Molybdenum Benzylidyne Complexes for Olefin Metathesis Reactions

Article abstract of DOI:10.1021/acs.organomet.0c00491

The molybdenum benzylidynes [ArCMo(OC(CF3)2CH3)3(1,2-dimethoxyethane)], where Ar = Ph (2a), p-(OCH3)C6H4 (2b), p-(CF3)C6H4 (2c), p-(NO2)C6H4 (2d), or 4-(NO2)-3-(CF3)C6H3 (2e), and [p-(NO2)C6H4CMo(OC(CF3)2CH3)3] (2f) catalyze the ring-closing metathesis (RCM) reaction of diallyl N-tosylamide (3) to produce 1-tosyl-2,5-dihydro-1H-pyrrole (4) and ethylene. The scope of RCM catalytic activity of 2e, cross-metathesis of 1-hexene, and ring-opening metathesis polymerization of cyclooctene were explored. The X-ray crystal structure of 2e was determined. Variable-temperature 1H NMR spectra revealed the formation of intermediates during the reaction of 3 with 2f and the reforming of 2f after completion of the reaction. The use of 13C-labeled Mo benzylidyne did not show transfer of the carbon atom next to Mo to any of the products.

Full text of DOI:10.1021/acs.organomet.0c00491

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Molybdenum Benzylidyne Complexes for Olefin Metathesis
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Sergey Chuprun, Carlos M. Acosta, Logesh Mathivathanan, and Konstantin V. Bukhryakov*
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ABSTRACT: The molybdenum benzylidynes [ArCMo(OC-
(CF3)2CH3)3(1,2-dimethoxyethane)], where Ar = Ph (2a), p-
(OCH3)C6H4 (2b), p-(CF3)C6H4 (2c), p-(NO2)C6H4 (2d), or 4-
(NO2)-3-(CF3)C6H3 (2e), and [p-(NO2)C6H4CMo(OC-
(CF3)2CH3)3] (2f) catalyze the ring-closing metathesis (RCM)
reaction of diallyl N-tosylamide (3) to produce 1-tosyl-2,5-dihydro-
1H-pyrrole (4) and ethylene. The scope of RCM catalytic activity of
2e, cross-metathesis of 1-hexene, and ring-opening metathesis
1
polymerization of cyclooctene were explored. The X-ray crystal structure of 2e was determined. Variable-temperature H NMR
spectra revealed the formation of intermediates during the reaction of 3 with 2f and the reforming of 2f after completion of the
reaction. The use of 13C-labeled Mo benzylidyne did not show transfer of the carbon atom next to Mo to any of the products.
spectroscopically.1 Additionally, a catalytic reaction based on
ynene [2 + 2] cycloaddition was used to prepare cyclic cis-
reaction between a metal carbyne and an alkene (ynene)1
syndiotactic polynorbornenes.
INTRODUCTION
Catalytic transformations based on the [2 + 2] cycloaddition
■
A similar trianionic pincer molybdenum benzylidyne
complex, [OCO]3−Mo, was synthesized, but its reactivity
toward alkenes was not discussed.14 Generally, Mo carbyne
complexes do not react with alkenes. This orthogonality has
been used in the total synthesis of many natural products
containing both alkene and alkyne moieties, where only alkyne
metathesis occurred.15−22
represent an underdeveloped reaction class among olefin
(enene),2−4 alkyne (ynyne),5−8 and enyne9 metathesis trans-
formations (Scheme 1). The first example of metathesis
Scheme 1. Four Types of [2 + 2] Cycloaddition Reactions
Involving Metal Carbene and Carbyne Complexes
Recently, Fischer found that electron-donating or -with-
drawing substituents on the benzylidyne [p-R-C6H4C
Mo(OC(CF3)2CH3)3] can significantly affect the initiating
step in ring-opening alkyne metathesis polymerization
(ROAMP).23 Density functional theory calculations have
shown that the rate-determining step in the ROAMP initiation
is associated with cycloelimination, where the carbon atom
next to Mo at the benzylic α-position has a negative charge.
