2 research outputs found
Computational and Experimental Studies of Regioselective S<sub>N</sub>Ar Halide Exchange (Halex) Reactions of Pentachloropyridine
The Halex reaction
of pentachloropyridine with fluoride ion was
studied experimentally and computationally with a modified ab initio
G3MP2B3 method. The G3 procedure was altered, as the anionic transition
state optimizations failed due to the lack of diffuse functions in
the small 6-31G* basis set. Experimental Halex regioselectivities
were consistent with kinetic control at the 4-position. The reverse
Halex reaction of fluoropyridines with chloride sources was demonstrated
using precipitation of LiF in DMSO as a driving force. Reverse Halex
regioselectivity at the 4-position was predicted by computations and
was consistent with kinetic control. Scrambling of halide ions between
chlorofluoropyridines was catalyzed by <i>n</i>-Bu<sub>4</sub>PCl, and the products of these reactions were shown to result from
a combination of kinetic and thermodynamic control. Comparison of
the C–F and C–Cl homolytic bond dissociation energies
suggests that an important thermodynamic factor which controls regioselectivity
in this system is the weak C2–Cl bond. The differences between
Δ<i>H</i>° values of chlorofluoropyridines can
be explained by a combination of three factors: (1) the number of
fluorine atoms in the molecule, (2) the number of fluorine atoms at
the C2 and C6 positions, and (3) the number of pairs of fluorine atoms
which are ortho to one another
Nucleophilic Deoxyfluorination of Phenols via Aryl Fluorosulfonate Intermediates
This
report describes a method for the deoxyfluorination of phenols
with sulfuryl fluoride (SO<sub>2</sub>F<sub>2</sub>) and tetramethylammonium
fluoride (NMe<sub>4</sub>F) via aryl fluorosulfonate (ArOFs) intermediates.
We first demonstrate that the reaction of ArOFs with NMe<sub>4</sub>F proceeds under mild conditions (often at room temperature) to afford
a broad range of electronically diverse and functional group-rich
aryl fluoride products. This transformation was then translated to
a one-pot conversion of phenols to aryl fluorides using the combination
of SO<sub>2</sub>F<sub>2</sub> and NMe<sub>4</sub>F. Ab initio calculations
suggest that carbon–fluorine bond formation proceeds via a
concerted transition state rather than a discrete Meisenheimer intermediate