bluuebunny/crossref_metadata_2025 huggingface dataset:
Kinetics and mechanism of the formation of N-vinyl pyridinium cations in elimination reactions in aqueous base
Authors: Bunting John W. , Toth Andrea , Kanter James P.
Published: May 2006

The rates of the elimination reactions of N-(2-bromoethyl) pyridinium cations (1) and N,N′-ethylene bispyridinium dications (3) to give the corresponding N-vinyl pyridinium cations (2) have been measured spectrophotometrically in basic aqueous solutions (ionic strength 0.1, 25 °C) for a variety of substituents in the pyridine rings of each of these classes of pyridinium cation. The reaction kinetics are first order in 1 or 3 and first order in hydroxide ion. Brønsted-type plots of the second-order rate constants (kOH) as a function of the basicity (as pKBH) of the corresponding substituted pyridine are nonlinear for each of 1 and 3 and can be interpreted in terms of E1cb reaction mechanisms. For 1, the Brønsted-type plot displays two distinct "concave down" linear regions; rate-determining deprotonation for pKBH < 5.16 (slope = −0.30), and a change in rate-determining step to bromide ion departure for pKBH > 5.16 (slope −0.58). For 3, the Brønsted-type plot appears to be smoothly curved for symmetrically disubstituted bispyridinium dications, as a consequence of the multiple substituent effects upon each step of the E1cb reactions of these dications. However, log kOH for 3 is a smooth linear function of the previously reported log kOH for the E1cb reactions of N-(2-cyanoethyl) pyridinium cations over a range in which a change in rate-determining step has been directly demonstrated for these latter cations. Thus a change in rate-determining step as a function of pyridine basicity is also required within the E1cb mechanism for 3. The E1cb reactions of 1 are approximately 104-fold faster than the corresponding hydroxide ion catalyzed E2 eliminations from 2-phenylethyl bromides that are isoelectronic with 1.
Phosphorus-31 chemical shieldings in a fused cis-1,2,3-benzothiadiphosphole: a dipolar NMR study
Authors: Wu Gang , Wasylishen Roderick E. , Power William P. , Baccolini Graziano
Published: May 2006

Phosphorus-31 NMR static powder spectra and high-resolution magic angle spinning spectra have been obtained for a new heterocyclic compound, cis-2,10-dimethyl[1,2,3]benzothiadiphospholo[2,3b][1,2,3]benzothiadiphosphole (1), which contains a P(III)—P(III) single bond. The homonuclear 31P–31P dipolar interaction manifests itself in both the magic angle spinning spectra and the non-spinning line shape. Under the AX spin pair approximation, analysis of the spinning sidebands in the MAS experiment yields a full characterization of the two 31P chemical shielding tensors. This approximation is confirmed by the exact powder line shape simulation for a homonuclear spin pair. Analysis of the dipolar subspectra also yields the absolute sign of 1J(P,P), which is found to be negative. Keywords: phosphorus–phosphorus single bond, chemical shielding tensors, dipolar NMR, MAS, static line shape.
Syntheses and antibacterial activity of novel 6-fluoro-7-(gem-disubstituted piperazin-1-yl)-quinolines
Authors: Chu Daniel T. W. , Claiborne Akiyo K. , Clement Jacob J. , Plattner Jacob J.
Published: May 2006

A series of quinoline and naphthyridine antibacterial agents possessing an acyclic or cyclic gem-disubstituted piperazine substituent at the C-7 position have been prepared and evaluated in vitro and in vivo for antibacterial activity against a variety of Gram-positive and Gram-negative organisms. They are, however, not as active as quinolones or naphthyridines with a monosubstituted piperazine substituent at C-7. The chemical synthesis of these derivatives is also described.
Universal Gaussian basis functions in relativistic quantum chemistry: atomic Dirac–Fock–Coulomb and Dirac–Fock–Breit calculations
Authors: Malli G. L. , Silva A. B. F. Da , Ishikawa Yasuyuki
Published: May 2006

Matrix Dirac–Fock–Coulomb and Dirac–Fock–Breit self-consistent field calculations are performed for a number of neutral atoms. He (Z = 2) through Xe (Z = 54), using the universal Gaussian basis set (18s, 12p, 11d) reported recently by Da Silva etal. The total Dirac–Fock–Coulomb, the Dirac–Fock–Breit, and the Breit interaction energies calculated with this universal Gaussian basis set are in good agreement with the corresponding values obtained by using an extensive well-tempered Gaussian basis set for the He through Ca (Z = 20) atoms. Although this universal Gaussian basis set is inadequate for the calculation of total Dirac–Fock–Coulomb and Dirac–Fock–Breit energies for the Kr, Sr, and Xe atoms, the Breit interaction energies calculated with this basis for these three atoms are in very good agreement with the corresponding Breit interaction energies obtained by using the extensive well-tempered Gaussian basis sets. Work is in progress to generate a more extensive and energetically better universal Gaussian basis set for He through Xe for its use in non-relativistic Hartree–Fock as well as Dirac–Fock self-consistent field calculations on polyatomics involving heavy atoms.
A study of the nonstationary character of the methanol-to-hydrocarbons conversion
Authors: Kanazirev V. , Tsoncheva T.
Published: May 2006

An important characteristic of the methanol-to-hydrocarbons reaction is its autocatalytic behaviour under certain conditions. In this paper, self-acceleration in the conversion of MeOH over various zeolites, e.g., H-ZSM-5, H-CaY, H-offretite, and H-mordenite, in a flow reactor is examined. Self-acceleration is considered as a step of the nonstationary state changes taking place as a result of the interaction between the reaction medium and the catalyst. The effect on this process of factors such as partial pressure of methanol and the presence of hydrogen in the reaction mixture are investigated. Conclusions regarding the mechanism of C—C bond formation are considered. The course of the nonstationary process may be used for preliminary selection of catalysts for synthesis of hydrocarbons from methanol.
Phosphonolipids. 3. Phosphonic acid analogues of phosphatidylinositol and related materials
Authors: Pratt Clifford , Liu Yue-Jin , Chu Ting-Yi , Melkonian Karin , Tropp Burton E. , Engel Robert
Published: May 2006

