A unique ruthenium carbyne complex: A highly thermo-endurable catalyst for olefin metathesis

Article abstract of DOI:10.1002/adsc.201200119

A cationic ruthenium carbyne complex was prepared and was found to initiate olefin metathesis reactions with good activities, which throws a new light on the design of a new type of ruthenium catalyst for RCM reactions. More importantly, no double bond isomerized by-product was observed even at elevated temperatures in reactions catalyzed by the new carbyne complex. A mechanism involving the in situ conversion of the ruthenium carbyne to a ruthenium carbene complex via addition of an iodide to the carbyne carbon was also proposed.

Full text of DOI:10.1002/adsc.201200119

FULL PAPERS
DOI: 10.1002/adsc.201200119
A Unique Ruthenium Carbyne Complex: A Highly
Thermo-endurable Catalyst for Olefin Metathesis
Mingbo Shao,a Lu Zheng,a Weixia Qiao,a Jingjing Wang,a and Jianhui Wanga,*
a
Department of Chemistry, College of Science, Tianjin University, Tianjin 300072, Peopleꢀs Republic of China
Fax : (+86)-22-2789-2497; e-mail: wjh@tju.edu.cn
Received: February 14, 2012; Revised: June 28, 2012; Published online: September 28, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200119.
Abstract: A cationic ruthenium carbyne complex by the new carbyne complex. A mechanism involving
was prepared and was found to initiate olefin meta- the in situ conversion of the ruthenium carbyne to
thesis reactions with good activities, which throws a ruthenium carbene complex via addition of an
a new light on the design of a new type of ruthenium iodide to the carbyne carbon was also proposed.
catalyst for RCM reactions. More importantly, no
double bond isomerized by-product was observed Keywords: olefin isomerization; olefin metathesis;
even at elevated temperatures in reactions catalyzed ring-closing metathesis; ruthenium carbynes
Introduction
a catalyst that does not bring about isomerization re-
action for the application of olefin metathesis reaction
Olefin metathesis, especially ring-closing metathesis under harsh reaction conditions.
(RCM), has evolved into an important method for
In contrast with the ruthenium carbene complexes,
carbon-carbon bond formation in modern organic ruthenium carbyne complexes exhibiting catalytic ac-
synthesis.[1] Its success has largely relied upon the dis- tivity towards olefin metathesis are extremely rare
covery of the well-defined modern ruthenium cata- and the nature of their catalytic behaviour remains
lysts 1 and 2[2] and the parallel development of cata- elusive. Although many ruthenium carbyne complexes
lysts 3–8 (Figure 1).[3] The repertoire of such catalysts
is still expanding[4] as exemplified by the recent addi-
tion of several ruthenium-based complexes with en-
hanced catalytic performance, such as 9,[5a] 10,[5b] and
other
[(H2IMes)Cl2Ru=CH
ruthenium
carbene
complexes
[A=B(C6F5)4,
A
ACHTUNGTRENNUNG
BF4, OTf, BPh4].[5c–f] A carbene ligand coordinated to
the ruthenium center is a constituent of the catalysts
currently available for olefin metathesis.
These well-defined ruthenium carbene-based olefin
metathesis catalysts are generally highly selective for
olefin metathesis. However, there has been an in-
creasing number of reports of olefin isomerization
when the catalysts were used under harsh reaction
conditions, such as high temperature, high dilution,
long reaction time, and forced high turnovers.[6] The
resulting side products are usually difficult to remove
via standard purification techniques. A large amount
of additives, such as 1,4-benzoquinone, and maleic an-
hydride, has to be added into the reaction system to
prevent the undesired isomerization reaction,[6d]
which brings additional problems for purification of
the final products. Hence, it is important to develop Figure 1. Catalysts for olefin metathesis.
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have been prepared,[7] only few ruthenium carbyne
complexes were reported to catalyze the olefin meta-
thesis reaction. For example, the ring-opening meta-
thesis polymerization (ROMP) of cyclic olefin sub-
strates is catalyzed by a carbyne complex.[8] Some of
the carbyne complexes are known for catalyzing the
dimerization of alkynes rather than for alkyne meta-
thesis.[7a] There is a general belief that ruthenium car-
byne complexes are inert towards olefin metathesis.[9]
It is also believed that, to initialize an olefin metathe-
sis reaction, these complexes must be first converted
into allenylidene- or alkylidene-ruthenium com-
plexes.[10] To the best of our knowledge, none of the
ruthenium carbyne complexes synthesized so far has
been directly used in ring-closing olefin metathesis.
