Angewandte
Communications
Chemie
Figure 2. X-ray crystal structures of 15a and 15 f. Ellipsoids are set at
[
37]
3
0% probability; hydrogen atoms are omitted for clarity.
shorter than RuÀC
in complex 2 (1.981(5) ꢀ). Surpris-
bonds in complexes 15 were found to be
carbene
ingly, the RuÀC
carbene
significanly longer than RuÀC
in 12a and very similar to
carbene
the RuÀC
distances in 5 (2.105(4) and 2.091(4) ꢀ). For
carbene
Figure 3. Reaction profiles for RCM of DEDAM with catalysts 12g,
15a–h.
example, the distances in 15 f are RuÀC7
2.093(11) and
carbene
RuÀC46
2.103(11) ꢀ.
carbene
To examine the possible initiation pathway of 15, we
reacted 15a and 15 f with doubly chelating olefin 13. Clean
conversion of 15 f to complex 12 f was observed within 1 h in
the presence of 1.2 equiv of 13 (toluene, 608C), and the
product was isolated in 86% yield. Under the same conditions
no conversion of 15a was observed within two days. However,
in the presence of 2 equiv of CuCl, complex 12a was formed
within 30 minutes. This results strongly support the classical
mechanism in which active, 14-electron species are formed via
neutral ligand release and also prove that for 15a dissociation
of CAAC is the rate-limiting step. NMR kinetic studies were
undertaken to gain more information about the possible
mechanism of initiation of 15 f. The rate of the reaction
The structure of the CAAC ligand proved to have
a dramatic influence on the initiation rate. Complexes 15a–
d exhibited a very low (or lack of) activity, which could be
expected in the case of (pre)catalysts bearing two strongly
binding CAACs. These complexes can be activated by CuCl
(Supporting Information). Unexpectedly, complexes 15e–h
showed a promising, moderate-to-high activity under these
mild conditions.
[28]
For the complexes with a symmetric substitution of the N-
aryl ring (15a–c, 15 f), the ratio of isomers in the solution can
1
be conveniently determined by H NMR based on the
chemical shift of the characteristic proton of the indenylidene
[30]
between 15 f and olefin 13 ([D ]toluene, 608C) proved to be
ligand (see the Supporting Information for details). Crystal
8
1
independent of the concentration of 13 (see the Supporting
structures and H NMR spectra of 15 f and 15c suggest that
Information) and was in good agreement with the stability of
introduction of a phenyl ring into the quaternary carbon atom
favors the formation of a rotamer having N-aryl groups on the
opposite side with respect to the coordination pyramid base.
These complexes are more active than their close analogues
which contain methyl instead of a phenyl substituent (15a,
15b) and which exist mainly in the rotamer having N-aryl
groups on the same side with respect to the coordination
pyramid base. However, specific rotamer does not ensure
high activity, as exemplified by the poor conversion of 17
obtained with 15c. The electronic properties of CAACs in
1
5 f in solution (which appears to be limited by the high
[
25]
lability and low stability of CAAC). Experimental results
suggest that a dissociative mechanism operates for 15 f
(
Scheme 3). This, however, cannot be extrapolated to all
complexes 15, and further theoretical and kinetic studies are
needed to shed more light on the initiation mechanism of
[
29]
these (pre)catalysts.
1
5a and 15g must be similar since these ligands differ only in
the position of one methyl group in the N-aryl substituent. At
the same time, a striking difference in activity of 15a and 15g
was observed. Therefore, the most plausible explanation for
the very broad range of activities covered by catalysts 15 is
that the degree of steric repulsion between two CAACs
[
31]
determines the initiation rate.
Next, ethenolysis of methyl oleate 7 was performed. Also
in this case, 12g was used for the comparative reasons.
Initially, reactions were run using 10 ppm catalyst loading
in neat MO at 150 psi of ethylene (99.99% purity). The
catalysts were compared at the conditions at which they were
most efficient. Catalyst 12g provided maximum TON at 408C
in just 2 h. The bis(CAAC)Ru indenylidene complexes 15
Scheme 3. Proposed mechanism for initiation of 15 f.
[
32]
Complexes 15a–h and highly efficient in ethenolysis
reaction complex 12g were tested in the benchmark ring
closing metathesis (RCM) of diethyl diallylmalonate
(
DEDAM, 17, Figure 3), to compare their activity.
Angew. Chem. Int. Ed. 2016, 55, 1 – 7
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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