摘要:

Decomposition of γ-unsaturated substituted peroxides in a good hydrogen donor solvent can lead to two diastereoisomeric oxetans. An intramolecular 1,5 hydrogen shift followed by a cyclization may compete with the homolytic intramolecular substitution following the first addition step. The factors governing this never-reported competition are determined.Keywords: unsaturated peroxides, oxetans, homolytic intramolecular substitution, intramolecular 1,5 hydrogen shift.

ISSN:0008-4042    

DOI:10.1139/v95-024

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Reaction of aryldiazonium salts with methyl 3-aminocrotonate (1) affords high yields of the methyl 2-arylhydrazono-3-oxobutanoates (4); analogous diazonium coupling with 3-aminocrotononitrile (2) gives the 2-arylhydrazono-3-oxobutanenitriles (5). The hydrazones are the product of diazonium coupling at the C2-vinylic carbon, concomitant with hydrolysis of the 3-amino substituent to the 3-oxo derivative; there is no evidence for the formation of a triazene (6), which would be the product of N-coupling. All hydrazones (4a–eand5a–d) have been fully characterized by IR and1H and13C NMR spectroscopy; the NMR spectra of the methyl 2-arylhydrazono-3-oxobutanoates (4) suggest the presence of two isomeric intramolecularly H-bonded forms in solution. Selected compounds were further characterized by elemental analysis and mass spectrometry. A mechanism is proposed for the conversion of1or2into4or5, and these observations are compared with previously reported observations of diazonium coupling reactions with unsaturated systems.Keywords: hydrazone, diazonium, aminocrotonate, aminocrotononitrile, hydrogen bonding.

ISSN:0008-4042    

DOI:10.1139/v95-025

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Electrochemical behaviour of the medicinally important 4-(4′-sulphonamoyl)hydrazono-1-phenyl-3-methyl-2-pyrazolin-5-ones has been studied at d.m.e. and glassy carbon electrodes. At d.m.e., all six compounds exhibited a single, well-defined, four-electron wave in the pH range 2.5–12.0. Polarographic four-electron wave was found to be diffusion-controlled and irreversible. Similarly, cyclic voltammetry of these compounds at glassy carbon electrode exhibited a single peak. Peak potential shows shifts towards negative potential with pH, with linear segments up to pH 8.2 and are practically pH independent at higher pH values. An anodic peak at far-off positive potential was observed in the reverse scan, indicating the irreversible nature of the electrode process. Controlled potential electrolysis and coulometric studies gave the value ofnas 4.0 ± 0.1 in the pH range 2.5 to 10.0. Out of the two major end products formed, one was identified as sulphanilamide and the other as 1-phenyl-3-methyl-4-amino-2-pyrazolin-5-one on the basis of IR and NMR studies. On the basis of DCP, LSV, CV, CPE, coulometry and spectral analysis, a mechanism has been postulated for the reduction of these compounds at d.m.e. and glassy carbon electrodes.Keywords: polarography, cyclic voltammetry, antibacterials, pyrazolin-5-ones.

ISSN:0008-4042    

DOI:10.1139/v95-026

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Six derivatives of 5-phenyl-3,4-dihydro-2H-1,4-oxazin-2-ones1were synthetized from α-amino acids. These compounds are precursors of six-membered cyclic azomethine ylids involving one stereocenter. The 1,3-dipolar species react withN-methyl andN-phenylmaleimides, leading diastereospecifically to cycloadducts where the approach of the dipolarophile occurs from the less hindered side of the 1,3-dipole and with the maleimido moiety in anexoposition. Unfortunately, racemization was observed during the cycloaddition reaction, presumably owing to reversible imine–enamine transformation.Keywords: α-amino acids, cyclic azomethine ylids, diastereospecificity.

