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phosphine complexes, due to more orbital mixing with the Ru component when
not partially donated to a phosphine. This effect could be responsible for the
greater affinity of the 14-electron intermediates derived from NHC complexes to
bind olefins over phosphines. This cursory explanation is speculative, although
theoretical calculations could support the notion or shed light onto other factors
controlling the ligand binding.
These data in total have led to the revising of the ADMET mechanism with
ruthenium-based catalysts to include the very important aspect of phosphine dis-
sociation (Scheme 6.27). In this mechanism
k
3
,
k
4
,
k
7
, and
k
8
are assumed to be
very fast, such that the dissociative olefin-phosphine exchange is the rate-deter-
mining process [100].
6.6.3
Role of the Carbene: the Special Case of the Methylidene
In addition to the effects of the phosphine and the L ligand, the carbene fragment
has an impact on the metathesis activity of these complexes. As established above,
phosphine dissociation can be the rate-limiting step in the initiation (first turn-
over), especially for the NHC complexes, and thus it is difficult to directly com-
pare the metathesis activity of different carbenes in the absence of the influence
of phosphine dissociation. One obvious trend is that the methylidenes are particu-
larly less reactive than other carbenes for this catalyst series. This has great im-
pact on the ADMET reaction since the methylidene is involved in each turnover.
Phosphine dissociation is not rate limiting for the first-generation complexes
[101], and as such some insight is gained by examining the initiation rates for
these complexes. Selected results are given in Tabs. 6.4 and 6.5.
For reaction of the various ruthenium carbenes with either the terminal olefin
1-hexene [103] or ethyl vinyl ether [101], the rate of reaction increases in the order
methylidene < vinyl carbene < benzylidene < alkylidene. The differences in rates
were quite significant, especially for the methylidene, which is quite slow to initi-
ate. The data also show that internal olefins are slower to react, but only by about
an order of magnitude; the
cis
isomers being more readily metathesized than the
Tab. 6.4
Kinetics of reaction of olefins with ruthenium carbenes as monitored by NMR spectro-
scopy [103].
Carbene
a)
Temperature [
C]
Olefin
Relative rate
7
1-hexene
[Ru] =CHPh (
6
)
1.00
7
1-hexene
[Ru] =CHEt
5.52
35
1-hexene
[Ru] =CHPh (
6
)
8.5
35
1-hexene
[Ru] =CH
2
(
7
)
0.4
35
cis
-3-hexene
[Ru] =CHPh (
6
)
0.51
35
trans
-3-hexene
[Ru] =CHPh (
6
)
0.20
a)
[Ru] =RuCl
2
(PCy
3
)
2
.
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