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5.2.4.3
Ligand and Structural Variation in Ruthenium-Based Initiators
Variations in and Substitution of Phosphine-Ligands
The reaction of diazo-compounds with ruthenium arene complexes has been used
for the
in situ
generation of ruthenium alkylidenes from [RuCl
2
(
p-
cymene)]
2
, tricy-
clohexylphosphine and trimethylsilyldiazomethane [191]. The resulting initiators
were again active in the polymerization of 2,3-difunctionalized NBDEs and their
7-oxa-analogues with high
trans-
stereoselectivity [192, 193]. Finally, following the
diazo-approach, the synthesis of an internally oxygen-chelated ruthenium alkyli-
dene has to be mentioned (Scheme 5.14).
This compound, Cl
2
Ru(CH-2-i-Pr-O-C
6
H
4
))(PCy
3
), is realized by reaction of
Cl
2
Ru(PPh
3
)
3
with 2-i-propoxyphenydiazomethane/PCy
3
. Alternatively, this com-
plex, which turned out to be highly stable and even chromatographically recycl-
able, may be prepared by reaction of Cl
2
Ru(CHPh)(PCy
3
)
2
with 2-i-propoxystyrene.
So far, this compound, known as the “Hoveyda catalyst”, has only been used in
RCM [194]. More recently, the synthesis of entirely water-soluble analogues has
been reported (Fig. 5.5) [195-197].
With these systems, initiation of various substituted NBEs and 7-oxanorborn-5-
enes occurs rapidly and quantitatively, yet polymerizations do not proceed in a liv-
ing manner. In order to obtain truly living systems, investigations in terms of sta-
bility versus Brønsted acids such as DCl and bases were carried out [198]. Interest-
ingly, and in contrast to the behavior of “classical” water-soluble systems such as
Scheme 5.14
Synthesis of an internally stabilized and chromatographi-
cally recoverable ruthenium alkylidene.
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