Electron‐transfer Reactions of Aromatic Compounds

Abstract

Aromatic molecules are inclined to undergo electron-transfer reactions. The additional charges introduced by the electron-transfer process are efficiently stabilized by delocalization. Nevertheless rearrangements of the molecular skeleton or followup reactions can occur. The course of the electron-transfer reactions and the properties of the species thus formed are the subject of this section.In particular, we concentrate on the species formed after an one-electron transfer reaction. Starting from closed-shell diamagnetic precursors, paramagnetic stages, i.e. radical cations or radical anions are formed in this first step. To understand the properties and reactivity of these species, detailed knowledge in terms of charge and spin delocalization and electronic structure is an important prerequisite. This information is preferably derived from electronic and paramagnetic resonance (EPR and electron-nuclear multiple resonance-techniques such as ENDOR or Triple spectroscopy). It has recently been shown that the experimental results can be substantiated by the use of quantum-chemical calculations. In addition to Hartree-Fock-based ab initio procedures, calculations on the density-functional level of theory have been established as rather efficient tools. Therefore, before representing examples of electron-transfer-generated radicals a short survey of the computational methods is given.Another substantial factor directing the kinetic and thermodynamic stability of charged radicals is ion pairing. This phenomenon, although well established for many years, is often not directly distinct in experiments. To establish the importance of ion pairing, or, in other words, supramolecular interactions, a separate introductory chapter is dedicated to these aspects.The references are selected particularly from recent publications, without, however, ignoring some 'classical' contributions. In some sections topics are included which do not strictly represent aromatic molecules but involve interactions of ztype orbitals.

Photoinduced oxidative activation of electron-rich arenes: alkenylation with H2 evolution under external oxidant-free conditions

“… 1 Electron-rich aromatic compounds are inclined to undergo electron-transfer reactions. 2 One-electron oxidation of aromatic compounds would lead the generation of the corresponding arene radical cations that are very reactive species and tend to undergo follow-up reactions, such as rearrangement, 3 trapping nucleophiles 1d , 4 or dimerization, 5 to yield more persistent radical cations. Through a π radical cation of arenes, the biaryl dehydrodimerization of aromatic compounds has been well-studied under anodic or metal-ion oxidation conditions.…”

Benzophenone−Phenylthioacetic Acid Tetraalkylammonium Salts as Effective Initiators of Free-Radical Photopolymerization of Vinyl Monomers, Mechanistic Studies

“…The first factor is the precise nature of the reacting partners taking part in the electron-transfer process. The second is the efficiency of any subsequent fragmentations yielding secondary radicals and radical ions − that are often much more effective in initiating polymerizations than the primary radicals. Therefore, a detailed knowledge of such primary and secondary photochemical mechanisms is critical for the design of practical systems of initiating polymerizations.…”