bluuebunny/crossref_metadata_2025 huggingface dataset:
The synthesis and characterization of the pendant-armed ligand N,N′-bis(2′-pyridylmethyl)-1,7-dithia-4,11-diazacyclotetradecane (L4) and crystal structures of L4 and the copper(II) complex [Cu(L4)](ClO4)2  Crystal structure of the nickel(II) complex of N-(2′-pyridylmethyl)-1,4,7-trithia-11-azacyclotetradecane (L2), [Ni(L2)(CH3CN)](ClO4)2·CH3CN
Authors: Chak B CM , McAuley A
Published: May 2006

The synthesis and characterization of the mixed donor ligand N,N′-bis(2′-pyridylmethyl)-1,7-dithia-4,11-diazacyclotetradecane (L4) containing two pendant pyridine arms is described. The macrocycle formation involves reaction of N-tosylbis(2-bromoethyl)amine with N-tosylbis(3-mercaptopropyl)amine at high dilution followed by detosylation at the nitrogen atoms using LiAlH4 in view of the presence of the thioether groups. An X-ray study of the ligand shows the sulphur atoms lie exo to the macrocyclic ring. The nickel(II) and copper(II) complexes of L4 have been characterized and the X-ray structure of the Cu(II) ion exhibits a five-coordinate copper ion despite the presence of six available donor atoms. In the case of ligand N-(2-pyridylmethyl)-1,4,7-trithia-11-azacyclotetradecane (L2), with a single pendant N donor, the Ni(II) complex is distorted octahedral with a molecule of acetonitrile in the sixth site. Details of UVvis, ESR spectra, and electrochemical studies on the complexes are presented.Key words: synthesis, pendant-armed, mixed donor S,N macrocycles, copper, nickel.
Synthesis and characterization of N-silylated, C1-symmetric benzamidinates of lithium, sodium, and tin(II)
Authors: Antolini Floria , Hitchcock Peter B , Khvostov Alexei V , Lappert Michael F
Published: May 2006

The synthesis and characterization of complexes obtained from the reactions between Li[N-t-Bu(SiMe3)] (A) or the sodium analogue Na[N-t-Bu(SiMe3)] (B) and the cyanoarene RCN (R = Ph or 4-MeOC6H4) are discussed. These are the THF adduct [Li{µ-cis-N(t-Bu)C(Ph)N(SiMe3)}(THF)]2 (1), the TMEDA adduct Li[N(t-Bu)C(Ph)N(SiMe3)](TMEDA) (2), the neutral ligand-free lithium benzamidinate Li[N(t-Bu)C(C6H4OMe-4)N(SiMe3)] (3), and the THF adduct Li[N(t-Bu)C(C6H4OMe-4)N(SiMe3)](THF) (3a). The preparation and structure of the crystalline compound [Na{µ-cis-N(t-Bu)C(Ph)N(SiMe3)}(OEt2)]2 (4) is described. From the lithium benzamidinate 1 and Sn(II) chloride the tin(II) complex [Sn{N(t-Bu)C(Ph)N(SiMe3)}2] (5) was obtained. The molecular structures of the crystalline compounds 1, 4, and 5 were established by X-ray diffraction. In 1 and 4 the benzamidinato ligand is both chelating and bridging, with the Me3Si-substituted nitrogen atom as the bridging site. The central planar (MN)2 four-membered ring is a rhombus in 1, with almost equal LiN bond lengths, whereas in 4 the bonds to Na(1) are significantly longer than those to Na(2). In 5, the ligand is N,N′-chelating. Key words: alkali metals, tin(II), benzamidinates, NMR spectra, X-ray structures.
Metallocenes as references for the determination of redox potentials by cyclic voltammetry  Permethylated iron and cobalt sandwich complexes, inhibition by polyamine dendrimers, and the role of hydroxy-containing ferrocenes
Authors: Aranzaes Jaime Ruiz , Daniel Marie-Christine , Astruc Didier
Published: May 2006

Measurement of the redox potentials (E1/2) of ferrocene (1), decamethylferrocene (2), decamethylcobaltocene (3), and [FeICp*(η6-C6Me6)] (4) (Cp* = η5-C5Me5) in DMF, CH3CN, THF, DME, CH2Cl2, and DMSO were recorded vs. the saturated calomel electrode (SCE), showing that the E1/2 value of 1 was more solvent dependent than those of 24, and that the difference in E1/2 values among 24 was solvent independent, unlike those found between 1 and 24. This shows that 24 are much more reliable internal references for determining redox potentials than 1, but using data from this article, one can convert the literature values reported vs. 1 into values vs. 2, 3, or 4. The electrochemistry of 1 and 2 is all the more inhibited by polyamine dendrimers as the dendrimer generation is higher. On the other hand, ferrocenes or other iron sandwich compounds containing hydroxy groups can reestablish reversible cyclic voltammograms. It is shown, using a tetra(ferrocenylmethylamine), that this phenomenon is due to hydrogen bonding between the amido and phenol groups rather than bonding of hydroxy groups to the electrode surface.Key words: ferrocene, reference, electrochemistry, metallocene, sandwich complex, iron, dendrimer, passivation, hydrogen bond.
Concerted and stepwise mechanisms in the DielsAlder and Michael reactions of furans with methyl 3-nitroacrylate  Experimental and theoretical studies
Authors: Itoh Kuniaki , Kitoh Kazue , Kishimoto Shigehisa
Published: May 2006

