We have found O(2)-substituted diazeniumdiolates, compounds of structure R(2)N-N(O)=NOR' that are under development for various possible pharmaceutical uses, to be rather photosensitive. With R = ethyl and R' = methyl, benzyl, or 2-nitrobenzyl, the observed product distributions suggest that two primary pathways are operative. A minor pathway involves the extrusion of nitrous oxide (N(2)O) with simultaneous generation of R(2)N(*) and R'O(*), which may then form amines, aldehydes, and alcohols. The major reaction pathway is an interesting photochemical cleavage of the N=N bond to form a nitrosamine (R(2)NN=O) and an oxygen-substituted nitrene (R'ON). The intermediacy of the O-nitrene was inferred from the production of abundant oxime, via rearrangement of the O-nitrene to a C-nitroso compound (R'ON --> O=NR'), and subsequent tautomerization to the more stable oxime. Involvement of the O-nitrene was confirmed by trapping with 2,3-dimethyl-2-butene to form the aziridine and with oxygen to generate the nitrate ester. 2-Nitro substitution on the benzyl derivative had surprisingly little effect on the reaction course. For each compound examined, minor amounts of nitric oxide (NO), presumably produced by secondary photolysis of the nitrosamine, were observed. Time-resolved infrared experiments provided additional support for the above reaction pathways and confirmed that the nitrosamine is a primary photoproduct. We have also found that the relative contributions of the reaction pathways can be altered in certain derivatives. For example, when R' = 2,4-dinitrophenyl, the contribution of the nitrosamine/O-nitrene-forming pathway was diminished. Pharmacological implications of these results are discussed.
The pp* triplet excited states of 3-phenyl-2-cyclohexenone (1) and 3-phenyl-2-cyclopentenone (2) have been observed by time-resolved infrared (TRIR) spectroscopy. The reaction of various alkenes with the triplet excited state of 2 has also been studied by TRIR spectroscopy, and the partitioning between fragmentation and ring closure of the biradical intermediates involved has been estimated. Within a series of structurally related biradicals, more stable ones undergo ring closure more efficiently. Scheme 1 Representative enone-alkene [2 + 2] reactions. y Dedicated to Professor Dr Z. R. Grabowski and Professor Dr J. Wirz on the occasions of their 75th and 60th birthdays. z Electronic supplementary information (ESI) available: Optimized geometries and ab initio-calculated IR frequencies and intensities for cyclic enones. See
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