Ligand versus Complex: C-F and C-H Bond Activation of Polyfluoroaromatics at a Cyclic (Alkyl)(Amino)Carbene.

Abstract

C-F and C-H bond activation reactions of polyfluoroaromatics at the cyclic (alkyl)(amino)carbene (cAAC) cAACmethyl (1) are reported. Studies on the C-F bond activation using the cAAC-stabilized nickel(0) complex [Ni(cAACmethyl )2 ] (2) have shown that 2 does not react with fluorinated arenes. However, these investigations led to the observation of C-F bond cleavage of perfluorinated arenes by the carbene ligand cAACmethyl (1) itself. The reaction of 1 with C6 F6 , C6 F5 -C6 F5 , C6 F5 -CF3 , and C5 F5 N afforded the insertion products of cAAC into one of the C-F bonds of the substrate, that is, the C-F bond activation products (cAACmethyl )F(Arf ) (Arf =C6 F5 4 a, C6 F4 -C6 F5 4 b, C6 F4 -CF3 4 c, C5 F4 N 4 d). These products decompose readily upon heating to 80 °C within a few hours in solution with formation of ionic iminium salts [(cAACmethyl )(Arf )][X] 6 a-d or neutral alkenyl perfluoroaryl imine compounds 7 a-d. The compounds (cAACmethyl )F(Arf ) 4 a-d readily transfer fluoride, which has been exemplified by the fluoride transfer of all compounds using BF3 etherate as fluoride acceptor. Fluoride transfer has also been achieved starting from (cAACmethyl )F(C6 F4 -CF3 ) (4 c) or (cAACmethyl )F(C5 F4 N) (4 d) to other selected substrates such as trimethylchlorosilane, benzoyl chloride and tosyl chloride. Instead of C-F bond activation, insertion of the cAAC into the C-H bond was observed if 1 was treated with the partially fluorinated arenes C6 F5 H, 1,2,4,5-C6 F4 H2 , 1,3,5-C6 F3 H3 , and 1,3-C6 F2 H4 . The compounds (cAACmethyl )H(Arf ) (Arf =C6 F5 12 e, 2,3,5,6-C6 F4 H 12 f, 2,4,6-C6 F3 H2 12 g and 2,6-C6 F2 H3 12 h) have been isolated in good yields and have been characterized including X-ray analysis. Fluorobenzene C6 FH5 (pKa ≈37), the least C-H acidic fluoroarene used in this study, does not react. In order to investigate the scope and limitations of this type of cAAC C-H bond activation reaction, cAACmethyl (1) was treated with several other reagents of different C-H acidity such as imidazolium salts, imidazoles, esters, and trimethylphosphine. These investigations led to the isolation and characterization of the compounds [(cAACmethyl )H(R2 ImMe2 )]X (13 a,b), (cAACmethyl )H(ImR2 ) (14 a-c), (cAACmethyl )H(CH(COOCH3 )2 ) (15 b) and (cAACmethyl )H(CH2 -PMe2 ) (16). Deprotonation of [(cAACmethyl )H(Me2 ImMe2 )][BF4 ] (13 a) at the cAAC carbon atom using KHMDS as a base led to isolation and structural characterization of the cAACmethyl -NHC heterodimer (17).