The DielsAlder reactions of the furans (1a1c) and methyl 3-nitroacrylate (2) produce the cycloadducts via a concerted mechanism. Then, the products subsequently undergo retro-DielsAlder reactions to give the Michael adducts via a stepwise mechanism. We have used both experimental and theoretical methods to investigate the mechanism for, and the regio- and stereo-selectivity in, the DielsAlder and Michael reactions. To account for the selectivity observed in these additions, we examined the frontier molecular orbitals involved, and to locate the transition states (TS), we employed a B3LYP/6-31G* level density functional calculation. We were able to explain the regio- and stereo-selectivity of the four possible DielsAlder adducts observed in the experiments by a comparison of their calculated activation energies, which ranged from 11 to 18 kcal/mol (1 cal = 4.184 J). The s-cis forms of the dienophile (2) were found to be more stable than the s-trans forms, both in the ground state and in the transition state. In addition, the endo transition structures for the ester group exhibited a lower energy barrier by 0.3 kcal/mol than their exo counterparts, a value that is in disagreement with the experimental results. The presence of the nitro group in 2 may play an important role in determining the selectivity. Our attempts to find a stepwise mechanism leading to the DielsAlder adduct via a zwitterion were unsuccessful. We did find that two stepwise processes lead to the formation of Michael adducts generated via an electrophilic attack by 2 at the α position of 1b and 1c, and an intramolecular proton-transfer mediated by the formation of a four-membered structure. The potential energies for these reactions showed values in the following ranges: 17 kcal/mol for 5a and 5b, and 11 to 12 kcal/mol for 7a and 7b for the first step, and, for the proton-transfer step, 4851 kcal/mol for 5a and 5b, and 4145 kcal/mol for 7a and 7b. Solvent effects in chloroform on the Michael addition did not appear in the electrophilic attack step (less than 4 kcal/mol), but the transition state of the later process was stabilized by 613 kcal/mol.Key words: DielsAlder reactions, Michael reactions, density functional method, furan, retro-DielsAlder reactions, solvent effect.
Nitration of 3-aryl-4-acetylsydnones: preparation of 3-(3-nitroaryl)sydnones by using acetyl group as a blocking group
Authors: Tien Hsien-Ju , Lin Shaw-Tao , Sheu Jinn-Tsair
Published: May 2006

The 3-(3-nitroaryl)sydnones were prepared in good yields by the nitration of 3-aryl-4-acetylsydnones followed by deacetylation with aqueous barium hydroxide.
β-Deuterium kinetic isotope effects for identity processes: bromide ion substitution at 1-bromo-1-arylethanes and 2-bromooctane
Authors: Stein Allan R.
Published: May 2006

While deuterium kinetic isotope effects for solvolyses have been extensively studied, other nucleophilic substitutions have received less attention, and identity processes, that is, substitutions where the nucleophile and leaving group are the same, have rarely been examined. Identity reactions must pass through a truly symmetrical stage, a transition state or an intermediate, so that data will be of interest to both theoretical and experimental chemists. Values of kH/kD have been determined by polarimetry for bromide exchange– racemization at ArCHBrCH3/CD3 (Ar = C6H5,4-Br- and 4-Me-C6H4, and 3,4,-dimethyl-C6H3) in acetone, acetonitrile, and nitromethane. Observed values are analogous to values seen in solvolyses. They range from 1.01 to 1.35 and, in some cases, increase markedly as the concentration of Bu4NBr decreases. Solvolyses are either first order or pseudo first order whereas plotting observed racemization rate versus [Bu4NBr] allows separation of first- and second-order components; those species giving more stable carbocations in the more dipolar solvents, the systems showing kH/kD variation with Br− concentration, alone show an appreciable first-order component. The second-order kH/kD ratio averages 1.062 ± −0.018 at temperatures ranging from 25 to 50 °C for all substrates in the three solvents, very analogous to the values seen for racemization of 1,1,1-d3-2-bromooctane or solvolysis of ethyl substrates but considerably lower than the typical solvolysis values of 1.15–1.25 for secondary, and 1.35–1.5 for tertiary substrates. The first-order kH/kD values obtained are higher, 1.1–1.5. These and other results are discussed.
A mild and efficient methodology for the synthesis of 5-halogeno uracil nucleosides that occurs via a 5-halogeno-6-azido-5,6-dihydro intermediate
Authors: Kumar Rakesh , Wiebe Leonard I. , Knaus Edward E.
Published: May 2006

A mild and efficient methodology for the synthesis of 5-halogeno (iodo, bromo, or chloro) uracil nucleosides has been developed. 5-Halo-2′-deoxyuridines 4a–c (84–95%), 5-halouridines 7a–c (45–95%), and 5-haloarabinouridines 8a–c (65–95%) were synthesized in good to excellent yields by the reaction of 2′-deoxyuridine (2), uridine (5), and arabinouridine (6), respectively, with iodine monochloride, or N-bromo (or chloro)succinimide, and sodium azide at 25–45 °C. These C-5 halogenation reactions proceed via a 5-halo-6-azido-5,6-dihydro intermediate (3), from which HN3 is eliminated, to yield the 5-halogeno uracil nucleoside. The 5-halo-6-azido-5,6-dihydro intermediate products (10a, 10b) could be isolated from the reaction of 3′,5′-di-O-acetyl-2′-deoxyuridine (9) with iodine monochloride or N-bromosuccinimide and sodium azide at 0 °C. The isolation of 10a, 10b indicates that the C-5 halogenation reaction proceeds via a 5-halo-6-azido-5,6-dihydro intermediate.
Acid-catalyzed reactions of six- and seven-membered cyclic hemiperacetals and peracetals and of related bicyclic ozonides
Authors: Griesbaum Karl , Kiesel Gilbert , Mertens Henri , Krieger-Beck Petra , Henke Henning
Published: May 2006

HCl-catalyzed reactions of the hemiperacetals 3-methoxy-3-methyl-7-hydroxy-1,2-dioxepane (1a) and 3-methoxy-3-methyl-6-hydroxy-1,2-dioxane (1b) and of the related ozonides 1-methyl-6,7,8-trioxabicyclo[3.2.1]octane (2a) and 1-methyl-5,6,7-trioxabicyclo[2.2.1]heptane (2b) gave peroxidic monocyclic (11—13), bicyclic (14), and (or) tricyclic (15) products. By contrast, reactions of the peracetals corresponding to 1a and 1b, viz., 3-methyl-3,7-dimethoxy-1,2-dioxepane (13a) and 3-methyl-3,6-dimethoxy-1,2-dioxane (13b) gave only non-peroxidic products. Reactions of the persubstituted peracetals 3,6-dimethoxy-3,6-dimethyl-1,2-dioxane (25) and 3,6-dimethoxy-3-isopropyl-6-methyl-1,2-dioxane (27) also gave non-peroxidic products, which resulted from fragmentation of the carbon skeletons.
Study of the reaction between conjugated azoalkenes and α-unsubstituted- or α-substituted-β-dicarboxylate derivatives: an improved preparation of unknown polyfunctionalized 1-amino-1H-pyrrol-2(3H)-ones
Authors: Attanasi Orazio A. , Crescentini Lucia De , Serra-Zanetti Franco , Foresti Elisabetta
Published: May 2006

