Browsing by Author "Frija, L."
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- Genesis of rare molecules using light-induced reactions of Matrix-Isolated TetrazolesPublication . Frija, L.; Cristiano, Maria Lurdes Santos; Gómez-Zavaglia, A.; Reva, I. D.; Fausto, R.Tetrazoles exhibit a very rich photochemistry, strongly influenced by the nature of substituents in the tetrazolic ring. Photolysis of representative tetrazoles trapped in a rigid environment of solidified noble gases at cryogenic temperatures (usually argon at 10–15 K) results in photofragmentation of the monomeric species with a wide range of exit channels. Since the obtained fragments are generally confined to the matrix cage where they are formed, no subsequent cross-reactions involving species resulting from photolysis can occur, strongly reducing the number of possible photoproducts in comparison with gas phase or solution studies. These conditions introduce a useful simplification for the interpretation of the reaction mechanisms and enable spectroscopic characterization of novel and/or highly reactive molecules. In this review, we provide an updated report on the photolysis of matrix-isolated tetrazoles, focusing on the scope and usefulness of this methodology for generation of rare molecules and investigation of photocleavage pathways. Special emphasis is placed on mechanistic interpretations and characterization of rare molecules and on the relevance of conformation and tautomerism on the photochemistry of the studied compounds.
- Heteroaromatic benzyl ethers as intermediates for palladium-catalysed transfer hydrogenolysis of benzyl alcoholsPublication . Araujo, Nuna C. P.; Brigas, Amadeu; 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.
- Infrared spectrum and UV-induced photochemistry of matrix-isolated 5-methoxy-1-phenyl-1H-tetrazolePublication . 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 compound, the calculated spectrum showing a very good agreement with the experimental data. In situ UVirradiation (λ > 235 nm) of the matrix-isolated 5MPT induced unimolecular decomposition of the compound, which led mainly to production of methylcyanate and phenylazide, this latter further reacting to yield, as final product, 1-aza-1,2,4,6-cycloheptatetraene. 3-Methoxy-1-phenyl-1Hdiazirene was also observed experimentally as minor product, resulting from direct elimination of molecular nitrogen from 5MPT.
- Investigation of structural effects on the reactivity of heterocyclic bioactive compoundsPublication . Frija, L.; Cristiano, Maria Lurdes Santos; Lourenço, Rui Fausto Martins Ribeiro da SilvaIn the course of this investigation, structural effects on the reactivity of a series of heterocyclic compounds, in particular tetrazole and benzisothiazole derivatives, submitted to different chemical environments, were explored. The photochemistry of several representative tetrazoles was considered in solution and for the compounds trapped in a rigid matrix of solidified argon at cryogenic temperatures. UV-excitation resulted in photofragmentation of tetrazoles with a wide range of exit channels. Important mechanistic questions concerning the photodecomposition of allyl-tetrazolyl derivatives in different solvents were answered, and new synthetic methodologies for the preparation of heterocyclic compounds, such as pyrimidinones or oxazines, from allyl-tetrazoles, were developed. For the matrix-isolated compounds, since the obtained fragments in general stay in the matrix cage where they are formed, no subsequent cross-reactions involving species resulting from photolysis of different reactant molecules can occur. This fact introduced a useful simplification for the interpretation of the reaction mechanisms. FTIR spectroscopy provided experimental frequencies and intensities of characteristic absorptions of the matrix-isolated chemical species, both for reagents and photoproducts. The analysis of experimental data was assisted by their direct comparison with the vibrational spectra theoretically calculated for the single molecule in vacuum. Spectroscopic characterization of a number of relatively unusual or highly reactive molecules, formed from photolysis of matrix-isolated tetrazoles, is presented for the first time. Novel tetrazolyl and benzisothiazolyl naphthylmethylic ethers were synthesized and the development of experimental conditions for their palladium-catalysed hydrogenolysis, using a hydrogen donor or molecular hydrogen, was carried out successfully. Structural effects on the reactivity of the heteroaromatic ethers were investigated. Furthermore, new benzisothiazole-tetrazolyl derivatives differing on the spacer-group used for linkage of the two heterocyclic systems were designed, produced and tested as multidentate ligands in coordination reactions with transition-metal complexes.
