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Paper | Regular issue | Vol 27, No. 1, 1988, pp.113-124
Published online, 1st January, 1970
DOI: 10.3987/COM-87-4267
Photochemistry of Pyridine N-Oxides. Trapping of an Intermediate with Amines

Angelo Albini, Elisa Fasani, and Christian Lohse

*Department o f Organic Chemistry, University of Pavia, Via Taramelli 10, 27100 Pavia, Italy

Abstract

The photochemistry of pyridine N-oxide (1a) and the corresponding 4-benzyl (1b),4-phenyl (1c), and 2,6-dimethyl (1d) derivatives has been investigated or reinvestigated. Substrates 1a and 1b yield a small amount of the corresponding 2-acylpyrroles (5a,b) as the only isolable product. 1d yields 5d in moderate yield and 1c yield 5c in aprotic and the 2-pyridone 7c in protic solvents. In the presence o f diethylamine isomeric 5-diethylaminopentadienenitriles (3,4) are obtained in moderate yield along with deoxygenated pyridines (6). Under this condition no 5 is formed, but the yield of 7 is unchanged. These reactions are most economically accounted for by admitting that nitrene 11 is formed as an intermediate and undergoes rearrangement to pyrroles 5 or polymerization to tars, or it can be trapped by amines (proton abstraction followed by nucleophilic substitution) to yield products 3 and 4. The lactams 7 arise directly from the N-oxide singlet excited state, which is also involved in the deoxygenation by amines.