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found to be more resistant, to harsh reaction conditions
(temperature and functional group tolerance),[15] than their
benzylidene counterparts. Nevertheless, their catalytic activi-
ties have been scarcely examined, especially the SIMes-con-
taining complex 7 (SIMes=1,3-bis(2,4,6-trimethylphenyl)-
4,5-dihydroimidazol-2-ylidene)for which no evaluation in
catalysis has been reported so far.
with a more hindered substrate, diethyl allymethallylmalo-
nate 10 (Figure 2). The reaction trend observed for substrate
10 with precatalysts 6 and 7 was similar to the trend ob-
Results and Discussion
Kinetic studies and mechanism: Historically, metathesis cat-
alysts have been tested in metathesis transformations such
as RCM, without significant optimization of reaction condi-
tions (for instance high temperatures have been generally
used). The report of high isolated yields is not satisfactory
to determinate catalyst efficiency (this relationship is too
often used). Moreover, in numerous instances, such studies
do not report reaction conditions, often omitting catalyst
loading and reaction time information. Consequently, a
comprehensive and useful comparison between metathesis
catalysts proves practically difficult or impossible.
Figure 2. RCM of substrate 10 with precatalyst 5–7 (1 mol%)in CH 2Cl2
~
*
^
at RT; ( , 5) , ( , 6)and ( , 7).
To evaluate efficiently the activity of precatalysts 5–7 in
RCM, several kinetic studies were carried out. We initiated
our study with diethyl diallylmalonate 8 as a model sub-
strate and performed room-temperature reactions with low
precatalyst loadings (1 mol%), to slow the rate of RCM re-
action to obtain an accurate measurement of conversion
(Figure 1). Under these conditions, precatalyst 5 was found
served for the previous substrate (diene 8), a constant reac-
tion rate was observed during the RCM. A 36 hour reaction
time led to only ꢀ50% conversion with 6, and in 10 hours
full conversion was obtained with 7. Whereas, the phos-
phine-containing precatalyst 5 led to a significantly different
performance. Instead of 10 min for substrate 8, 10 hours
were necessary to complete the RCM for substrate 10. Inter-
estingly, after a fast initiation, the reaction rate decreased,
this indicates a degree of degradation in the active species.
These kinetic studies performed on 8 and its trisubstituted
analogue 10 have shown that the efficiency of 5 is closely re-
lated to the steric hindrance of the substrate. Whereas, the
activities of the NHC-containing complexes were found to
be comparable if used with substrates 8 and 10. These obser-
vations suggest different rate-determining steps for each cat-
alyst generation (Figure 3).[16] According to previous mecha-
nistic studies,[17] ruthenium olefin metathesis catalysts in-
volves 14 electron complexes B and E, which are the active
species and their formation represents the rate-determining
step of the reaction. The formation of B, by dissociation of a
phosphine ligand from precatalyst A, precedes the olefin co-
ordination and a first metathesis that leads to complex C.
This activation step (formation of B)is not related to the
nature of the substrate and corresponds to the limiting step
for NHC-containing precatalysts. This explains why, for sev-
eral substrates, a thermal activation of complexes 6 and 7 is
required, whereas 5 reacted faster (vide supra). The forma-
tion of the metallacyclobutane D, and its conversion to E
upon the extrusion of RCM product, is directly related to
the steric hindrance of the substrate and constitute the limit-
ing step for phosphine-containing precatalysts.
Figure 1. RCM of substrate 8 with precatalyst 5–7 (1 mol%)in CH Cl2 at
2
~
*
^
RT; ( , 5) , ( , 6)and ( , 7).
to be extremely efficient, only a few minutes were necessary
to complete the reaction, whereas 6 and 7 showed slow and
moderate reaction kinetics, respectively. Although the activi-
ties of 6 and 7 persisted over a few hours at a constant rate,
10 hours were required for 7 to reach a full conversion and
a reaction with 6 over 36 hours led to only a 72% conver-
sion.
In an attempt to establish if these catalysts behaved simi-
larly for various substrates, we carried out similar studies
The significant difference in activity between IMes- and
SIMes-containing precatalysts is still not clearly explained.
8030
ꢁ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 8029 – 8036