Triplet states mediating hydrogen abstraction in 4-acylpyrimidines, 2-acylpyridines, 2-acylpyrazines, and 3-acylpyridazines

Article abstract of DOI:10.1021/ja00031a044

Irradiation of 9 leads to hydrogen abstraction by N(1) and fragmentation to 8 from a triplet with E_T ～78 kcal/mol. Irradiation of 2-acylpyridines (10) leads to abstraction by both nitrogen and oxygen (cf. eq 4), with the same Stern-Volmer k_qτ for the two processes. Irradiation of 2-acylpyrazines (11) can lead to abstraction by either nitrogen (φ₂₇ 0.77 from 11b) or oxygen (φ_11a 0.95 from 11f)- 3-Acylpyridazine 12d is unreactive on direct irradiation or triplet sensitization with sensitizer E_T ～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.