Electron and hydrogen transfer in organic photochemical reactions † Norbert Hoffmann a * Electron and hydrogen transfers are basic steps in many chemical reactions. Photochemical or electronic excitation significantly influences the reactivity of chemical compounds and thus also these transfer processes. Furthermore, the formation of typical in- termediates has a decisive impact on the outcome of the transformations. Based on these properties of photochemical processes, efficient homogeneous and heterogeneous photocatalytic transformations for application to organic synthesis have been devel- oped. Efficient reactions without any chemical activation but induced by photochemical electron transfer can also be carried out. The photon acts as a traceless reagent. In the context of photochemical hydrogen transfer, two processes are frequently encoun- tered. The two particles the electron and the proton are transferred either simultaneously or in two steps. In the latter case, the electron is transferred first and the proton follows. Both processes are applied to organic synthesis. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: heterogeneous photocatalysis; homogeneous photocatalysis; hydrogen abstraction; photochemical electron transfer; radical ions; radical reactions; stereo and regioselectivity; α,β-unsaturated carbonyl compounds INTRODUCTION Photochemical reactions start with absorption of light by a chromophore. Molecules become electronically excited. The electronic configuration of chemical compounds is thus changed. [1,2] Because the chemical properties are determined by the electronic configuration, the photochemical reactivity of compounds is significantly different from that one of the ground state. Frequently, the photochemical reactivity is even complementary to the thermal reactivity. Such phenomena are conveniently described by means of potential hypersurface topology. [3] These fundamental properties of photochemical reactions make them particularly interesting for application to organic synthesis. Products and entire compound families be- come easily available that cannot be synthesized by conven- tional methods of organic synthesis. [4,5] Many photochemical reactions are carried out without chemical activation with acids, bases, metals or different catalysts that limits the forma- tion of side products or waste. They are particularly attractive in the context of sustainable or “green” chemistry. [6,7] In this context, the photon is considered as a traceless reagent. [8] Photochemical transformations are also carried out in particu- larly optimized reactor systems such as micro reactors or continuous flow reactors that offer perspectives for a more extensive application in industry. [7,9] Electronic excitation also has an influence on redox poten- tials [10,11] and on acidity and basicity of chemical compounds. [12] Electron [13] and hydrogen transfers [14] are described with the Marcus theory. The present article deals with hydrogen transfer and the fact that the two corresponding particles, the electron and the proton, are transferred in different ways. In a very general context, these transfers are discussed with the concept of proton- coupled electron transfer. [15–17] As far as application to organic synthesis is concerned, the way in which the particles are trans- ferred has a significant influence on the outcome of the reaction. This mechanistic question is therefore of central interest. In organic photochemical reactions, most frequently, two types of mechanisms are discussed. A hydrogen atom may be transferred from a hydrogen donor to a photochemically excited species in one step. The electron and the proton are transferred almost simulta- neously (Fig. 1, Eqn 1). Or the electron is transferred first, leading to the formation of a radical ion pair. The proton follows in an acid base reaction (Eqn 2). In contrast to many types of proton-coupled elec- tron transfer, both particles are transferred from the same site of the hydrogen donor to the same site of the acceptor. In a narrow def- inition, this is called “hydrogen atom transfer. ” [17,18] In the first case (Eqn 1), the electron transfer is endergonic while the overall reaction is exergonic. In the second case (Eqn 2), both steps are exergonic. Beyond these mechanisms, proton exchange between photo- chemically generated intermediates, for example, a radical ion and the reaction medium may occur, which also leads to unusual final products when compared with similar ground state reactions. In the present article, examples of such processes are discussed that have a significant influence on the outcome and are therefore interesting in the context of an application to organic synthesis. PHOTOREDOX CATALYTIC REACTIONS α,β-Unsaturated carbonyl or carboxyl compounds are very valuable synthons in organic chemistry. Addition of nucleophiles * Correspondence to: N. Hoffmann, CNRS, Université de Reims Champagne-Ardenne, ICMR, Equipe de Photochimie, UFR Sciences, B.P. 1039, 51687 Reims, France. E-mail: norbert.hoffmann@univ-reims.fr † This article is published in Journal of Physical Organic Chemistry as a special issue on the International Symposium on Reactive Intermediates and Unusual Molecules 2014 on Physical Organic Chemistry by Robert Moss (Rutgers University USA) and Anna Gudmundsdottir (University of Cinncinnati, USA). a N. Hoffmann CNRS, Université de Reims Champagne-Ardenne, ICMR, Equipe de Photochimie, UFR Sciences, B.P. 1039, 51687, Reims, France Special Issue Review Received: 21 July 2014, Revised: 20 August 2014, Accepted: 24 August 2014, Published online in Wiley Online Library: 10 October 2014 (wileyonlinelibrary.com) DOI: 10.1002/poc.3370 J. Phys. Org. Chem. 2015, 28 121–136 Copyright © 2014 John Wiley & Sons, Ltd. 121