Publication 625

J. Org. Chem. 73, 2607-2620, 2008. DOI: 10.1021/jo702513w S0022-3263(70)02513-6
Pentakis(trifluoromethyl)phenyl, a Sterically Crowded and Electron-withdrawing Group: Synthesis and Acidity of Pentakis(trifluoromethyl)benzene, -toluene, -phenol, and -aniline

University of Tartu, Institute of Chemistry, 2 Jakobi St., 51014 Tartu, Estonia, Institute of Organic Chemistry, Ukrainian National Academy of Sciences, 5 Murmanskaya St., Kiev 02094, Ukraine, Institute of Inorganic & Physical Chemistry, University of Bremen, Leobener St., D-28334 Bremen, Germany, University of Tartu, Institute of Technology, 1 Nooruse St., 50411 Tartu, Estonia, Kyushu University, Institute for Materials Chemistry and Engineering, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan, and Université Paris 7 Denis Diderot, Electrochimie Moleculaire, UMR CNRS 7591, Case courrier 7107, 2 place Jussieu, F-75251 Paris, Cedex 05, France

A general route to functionalized pentakis(trifluoromethyl)phenyl (C₆(CF₃)₅) derivatives, promising building blocks for designing novel stable carbenes, radical species, superacids, weakly coordinating anions and other practically and theoretically useful species, is presented. This pertrifluoromethylation route proceeds via conveniently pregenerated (trifluoromethyl)copper (CF₃Cu) species in DMF, stabilized by addition of 1,3-dimethyl-2-imidazolidinone (DMI). These species react with hexaiodobenzene at ambient temperature to give the potassium pentakis(trifluoromethyl)phenoxide along with hexakis(trifluoromethyl)benzene and pentakis(trifluoromethyl)benzene in a combined yield of 80%. A possible reaction pathway explaining the formation of pentakis(trifluoromethyl)phenoxide is proposed. Pentakis(trifluoromethyl)phenol gives rise to easily functionalized pentakis(trifluoromethyl)chlorobenzene and pentakis(trifluoromethyl)aniline. Pertrifluoromethylation of pentaiodochlorobenzene and pentaiodotoluene allows straightforward access to pentakis(trifluoromethyl)chlorobenzene and pentakis(trifluoromethyl)toluene, respectively. XRD structures of several C₆(CF₃)₅ derivatives were determined and compared with the calculated structures. Due to the steric crowding the aromatic rings in all C₆(CF₃)₅ derivatives are significantly distorted. The gas-phase acidities (G_acid) and pK_a values in different solvents (acetonitrile (AN), DMSO, water) for the title compounds and a number of related compounds have been measured. The origin of the acidifying effect of the C₆(CF₃)₅ group has been explored using the isodesmic reactions approach.