(CAAC)Pd(py) Catalysts Disproportionate to Pd(CAAC)₂

Area of Science:

• Organometallic Chemistry
• Catalysis
• Organic Synthesis

Background:

• Palladium complexes with N-heterocyclic carbene (NHC) ligands are established cross-coupling precatalysts.
• Pyridine serves as a common ancillary ligand in these systems.

Purpose of the Study:

• To investigate palladium complexes featuring cyclic (alkyl)(amino)carbene (CAAC) ligands as alternatives to NHC ligands.
• To evaluate the catalytic performance of these CAAC-palladium complexes in cross-coupling reactions and diazene formation.

Main Methods:

• Synthesis and characterization of palladium complexes bearing CAAC and pyridine ligands.
• Evaluation of catalytic activity in Buchwald-Hartwig amination using aryl chlorides.
• Assessment of catalytic performance in diazene formation from anilines in the presence of dioxygen.

Main Results:

• The reduced form, (CAAC)Pd(py), disproportionates into (CAAC)2Pd0 and palladium nanoparticles.
• CAAC-palladium complexes exhibit potent catalytic activity in Buchwald-Hartwig amination of aryl chlorides at 60 °C.
• These complexes effectively catalyze diazene formation from anilines under aerobic conditions.
• Catalytic activity in cross-coupling is comparable to NHC-supported complexes, but CAAC complexes excel in diazene synthesis.

Conclusions:

• Cyclic (alkyl)(amino)carbene ligands can effectively replace N-heterocyclic carbene ligands in palladium precatalysts.
• CAAC-palladium complexes are highly efficient catalysts for both Buchwald-Hartwig amination and aerobic diazene formation.
• These findings expand the scope of ligands for palladium-catalyzed cross-coupling and related transformations.