In the presence of sodium methoxide in catalytic amounts, conjugated azoalkenes easily react with α-unsubstituted-or α-sub-stituted-β-dicarboxylate derivatives to give at first hydrazonic derivatives, by 1,4-conjugate addition, and, by increasing sodium methoxide up to stoichiometric amounts, new polyfunctionalized 1-amino-1H-pyrrol-2(3H)-ones.
Molecular shape effects on electron and ion behavior in dielectric liquids: cis- and trans-butene-2 between the freezing and critical points.
Authors: Sasaki Shinichi , Freeman Gordon R.
Published: May 2006

not available
Reverse pulse voltammetry and polarography: a general analytical solution
Authors: Camacho Luis , Ruiz Juan José , Serna Carmen , Molina Angela , Martínez-Ortiz Francisco
Published: May 2006

A rigorous analytical solution to reverse pulse techniques for a plane electrode expanding with an arbitrary power of time is presented. Our results include the plane approximation for SMDE as well as the expanding plane electrode model for DME. In the case of SMDE, our results have been contrasted with those obtained by numerical solution of the problem, obtaining identical results. Finally, we have compared our treatment with previous solutions in literature.
X-ray structures of triphenylphosphine and 1,2,5-triphenylphosphole products with dimethyl acetylenedicarboxylate tetramer
Authors: Hocking Martin B. , van der Voort Maarschalk Frances W.
Published: May 2006

A tetramer of dimethyl acetylenedicarboxylate, tetramethyl 4-methoxy-5-[1,2,3-tris(methoxycarbonyl)-2-cyclopropen-1-yl]-7-oxabicyclo[2.2.1]hepta-2,5-diene-1,2,3,6-tetracarboxylate 3, was recovered from old stocks of the monomer, and was also prepared thermally from the monomer by a variation of a previous method. NMR data and an X-ray crystal structure were determined for a red 1:1 adduct of this ester with triphenylphosphine. This red adduct, tetramethyl 4-methoxy-5-[1,2,3-tris(carbomethoxy)-3-triphenylphosphoranylidenepropen-1-yl]-7-oxabicyclo[2.2.1]hepta-2,5-diene-1,2,3,6-tetracarboxylate 5, crystallized in the triclinic space group P1 (No. 2) with two molecules in the unit cell (a = 12.315(2) Å, b = 12.321(2) Å, c = 14.652(2) Å, α = 110.60(1)°, β = 90.62 (1)°, and γ = 103.22(1)°). Refinement converged at R = 0.0694 (Rw = 0.0986) for 542 parameters using 4714 reflections with I > 2σ(I). Triphenylphosphine oxide did not react with tetramer 3. Reaction of the tetramer of dimethyl acetylenedicarboxylate 3 with 1,2,5-triphenylphosphole gave an orange product with concomitant loss of a furan triester. NMR data confirmed that this was not a simple adduct, and examination of a crystal by X-ray established the structure as the dichloromethane complex of tetramethyl 1,9,10-triphenyl-1-phospha(V)tricyclo[5,2,1,05,10] deca-1,3,5,8-tetraene-2,3,4,6-tetracarboxylate 10. This orange product crystallized in the monoclinic space group P21/c (No. 14) with four molecules in the unit cell (a = 10.056(2) Å, b = 14.280(1) Å, c = 23.892(3) Å, α = 90.0°, β = 94.15(1)°, and γ = 90.0°). Refinement converged at R = 0.0683 (Rw = 0.0710) for 540 parameters using 3180 independent reflections with I > 3σ(I). The tetramer 3 did not react with 1,2,5-triphenylphosphole-1-oxide quickly, but after 9 months gave a white, probably polymeric, product.
Absolute rate constant measurements for the reaction of atomic hydrogen with acetone, 2-butanone, propionaldehyde, and butyraldehyde in aqueous solution
Authors: Mezyk Stephen P. , Bartels David M.
Published: May 2006

Arrhenius parameters for the reaction of hydrogen atoms with acetone, 2-butanone, propionaldehyde, and butyraldehyde in aqueous solution have been directly measured by electron paramagnetic resonance free induction decay (FID) attenuation measurements. For these compounds, absolute scavenging rate constants at 25 °C of (1.97 ± 0.10) × 106, (3.82 ± 0.32) × 107, (8.11 ± 0.47) × 107, and (1.14 ± 0.07) × 108 dm3 mol−1 s−1 with corresponding activation energies of 30.7 ± 1.3 (7.5–52.8 °C), 40.1 ± 0.7 (8.1–74.5 °C), 22.2 ± 0.9 (8.0–39.0 °C) and 23.8 ± 0.8 (5.4–67.7 °C) kJ mol−1 were measured, respectively. Competition kinetic measurements have established that greater than 95% of H atom reaction with 2-butanone is by abstraction.
The anomeric effect in anionic systems XHm-CH2-NH− and XHm-CH2-CH2−, with XHm = F, OH, NH2 and CH3
Authors: Scott Camilla , Grein Friedrich
Published: May 2006

Ab initio geometry optimizations were performed at the RHF/6-31 + G* level on anionic acetal-like systems of the form XHm-CH2-NH− and XHm-CH2-CH2−, with XHm = F, OH, NH2, and CH3. Using Fourier analysis combined with energy decomposition methods, the electronic part of the anomeric stabilization, e, has been evaluated. For the NH− systems, [Formula: see text]was calculated to be −16.5 for F, −5.2 for OH, −2.7 for NH2, and −1.0 for CH3. For the CH2− systems the [Formula: see text] values are −8.1 for F, −5.3 for OH, −3.0 for NH2 and −1.2 for CH3. The units are kcal/mol. Comparing the trends in calculated energies, bond distances, and charges with those obtained for the model compounds CH2-NH and CH2-CH2, the anomeric effect can be understood as the tendency of the anion to form a π-bond in the CH2-NH− orCH2-CH2− portion of the molecule, caused by charge transfer from this portion to XHm. The CH3-CH2-NH− and CH3-CH2-CH2− systems form an exception.
Characterization of the capsular antigen of Streptococcus pneumoniae serotype 35B
Authors: Beynon Linda M. , Richards James C. , Perry Malcolm B. , Kniskern Peter J.
Published: May 2006