A convergent synthesis of an isosteric phosphonic acid analogue of phosphatidylinositol has been accomplished in which a non-hydrolyzable P—C—C linkage is present in place of the normal P—O—C esteric linkage joining the phosphate and diacylglycerol portions of the molecule. The synthetic route used provides the configuration at each stereogenic center to correspond to that present in the biologically generated phospholipid. In addition, the approach provides asymmetric introduction of acyl functions, placing saturated and unsaturated acyl groups in the terminal and internal positions respectively of the backbone portion of the analogue, corresponding to that present in the biologically generated phospholipid.
Theoretical studies applicable to the design of novel anticonvulsants: an AM1 molecular orbital structure–activity study of dihydropyridine calcium channel antagonists
Authors: Bikker Jack Andrew , Weaver Donald Fredric
Published: May 2006

The development and synthesis of anticonvulsant new chemical entities that are distinct from the cyclic ureides in current clinical use is a continuing neuropharmacologic priority. The design of neuronal specific dihydropyridines active at the L-type calcium channel protein represents a rational approach to this design problem. To provide the structural data required for the design of anticonvulsant dihydropyridines, an AM1 semi-empirical molecular orbital study has been undertaken. Forty-six dihydropyridine calcium channel antagonists have been fully optimized at the AM1 level. Each of the 46 analogues was considered in six conformations to provide a systematic evaluation of changes in ester and phenyl ring orientation. The calculational validity of the AM1 Hamiltonian when applied to dihydropyridines was demonstrated by comparing AM1-optimized structures to experimental (X-ray crystallographic) and abinitio molecular orbital (STO 3G basis set) geometries. For each dihydropyridine, 79 AM1-derived geometric and electronic descriptors were obtained. The resulting descriptor matrix comparing structural descriptors with biological activity was statistically reduced to provide regression and discriminant structure–activity models for dihydropyridine calcium channel antagonism.
Study of the aqueous reactions of K[Pt(DMSO)Cl3] with nitrile ligands and crystal structures of Cl(DMSO)Pt(μ-C4H8NO)2Pt(DMSO)Cl and Cl(DMSO)Pt(μ-C5H10NO)2Pt(DMSO)Cl
Authors: Rochon Fernande D. , Boughzala Radia , Melanson Robert
Published: May 2006

The aqueous reactions of K[Pt(DMSO)Cl3] with nitriles were studied in different conditions of pH. At normal pH (≈2.4), the ligands RCN, where R is an alkyl group, produced the yellow complexes Pt(DMSO)(R-CN)Cl2. Infrared spectroscopy suggested cis isomers. The 195Pt NMR spectra of the compounds show a resonance around −3000 ppm. The same reactions in neutral medium (pH adjusted to ~7 with NaOH) produced acetamido-bridged dimers (DMSO)ClPt(μ-RCONH)2Pt(DMSO)Cl with all the ligands studied: propionitrile, butyronitrile, dimethylacetonitrile, trimethylacetonitrile, fluoroacetonitrile, chloroacetonitrile, bromoacetonitrile, and dibromoacetonitrile. The results of 195Pt NMR spectroscopy show a head-to-tail isomer, since only one signal around −2750 ppm is observed for each complex. These dimeric compounds are all blue or green depending on the ligand. The same dimers are obtained with all the haloacetonitrile ligands when the pH is not adjusted (≈2.4). The crystal structures of Cl(DMSO)Pt(μ-C4H8NO)2Pt(DMSO)Cl (1) and Cl(DMSO)Pt(μ-C5H10NO)2Pt(DMSO)Cl (2) were determined. The results confirm that the dimers are head-to-tail isomers with the DMSO ligands located in trans position to the O-acetamido atoms. The two crystals belong to the Pna21 space group with a = 11.112(4), b = 20.414(9), c = 10.161(4) Å for 1 and a = 11.607(6), b = 20.113(11), and c = 10.641(5) Å for 2. The structures were refined to R = 0.054 for 1 and R = 0.056 for 2. The Pt—Cl bond lengths are 2.328(10), 2.293(10) Å for 1 and 2.317(9), 2.317(10) Å for 2, while the Pt—S bond lengths are 2.173(10), 2.193(10) for 1 and 2.180(9), 2.176(9) Å for 2. The Pt—O bonds vary from 1.91(2) to 2.08(2) and the Pt—N from 1.88(2) to 2.03(2) Å. The dihedral angle between the two Pt(II) planes is 36°.
Electrochemical studies on arylhydrazonomesoxalonitriles in nonaqueous media
Authors: Abou-Elenien G. M. , Ismail N. A. , Hassanin M. M. , Fahmy A. A.
Published: May 2006

The substituted arylhydrazonomesoxalonitriles and N-methyl phenylhydrazonomesoxalonitrile are studied electrochemically in benzonitrile (BN) employing different voltammetric and coulometric techniques. They are oxidized in a single irreversible one-electron transfer leading to a dimerization reaction. The reduction process occurs in two irreversible one-electron uptakes followed by proton abstraction from the solvent to give the saturated azomethine linkage.
An efficient and general synthesis of alkyl alkynyl sulfoxides
Authors: Kabanyane Sidima T. , MaGee David I.
Published: May 2006

A convenient one-step method for the preparation of alkynyl phenyl sulfides using diphenyl disulfide has been developed. The oxidation of the corresponding sulfides to the sulfoxides, and not sulfones, was achieved by using trans-(phenylsulfonyl)-3-phenyloxaziridine. A variety of other oxidizing reagents was also investigated.
Fused heterocycles derived from pseudoephedrine and ephedrine
Authors: Farfán Norberto , Santillan Rosa L. , Castillo Dolores , Cruz Reyna , Joseph-Nathan Pedro , Daran Jean-Claude
Published: May 2006

