Zwitterionic Palladium Complexes: Room-Temperature Suzuki–Miyaura Cross-Coupling of Sterically Hindered Substrates in an Aqueous Medium

A series
of new imidazolium chlorides were straightforwardly prepared
from the reactions between chloroacetone and imidazole derivatives.
Deprotonation of the methylene proton next to the ketone group in
these salts by pyridine led to the formation of a monodentate ligand
that coordinated to palladium, readily forming zwitterionic anionic
palladium pyridine complexes bearing a formal positive charge on the
ligand ancillary. The pyridine ligand in the zwitterionic complexes
can be facilely replaced by phosphine ligands. Seven of these new
complexes were successfully characterized by X-ray crystallography.
The zwitterionic phosphine complexes were highly efficient in catalyzing
room-temperature Suzuki–Miyaura reactions between sterically
hindered aryl chlorides and arylboronic acids in an aqueous medium.