Vanadium-Catalyzed Cross Metathesis: Limitations and Implications for Future Catalyst Design

Article abstract of DOI:10.1021/acs.organomet.9b00362

Self-metathesis of terminal olefins using vanadium(V) alkylidenes is presented. Under various reaction conditions, incomplete conversion is observed due to decomposition of the metallocyclobutane intermediate via β-hydride elimination. The activity was observed to decline when a more electron withdrawing, less sterically bulky ligand was used, in contrast to trends observed in ring-opening metathesis polymerization with vanadium catalysts. These results provide insight into the current limitations of olefin metathesis with vanadium catalysts, as well as guidance for catalyst development.

Full text of DOI:10.1021/acs.organomet.9b00362

Communication
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Vanadium-Catalyzed Cross Metathesis: Limitations and Implications
for Future Catalyst Design
Wesley S. Farrell*
Chemistry Department, United States Naval Academy, 572M Holloway Road, Annapolis, Maryland 21402, United States
S
* Supporting Information
ABSTRACT: Self-metathesis of terminal olefins using vanadium(V) alkylidenes is
presented. Under various reaction conditions, incomplete conversion is observed due
to decomposition of the metallocyclobutane intermediate via β-hydride elimination.
The activity was observed to decline when a more electron withdrawing, less sterically
bulky ligand was used, in contrast to trends observed in ring-opening metathesis
polymerization with vanadium catalysts. These results provide insight into the current
limitations of olefin metathesis with vanadium catalysts, as well as guidance for catalyst
development.
lefin metathesis (OM) is one of the most valuable and
which can lead to the production of undesired byproducts
Oversatile methods for the construction of new carbon− which are difficult to remove by common purification
techniques. In the case of ruthenium catalysts, several pathways
originating from alkylidene loss have been explored,28 and
nanoparticles have been implicated as well.29 Identifications of
new species which do not lead to isomerization are therefore
highly desirable.
carbon bonds, with many applications being found in organic
and polymer chemistry.1−9 Although well-defined catalysts for
OM have been known for quite some time,10,11 new
compounds capable of catalyzing previously challenging
transformations are still being reported. Notably, stereo-
selective, including Z-selective, cross metathesis (CM) has
recently become more facile with the development of Ru-based
catalysts,12−22 while Mo-based catalysts have allowed for Z-
selective CM with alkenes bearing electron-withdrawing
groups (Cl, Br, CF3) directly on the olefinic carbon.23−25 As
these select examples demonstrate, the scope and utility of OM
will continue to grow with continued catalyst development.
Nomura and co-workers have elegantly demonstrated that
vanadium(V) alkylidene complexes may be used to initiate
ring-opening metathesis polymerization (ROMP) of cyclic
olefins in a controlled manner. In these studies, ligands which
decrease electron density at vanadium increase the rate of
polymerization, while bulky alkoxides (e.g. C(CF3)3) produce
polyalkenemers with Z selectivity up to 98%.5,26 Impressively,
high molar mass polyalkenemers with narrow dispersities may
even be obtained from low-strain monomers (e.g. cyclopentene
and cycloheptene).27
Given that new catalysts will be required to continue the
progress of OM research, and these recent developments in
ROMP using vanadium alkylidenes, investigation into other
OM reactions with vanadium compounds is warranted. While
it is uncertain that vanadium alkylidenes will meet or surpass
the activity, efficiency, and/or ease of handling of established
ruthenium catalysts, such compounds may complement these
systems by meeting other challenges. For example, isomer-
ization is an issue often faced in the CM of terminal olefins,
Herein, an evaluation of the homocoupling of terminal
olefins with two vanadium complexes, (N-2,6-X2C6H3)-
(OC6Cl5)[P(CH3)3]2V[CHSi(CH3)3], X = CH3 (1), F (2),
is reported. These species were chosen given that vanadium
catalysts bearing the perchloroaryloxide ligand (OC6Cl5) have
demonstrated the highest ROMP activity to date.15 While it is
true that high ROMP activity does not necessarily translate to
high self-metathesis (SM; i.e. CM between two identical
olefins) activity, established catalysts were chosen so that direct
comparison to other OM reactions may be made (i.e., no
uncertainty regarding the overall metathesis activity). Inves-
tigation into electronic and steric effects is also reported, which
was achieved through substitution of the imido ligand, and
decomposition pathways have been probed. The results of this
study are not intended to constitute a comprehensive evaluation of
SM with vanadium alkylidenes but rather to serve as an initial
survey meant to provide guidance for future catalyst development.
