Silver‐Catalyzed Oxidative Activation of Benzylic C?H Bonds for the Synthesis of Difluoromethylated Arenes

A mild and catalytic method to form difluoromethylated arenes through the activation of benzylic C H bonds has been developed. Utilizing AgNO₃ as the catalyst, various arenes with diverse functional groups undergo activation/fluorination of benzylic C H bonds with commercially available Selectfluor reagent as a source of fluorine in aqueous solution. The reaction is operationally simple and amenable to gram‐scale synthesis.     

Ion‐Pairing of Phosphonium Salts in Solution: C?H⋅⋅⋅Halogen and C?H⋅⋅⋅π Hydrogen Bonds

The ¹H NMR chemical shifts of the C(α)? H protons of arylmethyl triphenylphosphonium ions in CD₂Cl₂ solution strongly depend on the counteranions X^?. The values for the benzhydryl derivatives Ph₂CH? PPh₃⁺ X^?, for example, range from δ_H=8.25 (X^?=Cl^?) over 6.23 (X^?=BF₄^?) to 5.72 ppm (X^?=BPh₄^?). Similar, albeit weaker, counterion‐induced shifts are observed for the ortho‐protons of all aryl groups. Concentration‐dependent NMR studies show that the large shifts result from the deshielding of the protons by the anions, which decreases in the order Cl^? > Br^? ? BF₄^? > SbF₆^?. For the less bulky derivatives PhCH₂? PPh₃⁺ X^?, we also find C? H???Ph interactions between C(α)? H and a phenyl group of the BPh₄^? anion, which result in upfield NMR chemical shifts of the C(α)? H protons. These interactions could also be observed in crystals of (p‐CF₃‐C₆H₄)CH₂? PPh₃⁺ BPh₄^?. However, the dominant effects causing the counterion‐induced shifts in the NMR spectra are the C? H???X^? hydrogen bonds between the phosphonium ion and anions, in particular Cl^? or Br^?. This observation contradicts earlier interpretations which assigned these shifts predominantly to the ring current of the BPh₄^? anions. The concentration dependence of the ¹H NMR chemical shifts allowed us to determine the dissociation constants of the phosphonium salts in CD₂Cl₂ solution. The cation–anion interactions increase with the acidity of the C(α)? H protons and the basicity of the anion. The existence of C? H???X^? hydrogen bonds between the cations and anions is confirmed by quantum chemical calculations of the ion pair structures, as well as by X‐ray analyses of the crystals. The IR spectra of the Cl^? and Br^? salts in CD₂Cl₂ solution show strong red‐shifts of the C? H stretch bands. The C? H stretch bands of the tetrafluoroborate salt PhCH₂? PPh₃⁺ BF₄^? in CD₂Cl₂, however, show a blue‐shift compared to the corresponding BPh₄^? salt.     

Rhodium(III)‐Catalyzed Activation of C?H Bonds and Subsequent Intermolecular Amidation at Room Temperature

Disclosed herein is a Rh^III‐catalyzed chelation‐assisted activation of unreactive C H bonds, thus enabling an intermolecular amidation to provide a practical and step‐economic route to 2‐(pyridin‐2‐yl)ethanamine derivatives. Substrates with other N‐donor groups are also compatible with the amidation. This protocol proceeds at room temperature, has a relatively broad functional‐group tolerance and high selectivity, and demonstrates the potential of rhodium(III) in the promotive functionalization of unreactive C H bonds. A rhodacycle having a SbF₆⁻ counterion was identified as a plausible intermediate.     

Rhodium(III)‐Catalyzed Activation of C?H Bonds and Subsequent Intermolecular Amidation at Room Temperature

Disclosed herein is a Rh^III‐catalyzed chelation‐assisted activation of unreactive C H bonds, thus enabling an intermolecular amidation to provide a practical and step‐economic route to 2‐(pyridin‐2‐yl)ethanamine derivatives. Substrates with other N‐donor groups are also compatible with the amidation. This protocol proceeds at room temperature, has a relatively broad functional‐group tolerance and high selectivity, and demonstrates the potential of rhodium(III) in the promotive functionalization of unreactive C H bonds. A rhodacycle having a SbF₆⁻ counterion was identified as a plausible intermediate.