The Photochemistry of Polydonor-Substituted Phthalimides:  Curtin-Hammett-Type Control of Competing Reactions of Potentially Interconverting Zwitterionic Biradical Intermediates

The results of studies designed to obtain information about the factors that control the chemical efficiencies/regioselectivities and quantum yields of single electron transfer (SET)-promoted reactions of acceptor−polydonor systems are reported. Photochemical and photophysical investigations were carried out with bis-donor tethered phthalimides and naphthalimides of general structure N-phthalimido- and N-naphthalimido-CH₂CH₂-D-CH₂CH₂-NMsCH₂-E (E = SiMe₃ or CO₂NBu₄ and D = NMs, O, S, and NMe). These substrates contain common terminal donor groups (NMsCH₂SiMe₃ or NMsCH₂CO₂NBu₄) that have known oxidation potentials and cation radical fragmentation rates. Oxidation potentials and fragmentation rates at the other donor site in each of these substrates are varied by incorporating different heteroatoms and/or substituents. Photoproduct distribution, reaction quantum yield, and fluorescence quantum yield measurements were made. The results show that photocyclization reactions of α-trimethylsilylmethansulfonamide (E = SiMe₃)- and α-carboxymethansulfonamide (E = CO₂NBu₄)-terminated phthalimides and naphthalimides that contain internal sulfonamide, ether, and thioether donor sites (D = NMs, O, or S) are chemically efficient (80−100%) and that they take place exclusively by a pathway involving sequential photoinduced SET (zwitterionic biradical desilylation or decarboxylation) biradical cyclization. In contrast, photoreactions of α-trimethylsilylmethansulfonamide- and α-carboxymethansulfonamide-terminated phthalimides and naphthalimides that that contain an internal tertiary amine donor site (D = NMe) are chemically inefficient and follow a pathway involving α-deprotonation at the tertiary amine radical cation center in intermediate, iminium radical-containing, zwitterionic biradicals. In addition, the quantum efficiencies for photoreactions of α-trimethylsilylmethansulfonamide- and α-carboxymethansulfonamide-terminated phthalimides are dependent on the nature of the internal donor (eg., φ = 0.12 for D = NMs, E = SiMe₃; φ = 0.02 for D = S, E = SiMe₃; φ = 0.04 for D = NMe, E = SiMe₃). The results of this effort are discussed in terms of how the relative energies of interconverting zwitterionic biradical intermediates and the energy barriers for their α-heterolytic fragmentation reactions influence the chemical yields and quantum efficiencies of SET promoted photocyclization reactions of acceptor−polydonor substrates.