PET processes between ground and excited states of donor-acceptor pairs lead to the formation of radical ion pairs which upon disproportionation gives neutral radicals and/or ions required for synthetic purposes. A host of new reactions has been uncovered, the nature of which is governed by the chemical properties and reactivity profiles of these reactive intermediates. Our objective in this chapter will be to enlist critical examples from the last ten years dealing with the radical ion reactions of synthetic importance initiated by photoexcitation of donor-acceptor pairs.

Photoinduced electron transfer (PET) in organic synthesis

In recent years, electron transfer has been a central focus of general interest in a variety of fields of chemistry, particularly in organic photochemistry, since photoexcitation of organic molecules is a convenient way to generate the ion radicals which are very reactive to undergo versatile chemical reactions of mechanistic and synthetic significance. A recent Yearly Review covers the literature on photochemical electron-transfer reactions and exciplexes from 1983 to the end of June 1984 (Creed and Caldwell, 1985). Other reviews have appeared on general aspects of electron transfer in photochemistry (Kavarnos and Turro, 1986), on proton transfer of ion-radical pairs following photoinduced electron transfer (Lewis, 1986), on photoaddition reactions of aromatic compounds by way of exciplexes and electron transfer (McCullough, 1987). on structures and chemical reactivities of photogenerated organic cation radicals studied by means of CIDNP techniques (Roth, 1987), on organic photochemistry involving charge-transfer and electron-transfer events (Mattay, 1987), on electron transfer of specific molecules (Balzani, 1987), and on general electron-transfer photochemistry (Fox and Chanon, 1988). Several reviews are also mentioned in the appropriate sections below. This review covers the literature on electrontransfer organic and bioorganic photochemistry mainly from 1985 to April 1988, whereas earlier works are also included as complementary references. However, this review does not describe general and comprehensive aspects of electron-transfer photoreactions of organic molecules but specialized topics on photoinduced electron transfer in homogeneous solution, because the authors wish to avoid duplication of earlier reviews. The topics which have been excluded are photooxidations, polymer and polymerization photochemistry, and photochemistry in organized media and on solid surfaces, even though electron transfer is involved as an essential mechanistic pathway.

Electron‐transfer organic and bioorganic photochemistry

This review deals with the recent advances in the photochemistry of cyclopropanes, methylenecyclopropanes, and vinylidenecyclopropanes cis–trans Photoisomerization of 1,2-diarycyclopropanes via excited singlet and triplet states and radical cations, photochemical polar addition to arylcyclopropanes, and photooxygenation of arylcyclopropanes and methylenecyclopropanes giving cyclic peroxides are described The new photochemistry of vinylidenecyclopropanes including cis–trans photoisomerization, (3+2) photocycloadditions, and photorearrangements is also discussed

Photochemistry of cyclopropanes, methylenecyclopropanes, and vinylidenecyclopropanes

The primary aim of this chapter is to provide a review of the chemistry of PET generated aromatic radical ions in solution. Extensive delocalization of the unpaired electron makes these species relatively stable. Thus it is possible to exploit both aspects of their reactivity (as radicals and as ions) and to intercept them both with electrophiles (or respectively with nucleophiles) and with radicals. Other possibilities are that the original radical ions recombine or that one of them (or both) fragments either α or β to the ring yielding aryl (or respectively benzyl) radicals, which can then be trapped. Examples of all these mechanisms are given, and the synthetic potential offered by PET induced reactions of aromatics is discussed.

PET-reactions of aromatic compounds

Construction of Isoxazolidines through Formal [3+2] Cycloaddition Reactions of in situ Generated Nitrosocarbonyls with Donor–Acceptor Cyclopropanes: Synthesis of α-Amino γ-Butyrolactones

9,10-Dicyanoanthracene-sensitized photoreaction of electron-rich 1,2-diarylcyclopropanes with NO in acetonitrile afforded 3,5-diaryl-2-isoxazolines in good yields. The cyclopropanes bearing different aryl-substituents gave a mixture of two isomeric 2-isoxazolines. Mechanistic features of this reaction are described.

Katsuhiko Yoshida

Papers

9,10-Dicyanoanthracene-sensitized NO insertion into cyclopropane ring via photoinduced electron transfer.