Electrochemical and Chemical Syntheses of Trifluoromethylating Reagents and Trifluoromethyl Substituted Compounds

Abstract The electroreduction of the halofluoromethanes CF 3 Br, CF 2 Br 2 and CF 2 BrCl has been studied in high‐pressure stainless steel autoclaves at different cathodes [Pt, steel (V2A, V4A), glassy carbon (GC)] and in various solvent‐supporting electrolyte systems (SSE), e.g. DMF/[Bu 4 N]Br, NMP/[Bu 4 N]BF 4 etc. The reduction potentials for CF 3 Br increase from Pt (–1.6 V) < V2A (–1.8 V) < GC (–2.1 V) and are lower for CF 2 Br 2 and CF 2 BrCl suggesting a reductive cleavage of C‐X bonds as the first step. CF 2 Br 2 and CF 2 BrCl show a two‐step reduction in accord with the C–X bond energies (C–F > C–Cl > C–Br) and the “Perfluoro‐effect”. The electrolysis of CF 3 Br in different SSE‐systems with sacrificial zinc or cadmium anodes has been reinvestigated with our experimental set‐up to elucidate the influence of the experimental conditions on the type and ratio of the products. The observed products CF 3 MBr·42L and (CF 3 ) 2 M·42L (M = Zn, Cd; L = DMF or AN) are the same as in the previous investigations, but are obtained in different ratios, as a rule caused by a parallel chemical corrosion of the respective anodes. By using aluminium as sacrificial anode no CF 3 Al compounds are formed. The CF 3 species generated by electroreduction of CF 3 Br react with the solvents via hydrogen abstraction and formation of CF 3 H. The current yield with respect to the dissolution of the Al anode reaches 120 % indicating a considerable chemical corrosion in addition to the anodic oxidation. This result enabled a one‐pot trifluoromethylation reaction of NMP as organic carbonyl substrate and solvent with CF 3 Br and aluminium powder (ratio 3 : 2) at higher temperatures (> 70 °C). The complete reaction of CF 3 Br to give CF 3 H and 1‐methyl‐2‐trifluoromethyl‐4,5‐dihydropyrrol allowed the isolation of the latter by vacuum condensation and distillation in 45 % yield, rel. to the CF 3 Br used. Gallium and indium were also applied as sacrificial anodes in combination with CF 3 Br as substrate. In both cases, anodic current yields of about 280 % indicated an extreme chemical corrosion together with cathodic metal depositions corresponding to the cathodic current yield. These deposits – in contrast to those of Zn and Cd – do not react with CF 3 Br in Grignard ‐type conversions to CF 3 Ga and CF 3 In compounds. So, the observed products (CF 3 ) n MBr 3–n ·L (M = Ga, In; n 1‐3; L = DMF, NMP) are obviously formed by chemical corrosion of the electro‐activated anodes. Finally, electrochemical and chemical trifluoromethylations were successfully carried out, using R 3 SiCl (R = Me, Vi, Ph), Me 3 M′Cl (M′ = Ge, Sn) and aluminium anodes or Al‐powder. The products were characterized either after isolation or in the product solutions by NMR‐spectroscopic investigations.