Intramolecular photochemical hydrogen abstraction in 2-alkylpyrazines and 2-alkylpyridines
摘要:
Photochemical;hydrogen abstraction in 2-alkylpyrazines 5-8 and 2-alkylpyridines 9-11 proceeds analogously to the process of eq 1. Hydroxylic solvent causes quantum yields for pyrazine fragmentation products (Phi(p),) to increase about 1 order of magnitude. Pyridine fragmentation takes place from both singlet and triplet states. Pyrazine fragmentations follow Stern-Volmer kinetics with modest bond strength selectivity (1 degrees:3 degrees similar to 1:17 for 5a and 5e). Methyl substitution lowers the rate of abstraction in pyrazines and trifluoromethyl substitution increases Phi(p), in;pyridines. It is suggested that these effects reflect changes in the n pi* and pi pi* character of the reactive excited states.