Photochemical hydrogen abstraction by singlet and triplet nπ* states of aromatic nitrogen: Fragmentation of 4-alkylpyrimidines and 2-alkylquinolines

Article abstract of DOI:10.1021/ja00166a033

On either direct irradiation in water or triplet-sensitized irradiation in acetone, pyrimidines 1a-c undergo hydrogen abstraction from an nπ* triplet state, fragmentation, and rearomatization to form 5 (eq 1). Similarly, direct irradiation of quinolines 2a-c in benzene or tert-butyl alcohol furnishes 6 from an nπ* singlet state. Quantum yields for products (Φ₅ and Φ₆) and Stern-Volmer quenching studies on these reactions (Tables I and II) provide mechanistic information concerning abstraction by aromatic nitrogen. Unlike Φ for abstraction by carbonyl triplets, Φ₅ and Φ₆ do not increase in hydrogen-bonding solvent, and there is little rate difference in transferring primary, secondary, and tertiary hydrogen in the initial step. Singlet and triplet abstractions show little difference in selectivity, but the singlet reaction is ? 10⁴ faster. Concentration studies show that fragmentation of 1a-c is self-quenched, while reaction of 2a-c is enhanced with increasing concentration.