ConspectusThe chemistry of hypervalent iodine(III) compounds
has gained great interest over the past 30 years. Hypervalent iodine(III)
compounds show valuable ionic reactivity due to their high electrophilicity
but also express radical reactivity as single electron oxidants for
carbon and heteroatom radical generation. Looking at ionic chemistry,
these iodine(III) reagents can act as electrophiles to efficiently
construct C–CF3, X–CF3 (X = heteroatom),
C–Rf (Rf = perfluoroalkyl), X–Rf, C–N3, C–CN, S–CN, and C–X
bonds. In some cases, a Lewis or a Bronsted acid is necessary to increase
their electrophilicity. In these transformations, the iodine(III)
compounds react as formal “CF3+”,
“Rf+”, “N3+”, “Ar+”, “CN+”, and “X+” equivalents. On the other
hand, one electron reduction of the I(III) reagents opens the door
to the radical world, which is the topic of this Account that focuses
on radical reactivity of hypervalent iodine(III) compounds such as
the Togni reagent, Zhdankin reagent, diaryliodonium salts, aryliodonium
ylides, aryl(cyano)iodonium triflates, and aryl(perfluoroalkyl)iodonium
triflates. Radical generation starting with I(III) reagents can also
occur via thermal or light mediated homolysis of the weak hypervalent
bond in such reagents. This reactivity can be used for alkane C–H
functionalization. We will address important pioneering work in the
area but will mainly focus on studies that have been conducted by
our group over the last 5 years.We entered the field by investigating
transition metal free single electron reduction of Togni type reagents
using the readily available sodium 2,2,6,6-tetramethylpiperidine-1-oxyl
salt (TEMPONa) as an organic one electron reductant for clean generation
of the trifluoromethyl radical and perfluoroalkyl radicals. That valuable
approach was later successfully also applied to the generation of
azidyl and aryl radicals starting with the corresponding benziodoxole
(Zhdankin reagent) and iodonium salts. In the presence of alkenes
as radical acceptors, vicinal trifluoromethyl-, azido-, and arylaminoxylation
products result via a sequence comprising radical addition to the
alkene and subsequent TEMPO trapping. Electron-rich arenes also react
with I(III) reagents via single electron transfer (SET) to give arene
radical cations, which can then engage in arylation reactions. We
also recognized that the isonitrile functionality in aryl isonitriles
is a highly efficient perfluoroalkyl radical acceptor, and reaction
of Rf-benziodoxoles (Togni type reagents) in the presence
of a radical initiator provides various perfluoroalkylated N-heterocycles (indoles, phenanthridines, quinolines, etc.).
We further found that aryliodonium ylides, previously used as carbene
precursors in metal-mediated cyclopropanation reactions, react via
SET reduction with TEMPONa to the corresponding aryl radicals. As
a drawback of all these transformations, we realized that only one
ligand of the iodine(III) reagent gets transferred to the substrate.
To further increase atom-economy of such conversions, we ide...