In this article, we report a stable cationic ruthenium
carbyne complex 11 capable of initiating olefin meta-
thesis, including both ring-closing and cross metathe-
sis reactions.
Figure 2. Molecular structure of the ruthenium carbyne 11
(50% thermal ellipsoid plot; hydrogens have been omitted
for clarity; see Supporting Information for details).
Results and Discussion
The complex 11 was prepared in good yield (>95%)
via oxidation of 2 with an excess amount of I2 in
CH2Cl2 at 258C (Scheme 1).
Unlike its bisphosphine analogues, 11a and 11b,[7a]
the new Ru-carbyne complex does not exist as a neu-
tral, six-coordinate Ru-carbyne structure, which may
be due to the large steric effect of the N-heterocar-
bene preventing the coordination of the iodide ligand
to the ruthenium center. The molecular structure of
11 was established by X-ray diffraction (Figure 2).[11]
The structure reveals the presence of an Ru-carbyne
bond with a typical bond length of 1.600 ꢂ, which is
in agreement with that of previously reported Ru-car-
byne complexes.[7]
In sharp contrast to other Ru-carbyne complexes,
compound 11 act as an effective catalyst for RCM re-
actions. The RCM activity of 11 was initially exam-
ined using the N-protected diallylamine 12 as a test
substrate. The kinetic behaviour of RCM of 12 cata-
Figure 3. Kinetic curves for the RCM of diene 12 using 11
as a precatalyst: a) in CH2Cl2 at 408C; b) in the presence of
1.5 mol% Fe in CH2Cl2 at 408C; c) in the presence of
2.5 mol% Fe in CH2Cl2 at 408C; d) in the presence of
5.0 mol% Fe in CH2Cl2 at 408C; e) in toluene at 1008C; f)
in the presence of 5.0 mol% Fe in toluene at 1008C. All re-
actions used 2.5 mol% of 11. See the Supporting Informa-
tion for details.
lyzed by 11 (2.5 mol%) under a variety of conditions
is illustrated in Figure 3. When CH2Cl2 at reflux was
used as the solvent (0.10M), the reaction proceeded
slowly to give the desired RCM product in low yield
(Figure 3, a). Many reagents such as Sn, Fe, Zn, or
SnCl2 were found to increase the reaction rate, and
Scheme 1. Synthesis of ruthenium carbyne complex 11.
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A Unique Ruthenium Carbyne Complex: A Highly Thermo-endurable Catalyst
Table 1. Olefin metathesis reactions catalyzed by Ru-carbyne complex 11.
[a]
In toluene(c=1.0 mol/L) at 1008C in the presence of 5.0 mol% Fe under nitrogen.
[b]
1
Conversions were determined by H NMR analysis of the crude reaction mixture, isolated yields were given in paren-
theses.
ACHTUNGTRENNUNG
among these Fe powder was the best. Thus, the addi- heterocyclic structures with a di- or tri-substituted
tion of iron powder of up to two equivalents double bond proceeds cleanly in the presence of 11
(5.0 mol% relative to the substrate) clearly accelerat- (1.0–2.0 mol%) and Fe (5.0 mol%) in toluene at
ed the reaction (Figure 3, b–d), which might be due to 1008C (entries 1–11, Table 1). The RCM of enyne
À
the role of iron as an efficient scavenger of the I5
substrates 34 and 36 also proceeded uneventfully (en-
that is present in the reaction system.[12] When the re- tries 12 and 13, Table 1). However, only a minimal ac-
action was carried out in toluene (0.10M) at 1008C, tivity was observed for 11 towards the RCM of struc-
the catalytic activity of 11 was sharply increased and turally more challenging substrates such as the tetra-
a complete conversion (>98%) was observed in substituted diene 38. The complex 11 is also an effec-
40 min (Figure 3, e) and the presence of Fe tive catalyst for typical cross-metathesis reactions
(5.0 mol%) in the reaction medium, could facilitate (entry 15, Table 1) such as the one between olefins 40
complete conversion in 20 min (Figure 3, f).
