: January 2005

Reaction of aryliminodimagnesium [ArN(MgBr)2, IDMg] with nitrobenzene (Ar1NO2) in tetrahydrofuran (THF) gives unsymmetrical (unsym) azoxybenzene, which is deoxygenated to give unsym‐azobenzene. The reaction is utilized for the independent preparation of ONN and NNO isomers of unsym‐azoxy compounds. The effects of the relative value of the difference between oxidation and reduction potentials of para‐substituted reactants, special effects of ortho‐substituents of the substrate and also effects of structure and concentration of mono‐ and bis‐IDMg reagents were studied previously, and ‘cooperation of their aggregate excess needed for product formation’ was proposed. This unfamiliar concept for reactions of magnesium reagents was studied further. Supporting evidence was obtained from the retarding effect of trimethylene chains of nitro [3.3] metacyclophanes and the derived azoxy [3.3] metacyclophanes and also from the effect of the length of bis‐IDMg's central α,ω‐polymethylenedioxy chain appropriate for azoxy deoxygenation. By comparison with deoxygenation by X3P reagents (X = EtO, Me2N), the general needs of cooperation of excess magnesium reagents were confirmed and its role in their reactions is discussed.
Exploratory photochemistry of alkylbromo‐ and alkylfluorodiazirines. Excited‐state hydrogen migration and carbene formation
Authors: Chidester Wendy , Modarelli David A. , White Walter R. , Whitt David E. , Platz Matthew S.
Published: January 2005

Laser flash photolysis of a series of alklylbromo‐ and alkylfluorodiazirines in pentane at ambient temperature generates alkylhalocarbenes by decomposition of the diazirine excited states. The halocarbenes can be intercepted with pyridine to form ylides. The ylides absorb intensely between 350 and 400 nm and are fairly long lived (τ » 10 μs), making them convenient probes of the yield and dynamics of the carbene. The yield of the ylides increases with increasing pyridine concentration up to 1·5 M. At pyridine concentrations > 1·5 M the yield of ylide is saturated, signifying that every carbene generated in a laser pulse is captured by pyridine prior to reaction with solvent or intramolecular rearrangement. The yield of trappable carbene generated from alkylbromodiazirines closely tracks the bond dissociation energy of the CH bond adjacent to the diazirine moiety. The data indicate that the excited states of the alkylbromodiazirines suffer CH migration (or CC migration with cyclobutylbromodiazirine) and nitrogen extrusion in competition with carbene formation. The yield of trappable carbene derived from the alkylfluorodiazirines is independent of the bond dissociation energy of the adjacent CH bond. This is probably a consequence of the great thermodynamic stability of α‐fluorocarbenes.
Intramolecular fluorescence quenching and exciplex formation in ω‐(1‐pyrenyl)alkyl para‐substituted benzoates
Authors: Kawakami Jun , Iwamura Michiko
Published: January 2005

Measurements of absorption and emission spectra, fluorescence quantum yields (Φf) and lifetimes (τf) of ω‐(1‐pyrenyl)alkyl para‐substituted benzoates PnX (X = H, Cl, CF3, CN, NO2, with n = 1–4) were carried out in solvents of various polarity. Intramolecular interaction in the ground state is not observed in any of these compounds. PnCN (n = 2–4) shows an intramolecular exciplex emission in solvents of low to high polarity. The broad, structureless emission at longer wavelength observed in ethyl acetate solution of P1CN is ascribed to an ‘exciplex‐type’ emission which does not require actual overlapping of the two chromophores. P2CF3 also shows an exciplex emission in solvents of medium to high polarity. The solvent dependence of both Φf and τf increases as the electron‐withdrawing ability of the para‐sustituents increases. The relationship between fluorescence quenching by electron transfer and para‐substituent of PnX is discussed by means of the free energy for electron transfer, ΔGET, obtained from the oxidation and reduction potentials of pyrene and methyl para‐substituted benzoates in acetonitrile, respectively. PnH, with positive ΔGET, does not show a solvent dependence of Φf, except for P1H, in which ca 40% of the fluorescence is quenched in acetonitrile. PnCl, with slightly negative ΔGET, shows more efficient quenching, but does not show exciplex emission. PnCN, PnCF3 and PnNO2 have ΔGET values between −0·36 and −0·65 eV, and their fluorescence is fairly efficiently quenched. The fluorescence of PnCN is concluded to be strongly quenched by intersystem crossing from the singlet exciplex to the locally excited pyrene, and by electron transfer from the pyrene part to the benzoate part. That the formation of a singlet exciplex is necessary for intersystem crossing in bichromophoric compounds containing pyrene is thus clearly illustrated.
Kinetic study of the formation and rupture of stable tetrahedral intermediates. CO, CN and CS bond formation
Authors: Núñez Oswaldo , Rodríguez José , Angulo Larry
Published: January 2005

