Introduction
<P>Developments in two seemingly unconnected fields have given rise
to a new family of powerful catalysts with many applications in
olefin metathesis. On the one hand, the progress made over the past
decade in the synthesis of
N-heterocyclic
carbenes (NHCs) and their complexes, as well as the catalytic applications
of these systems, has been impressive.
[
1]
[
2]
On the other hand, the
achievements in the evolution of homogeneous catalysts for olefin
metathesis have turned this reaction into a common C-C
coupling process
[
3]
with various
possibilities including ring-opening metathesis polymerization (ROMP),
ring-closing metathesis (RCM), acyclic diene metathesis polymerization (ADMET),
ring-opening metathesis (ROM), and cross-metathesis (CM or XMET).</P><P>Herrmann et
al.
[
4]
were the first
to synthesize a modified Grubbs catalyst that included
N-heterocyclic carbene ligands. The bis(NHC)-substituted
derivatives
2 showed a minor improvement
in terms of catalytic activity compared to conventional Grubbs catalyst
1. Soon after, papers concurrently published
by Nolans’,
[
5]
Grubbs’,
[
6]
and Herrmanns’
[
7]
groups described the synthesis
of mono(NHC)-substituted complexes such as those of type
3 and
4. This
new family of compounds has been called the second generation Grubbs
catalysts.</P>
<P>The strong σ donor character of NHC ligands facilitates
the dissociation of the phosphine in compounds
3 and
4 due to the
trans effect,
although the key step for olefin metathesis consists of the coordination
of the olefin to the unsaturated ruthenium complex.
[
8]
These new compounds (
3 and
4), which
are reasonably air- and water-stable, are more active than
2 or
1 and can
also tolerate the presence of different functional groups (with
the exception of basic ones like nitriles and amines).
[
3a]
The enhanced activity
of these compounds means that the reactions can be carried out under
mild conditions. Three years after its synthesis,
[
6b]
complex
4a [R = Mes
(mesityl), R′ = Cy] has
become commercially available
[
9]
owing
to its wide-ranging applications.</P><P>Compound
4a was synthesized from
1 and the saturated 1,3-dimesityl-2,3,4,5-tetrahydro-1
H-imidazol-2-ylidene, which was protected
as the alkoxide to generate the free carbene ligands in situ.</P>