The formation of heterobicycles by condensation of (1R, 2R)-(−)-pseudoephedrine or (1R, 2S)-(−)-ephedrine with glyoxal is reported. In the case of pseudoephedrine, the nuclear magnetic resonance data of the crude reaction mixture indicate the presence of three isomeric compounds (5, 6, and 7), which were separated by fractional crystallization, while ephedrine afforded the oxazino-oxazine 8 and the known (5S*,6R*)-4,5-dimethyl-6-phenyl-1,4-oxazin-2-one (9). Conclusive spectroscopic evidence for the structures of the new compounds having the cis-[1,4]oxazino[3,2-b]-1,4-oxazine (5 and 8) and the 2,2′-bioxazolidine type structures (6 and 7) was obtained by measurement of the 13C satellite coupling constants in the 300 MHz 1H nmr spectra, combined with the evaluation of dihedral angles using a modified Karplus type relationship. Unequivocal 1H and 13C nmr spectral assignments for all new compounds were done using two-dimensional carbon–proton correlation experiments. Syntheses of the known 5a,6,11a,12-tetrahydro[1,4]benzoxazino[3,2-b]benzoxazine (3a) whose crystal structure has been previously determined, as well as of its N,N′-diisopropyl derivative 3b, allowed us to corroborate the above observations for the six-membered ring heterocycles. Moreover, the structures of 3b and 7 were independently verified by single crystal X-ray diffraction studies, thus validating the structural conclusions deduced from nmr satellite multiplicity measurements.
The effect of Lewis acids on the intramolecular Diels–Alder reaction of the furan diene
Authors: Rogers Christine , Keay Brian A.
Published: May 2006

A general method is described for effecting the intramolecular Diels–Alder reaction of the furan diene in which the side arm connecting the diene to the dienophile contains four carbon atoms. The use of 1.1 equivalents of methylaluminum dichloride at −78 °C for 2–8 h shifts the Diels–Alder equilibrium towards the products and provides the oxatricyclo adducts in good to excellent yield. Catalytic quantities of methylaluminum dichloride (10 mol%) provided a higher quantity of adduct than excess Lewis acid when the enone was substituted with alkyl groups. The scope was extended to include a precursor containing a five carbon atom side arm, and two examples containing acetylenic dienophiles that were activated by a carbonyl moiety on the side arm. Precursors having a four carbon atom side arm provided only oxatricyclo adducts having the side arm syn to the oxygen bridge. The assignment of the stereochemistry of the oxatricyclo adducts is discussed in detail.
Vinyl ether hydrolysis XXVIII. The mechanism of reaction of methyl α-(2,6-dimethoxyphenyl)vinyl ether
Authors: Jones J. , Kresge A. J.
Published: May 2006

The acid-catalyzed hydrolysis of methyl α-(2,6-dimethoxyphenyl)vinyl ether in aqueous solution at 25 °C occurs with the hydronium ion catalytic coefficient [Formula: see text] and gives the solvent isotope effect [Formula: see text] this indicates that reaction occurs by rate-determining proton transfer from the catalyst to the substrate to generate an alkoxycarbocation intermediate. An oxygen-18 tracer study shows further that, despite the steric hindrance provided by its two ortho substituents, this cation then reacts by addition of water to the cationic carbon atom to generate a hemiacetal, and not by nucleophilic attack of water on the methyl group remote from the carbocationic center:[Formula: see text]
Enantioselective synthesis of epoxides via Sharpless epoxidation of alkenylsilanols
Authors: Chan T. H. , Chen L. M. , Wang D. , Li L. H.
Published: May 2006

Enantioselective synthesis of simple epoxides can be achieved by Sharpless epoxidation of alkenylsilanols followed by protodesilylation of the chiral epoxysilanols. The approach has been applied to the synthesis of frontalin.
Synthesis, NMR spectroscopy, and crystal structure of [9](N6,9)-6-aminopurinophane
Authors: Capretta Alfredo , Hunter Howard N. , Frampton Christopher S. , Bell Russell A.
Published: May 2006

[9](N6,9)-6-Aminopurinophane, 9, was synthesized by Mitsunobu coupling of 9-azidononanol, 5, and 6-chloropurine, 6. Reduction of the azide allowed for intramolecular nucleophilic displacement of chloride by the resultant amine and cyclization to the cyclophane. Variable temperature proton NMR showed the presence of two conformers below −25 °C separated by an activation barrier having a ΔGc≠ = 50.2 ± 2.5 kJ mol−1. The conformers arose from partial rotation about the C6—N6 bond, with the anti conformation assigned to the major isomer, 9a, and the syn conformation to the minor, 9b. The crystal structure of the [9](N6,9)-6-aminopurinophane, C14H21N5, was determined at 173 K. The crystals are monoclinic, of space group P21/n, with a = 16.941(3) Å, b = 8.512(2) Å, c = 19.300(2) Å, β = 95.90(1)°, V = 2769(1) Å3,Dc = 1.24 g cm−3 Dm = 1.27 g cm−3, for Z = 8, λ(CuKα) = 1.540598 Å. R1 = 0.0364, R2 = 0.0395 for 2846 reflections, ((R1) = 0.0319, R2 = 0.0365 for 2556 reflections with 1 > 2.5σ(I)). The purine ring shows small but significant distortions from planarity. The N(1)—C(6)—N(6)—C(1′) fragment adopts an anti conformation, which is twisted by approximately 35° from the plane of the purine ring, and the exocyclic 6-nitrogen is partially pyramidalized. Such pyramidalization is consistent with the Bader–Wiberg theory of barriers to rotation about bonds.
Antibody–oligosaccharide interactions: the synthesis of 2-deoxy-α-L-rhamnose containing oligosaccharide haptens related to Shigella flexneri variant Y antigen
Authors: Hanna H. Rizk , Bundle David R.
Published: May 2006

