Transition metal catalysts are an essential tool in cross-coupling reactions. Advances to improve these catalysts rely on tuning spectator ligands, with highly σ-donating N-heterocyclic carbene (NHC) complexes being among the most productive catalysts. By increasing the donating ability of NHCs, more electron density is placed on the metal to facilitate oxidative addition. While only a few examples exist, monosubstituted NHCs have the benefit of being less sterically hindered as well as increasing the electron density on the metal center after deprotonation of the imidazolium N–H. In the Michaelis lab, we have created a new class of monosubstituted NHC complexes of palladium by phosphinylating our monosubstitued NHC ligand and then going through a pseudo-oxidative addition to insert palladium(II) chloride into the carbene-phosphorous bond. These new complexes include a range of aryl, alkyl, and benzyl-substituted NHCs. In catalysis, these complexes perform Suzuki-Miyaura and Sonogashira cross-coupling reactions with exceptionally low catalyst loadings and tolerate a wide range of functional groups. In the Suzuki-Miyaura reaction, catalyst loading can be lowered into the 1-10 ppm range and achieve a total of 250,000 turnovers.

Monosubstituted, anionic N-heterocyclic carbene complexes of palladium and their reactivity in cross coupling reactions

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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