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(a)
(c)
(b)
Fig. 5.6
Tris(pyrazolyl)borate- (a), cyclopentadienyl- (Cp-) (b), and penta-
methyl-Cp- (c) based Grubbs-type catalysts.
hydridotris(pyrazolyl)borate (Tp), cyclopentadienyl (Cp) and pentamethylcyclopenta-
dienyl
(=pentamethyl-Cp)
(Fig. 5.6)
[208-212]
and indenyl
ligands
[213].
1
-O
2
CCHPh
2
)Ru
(=CHPh) is a (poor) catalyst for the ROMP of NBE without a co-catalyst, while
molybdenum alkylidenes prepared from the Tp-ligand of the formula
Mo(Tp)(CHCMe
2
Ph)(N-2,6-i-Pr
2
-C
6
H
3
)(OTf) require a co-catalyst (AlCl
3
) in order
to be ROMP active [214].
The most outstanding achievements in catalyst design have been accomplished
by the use of N-heterocyclic carbenes (NHCs), a class of ligands developed by
Wanzlick some 40 years ago [215]. Almost 20 years passed by from the synthesis
of the first NHC-based-metal complexes [216, 217] to the isolation of the first
stable, free NHC by Arduengo [218, 219]. Meanwhile their chemistry is well-un-
derstood [220-222], yet their enormous potential in catalysis has been recognized
at a comparatively late stage [223-241]. NHCs are prepared either from glyoxal
[242], by reduction of intermediary imidazole-2(3
H
)-thiones [243] or, in the case of
the dihydro-analogues, from ethylenediamines and ethyl ortho-formate [244].
From the present point of view, the use of NHCs in ruthenium-based ROMP and
RCM must be considered as a milestone in catalyst research [245].
Herrmann and co-workers first reported on the synthesis of mononuclear as
well as binuclear NHC-based ruthenium alkylidenes [235] (Scheme 5.17).
An increased ROMP activity compared to mononuclear bis(phosphine)ruthe-
nium alkylidenes [246] was found. A real breakthrough was the reaction of
Cl
2
Ru(CHR)(PPh
3
)
2
(R=Ph, CHCMe
2
) with imidazoline-2-ylidenes [225, 227, 247]
for the synthesis of ruthenium-based
mixed ligand
systems of the general formula
Cl
2
Ru(CHR)(PR
3
)(NHC) [231]. These imidazoline-2-ylidene-derived initiators
turned out to possess dramatically elevated ROMP activities compared to the par-
In this
context
it
is worth mentioning that
(Tp(PPh
3
)(
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