The specific capsular antigen of Streptococcuspneumoniae serotype 35B was shown by a combination of 2D NMR methods and mass spectrometric and classical carbohydrate chemical techniques to be a high molecular weight polymer containing D-galactose, D-glucose, 2-acetamido-2-deoxy-D-galactose, and ribitol (2:1:1:1). The pentasaccharide repeating unit is polymerized through phosphate diester linkages to give the structure,[Formula: see text]Seventy percent of the β-D-Galf residues glycosidically linked to the ribitol units carry an O-acetyl substituent. Keywords: Streptococcuspneumoniae, capsular antigen, polysaccharide structure.
Dehydrobromination of 1,1-diaryl-1,2-dibromoalkanes catalyzed by iron and iron bromides. An ion-pair-mediated reaction?
Authors: Suárez Angela R. , Suárez Alejandra G. , Martín Sandra E. , Mazzieri María R.
Published: May 2006

The participation of ion-paired Lewis acid/Lewis base in the dehydrohalogenations of 1,2-dibromo-1,1 -diarylalkanes catalyzed by Fe0 and by Fe(II) and Fe(III) bromides was investigated. The reactivities of the catalyst employed were correlated with their chemical hardness. The influence of para substituents on the phenyl rings of the substrates and the results obtained in Lewis-acid-catalyzed and thermal dehydrobromination of the same compounds were compared. The experimental results could not be explained in terms of a Lewis-acid-catalyzed dehydrobromination. Keywords: dehydrohalogenation, bromoalkanes, Lewis acid catalysts, ion-pair mechanism.
Electrochemical behavior of amidine hydrochlorides and amidines
Authors: Daoust Benoit , Lessard Jean
Published: May 2006

The electrochemical behavior of N,N-dimethyl-N′-phenylformamidine hydrochloride was studied on a platinum electrode. The oxidation peak at +0.90 V vs. Ag/Ag+ 0.01 M (in CH3CN −0.1 M LiClO4) was assigned to the oxidation of the chloride anion. N,N-Dimethyl-N′-(4-chlorophenyl)formamidine and N,N-dimethyl-N′-(2-chlorophenyl)formamidine were isolated from the preparative electrolysis of this amidine hydrochloride. The electrochemical behavior of N-phenylbenzamidine hydrochloride and N-phenylcyclohex-3-enecarboxamidine hydrochloride was also studied at platinum and at vitreous carbon. Cyclic voltammetry of a number of amidines was performed. Only N-arylbenzamidines showed a reduction peak at potentials less negative than −3.0 V vs. Ag/Ag+ 0.01 M (discharge potential of the medium at vitreous carbon). Preparative electrooxidations of all amidines studied were unsuccessful because of strong and rapid passivation of the anode. Keywords: amidine, electrochemical oxidation, chloride, amidine hydrochloride.
Experimental and AM1 calculational studies of the deprotonation of bicyclo[2.2.2]octane-2,5-dione and bicyclo[2.2.2]octane-2,6-dione: a study of homoconjugation, inductive, and steric effects on the rates and diastereoselectivities of α enolization
Authors: Werstiuk Nick Henry , Roy Chandra Deo
Published: May 2006

The kinetics of NaOD-catalyzed H/D exchange (enolization) at C3 α to the carbonyl group of bicyclo[2.2.2]octane-2,5-dione (1) and bicyclo[2.2.2]octane-2,6-dione (2) have been studied in 60:40 (v/v) dioxane–D2O at 25.0 °C. The second-order rate constants for exchange are (9.7 ± 1.5) × 10−1 and (3.4 ± 1.2) × 10−5 L mol−1 s−1 for 1 and 2, respectively. Thus, 1, exchanges 76 times faster than bicyclo[2.2.2]octan-2-one (3) (k = (1.27 ± 0.02) × 10−2 L mol−1 s−1), but the 2,6-dione 2 unexpectedly is much less reactive (2.7 × 10−3) than the monoketone. Unlike the large exo selectivity of 658 observed in the case of bicyclo[2.2.1]heptan-2-one, small and opposite selectivities, exo (1.2) for 1 and endo (2.1) for 2, are found for the isomeric [2.2.2] ketones. The results indicate that the incipient enolate of 1 is stabilized by a polar effect of the β carbonyl group at C5, not by homoconjugation. The source of the surprising low reactivity of 2 is unknown at this stage. The small diastereoselectivities, exo (1.2) for 1 and endo (2.1) for 2, correlate with relative energies of the diastereomeric pyramidal enolates calculated with AM1. Keywords: enolization, bicyclo[2.2.2]octane-2,5-dione, bicyclo[2.2.2]octane-2,6-dione, AM1, thermodynamic acidities.
The hydration of mesitylketene in aqueous solution: detection of acid catalysis for an aromatic ketene
Authors: Andraos J. , Kresge A.J. , Schepp N.P.
Published: May 2006

Mesitylketene was generated flash photolytically in aqueous solution by the photo-Wolff reaction of 2,4,6-trimethyldiazoacetophenone and also by rearrangement of mesitylynol obtained through photodecarbonylation of mesitylhydroxycyclopropenone, and rates of hydration of this ketene were measured in dilute perchloric acid, sodium perchlorate, and sodium hydroxide solutions as well as in concentrated sodium perchlorate and perchloric acid solutions. In dilute solution only an uncatalyzed reaction and a sodium-hydroxide-catalyzed process were observed, both of which could be attributed to nucleophilic attack, by water and by hydroxide ion, respectively, at the ketene carbonyl carbon atom. In concentrated sodium perchlorate solutions, a mild decrease in reaction rate with increasing salt concentration was observed, as expected on the basis of decreasing water activity and a consequent slowing of the uncatalyzed reaction. A similar mild decrease was found in perchloric acid solutions up to [Formula: see text] but this then gave way to a rate increase that became dominant above [Formula: see text] This appearance of acid catalysis indicates a change in reaction mechanism from nucleophilic attack of water to an electrophilic process involving rate-determining protonation on the β-carbon atom of the ketene group. Analysis of the acid-catalyzed reaction rate by the Cox–Yates method gives the catalytic coefficient [Formula: see text] This, when compared with [Formula: see text] for ketene itself, shows that the mesityl group retards acid-catalyzed hydration by a factor of 2200, and consequently the acid-catalyzed reaction of this, and other aromatic ketenes as well, becomes apparent only under strongly acidic conditions. Keywords: mesitylketene, ketene hydration, acid catalysis, Cox–Yates excess acidity correlation.
Synthesis of 5-substituted pyrrole-2-carboxaldehydes. Part I. Generation of formal 5-lithiopyrrole-2-carboxaldehyde equivalents by bromine–lithium exchange of 2-bromo-6-(diisopropylamino)-1-azafulvene derivatives
Authors: Berthiaume Sylvie L. , Bray Brian L. , Hess Petr , Liu Yanzhou , Maddox Michael L. , Muchowski Joseph M. , Scheller Markus E.
Published: May 2006

