The Evolution of Pd⁰/Pd^II-Catalyzed Aromatic Fluorination

Area of Science:

• Organic Chemistry
• Catalysis
• Medicinal Chemistry

Background:

• Aromatic fluorides are vital components in pharmaceuticals and agrochemicals.
• Existing methods for aromatic fluoride synthesis, like Balz-Schiemann and Halex reactions, suffer from harsh conditions, limited functional group tolerance, and narrow substrate scope.
• Transition metal catalysis offers alternatives, but a broadly applicable C-F cross-coupling method remains a significant challenge, particularly due to difficulties in aryl-fluoride reductive elimination from palladium(II) intermediates.

Purpose of the Study:

• To develop a novel, broadly applicable palladium-catalyzed C-F cross-coupling reaction for the synthesis of aromatic fluorides.
• To overcome limitations of existing methods by enabling milder conditions and wider substrate scope.
• To address challenges associated with aryl-fluoride reductive elimination and improve the practicality of aromatic fluorination.

Main Methods:

• Discovery and development of specific biaryl monophosphine ligands that promote challenging Ar-F reductive elimination from Pd(II) intermediates.
• Investigation of an in situ ligand modification process involving substrate incorporation into the ligand scaffold.
• Synthesis of 'premodified' ligands and identification of a superior ligand for expanded substrate scope, including (hetero)aryl triflates and bromides.
• Development of a new Pd(0) precatalyst that bypasses inefficient in situ Pd(II) reduction.
• Design of a preformed wax capsule to isolate air-sensitive catalysts and reagents, enabling glovebox-free benchtop operation.
• Mechanistic studies, including deuterium labeling, to understand the formation of regioisomeric fluoride side products via deprotonation and Pd-benzyne intermediates.
• Design of a new ligand to minimize regioisomer formation and facilitate room-temperature Ar-F reductive elimination.

Main Results:

• A novel palladium-catalyzed C-F cross-coupling process was successfully developed.
• The use of specific biaryl monophosphine ligands enabled challenging Ar-F reductive elimination.
• A premodified ligand significantly expanded the substrate scope to include (hetero)aryl triflates and bromides.
• A new Pd(0) precatalyst improved efficiency and reduced byproducts.
• A wax capsule technology allowed for air-sensitive reagent handling and benchtop use, enhancing practicality.
• Mechanistic insights revealed side product formation pathways, leading to the design of a new ligand that substantially reduced regioisomer formation.

Conclusions:

• The developed palladium-catalyzed C-F cross-coupling method represents a significant advancement in aromatic fluoride synthesis.
• The new ligand and catalyst system, coupled with innovative handling technology, offers a more practical, efficient, and versatile approach compared to traditional methods.
• Further ligand design based on mechanistic understanding can minimize side reactions and enable even milder reaction conditions, such as room-temperature Ar-F reductive elimination.