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- Title
Ligand versus Complex: C−F and C−H Bond Activation of Polyfluoroaromatics at a Cyclic (Alkyl)(Amino)Carbene.
- Authors
Paul, Ursula S. D.; Radius, Udo
- 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=C6F5 4 a, C6F4−C6F5 4 b, C6F4−CF3 4 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 BF3 etherate 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=C6F5 12 e, 2,3,5,6-C6F4H 12 f, 2,4,6-C6F3H2 12 g and 2,6-C6F2H3 12 h) have been isolated in good yields and have been characterized including X-ray analysis. Fluorobenzene C6FH5 (p Ka ≈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).
- Subjects
CHEMICAL bonds; CARBENES; NICKEL isotopes; AROMATIC fluorine compounds; IMINES
- Publication
Chemistry - A European Journal, 2017, Vol 23, Issue 16, p3993
- ISSN
0947-6539
- Publication type
Article
- DOI
10.1002/chem.201605950