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

Abstract

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 alpha-trimethylsilylmethansulfonamide (E = SiMe3)- and alpha-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 alpha-trimethylsilylmethansulfonamide- and alpha-carboxymethansulfonamide-terminated phthalimides and naphthalimides that that contain an internal tertiary amine donor site (D = NMe) are chemically inefficient and follow a pathway involving alpha-deprotonation at the tertiary amine radical cation center in intermediate, iminium radical-containing, zwitterionic biradicals. In addition, the quantum efficiencies for photoreactions of alpha-trimethylsilylmethansulfonamide- and alpha-carboxymethansulfonamide-terminated phthalimides are dependent on the nature of the internal donor (eg., phi = 0.12 for D = NMs, E = SiMe3; phi = 0.02 for D = S, E = SiMe3; phi = 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 alpha-heterolytic fragmentation reactions influence the chemical yields and quantum efficiencies of SET promoted photocyclization reactions of acceptor-polydonor substrates.

A Green and Highly Efficient Naphthalimide Visible Photoinitiator with an Ability Initiating Free Radical Polymerization under Air

“…Third, from the molecular motif view of point, the methylene ether linkage in ND‐O‐EA generally acts as a hydrogen donor. Moreover, the methylene ether linkage in the side chain is favorable to form an intramolecular exciplex or a radial ion pair and then generate radicals by intramolecular proton transfer . At the same time, this unit could improve the compatibility between the photoinitiator and the monomer…”

Direct and indirect single electron transfer (SET)-photochemical approaches for the preparation of novel phthalimide and naphthalimide-based lariat-type crown ethers

“…Previous studies [31–34 62–67] in our laboratories resulted in the development, mechanistic elucidation, and synthetic application of various kinds of SET-promoted photocyclization reactions of α-trialkylsilyl donor-linked imide acceptor systems and led to an understanding of the factors controling the chemical selectivities and efficiencies. For example, we have demonstrated that intramolecular SET-photochemical reactions of linked α-trimethylsilyl n-electron donor-phthalimides/naphthalimides produce functionalized macrocyclic poly-ethers, -thioethers, -amides, and -peptides via the intermediacy of interconverting zwitterionic biradicals 21 and 22 .…”

“…The biradicals 24 generated by silyl transfer from the zwitterionic biradicals undergo C–C bond formation to form macrocyclic products 23 (Scheme 5). In addition, these studies have shown that the length and nature of the chain linking the α-trimethylsilyl n-electron donor centers and the arylimide acceptors play important roles in controlling the rate of formation of the zwitterionic biradicals 21 and 22 and, thus, affecting the yield of the macrocyclic products [31,33–34 65–67]. Importantly, these efforts provided a solid foundation for the design of new strategies for the preparation of interesting members of the crown ether family.…”

“…In another case, where the intrasite-SET is more rapid than α-fragmentation at the cation radical centers, an equilibrium is attained between the zwitterionic biradicals, the mol fraction of each being controlled by the redox potential of each donor site. Moreover, in these cases, the product yields will rely on the relative rates of the competing fragmentation reactions ( k E1 vs k E2 ) as a result of the fact that the energy barriers for these processes are higher than that of intrasite-SET [31,62–67]. As a result, the number, location, types, and reactivity of zwitterionic radical centers arisen by either direct or intrasite-SET will influence the efficiencies of redox reactions of polydonor-imide acceptor systems.…”

“…In these studies, we assumed that photoirradiation of the bis-donor-linked bis-phthalimides could result in the formation of various interconverting zwitterionic biradicals through SET from donor sites in either oxygen-containing chain to either phthalimide acceptor. Based on observations made in earlier studies that the SET process displays a distance-dependence between donor and acceptor, we anticipated that the most efficient reaction pathway would involve a SET between the closest donor and acceptor moieties rather than the more distant pairs and that this event would selectively and sequentially produce zwitterionic biradicals related to 64 and 67 [31–32 67]. Based on this reasoning, we predicted that bis-crown ethers rather than their cross bridged counterparts would be generated in photoreactions of the α-silyl ether-terminated, polyethyleneoxy-linked bis-phthalimides 56 – 59 .…”