Sun Wha Oh

Single Electron Transfer-Promoted Photocyclization Reactions of Linked Acceptor−Polydonor Systems: Effects of Chain Length and Type on the Efficiencies of Macrocyclic Ring-Forming Photoreactions of Tethered α-Silyl Ether Phthalimide Substrates

Results of an investigation, aimed at gaining information about the factors governing the efficiencies of single electron transfer (SET)-promoted photocyclization reactions of linked acceptor-polydonor systems, are described. One set of substrates used in this effort includes alpha-trimethylsilyl ether terminated, polymethylene- and polyethylenoxy-tethered phthalimides and 2,3-naphthalimides. Photocyclization reactions of the polyethylenoxy-linked phthalimides and naphthalimides were observed to take place in higher chemical yields and with larger quantum efficiencies than those of analogs containing polymethylene tethers of near equal length. These findings show that the rates of formation of 1,omega-zwitterionic biradicals that serve as key intermediates in the photocyclization processes are enhanced in substances that contain oxygen donor sites in the chain. The findings suggest that these donor sites facilitate both initial SET to acceptor excited states and ensuing intrachain SET, resulting in migration of the cation radical center to the terminal alpha-trimethylsilyl ether position. In addition, an inverse relationship was observed between the quantum yields of photocyclization reactions of the tethered phthalimides and naphthalimides and the length of the polyethylenoxy chain. Finally, the roles played by chain type and length in governing photoreaction efficiencies were investigated by using intramolecular competition in photoreactions of polyethylenoxy and polymethylene bis-tethered phthalimides. Mechanistic interpretations and synthetic consequences of the observations made in this study are discussed.

Single electron transfer-induced photocyclization reactions of N-[(N-trimethylsilylmethyl)aminoalkyl]phthalimides

Abstract Studies have been conducted to explore single electron transfer (SET) induced photocyclization reactions of N -[( N -acetyl- N -trimethylsilylmethyl)amidoalkyl]phthalimides (alkyl≡ethyl, n -propyl, n -pentyl and n -hexyl) and N -[( N -mesyl- N -trimethylsilylmethyl)amidoethyl]phthalimide. Photocyclizations occur on irradation of these substances in methanol in modest to high yields to produce cyclized products in which the phthalimide carbonyl carbon has become bonded to the carbon of the side chain in place of the trimethylsilyl group. A mechanism for these photocyclization reactions involving intramolecular SET from nitrogen in the α-silylamido group to the singlet excited state of the phthalimide followed by desilylation of the intermediate α-silylamido cation radical and cyclization by radical coupling is proposed. Furthermore, photoreactivity of N -[( N -methyl- N -trimethylsilylmethyl)aminoethyl]phthalimide differs from the other members of this series. Here a route involving triplet hydrogen atom abstraction predominates over that involving sequential singlet SET-desilylation. A relationship between the quantum efficiencies for reactions of the phthalimides with various α-silyl- n -electron donors and the oxidation potentials of the electron donors has been noted. The results suggest that the rate of desilylation of the cation radical intermediate is an important factor in determining the quantum efficiency of the SET-induced photoreaction and that the desilylation rates are directly proportional to the oxidation potentials of the donors. The efficient and regioselective cyclization reactions observed for photolyses in methanol represent synthetically useful processes for construction of medium and large ring heterocyclic compounds.

Photoaddition Reactions of 1,2-Diketones with Silyl Ketene Acetals. Formation of β-Hydroxy-γ-ketoesters

Photochemical reactions taking place between 1,2-diketones and silyl ketene acetals and their excited state reaction mechanisms have been explored. Irradiation of benzene, acetone, or acetonitrile solutions containing 1,2-diketones and silyl ketene acetals is observed to promote formation of 1,4-dioxenes, resulting from [4 + 2]-cycloaddition, oxetanes, arising by Paterno-Buchi processes, and beta-hydroxy-gamma-ketoesters, generated by SET-promoted Claisen-type condensation. These competitive pathways leading from the excited states of the 1,2-diketones to these products are influenced by solvent polarity and the nature of the silyl ketene acetal and 1,2-diketone. The Claisen-type condensation process, following an SET desilylation pathway and predominating when the photoreactions are carried out in the polar solvent acetonitrile, represents an efficient method to prepare a variety of diversely substituted beta-hydoxy-gamma-ketoesters.

Comparison of photomacrocyclization reactions of trimethylsilyl- and tributylstannyl-terminated phthalimido- and maleimido-polyethers

Abstract SET-promoted photomacrocyclization reactions of trimethylsilyl- and tributylstannyl-terminated phthalimido- and maleimido-polyethers were investigated. The results indicate that the excited state cyclization processes, which take place via sequential SET-destannation pathways, produce macrocyclic polyethers more efficiently than those involving sequential SET-desilylation routes do. In addition, differences in product distributions obtained from photoreactions of trimethylsilyl- and tributylstannyl-terminated maleimido-polylethers suggest that more than one mechanistic pathway is followed in excited-state reactions of the tin-containing substrates.