- Mechanistic Investigations into the photochemistry of 4-allyl-tetrazolones in solution: a new approach to the synthesis of 3,4-dihydro-pyrimidinonesPublication . Frija, L.; Khmelinskii, Igor; Cristiano, Maria Lurdes SantosPhotolysis (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 the solvent cage stabilization due to formation of hydrogen bonds. The viscosity of alcohols is directly related to the cage effects observed. The photocleavage of 4-allyl-tetrazolones leads probably to a caged triplet radical pair. This hypothesis is confirmed by the solvent viscosity effect on the photolysis quantum yields. Additionally, dissolved molecular oxygen sensitizes the formation of pyrimidinones, as should be expected for a triplet intermediate that can only form the product molecule after T-S conversion, which is accelerated by oxygen.
- Molecular structure, vibrational spectra and photochemistry of 2-methyl-2 h -tetrazol-5-amine in solid argonPublication . 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: (1) tautomerization to mesoionic 3-methyl-1H-tetrazol-3-ium-5-aminide; (2) nitrogen elimination, with production of 1-methyl-1H-diazirene-3-amine; (3) ring cleavage leading to production of methyl azide and cyanamide. Following the primary photoproducts, secondary reactions were observed, leading to spectroscopic observation of methylenimine and isocyanidric acid.
- Molecular structure, vibrational spectra and photochemistry of 5-mercapto-1-methyltetrazolePublication . 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 leading to production of methyl isothiocyanate and azide; (3) simultaneous elimination of nitrogen and sulphur with production of N-methylcarbodiimide. Following these photoprocesses, subsequent reactions occur, leading to production of methyl diazene, carbon monosulphide and nitrogen hydride. Spectroscopic evidence of the production of the above-mentioned chemical species is provided.
- Novel efficient synthesis of 3,4-dihydro-6-substituted-3-phenylpyrimidin-2(1H)-onesPublication . Frija, L.; Khmelinskii, Igor; Cristiano, Maria Lurdes SantosA new attractive and convenient strategy for the synthesis of 3,4-dihydro-6-substituted-3-phenylpyrimidin-2(1H)-ones is described. Photolysis (k = 254 nm) of 4-allyl-tetrazolones in alcoholic solutions produces the corresponding pyrimidinones as the sole product in nearly quantitative yields, with simultaneous extrusion of molecular nitrogen. Work-up procedures consist in the simple evaporation of the solvent under reduced pressure, in mild conditions. Heats of reaction for the photocleavage process of 4-allyltetrazolones were calculated, indicating high stability of the resulting products.
- Palladium-catalysed reduction of heteroaromatic naphthyl ethers: structural effects on reactivityPublication . 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 molecular orbital DFT(B3LYP)/6-311G(d) calculations, for all ethers. It can be concluded from this investigation that 5-chloro-1-phenyltetrazole 2a acts as a better derivatizing agent for naphthyl methanols than 3-chloro-1,2-benzisothiazole-11-dioxide 2b, this contrasting with what has been observed with phenols, allylic and benzylic alcohols.
- Photochemical ring-opening and intramolecular hydrogen shift reactions in sulfur analogues of a-pyronePublication . 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 compounds. The ring-opening photoreaction in PT represents the first reported case of an R-bond cleavage in a compound with a CdS group attached to a six-membered ring, in which the internal strain practically does not exist, whereas the corresponding reaction in TP (a cleavage of a C-S bond in the R position with respect to a carbonyl group) is now reported for the first time. Following the ring-opening reactions, isomerization processes and intramolecular hydrogen shift reactions were observed, enabling production of TP from PT and vice versa. A detailed study of such processes was undertaken, and kinetical and mechanistical data are presented and discussed.