Chemistry Reference
In-Depth Information
Tab. 6.5
Initiation kinetics as measured by reaction of the carbene with ethyl vinyl ether [101].
Carbene
a)
Temperature [
C]
Relative rate
10
[Ru] =CHPh (
6
)
1.0
40
[Ru] =CH2 (
7
)
0.85
0
[Ru] =CHEt
0.54
25
[Ru] =CHCH=C(CH
3
)
2
(
5
)
1.0
35
[Ru]*=CHPh
0.46
10
[Ru]**=CHPh
3.3
85
[Ru]*=CH
2
<1.0
a)
[Ru] =RuCl
2
(PCy
3
)
2
; [Ru]*=RuCl
2
(PCy
3
)(SIMes); [Ru]**= RuCl
2
(PPh
3
)(SIMes).
trans
by a factor of about two. The reasons for these trends are both steric and
electronic in nature. The alkylidene moiety, being electron donating and some-
what sterically bulky, probably assists the dissociation of a phosphine ligand and
stabilizes the formal Ru(IV) metallacycle intermediate, thus increasing the rate of
reaction with an olefin.
It is notable that the methylidenes, especially the second-generation methyli-
dene, are relatively unreactive towards metathesis. In fact, the second-generation
methylidene complex polymerizes cyclooctadiene by ROMP
4 orders of magnitude
slower
than benzylidene
10
[101]. Thus, the unique properties of the methylidene
complex discussed above are even more apparent for the second-generation cata-
lyst series, due to the extremely slow phosphine dissociation of the methylidene
complexes.
These results indicate that the methylidene complex is the “bottleneck” in the
ADMET reaction with Grubbs catalysts, which suggests that performing the AD-
MET reaction on internal olefins may be beneficial. For example, ADMET with
2,10-dodecadiene versus 1,9-decadiene would completely avoid formation of the
methylidene and removal of 2-butene would be the driving force of the reaction
(Scheme 6.28).
Ruthenium alkylidenes would be the only propagating species. This theory has
yet to be tested, however, because the rates of reaction of the benzylidene with in-
ternal olefins are lower than that with 1-hexene (Tab. 6.4). With the advent of the
second-generation catalysts, however, the benefit of avoiding the methylidene may
outweigh the reduced rate of reaction with internal olefins.
The other strategy to counteract the slow kinetics of the methylidene is to modi-
fy the phosphine in order to make the methylidene phosphine more labile. This
Scheme 6.28
ADMET of internal dienes.
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