The first known lithiated 1-azafulvene derivatives, e.g., 8 and 13a, were generated by a low-temperature bromine–lithium exchange procedure with tert-butyllithium. These lithio species show substantial stability at ≤ −90 °C because, at these temperatures, the sterically demanding 6-diisopropylamino moiety, unlike the dimethylamino group, completely inhibits nucleophilic addition to C-6. At higher temperatures, the addition of tert-butyllithium to C-6 is significant and it can become the dominant process. The lithio species 8 is a useful formal equivalent of 5-lithiopyrrole-2-carboxaldehyde since, on reaction with electrophilic reagents and subsequent hydrolysis, a wide variety of regiochemically pure 5-substituted pyrrole-2-carboxaldehydes is formed. The 6-dialkylamino-1-azafulvenes described herein exist predominantly or exclusively as the syn conformer in solution at room temperature. This conformational preference is confirmed by a significance NOE effect between H-4 and H-6 in the parent diisopropylamino compound 3f. The origin of the syn conformational preference stems from a substantial contribution of the charge-separated form 16 to the ground state structure of these compounds, a phenomenon that is strongly supported by variable temperature NMR measurements on 2-bromo-6-diisopropylamino-1-azafulvene (3c). Keywords: 2-bromo-6-diisopropylamino-1-azafulvenes, stereochemistry, lithiation, pyrrole-2-carboxaldehydes.
Photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction. Part 9: methanol-2,6-dimethyl-1,6-heptadiene, and 1,4-dicyanobenzene
Authors: Connor Dennis A. , Arnold Donald R. , Bakshi Pradip K. , Cameron T. Stanley
Published: May 2006

The photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction of methanol, 2,6-dimethyl-1,6-heptadiene, and 1,4-dicyanobenzene yields three distinct types of 1:1:1 adducts: an acyclic product, 4-(1-methoxymethyl-1,5-dimethyl-5-hexenyl)benzonitrile (8, 5%); a cis–trans pair of cyclohexanes, 4-(3-methoxymethyl-1,3-dimethylcyclohexyl)benzonitrile (9cis (12%) and 9trans (11%)); and a cis–trans pair of cycloheptanes, 4-(4-methoxy-1,4-dimethylcycloheptyl)benzonitrile (10cis (12%) and 10trans (10%)). Variation in the concentration of the nucleophile, methanol, and codonor, biphenyl, affects the product ratio and it has been possible to establish the mechanisms for the formation of these products. The acyclic product is formed by a typical photo-NOCAS reaction, that is, addition (anti-Markovnikov) across one of the heptadiene double bonds. The cyclohexane products are formed following 1,6-endo cyclization of the intermediate β-alkoxy radical. And the cycloheptane products result from 1,7-endo,endo cyclization of the initially formed 2,6-dimethyl-1,6-heptadiene radical cation. Comparison of the relative rates of these cyclization processes can be made with those of the next smaller homolog, 2,5-dimethyl-1,5-hexadiene. Keywords: photochemistry, photoinduced electron transfer, radical ions, radicals, cyclization.
Aqueous nonelectrolyte solutions. Part XIII. Ice and hydrate freezing points of aqueous ethylene oxide solutions and the formula of congruent ethylene oxide hydrate
Authors: Glew David N. , Rath Norman S.
Published: May 2006

Ice freezing temperatures and hydrate formation temperatures have been measured by the dynamic cooling method for aqueous ethylene oxide (EO) solutions containing from 0 to 95 mol% EO. The ice and the congruent hydrate freezing temperatures exhibited standard errors on a single measurement of 0.004 °C and 0.013 °C, respectively. The ice–hydrate eutectic temperature was observed at −2.107 °C with standard error 0.001 °C and composition 1.991 mol% EO with standard error 0.008 mol% EO. The congruent hydrate was found to freeze at 11.083 °C with standard error 0.002 °C and composition 12.64 mol% EO with standard error 0.02 mol% EO. The formula of the congruent hydrate was EO•6.91H2O with standard error 0.013 mol water/mol EO. Only a single hydrate was found over the whole composition range down to −26 °C: the shoulder of the hydrate freezing curve above 40 mol% EO resulted from the high activity coefficients to dilute water in concentrated EO solutions. Equations and best values for the ice freezing temperatures and the hydrate formation temperatures together with their standard errors were evaluated by the method of least squares. Keywords: clathrate hydrate of ethylene oxide, freezing of water – ethylene oxide, ethylene oxide hydrate.
Extensions of the applicability of the MMX molecular modelling system to determination of barriers to rotation of π-bonded ligands
Authors: Polowin Joel , Poe Robert , Baird Michael C.
Published: May 2006

The commercially available molecular mechanics package PCMODEL, which has been shown to be very useful for determining conformational energy profiles for rotation of σ-bonded ligands, cannot be utilized to determine barriers to rotation of π-bonded ligands because of limitations in the way the π-bonding interactions are defined. This paper describes a partially successful modification of the dihedral driver of PCMODEL that makes possible reasonable calculations of the conformational energy profiles for rotation of the π-bonded arene ligands in the compounds (η6-arene)Cr(CO)2(PPh3) (arene = C6H6, C6Me6) and of the π-bonded ethylene ligands in [PtCl3(η2-CH2=CH2)]− and [(η5-C5H5)Re(NO)(PPh3)(η2-CH2=CH2)]+. The steric barriers to ethylene rotation in the latter two complexes were found to be small relative to the electronic barriers to ligand rotation. Keywords: molecular mechanics, olefin rotation, conformational energy profile.
Tautomérie du 2,5-dimercapto-1,3,4-thiadiazole et synthèse de thiadiazoloacridiniques
Authors: Amiel Pascale , Mahamoud Abdallah , Brouant Pierre , Galy Jean Pierre , Barbe Jacques , Karolak-Wojciechowska Janina , Posel Maciej
Published: May 2006

