In the last few decades there has been a big push for more environmentally friendly ways of living. This does not stop at chemistry. In electrochemistry the term “green chemistry” has been coined. Organic synthesis through electrochemistry is expected to be one of the most useful reactions in green chemistry, as no toxic reagents or products are needed or produced. It also has less waste when it is compared to other traditional forms of organic synthesis. Green organic synthesis is one of the most promising fields in organic synthetic processes (1). The reactions are said to be green as an electric current is used as opposed to stoichiometric redox species. The use of an electric current scales up the reaction easily too. Organic synthesis via electrochemistry is expected to be widely used in the future as it keeps toxic reagents out and energy consumption is reduced with this process. (2) An electrochemical process can be stopped and the reaction can be controlled. Typically water is used as a solvent. There are three main components for an electrolysis process. These are an anode, cathode and an electrolyte. A Hofmann rearrangement has been successfully performed via green electrolysis. The Hofmann rearrangement converts a primary amide into an amine. When using benzamide as the starting product in this process an amine is produced. In this reaction the anode was platinised with some titanium. The cathode that was used was graphite. A hypohalite is needed in the reaction conditions to help form the product. A hypohalite is an oxyanion containing a halogen of +1 oxidation state. Examples include OBr and OCl. During the electrolysis stage of the reaction NaCl, NaOH and Cl2 are produced, as well as a hypochlorite. This helps produce the final product. This product is said to be diazotised and it is coupled to naphtol which produces dyes. The dyes that are produced are identified by UV Vis absorption. Green chemistry also cuts costs as for example when polyaniline is synthesised, positive chloride ions are used with a sodium chloride solution instead of using expensive and sometimes dangerous oxidising agents.
Due to pollution of the environment, several principles for green chemistry have been developed. These are listed below in figure (2). The principles listed below indicate that green chemistry must be safe, raw materials must be used and solvent should also be used. These principles are now used in a number of processes in chemistry. These include the Hofmann rearrangement as previously mentioned, decolourisation of dyes and the production of polyaniline. In each example stated, water is used as the solvent, the chemicals that are used are nontoxic and the reagents used are relatively cheap when compared to non-green processes. Cheap electrodes such as steel, silver and copper are used. In some cases, the waste materials from reactions can also be used. They have been used as reducing agents. An example of this is reducing silver ions produced from silver nano particles by a neem extract. (3)
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Umpolung chemistry refers to the chemical modification of a functional group to reverse the polarity of that group. This modification allows reactions to proceed. In electrochemistry electrons are added to electron poor groups. This helps to create new nucleophiles. By removing electrons from electron rich groups, electrophiles can be created. Electrochemistry allows selective induction of electrons. When the electrons are added or removed a reactive intermediate is formed. The intermediates can be trapped to help complete the reaction by coupling two nucleophiles or two electrophiles. These types of reaction help develop brand new synthetic methods for complicated organic molecules. Cathodic reductions are widely used in organic synthesis. The carbonyl group is often described as the most important functional group in organic chemistry. This is due to the polarisation of the carbonyl group leaving it open for Nucleophilic attack. A number of functional groups can be used for cathodic reductions instead of carbonyl groups. These include esters, nitriles and aryl groups. They act as building blocks, which can be useful for the synthesis of a final product. Many functional groups take part in reductive coupling reactions. Mediators have been highlighted in these reactions. These are redox reagents that are generated in an electrochemical reaction. They are regenerated at the electrode. For example, mediators occur in the Diels- Alder reaction. Maleic anhydride acts as both a mediator and a dienophile.
Anodic oxidations are useful too in electrochemistry. These form new bonds and increase the functionality of the product formed. They can also protect the functionality of the product. The product that is formed can even be further manipulated. There is a huge potential to synthetic development in anodic reactions that are generated through umpolung reactions. The Kolbe electrolysis reaction is a good example of an anodic reaction. It is an important carbon carbon forming reaction. Anodic oxidations of neutral substartes lead to a reactive intermediate that can undergo an elimination reaction or a trapping reaction as mentioned before. The elimination reactions regularly lead to a reactive cation species that can trap nucleophiles. An example of this reaction is the oxidative generation of acyliminium ions from an amide.