ISSN:0008-4042    

DOI:10.1139/v95-027

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The photochemistry and photophysics of 1-phenylcyclobutene and five aryl-substituted derivatives have been studied in various solvents at room temperature. All six compounds fluoresce with quantum yields in the 0.2–0.3 range in cyclohexane and acetonitrile solution. 1-Phenylcyclobutene undergoes [2+2]-cycloreversionto yield phenylacetylene upon photolysis in either hydrocarbon or acetonitrile solution, and undergoes (Markovnikov) solvent addition upon irradiation in methanol solutionin addition to cycloreversion. Triplet sensitization and quenching experiments indicate that cycloreversion and methanol addition are both excited singlet state processes. None of the six compounds studied undergo ring opening to the corresponding 2-aryl-1,3-butadiene in detectable yield. Quantum yields for cycloreversion in cyclohexane, acetonitrile, and methanol solution and methanol addition have been determined for the six compounds, along with excited singlet state lifetimes. The quantum yields and rate constants for cycloreversion and methanol addition are both enhanced by substitution with electron-donating groups. The variation in the rate constant for [2+2]-cycloreversion with substituent indicates that there is substantial dipolar character developed in the cyclobutenyl σ-bond framework during the reaction, in almost exact correspondence with that developed in the π system during photoprotonation. No deuterium scrambling is observed in 1-phenylcyclobutene-2,4,4-d3after photolysis in pentane solution to ca. 80% conversion, indicating that skeletal rearrangements leading to cyclopropyl carbenes do not occur in the direct photolysis of arylcyclobutene derivatives. A pericyclic mechanism for the photocycloreversion reaction is suggested. Triplet–triplet absorption spectra and triplet lifetimes of 1-phenyl-, 1-(para-methylphenyl)-, and 1-(para-trifluoromethylphenyl)cyclobutene in hydrocarbon solution are also reported.Keywords: photochemistry, cyclobutene, fluorescence, [2+2]-cycloreversion, substituent effects, nanosecond laser flash photolysis, lifetime, triplet state, styrene, photoaddition

ISSN:0008-4042    

DOI:10.1139/v95-028

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Chemiluminescence of the vibrationally excited HF†and of the electronically excited HSO* in the2A′ state were observed in the elementary reactions of F + H2S and HS + O3. In the F + H2S reaction, the vibrational populations of HF†in ν = 3 and 4 were found to be nonstatistical but the rotational distribution in the ν = 4 state was found to be Boltzmann-like with a rotational temperature of 700 K, confirming similar data obtained by different methods. The HSO* emission was observed in the HS + O3elementary reaction. The spectrum of HSO* characterized by broad vibrational bands indicates nonstatistical excitation for the rotational and vibrational states.Keywords: chemiluminescence, internal energy distribution, F + H2S, HS + O3, HF†, HSO*.

ISSN:0008-4042    

DOI:10.1139/v95-029

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The reaction of 1,3-adamantanedicarbonamide with chlorocarbonylsulfenyl chloride in hot toluene gives two new oxathiazolone derivatives that have had their structures confirmed crystallographically: 1,3-bis(1,3,4-oxathiazol-2-on-5-yl)-adamantane and 1 -cyano-3-(1,3,4-oxathiazol-2-on-5-yl)-adamantane. (Crystal data: C10H14(COC(O)SN)2:P21/m,a = 10.447(2),b = 7.119(3),c = 10.684(2) Å, β = 112.98(1)°,V = 731.5(3) Å3,z = 2,Dc = 1.536 g cm−3,R = 0.034,Rw = 0.029; (NC)C10H14(COC(O)SN):P21/c,a = 9.153(2),b = 19.134(2),c = 7.130(3) Å, β = 105.38(2)°,V = 1204(1) Å3,z = 4,Dc = 1.447 g cm−3,R = 0.038,Rw = 0.035.) The reaction of the precursor with dimethyl acetylenedicarboxylate (DMAD) in refluxing chlorobenzene gave a mixture that was resolved into pure compounds by Kugelrohr sublimation; these were shown to be sulfur, tetramethyl thiophenetetracarboxylate, 1,3-adamantanedinitrile, 1,3-bis(4,5-bis(methoxycarbonyl)-isothiazol-3-yl)-adamantane, and 1-cyano-3-(4,5-bis(methoxycarbonyl)-isothiazol-3-yl)-adamantane. The structures of the two new isothiazole derivatives were confirmed crystallographically. (Crystal data: C10H14(CNS•DMAD)2:Pca21,a = 15.857(4),b = 11.562(4),c = 13.602(4) Å,V = 2494(2) Å3,z = 4,Dc = 1.424 g cm−3,R = 0.057,Rw = 0.038; C10H14(CN)(CNS•DMAD):P21/c,a = 19.683(2),b = 11.566(3),c = 7.727 (3) Å, β = 93.30(2)°,V = 1756(1) Å3,z = 4,Dc = 1.363 g cm−3,R = 0.079,Rw = 0.076.) The pattern of reactivity indicates that thesyntheticequivalentof the bis(nitrile sulfide) may be present in solution but has a very short lifetime and failed to react at all with 1,3-adamantanedinitrile.Keywords: oxathiazolone, nitrile sulfide, cycloaddition, isothiazole.

