Electrochemical Hydrodefluorination Of Fluoroaromatics

Fluorinated organic compounds attracted great attention during the last decades becauseof their extensive application in modern pharmaceuticals, agrochemicals, materials science,and polymer chemistry. C-F bonds are highly thermodynamic stableand kinetic inert as theconsequence of the small size and high electronegativity of fuorine atom and these make theC F bond the most inert covalent single bond. As a result, these molecules are highlypersistent and difficult to degrade. The abandoned fluorinated compounds accumulated toomuch on the earth are threatened to the environment. In recent years, it has been attractedextensive interests to developeffective methods for selective C-F bond activation andfunctionalization. This is not only a way to higher-value fluorinated compounds but alsomeaningful to the degradation of fluorinated inert substances.The recent development of C-F bond activation and functionalization of aromatic andalkenyl and aliphatic fluorides are reviewed in this paper. The simplest activation of C-F bondis hydrodefluorination (HDF), which is the conversion of a carbon–fluorine bond into acarbon–hydrogen bond. The latest progresses of hydrodefluorination reactions mediated bytransition metals were elaborated in this thesis.Organic electrochemical synthesis has received signifcant interest from both academia andindustry because of its mild reaction conditions, easy to handle, environmental friendly,etc.Previous work on electrochemical hydrodefluorination of fluoroaromatics had to use toxicmercury pool as electrode, and the yield was less than76%.Furthermore, thearomatic rings ofthe products were easy to be over reduced to cyclohexane, cyclohexadiene andcyclohexenederivatives. As the recent work of hydrodefluorination and our interests in electrochemicalsynthesis, we managed to achieve the electrochemical hydrodefluorination of aromaticfluorides with nontoxic platinum electrode, realizing high yield and high chemical selectivity.Our project focuses on the electrochemical behaviors of aromatic fluorides. Extensivestudies were directed to optimize the reaction conditions on current density, electrode,additive and solvent. The studies have shown that in an undivided cell with platinum foil andplatinum net as anode and cathode, instead of mercury pool electrodes, NaBH4as additive, inthe electrolyte of0.2M Bu4NBF4of diglyme, under a constant current, a wide variety ofaromatic fluorides could undergo hydrodefluorination in good yields (70%-99%) and withhigh chemical selectivity. Under optimal conditions, the aromatic ring is not further reduced.This subject was the first to use sodium boron hydride as additives, platinum-platinum as the electrodes and achieved the electrochemical hydrodefluorination of fluoroaromatics at roomtemperature with high yield of99%, realizing quantitative conversion of this reaction.