A comparative study of anodic fluorination of N-alkyl and N,N-dialkyl phenylacetamides in Et3N · 4HF medium

Anodic fluorination of N-alkyl and N,N-dialkyl phenylacetamides (alkyl = methyl, ethyl, propyl and n-butyl) was carried out in Et3N · 4HF medium. Effects of current density, quantity of electric charge and alkyl chain length on the overall conversion efficiency and selectivity of fluorinated products are reported. 1H NMR, 19F NMR and GC/MS were employed for product identification and characterization. Under galvanostatic condition, N-alkyl phenylacetamides lead to predominantly monofluoro derivative at ortho position in the aromatic ring. Mono and difluoro active methylene derivatives and p-fluoro compounds were also formed in smaller quantities. The selectivity was low in the case of N,N-dialkyl phenylacetamides. On increasing the electricity passed beyond 2 F/mol, the o-fluoro compound produced 1,4 addition compounds followed by further chemical and electrochemical transformations. Cyclic voltammetric studies indicated significant adsorption of N,N-dialkyl compounds. Potentiostatic electrolysis lead to predominantly side chain fluorination at the active methylene group. The product distribution under different experimental conditions is explained on the basis of reactant adsorption and protonation at the amide group.