Photosensitized Oxidations of Substituted Pyrroles: Unanticipated Radical-Derived Oxygenated Products
journal contributionposted on 02.10.2009, 00:00 by Mariza N. Alberti, Georgios C. Vougioukalakis, Michael Orfanopoulos
Photooxidation of pyrrole adducts 7−10 has been investigated in order to establish a general reaction pattern and mechanism for the formation of the resulting oxygenated products. The reactions were performed in several solvents utilizing both type I and type II sensitizers. In most cases, photooxidations gave complex mixture of products. Among these products, 5,5- or 6,5-bicyclic lactams (11, 15, and 19), maleimide 12 unsaturated γ-lactams (16 and 20), 5-hydroxylactams (13, 17, and 21), and 5-methoxylactams (14, 18, and 22) were isolated and characterized. Photooxidation of 2,5-dimethyl-substituted pyrrole 10 in aprotic solvents unexpectedly afforded aldehyde 23 as the major product. Moreover, photooxidation of pyrrole adduct 10 in protic solvents exclusively gave the unprecedented solvent-trapped products 24−27. The formation of products 11−22 was rationalized by the intermediacy of a common endoperoxide intermediate, which could be formed by both type I and type II mechanisms. Compounds 23−27 were most probably formed via an electron-transfer mechanism.