Photocyclization of β-oxo-amides. Possible electron transfer from amide nitrogen to excited ketone carbonyl

Abstract

Benzoylacetamide (1a) undergoes unquenchable photocyclization to give pyrrolidin-2-one (2a) in benzene. Upon irradiation p-phenylbenzoylacetamide (1d) and 2-naphthoylacetamide (1e) also undergo cyclization to give the pyrrolidin-2-ones (2d) and (2e), respectively. The quenchable portion of the photoreaction of (1a) is greater in methanol than in benzene. Benzoylacetamides having a 2-methyl substituent exhibit similar behaviour although the quenchable portion of the reaction increases with an increase in the number of the 2-methyl substituents. The photoreaction of 2,2-dimethylbenzoylacetamide (1c) is completely quenched by piperylene in methanol. Quantum yields also increase with introduction of a substituent into β-oxo-amides except in the photocyclization of 2-naphthoylacetamides. Irradiation of N-benzyl-N-methyl-β-oxo-amides (1g–i) affords, predominantly, 1-benzylpyrrolidin-2-ones, which are produced via methyl hydrogen abstraction; 2-methyl-β-oxo-amide (1j) however gives 1-methylpyrrolidin -2-one (2j) exclusively, which is formed via benzylic hydrogen abstraction. It is concluded that β-oxo-amides which have no substituents on the 2-position undergo photocyclization mainly through electron transfer from amide nitrogen to singlet-excited ketone carbonyl followed by δ-proton transfer, while β-oxo-amides which carry two methyl substituents on the 2-position undergo the cyclization via normal δ-hydrogen abstraction from the n,π* triplet state. The enol form of β-oxo-amides acts as internal filter in the photoreactions.