Optimizing Electron Transfer Mediators Based on Arylimidazoles by Ring Fusion: Synthesis, Electrochemistry and Computational Analysis of [9,10-D]Phenantroimidazoles

One approach to increase the efficiency of electroorganic synthesis is to use electron transfer catalysts in an indirect electrolysis (1). Since the electron transfer step is shifted from a heterogeneous to a homogeneous process, the kinetic inhibition associated with the electron transfer from electrode to substrate can be eliminated (2). Typically, higher and/or totally different selectivity is achieved. The electron transfer can occur even against a potential gradient if one or more of the subsequent steps is an irreversible chemical reaction (1,2). In many cases, a mediator can help to suppress undesired side reactions and electrode passivation.

A new class of metal-free, easy to synthesize redox catalysts based on the triarylimidazole framework 1 was developed in our laboratory (see Figure, left) (3). Since the electronic character of the substituents strongly affects the oxidation potential, the redox properties of the mediators can be tuned within a wide range and tailored to catalyze specific redox reactions (4). The successful conversion of several model compounds using catalytic amounts of 1 demonstrates that triarylimidazoles can be useful mediators for the activation of benzylic C-H bonds under mild conditions.

With a simple structural modification of 1 a significant improvement of the redox properties can be achieved (5). By connection of the ortho-carbons of the aromatics positioned at C-4 and C-5, fused framework 2 is generated (see Figure, right). The distortion from planarity is thereby strongly decreased and the influence of the substituents on the redox properties is enhanced. This modification not only opens up a much broader range of available redox potentials for the resulting [9,10-d]phenantroimidazoles (2), but also leads to improved stability of the corresponding radical cation. These concepts were verified with eight new [9,10-d]phenantroimidazole derivatives, using cyclic voltammetry and DFT calculations. An excellent linear correlation of the calculated effective ionization potentials with the experimental oxidation potentials was obtained, allowing for an accurate prediction of oxidation potentials of derivatives yet to be synthesized. Moreover, high catalytic activity was found for electrooxidative C-H activation reactions.

References:

1. J.-M. Savéant, Chem. Rev. 108, 2348 (2008).

2. E. Steckhan, Angew. Chem. Int. Ed. Engl. 25, 683 (1986).

3. C.-C. Zeng, N.-T. Zhang, C. M. Lam and R. D. Little, Org. Lett. 14, 1314 (2012).

4. N.-T. Zhang, C.-C. Zeng, C. M. Lam, R. K. Gbur and R. D. Little, Org. Chem. 78, 2104 (2013).

5. R. Francke and R. D. Little, manuscript submitted (2013).