Mechanistic Insights on Concentrated Lithium Salt/Nitroalkane Electrolyte Based on Analogy with Fluorinated Alcohols

Abstract

Fluorinated alcohols such as 1,1,1,3,3,3-hexafluoro2propanol (HFIP) and 2,2,2-trifluoroethanol (TFE) have emerged as powerful solvents in oxidation chemistry including hole catalysis. In this paper, we describe the similarity of lithium salt/ nitroalkane electrolytes with fluorinated alcohols in electrosynthesis. Based on the results from electrosynthesis, Raman and nuclear magnetic resonance spectroscopy and cyclic voltammetry (CV), we have demonstrated that the combination of lithiumRecently, there has been a significant interest in redox-based organic reactions. Especially, the development of a redox process using homogeneous photoredox catalysts, [1,2] heterogeneous photocatalysts [3] and electrochemistry [4][5][6] is of great interest from the viewpoint of safety and sustainability. These systems are also advantageous compared to chemical redox reactions in terms of price of the reagents and easy purification of products since no quantitative reagents are required. Photoredox and electrochemical reactions enable unique chemical transformations, which conventional chemical methods are not capable of achieving.In previous work, we demonstrated that hole catalysis, where electrophilic radical cations generated by single-electron transfer function as a catalyst, is heavily affected by anions. [7] In that study, Ru-catalyzed radical cation Diels-Alder reaction was chosen as a model reaction of hole catalysis. The reaction efficiency, which was estimated from the product yields after generating a certain amount of radical cation, was found to be lowered in the presence of additional salts. It was also demonstrated that the addition of a fluorinated alcohol such as 1,1,1,3,3,3-hexa-fluoro2-propanol (HFIP), which is known to bind anions, can nullify the effect of anions and preserve the efficiency of the reaction.A very similar concept was reported by Yoon and co-workers, where the effect of anions in the Ru-catalyzed radical cation [a]

Oxidation Potential Gap (ΔEox): The Hidden Parameter in Redox Chemistry

“…Previously, we revealed that HFIP could trap anionic species via its hydrogen bond acceptor property, generating reactive “intact” radical cations under electrochemical conditions [4b] . Furthermore, we recently reported that the concentrated MeNO 2 –LiClO 4 system exhibits the identical acceleration effect via its unique solvent clustering, which was detected using Raman spectroscopy [7e] . Therefore, the MeNO 2 ‐HFIP mixed solvent system is a suitable medium for the intramolecular cross‐coupling reactions of N ‐benzoyl indolines, as demonstrated by the successful total syntheses of pyrrolophenanthridone alkaloids [10] .…”

Electrosynthesis Governed by Electrolyte: Case Studies that Give Some Hints for the Rational Design of Electrolyte

“…We analyzed the LiX/nitroalkane electrolyte using Raman spectroscopy. 22 Fig. 5b shows representative data for LiClO 4 /CH 3 NO 2 electrolyte in the concentration range of 0.1 ~ 2.0 M. A broad absorption band at 942 cm -1 appeared with increasing concentration of LiClO 4 (Fig.…”

“…4). 22 To highlight the difference in efficiency of hole catalysis depending on the concentration of Li salts in the electrolytes, the electrolysis was intentionally stopped after the passage of 0.03 F/mol of charge (equals to 0.03 electron oxidation per trans-anethole). In other words, the reaction was quenched before the complete consumption of the starting materials.…”

Radical cation Diels–Alder reactions of arylidene cycloalkanes

“…We have developed radical cation cycloadditions using (photo)electrochemical single-electron transfer in lithium per- chlorate (LiClO 4 )/nitromethane (CH 3 NO 2 ) solution [36][37][38][39][40][41][42][43][44]. During the course of our studies, we found that the TiO 2 photoelectrochemical approach was more beneficial than simple electrochemistry in most cases, probably because both single-electron oxidation and reduction are made possible at the same surface [45].…”