and 41, to give the product 42 E isomer only, indicat-
Further study showed that the catalytic reaction ing 11 as a highly stereoselective catalyst. These stud-
proceeds with better yield on increasing the concen- ies clearly establish that 11 is an olefin metathesis cat-
tration of substrates to 1.0M. Under these conditions, alyst with moderate to good activity, and is very com-
the RCM of variety of substrates leading to the for- parable with many of the other well known olefin
mation of five-, six-, and seven-membered carbo- and
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metathesis catalysts in terms of substrate scope and tected by GC-MS) in the crude reaction product of
functional group tolerance.[13]
the CM of styrene (see the Supporting Information
The observation of the unique catalytic activity of for details). Although the isolation of the key inter-
ruthenium carbyne to olefins prompted us to do more mediates was unsuccessful, the above mentioned evi-
experiments to uncover the reaction mechanism of dence clearly points to the mechanism we proposed.
11. This work was started with the attempt to trap
A comparative study on the behaviour of 11 and
any key intermediate by stoichiometric reaction of 11 several well known catalysts such as 1, 2 and 5 re-
with styrene, but this failed to reveal any mechanistic vealed an appealing feature of the Ru-carbyne com-
implications. Instead styrene was converted to (E)- plex 11: the lack of tendency to bring about double
1,2-diphenylethene quickly and completely, with no bond isomerization, a cumbersome phenomenon in
key intermediate being observed either during or olefin metathesis by Ru-carbene-based catalysts.[6] Al-
after the reaction by NMR or TLC, indicating that though the RCM of tosylamide dienes 12, 14, 16, 20
the key intermediates may be formed in very low con- using the complexes 1, 2, or 5 in CH2Cl2 at room tem-
centrations that could not be observed by convention- perature give the desired ring closing products clean-
al tools. Further studies showed that the deprivation ly, it was found that these reactions in toluene at ele-
À
of I5 , and hence IÀ, by anionic exchange with AgBF4 vated temperature (1008C) resulted in the formation
completely shuts down the activity of 11. This obser- of an undesired secondary product in each case
vation, coupled with the fact that metallic Fe could through the isomerization of the initially formed ring-
accelerate the metathesis reaction (vide supra), points closure product (Table 2, entries 1–4). However, the
to the important role played by IÀ. Based on these same reactions proceeded uneventfully with precata-
preliminary studies on the role of 11 in olefin meta- lyst 11 under otherwise noted identical conditions
thesis, a catalytic mechanism involving the in situ con- with essentially quantitative formation of 13, 15, 17,
version of 11 to the a-iodobenzylidene complex 44, and 27, without any complication due to the double
resulting from the nucleophilic addition of IÀ to either bond isomerization of the primary products. Additives
11 or its resonance structure 43 (Scheme 2) was thus were used with 2 and 5 to prove the higher selectivity
proposed.[14] Although addition to carbon-metal triple of precatalyst 11. When 5 mol% I2 or FeCl3 was used
bonds by heteronucleophiles is well-documented,[15] as the additive with 5, the RCM reaction of diene 12
the formation of 44 from 11 represents a rare but po- did not reach completion (86% with I2, and only 5%
tentially general approach for the formation of halo- with FeCl3). Under similar conditions, when 5 mol%
carbene complexes. To further support this mechanis- Fe was used with 2 and 5, the isomerization product
tic proposal, a sample of 11 was heated to 1008C in was detected in a large amount (31% for 2, and 41%
toluene for 30 min and subjected to MS-MS analysis. for 5). Only FeCl2 has the tendency to prevent the un-
A signal with m/z=939.12 corresponding to [44ÀCl]+ desired isomerization reaction (no 13a was observed
(calcd.: m/z=939.26) was observed, suggesting the for 2, and 2% was observed for 5). It is worthy of
formation of 44 as a putative active intermediate. The note that the RCM reaction of 46 in CH2Cl2 at 408C,
latter is expected to react with an olefin to generate using 1, 2, or 5 as catalyst, also resulted in the forma-
the 14-electron Ru complex 45 which participates in tion of an undesired product of 47b in significant
subsequent catalytic cycles. This hypothesis was fur- amounts (Table 2, entry 5). In another example, the
ther proved by the formation of a-iodostyrene (de- RCM reaction of 46 proceeded smoothly using 11 as
catalyst in toluene at elevated temperature (1008C) to
give the desired product 47 in good yield (85%)
(Table 2, entry 5) without any complication of 47b.