Compound 1 was employed to screen several reaction
conditions with three terminal olefins, 1-hexene, allylbenzene,
and allyltrimethylsilane (allyl-TMS), as summarized in Scheme
1.30 The results of these screenings are summarized in Table 1.
In each case, a catalyst loading of 2 mol % was employed. In
the case of 1-hexene, the highest conversion was observed
Received: May 29, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00362
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
Scheme 1. Homocoupling of Terminal Olefins
Scheme 2. Intermediates of CM
when the reaction was performed neat. Extending the reaction
time from 0.5 to 1.0 h did not increase conversion, indicating
premature catalyst deactivation before 100% conversion is
achieved (vide infra). Increasing the temperature to 40 °C
increased the conversion slightly to 81%. Repeating the
reaction in tetrahydrofuran (THF) and benzene both resulted
in decreased maximum conversion, again with little difference
between 0.5 h and longer reaction times. Conversions were
significantly higher in the coordinating solvent THF relative to
noncoordinating benzene, perhaps due to stabilization of the
transient methylidene (vide infra). In all cases, the E:Z ratio of
isomers is near the expected thermodynamic ratio, indicating
no stereoselectivity. Slow cis/trans isomerization was observed
when 1 was reacted with excess cis-4-octene at room
temperature; however, the rate indicates that isomerization
likely does not play a role in the E:Z ratio observed in
homocoupling.30
the reaction in THF led consistently to minimal conversion,
although 12% conversion was observed with mild heating in
THF. No conversion was observed in benzene. Allyl-TMS
showed moderate conversion at room temperature, with a
longer reaction time leading to slight increases in conversion.
Increasing the temperature to 60 °C also increased the
conversion to 40%. Interestingly, the E:Z ratio was nearly
50:50 for this substrate, indicating that the increased bulk leads
to more preferential formation of the Z isomer.
It is interesting to note the relative reactivity between the
substrates examined. Allyl-TMS is significantly bulkier than
allylbenzene; however, conversions to its homocoupled
product were higher than those of allylbenzene. This indicates
both an electronic and steric effect on reactivity with 1. The
less electron rich olefin of allylbenzene31 is less likely to
undergo SM than that of 1-hexene or allyl-TMS, indicating a
preference of 1 for more electron rich olefins. Sterics clearly
also play a role, however, with the much less bulky 1-hexene
exhibiting the highest activity of all. Finally, minimal
isomerization occurred during the SM of allyl-TMS, as
evidenced by the presence of <1% 1,3-bis(trimethylsilyl)-1-
propene in the isolated product by 1H NMR (Figure S5).30 Z-
Selective Ru-based catalysts have been reported to yield this
Attempts to perform SM with 1 and neat allylbenzene
provided perplexing results, with conversions of 9−44%
observed with no consistency between identical runs or for
different reaction times. Presumably, this may be due to the
formation of a solid product which prevents adequate mixing,
leading to inconsistent reactions in each run. Performance of
Table 1. SM of 1-Hexene, Allylbenzene, and Allyltrimethylsilane Using {[N-2,6-
a
(CH3)2C6H3](OC6Cl5)[P(CH3)3]2V[CHSi(CH3)3]} (1)
b
c
b
entry
solvent (4 M)
substrate
time (h)
temp (°C)
conversn (%)
yield (%)
E:Z
1
2
3
4
5
6
7
8
9
10
neat
neat
neat
THF
THF
THF
THF
benzene
benzene
benzene
THF
THF
THF
1-hexene
1-hexene
1-hexene
1-hexene
1-hexene
1-hexene
1-hexene
1-hexene
0.5
0.5
1.0
0.5
1.0
1.0
2.0
0.5
1.0
2.0
0.5
1.0
2.0
2.0
1.0
2.0
0.5
1.0
2.0
2.0
25
40
25
25
25
40
25
25
25
25
25
25
25
40
25
25
25
25
25
60
77
81
75
43
55
57
55
28
29
32
0
5
1
12
0
0
65
69
71
24
23
49
35
89:11
89:11
88:12
91:9
89:11
88:12
88:12
91:9
1-hexene
1-hexene
91:9
92:8
d
11
allylbenzene
allylbenzene
allylbenzene
allylbenzene
allylbenzene
allylbenzene
allyl-TMS
allyl-TMS
allyl-TMS
allyl-TMS
d
12
d
88:12
87:13
13
14
THF
d
15
d
benzene
benzene
neat
neat
neat
16
e
17
18
19
20
15
23
27
40
14
22
27
31
52:48
52:48
52:48
56:44
e
e
e
neat
a
All runs were performed with 2 mol % of 1 as the catalyst in a sealed vial with constant stirring. All values are the average of two separate
b
c
d
e
1
experiments. Determined by H NMR analysis. Isolated yield. One reaction performed. Allyltrimethylsilane.