Some 2(3H)-thione-5-alkylthio-1,3,4-thiadiazoles were prepared with a view to arylating these compounds with 9-chloroacridines. Using pyridine as solvent and base, this arylation led to the 2-thione-(N)3-acridinyl-5-alkylthio-1,3,4-thiadiazoles. Molecular structures of the latter were determined either by NMR spectroscopy or by referring to X-ray crystallography of the 5-(diethylaminoethylthio)-1,3,4-thiadiazole-2-thione. Thus, with respect to the tautomeric equilibrium of the compound investigated, the thione group was detected either in the solid state or in solution. In contrast, 2-acridinylthio-5-alkylthio-1,3,4-thiadiazoles were prepared by using the sodium salt of 2-mercapto-5-alkylthio-1,3,4-thiadiazoles as starting material. Keywords: acridines, thiadiazoles, tautomerism.
The crystal and molecular structure of chlorotriphenyl(isoquinolinium-1-carboxylato)tin(IV)
Authors: Smith Frank E. , Khoo Lian Ee , Chia Lian Sai , Hynes Rosemary C.
Published: May 2006

The synthesis and crystal structure of the 1:1 complex formed between triphenyltin chloride and isoquinoline-1-carboxylic acid ([Ph3SnCl:C10H7NO2]) is described. The tin(IV) atom is found to be five coordinate, being bound to three phenyl groups, the chlorine atom, and an oxygen from isoquinoline-1-carboxylic acid. The geometry around the tin atom is that of a trigonal bipyramid, with the three phenyl groups occupying the equatorial positions, and the apical sites taken by the chlorine atom and a carboxyl oxygen from the isoquinoline-1-carboxylic acid. The crystals are triclinic, space group [Formula: see text] with a = 17.932(12) Å, b = 17.935(15) Å, c = 18.686(12) Å, α = 63.16(5)°, β = 81.70(6)°, γ = 68.45(6)°, V = 4986(6)(10) Å3Z = 8, and Dcalc = 1.486 Mg m−3 The final residual is RF = 0.050, on 8375 reflections with Inet > 2.5σ(Inet). Two of the cell dimensions are equivalent and there are four molecular units in the unit cell, so precautions are described for ensuring that correct space-group determination had been made and that some concealed symmetry in the final structure had not been overlooked. Keywords: triorganotin, complex, isoquinoline-1-carboxylic acid, fungicidal activity.
Electrochemistry of methylidyne and 1,2,3,5-dithiadiazole complexes of iron and nickel and a reinterpretation of the composition of "PhCN2S2Fe2(CO)6"
Authors: Boeré René T. , Moock Klaus H. , Klassen Vicki , Weaver Joel , Lentz Dieter , Michael-Schulz Heike
Published: May 2006

The title compounds have been studied in CH2Cl2 by cyclic voltammetry. Nonacarbonyldi(μ3-methylidyne)triiron, [Fe3(CO)9(μ3-CX)2] (X = F, Cl, Br, H), undergo chemically reversible 1e− reduction (−0.72 to −0.88 V vs. SCE) and irreversible oxidation (+1.59 to +1.72 V); the compound for which X = F displays a second, irreversible reduction (−1.96 V). Using reduction potentials of comparable μ3-E (E = S, Se, NPh, PPh) clusters, μ3-methylidyne ligands are shown to be strongly basic, comparable to PPh and NPh and more basic than S or Se. The methylidyne clusters are both more difficult to oxidize and reduce than isomeric alkyne clusters [Fe3(CO)9(C2R2)], indicative of greater thermodynamic stability. The complexes previously formulated as [Fe2(CO)6(S2N2CC6H4X)] (X = H, CF3, OCH3) are reversibly reduced (−1.59 to −1.70 V) and irreversibly oxidized (+1.0 to 1.3 V), unlike free dithiadiazole radicals, which are reversibly oxidized around +0.8 V. This behaviour is not consistent with the ring-centered free radical claimed for the case where X = H; instead we find conclusive evidence that these are diamagnetic complexes of the 3-H-1,2,3,5-dithiadiazolines, [Fe2(CO)6(S2N{NH}CC6H4X)] (νNH = 3376−3381 cm−1;δNH = 6.55−7.1 ppm). Dicyclopentadienyl-dithiadiazoledinickel, [Ni2Cp2(S2N2CC6H5)], is both reversibly reduced (−0.79 V) and oxidized (+0.45 V); and irreversibly oxidized at +1.9 V. This is consistent with an unpaired electron, and only the nickel cluster has an ESR signal (g = 2.0410, no resolved hyperfine splitting). Extended Hückel theory locates the free electron in a SOMO restricted to Ni, C, and S atoms. Keywords: electrochemistry, organometallic, methylidyne, dithiadiazole, dithiadiazoline.
Syntheses and interfacial characterization of graft copolymers from styrene and 2-hydroxyethyl methacrylate that comprise either trunks or branches
Authors: Yoshida Hidekazu , Itsuno Shinichi , Ito Koichi.
Published: May 2006

Polystyrene (PS) and poly(2-hydroxyethyl methacrylate) (PHEMA) macromonomers with either p-vinylbenzyl or methacrylate end groups were prepared by living anionic polymerizations of styrene and 2-(trimethylsilyloxy)ethyl methacrylate, respectively, followed by appropriate end-capping. They were radically copolymerized with 2-hydroxyethyl methacrylate and styrene, respectively, to afford PS-g-PHEMA and PHEMA-g-PS, having PS trunks with PHEMA branches or the reverse architecture. 1H NMR spectra of the graft copolymers in deuterated methanol showed their micelle formation with PS and PHEMA chains as the core and the shell, respectively. In chloroform the reverse micelles were observed. Contact angle measurements of water on the graft copolymer films revealed that PS chains were generally dominant on the free surface. The films either cast from methanol or after immersion into water, however, were apparently wettable, indicating dominant PHEMA chains on the surface. Keywords: styrene, 2-hydroxyethyl methacrylate, graft copolymer, micelle formation, wettability.
New amphiphilic copolymers derived from hydrophobically modified vinyl saccharides
Authors: Klein Joachim , Kunz Markwart , Guderjahn Lutz
Published: May 2006

