Mostrando recursos 1 - 20 de 45

  1. Photoactive materials applicable to imaging systems

    Cristiano, Maria Lurdes Santos; Johnstone, Robert A. W.; Pratt, M. J.; Wade, M. J.
    Organic photoprecursors of amines are provided for use in photosensitive imaging systems, ?nding particular application in the preparation of lithographic printing plates. Said photoprecursors generate free amines on exposure to long Wavelength UV or visible radiation, have high solubility in organic solvents, and include photolabile 2-nitrobenZyl functional groups. Methods for the synthesis of the photo precursors are described, and the use of the said photopre cursors for the production of printing plates using both positive Working and negative Working techniques are dis cussed.

  2. Bond energy/eond order relationships for N-O linkages and a quantitative measure of ionicity: the rôle of nitro groups in hydrogen bonding

    Johnstone, Robert A. W.; Loureiro, Rui M. S.; Labat, G.; Cristiano, Maria Lurdes Santos
    The nitro group is active in metabolic systems and can be found as an integral part of a number of useful curative drugs and many toxic substances. The basis for much of this activity is not fully understood. It is not necessarily caused directly by through-bond electronic effects but may also be due to direct H-bonding to nitro or to indirect interference by the nitro group with existing H-bonding. An unusual effect of a nitro substituent on kinetic results from urethane addition/elimination reactions (Scheme 1) has been ascribed to some form of self-association, which was neither specified nor quantified. To...

  3. Molecular structure, vibrational spectra and photochemistry of 5-mercapto-1-methyltetrazole

    Gómez-Zavaglia, A.; Reva, I. D.; Frija, L.; Cristiano, Maria Lurdes Santos; Fausto, R.
    In this work, 5-mercapto-1-methyltetrazole was studied by low temperature matrix-isolation and solid-state infrared spectroscopy, DFT(B3LYP)/6-311CCG(d,p) calculations and photochemical methods. In the low temperature neat solid phase and isolated in an argon matrix, the compound was found to exist in the 1-methyl-1,4-dihydro-5H-tetrazole-5-thione tautomeric form. The infrared spectra of the compound were fully assigned and correlated with structural properties. In situ UV-irradiation (lO235 nm) of the matrix-isolated monomer is shown to induce different photochemical processes, all of them involving cleavage of the tetrazole ring: e.g. (1) molecular nitrogen expulsion, with production of 1-methyl-1H-diazirene-3-thiol, which is produced in two different conformers; (2) ring cleavage...

  4. Photochemistry of 1-phenyl-tetrazolone isolated in solid argon

    Gómez-Zavaglia, A.; Reva, I. D.; Frija, L.; Cristiano, Maria Lurdes Santos; Fausto, R.
    The molecular structure, vibrational spectra, tautomerism and photochemistry of the derivative of tetrazole, 1-phenyl-tetrazolone (C7H6N4O; PT) have been studied by FT-IR matrix isolation spectroscopy and DFT/B3LYP/6-311++G(d,p) calculations. Among the five structures in which PT could be expected to exist (two keto tautomers, one mesoionic olate-form and two different conformers of the hydroxyl tautomer), only the most stable species, 1-phenyl-1,4-dihydro-5H-tetrazol-5-one, could be experimentally observed in low temperature argon matrices. Monomers of this tautomer give rise to an IR spectrum that fits nicely the calculated spectrum obtained at the DFT/B3LYP/6-311++G(d,p) level of theory. In situ UV irradiation (λ > 235 nm) of...

  5. Palladium-catalysed reduction of heteroaromatic naphthyl ethers: structural effects on reactivity

    Frija, L.; Cristiano, Maria Lurdes Santos; Guimarães, Emanuel M. O.; Martins, Nelson C.; Loureiro, Rui M. S.; Bickley, Jamie F.
    Tetrazolyl and benzisothiazolyl naphthylmethylic ethers 3 and 4(a–e) are stable crystalline compounds that can be synthesised in high yields by reaction of the corresponding naphthyl methanols (1a–e) with the derivatizing agents 2a and 2b. Experimental conditions for palladium-catalysed hydrogenolysis of ethers 3, 4, with a hydrogen donor and with molecular hydrogen, were investigated. Analysis of the structure and reactivity indicates that naphthylmethylic ethers 3 and 4 are structurally similar to the corresponding benzyloxyderivatives around the ether bond but exhibit different reactivity. Structural analysis for these compounds is based on crystallographic structure determinations, for 5-(2-naphthylmethoxy)-1-phenyltetrazole (3a) and 3-(2-naphthylmethoxy)-1,2-benzisothiazole 1,1-dioxide (4a), and...

