Fe-Promoted C-F Activation of Aryl Fluorides Enables Heck-Type Coupling with Alkenes and One-Pot Synthesis of Indenes

Abstract

Metal-mediated aryl fluoride activation presents challenges because of the thermodynamically robust C-F bond but also provides opportunities to develop C-C coupling reactions with this unconventional electrophile. Here, we report that the metallanucleophile, K[CpFe(CO)2] (KFp), is readily arylated by aryl fluorides to provide CpFe(CO)2Ar (FpAr) complexes at ambient conditions in the absence of any catalyst, contrary to previous literature reports. This C-F activation likely proceeds by a SNAr mechanism rather than the more common oxidative addition pathway for metal-mediated C-F cleavage. Facile access to FpAr derivatives has enabled further development of Fe-promoted coupling reactions of aryl fluorides with alkenes and alkynes. Under stoichiometric Fe conditions, the first documented examples of Heck-type coupling reactions with aryl fluoride electrophiles are reported herein. Various aryl fluorides were found to couple with olefins to provide E-alkene products; aliphatic derivatives unexpectedly underwent reduction to 1,2-disubsituted ethane products to varying extents. Aryl fluorides were also found to couple with 5-decyne to yield indene derivatives in a one-pot manner; indeneone and indanone products were also observed in selected cases. A mechanistic investigation to identify the apparently potent hydride donor generated under these conditions was conducted, leading to a mechanistic hypothesis involving the intermediacy of metal carbonyl-capped, triple-decker ferrocene compounds as reductants. Despite the involvement of these diiron intermediates limiting stoichiometric product yields, the observed C-C coupling reactions expand the limits of organofluorine chemistry.