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Tab. 6.2
Phosphine exchange rates for various ruthenium complexes in toluene-
d
8
at 80
C [101].
Complex
a)
Exchange rate [s
-1
]
6
9.6
[Ru] =CHEt
19.4
5
0.33
10
0.13
11
7.5
a)
[Ru] =RuCl
2
(PCy
3
)
2
.
To confirm that a dissociative ligand exchange mechanism is in operation, the
kinetics of the rapid reaction of these complexes with ethyl vinyl ether was stud-
ied by NMR and UV/VIS spectroscopies. The results were in complete agreement
with a dissociative ligand exchange mechanism for all complexes in this series,
with the possible exception of the diiodo derivatives [101].
The results of the phosphine dissociation experiments for the various ruthe-
nium complexes were surprising in that the second-generation catalysts exchange
phosphine ligands slower than the first-generation analogs (Tab. 6.2).
At first the unprecedented activity of the second-generation catalysts was
thought to be a result of faster phosphine dissociation [95]. This result indicated
that the phosphine dissociation kinetics as well as the metathesis activity of the re-
sulting 14-electron intermediate must be considered to rationalize the activity of
these catalysts. Interestingly, returning to PPh
3
is valuable for the NHC com-
plexes, in that the phosphine exchange rate for complex
11
is almost that of com-
plex
6
[101].
The effects of this kinetic situation were clearly demonstrated in the context of
ADMET. In DP versus time curves (Fig. 6.4) for the ADMET of 1,9-decadiene with
complexes
6
and
10
a conspicuous induction period is witnessed for
10
that is ab-
sent for
6
.
6
Fig. 6.4
DP vs. time data at low con-
version (monomer to dimer) for
ADMET of 1,9-decadiene at 45
C
showing the induction period of cata-
lyst
10
. (Reprinted with permission
from Lehman, Jr., S. E., Wagener, K. B.,
Macromolecules
2002
,
35
, 48-53.)
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