  6. Intramolecularity of the thermal rearrangement of allyloxytetrazoles to N-allyltetrazolones†

    Cristiano, Maria Lurdes Santos; Johnstone, Robert A. W.
    The intramolecularity of the thermal rearrangement of 1-aryl-5-allyloxy-1H-tetrazoles 1 to 1-aryl-4-allyl-1,4-dihydrotetrazol-5-ones 2 has been investigated through cross-over studies: the results support the hypothesis for a concerted sigmatropic rearrangement occurring through a highly polar transition state, in which a partially positively charged allyl group migrates from oxygen to nitrogen, without leaving the solvent cage.

  7. First observation of Chapman rearrangement of a pseudosaccharyl ether in the solid state: the thermal isomerization of 3-(methoxy)-1,2-benzisothiazole 1,1-dioxide revisited

    Almeida, R.; Gómez-Zavaglia, A.; Kaczor, A.; Cristiano, Maria Lurdes Santos; Eusébio, M. E. S.; Maria, T. M. R.; Fausto, R.
    3-(Methoxy)-1,2-benzisothiazole 1,1-dioxide, a pseudosaccharyl ether, was long ago known to undergo a thermal Chapman-like [1,30]-isomerization to the corresponding N-methyl pseudosaccharin at temperatures above its melting point (ca. 184 C) [Hettler H., Tetrahedron Lett. 1968, 15, 1793]. In the present study, it is shown that this rearrangement can also take place in the solid state, at temperatures as low as 150 C. This was the first observation of a Chapman-like [1,30]-isomerization in pseudosaccharyl ethers in the solid state. The study has been carried out by a multidisciplinary approach using temperature dependent infrared spectroscopy, differential scanning calorimetry (DSC), and polarized light thermomicroscopy,...

  8. The chapman-type rearrangement in pseudosaccharins: the case of 3-(methoxy)-1,2-benzisothiazole 1,1-dioxide

    Kaczor, A.; Proniewicz, L. M.; Almeida, R.; Gómez-Zavaglia, A.; Cristiano, Maria Lurdes Santos; Matos Beja, A. M.; Ramos Silva, M.; Fausto, R.
    The thermal Chapman-type rearrangement of the pseudosaccharin 3-(methoxy)-1,2-benzisothiazole 1,1-dioxide (MBID) into 2-methyl-1,2-benzisothiazol-3(2H)-one 1,1-dioxide (MBIOD) was investigated on the basis of computational models and knowledge of the structure of the reactant and product in the isolated and solid phases. X-ray diffraction was used to obtain the structure of the substrate in the crystalline phase, providing fundamental structural data for the development of the theoretical models used to investigate the reaction mechanism in the condensed phase. The intra- and different intermolecular mechanisms were compared on energetic grounds, based on the various developed theoretical models of the rearrangement reactions. The energetic preference (ca. 3.2...

  9. Photochemistry and vibrational vpectra of matrix-isolated 5-ethoxy-1-phenyl-1 H -tetrazole

    Frija, L.; Reva, I. D.; Gómez-Zavaglia, A.; Cristiano, Maria Lurdes Santos; Fausto, R.
    A combined matrix isolation FT-IR and theoretical DFT(B3LYP)/6-311++G(d,p) study of the molecular structure and photochemistry of 5-ethoxy-1-phenyl-1H-tetrazole (5EPT) was performed. A new method of synthesis of the compound is described. Calculations show three minima, very close in energy and separated by low-energy barriers (less than 4 kJ mol-1), in the ground-state potential energy profile of the molecule. The method of matrix isolation enabled the reduction of the number of populated conformational states in the experiment at low temperature due to the effect known as conformational cooling. As a result, the spectrum of the as-deposited matrix of 5EPT closely matches that of...

  10. Diels–alder/thiol–olefin co-oxygenation approach to antimalarials incorporating the 2,3-dioxabicyclo[3.3.1]nonane pharmacophore

    O'Neill, Paul M.; Verissimo, Edite; Ward, Stephen A.; Davies, Jill; Korshin, Edward E.; Araújo, Nuna C.; Pugh, Matthew D.; Cristiano, Maria Lurdes Santos; Stocks, Paul A.; Bachi, Mario D.
    Abstract—A Diels–Alder/thiol–olefin co-oxygenation approach to the synthesis of novel bicyclic endoperoxides 17a–22b is reported. Some of these endoperoxides (e.g., 17b, 19b, 22a and 22b) have potent nanomolar in vitro antimalarial activity equivalent to that of the synthetic antimalarial agent arteflene. Iron(II)-mediated degradation of sulfone-endoperoxide 19b and spin-trapping with TEMPO provide a spin-trapped adduct 25 indicative of the formation of a secondary carbon centered radical species 24. Reactive C-radical intermediates of this type may be involved in the expression of the antimalarial effect of these bicyclic endoperoxides.