Electron-withdrawing substituents stabilize the negative charge
buildup in the transition state and as a result increase the rate
of the initiation step. The rate of propagation is mostly
unaffected by substituents on the initial benzylidyne, since a
new benzylidyne forms after each catalytic cycle.
involving alkenes catalyzed by a well-defined tungsten
alkylidyne complex, [Cl3(dme)WC−CMe3], was reported
in 1986 by Weiss.10,11 Although this complex successfully
polymerized cyclopentene and catalyzed the cross-metathesis
of 1-octene, the structure of the active species was not
elucidated. Traces of water in the reaction mixture should be
considered since alkylidyne complexes can react with water to
form oxoalkylidenes, which are active catalysts for olefin
metathesis.12,13 Three decades after Weiss’ discovery, Veige
showed that a trianionic pincer alkylidyne complex,
[OCO]3−W, can reversibly react with ethylene to form
metallacyclobutene, with the structure of the latter confirmed
The observation by Fischer, the first spectroscopic evidence
of a metallacyclobutene from Viege’s lab, and the importance
Received: July 21, 2020
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© XXXX American Chemical Society
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of Mo carbynes in modern organic and organometallic
chemistry and catalysis24−30 have led to our interest in
examining the influence of the electron-donating or -with-
drawing substituents on the benzylidyne [ArCMo(OC-
(CF3)2CH3)3] with regard to its reactivity toward alkenes. We
have now prepared Mo benzylidyne complexes that are
reactive toward olefins. To our knowledge, we report the
first examples of ring-closing metathesis (RCM), cross-
metathesis (CM), and ring-opening metathesis polymerization
(ROMP) of alkenes catalyzed by well-defined Mo benzylidyne
complexes.
RESULTS AND DISCUSSION
■
The reaction between 131,32 and R1R2C6H3CC−TMS
provided a series of benzylidyne complexes 2a−e (Scheme
2) utilizing a method analogous to the published procedure.12
Scheme 2. Synthesis of Mo Benzylidyne Complexes 2a−e
with Isolated Yields
Figure 1. Drawing of the X-ray structure of 2e.
a
Table 1. Metathesis Activity of Mo Carbynes toward 3
b
entry
Mo complex
conv. to 4 (%)
1
2
3
4
5
6
7
8
1
<1
61
13
64
85
>99
>99
<1
2a
2b
2c
2d
2e
c
During the synthesis, a limitation of the above reaction became
apparent, as substrates containing the NO2 group at the ortho
position with respect to the alkyne did not lead to benzylidynes
(see the Supporting Information for specific examples). The
2e
2e
d
a
b
c
d
1
0.2 M 3. Determined by H NMR spectroscopy. 1 mol % 2e.
equiv of water was added.
1
1
structures of 2a−d were confirmed by H NMR spectrosco-
py.23
Orange prisms of 2e suitable for X-ray crystallography were
obtained from a saturated toluene solution at −35 °C. The Mo
center adopts a pseudo-octahedral geometry with Mo located
0.45 Å above the equatorial O4 plane. The X-ray crystal
structure of 2e (Figure 1) confirms the presence of a C(1)
Mo(1) triple bond with a bond length of 1.749(4) Å and a
C(2)−C(1)−Mo(1) angle of 176.9(3)°. One 1,2-dimethoxy-
ethane (DME) molecule is coordinated to the Mo center, with
one of the O atoms located trans to the benzylidyne. The bond
lengths of the Mo(1)−O(4) cis and Mo(1)−O(5) trans to the
carbyne are 2.226(3) and 2.399(4) Å, respectively. Three
hexafluoro-tert-butoxide ligands occupy meridional sites
featuring Mo(1)−O(1), Mo(1)−O(2), and Mo(1)−O(3)
distances of 1.946(3), 1.906(3), and 1.955(3) Å. The presence
of the electron-withdrawing CF3 group in 2e has a small effect
on the C(1)Mo(1) triple bond compared with 2d (1.754(7)
Å) and a C(2)−C(1)−Mo(1) angle of 174.8(6)°, but the
changes in the bond lengths for Mo(1)−O(1), Mo(1)−O(2),
and Mo(1)−O(3) are more pronounced (1.958(4), 1.922(4),
and 1.952(5) Å, respectively) because of the weaker π-
donating character of the carbyne in 2e.23
on the substituents on the benzylidyne. For instance, electron-
withdrawing groups accelerate the reaction. Complex 2e
exhibits high RCM catalytic activity at 1 mol % (entry 7,
Table 1), comparable to those of some Ru-33 and W-based34
alkylidene catalysts. The addition of water does not lead to the
formation of a catalytically active Mo oxobenzylidene under
the reaction conditions (entry 8, Table 1); instead, only
decomposition of the catalyst was observed by 1H NMR
spectroscopy.