The synthesis and characterization of new amphiphilic copolymers derived from copolymerization of hydrophobically modified so-called vinyl saccharides with 1-vinyl-2-pyrrolidone are described. The vinyl saccharides were prepared in a two-step synthesis. In the first step hydrophobically modified saccharide derivatives were synthesized without protecting procedures by direct reductive amination of the reducing keto-disaccharide isomaltulose (Palatinose®) with n-dodecylamine. In the second step the resulting N-dodecylaminopolyols were coupled with allyl glycidyl ether to give polymerizable amphiphilic vinyl saccharides. The copolymerizations with 1-vinyl-2-pyrrolidone were performed as free radical aqueous solution polymerizations. The characterization of the copolymers was carried out by measuring the viscosity and light scattering in water as well as in trichloromethane, in addition to surface tension measurements and solubilization studies. The solution properties of the copolymers in water and trichloromethane differ remarkably. While the values for the intrinsic viscosities in both solvents are comparable, the weight-average molecular weights in water are much higher than those in trichloromethane, indicating the formation of aggregates in aqueous solution. The vinyl pyrrolidone copolymers are also very surface active, lowering the surface tension of water to as low as 30 mN/m. The combination of measurements of surface tensions and solubilization studies indicates that their hydrophobic association behaviour is formally comparable to those of low molecular weight surfactants. Keywords: vinyl saccharides, isomaltulose, hydrophobic association, amphiphilic polymers.
Synthesis of cyclic and acyclic imides, sulfonimides, andN-carbobenzyloxyamides by ketene trapping
Authors: MaGee David I. , Ramaseshan Mahesh , Leach James D.
Published: May 2006

A convenient and general method for the synthesis of cyclic and acyclic imides, sulfonimides, and N-benzyloxycarbonyl amides via the inter-and intramolecular trapping of ketenes has been developed. Heating the corresponding amido, sulfonamido, and carbamamido ethyl ethynyl ethers at 150 °C for 3 h produced the desired compounds in good to excellent yields. Keywords: ketenes, imides, sulfonimides, N-benzyloxycarbonyl amides.
Protonation of porphocyanine – a new expanded tetrapyrrolic macrocycle
Authors: Xie Lily Yun , Dolphin David
Published: May 2006

The porphocyanine nucleus contains six nitrogen atoms, four of which are theoretically capable of accepting a proton in acidic solution. However, only the monoprotonated and diprotonated forms have been observed. These two forms were characterized by their visible and fluorescence spectra. pKa values of 6.0 ± 0.1 and 4.4 ± 0.1 were determined by spectrophotometric titration. The monoprotonated monocationic form can be observed in methanol. Keywords: porphocyanine, protonation, octaethylporphocyanine, octaethylporphyrin.
A deuterium NMR study of guest molecular dynamics of acetone in two organic inclusion compounds
Authors: Sidhu Paul S. , Bell Jason , Penner Glenn H. , Jeffrey Kenneth R.
Published: May 2006

Deuterium nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation times (T1) are used to investigate the dynamics of the guest molecule, acetone, in tris(5-acetyl-3-thienyl)methane (TATM) and cyclotriveratrylene (CTV) inclusion compounds. 13C CPMAS powder NMR spectra were obtained for each clathrate, to verify inclusion. In acetone: TATM, the guest molecule is undergoing twofold reorientation about the CO bond, exchanging the two methyl groups. An activation energy of 20 (± 1.4) kJ/mol, for the two-site jump motion, was found, independently, from deuterium NMR spectra an T1 measurements. Acetone in CTV performs the same type of motion as acetone in TATM. Activation energies of 25.0 (± 3.2) kJ/mol and 24.1 (± 0.5) kJ/mol were determined using the same two techniques. both inclusion compounds, the rate of methyl rotation within the acetone molecule is greater than 108 Hz even at the lowest temperature measured (84 K). Analytical expressions for the spin-lattice relaxation time (T1), for a twofold jump, were derived. Calculated values of the effective quadrupolar coupling constant and T1min for the guests agree very well with the experimental data. The 84 K spectrum of acetone:TATM unexpectedly shows some asymmetry, the origin of which is discussed. Finally, these two clathrates are compared to the recently examined acetone: tri-ortho-thymotide inclusion compound. Key words: inclusion compounds, deuterium NMR, solid state NMR spectroscopy, molecular dynamics.
Molar excess volumes of binary liquid mixtures: 2-pyrrolidinone with C6–C10n-alkanols
Authors: García Begoña , Hoyuelos Francisco J. , Alcalde Rafael , Leal José M.
Published: May 2006

The excess volumes VE of the binary mixtures 2-pyrrolidinone–(C6–C10) n-alkanols have been calculated from density measurements over the whole composition range and the 298.15–318.15 K temperature range. The excess volumes were independent of temperature, but changed noticeably with the chain length of the alkanol. The VE values were only positive starting from heptanol, with the maximum value, 0.239 cm3 mol−1, for equimolar decanol. The observed changeover VE < 0 to VE > 0 suggests that the steric effect is primarily responsible for the positive contributions to VE. The thermal expansion coefficients α were evaluated from the variation of densities with temperature. Key words: excess volumes, 2-pyrrolidinone, n-alkanols, liquid mixtures, hydrogen bonding.
Peptaibol metabolites of Tolypocladium geodes
Authors: Tsantrizos Youla S. , Pischos Sotiria , Sauriol Françoise , Widden Paul
Published: May 2006

Three antibiotic peptides, LP237-F8 (1), F5 (2), and F7 (3), were isolated from the liquid culture of the fungus Tolypocladium geodes. Chemical shift assignments of the 1H and 13C NMR resonances and sequencing of these metabolites were achieved by extensive high-field 2D NMR spectroscopy. The N-terminal of peptides 1 and 2 is protected with an octanoyl (Oc) fatty acid unit, whereas that of peptide 3 is protected with a decanoyl (Dec) unit. The C-terminal of all three peptides is protected with the amino alcohol leucinol (Lol). All three metabolites contain the common amino acids Ala, Phe or Tyr, Pro, and Gln, as well as the unusual amino acid α-aminoisobutyric acid (Aib). In addition, peptides 1 and 2 contain the amino acid α-amino-α-ethyl-n-pentanoic acid (α-ethylnorvaline, EtNor), which has not been previously reported as a constituent of a natural product. Metabolites 1, 2, and 3 are new members of the class of natural products known as peptaibols. Key words: Tolypocladium geodes, peptaibols, leucinol, α-amino-α-ethyl-n-pentanoic acid.
The effect of ortho and para substituents on the formation of the E and Z isomers of the arylhydrazones obtained from diazonium coupling with methyl 3-aminocrotonate and 3-aminocrotononitrile
Authors: Jollimore Jason V. , Vacheresse Marc , Vaughan Keith , Hooper Donald L.
Published: May 2006

