Photoinduced Electron Transfer Cyclizations via Radical Ions

Abstract

Although photoinduced electron transfer reactions occupy a central position in the chemistry of life [1] their application to organic synthesis started only a few decades ago [2]. At first glance this is somewhat surprising since analogous reactions via neutral radicals are already known for a long time [3]. The reason for this delayed development of photoinduced electron transfer (PET) reactions may be simply related to the role of photochemistry in chemistry in general. Although the beginnings of modern organic photochemistry were laid more than 100 years ago [4], the fundamental understanding of electron transfer in general [5] and of PET processes in particular [1] developed relatively late in the middle of the twentieth century. Therefore, many photochemical transformations that are initiated by electron transfer were originally thought to proceed via homolytic steps. Meanwhile, the basis of photoinduced electron transfer as well as the mechanisms of fundamental processes such as PET-sensitization, co-sensitization or medium and salt effects are widely understood [6]. As a consequence the number of applications of PET reactions in organic synthesis has increased dramatically as noticeable from a series of summarizing reviews and book chapters [2,7]. Among these applications are for example cycloadditions [8], fragmentations [9], macrocyclizations [10], and addition reactions [9], respectively. In order to keep this chapter within an acceptable length we will restrict ourselves to intramolecular cyclization reactions of radical ions. Readers interested in other aspects of photo- reactions involving radical ions may refer to the above mentioned reviews [2,7-10].