Org Biomol Chem 2008 Mar 14;6(6):1046-55. Epub 2008 Feb 14.
Department of Chemistry, Biochemistry and Pharmacy, F.C.T. and CCMAR, University of Algarve, 8005-039 Faro, Portugal.
The 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.