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 photoinduced 3,4 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, ∆G C 0 (eq 1) (BDFE: R-X bond dissociation free energy, E 0 s: standard potentials of the subscript couples). The influence of these three parameters on the mechanism has been illustrated by several experimental examples. 1d,2,6 There are also a few borderline cases where, as shown in Figure 1, for the same cleaving acceptor molecule, a transition between concerted and stepwise mechanisms has been observed upon increasing the driving force of the reaction by decreasing the electrode potential or the standard potential of the donor in electrochemical 6c,7 and homogeneous examples, 1c,8 respectively. The observation of such mechanism transitions demonstrates that the occurrence of a concerted mechanism is related not merely to the fact that the intermediate does not exist, but rather to the energetic advantage of one pathway over the other. It also allows a clear-cut experimental distinction between the two mechanisms in situations where the kinetics of the stepwise process is controlled by the initial electron-transfer pathway and is therefore not easy to distinguish from the concerted mechanism. In cyclic voltammetry, 9 when the wave remains irreversible whatever the scan rate, mechanism diagnosis is based on the value of an apparent transfer coefficient, R that may be derived from the variation of the peak potential, E p , with the scan rate, V, or from the value of the peak width, E p/2 -E p , When the value of R reaches 0.5 or less, the rate-determining step is an electron-transfer step and R is then a true transfer coefficient (symmetry factor). The rate-determining electrontransfer step may then be either the initial electron transfer of the stepwise pathway or the dissociative electron transfer of the concerted pathway. In both cases, the activation free energy is expected to be a quadratic function of the standard free energy of the reaction and thus R varies lin...

Electrocatalysis and electron transfer mechanisms in the reduction of organic halides at Ag

“…In analogy with chlorides and bromides, which have been more extensively investigated [3,33,34], we may assume a concerted DET mechanism for the aliphatic iodides 1b, 2b and 4c at both electrodes. As to the aromatic compound 6c, DET of iodobenzenes is reported to be a borderline between concerted and stepwise mechanisms, with the possibility of a passage from one mechanism to the other, depending on the reaction conditions [35]. Therefore, we may assume a borderline situation for 4−iodotoluene.…”

Reduction of substituted benzonitrile pesticides

“…Carbon-halogen cleavage has been studied extensively in nonaqueous solvents for a number of substituted benzyl bromides, substituted iodobenzene, bromobenzene, and chlorobenzene derivatives [8][9][10][11][12]. The electrochemical reduction of a series of halogenated benzonitriles in DMF involves the initial one-electron reduction yielding the anion radical with a rapid loss of the halide ion to give the corresponding cyanophenyl radical [12].…”

Electron Transfer to Benzenes by Photoactivated Neutral Organic Electron Donor Molecules

“…Thus, we have shown that the neutral ground-state organic electron donor 1 [7,8] (Scheme 1) reduces iodoarenes [9,10] to aryl radicals [11] while the stronger donors 2 [12] and 3 [13] under milder conditions, afford aryl anions from the same substrates through transfer of two electrons. These donors also reduce sulfonamides, acyloin derivatives, and Weinreb amides.…”

“…Thus, we have shown that the neutral ground-state organic electron donor 1 [7,8] (Scheme 1) reduces iodoarenes [9,10] to aryl radicals [11] while the stronger donors 2…”