  11. Infrared spectrum and UV-induced photochemistry of matrix-isolated 5-methoxy-1-phenyl-1H-tetrazole

    Gómez-Zavaglia, A.; Reva, I. D.; Frija, L.; Cristiano, Maria Lurdes Santos; Fausto, R.
    The molecular structure, vibrational spectra and photochemistry of 5-methoxy-1-phenyl-1H-tetrazole (5MPT) were studied by matrix isolation infrared spectroscopy and DFT(B3LYP)/6-311++G(d,p) calculations. According to the calculations, only one double degenerated-by-symmetry minimum exists in the ground state PES of the compound. In this structure, the dihedral angle between the two rings (phenyl and tetrazole) is ca. 30◦, whereas the methoxyl group stays nearly in the plane of the tetrazole ring. In consonance with the theoretical predictions, only one molecular species was experimentally observed in the as-deposited argon matrices. Theoretical calculations were also used to help in assignment of the experimental spectrum of the...

  12. Mechanistic Investigations into the photochemistry of 4-allyl-tetrazolones in solution: a new approach to the synthesis of 3,4-dihydro-pyrimidinones

    Frija, L.; Khmelinskii, Igor; Cristiano, Maria Lurdes Santos
    Photolysis (ì ) 254 nm) of 4-allyl-tetrazolones 2a-c was carried out in methanol, 1-propanol, 1-hexanol, acetonitrile, and cyclohexane. The sole primary photochemical process identified was molecular nitrogen elimination, with formation of pyrimidinones 6a-c. Following the primary photocleavage, secondary reactions were observed in acetonitrile and cyclohexane, leading to phenyl-isocyanate (7), aniline (9), and 1-phenylprop-1-enyl-isocyanate (10a). In alcoholic solutions, the primary products, 6a-c, remained photostable even under extended irradiation, making possible the isolation of 3,4-dihydro-pyrimidinones as stable compounds in very high yields. The observed photostability of pyrimidinones 6a-c in alcohols is ascribed to the excited state quenching via reversible proton transfer, facilitated by...

  13. Photochemical ring-opening and intramolecular hydrogen shift reactions in sulfur analogues of a-pyrone

    Breda, S.; Reva, I. D.; Lapinski, L.; Cristiano, Maria Lurdes Santos; Frija, L.; Fausto, R.
    A combined matrix isolation FTIR and theoretical DFT(B3LYP)/6-311++G(d,p) study of the photochemistry of sulfur analogues of R-pyrone [2H-thiopyran-2-one (TP) and 2H-pyran-2-thione (PT)] was carried out. The vibrational spectra of monomers of the compounds isolated in low-temperature argon matrixes were studied experimentally and assigned completely on the basis of theoretical calculations. UV irradiation (ì > 337 nm) of the studied compounds isolated in low-temperature matrixes results mainly in the ring-opening reaction by means of the cleavage of the R-bond. Other photoprocesses, not involving the R-bond-cleavage step (such as generation of Dewar valence isomer), correspond to the minor reaction channels in both studied...

  14. Molecular structure, vibrational spectra and photochemistry of 2-methyl-2 h -tetrazol-5-amine in solid argon

    Gómez-Zavaglia, A.; Reva, I. D.; Frija, L.; Cristiano, Maria Lurdes Santos; Fausto, R.
    In this work, the molecular structure, infrared spectrum and UV photochemistry of 2-methyl-2H-tetrazol-5-amine (2MTA) isolated in solid argon (10 K) were investigated. The experimental studies were supported by extensive DFT(B3LYP)/6-311++G(d,p) calculations. The infrared spectrum of matrix-isolated 2MTA was fully assigned and correlated with structural properties. Taking into consideration the observed frequency of the NH2 wagging mode, it is suggested that, in the matrixes, the amine group becomes slightly more planar than in the gas phase, due to matrix-packing effects. In situ UV irradiation (ì > 235 nm) of the matrixisolated 2MTA monomer is shown to induce three main primary photochemical processes:...

  15. Heteroaromatic benzyl ethers as intermediates for palladium-catalysed transfer hydrogenolysis of benzyl alcohols

    Araújo, Nuna C.; Brigas, Amadeu F.; Cristiano, Maria Lurdes Santos; Frija, L.; Guimarães, Emanuel M. O.; Loureiro, Rui M. S.
    A chemoselective and easy to carry procedure for the hydrogenolysis of benzyl alcohols is described. Benzyl alcohols are readily converted into tetrazolyl and benzisothiazolyl ethers that can be catalytically hydrogenolysed to toluenes over palladium-on-charcoal using hydrogen donors. The effect of electronwithdrawing heteroaromatic substituents is interpreted on the basis of chrystallographic structure determinations and molecular orbital calculations.