B
DOI: 10.1021/acs.organomet.9b00362
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Communication
Table 2. SM of 1-Hexene, Allylbenzene, and Allyltrimethylsilane Using {(N-2,6-
X2C6H3)(OC6Cl5)[P(CH3)3]2V[CHSi(CH3)3]}, X = CH3 (1), F (2), under Optimized Conditions
a
b
c
b
entry
catalyst
substrate
solvent (4 M)
time (h)
temp (°C)
conversn (%)
yield (%)
E:Z
1
2
3
4
5
6
1
2
1
2
1
2
1-hexene
1-hexene
allylbenzene
allylbenzene
allyl-TMS
neat
neat
THF
THF
neat
neat
0.5
0.5
2.0
2.0
2.0
2.0
40
40
40
40
60
60
81
67
12
8
40
23
69
58
89:11
86:14
87:13
87:13
56:44
60:40
d
25
11
d
allyl-TMS
a
All runs were performed with 2 mol % of the catalyst in a sealed vial with constant stirring. All values are the average of two separate experiments.
b
c
d
1
Determined by H NMR analysis. Isolated yield. Allyltrimethylsilane.
same impurity, which arises from CM of allyl-TMS and its
isomerization product, 1-(trimethylsilyl)-1-propene; however,
significantly higher yields of 5% were observed in the isolated
products.17 Efforts to suppress this isomerization were beyond
the scope of this study and were not attempted. The direct
observation of isomerization products (2-hexene, 1-(trimethyl-
silyl)-1-propene, 1-phenylpropene) in crude reaction mixtures
was not successful, and no isomerization product was observed
in the product isolated from SM of 1-hexene.
Scheme 3. Possible Decomposition Pathways
In order to probe the reason for the incomplete conversion
of the terminal olefins to the desired products, SM of 1-hexene
(20 equiv) with 1 in benzene-d6 was performed and followed
1
by H nuclear magnetic resonance (NMR) spectroscopy. No
difference in conversion between the initial 1H NMR and 12 h
time point was observed, consistent with the observations
above that longer reaction times did not lead to increased
conversion. After addition of 1-hexene, no alkylidene
while the other is propylene (R = H, Scheme 3), which
indicates that the prominent decomposition at play in the SM
described here is in fact β-hydride elimination rather than
bimolecular coupling. Specifically, propylene was detected in
43% yield relative to the initial concentration of 1. This value
should be viewed as a lower limit for the amount of β-hydride
elimination that occurred, given that propylene certainly
escaped to the headspace of the NMR tube, preventing it
from being accurately measured. Therefore, this evidence for
β-hydride elimination cannot fully exclude the possibility for
other decomposition pathways which were not detected in the
present work. Complete quantification beyond this lower limit
for β-hydride elimination is beyond the scope of this study but
will be reported in due course. Presumably, the decomposition
observed here is not at play during ROMP given the larger
steric bulk of substituents on each carbon atom of the
metallocyclobutane, preventing proper orientation for β-
hydride elimination.