A series of di- and trisaccharide glycosides based on the α-L-Rha(1 → 3)β-D-GlcNAc and α-L-Rha(1 → 3)α-L-Rha(1 → 3)β-D-GlcNAc elements have been synthesized to locate the minimal oligosaccharide determinant of the Shigella flexneri O-polysaccharide, which is built from a tetrasaccharide repeating unit, [ → 2) α-L-Rhap(1 → 2)α-L-Rhap(1 → 3)α-L-Rhap(1 → 3)β-D-GlcNAcp(1-]n. These compounds also serve to identify the carbohydrate surface of the Shigella antigen that interacts with a monoclonal antibody, currently the subject of crystallographic studies. Two strategies utilizing suitably protected glycals 1 and 19 were employed to obtain analogs bearing either terminal or glycosylated 2,6-dideoxy-α-L-arabino-hexopyranosyl (2-deoxy-α-L-rhamnopyranosyl) residues. N-Iodosuccinimide activation of the glycals in the presence of selectively protected mono- and disaccharide alcohols afforded 2-deoxy-2-iodo-α-L-rhamnopyranosides and these were ultimately reduced during deprotection stages to afford the desired functionality. Di-O-acetyl L-rhamnal 1 reacted with monosaccharides 2 and 7, and with disaccharide 11, to yield disaccharides 4 and 8, and trisaccharide 12, each bearing a terminal 2-deoxy-α-L-rhamnopyranosyl residue. The selectively protected 3-O-benzoyl-4-O-benzyl-L-rhamnal 19 was synthesized from L-rhamnal and used to prepared trisaccharide 22, which contained an internal 2-deoxy-2-iodo-α-L-rhamnopyranosyl unit. Removal of protecting groups gave the oligosaccharides 6, 10, 14, and 23. Oligosaccharides that contained a 2-deoxy-α-L-rhamnopyranosyl residue showed enhanced inhibitory power: in the case of trisaccharide 23 a 1.8 kcal mol−1 relative increase in free energy of binding compared to a larger pentasaccharide epitope, α-L-Rhap(1 → 2)α-L-Rhap(1 → 3)α-L-Rhap(1 → 3)β-D-GlcNAcp(1 → 2)α-L-Rhap-1 → OMe. These data suggest that the rhamnose O–2 hydroxyl of residue C points toward and has important interactions with binding site amino acids.
Characterization of the 2,2′-bipyridine chelates of lanthanum(III) in acetonitrile. A 1H, 13C, 17O, and 139La NMR study
Authors: Fréchette Monique
Published: May 2006

We have investigated the reaction between La(NO3)3•6H2O and 2,2′-bipyridine (bipy) in acetonitrile by means of 139La, 17O, 13C, and 1H NMR spectroscopy. The 139La spectra show up to three different sites with variable intensities attributed to solvated La(III) ions and to two complex species with one or two bipy moieties in the La(III) coordination sphere. 17O NMR indicates that during the complexation reaction, the coordinated water is totally displaced from the coordination sphere whereas the nitrate groups remain coordinated. Proton NMR data confirm the water expulsion. In addition, the proton chemical shifts of bipy, in comparison with the 13C NMR data, clearly show an anisotropic shielding when two bipy are coordinated to La(III). Therefore, the two bipy molecules are in close proximity in the coordination sphere. From the NMR results it has been shown that, during the complexation process, five different La(III) species are formed: [La(NO3)3(CH3CN)4], [La(NO3)3(CH3CN)3(H2O)], [La(NO3)3(bipy)(CH3CN)2], [La(NO3)3(bipy)(CH3CN)-(H2O)], and [La(NO3)3(bipy)2]. Each La(III) complex has a coordination member of 10, with NO3− and bipy acting as chelating ligands. Furthermore, the structure of the La(NO3)3(bipy)2 complex in solution shows similarities with the solid state compound. Finally, comparisons are made with the La(III) complexes previously obtained with 1,10-phenanthroline.
Activity coefficients of NaNO3 in (Mg, Ca, Sr, and Ba)(NO3)2 + H2O systems at 298 K by EMF methods
Authors: Smith Stephen N. , Sarada S. , Palepu Ramamurthy
Published: May 2006

The activity coefficients of NaNO3 in Mg(NO3)2, Ca(NO3)2, Sr(NO3)2 and Ba (NO3)2 were determined at constant total ionic strength of 0.1, 0.5, 0.75, 1.0, 1.5, and 2.0 mol kg−1 at 298 K using EMF methods. The experimental activity coefficients were analyzed using four different formalisms, namely, Reilly–Wood–Robinson, Scatchard, Pitzer, and Harned equations, and the interaction parameters were evaluated. Excess Gibbs free energy of mixing and trace activity coefficients were calculated and the results are discussed.
Preparation and characterization of the osmium cluster complexes H2Os3(CO)8L(EtFcPCMe) (L = CO or PEt2Fc) and H2Os3(CO)8[(η-C5H3PEt2)Fe(η-C5H4)]
Authors: Cullen William R. , Rettig Steven J. , Zheng Tu Cai
Published: May 2006

Thermolysis of Os3(CO)11(PEt2Fc) (Fc = (η-C5H4)Fe(η-C5H5)) in octane for 8 h affords H2Os3(CO)9(EtFcPCMe), 10, and H2Os3(CO)8[(η-C5H3PEt2)Fe(η-C5H4)], 9e, in 15 and 65% yield, respectively. Heating Os3(CO)12 and PEt2Fc in 1:2 molar ratio in xylene for 10 h gives two isomers of H2Os3(CO)8(PEt2Fc)(EtFcPCMe), 11, in about 40% yield. Two crystalline modifications of the complex 9e have been obtained from the same solvent: 9eA, triclinic, a = 15.653(1), b = 15.799(1), c = 11.257(1) Å, α = 90.388(7)°, β = 109.740(6)°, γ = 82.445(6)°, Z = 4 (two molecules per asymmetric unit), space group [Formula: see text]; and 9eB, monoclinic, a = 9.889(2), b = 13.632(3), c = 18.927(4) Å, β = 97.36(2)°, Z = 4, space group P21/n. The structures were solved by heavy atom methods and were refined by full-matrix least-squares procedures to R = 0.033 and 0.037 for 8131 and 5145 reflections with I ≥ 3σ(I), respectively.
The photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction (Part 4): methanol–olefins, methyl 4-cyanobenzoate
Authors: McMahon Kevin , Arnold Donald R.
Published: May 2006

