Substituent Effects on Photochemical Hydrogen Abstraction in 2-Acylpyridines, 2-Acylpyrazines, and 4-Acylpyrimidines

Article abstract of DOI:10.1021/jo00087a032

Stern-Volmer quenching of the photochemistry of 1c indicates that N- and O-abstraction (eqs 1 and 2, respectively) are quenched at different rates (Figure 1).For quenching of 2c k_qτ is 157 M^-1 and for 3c, 64 M^-1.When 1c is sensitized with triplet sensitizers of increasing E_T,N-abstraction increases (Table 1).These data indicate that N- and O-abstraction in 1c take place from distiguishable triplet states.Survey of Φ_p's of ring-substituted ketones 1b-d, 6d, 7a,b,d, 8b, 9b,d, 10b, and 11d demonstartes the effect of substitution on the competition between N- and O-abstraction (Table 2).For methyl- and dicyano-substituted ketones, the results can be understood simply in terms of shifts in E_T of the n?^* and ??^* states of the heterocycle.The photochemistry of all these ketones requires consideration of interactions among three triplet states.