N-Heterocyclic carbenes: deducing σ- and π-contributions in Rh-catalyzed hydroboration and Pd-catalyzed coupling reactions

The effect of tuning the electronic properties of N-heterocyclic carbene (NHC) ligands was evaluated in multiple, mechanistically distinct, metal-mediated reactions. Hydroboration and Heck reactions, catalyzed by Rh–NHC and Pd–NHC complexes, respectively, were found to result in yields that were up to ten times lower when π-withdrawing substituents were incorporated into the NHC backbone relative to analogues bearing σ-withdrawing groups.