bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

AbstractReaction 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

AbstractLaser 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

AbstractMeasurements 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

AbstractMeasured 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

AbstractTransfer 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

AbstractCleavage 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

AbstractThe 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

AbstractSolvent 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

AbstractThe 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

AbstractThe 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

AbstractStructural 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

Abstract2,2‐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

AbstractThe 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

AbstractAcidities 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.
Kinetics and mechanism of the aminolysis of benzoic anhydrides
Authors: Choon Lee Byung , Hyun Yoon Ji , Gyu Lee Cheal , Lee Ikchoon 
Published: January 2005

AbstractNucleophilic substitution reactions of benzoic anhydrides, in which one of the rings is substituted, with anilines were investigated in methanol. The product‐formation step coincides with the rate–limiting step so that the two rate constants, kXZ and kXZ, for the competitive reaction pathways can be dissected. The two cross‐interaction constants, ρXY and ρXY, especially an unusually large magnitude of the latter, indicate that the reaction proceeds by a frontside SN2 attack on either one of the caronyl carbon with a strong interaction between the nucleophile (X) and the leaving group (Z). The mechanism is also supposed by the trends in the activation parameters.
Anomalous polar substituent effect on 15N NMR chemical shifts of Aniline derivatives. Acylanilides and related compounds
Authors: Yuzuri Tomoaki , Wada Hajime , Suezawa Hiroko , Hirota Minoru 
Published: January 2005

AbstractThe 15N chemical shifts of various m‐ and p‐substituted anilides and related derivatives, ZNHC6H4Y [Z = C6H5CO, CH3CO, CF3CO, CH3SO2, (C6H5O)2PO and H], were correlated with σ− constants. With all series of anilides investigated, the plotted points for the m‐ and p‐substituted derivatives tend to be separated from each other and to lie on two different regression lines. Reinvestigation from this point of view revealed that similar separations occur generally with substituted aniline derivatives. The anomalous polar substituent effect was ascribed to the very large contribution of a resonance effect. In addition, the local π‐polarization effect causes a negative slope for the m‐substituted series in the case of carboxylic anilides.
Water‐Catalysed hydrolysis of p‐nitrobenzyl cellulose xanthate
Authors: Humeres Eduardo , Sequinel Luiz Fernando , Nunes Mauricéa , Oliveira Célia M. S. , Barrie Patrick J. 
Published: January 2005

AbstractThe spontaneous hydrolysis of p‐nitrobenzyl cellulose xanthate (CelXNB) with a degree of substitution (DS) in the range 2–9 was studied in 10% aqueous ethanol at pH 10, and was followed spectrophotometrically by the appearance of p‐nitro‐α‐toluenethiol, in a continuous‐flow system where the reactor was shaken. CelXNB was characterized by solid‐state 13C NMR spectra. The reaction occurs through two parallel processes due to two xanthate ester groups with different reactivities. The fast hydrolysis was ascribed to the reaction of the C‐2 + C‐3 isomers, whereas the slow hydrolysis was due to the C‐6 isomer. The percentage of the latter is much higher than C‐2 + C‐3. The solvent isotope effect of the fast hydrolysis (k′/)k′ 11 was 2·22 ± 0·16 and the proton inventory indicated that there is only one proton transfer involved in the transition state, where a second water (or a neighbouring OH group) acts as a general base. The entropy of activation of the fast hydrolysis was only 3·3 ± 0·8 e.u., suggesting that the water molecules involved are highly oriented with respect to the coordinates required to reach the transition state. It is proposed that they form part of the three‐dimensional hydrogen‐bonded ice‐like structure that involves the cellulose matrix.
Stereochemistry and kinetics of methoxide ion substitution in (Z)‐ and (E)‐O‐methylbenzohydroximoyl cyanides
Authors: Johnson James Elver , Todd Susan L. , Gardner Jana L. , Gardner Toni M. , Buck Patricia , Ghafouripour Abdolkarim , Zimmerman Wayne 
Published: January 2005

AbstractThe stereochemistry and kinetics of the methoxide ion substitution reactions (Z)‐ and (E)‐methylbenzohydroximoyl cyanidc [PhC(CN)NOCH3] were investigated. The reaction of the (Z)‐hydroximoyl cyanide with sodium methoxide in DMSO–methanol (9:1) solution at 44·8°C gives a mixture of methyl (Z)‐O‐methylbenzohydroximate [PhC(OCH3)NOCH3] and the O‐methyloxime of α‐ketophenylacetamide PhC(CONH2)NOCH3. The (E)‐hydroximoyl cyanide undergoes methoxide ion‐catalyzed isomerization to the E‐isomer faster than it undergoes nucleophilic substitution. These observations were interpreted in terms of an addition–elimination mechanism in which the rate‐limiting step is elimination of the nucleofuge (AN + DN#).