Measured pseudo‐first‐order rate constants for intramolecular formation of tetrahedral intermediates from N‐2‐hydroxyethylphthalimide, N‐2‐aminoethylphthalimide and N‐2‐thioethylphthalimide at pH > 6 are reported. The reaction is specific and general base catalysed, with β(Brønsted) values 0·44, 0·52 and 0·52 respectively. From a plot of log kb (general base rate constants) vs γ′ [the affinities of EtXH (X = O, NH, S) toward the carbonylic carbon], β′nuc values of 0·01 (with OH− as specific base), 0·25 (with imdidazole as general base) and 0·27 (with HPO as general base were obtained). The observed relationships pxy = ϑβ′/ − ϑpKa = −ϑ(Brønsted)/ϑγ′ = 0·03 is supported by the predictions of an energy contour diagram, which, on extrapolation to a non‐stable tetrahedral intermediate, predicts a late and slightly protonated transition state for the cleavage process. At pH < 3, these intermediates cleave to yield only the corresponding diacylimides. These reactions are general base and acid catalysed with β > 0·3 and α < 0·1. A fast equilibrium between the intermediate and its N‐protonated (amide) form is reached. The general base rupture of the latter is faster than that of the corresponding non‐protonated intermediate by a factor of ca 109 − 1010‐fold.
Methyl transfer from MeCo(III)Pc to thiophenoxide
Authors: Galezowski Wlodzimierz , Lewis Edward S.
Published: January 2005

Transfer of the cobalt‐bound methyl in MeCo(III)Pc to thiophenoxide ion was studied (H2Pc is the planar macrocyclic phthalocyanine; the cobalt is held in the center in this plane). In dimethylacetamide solution, the reaction is rapid, requiring stopped flow for the kinetics, and yielding MeSPh and Co(I)Pc− in good yield. The kinetics are not simple second order, but instead approach a constant rate at high [PhS−], attributed to the reversible formation of an inert complex with PhS− occupying the vacant octahedral site in MeCo(III)Pc, on the other side of the phthalocyanine plane from the methyl group. The kinetics allow the estimation of the equilibrium constant, K, and the SN2 rate constant, k, which at 25°C have values of ca. 9·4 × 103 l mol−1 and 1·8 × 104 l mol−1, respectively. Although these values are rough, the ratio k/K is firm at 1·91 ± 0·02 s−1; this is the limit of the rate at high [PhS−]. An alternative mechanism, which is entirely consistent with the kinetics, involves a rate‐determining homolysis of the CoS bond of the same complex. The mechanism is not favored because the product yields are high for a radical combination process and alternative chain processes are kinetically unacceptable. Further, the rate constant is about what would be expected from the reactivity of other nucleophiles in SN2 reactions. Further arguments in favor of the SN2 mechanism are presented. This transfer of the methyl group from Co to S is part of the possible analogy to the vitamin B12‐promoted methionine synthesis in nature. The other step in the biological, enzymatic process is the transfer of methyl from the nitrogen of N‐methyltetrahydrofolate to cobalt. An attempt to model this with the very reactive N‐methyl‐2,6‐dichloropyridinium ion was unsuccessful; the reaction took an entirely different course, presumably initiated by electron transfer, but leading to substantial loss of Cl− from the pyridine. No more than 0.5% methyl transfer took place. This system does mimic well the complete natural enzymatic process.
Cleavage rates for radiolysis‐produced radical anions of naphthylmethyl phenyl ethers and naphthyl benzyl ethers
Authors: Guthrie Robert D. , Patwardhan Manjiri , Chateauneuf John E.
Published: January 2005

Cleavage reactions of α‐ and β‐naphthylmethyl phenyl ethers and of α‐ and β‐naphthyl benzyl ethers were studied by pulse radilysis. Transient spectra indicate that reactions occur via electron capture followed by cleavage of the resultant radical anions to give arylmethyl radicals and aryloxide ions. Product studies of extensively irradiated samples are consistent with this scheme and show patterns which are informative for radiation studies in general. The reactions were studied in several solvents. The behavior of transient spectra obtained in acetonitrile for these ethers shows clearly that radical anions of the naphthylmethyl ethers cleave more rapidly than do the radical anions of the naphthyl benzyl ethers.
Anionic products of the two‐electron reduction of aromatic nitriles in liquid ammonia
Authors: Bilkis Isaak I. , Vaganova Tamara A. , Panteleeva Elena V. , Salnikov Georgij E. , Tananakin Andrej P. , Mamatyuk Victor I. , Shteingarts Vitalij D.
Published: January 2005

