High-Valent Cobalt-Difluoride in Oxidative Fluorination of Saturated Hydrocarbons

Abstract

The heme paradigm where Fe=O acts as the C−H oxidant and Fe−OH rebounds with the formed carbon-centered radical guides the design of the prototypical synthetic hydroxylation catalyst. We are exploring methods to evolve beyond the metal-oxo oxidant and hydroxide rebound, to incorporate a wider array of functional group. We have demonstrated the application of Co^II(OTf)₂ (10 mol% catalyst; OTf=trimfluoromethanesulfonate) in combination with polydentate N-donor ligands (e. g. BPMEN=N,N′-dimethyl-N,N′-bis(pyrid-2-ylmethyl)ethane-1,2-diamine) and Selectfluor in the oxidative fluorination of saturated hydrocarbons in high yields. The addition of CsF to the reaction mixture induced near-quantitative yields of fluorinated saturated hydrocarbons (>90 % yield of fluorinated product). For 1-hydroxy, 1-acetyl, 1-carboxy-, and 1-acetamido-adamantane, we demonstrated selective fluorination at the 3-position. We propose two mechanisms for the Co^II-catalyzed reaction: either (i) an N-radical, derived from Selectfluor, acted as the C−H oxidant followed by radical rebound with Co^III−F; or (ii) a Co^IV−(F)₂ species was the C−H oxidant followed by radical rebound with Co^III−F. Our combined spectroscopic, kinetic, and chemical trapping evidence suggested that an N-radical was not the active oxidant. We concluded that a Co^IV−(F)₂ species was the likely active oxidant and Co^III−F was the likely F-atom donor to a carbon centered radical producing a C−F bond.