Hydroxyl-functionalized triazolylidene-based PEPPSI complexes: metallacycle formation effect on the Suzuki coupling...

Area of Science:

• Organometallic Chemistry
• Catalysis
• Organic Synthesis

Background:

• PEPPSI (Pyridine-Enhanced Precatalyst Preparation Stabilization and Initiation) complexes are widely used in catalysis.
• N-heterocyclic carbenes (NHCs) are important ligands in palladium-catalyzed cross-coupling reactions.
• Hydroxyl functionalization offers potential for tuning catalyst properties.

Purpose of the Study:

• To synthesize and characterize novel hydroxyl-functionalized 1,2,3-triazolylidene-based PEPPSI complexes.
• To evaluate the catalytic activity of these new complexes in Suzuki cross-coupling reactions.
• To investigate the role of the hydroxyl group in the catalytic mechanism.

Main Methods:

• Synthesis and full characterization of hydroxyl-functionalized PEPPSI complexes (2a-c).
• Catalytic testing in Suzuki cross-coupling of aryl chlorides/amides with boronic acids under basic conditions.
• Comparison of catalytic efficiency with ether-functionalized analogues (3, 4) and a commercial NHC-Pd complex (5).
• Isolation and characterization of metallacycles (6a-b) formed by deprotonation.

Main Results:

• Complexes 2a-c exhibited notably increased catalytic efficiency in Suzuki cross-coupling compared to analogues 3, 4, and 5.
• The catalytic performance was superior under basic conditions, suggesting the importance of hydroxyl group deprotonation.
• Deprotonation of complexes 2a-b with sodium tert-butoxide yielded metallacycles 6a-b.
• Metallacycles 6a-b are proposed as the active catalytic species.

Conclusions:

• Hydroxyl-functionalized 1,2,3-triazolylidene-based PEPPSI complexes are effective catalysts for Suzuki cross-coupling.
• Deprotonation of the hydroxyl moiety is crucial for enhancing catalytic activity.
• The formation of metallacycles is likely key to the catalytic cycle in these systems.