Even though elimination reactions are used sucessfully in syntheses, trapping reactions most clearly shows the use of anodic reactions in starting an umpolung reaction. By oxidising a substrate with two nucleophiles, a radical cation. This is followed by a coupling between two nucleophiles. This opens up a new synthetic strategy for forming ring systems. For example a bicycylic ring system is formed from anodic coupling of an enol ether with am allysilane. This is quite a unique approach to ring formation. Recent studies into electrochemical reactions has discovered new synthetic routes to complicated molecules. It’s possible with further development that electrochemistry will be the primary tool for synthetic chemists and that there may be no need for the development of stoichiometric methods.(4)
As stated before electrochemical reactions allow selectivitiy within a reaction with regards to inudction or withdrawl of electrons. These types of reactions are becoming popular again as fuels and important chemicals can be formed from biomass and they are also good for the environment as stated already. Notabale examples of its uses would be the formation of octane from levulinic acid or furan from residues in the paper industry. The best example of organic synthesis is the oxidation of organic acids to an alkane. This is widely known as a Kolbe electrolysis which was discovered in the 19th century. Several variables of Kolbe electrolysis were tested for decades. These variables included pH and the concentration of reactant. These were tested to see what influence they had on Kolbe electrolysis, but as one variable was changed another key variable would change simultaneously. An example of this is when the concentration of the reactant was increased, the number of ions would increase too. Also if the temperature was increased the movement of ions accelerated which would increase the electrolytic conductivity. Valeric acid has been studied sucessfully as an organic acid in kolbe electrolysis as it is soluble in water. From the studies it was noted that electrolyte salts had different effects on the electrolytic conductivity. It was found that Na2SO4 increased the current flow at an anode, while KNO3 had only a small effect. Na2SO4 greatly increased the yield in Kolbe electrolysis of Valeric acid. This reaction with Na2SO4 was both energetically and economically favourable. From the results on several Ph values it was found that Kolbe electrolysis was best performed at an acidic value of pH 5 and an alkaline value of pH 11. From the studies it was found that Kolbe electrolysis should also be completed at a high electric conductivity which is attained by increasing the reactant concentration. In kolbe electrolysis it is beter to increase the concentration of a reactant as opposed to adding electrolytes. It is only necessary to add electrolytes when long chaoned organic acids are used as solubilut decreases with increasing chain length.(5)
It has also been found that five and six membered rings can be formed through kolbe electrolysis. It is done through radical cyclisation. As like the reactions mentioned before, this reaction is environmentally friendly too. Over the past few years the use of radical reactions in synthetic chemistry has increased at a huge rate. Importantly the formation of five and six membered rings has become a popular synthetic process aswell as being an effiecient process. Complicated ring structures can now me formed from readily from radical cyclisations. Historically radical transformations were carried out with large volumes of toxic reagents or expensive hydride sources like trimethylsilane. Electroloysis has put a halt to this problem as radical species are now formed from electrochemistry. Primary radicals can now be made from electroreductive techniques using nickel or cobalt. Hans Schafer discovered that cyclic compounds could be obtained by the use of unsaturation in the starting materials. This is illustarted in figure (5). The use of this method was regularly ignored due to its average yields. It was found that yields increased when an electron withdrawing group was added to an alkenyl group in the starting material. Yields were also further improved by using platinum electrodes and a high dilution of Methanol. Switching electric supply also stopped the formation of a black layer on the electrodes. These were noted as the optimal reaction conditions. The use of an EWG showed huge differences in the results. In one case shown below in figure (6) , it increased the yield from 35% to 90%. A five membered ketal ring was only formed in he 90% yield. There were two products formed without the EWG present. A cyclic structure and a linear product were formed.(6)32956501548765Fig (6) the contrasting yields are shown above. Note only a cyclic product is formed when an EWG is present in the starting material.
References
- https://www.jstage.jst.go.jp/article/electrochemistry/81/5/81_13-5-OT0064/_pdf
- https://medcraveonline.com/IJBSBE/IJBSBE-03-00063.pdf
- http://www.scielo.mec.pt/scielo.php?script=sci_arttext&pid=S0872-19042016000500003
- https://www.electrochem.org/dl/interface/wtr/wtr02/IF12-02-Pages36-42.pdf
- http://ze5mw2yz8y.search.serialssolutions.com/?ctx_ver=Z39.88-2004&ctx_enc=info%3Aofi%2Fenc%3AUTF-8&rfr_id=info%3Asid%2Fsummon.serialssolutions.com&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.atitle=The+Dilemma+of+Supporting+Electrolytes+for+Electroorganic+Synthesis%3A+A+Case+Study+on+Kolbe+Electrolysis&rft.jtitle=CHEMSUSCHEM&rft.au=Stang%2C+C&rft.au=Harnisch%2C+F&rft.date=2016-01-08&rft.pub=WILEY-V+C+H+VERLAG+GMBH&rft.issn=1864-5631&rft.eissn=1864-564X&rft.volume=9&rft.issue=1&rft.spage=50&rft.epage=60&rft_id=info:doi/10.1002%2Fcssc.201501407&rft.externalDBID=n%2Fa&rft.externalDocID=000367826500006¶mdict=en-UKhttps://www.thieme-connect.de/products/ejournals/html/10.1055/s-0028-1083547
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