Dicyanobenzene-1,4 (1) and -1,2 are known to undergo substitution upon irradiation, in the presence of an olefin, in acetonitrile–methanol (3:1) solution. The products are 1:1:1 (methanol:olefin:aromatic) adducts, substituted on the aromatic ring with loss of a cyano group. This reaction, referred to as the photo-NOCAS (nucleophile–olefin combination, aromatic substitution) reaction, has been shown to be fairly general with regard to the olefin and the nucleophile that can be incorporated. Less is known about the scope of the reaction incorporating other electron-withdrawing substituted aromatic molecules. The purpose of this study was to determine if methyl 4-cyanobenzoate (10) would also take part in this reaction, to form 4-substituted aromatic esters. Irradiation of acetonitrile–methanol solutions of 10 and olefins 2,3-dimethyl-2-butene (2) and 1-methylcyclohexene (5) gave cyclic imine esters, 11 and 13, respectively, instead of photo-NOCAS products. The photo-NOCAS products were obtained when the codonor biphenyl (4) was added to the irradiation mixture. Formation of the cyclic imine ester is attributed to excitation of the charge-transfer complex formed between 10 and the olefin. The addition of biphenyl (4) serves to generate the contact radical ion pair (CRIP) upon irradiation of the charge-transfer complex between 10 and 4. This CRIP can dissociate to the solvent-separated radical ions, the radical cation of 4 can accept an electron from the olefin, and the olefin radical cation can go on to give the photo-NOCAS products. Irradiation of a solution of 10 and 2 in nonpolar solvent (benzene) gave the oxetane, believed to arise from the exciplex. In addition to photo-NOCAS products from 10, 4-cyanophenylketones 17 and 23 are also formed by attack of the β-alkoxyalkyl radical at the carboxyl carbonyl. The differences in behaviour between 1,4-dicyanobenzene (1) and methyl 4-cyanobenzoate (10) under these reaction conditions are described and explained.
Aryl 1,5-dithia-2,4,6,8-tetrazocines. A synthetic, electrochemical, and structural investigation
Authors: Boeré René T. , Moock Klaus H. , Derrick Sean , Hoogerdijk William , Preuss Kathryn , Yip Judy , Parvez Masood
Published: May 2006

New preparative routes to the title compounds are reported in which the yields are substantially improved over previous methods. In one method, reaction of O2 with a mixture of XC6H4CN2S2+Cl− and Ph3Sb leads to the products in 40–60% yield. In another, the reaction of the same salts with silylated amidines XC6H4C{NSiMe3}N(SiMe3)2 in a 1:1 mol ratio leads to the same compounds in 18–65% yield. The syntheses of 3,7-(XC6H4)2{CN2S2N2C}, X = H, 4-CH3, 4-CH3O, 4-Cl, 4-CF3, and 3-CF3, are reported. Mixed derivatives were also prepared, but not isolated as pure compounds. The crystal structure of the 4-CF3 compound has been determined by X-ray diffraction; triclinic, [Formula: see text] with Z = 1; cell dimensions: a = 6.835(2), b = 9.936(3), c = 6.320(2) Å, α = 96.44(3)°, β = 102.88(3)°, γ = 88.42(3)°. R = 3.4%, Rw = 3.5% for 1100 data. The eight-membered ring is flat with equivalent bond lengths between any two types of atoms, indicative of a delocalized electron distribution. The 10π central aromatic rings have been oxidized and reduced in CH2Cl2 using voltammetric methods. The reversible one-electron reduction step gives stable 11π species. The resistance to oxidation of the title compounds is contrasted to the ease of oxidizing 3,7-[(CH3)2N]2{CN2S2N2C}, in accordance with earlier chemical studies.
Fractionation of woodmeal by prehydrolysis and thermal organosolv. Alkaline depolymerization, chemical functionality, and molecular weight distribution of recovered lignins and their fractions
Authors: Thring Ronald W. , Chornet Esteban , Overend Ralph P.
Published: May 2006

Thermal organosolv glycol lignins and their fractions have been characterized by means of elemental composition, molecular weight distribution, and 1H and 13C NMR spectroscopy. Fractionation of each lignin by sequential solvent extraction produced fractions of increasing molecular weight and polydispersity. Structures in the highest molecular weight fractions were found to be linked by a high proportion of β-O-4 type bonds, whilst the lowest molecular weight fractions consisted of a high content of saturated aliphatic side-chain structures. A noticeable difference in the content of carbohydrate contaminants in both starting lignins indicated the formation of relatively stable lignin–carbohydrate complexes, especially in the lignin recovered from pretreated wood. In addition, depolymerization of the lignins and their fractions to monomeric compounds was explored using alkaline hydrolytic and nitrobenzene oxidative routes. The highest molecular weight fractions from each lignin were identified as the best candidates for production of useful monomeric phenolic compounds.
Laser flash photolysis study of the photochemistry of ortho-benzoylbenzaldehyde
Authors: Netto-Ferreira J.C. , Scaiano J.C.
Published: May 2006

Ketene-enols 4 and 5 have been generated by laser flash photolysis of ortho-benzoylbenzaldehyde (3) and kinetically and spectroscopically characterized. In benzene or acetonitrile, the E ketene-enol, 4, shows absorption at 340 and 400 nm and a lifetime in excess of 1 ms, whereas the Z ketene-enol, 5, shows maxima at 360 and 430 nm and a lifetime of only 1.5 μs. At shorter time scales we observed a weak absorption (λmax = 580 nm) tentatively assigned to biradical 6 with a lifetime of 140 ns. The E ketene-enol is readily quenched by oxygen, dienophiles, methanol, and water, with quenching rate constants ranging from 3.6 × 103 M−1 s−1 (for methanol as a quencher) to 2.2 × 108 M−1 s−1 (for diethylketomalonate). At high water concentrations (typically > 10 M) a new species, 7, was detected showing maximum absoiption at 510 nm and a growth lifetime of 7 μs. In deuterated water and using the same concentration as before we observed a formation lifetime for 7 of 10 μs, which results in an isotope effect of ~ 1.5. It is proposed that 5 is the main precursor for 7. Steady-state irradiation of 3 in deaerated methanol leads to the formation of dihydroanthraquinone (9), a strongly colored and fluorescent (λmax = 475 nm, τn = 29 ns) species, whereas 3-phenylphthalide (2, R = Ph) is the main product when the irradiation is performed in benzene. Steady-state quenching of product formation by diethyl ketomalonate gives a Stern–Volmer constant of 380 M−1 from which we conclude that 5 is the ketene-enol responsible for product formation, in agreement with the laser flash photolysis results.
1992 Alfred Bader Award Lecture Probing the specificity of synthetically useful enzymes
Authors: Jones J. Bryan
Published: May 2006

Factors involved in controlling and predicting the structural specificity and stereospecificity of synthetically useful enzymes, including pig liver esterase, subtilisin BPN′ and Carlsberg, α-chymotrypsin, and an L-lactate dehydrogenase, are discussed.
Synthesis of functionalized hexahydro-, octahydro-, and decahydro-1H-phenalenes via Diels–Alder reactions of 1-methylene-4a-methoxycarbonyl-1,2,3,4,4a,5,6,7-octahydronaphthalene and related dienes
Authors: Piers Edward , Friesen Richard W. , Kao Paul , Rettig Steven J. , Trotter James
Published: May 2006

