In recent years organic electrosynthesis has become recognized as a promising green methodology for organic chemistry. It can fulfill several important criteria that are needed to develop environmentally compatible processes. Using electric current as reagent, it can be used to replace toxic or dangerous oxidizing or reducing reagents. Moreover, the total energy consumption can be reduced and unstable and hazardous reagents can be generated in situ. These are just a few of the most important attributes that render electrochemistry environmentally relevant. Recent group-developments in this field comprise electrocatalytic reactions, electrochemical reactions that use renewable starting materials (biomass) and green electrolytes based on micro- and nano-emulsions or ionic liquids.

Within the field of organic electrochemistry we are currently focused on the following aspects:

1. Novel Mediators for Indirect Electroorganic Synthesis

2. Green Electrolytes and Waste Reduction

3. Methods Development with Applications to Synthesis

4. Synthesis of Complex Molecules Using Electrosynthetic Transformations

1. Novel Mediators for Indirect Electroorganic Synthesis

Heterogeneous electrochemical reactions are usually hindered due to the kinetic inhibition of the electron transfer between the electrode surface and the substrate. Electrochemically generated redox agents can widen the scope of applications, as the crucial reaction step is shifted from a heterogeneous to a homogeneous electron transfer (indirect electrolysis).[1] If the following step is an irreversible chemical reaction, the electron transfer reaction can even occur against a potential gradient. Therefore, much higher or totally different selectivity can be achieved with lower energy consumption. Preferably, a reversible redox couple is interposed between an electrode and the desired reaction in order to avoid reagent waste and difficult separation procedures (see Scheme, example for oxidative mediator).

Previously, transition metal or halogen compounds were used as redox catalysts, as oxidation states can be easily changed. In recent years progress was also made with organic electron transfer reagents. For example, p-substituted triarylamines developed by Steckhan et al. form stable radical cations upon anodic oxidation and can therefore be employed for a wide range of oxidative transformations with high selectivity. Another prominent example is the anodic regeneration of catalytically employed (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO reagent), which can be used for a variety of oxidation reactions.

Our current efforts to develop more efficient redox mediators are focused on a new class of metal-free, easy to synthesize redox catalysts based on a triarylimidazole framework. With those synthesized thus far, one can access a potential range of ca. 700 mV, depending on the substitution pattern on the aryl units. They have already proved to be useful mediators for the activation of benzylic C–H bonds under mild conditions (see Scheme).[2-4]

2. Green Electrolytes

We have designed, synthesized and are testing a new recyclable “polymeric ionic liquid (PIL) and carbon nanoparticle composite” in order to overcome separation and waste problems that are associated with the use of common supporting electrolytes in organic electrosynthesis. The attractive interactions that exist between the cation of an ionic liquid and the surface of carbon nanoparticles are utilized in order to generate the composite material. This novel type of supporting electrolyte combines the features of a polymeric ionic liquid to serve as an electrolyte and the properties of the Super P carbon black to generate a dispersion. Electrolysis can now be performed without adding any other supporting electrolyte and to efficiently and simply recover and reuse the composite in subsequent electrolysis.

3. Methods Development with Applications to Synthesis

In order to establish alternatives to traditional C,C-bond formation and cyclization reactions in organic chemistry we are interested in the development of electroreductive cyclization (ERC) reactions.[3] This process is characterized by ? bond formation between two formally electrophilic carbons, often, but not always, the ?-carbon of an electron deficient alkene, and the carbonyl carbon of a remotely tethered subunit G (see Scheme below). To accomplish this transformation requires the reduction of an electrophore, for example an electron deficient alkene as depicted below. The resulting radical anion, or the carbanion which is obtained by protonation and the addition of a second electron, possesses nucleophilic character, thereby facilitating the cyclization reaction.

During our efforts to optimize ERC reactions we found that using catalytic nickel(II) salen as a mediator proved to be very efficient.[5] The transformation of bisenoates (see example in scheme below) were achieved in good yields using either a mercury pool or an environmentally preferable reticulated vitreous carbon (RVC) cathode. These examples represent the first instances wherein a nickel salen complex has been used in this manner.

4. Synthesis of Complex Molecules Using Electrosynthetic Transformations

In one example from our group the total synthesis of daucene was completed using an electroorganic conversion in the key step.[6] It was the first time that housane-derived cation radicals have been used as the key intermediate in the synthesis of a natural product. The term “housane” refers to molecules possessing a bicyclo[2.1.0]pentane core (see Scheme, below). The transformation used in the construction involves an oxidation to generate the cation radical via either a chemically or an electrochemically mediated electron transfer, the latter process using tris(p-bromophenyl)amine as the mediator. The two methods were compared, and guiding principles were formulated to assist in deciding how best to implement each. A mechanistic proposal calling for the use of a catalytic quantity of the electrochemical mediator and the consumption of exceptionally small quantities of current is advanced.

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