Reaction of arene diazonium salts with 3-aminocrotononitrile or methyl 3-aminocrotonate affords the 2-aryl-hydrazono-3-oxobutanenitrile (1 or 3 and 3′) or the methyl 2-arylhydrazono-3-oxobutanoate (2 and 2′ or 4 and 4′). A series of these hydrazones has been prepared with a range of electron-withdrawing and -donating substituents in the ortho or para position of the aryl moiety. The hydrazones have been characterized by spectroscopic methods, with emphasis on the 1H NMR spectra, which have been used to determine the configuration of the hydrazones as E or Z or a mixture of the two. The para-substituted hydrazononitriles (1) are formed as a single species, namely the Z isomer, whereas the ortho isomers are formed as a mixture of E and Z configurations (3 and 3′). The hydrazonobutanoates (2 and 2′ or 4 and 4′) are formed as E/Z mixtures regardless of the position of the substituent in the aryl moiety. Complete assignments of all signals in the 1H NMR spectra have been made on the basis of the ability of the various substituents to participate in intramolecular hydrogen bonding and a mechanism is proposed to account for the variations in the proportions of E and Z isomers and the effect of the nature of the substituent on this ratio. 13C NMR spectra of selected hydrazones have been recorded as an aid to structure assignment. Key words: hydrazone, diazonium, NMR spectroscopy, E/Z isomers, crotonic acid derivatives.
2-Azabuta-1,3-diene-4-carbonitriles: stereoselective synthesis and nucleophilic substitution at the carbon–nitrogen double bond
Authors: Lorente Antonio , Casillas Marta , Gomez-Sal Pilar , Manzanero Antonio
Published: May 2006

The synthesis of (E)-1-methoxy-2-azabuta-1,3-diene-4-carbonitriles was performed by methylation of N-alkenylamides 9 and 11. The Z isomers were obtained by treatment of (E)-1-methylthio-2-azabuta-1,3-diene-4,4-dicarbonitriles with sodium methoxide in methanol. We also describe the reactions of (E)-1-methylthio-2-azabuta-1,3-diene-4,4-dicarbonitriles with pyrrolidine, which afforded 1-(1-pyrrolidinyl) derivatives 20, 21, and 23. X-ray crystallographic analyses of 21 and 23 established the E stereochemistry of the C—N double bond. Key words: 2-azabuta-1,3-diene-4-carbonitriles: stereoselective synthesis, nucleophilic substitution and X-ray diffraction; N-alkenylamides: methylation.
Quantum yield of singlet dioxygen and elucidation of the quenching mechanism for the reaction between the doublet excited states of tris(bipyridine)chromium(III) and triplet dioxygen
Authors: Tiyabhorn Acharee , Zahir K. Omar
Published: May 2006

The formation of singlet dioxygen (1O2) in the reaction between the doublet excited states of tris(bipyridine)chromium(III) and triplet dioxygen in deuterium oxide was detected using near-infrared luminescence corresponding to singlet dioxygen decay. In addition, luminescence intensity measurements were used to determine the quantum yield of singlet dioxygen produced. A value of 0.86 ± 0.08 was obtained for the yield of 1O2. Based on these quantum yield measurements, it is proposed that the major quenching mode is energy transfer quenching. However, considering that the 1O2 yield is less than unity and electron transfer quenching is not a viable pathway, it is proposed that, in addition to the energy transfer quenching, physical quenching involving spin-catalyzed deactivation of Cr(bpy)33+ may also be operative as a minor quenching pathway. Key words: Cr(bpy)33+, singlet dioxygen, quenching, quantum yield.
The photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction. Part 11: Involving (R)-(+)-α-terpineol and (R)-(+)-limonene, substituting on 1,4-dicyanobenzene
Authors: Arnold Donald R. , Connor Dennis A. , McManus Kimberly A. , Bakshi Pradip K. , Cameron T. Stanley
Published: May 2006

Irradiation of an acetonitrile–methanol (3:1) solution of 1,4-dicyanobenzene (1), biphenyl (5), and (R)-(+)-limonene (21) leads to formation of the 1:1:1 (methanol:21:1) photo-NOCAS adducts: 4-[(1R,2S,4R)-4-isopropenyl-2-methoxy-1-methylcyclohexyl]benzonitrile (23, 30%), 4-[(1S,2R,4R)-4-isopropenyl-2-methoxy-1-methylcyclohexyl]benzonitrile (24, 2%), 4-[(1R,2R,5R)-5-isopropenyl-2-methoxy-2-methylcyclohexyl]benzonitrile (25, 3%), and 4-[(1S,2S,5R)-5-isopropenyl-2-methoxy-2-methylcyclohexyl]benzonitrile (26, 1%). When an acetonitrile solution (no added methanol) of 1,4-dicyanobenzene (1), biphenyl (5), and (R)-(+)-α-terpineol (22) was irradiated under these conditions, the products were the cyclized 1:1 (22:1) photo-NOCAS adducts: (1R,2S,5R)-2-(4-cyanophenyl)-2,6,6-trimethyl-7-oxabicyclo[3.2.1]octane (27,21%) and (1S,4R,6R)-6-(4-cyanophenyl)-1,3,3-trimethyl-2-oxabicyclo[2.2.2]octane (28, 2%). Structural assignments were based primarily upon detailed analysis of 1H and 13C nmr spectra and, for four of the products (24, 26, 27, and 28), structures were firmly established by X-ray crystallography. The mechanism for the formation of these products is discussed, with emphasis on the intramolecular reactions of the intermediate alkene radical cations. Molecular mechanics (MM3) calculations gave information regarding the structure and energy of the conformers of 21 and 22 that was useful for predicting/explaining the observed reactivity on the basis of approach vector analysis; the transition state for cyclization incorporates the nucleophile and the alkene radical cation carbon atoms at the vertices of an obtuse triangle orthogonal to the plane of the π-system. Key words: photoinduced electron transfer, radical cations, cyclization, terpenes.