The results of a study of Diels–Alder reactions of the bicyclic dienes 6–8 with a variety of dienophiles are reported. Although 6 and 7 undergo cycloaddition reactions smoothly and efficiently, thermal Diels–Alder reactions of 8 are generally sluggish or, under the conditions investigated, do not proceed at all. Additions of tetracyanoethylene (TCNE) to 6–8 are highly face-selective, with preferential attack of the dienophile on the side of the dienes opposite to the angular methoxycarbonyl group. Reaction of 7 with maleic anhydride (MAN) is completely face-selective and proceeds preferentially via an endo transition state. Diels–Alder reactions of 6 and 7 with methyl acrylate (MAC) and nitroethylene (NE) are entirely regioselective, but the face-selectivities, which vary from ~2:1 to ~3:1, are rather low. The use of this chemistry as a method for the synthesis of functionalized, stereochemically defined, perhydro-1H-phenalenes is demonstrated.
Light-induced change of the molecular charge in a spironaphthoxazine compound
Authors: Rys Paul , Weber Ruth , Wu Qinglan
Published: May 2006

To evaluate the experimental conditions for the light-induced change of the molecular charge, the dependence of the photochromic reaction behaviour of the indolino spiro naphthoxazine compound 1,3,3-trimethyl-spiro[2H-indol 2,3′-[3H]naphth[2,1-b]-[1,4]oxazine] on the pH value of the solution is investigated. In the absence of UV light an acid–base equilibrium between the spiro form and a protonated closed form is established. By irradiation under appropriate acidic conditions the spiro form can be transformed into an open cationic form through the merocyanine form. Between the two open forms an acid–base equilibrium is established. Except for the closed protonated form the structures are confirmed by mean of NMR. The kinetics of the thermal ring-closing reaction of both forms are investigated by flash photolysis. It is shown that the ring-closing reaction proceeds exclusively via the merocyanine form. The pK values determined for both equilibria give the pH range, in which the light-induced change of the molecular charge occurs.
The synthesis of the isomeric components of San Jose scale pheromone — an illustration of a stereospecific synthesis of trisubstituted alkenes
Authors: Alderdice Margot , Spino Claude , Weiler Larry
Published: May 2006

The three isomeric components of the San Jose scale pheromone, 5–7, have been synthesized from a common β-keto ester intermediate. A study of the alkylation of the dianion of methyl acetoacetate with a series of alkylating agents with the same carbon skeleton has been carried out. The trisubstituted alkenes in 5 and 6 have been synthesized stereospecifically via a copper-catalyzed coupling of a methyl Grignard reagent with the E or Z enol phosphate from the alkylated β-keto ester. In the case of the Z enol derivative, the coupling reaction was carried out on the diethyl- and diphenylphosphates, and the enol triflate. The diethyl enol phosphate gave the highest stereoselectivity. The synthetic pheromones were attractive to San Jose scale in the field.[Formula: see text]
Local and framework stereochemical markers in vibrational circular dichroism: 1,2- and 2,3-dimethylaziridines
Authors: Yang Danya , Eggimann Thomas , Wieser Hal , Rauk Arvi , Shustov Gennadii
Published: May 2006

The vibrational circular dichroism (VCD) spectra of 2,3-dimethylaziridine 1 and 1,2-dimethylaziridine 2 were measured experimentally and interpreted with the help of ab initio Vibronic Coupling Theory (VCT) both in the common origin (CO) and distributed origin (DO) gauge. The absolute magnitudes of the VCD intensities by the VCT-CO method in the mid-IR range are closer to the experimental measurement. Both VCT-DO and VCT-CO methods, however, predict the same signs for almost all VCD bands. In general, both VCT-DO and VCT-CO methods predict VCD and IR spectra that are both in good agreement with the experimental spectra. The VCT method together with the 6-31G*(0.3) basis set can be employed with confidence in determining the absolute configuration of a chiral molecule. The theoretical and experimental VCD spectra of 1 and 2 were compared with the VCD spectra of the isostructural 2,3-dimethyloxirane and -thiirane. One of the Me rocking motions, in the wave number range 1050–1150 cm−1, was found to serve as a possible marker for the stereochemistry of the molecular skeleton. Two other modes were identified whose Cotton effects (CEs) are characteristic of the local asymmetry, and for which chirality rules were proposed. The first is a methyne vibration, ||CH, which is expected to have a (−) CE at a three-ring centre with (R) absolute configuration. The second is an umbrella motion of a methyl group, which is expected to have a (+) CE when attached to a three-ring centre with (R) absolute configuration.
Kinetics and mechanism of the oxidation of oxalate ion by tris(acetylacetonato)-manganese(III) and its hydrolytic derivatives in aqueous perchlorate media
Authors: Mukhopadhyay Subrata , Chaudhuri Swapan , Das Rina , Banerjee Rupendranath
Published: May 2006

In the pH range 6.6–8.6, [MnL2(H2O)2]+ and [MnL2(H2O)(OH)] (HL = acetylacetone) oxidize oxalate ion (ox2−) to CO2 through the inner-sphere intermediates [MnL2(ox)]− and [MnL2(OH)(ox′)]2−, where ox′ is a half-bonded (unidentate) oxalate ion. Their rate constants of decomposition are 1.0 × 10−4 s−1 and 11.2 × 10−2 M−1 s−1 at 30 °C and at I = 1.0 M (NaClO4). Decomposition of these mixed complexes produces free radicals, presumably CO2−, which is further oxidized to CO2 by another Mn(III) in a fast step. At pH 4.2, [Mn(ox)3]3− is produced quantitatively when [ox]0 ≥ 0.12 M, which has been characterized spectrally, and its unimolecular decomposition rate constant k (= 2.7 × 10−4s−1 at 30 °C and I = 1.0 M) compares well with that reported earlier (2.44 × 10−4 s−1 at 25 °C and I = 1.0 M).
Stereochemistry and chiroptical properties of 1,3-dialkylaziridinones (α-lactams). Chiral rules for the nonplanar amide chromophore
Authors: Shustov G. V. , Kachanov A. V. , Chervin I. I. , Kostyanovsky R. G. , Rauk A.
Published: May 2006