The reduction of 9‐cyanoanthracene by two equivalents of potassium in liquid ammonia was shown to yield the 9‐cyanoanthracene dianion, whereas 1‐naptho‐ and benzonitrile gave the cyanodihydroaryl anions corresponding to the protonation of nitrile dianions at a position para to the cyano group. The 9‐cyanoanthracene dianion underwent the same transformation in the presence of a stronger protonating agent, methanol. According to 13C NMR spectral data of the generated species, the cyano group extracts the negative charge from the π‐electronic system: ca 0·20:0·25 e in the case of the 9‐cyanoanthracene dianion and ca 0·14:0·17 e in the case of cyanodihydroaryl anions. These estimations and the general NMR pattern of π‐charge distribution in all the anionic species under investigation are in accordance with data from quantum molecular orbital calculations at the PM3 and INDO levels, being reflected by the fairly good linear relationships between the changes of ring carbon chemical shifts on going to the anionic species from the respective neutral precursors on the one hand and the calculated π‐charges on the other. The para‐orienting effect of the cyano group in the protonation of nitrile dianions is discussed in terms of the π‐charge distribution in the starting dianion and the tendency to form a most stable cyanodihydroaryl anion isomer.
Competition between inter‐ and intramolecular hydrogen bonding in molecules with donor and acceptor groups. Solvatochromic and thermochromic evidence in n‐(nitrophenyl)alkylenediamines
Authors: Giacomelli L. , Cattana R. , Anunziata J. , Silber J. J. , Hedrera M. , Salerno A. , Perillo I.
Published: January 2005

Solvent effects on the absorption spectra of N‐(p‐nitrophenyl)dimethylenediamine (Ia), N‐(p‐nitrophenyl)trimethylenediamine (Ib), N‐(p‐nitrophenyl)tetramethylenediamine (Ic). N‐methyl‐N‐(p‐nitrophenyl)tetramethylenediamine (II), N‐butyl‐p‐nitroaniline (III) and N‐(o‐nitrophenyl)trimethylenediamine (IV) were studied at different temperatures. Whereas II, III and IV do not show any variation in their spectra characteristics with changes in temperature, I shows a hypsochromic shift with a hypochromic effect when the temperature is increased. The Kamlet and Taft solvatochromic comparison method was applied. A strong effect of the β parameter on I and III was interpreted as being due to the hydrogen bond donor ability of the H atom in the aromatic amino groups. In I, the β influence increases with increase in temperature. These facts are explained by proposing the formation of intramolecular hydrogen bonds between amine groups in all compounds I, besides the intermolecular interactions between compounds I and the solvent. On the other hand, the values of vo, s and b for Ib are smaller than the corresponding values for Ia and Ic. Since in Ib a six‐membered ring may be formed, a more stable bond is expected. Comparative 1H NMR of the aniline hydrogen for I and IV (in non‐hydrogen bond acceptor solvents) shows a particular downfield chemical shift for I which suggests hydrogen bond formation. Since this effect is independent of concentration, the hydrogen bond is assumed to be intramolecular, in agreement with solvatochromic and thermochromic studies. These conclusions were corroborated by IR spectroscopy in the solid state and in chloroform solutions.
The volume profile for a cheletropic reaction
Authors: Isaacs Neil S. , Laila Abdulhameed
Published: January 2005

The volumes of activation and of reaction were measured for the reaction between sulphur dioxide and 2,3‐dimethylbuta‐1,3‐diene to form 3,4‐dimethylsulpholene. Values of ΔV and ΔV* are −33 and −35 cm3 mol−1, respectively in accordance with a concerted reaction having a very product‐like transition state.
Secondary α‐deuterium isotpe effect on the hydration of p‐nitrobenzaldehyde
Authors: Pires José Ricardo , Stachissini Antonia Sonia , Amaral Luciano Do
Published: January 2005

