PHOTOCYCLIZATION OF ALPHA-KETO AMIDES IN HOMOGENEOUS SOLUTION AND AQUEOUS CYCLODEXTRIN MEDIA - THE ROLE OF ZWITTERIONS AND DIRADICALS IN PHOTOINDUCED ELECTRON-TRANSFER REACTIONS

A mechanistic investigation of the photoreaction of the N,N-disubstituted benzoyl amides, PhCOCONR'R" 1-4 (1: R' = C2H5, R" = Ph; 2: R' = R" = C2H5; 3: R' = R" = CH2Ph; 4: R' = R" = i-C3H7), in homogeneous solutions (water, methanol) and aqueous cyclodextrin is reported. The chief photoproducts, oxazolidin-4-ones, 3-hydroxyazetidin-2-ones, mandelamides, and carbonyl compounds, can all be accounted for by a mechanism involving single electron transfer quenching of the "ketone" excited state with the amide serving as an electron donor. The first quenching product, a zwitterion Z1, undergoes rapid and irreversible decay to a diradical, D, and a second zwitterion, Z2, by a sequence of proton transfer followed by a second intramolecular electron transfer. Formation of the observed products can be accounted for by decay or interception of D or Z2. Although D and Z2 appear to be resonance contributors to a single structure, calculations suggest that their relative importance varies greatly with configuration, such that a "planar" structure is predominantly zwitterionic while partially or largely twisted structures should be predominantly diradical. The application of this principle leads to a consistent interpretation of the varied product distribution that occurs as substituents on the amide and the reaction medium are varied.