Enantioselective Aryl-Iodide-Catalyzed Wagner–Meerwein Rearrangements

We report a strategy
for effecting catalytic, enantioselective
carbocationic rearrangements through the intermediacy of alkyl iodanes
as stereodefined carbocation equivalents. Asymmetric Wagner–Meerwein
rearrangements of β-substituted styrenes are catalyzed by the C2-symmetric aryl iodide 1 to provide
access to enantioenriched 1,3-difluorinated molecules possessing interesting
and well-defined conformational properties. Hammett and kinetic isotope
effect studies, in combination with computational investigations,
reveal that two different mechanisms are operative in these rearrangement
reactions, with the pathway depending on the identity of the migrating
group. In reactions involving alkyl-group migration, intermolecular
fluoride attack is product- and enantio-determining. In contrast,
reactions in which aryl rearrangement occurs proceed through an enantiodetermining
intramolecular 1,2-migration prior to fluorination. The fact that
both pathways are promoted by the same chiral aryl iodide catalyst
with high enantioselectivity provides a compelling illustration of
generality across reaction mechanisms in asymmetric catalysis.