Remote Substitution on N-Heterocyclic Carbenes Heightens the Catalytic Reactivity of Their Palladium Complexes

A new series of exceptionally bulky N-heterocyclic carbene (NHC) ligands is based on 1,3-bis-2′,6′-diisopropylphenylimidazol-2-ylidene (IPr), through substitution of large groups at the para (4′) position of the aryl rings. π-Cinnamylpalladium chloride complexes with these ligands are precatalysts for Suzuki–Miyaura coupling. Large triarylmethyl substituents at the 4′ position of the NHC give significantly greater catalyst activity. Although the %V_bur parameter has been proposed as a general measure of the size of bulky ligands, the sterically induced enhancement of the catalytic rate does not correlate with the %V_bur parameter; instead, solid angles are more useful in quantifying the differences between ligands because they account for bulk more distant from the metal. Overall, these results show that the use of distant steric hindrance can have a distinct positive influence on catalytic ability.