The secondary α‐deuterium isotpe effect on the equiliburium constant (KD/KH) for the hydration of p‐nitrobenzaldehyde is 1·39 ± 0·05. The Kinetic secondary α‐deuterium isotope effect (KD/KH) for the addition of water to p‐nitrobenzaldehyde is 1·18 ± 0·07. This result demonstrates that there is extensive carbon–oxygen bond formation in the transition state for addition of water to the carbonyl group of p‐nitrobezaldehyde.
Quantitative description of structural effects on melting points of substituted alkanes
Authors: Charton Marvin , Charton Barbara
Published: January 2005

Structural effects on the melting points of substituted alkanes, both branched and unbranched, ranging in length from 1 to 19 carbon atoms, were successfully correlated with a variant of the intermolecular force equation which included a variable capable of accounting for the packing energy contribution of the alkyl group. The final data set had 366 data points. The regression equation obtained accounted for 91·85% of the variance of the data with a standard error of 17·9 K. Twenty‐nine data points could not be included in the correlation; nine of these are carboxamides. The contributions of the polar variables are slightly larger than those of the nonpolar variables while the contribution of structural variation in the substituent is greater than that in the alkyl group. It will be necessary to determine appropriate values of the packing parameter if compounds with chains longer than 20 carbon atoms are to be included in the model.
Number and structure of solvolysis intermediates. Part 3.SN1 solvolysis of 2,2‐Dimethyl‐1‐(p‐Methoxyphenyl)Propyl p‐Nitrobenzoate: Mechanism of the common ion salt effects arising at the stage of the second ion‐pair intermediate
Authors: Kinoshita Tomomi , Itoh Masaaki , Shibayama Koichi , Takeuchi Ken'ichi
Published: January 2005

2,2‐Dimethyl‐1‐(p‐methoxyphenyl)propyl p‐nitrobenzoate (ROPNB) was subjected to solvolysis in phenol in the presence of tetrabutylammonium [carboxy‐13C]‐p‐nitrobenzoate, in which the ‘common ion rate depression’ was confirmed to arise at the stage of the second ion‐pair intermediate (Int‐2). The unchanged substrate recovered at 46% reaction contained the isotopically labelled leaving group, indicating the occurrence of common ion exchange to the extent of 41–46%. In the solvolysis of the optically active substrate under identical conditions, the unchanged substrate was recovered with 51·4% racemization and ROPh was produced with slightly (1·56%) retained configuration, similarly to the solvolysis in the absence of the common ion salt. These isotope‐tracer and stereochemical outcomes indicate that the common ion exchange in this solvolysis system should be attributable to the retentive nucleophilic attack on Int‐2 by the common ion salt via a quadrupole (four‐centre ion pair) transition state, accompanying the common ion rate depression, and they suggest that the special salt effect also should proceed by an analogous anion‐exchange mechanism to that for the common ion effects.
Conformational properties of disulphide bridges. 2. Rotational potentials of diethyl disulphide
Authors: GÖRbitz Carl Henrik
Published: January 2005

The conformational properties of diethyl disulphide, a model compound for the disulphide bridges in peptides and proteins, were studied with ab initio methods. Stationary point structures were optimized at the HF/6–31G* level with consideration of electron correlation in subsequent single‐point MP2 calculations. The six energy minima were also optimized at the MP2/6–31G* level with calculation of zero‐point vibrational frequencies and thermal corrections. Additional single‐point MP2 energy calculations employed larger basis sets up to 6–311G(2d, p). With positive disulphide chirality, the global energy minimum is a ‘spiral’ conformation with gauche + C–C–S–S torsion angles. The further stability order for energy minima deviates from previous ab initio results. In particular, the extended trans,trans conformer is subject to a significant relative destabilization on inclusion of electron correlation in the calculations and is only the fifth most stable energy minimum with estimated ab initio ΔH298 = 5·13 kJ mol−1. The results presented are relevant for the discussion of conformational properties for the structurally equivalent disulphide bridges in polypeptides and calculations of relative energies with molecular mechanics methods.
AB initio and semi‐empirical investigation of gas‐phase carbon acidity
Authors: Saunders William H.
Published: January 2005

Acidities of a representative range of carbon acids were calculated by various levels of ab initio theory and by the semi‐empirical methods AM1 and PM3 in order to ascertain what level is necessary in order to obtain consistently reliable results. The semi‐empirical methods give good agreement with experiment in about a third of the cases, but are significantly, and sometimes seriously, in error for the rest. The 3–21G ab initio method consistently underestimates acidities by a substantial amount. The best results were obtained at the MP2/6–31 + G*//6–31 + G* level. Higher level Møller–Plesset corrections worsened the agreement with experiment.