Electrochemical oxidation and reduction of hydroxyalkenes

Abstract

Electrochemistry is the chemistry that studies all those processes that involve the transfer of electrons and organic electro-synthesis is that branch that exploits electricity for the synthesis of valuable chemical compounds. It is considered an environmentally friendly approach but it has never been largely used in synthetic organic laboratories because of lack of standardised equipment and protocols. For this reason, nowadays the use of flow equipment and flow electrolysis cells is gaining a lot of interest and is helping the development of new electrosynthetic processes. Heterocycles are one of the most important and useful groups of organic compounds; they are found in bio-active natural compounds, agrochemical and pharmaceutical compounds. Because of that, their preparation in an efficient and economical way has been the main target in industry. Because the common approaches usually require expensive or toxic reagents, severe conditions, long reaction time, new more sustainable and convenient approaches are required. So electrochemical methods can be an interesting and environmentally friendly way to synthetize different heterocyclic structures. Recently, there has been much interest in the use of flow systems in organic synthesis, especially flow electrochemical reactors. This interest is due to the ability of microflow approach to give high selectivity and high conversion in a single pass. The main target of my first work was the synthesis of substituted tetrahydrofurans and lactones using an Ammonite electrochemical flow reactor. The approach targets a reagent free oxidative cyclisation of styrene-derivatives using microfluidic electrolysis cells. Different substrates have been tested with moderate to good yields and some mechanistic insight have been given with regard to this process. While working on the oxidative cyclisation we discovered a side-reaction involving the reduction of the styrene double bond to single bond. Considering the importance of such reaction in synthetic chemistry and after a careful literature review we decided to focus our attention on developing an electrolysis method in the Ammonite microfluidic reactor for the reduction of styrene-double bond. Our method does not require any metal catalyst and no flammable hydrogen gas. Once optimised the electrolysis conditions, the method has been applied to different substrates, in particular on the synthesis of Gigantol a natural compound with medicinal properties. While working on the substrate scope a defluorination side-reaction was observed for substrate bearing fluorine atoms on their structure and this could be a topic to further explore in future considering the importance of such reaction especially in medicinal chemistry.

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ORCID for Richard Brown: orcid.org/0000-0003-0156-7087

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