title: 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 C6F6, C6F5-C6F5, C6F5-CF3, and C5F5N 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=C6F54 a, C6F4-C6F54 b, C6F4-CF34 c, C5F4N 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 BF3etherate as fluoride acceptor. Fluoride transfer has also been achieved starting from (cAACmethyl)F(C6F4-CF3) (4 c) or (cAACmethyl)F(C5F4N) (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 C6F5H, 1,2,4,5-C6F4H2, 1,3,5-C6F3H3, and 1,3-C6F2H4. The compounds (cAACmethyl)H(Arf) (Arf=C6F512 e, 2,3,5,6-C6F4H 12 f, 2,4,6-C6F3H212 g and 2,6-C6F2H312 h) have been isolated in good yields and have been characterized including X-ray analysis. Fluorobenzene C6FH5(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(R2ImMe2)]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(Me2ImMe2)][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).
authors.affiliation: Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074, Würzburg, Germany.
authors.forename: Ursula S D
authors.initials: US
authors.lastname: Paul
authors.affiliation: Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074, Würzburg, Germany.
authors.forename: Udo
authors.identifier: http://orcid.org/0000-0002-0759-1806
authors.initials: U
authors.lastname: Radius
keywords: C−F bond activation; C−H bond activation; cAAC ligands; carbenes; fluorine transfer