C-X (X = N, O) cross-coupling reactions catalyzed by copper-pincer bis(N-heterocyclic carbene) complexes

Jennifer L. Minnick, Doaa Domyati, Rachel Ammons, Laleh Tahsini

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Abstract

Over the last two decades, N-heterocyclic carbene (NHC)-copper catalysts have received considerable attention in organic synthesis. Despite the popularity of copper complexes containing monodentate NHC ligands and recent development of poly(NHC) platforms, their application in C-C and C-heteroatom cross-coupling reactions has been limited. Recently, we reported an air-assisted Sonogashira-type cross-coupling catalyzed by well-defined cationic copper-pincer bis(NHC) complexes. Herein, we report the application of these complexes in Ullmann-type C-X (X = N, O) coupling of azoles and phenols with aryl halides in a relatively short reaction time. In contrast to other well-defined copper(I) catalysts that require an inert atmosphere for an efficient C-X coupling, the employed Cu(I)-pincer bis(NHC) complexes provide good to excellent yields in air. The air-assisted reactivity, unlike that in the Sonogashira reaction, is also affected by the base employed and the reaction time. With Cs2CO3 and K2CO3, the oxygen-generated catalyst is more reactive than the catalyst formed under argon in a short reaction time (12 h). However, the difference in reactivity is compromised after a 24 h reaction with K2CO3. The efficient pincer Cu-NHC/O2/Cs2CO3 system provides great to excellent cross-coupling yields for electronically diverse aryl iodides and imidazole derivatives. The catalyst scope is controlled by a balance between nucleophilicity, coordinating ability, and the steric hindrance of aryl halides and N-/O-nucleophiles.

Original languageEnglish
Article number12
JournalFrontiers in Chemistry
Volume7
Issue numberJAN
DOIs
StatePublished - 1 Jan 2019
Externally publishedYes

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Keywords

  • Air-assisted cross-coupling
  • Biarylethers
  • Copper-oxygen catalysts
  • Cu-pincer NHC complexes
  • N-aryl imidazoles
  • Ullmann-type C-X coupling

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