Structural features, configurational stability, and chiroptical properties of the nonplanar amide group in α-lactams were investigated by means of ab initio (6-31 + G*) molecular orbital calculations on (1R)-aziridinone 1, (1R)-1-methylaziridinone 2, (1R,3R)-3-methylaziridinone 3, (1R,3R)-1,3-dimethylaziridinone 4a, its cis diastereomer (1S,3R)-1,3-dimethylaziridinone 4b, and (1R,3R)-3-tert-butyl-1-methylaziridinone 5, and by experimental CD spectra of 1-tert-butyl- and 1-(1′-adamantyl)-substituted 3(R)-3-tert-butylaziridinones 6 and 7. The nitrogen inversion barriers of 4a and 4b are 2.8 and 1.6 kcal mol−1, respectively. The lowest excited singlet state of all of the aziridinones is a valence state (the nO–πCO* transition); the second is a Rydberg state (the nN–3s transition). The signs of the first and second Cotton effects in the CD spectra of the compounds 6 and 7 coincide with the calculated ones for 1 and 2 and the trans isomers 3, 4a, and 5. According to the calculated and experimental data for aziridinones 1–7 as well as to the well-known data for other nonplanar amides, the sign of the first Cotton effects is determined by the intrinsic chirality of the nonplanar amide chromophore and obeys a spiral rule. For cases where the chromophore has the conformation around the N—C(O) bond, which is close to the antiperiplanar, a reverse octant rule is proposed.
On the conformational preferences of the dehydrochlorination of α-chlorosulfoxides
Authors: Schwan Adrian L. , Roche Michael R. , Gallagher John F. , Ferguson George
Published: May 2006

Several acyclic α-chlorosulfoxides have been shown to undergo a γ-dehydrochlorination upon treatment with LDA. The proposed immediate products of γ-dehydrochlorination, thiirane-S-oxides, are unstable under the basic conditions and react further with the LDA; the isolated products are usually E-alkenes and (or) E-vinyl sulfoxides. Some of the proposed intermediate thiirane-S-oxides, compounds 6, 7, 8, and 18, were synthesized independently and treated with one equivalent of LDA in order to mimic the second step of the overall dehydrochlorination/ring opening sequence. The products obtained from the reactions of compounds 6 and 18 compared favourably with those products which were believed to arise from certain conformations of α-chlorosulfoxides 1kB and 1e, respectively. The addition of one equivalent of LDA to 1kA afforded a mixture containing thiirane-S-oxide 8, which is proposed as the immediate product of γ-dehydrochlorination of 1kA. The configurations of 1kB and 1hA were both shown to be threo by X-ray crystallographic studies. Those conformations which are preferred for the dehydrochlorination possess a geometry where the sulfinyl oxygen is anti to any of the substituents of the ring carbons.
New dihydrogen complexes: the synthesis and spectroscopic properties of iron(II), ruthenium(II), and osmium(II) complexes containing the meso-tetraphos-1 ligand
Authors: Bautista Maria Teresa , Earl Kelly Anne , Maltby Patricia Anne , Morris Robert Harold , Schweitzer Caroline Theresia
Published: May 2006

The synthesis and properties of dihydrogen complexes trans-[MH(H2)L]+, M = Fe, Ru, Os, which contain the ligand meso-tetraphos-1, S,R-Ph2PCH2CH2P(Ph)CH2CH2P(Ph)CH2CH2PPh2 (L) are described. There are interesting possibilities of isomerism in such trans complexes because the axial binding sites at the metal are different, one being surrounded by four phenyl groups and the other by two phenyl groups. The osmium complex is prepared in an unusual reaction of cis-β-Os(Cl)2L with H2 (1 atm) and NaBPh4 (1 mol) in THF or by the reaction of trans-OsH(Cl)L with NaBPh4 and H2. The iron and ruthenium complexes were made by a reaction of HBF4 with complexes trans-M(H)2L that have inequivalent trans hydrides. The ruthenium complex was also prepared starting from isomers of trans-RuH(Cl)L. The H—H distance in the rapidly spinning dihydrogen ligand has been calculated from T1(min) data to be 0.88, 0.89, and 0.99 Å for the complexes of Fe, Ru, Os, respectively. The presence of the H—D bond in the isotopomers trans-[MH(HD)L]+ and trans-[MD(HD)L]+ is also confirmed by the observation of 1JHD coupling constants of 32, 33.5, and 26.4 Hz for Fe, Ru, and Os, respectively. There is no rapid intramolecular H atom exchange in these complexes in contrast to those with di-tert-phosphine ligands like [MH(H2)(dppe)2]+ or to the trihydride Re(H)3L. Described also are the properties of the precursor complexes including cis-β- and trans-Ru(Cl)2L and derivatives of the dihydrogen complexes trans-[MH(L′)L]+, L′ = CH3CN (on Ru and Os), PMe2Ph (on Ru), and CO (on Os). Trends in the NMR properties of isostructural complexes are reported.
Infrared chemiluminescence studies of the reactions of H atoms with CCl3, CF2Cl, and CH2CH2Cl radicals at 300 and 475 K: recombination–elimination vs. abstraction mechanisms
Authors: Arunan E. , Rengarajan R. , Setser D.W.
Published: May 2006

The reactions of H atoms with CCl3, CF2Cl, and CH2CH2Cl radicals have been studied in a flow reactor at 300 and 475 K by observation of the infrared emission from the HCl and HF products. These reactions were observed as secondary reactions from the H + CCl3Br, CF2ClBr, and CH2Cl–CH2I chemical systems. The conditions in the flow reactor were controlled so that the nascent vibrational distributions of HCl and HF were recorded. The pattern of vibrational energy disposal to HCl was used to differentiate between Cl atom abstraction and recombination–elimination mechanisms. The H atom reactions with CCl3 and CF2Cl radicals occur only via a recombination–elimination mechanism and give HCl(υ) or HF(υ) in a unimolecular step. Thus, the Cl atom abstraction reactions must have ≥3.0 kcal mol−1 higher activation energy than the recombination reaction. From observation of the ratio of the HCl and HF products from CHF2Cl*, the difference in threshold energies for HF and HCl elimination was determined to be ∼13 kcal mol−1. On the other hand, Cl atom abstraction does compete with recombination–elimination in the H + CH2CH2Cl reaction, the branching fraction is ∼0.3 at 300 K and ∼0.6 at 475 K.
Electron spin resonance study of the reaction of hydrogen atoms with methane
Authors: Marquaire Paul-Marie , Dastidar Ashok Ghose , Manthorne Kim C. , Pacey Philip D.
Published: May 2006

