Photosensitized oxidations of substituted pyrroles: unanticipated radical-derived oxygenated products.
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 gamma-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.
Alberti
Mariza N
MN
Department of Chemistry, University of Crete, 71003 Voutes, Heraklion, Crete, Greece.
Vougioukalakis
Georgios C
GC
Orfanopoulos
Michael
M
D005609:Free Radicals / Q000737:chemistry; D007769:Lactams / Q000138:chemical synthesis* / Q000737:chemistry; D015394:Molecular Structure; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D010777:Photochemistry; D011758:Pyrroles / Q000737:chemistry*; D013237:Stereoisomerism
D005609:Free Radicals; D007769:Lactams; D011758:Pyrroles; D010100:Oxygen