Carbonyl-Directed Aliphatic Fluorination: A Special Type of Hydrogen Atom Transfer Beats Out Norrish II

Recently,
our group reported that enone and ketone functional groups,
upon photoexcitation, can direct site-selective sp3 C–H
fluorination in terpenoid derivatives. How this transformation actually
occurred remained mysterious, as a significant number of mechanistic
possibilities came to mind. Herein, we report a comprehensive study
describing the reaction mechanism through kinetic studies, isotope-labeling
experiments, 19F NMR, electrochemical studies, synthetic
probes, and computational experiments. To our surprise, the mechanism
suggests intermolecular hydrogen atom transfer (HAT) chemistry is
at play, rather than classical Norrish hydrogen atom abstraction as
initially conceived. What is more, we discovered a unique role for
photopromoters such as benzil and related compounds that necessitates
their chemical transformation through fluorination in order to be
effective. Our findings provide documentation of an unusual form of
directed HAT and are of crucial importance for defining the necessary
parameters for the development of future methods.