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  • Investigations into the Mechanism of Solvolysis of 3-aryloxybenzisothiazoles
    Publication . Ismael, Amin; Gago, David J. P.; Cabral, Lília; Fausto, Rui; Cristiano, Maria De Lurdes
    The solvolysis of selected 3-aryloxybenzisothiazoles (6a-c; Figure 1) in alcohols has been theoretically investigated. The geometries of ethers 6a-c were fully optimized at the DFT(O3LYP) level, with the 6-31++G(d,p) and 6-311++G(3df,3pd) basis sets. Calculations including solvation effects were performed with the 6-31++G(d,p) basis set. Overall, theoretical values for bond lengths and angles around the central ether linkage in ethers 6a-c are very close, for the isolated molecule and in methanol, and are also very close to those obtained by X-ray crystallography, revealing that the nature of the substituent on the aryl system has a negligible effect on geometric parameters around the ether linkage. The same applies to charge distributions, predicted using the NPA approach. However, measured rate constants for the solvolysis of the same compounds in alcohols show that the rate is affected by the electron-withdrawing/-donating characteristics of the substituent on the aryl ring and by the polarity of solvent. Two general pathways were considered for the solvolysis of ethers 6: associative (addition-elimination) or dissociative (fragmentation-recombination) mechanisms. Molecular orbital calculations by means of polarized continuum model (PCM) reaction field predicted that solvolysis of ethers 6 prefers an addition-elimination mechanism. Calculations show also that a dissociative mechanism for the solvolysis of ethers 6a-c is energetically much more demanding than its addition-elimination counterpart and is therefore a highly improbable pathway for the solvolysis. In addition, it was found that the putative cation intermediate formed during a dissociative process should easily convert into its 2-cyanobenzenesulfone cation isomer, via cleavage of the S-N bond.
  • Structure and photochemistry of a novel tetrazole-saccharyl conjugate isolated in solid argon
    Publication . Ismael, Amin; Borba, A.; Duarte, L.; Giuliano, B. M.; Gómez-Zavaglia, A.; Cristiano, Maria Lurdes Santos
    A combined matrix isolation FTIR and theoretical DFT/B3LYP/6-311++G(3df,3pd) study of the novel synthesised tetrazole-saccharyl conjugate 2-[1-(1H-tetrazol-5-yl)ethyl]-1,2-benzisothiazol-3(2H)-one 1,1-dioxide [1-TE-BZT] was performed. In the gas phase, at room temperature, the compound exists as a mixture of six isomeric forms (four conformers of 1H tautomer and two conformers of 2H tautomer). According to theoretical calculations, conformers 1H were the most stable and the relative energies among the three most stable forms are lower than 4 kJ mol 1. These conformers benefit from stabilising intramolecular hydrogen bonds-like interactions involving the 1H of the tetrazole ring and the carbonyl oxygen of the saccharyl moiety. The photochemistry of 1-TE-BZT in solid argon was investigated and theoretical DFT/B3LYP/6-311++G(3df,3pd) calculations also helped in assignment of the experimental bands. A quick consumption of the compound occurred after irradiation of the matrix with UV laser light at k = 275 nm. Three photofragmentation pathways were proposed, one leading to 2-[1-(1H-diaziren-3-yl)ethyl]-1,2-benzisothiazol-3(2H)-one 1,1-dioxide and molecular nitrogen, a second one giving 2-(1,1-dioxide-3-oxo-1,2-benzisothiazol-2(3H)-yl)propanenitrile and azide, and a third one involving loss of azide from the tetrazole ring and decarbonylation of the saccharyl ring of 1-TE-BZT to give acrylonitrile and 7-thia-8- azabicyclo[4.2.0] octa-1,3,5-triene 7,7 dioxide. The comparison of the relative intensities of the bands of the photoproducts obtained from the three channels allowed us to consider the latter pathway, involving an unprecedented photocleavage of the benzisothiazole (saccharyl) ring, as the preferred photodegradation channel of 1-TE-BZT.