Finally, in ROMP with vanadium catalysts, addition of small
amounts (1−3 equiv) of P(CH3)3 in some cases led to higher
molar mass polymers of narrower dispersity, presumably
through coordination to coordinatively unsaturated vanadium
in the absence of an olefin.39 SM followed by NMR in the
present study did not show improved conversion when done in
the presence of 1 equiv of P(CH3)3, and conversion was found
to decrease further when 1 equiv of PCy3 was employed
instead.30 No improvement was observed when the reaction
was performed under static vacuum.
1
resonance could be found in the H NMR spectrum, and no
signals at all were found in the 51V NMR spectrum. Together
with the halt in conversion, these data indicate complete
decomposition of all catalytically active species.
Two key species are generated during SM, as depicted in
Scheme 2. It is unlikely that alkylidene A (Scheme 2) is the
source of decomposition, given the high activity and living
nature of 1 in the ROMP of cycloolefins such as cyclopentene,
which generates an analogous, albeit polymeric, species.27
Therefore, it seemed reasonable that the transient methylidene
B (Scheme 2) is the most likely species present to undergo
decomposition. Indeed, within group 5, methylidene com-
plexes are rare. Schrock first reported such a complex of
tantalum, [(C5H5)2Ta(CH3)(CH2)], which decomposed in a
bimolecular manner to form [(C5H5)2Ta(CH3)(C2H4)].32
Several other examples, along with their reactivity, have been
reported since this seminal discovery.33−36 Examples of
niobium methylidenes are also known, in which very bulky
aryloxide and siloxide ligands are employed to prevent such
bimolecular decomposition.35,37 Fogg has recently demon-
strated the effect of steric bulk on the bimolecular
decomposition of ruthenium catalysts, in which methylidenes
were found to decompose markedly faster than other moieties
explored.38 In addition to bimolecular coupling, β-hydride
elimination of the metallocyclobutane intermediate formed
from [2 + 2] addition of an olefin to the methylidene is also
possible (Scheme 3). In order to determine which pathway was
at work in the present study, which is of fundamental
importance for guiding future catalyst design, 1 was treated
On the basis of the conditions leading to maximum
conversion using catalyst 1, these reactions were repeated
using 2. The more electron poor catalyst 2 demonstrated
consistently lower conversions (Table 2), while E:Z ratios were
nearly identical. This is in stark contrast to ROMP, which
increases in rate as more electron withdrawing ligands are
1
with ethylene gas, and the reaction was followed by H NMR.
Immediately after the addition of ethylene, two new olefinic
products were observed (Figure S10). The first is vinyl-
trimethylsilane, the expected product of CM of 1 and ethylene,
C
DOI: 10.1021/acs.organomet.9b00362
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Communication
employed.26,27 The reason for the observed decrease in activity
may in fact be due to sterics, rather than electronics.
Replacement of the CH3 groups in 1 with F in 2 leads to a
reduction in steric bulk of the aryl imido ligand. This likely
allows for more facile adoption of the syn-coplanar geometry
necessary for β-hydride elimination, thus promoting decom-
position via this route. The products of decomposition display
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1
no signals in H or 51V NMR, making identification difficult
and beyond the scope of this communication.
These results serve as an initial evaluation of CM with
vanadium alkylidenes. Notably, the compounds evaluated
demonstrate minimal to no undesirable isomerization, without
the need for additives. The evaluation of decomposition
pathways should serve as a guide for future developments in
CM with vanadium. In order to increase conversion and utility,
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00362.
■
S
Experimental details and supporting spectra (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail for W.S.F.: wfarrell@usna.edu.
ORCID
Wesley S. Farrell: 0000-0003-0158-2463
Notes
The author declares no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by the Defense Threat Reduction
Agency Service Academy Research Initiative, Grant CB3260.
The summer salary for W.S.F. was provided by the Naval
Academy Research Council. W.S.F. wishes to thank Prof.
Shirley Lin and Prof. Amy H. Roy MacArthur for provision of a
Vacuum Atmospheres glovebox, as well as useful discussions.
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E
DOI: 10.1021/acs.organomet.9b00362
Organometallics XXXX, XXX, XXX−XXX