Why Nonheme Iron Halogenases Do Not Fluorinate C–H Bonds: A Computational Investigation

Selective
halogenation is necessary for a range of fine chemical
applications, including the development of therapeutic drugs. While
synthetic processes to achieve C–H halogenation require harsh
conditions, enzymes such as nonheme iron halogenases carry out some
types of C–H halogenation, i.e., chlorination or bromination,
with ease, while others, i.e., fluorination, have never been observed
in natural or engineered nonheme iron enzymes. Using density functional
theory and correlated wave function theory, we investigate the differences
in structural and energetic preferences of the smaller fluoride and
the larger chloride or bromide intermediates throughout the catalytic
cycle. Although we find that the energetics of rate-limiting hydrogen
atom transfer are not strongly impacted by fluoride substitution,
the higher barriers observed during the radical rebound reaction for
fluoride relative to chloride and bromide contribute to the difficulty
of C–H fluorination. We also investigate the possibility of
isomerization playing a role in differences in reaction selectivity,
and our calculations reveal crucial differences in terms of isomer
energetics of the key ferryl intermediate between fluoride and chloride/bromide
intermediates. While formation of monodentate isomers believed to
be involved in selective catalysis is shown for chloride and bromide
intermediates, we find that formation of the fluoride monodentate
intermediate is not possible in our calculations, which lack additional
stabilizing interactions with the greater protein environment. Furthermore,
the shorter Fe–F bonds are found to increase isomerization
reaction barriers, suggesting that incorporation of residues that
form a halogen bond with F and elongate Fe–F bonds could make
selective C–H fluorination possible in nonheme iron halogenases.
Our work highlights the differences between the fluoride and chloride/bromide
intermediates and suggests potential steps toward engineering nonheme
iron halogenases to enable selective C–H fluorination.