Syntheses, Structures, and Catalytic Activities of Hemilabile Thioether-Functionalized NHC Complexes

Four imidazolium ( 5a / b ) and benzimidazolium ( 6a / b ) salts with hemilabile alkyl-aryl thioether functions have been prepared via a straightforward and modular pathway in order to compare their reactivities toward palladation. Reaction of 5a / b with Pd(OAc) 2 gave complex product mixtures, whereas 6a / b afforded the desired bis(benzimidazolin-2-ylidene) complexes 8a / b in good yields. The difference in reactivities of benzimidazole versus imidazole derivatives was attributed to the presence of additional acidic protons at C4/5 positions of the imidazolium ring, leading to competing and unselective deprotonation reactions. The milder Ag-NHC transfer reaction, on the other hand, provided either mono- or bis(imidazolin-2-ylidene) complexes ( 9 or 7a / b ) in good yields depending on the ligand:metal ratio. The interesting hemilability in monocarbene complex 9 was investigated by spectroscopy and thioether displacement reaction with PPh 3, yielding the mixed NHC-PPh 3 complex 10 in high yields. An initial comparative catalytic study also reveals that the mixed-donor complex 10 exhibits the highest activity among the complexes tested.