Can Single-Electron Transfer Break an Aromatic Carbon−Heteroatom Bond in One Step? A Novel Example of Transition between Stepwise and Concerted Mechanisms in the Reduction of Aromatic Iodides

Abstract

Single-electron transfer is often accompanied by bond cleavage or bond formation reactions converting initially formed ion radicals into radicals. Electron transfer and bond breaking may involve a transient ion radical intermediate or occur in a concerted manner, as sketched in Figure 1.1 The question of the distinction between these two mechanisms and of the nature of the molecular factors that favor one mechanism over the other arises for thermal (electrochemical, homogeneous)2 as well as for photoinduced3,4 reactions.5 With aliphatic molecules, injection of an electron leads to a purely dissociative state, and accordingly, reductive cleavage follows a concerted mechanism. With molecules containing low lying orbitals, such as π* orbitals, able to host transitorily the incoming electron, reductive cleavage may follow one or the other of the two mechanisms. When the cleavage of the anion radical is fast, the rate-determining step of the stepwise pathway is the initial electron transfer. Under these conditions, the thermodynamic factor governing the competition between the two mechanisms is the standard free energy of anion radical cleavage, ∆GC 0 (eq 1)