Organic syntheses with electrochemically regenerable redox systems

The data on the use of mediator oxidation systems activated by electric current (anodic or parallel anodic and cathodic) in organic electrosynthesis are considered and generalised. Electrochemical activation of these systems permits successful application of catalytic versions and easy scaling of mediator-promoted processes. Chemical and environmental advantages of electrochemical processes catalysed by mediator oxidation systems are demonstrated. Examples of the application of organic and inorganic mediators for the oxidation of various classes of organic compounds under conditions of electrolysis are given.

https://iopscience.iop.org/article/10.1070/RC2009v078n02ABEH003886/pdf

Mediator oxidation systems in organic electrosynthesis

The synthesis of benzaldehyde derivatives by oxygenation of methylarenes is of significant conceptual and practical interest because these compounds are important chemical intermediates whose synthesis is still carried out by nonsustainable methods with very low atom economy and formation of copious amounts of waste. Now an oxygenation reaction with a 100% theoretical atom economy using a polyoxometalate oxygen donor has been found. The product yield is typically above 95% with no “overoxidation” to benzoic acids; H2 is released by electrolysis, enabling additional reaction cycles. An electrocatalytic cycle is also feasible. This reaction is possible through the use of an aqueous sulfuric acid solvent, in an aqueous biphasic reaction mode that also allows simple catalyst recycling and recovery. The solvent plays a key role in the reaction mechanism by protonating the polyoxometalate thereby enabling the activation of the methylarenes by an electron transfer process. After additional proton transfer and ox...

Oxygenation of methylarenes to benzaldehyde derivatives by a polyoxometalate mediated electron transfer-oxygen transfer reaction in aqueous sulfuric Acid.

Oxidation of toluene to benzaldehyde by Mn3+ and OH free radicals, generated in the anodic and cathodic reactions respectively, was carried out simultaneously in the cathodic and anodic compartments of a cell. The selectivity of benzaldehyde was very high in both the anodic and cathodic reactions. The maximum total current efficiency for benzaldehyde production in the paired electrooxidation was 171%.

Paired electro-oxidation. I. Production of benzaldehyde

An electrochemical microreactor for organic electrosynthesis has been investigated for the anodic synthesis of 4-methylanisole to 4-methoxy-benzaldehyde-dimethylacetal in methanol solution. Selectivity and conversion in the single-pass thin-gap flow reactor were examined as a function of the composition of the electrolyte solution, the flow rate and the applied current. The experimental results indicate that potassium fluoride currently used for industrial synthesis and providing higher yields than sodium perchlorate, exerts an influence on the reaction mechanism: high KF concentrations facilitate the undesired oxidation of the diacetal. Nevertheless, a feed solution containing 0.1 M anisole in 0.01 M KF can be converted at 90% in the 100 μm thin-gap cell with acceptable voltages and a measured selectivity of nearly 87%. The selectivity observed substantially higher than that typically observed in conventional electrochemical cells.

A thin-gap cell for selective oxidation of 4-methylanisole to 4-methoxy-benzaldehyde-dimethylacetal

The Mn3+-mediated anodic oxidation of toluene and some substituted toluenes (p-chloro-,p-xylene-,p-nitro-) in 60 wt % H2SO4 has been investigated with respect to reaction kinetics and process techniques. The mediators Co3+ in 3.5 N HNO3 and Ce4+ in 60 wt % H2SO4 are compared with Mn3+ in 60 wt % H2SO4 for unsubstituted toluene. The mediated oxidation of the toluenes proceeds in three well-distinguished steps: benzyl alcohol, benzaldehyde, benzoic acid. The last step, which destroys the desired product, the aldehyde, is crucial. Therefore, for continuous resaturation of the electrolyte with toluene and for extraction of benzaldehyde, the benzaldehyde content in the emulgated organic phase has to be kept below a critical value. For toluene,p-chlorotoluene and xylene Mn3+-mediated oxidation only becomes attractive with process temperatures above 60°C. The conversion ofp-nitrotoluene top-nitrobenzaldehyde can only be carried out by Co3+. The reactivities of the oxidants declines in the order: Co3+>Mn3+>Ce4+. Current efficiencies for the recuperation of all oxidants approaches 100%.

H. Wendt

Papers

Reaction kinetics and reaction techniques for mediated oxidation of methylarenes to aromatic ketones