J . Am. Chem. Soc. 1990, 112, 3940-3944
3940
Photochemical Hydrogen Abstraction by Singlet and Triplet nr* States of Aromatic Nitrogen: Fragmentation of 4-Alkylpyrimidines and 2-Alkylquinolines Sreedharan Prathapan, Sarah Loft, and William C. Agosta* Contribution from the Laboratories of The Rockefeller University, New York, New York 10021-6399. Received October 23, 1989 Abstract: On either direct irradiation in water or triplet-sensitized irradiation in acetone, pyrimidines la-c undergo hydrogen abstraction from an na* 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 nr* singlet state. Quantum yields for products (a5and @6) and Stern-Volmer quenching studies on these reactions (Tables I and 11) provide mechanistic information concerning abstraction by aromatic nitrogen. Unlike @ for abstraction by carbonyl triplets, 0,and @p6do 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 IO4 faster. Concentration studies show that fragmentation of la-c is self-quenched, while reaction of 2a-c is enhanced with increasing concentration. N
Photochemical abstraction of hydrogen by aromatic nitrogen has received much less study than the related abstraction by carbonyl oxygen. I n common with other fundamental properties of nitrogen heteroaromatics,l there is a lack of information in this area despite the biological importance of nitrogen-containing systems. In connection with related photochemical studies,z we have investigated intramolecular transfer of hydrogen to nitrogen in two series of heteroaromatics. The results provide information on selectivity among primary, secondary, and tertiary hydrogen that has long been available for ketones.’q4 The systems examined are 4-alkylpyrimidines (1) and 2-alkylquinolines (2).5 In each
y
1
2
1
1 as Rj =
b = CH,
product for side chains a-c. An earlier study of the photochemistry of various quinolines included 2b; both 6 and propylene were reported as products under preparative conditions, and the quantum yield for 6 (a,) was measured for 0.2 M 2b in benzene and in tert-butyl a l c ~ h o l . ~More recently, methide 7 has been identified spectroscopically as a transient on flash photolysis of
b, R = CHI,
R
-
2
H
2b.10.1 I
C,
R1 = R2 = H
case isopentyl (a), butyl (b), and propyl (c) side chains provide tertiary, secondary, and primary hydrogen, respectively, in a position from which abstraction by nitrogen can occur through a six-membered ring, analogous to the abstraction of y-hydrogen by carbonyl The overall reaction is illustrated in eq 1 for l a , and consists of 1,5 transfer of hydrogen, fragmentation
=