Although the exact mechanism for the prevention of
olefin isomerization/migration is unknown at this
time, it is highly likely that the released I2 during the
initiation stage of 11 (vide supra) can provide the re-
action system with a moderately oxidative atmos-
phere, and thus may play an important role in elimi-
nating the ruthenium hydride intermediate produced
from the decomposition of the ruthenium catalyst
while maintaining a low concentration of the in situ
generated 14-electron Ru complex 45.[16] The forma-
tion of the Fe2+ from the released I2 and the additive
Fe may also play an important role in preventing the
isomerization reaction (vide supra).
The ability of 11 to prevent olefin isomerization/mi-
gration makes it ideally suited for many olefin meta-
Scheme 2. Proposed mechanism for the olefin metathesis ac-
tivity.
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A Unique Ruthenium Carbyne Complex: A Highly Thermo-endurable Catalyst
Table 2. Comparation of 11 to 1, 2, and 5 for different substrates in model metathesis reactions at elevated temperature.
[a]
Except where noted, the catalytic reactions were performed in toluene (c=1.0 mol/L) at 1008C for 2 h under nitrogen,
and in the presence of 5 mol% Fe for 11.
Determined by H NMR analysis of the crude reaction mixture.
[b]
[c]
[d]
[e]
[f]
[g]
[h]
[i]
1
The reaction was conducted in the presence of 5 mol% Fe.
The reaction was conducted in the presence of 5 mol% FeCl2.
The reaction was conducted in the presence of 0.5 equiv. benzoquinone.
The reaction was conducted in the presence of 5 mol% I2.
The reaction was conducted in the presence of 5 mol% FeCl3.
Conversions were obtained by GC-MS analysis of the crude reaction mixtures.
In CH2Cl2, at 4À018C for 24 h.
[j]
c=1.5 mmolL for 48, 12 h.
[k]
[l]
tHE E/Z ratio was obtained by GC-MS analysis of the product.
The conversions were determined by GC-MS.
thesis reactions that require harsh reaction conditions. rization of allybenzene 50 (Table 3, entry 1). The use
For example, the RCM of 48 to form the 21-mem- of 2 and 5 as catalysts gave the desired product 51 in
bered macrocyclic lactone 49 was complicated by the low yield (7% for 2; 33% for 5) in CH2Cl2 at 408C,
formation of a by-product, the 20-membered cyclic along with the side products 1-[(E)-prop-1-enyl]ben-
lactone 49a, resulting from an initial isomerization of zene 50a (22% for 2; 1% for 5), (E)-1,3-diphenyl-
one of the double bonds in 48 when the Ru-carbene prop-1-ene 51a (33% for 2; 54% for 5), and (E)-1,2-
complex 3 was used as the catalyst. Performing the re- diphenylethene 51b (36% for 2; 6% for 5). The use of
action at elevated temperature using toluene as the 1 as the catalyst gave the desired product 51 in higher
solvent led to the formation of even more 49a as re- yield (62%), but the side products 1-[(E)-prop-1-
ported by Nolan.[6n] However, application of catalyst enyl]benzene 50a (13%), and the (E)-1,3-diphenyl-
11 in this reaction in toluene at 1008C gave 49 in 91% prop-1-ene 51a (11%) were also produced. When
yield without any indication for the formation of 49a 5 mol% 2 and 0.5 equiv. benzoquinone were used as
(Table 2, entry 6). Another example for the unique the catalyst, the product 51 was produced in 65%
application of 11 was found in the metathetical dime- yield in toluene at 1008C for 12 h, along with 2% by-
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Table 3. CM Reactions of 50, and 52 Catalyzed by 1, 2, 5, and 11.
[a]
Except where noted, the reactions were performed toluene (c=1.0 mol/L) at 1008C for 12 h, and in the presence of
5 mol% Fe for 11.
