Browsing by Author "Serpa, C."
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- Photochemistry of 1-allyl-4-aryltetrazolones in solution; structural effects on photoproduct selectivityPublication . Ismael, Amin; Serpa, C.; Cristiano, Maria Lurdes SantosThe photochemistry of tetrazolones derived from the carbocyclic allylic alcohols cyclohex-2-enol and 3-methylcyclohex-2-enol and from the natural terpene alcohol nerol was investigated in solution with the aim of assessing the effect of solvent and of structural constraints imposed by bulky allylic moieties on photoproduct selectivity and stability. Photolysis of tetrazolones derived from nerol and cyclohex-2-enol afforded the corresponding pyrimidinones as major products through a pathway that appears to be similar to that proposed for other 1-allyl-4-phenyl-1,4-dihydro-5H-tetrazol-5-ones derived from acyclic and unhindered allylic alcohols previously investigated but photolysis of the tetrazolone derived from the bulkier 3–methylcyclohex-2-enol 4c leads to formation of a benzimidazolone, indicating that, in this case, cyclization of the biradical formed upon extrusion of N2 involves the phenyl substituent and not the allylic moiety. Theoretical calculations (DFT(B3LYP)/3-21G*) were conducted to support the interpretation of the experimental results and mechanistic proposals. Laser flash photolysis experiments were conducted with the aim of clarifying the nature of the intermediate involved in the primary photocleavage process.
- Photochemistry of 5-allyloxy-tetrazoles: steady-state and laser flash photolysis studyPublication . Frija, L.; Khmelinskii, Igor; Serpa, C.; Reva, I. D.; Fausto, R.; Cristiano, Maria Lurdes SantosThe photochemistry of three 5-allyloxy-tetrazoles, in methanol, acetonitrile and cyclohexane was studied by product analysis and laser flash photolysis. The exclusive primary photochemical process identified was molecular nitrogen elimination, with formation of 1,3-oxazines. These compounds were isolated in reasonable yields by column chromatography on silica gel and were fully characterized. DFT(B3LYP)/6-31G(d,p)calculations predict that these 1,3-oxazines can adopt two tautomeric forms (i) with the NH group acting as a bridge connecting the oxazine and phenyl rings and (ii) with the –N=bridge and the proton shifted to the oxazine ring. Both tautomeric forms are relevant in the photolysis of oxazines in solution. Secondary reactions were observed, leading to the production of phenyl vinyl-hydrazines, enamines, aniline and phenyl-isocyanate. Transient absorption, detected by laser flash photolysis, is attributed to the formation of triplet 1,3-biradicals generated from the excited 5-allyloxy-tetrazoles. The 1,3-biradicals are converted to 1,6-biradicals by proton transfer, which, after intersystem crossing, decay to generate the products. Solvent effects on the photoproduct distribution and rate of decomposition are negligible.