  16. A kinetic investigation of the thermal rearrangement of allyloxytetrazoles to N-allyltetrazolones

    Cristiano, Maria Lurdes Santos; Johnstone, Robert A. W.
    The mechanism of the thermal rearrangement of 1-aryl-5-allyloxytetrazoles 1 to give 1-aryl-4-allyltetrazolones 2 in very high yield has been investigated through kinetic studies in one polar and one less polar solvent. The results suggest mainly a concerted [3,3] sigmatropic process, in which a partially positively charged allyl group migrates from oxygen to nitrogen, similar to the polar transition state found in the Claisen rearrangement.

  17. Semi-synthetic and synthetic 1,2,4-trioxaquines and 1,2,4-trioxolaquines: synthesis, preliminary SAR and comparison with acridine endoperoxide conjugates

    Araújo, Nuna C.; Barton, Victoria; Jones, Michael; Stocks, Paul A.; Ward, Stephen A.; Davies, Jill; Bray, Patrick G.; Shone, Alison E.; Cristiano, Maria Lurdes Santos; O'Neill, Paul M.
    A novel series of semi-synthetic trioxaquines and synthetic trioxolaquines were prepared, in moderate to good yields. Antimalarial activity was evaluated against both the chloroquine-sensitive 3D7 and resistant K1 strain of Plasmodium falciparum and both series of compounds were shown to be active in the low nanomolar range. For comparison the corresponding 9-amino acridine analogues were also prepared and shown to have low nanomolar activity like their quinoline counterparts.

  18. UV-induced photochemistry of matrix-isolated 1-phenyl-4-allyl-tetrazolone

    Frija, L.; Reva, I. D.; Gómez-Zavaglia, A.; Cristiano, Maria Lurdes Santos; Fausto, R.
    The photochemistry and molecular structure of 1-phenyl-4-allyl-tetrazolone (PAT) was studied by FT-IR matrix isolation spectroscopy and DFT(B3LYP)/6-311++G(d,p) calculations. The spectrum of matrix-isolated PAT monomers agrees well with the sum spectrum of three conformers predicted theoretically. UV irradiation (k > 235 nm) of matrix-isolated PAT induces three types of photofragmentation: (1) production of phenylazide and allyl-isocyanate, with phenylazide then losing N2 to yield 1-aza-1,2,4,6-cycloheptatetraene; (2) formation of phenyl-isocyanate and allylazide; (3) N2 elimination leading to formation of 1-allyl-2-phenyldiaziridin-3-one; this compound partially reacts further to form 1-allyl-1H-benzoimidazol-2(3H)-one. The observed photochemistry of the matrix-isolated PAT is distinct from the preferred photochemical fragmentation in solution, where 3,4-dihydro-3-phenylpyrimidin-2(1H)-one...

  19. Investigations into the mechanism of action of nitrobenzene as a mild dehydrogenating agent under acid-catalysed conditions

    Cristiano, Maria Lurdes Santos; Gago, David J. P.; Gonsalves, Antonio M. d'A. Rocha; Johnstone, Robert A. W.; McCarron, Moya; Varejão, Jorge M. T. B.
    Protonated nitrobenzene can be used to dehydrogenate a range of hydrocarbons, which already possess at least one double bond. Kinetic and spectroscopic results, together with known electrode potentials, yield approximate limits within which protonated nitrobenzenes can be expected to effect dehydrogenation of hydroaromatic compounds. A high yielding synthesis of benzo[ j ]fluoranthene is described.

  20. Structural effects on sigmatropic shifts in heteroaromatic allyl ethersElectronic supplementary information (ESI) available: selected crystal data for compound 7. See

    Araújo, Nuna C.; Barroca, Pedro M. M.; Bickley, Jamie F.; Brigas, Amadeu F.; Cristiano, Maria Lurdes Santos; Johnstone, Robert A. W.; Loureiro, Rui M. S.; Pena, Paula C. A.
    In contrast to the known thermal, exclusively [3,3], O- to N- rearrangement of allyl groups in phenyltetrazoles (1, Scheme 1), the comparable migration of the allyl group in pseudosaccharyl ethers (3; Scheme 2) has been shown to proceed through both [1,3]- and [3,3]-mechanisms, 4, 5; for the pseudosaccharyl derivative of the natural product myrtenol (6; Scheme 3) only the product 7 of a [1,3]-shift has been observed; crystallographic data and theoretical calculations provide an explanation of this ease of [1,3]-isomerization and the observed selectivity as being due to conformational constraints and electronic factors.

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