ISSN:0008-4042    

DOI:10.1139/v95-030

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The photochemistry of various mono- and dimethoxy-substituted α-bromoacetophenones has been investigated by laser flash photolysis in organic solvents. The short-lived excited singlet states cleave to yield bromine atoms and the corresponding methoxyphenacyl radicals with quantum yields ranging from 0.13 to 0.35. With the exception of 4-methoxy-α-bromoacetophenone (6), all other substrates yield readily detectable triplet states; these have π,π* character and are very poor hydrogen abstractors. Triplet decay does not contribute to methoxyphenacyl radical formation. While methoxyphenacyl radicals may have a spin density as high as 0.3 at the carbonyl oxygen, which accounts for the alkoxy-like visible band in their absorption spectrum, their chemical reactivity is dominated by the radical character at the carbon site. Methoxyphenacyl radicals are modest hydrogen abstractors; for example, reaction of6with 1,4-cyclohexadiene occurs with a rate constant of 2.6 × 105 M−1s−1, while for addition to the double bond in 1,1-diphenylethylene the rate constant is 9.4 × 107 M−1s−1. Additions to other double bonds are likely to be slower (e.g.,12 + 1,3-cyclohexadiene,k ≤ 105 M−1s−1). In contrast, reaction with oxygen occurs withkq = 2.5 × 109 M−1s−1. Under the conditions relevant for their participation in paper yellowing, the methoxyphenacyl radicals will be converted to peroxyl radicals. These probably play a key role in the oxidative photodegradation of lignin.Keywords: methoxyphenacyl radicals, α-bromoacetophenones, laser flash photolysis.

ISSN:0008-4042    

DOI:10.1139/v95-031

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

In this paper, some aspects of the polarography of indium(III) in azide/hydrazoic acid buffer are presented. The electrode process is mostly governed by diffusion in the complexing medium. Nevertheless, for controlled-potential coulometry (involving a large time window) a catalytic process was observed, ascribed to the reaction of the hydrazoic acid and indium amalgam. The stability of azide complexes of In(III) was studied polarographically, and the formation of four mononuclear species was confirmed by computational analysis of data obtained from polarograms of In(III) in different azide concentrations. The values of the global constants obtained are: β1 = 3.7 × 103 M−1, β2 = 8.6 × 105 M−2, β3 = 5.0 × 107 M−3, β4 = 2.1 × 109 M−4, in a constant ionic strength medium held at 2.0 M with NaClO4. These data are in good agreement with those obtained by potentiometry. The value found forE0′of the In(III)/In couple was −512.2 mV vs. calomel electrode (3 M NaCl), the mean diffusion coefficient of the complex being (6.1 ± 0.2) × 10−6 cm2s−1in perchlorate medium (T = 25 °C).Keywords: polarography, indium(III), azide, stability constants, catalytic wave.

ISSN:0008-4042    

DOI:10.1139/v95-032

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Regular polypeptide conformations include secondary structural motifs such as α-helices and β-strands. The occurrence of some regular conformation is usually deduced from a local analysis of dihedral angles. However, the value of a dihedral angle in itself does not provide any information on the conformation's "shape." This drawback can be circumvented with global, rather than local, macromolecular shape descriptors. Recently, fractal exponents have been proposed as a source of such descriptors. Yet, this approach does not fully capture all essential shape features, since protein backbones arenotfractal. In this work, we deal instead with a more "natural" characterization of the polymer's global shape that uses both the chain's geometry and "topology." For the geometry, we study the behaviour of molecular size and anisometry. For the chain's folding features, we study the self-entanglements in a polymer fold. We compute these descriptors for all relevant secondary structural motifs. By using self-entaglements and molecular geometry, we provide a view of secondary structure that is both conceptually appealing and also more discriminating than previous ones in the literature.Keywords: molecular shape analysis, protein secondary structure, self-entanglements.

ISSN:0008-4042    

DOI:10.1139/v95-033

出版商:NRC Research Press    

年代:1995

数据来源: NRC