Spectro-Photo-Electrochemical Study of Selected Flavine Derivatives for Application in Photocatalysis
Authors: Svoboda Jan , Ludvik Jiri 
Published: July 2021

The photocatalysis of chemical reactions in organic chemistry by the light of visible wavelengths has undergone great development in recent years, however many challenges remain. The effort to create reducing agent with power of alkali metals using light from the visible region of the spectrum remains unattained. Alkali metals are still used in some organic reactions as reducing agents despite the known risks and formation of undesirable by-products.

Flavin derivatives, which are the subject of this work, have the potential to accomplish these challenges. By the reduction and subsequent excitation by the visible light, flavine derivatives create long-lived excited states which behaves as extremely strong reduction agents (reduction potential comparable to sodium). These properties could be used to perform a number of hardly realizable reactions. Because it is a matter of redox processes and properties and the electron transfer, the electrochemical approach to this issue is very suitable. Most of published works deal with the use of flavines as oxidizing agents and by them catalyzed transformations. However, these substances have the potential to act as extreme reducing agents, which has not yet received such attention as their oxidizing abilities. It is therefore desirable to investigate their photo-reduction capabilities and their use in the field of organic synthesis.

The aim of this work is to study selected flavine derivatives using electrochemical and spectroscopic methods and use gained knowledge to generate extremely strong reducing agents in situ and then perform model reactions. Combining electrochemical reduction and photochemical excitation with subsequent chemical reaction in one reaction cell would significantly save time, amount of solvents and the overall cost of performing these otherwise hardly realizable reactions.

At first, selected flavine derivatives were characterized by cyclic voltammetry (using different materials of electrodes) and DC-polarography. The first reduction step of isoalloxazine and alloxazine derivatives is diffusion controlled and reversible with potential of -0,8 V (vs. SCE) for isoalloxazine and -1,3 V (vs. SCE) for alloxazine (both in acetonitrile). In the second stage of this work measured reduction potentials are used in potentiostatic electrolyses of flavine derivatives with aryl halides in one photo-electrochemical reaction cell. The cell is irradiated by visible light (450 nm) during the electrolysis. The combination of applied potential and visible light irradiation in one moment leads to electro-photocatalytic dehalogenation creating aryl radical. Afterwards, the products of model reactions are studied by spectroscopic methods like UV/VIS and NMR spectroscopy and correlated with quantum chemical calculations.

Derivatives of isoalloxazine, alloxazine and 5-deazaflavin have been selected as model substances, whose excited states may have the potential to reduce aryl halides forming an aryl radical, which may be directly used in organic synthesis. All experiments were performed in the non-aqueous solvent (acetonitrile) without the presence of oxygen.

Acknowledgement:

The financial support of the grant 19-22806S (Czech Science Foundation - GAČR) and the institutional support RVO: 61388955 are highly appreciated. The authors are grateful to Prof. R. Cibulka for introduction to this problem and for valuable discussion.

Figure 1