Fe(III)-Catalysis in Flow Enables Bimolecular Proton Transfer as an Alternative to Superelectrophiles in Carbonyl-Olefin Metathesis

Catalytic carbonyl-olefin metathesis has emerged as a new strategy for the direct formation of carbon–carbon double bonds. Successful approaches for ring-closing carbonyl-olefin metathesis exist, but limitations remain in accessing systems larger than 5-membered rings. Currently available strategies rely on Lewis acidic superelectrophiles as stronger catalysts, which require precious metal additives for their formation upon chloride abstraction from otherwise environmentally benign metal salts, such as FeCl3. Herein we report the development of a continuous flow reactor that overcomes this challenge and can access 6-membered rings using FeCl3 as the Lewis acid catalyst. The plug-flow reactor design is user friendly, benchtop amenable, and demonstrates up to 200x improved reaction efficiency over batch conditions. Computational investigations reveal that this transformation proceeds through an unprecedented bimolecular stepwise pathway via intermolecular proton transfer. These insights represent a significant advance for catalytic carbonyl–olefin metathesis and are expected to spur future developments in catalyst design and reaction scope.