Figure 3.9 Photochemistry by electron transfer sensitization.
Several aromatic molecules undergo no efficient photochemistry of their own. Thus, these molecules are well suited for use as electron transfer sensitizers. According to the substrate and the conditions chosen, they may form either complexes (or tight radical ion pairs) or free, solvated radical ions (Figure 3.9).
Electron transfer sensitization allows either the radical cation or the radical anion of an aromatic alkene to form as desired, which finally results in nucleo-phile addition with Markovnikov and anti-Markovnikov regiochemistry. In an apolar solvent, the tight radical ion pair undergoes a stereoselective reaction when the electron-accepting sensitizer is chiral (Figure 3.10).13
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