Res. Chem. Intermed.
, Vol. 28, No. 2,3, pp. 231–237 (2002)
Also available online - www.vsppub.com
EPR evidence of a triplet biradical in the photolysis
, A. FAUCITANO
, E. FASANI
, A. ALBINI
and A. RICCI
Dipartimento di Chimica Generale, Università di Pavia, V. le Taramelli 12, 27100 Pavia, Italy
Dipartimento di Chimica Organica, Università di Pavia, V. le Taramelli 12, 27100 Pavia, Italy
Received 24 October 2001; accepted 21 November 2001
Abstract—UV irradiation of nifedipine leads to of triplet biradicals that can be detected by means of
: Nifedipine; electron paramagnetic resonance; triplet; biradical.
Nifedipine (1) (1,4-dihydro-2,6-dimethyl-4-(2-nitrophenyl)-3,5-pyridinedicarboxy-
lic acid dimethyl ester) is a drug widely used in the therapy of hypertension.
In an extended project to study the radiolytic and photolytic behaviour of pharma-
ceutical formulations, special attention was devoted to this compound for its very
high light-sensitivity . The meta and para nitrophenyl isomers are much less pho-
tolabile. A strong suggestion of a structure–photoactivity relationship is indicated.
It is known, from the X-ray structure of 4-aryl-dihydropiridines, that the preferred
conformation places the ring planes orthogonal to each other . The rotational
barriers of the 4-aryl group around the ring–ring bond are low enough to permit
a free rotation on the NMR time scale at room temperature . Therefore the
ortho-nitro function in nifedipine is at reaction distance from an electron-rich
substrate (dihydropyridine ring) and a labile hydrogen (H-C
). These characteristics
make the nifedipine a valuable system for studying the intramolecular reactions of
the photoexcited nitrophenyl group. The reactive excited state is an n !
Its more important reactions are electron transfer from a suitable donor [4, 5] and
hydrogen abstraction .
Although the photochemistry of liquid solutions of nifedipine has been exhaus-
tively investigated by EPR spectroscopy , nothing is known about the initial
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