[b]
[c]
[d]
[e]
[f]
Ratios of the product were obtained by GC for 50; by isolated yields for 52.
In CH2Cl2 at 408C.
The ratios of Z/E isomer were obtained by GC-MS of the product mixtures.
In xylene at 1378C.
Only E isomers were detected by H NMR analysis of the product mixtures.
The reaction was conducted in the presence of 0.5 equiv. benzoquinone.
The reaction was conducted in the presence of 5 mol% I2.
The reaction was conducted in the presence of 5 mol% Fe.
The reaction was conducted in the presence of 5 mol% FeCl2.
The reaction was conducted in the presence of 5 mol% FeCl3.
1
[g]
[h]
[i]
[j]
[k]
product 50a. However benzoquinone produces addi- 2) is formed. Attempts to improve the yield of 53 by
tional problems for purification of the final product. carrying out the reaction at a higher reaction temper-
Under similar conditions, the use of 5 mol% 5 and I2 ature (1008C) in toluene failed and even more of the
give the desired product 51 in a yield of 76% along substrate was converted to 51a (25% for 1; 40% for
with 3% byproduct 51a. When 5 mol% Fe or FeCl2 2). Again, an improvement in the yield (85%) of the
was used as the additive with 5 mol% 5, there was no desired product 53 was achieved by the use of 11
product 51 formed. The use of 5 mol% FeCl3 and when the reaction was performed in xylene at 1378C,
5 mol% 5 gave 21% of the desired product 51, along which is because it can be used for a long time at high
with 48% 50a, 9% 51a, and 23% undefined com- temperatures
plexes. In contrast, with 11, the dimerization of 50
proceeded smoothly to give 51 in a higher yield
(79%) without any side products. The metathesis of Conclusions
the long-chain terminal olefin 52 to form the symmet-
ric internal olefin 53 was also found to be impaired The cationic complex 11 reported here is the first
due to the formation of the internal olefin 52a result- ruthenium-carbyne compound capable of catalyzing
ing from double bond migration (Table 3, entry 2). As RCM reactions. More importantly, no double bond
a result, substrate 52 could not be completely convert- isomerization by-products were formed even at ele-
ed to the desired product 53 when the CM reaction vated reaction temperatures in the reactions catalyzed
was conducted at a lower temperature (408C) in by 11, indicating the unique applications of 11. Re-
CH2Cl2 for 12 h, and even under these conditions, search on improving the catalytic activity of these
a certain amount of undesired 51a (9% for 1; 25% for types of complexes by varying the substituted groups
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A Unique Ruthenium Carbyne Complex: A Highly Thermo-endurable Catalyst
Cross-metathesis: To a mixture of alkene (0.10mmol) and
on the phenyl ring as well as the application of 11 in
alkyne metathesis is currently underway in our labo-
ratory.
cross-metathesis partner (0.20mmol) in toluene (0.10mL,
c=1.0M) was added solid 11 (0.0050mmol, 5.0 mol%)
under nitrogen. The resulting mixture was stirred at 1008C
for 12h. The solvent was removed under reduced pressure.
The crude product was purified by flash chromatography
(cyclohexane-CH2Cl2). The catalytic activity of ruthenium
carbyne 11 to 40 and 41 is shown in Table 1.
Experimental Section
Preparation of Ruthenium-Carbyne 11
Mechanism Study of 11 for RCM Reactions
Ruthenium-carbene complex 2 (100 mg; 118 mmol) and
iodine (179 mg; 707 mmol) were added into a flask under
N2. Dry dichloromethane (8.0 mL) was added into the mix-
ture by syringe under N2 and the resulting mixture was then
stirred for 30 min. After that, all manipulations were carried
out in air with reagent-grade solvents. The reaction mixture
was concentrated under vacuum. The solid was washed with
hexane until the hexane layer was colorless. The residue was
purified by flash column chromatography on neutral alumi-
num oxide using CH2Cl2 as eluant to give the desired prod-
uct 11 as a yellow solid; yield: 166 mg (95%). Anal. found
(calcd.) for C46H64Cl2I5N2PRu: C 37.27 (37.25), H 4.35
Catalyst 11 (1.0 mmol) was placed into a 5.0-mL round-
bottom flask under nitrogen, and toluene (1.0 mL) was
transferred at room temperature. The resulting solution was
stirred at 1008C for 0.5 h. Then the reaction mixture was in-
jected to a Thermo Finnigan LCQ Advantage instrument in
situ to examine the tandem mass spectrometry (MS-MS).
