Electron transfer photochemistry of bifunctional strained-ring and unsaturated systems

Abstract

Abstract:Organic radical cations, generated by electron transfer from bi-or trifunctional donor terpenes to photo-excited acceptors, undergo a variety of intra-and intermolecular reactions, including nucleophilic capture and sigmatropic shifts, deprotonation, intramolecular nucleophilic capture, and various cyclizations.The structures and reactions of organic radical cations have been the focus of much interest for the past two decades (ref. 1,2). A rich variety of substrates have been investigated by physical and chemical techniques to delineate changes in geometry upon oxidation, to assess the spin density distribution in the radical cations, and to elucidate parameters affecting their structures (ref. 2,3). Particularly the interactions between adjacent functions, conjugated or nonconjugated, pairs of olefinic a-systems or strained rings, or of strained ring moieties with olefinic fragments has attracted considerable interest (ref. 3).We discuss attempts to utilize light-induced radical cation reactions for synthetidpreparative purposes. We have studied photosensitized reactions of various donors with two or three functional groups, including unsaturated and strained ring moieties as well as an alcohol function as an intramolecular nucleophile. The sensitizers were chosen to give the putative electron transfer reaction either a marginal (-AG -0.1 eV) or a more substantial driving force (-AG > 0.5 eV). The driving force for radical ion formation can be further varied by using a variety of solvents with different polarities; the rates of back electron transfer can be varied by conducting the reactions with or without a co-sensitizer. One significant question in radical cation chemistry concerns the relationship between structure and reactivity. Since, the reactivity depends largely on I the distribution of unpaired spin and positive charge in the intermediate, we will discuss structural features of some radical cations derived from bicyclic vinylcyclopropanes. Structures of Vinylcyclopropane Radical CationsThe structures of vinylcyclopropane radical cations can be assigned by considering the frontier molecular orbital (FMO) of the cyclopropane unit and the interaction between the two moieties. Cyclopropane has a degenerate pair of in-plane e' orbitals (s, a); ionization results in a doubly degenerate 2E' state; first-order Jahn-Teller distortion leads to two non-degenerate electronic states, 2A1 and 2B2 (C2v symmetry) (ref. 4). The 2A1 component (orbital s singly occupied) relaxes to a "one-electron-bonded trimethylene" structure (type A) with one lengthened C-C bond. The alternative 2B2 component has the antisymmetrical molecular orbital a singly occupied; it gives rise to a structure with two lengthened C-C bonds resembling a "acomplex" (type B) (ref. 5).Radical cations of substrates containing strained rings and alkene groups may be stabilized by delocalization of spin and charge. We discuss the structures of four vinylcyclopropane radical cations in which the two functionalities are locked in either the ant...

The Protoadamantane Radical Cation

“…The largest peak on the final difference map was 0.672 e Ϫ / Å 3 . The calculated density for C 19 H 17 N 3 is 1.272 g/cm 3 and F(000) is 304 e Ϫ . The non-hydrogen atoms were refined anisotropically, while hydrogen atoms were placed in ideal positions with their coordinates and thermal parameters riding on the attached carbon atoms.…”

Hydrocarbon σ‐radical cations

“…The elimination of an electron from bonding molecular orbitals of saturated hydrocarbons gives so‐called σ‐radical cations,14–20 i.e., species that are characterized by partially broken and elongated σ CC and/or σ CH bonds 21. The hydrocarbons with the degenerate HOMOs undergo the Jahn–Teller9 distortions.…”

Cyclization of Terpenoid Dicarbonitrile Polyalkenes upon Photoinduced Electron Transfer to 1,4-Dicyano-2,3,5,6-tetramethylbenzene and Other Cyanoarenes

“…The role of radical ions in the photooxidation of unsaturated compounds has been the subject of intensive studies. − The singlet states of cyanoaromatics are used in this connection as efficient electron acceptors upon irradiation with UV light; the most widely employed acceptors are 1,4-dicyanobenzene ( A2 ) 8,13,18,28,29 and 9,10-dicyanoanthracene( A5 ). − ,− ,− ,, In the presence of an electron donor (D) in a polar solvent, the quantum yield and lifetime of 1 A* fluorescence decrease, as expected for an electron-transfer quenching mechanism (2a) …”