Overview
For decades, the dearomatization of arenes has been recognized as
a chemical transformation of fundamental importance. It provides the
connection between this robust and abundant source of hydrocarbons
and the alicyclic frameworks common to many biologically active products.
Thus, dearomatization methods have become powerful tools for organic
synthesis.
The chemical properties of arenes are influenced
by their coordination to a transition metal. For example, complexes
such as (η6-arene)Cr(CO)3+ and
its cationic analogs (e.g., FeCp+, RuCp+, Mn(CO)3+)
are susceptible to nucleophilic substitution or addition,
ultimately leading to the formation of substituted arenes or cyclohexadienes,
respectively. During the past three decades, the application of η6-arene
complexes to organic synthesis has been widely explored.
In the Harman Research Group, an alternative approach
to the dearomatization of arenes has been developed in which the aromatic
system may be activated toward organic transformations by η2-coordination.
Here, the metal-arene bond is stabilized primarily by the interaction
of filled metal dπorbitals with the
π* system of the arene, an interaction
having two important consequences for the activation of the aromatic
ring. Through πbackbonding,
the aromatic π system
becomes more electron-rich, similar to what is observed for organic
arenes bearing electron-donating
groups. Additionally, structural data for complexes containing η2-coordinated
aromatic rings show significant distortions in the bond lengths of
the ring (vide infra), consistent with a localization of πelectron
density. Together, these effects activate η2-bound
aromatic systems toward electrophilic rather than nucleophilic addition.
When the π base binds an aromatic
heterocycle such as a furan or pyrrole, novel tandem addition reactions
and cycloaddition reactions become possible.
Of the handful of transition metal systems that are known to form
stable η2 complexes with aromatic molecules, only d6 octahedral
metal complexes have been shown to enhance the reactivity of the
aromatic ligand toward electrophiles to date. For nearly a decade,
despite
our best efforts, this mode of arene activation was known only
for
the pentaammineosmium(II) system. However, in the past few years
a new generation of dearomatization agents have been developed
in our group based on a careful matching of the d5/d6 reduction potential
of rhenium(I), tungsten(0), and molybdenum(0) complexes with
that of pentaammineosmium(II) heterocycle such as a furan or pyrrole,
novel tandem addition reactions and cycloaddition reactions become
possible.
GO
TOP
