Conspectus
The efficient synthesis of organic compounds is an important field
of research, which sets the basis for numerous applications in medicine
or materials science. Based on the polarity induced by functional
groups, logical bond disconnections can be deduced for the elaboration
of organic compounds. Nevertheless, this classical approach makes
synthesis rigid, as not all bond disconnections are possible.
The concept of Umpolung has been therefore introduced: by inverting
the normal polarity of functional groups, new disconnections become
possible. Among the tools for achieving Umpolung, hypervalent iodine
reagents occupy a privileged position. The electrophilicity of the
iodine atom and the reactivity of the hypervalent bond allow access
to electrophilic synthons starting from nucleophiles. Nevertheless,
some classes of hypervalent iodine reagents can be too unstable for
many applications, in particular involving metal catalysis. In this
context, cyclic hypervalent iodine reagents, especially benziodoxolones
(BXs), have been known for a long time to be more stable than their
acyclic counterparts, yet their synthetic potential had not been fully
exploited.
In this Account, we report our efforts since 2008
on the use of
BX reagents in the development of new transformations in organic synthesis,
which showed for the first time their versatility as synthetic tools.
Our work started with electrophilic alkynylation, as alkynes are one
of the most important functional groups in organic chemistry, but
are usually introduced as nucleophiles. We used ethynylbenziodoxolones
(EBXs) in the direct alkynylation of nucleophiles, such as keto esters,
thiols, or phosphines. The reagents could then be applied to the gold-
and palladium-catalyzed alkynylation of C–H bonds on (hetero)arenes,
leading to a more efficient alternative to the Sonogashira reaction.
More complex reactions were then developed with formations of several
bonds in a single transformation. Gold- and platinum-catalyzed cyclization/alkynylation
domino processes gave access to new types of alkynylated heterocycles.
Multifunctionalization of olefins became possible through intramolecular
oxy- and amino-alkynylations. (Enantioselective) copper-catalyzed
oxy-alkynylation of diazo compounds led to stereocenters with perfect
atom economy. Finally, EBXs were also used for the alkynylation of
radicals generated under photoredox conditions.
Since 2013,
we then extended the use of BX reagents to other transformations.
Azidobenziodoxol(on)ess (ABXs) were used in the azidation of keto
esters, enol silanes, and styrenes. New more stable derivatives were
introduced. Cyanobenziodoxolones (CBXs) enabled the cyanation of stabilized
enolates, thiols, and radicals. Finally, new BX reagents were developed
for the Umpolung of indoles and pyrroles. They could be used in metal-catalyzed
directed C–H functionalizations, as well as in Lewis acid mediated
oxidative coupling to give functionalized bi(hetero)arenes.
In the past decade, our group and others have shown that BX reag...