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 pi-withdrawing substituents were incorporated into the NHC backbone relative to analogues bearing sigma-withdrawing groups. (C) 2008 Elsevier Ltd. All Fights reserved.