We explored the scope of RCM and the metathesis activity
of 2e for CM of hexene-1 and ROMP of cyclooctene. The
results are summarized in Scheme 3. Unlike 3, the tested
substrates require heating to speed up the reaction. The
substrates used to make 5 and 7−9 contain basic groups that
can compete with the alkene for coordination to Mo.
Additionally, 9 can form a chelating complex through both
oxygen atoms. The precursor for 6 is more hindered compared
with 3. Those factors can explain why heating is required to
produce 5−9. However, 1,7-octadiene (the substrate for 10)
and 1-hexene do not contain basic groups. Arguably, the
reason for the higher activity of 2e toward 3 is that the
coordination of the sulfone group in 3 through one of its
oxygen atoms35 to DME-free 2e (to be discussed later)
facilitates the intramolecular coordination of the alkene group
to Mo. However, this is not the case for internal olefins. Thus,
Next, the metathesis activity of complexes 1 and 2a−e in the
RCM reaction with diallyl N-tosylamide (3) was explored
(Table 1). The Mo benzylidyne complexes are active catalysts
in the RCM reaction of 3. The activity of the catalyst depends
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d
1
reaction was monitored by H NMR spectroscopy at variable
temperature starting from −40 °C. The resulting NMR spectra
are quite complex (Figure 2; the full 1H and 19F NMR spectra
Scheme 3. Catalytic Metathesis Reactions Initiated by 2e
a
b
c
5 mol % 2e. 24 h. Mn = 109700 g/mol, Mw/Mn = 1.77 by GPC
d
1
(THF vs polystyrene standards). Conversions by H NMR analysis
are indicated.
1
Figure 2. Aromatic region of the H NMR spectra for the RCM
ROMP of cyclooctene is slow at 80 °C in the presence of 5
mol % 2e. The resulting polymer has a high molecular weight
and polydispersity index based on gel-permeation chromatog-
raphy, which suggests that the rate of the propagation is higher
than that of initiation. Disubstituted olefin 11 does not react
with 2e even at elevated temperature. We conclude that
internal olefins do not readily react with Mo benzylidynes.
We attempted to elucidate the mechanism of RCM of 3
catalyzed by 2e. Monitoring the reaction of 3 in the presence
reaction of 3 with ∼1.5 mol % 2f at variable temperature. The
temperature and conversion toward 4 are indicated. Signals of
intermediates are indicated by *. Bottom: NMR signal of two H
atoms ortho to the nitro group in 2f.
are given in the Supporting Information). A few sets of signals
assigned to intermediates (indicated by * in Figure 2) were
observed, with the most pronounced changes at −10 °C, when
the reaction starts to accelerate. The assignment of new signals
to specific structures was not possible because of the low
concentrations and overlap with signals of the substrate and
products. It is worth mentioning that complex 2f partially
precipitates out from the reaction mixture at low temperature,
so the concentration of the catalyst varied slightly during the
experiment. However, the most significant result of this
experiment is the fact that after reaction completion, the
catalyst reformed its initial structure: when the reaction at 10
°C was 99% complete, all signals of the intermediates
disappeared, and only the catalyst 2f remained.
1
of 5 mol % 2e in C6D6 by H NMR spectroscopy did not
reveal any active intermediates. The catalyst remained intact
according to the 1H NMR spectra, while was 3 cleanly
converted to 4 and ethylene. A possible explanation is that the
dissociation of DME32,36 is the rate-limiting step, and the
following steps are too fast to be analyzed by the NMR
method. All attempts to prepare DME-free 2e failed. Heating
2e in high vacuum at 100 °C led to slow decomposition of the
complex. Reaction with ZnCl2 in benzene at 80 °C and several
cycles of dissolution/evaporation28 of 2e in toluene at 30 °C
did not show any significant loss of DME.