The important intermediates such as [15ÀCl]+ (calcd: m/z=
939.26, found 939.12) and [16ÀHÀCl]+ (calcd: m/z=532.13,
found 531.02) were observed, see the mass spectra for de-
tails.
Catalyst 11 (0.10 mmol) and styrene (1.0 mmol) were
placed into a 5-mL round-bottom flask under nitrogen. The
flask was evacuated and CHCl3 (1.0 mL) was transferred at
room temperature. The resulting solution was stirred at
408C for 1.0 h. After rapidly flash chromatography on silica
to remove the catalyst, the reaction mixture was injected to
TRACEDSQ GC-mass instrument to detect the reaction in-
termediates. The (1-iodovinyl)benzene intermediate was ob-
served in the reaction mixture, see the mass spectra for de-
tails.
1
(4.36), N 1.89 (1.91); H NMR (400 MHz, CDCl3): d=7.90–
7.94 (m, 1H, Ph para CH) 7.34–7.42 (m, 4H, Ph ortho CH,
meta CH), 4.16 (s, 4H, NCH2CH2N) 6.98 (s, 2H), 6.42 (s,
2H) 2.58 (s, 9H, H2Imes CH3), 2.45 (s, 6H, H2Imes CH3),
2.29 (s, 3H, H2Imes CH3), 1.843, 1.57–1.66, 0.98–1.15 (all m,
33H, PCy3); 13C NMR (100 MHz, CDCl3): d=298.05 (d),
204.2(d, JC,P=89.6 Hz), 140.6, 140.2, 138.4, 138.2, 137.0,
136.8, 136.4, 132.1, 130.5, 128.8, 66.2, 54.2 (d, JC,P =3.4 Hz),
52.6, 34.1, 34.0, 29.8, 27.7(d, JC,P=10.8 Hz), 25.7, 21.5, 21.4,
20.1, 18.6; MS (+ESI): m/z=847.13, calcd. for [M]+: 847.32.
General Procedures for the Kinetic Curves of 11
Acknowledgements
To a stirred solution of diene 12 (0.80 mmol, 200.4 mg) in
CH2Cl2 (8 mL) under nitrogen, catalyst 11 (0.02 mmol) was
added in a single portion at 408C and the reaction mixture
was stirred for 7 h at the same temperature. Aliquots
(0.20 mL), taken in regular intervals, were quenched imme-
diately with 0.10 mol/L PEI in CH2Cl2, and then the resul-
tant solution was passed through a short column to remove
the Ru metal residue using CH2Cl2 as eluant. Solvent from
the collected solution was evaporated under vacuum, and
We thank Q. Yao (Sphinx Scientific Laboratory, Sycamore,
Illinois) for discussions related to the catalytic mechanism
and M. M. Yusuff and J. L. Wynn for language assistance.
This work was supported by grants from the National Science
Foundation of China (NSFC 20872108 and 21072149).
References
1
the residue was analyzed by H NMR spectroscopy and the
conversion was obtained by comparing the ratios of the inte-
grals of starting materials with those of products. The com-
parative experiments are presented in Figure 2.
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General Procedures for Olefin Metathesis Reactions
RCM and enyne metathesis: A solution of 11 (0.001–
0.0025mmol, 1.00–2.50 mol%), and Fe powder (0.005mmol,
2.8mg, 5.0 mol%) was added into a solution of alkene
(0.10mmol) in toluene (0.10mL, c=1.0M) under nitrogen.
The resulting mixture was stirred at 1008C for 2–12h. Then,
the solvent was removed under reduced pressure to give
a crude product, which was purified by flash chromatogra-
phy (cyclohexane-CH2Cl2) to remove the Ru residue. Con-
1
versions were obtained from H NMR spectroscopy by com-
paring the ratios of the integrals of starting materials with
those of products. The catalytic activities of ruthenium-car-
byne 11 to a variety of substrates are shown in Table 1.
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À
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