Triplet states mediating hydrogen abstraction in 4-acylpyrimidines, 2-acylpyridines, 2-acylpyrazines, and 3-acylpyridazines
Irradiation of 9 leads to hydrogen abstraction by N(1) and fragmentation to 8 from a triplet with ET ~78 kcal/mol. Irradiation of 2-acylpyridines (10) leads to abstraction by both nitrogen and oxygen (cf. eq 4), with the same Stern-Volmer kqτ for the two processes. Irradiation of 2-acylpyrazines (11) can lead to abstraction by either nitrogen (φ27 0.77 from 11b) or oxygen (φ11a 0.95 from 11f)- 3-Acylpyridazine 12d is unreactive on direct irradiation or triplet sensitization with sensitizer ET ~70 kcal/mol; it furnishes a small amount of 12a on sensitization by acetone. Ketone 18 is recovered unchanged from irradiation under all conditions used with 12d. These observations suggest a correlation between the photochemistry of each of these compounds and the energy of the nπ* triplet of the heteroaromatic ring. The nature of the excited state(s) responsible for hydrogen abstraction by nitrogen and oxygen in these ketones is discussed.
Prathapan, Sreedharan,Robinson, Kevin E.,Agosta, William C.
p. 1838 - 1843
(2007/10/02)
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