Electronically tunable N-heterocyclic carbene ligands: 1,3-diaryl vs. 4,5-diaryl substitution

The catalytic activity of iridium-mediated transfer hydrogenation is readily tuned by electronic variation of the ligated tetraaryl-N-heterocyclic carbene and the installation of electron donating groups on the N-aryl substituents is more important than on the C-aryl substituents for effecting catalytic enhancement.