During this work, we noticed that the reaction of p-
NO2C6H4−CC−TMS with 1 produced 2d as the major
product (Scheme 2) along with a minor product that was
insoluble in toluene and benzene but had limited solubility in
dichloromethane and chloroform. That minor product was the
DME-free complex [p-NO2C6H4CMo(OC(CF3)2CH3)3]
(2f). Somewhat surprisingly, the reaction of 3 with 2 mol %
2f was complete in a few minutes at 22 °C. This supports the
assumption that the dissociation of DME is the rate-limiting
step for the reaction of 2e with 3.
Scheme 4 illustrates the proposed mechanism. The key step
is a [2 + 2] ynene cycloaddition with the formation of
metallacyclobutene 12. The ensuing cycloelimination gives
alkylidene 13, which is an active olefin metathesis catalyst.
Metallacyclobutane 14 produces 4 and methylidene 15. In the
last step, metallacycle 16 makes ethylene and regenerates
complex 2. The reaction of 2f with ethylene was attempted at
variable temperature, but the formation of 16 was not been
1
observed by H NMR spectroscopy.
To support the proposed mechanism, we prepared 13C-
labeled benzylidyne 2a-13C and tested its reactivity toward 3
(Scheme 5). We did not observe the formation of 13C-labeled
We attempted to elucidate possible intermediates in the
reaction of 3 with 2f. Since 2f is a highly active catalyst, the
C
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isolation of the catalyst rather than for the catalysis itself,
except for polymerization, where initiation should be faster
than propagation to achieve a low polydispersity index. Olefin
metathesis catalyzed by metal carbyne complexes provides an
opportunity to affect the catalysis through the carbyne ligands,
providing an additional tool to achieve desired properties.
Scheme 4. Proposed Mechanism Based on a [2 + 2] Ynene
Cycloaddition Reaction
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00491.
Detailed experimental procedures, NMR data and
spectra for all compounds, catalytic experiments, X-ray
data for 2e, and VT NMR spectra (PDF)
Cartesian coordinates for the structure 2e (XYZ)
Accession Codes
CCDC 2018870 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, U.K.; fax: +44 1223 336033.
Scheme 5. Reaction of 3 with 13C-Labeled 2a
AUTHOR INFORMATION
Corresponding Author
■
Konstantin V. Bukhryakov − Department of Chemistry and
Biochemistry, Florida International University, Miami, Florida
33199, United States; orcid.org/0000-0002-8425-9671;
Email: kbukhrya@fiu.edu
Authors
Sergey Chuprun − Department of Chemistry and Biochemistry,
Florida International University, Miami, Florida 33199,
United States
Carlos M. Acosta − Department of Chemistry and Biochemistry,
Florida International University, Miami, Florida 33199,
United States
Logesh Mathivathanan − Department of Chemistry and
Biochemistry, Florida International University, Miami, Florida
33199, United States
4 or ethylene by 13C NMR spectroscopy, even at higher
catalyst loadings. Thus, the intensity of olefinic carbon atoms
in both products is the same as in the reaction with unlabeled
2a (see the Supporting Information for details). Additionally,
the 13C benzylidyne peak stayed intact after the reaction. No
other peaks in the region characteristic for Mo carbene or
carbyne carbon resonance (200−400 ppm) were observed by
13C NMR spectroscopy.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00491
Notes
CONCLUSIONS
The authors declare no competing financial interest.
■
We have shown that Mo benzylidyne complexes can catalyze
ring-closing metathesis, cross-metathesis, and ring-opening
metathesis polymerization of alkenes. The active catalyst is a
four-coordinate Mo complex containing benzylidyne and three
hexafluoro-tert-butoxide ligands. The catalytic activity toward
olefins depends on the electron-withdrawing groups on the
benzylidyne and the nature of the olefin. The proposed
mechanism includes the formation of a metallacyclobutene
through [2 + 2] ynene cycloaddition and regeneration of the
ACKNOWLEDGMENTS
■
We are grateful to Florida International University and the
New Faculty Development Grant provided by the U.S. Nuclear
Regulatory Commission for financial support. We also thank
Professor Valentin Rodionov and his group for the MW
determination.
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1
■
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