CF3 oxonium salts, O-(trifluoromethyl)dibenzofuranium salts: in situ synthesis, properties, and application as a real CF3+ species reagent.
We report in situ synthesis of the first CF(3) oxonium salts, thermally unstable O-(trifluoromethyl)dibenzofuranium salts, which furthermore have different counteranions (BF(4)-, PF(6)-, SbF(6)-, and Sb(2)F(11)-) and ring substituents (tert-butyl, F, and OCH(3)), by photochemical decomposition of the corresponding 2-(trifluoromethoxy)biphenylyl-2'-diazonium salts at -90 to -100 degrees C. The yields markedly increased in the order of BF(4)- < PF(6)- < SbF(6)- < Sb(2)F(11)-. The CF(3) oxonium salts were fully assigned by means of (1)H and (19)F NMR spectroscopy at low temperature. The CF(3) salts decomposed to form CF(4) and dibenzofurans. The half-life times at -60 degrees C of the 2-tert-butyl salts having different counteranions were 29 min for BF(4)- salt 2d, 36 min for PF(6)- salt 2c, 270 min for SbF(6)- salt 2a, and 415 min for Sb(2)F(11)- salt 2b. Those at -60 degrees C of the Sb(2)F(11)- salts having different 2-substituents were 13 min for F salt 3b, 63 min for H (unsubstituted) salt 1b, and 415 min for tert-butyl salt 2b. Thus, the stability of the CF(3) oxonium salts increased in the order of BF(4)- < PF(6)- < SbF(6)- < Sb(2)F(11)- and F < H < tert-butyl, which is in accord with the increasing orders of the non-nucleophilicity of counteranions and the electron-donating effect of ring substituents. 2-tert-Butyl-O-(trifluoromethyl)dibenzofuranium hexafluoroantimonate (2a) was thus chosen and successfully applied as a real CF(3)+ species source to the direct O- and N-trifluoromethylations of alcohols, phenols, amines, anilines, and pyridines under very mild conditions. The thermal decomposition method with a mixture of diazonium salt 17a and aryl- or alkylsulfonic acids, pyridine, or pyridines having an electron-withdrawing group also afforded CF(3)O or CF(3)N products. The trifluoromethylation mechanism is discussed and an S(N)2 mechanism containing the transient formation of free CF(3)+ is proposed. Thus, the present study has demonstrated that the exceedingly reactive CF(3)+ species can be generated much easier than the CH(3)+ species, contrary to the common sense that CF(3)+ is extremely difficult to generate in solution.
Umemoto
Teruo
T
MEC Laboratory, Daikin Industries, Ltd., Miyukigaoka 3, Tsukuba, Ibaraki 305, Japan. teruo.umemeto@imandt.com
Adachi
Kenji
K
Ishihara
Sumi
S
D001391:Azo Compounds / Q000737:chemistry; D001572:Benzofurans / Q000138:chemical synthesis / Q000737:chemistry*; D005466:Fluorocarbons / Q000737:chemistry*; D006845:Hydrocarbons, Fluorinated / Q000138:chemical synthesis / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D008745:Methylation; D015394:Molecular Structure; D010777:Photochemistry; D012492:Salts / Q000737:chemistry*; D018036:Selenium Compounds / Q000737:chemistry; D013452:Sulfonium Compounds / Q000737:chemistry; D013464:Sulfuric Acids / Q000138:chemical synthesis / Q000737:chemistry*; D013696:Temperature; D013997:Time Factors
D001391:Azo Compounds; D001572:Benzofurans; D005466:Fluorocarbons; D006845:Hydrocarbons, Fluorinated; C523396:O-(trifluoromethyl)dibenzofuranium; D012492:Salts; D018036:Selenium Compounds; D013452:Sulfonium Compounds; D013464:Sulfuric Acids; C048813:potassium peroxymonosulfuric acid