The Photochemical Approach to Helicenes

In this chapter, the authors have decided to follow the nomenclature recommendations of IUPAC [1] for class names of organic compounds, which classifies helicenes as “ortho-fused polycyclic aromatic or heteroaromatic compounds in which all rings (minimum five) are angularly arranged so as to give helically shaped molecules, which are thus chiral.” Therefore, the following text includes [n]helicenes where n≥ 5 and the photoreaction is the very last step of their preparation, unless stated otherwise. Two basic photo-approaches (oxidative photocyclodehydrogenation and photoinduced elimination; for details, see Section 1.2) were used for the preparation of all helicenes listed in this chapter. The photochemically created bonds are highlighted in red in all figures. Non-oxidative photochemical approaches are discussed in specific cases. At the beginning of the chapter, some general features are mentioned, including the mechanism of the photocyclization, reaction conditions, attempts at asymmetric photosynthesis, and the synthetic approach to starting materials. The appropriate helical structures and their preparations are described in sections on carbo-, aza-, thia-, and phosphahelicenes and other helicenes. Helicene-like molecules, including dihydrohelicenes, are discussed separately, as well as photochemical transformations of helicenes.