The reaction: H + CH4 → CH3 + H2 has been investigated in a flow system between 348 and 421 K. Hydrogen atoms were generated in a microwave discharge, introduced to the reactor through a movable injector, and monitored by electron spin resonance. After an initial decay attributed to reaction with impurity, the hydrogen atom concentration decayed in a pseudo-first-order manner. Ethane was detected by gas chromatography, consistent with its formation by the following reaction: 2CH3 → C2H6. The amount of ethane formed at 421 K was only 0.015 times the amount of hydrogen atoms reacting. Most methyl radicals were assumed to have been removed by the process: H + CH3 + M → CH4 + M. Because of this process, two hydrogen atoms were removed each time the title reaction occurred. Applying this stoichiometric factor, the rate constant for the elementary reaction was calculated to be 2.5 × 103 L mol−1 s−1 at 348 K, increasing to 2.0 × 104 L mol−1 s−1 at 421 K. Most of the previous discrepancy between kinetics and thermochemistry has been eliminated; the exothermicity at 0 K was reduced to 0.8 ± 0.4 kJ mol−1, which corresponds to a standard heat of formation of the methyl radical of 145 kJ mol−1. Properties of the activation barrier have been inferred from the experimental data with the aid of transition state theory. The fitted barrier height was 63 ± 1 kJ mol−1, the average of five low-frequency vibrational term values was 640 ± 30 cm−1, and the characteristic tunnelling temperature was 500 ± 30 K.
Reaction dynamics studies of a simple tetraatomic system: AA + BA ↔ A + ABA
Authors: Koppe Stefan , Laurent Thomas , Naik Prakash D. , Volpp Hans-Robert , Wolfrum Jürgen
Published: May 2006

We have studied the reaction of translationally excited hydroxyl radicals with molecular hydrogen at different center-of-mass energies. H/D atoms produced in the reaction have been detected under single collision conditions by means of vacuum ultraviolet laser-induced fluorescence at the Lyman-α transition. By calibrating the H/D signals from the reaction against H atom signals from the H2S and HCl photolysis, respectively, the following absolute reactive cross sections were determined: OH + H2 → H + H2O: σR(0.17 eV) = (0.08 ± 0.03) Å2, σR(0.22 eV) = (0.60 ± 0.30) Å2; OH + D2 → D + HOD: σR(0.28 eV) = (0.22 ± 0.05) Å, σR(0.37 eV) = (0.43 ± 0.09) Å2. With translationally excited H atoms, we have also studied the reverse reaction system, H + D2O. In this case the OD product radicals were detected under single collision conditions with quantum state resolution by means of laser-induced fluorescence. By calibrating the OD signals from the reaction against OH signals from the H2O2 photolysis, absolute reaction cross sections were measured for H + D2O → OD + HD: σR(1.5 eV) = (0.07 ± 0.04) Å2, σR(1.8 eV) = (0.10 ± 0.03) Å2, and σR(2.2 eV) = (0.11 ± 0.03) Å2. At different center-of-mass collision energies nascent population distributions of the OD product fine-structure components were determined. It has been found that at all collision energies OD radicals are produced exclusively in their vibrational ground state, with only a small amount of the total available energy appearing in the rotational degree of freedom. A comparison of the dependence of the reaction cross section on the translational energy shows that relative reagent translation is more effective to promote reactivity in the reaction OH + H2 and OH + D2 than in the reaction H + D2O. This, together with a preferential population of the symmetric 2Π(A′) Λ-doublet state (with the unpaired π orbital lying in the plane of rotation) at high OD rotational quantum numbers, suggests that the reaction H + D2O → OD + HD proceeds through a planar transition state via a direct mechanism, where the OD moiety acts as a spectator, and where the reaction barrier is located at a "later" position of the reaction coordinate.
Vibrational specificity for charge transfer versus deactivation in N2+(υ = 0, 1, 2) + Ar and O2 reactions
Authors: Kato Shuji , Frost Michael J. , Bierbaum Veronica M. , Leone Stephen R.
Published: May 2006

Rate constants for charge transfer and vibrational deactivation in the N2+(X2Σg+, υ = 0, 1,2) + Ar and O2 reactions are directly measured by a state-resolved optical detection method. The novel, selected-ion flow tube, laser-induced fluorescence (SIFT–LIF) technique is used to study the vibrationally specific reactions at near-thermal collision energy. The total rate constant for N2+(υ = 1,2) + Ar increases by more than a factor of 40 relative to N2+(υ = 0). This enhancement is due exclusively to an increase in the charge transfer channel. The charge transfer rate constants for the N2+(υ) + Ar reaction are found to be almost identical for υ = 1 and υ = 2; this differs slightly from previous results at higher collision energies. The vibrational deactivation rate constant for the N2+(υ = 1) + Ar reaction is measured for the first time; the upper limit for the branching fraction is ≈3%, confirming that this reaction is a useful monitor for N2+(υ > 0). The total rate constant for N2+(υ = 1, 2) + O2 increases by factors of 2.6 and 3.3, respectively, relative to N2+(υ = 0). In contrast to the N2+ + Ar reaction, this enhancement is largely due to the occurrence of vibrational deactivation, which is found to be slightly faster for υ = 2 than for υ = 1. For N2+(υ = 2) + O2, the υ = 2 → 1 and υ = 2 → 0 vibrational deactivation channels are found to occur with comparable rates. The lack of substantial enhancement in the charge transfer channel in the N2+(υ) + O2 reaction by vibrational excitation (up to υ = 2) is in contrast to the observed translational enhancement, which opens a higher lying, endothermic O2+(a4Πu) product channel. These results are consistent with a short-range, curve-crossing mechanism that efficiently channels energy into the O2+(a4Πu) state.