Novel ruthenium-catalyst for hydroesterification of olefins with formates

Article abstract of DOI:10.1039/c4ob01246a

An alternative ruthenium-based catalyst for the hydroesterification of olefins with formates is reported. The good activity of our system is ensured by the use of a bidentate P,N-ligand and ruthenium dodecacarbonyl. A range of formates can be used for selective alkoxycarbonylation of aromatic olefins. In addition, the synthesis of selected aliphatic esters is realized. The proposed active ruthenium complex has been isolated and characterized. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ob01246a

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Novel ruthenium-catalyst for hydroesterification
of olefins with formates†
Irina Profir,a Matthias Beller*a and Ivana Fleischer*a,b
Cite this: Org. Biomol. Chem., 2014,
12, 6972
Received 16th June 2014,
Accepted 30th July 2014
DOI: 10.1039/c4ob01246a
www.rsc.org/obc
An alternative ruthenium-based catalyst for the hydroesterification
of olefins with formates is reported. The good activity of our
system is ensured by the use of a bidentate P,N-ligand and ruthe-
nium dodecacarbonyl. A range of formates can be used for
selective alkoxycarbonylation of aromatic olefins. In addition,
the synthesis of selected aliphatic esters is realized. The proposed
active ruthenium complex has been isolated and characterized.
The conversion of alkenes to esters by carbonylation is a
broadly applied and commercially relevant process. It is con-
ventionally performed by the reaction of an alkene with CO gas
and corresponding alcohols.1 However, due to the toxicity and
the intricate handling of CO, there is an increasing interest in
applying alternative carbonyl sources in such transformations.2
As an example, hydroesterification with formates proves to
be an attractive route to esters. Since Sneeden and co-workers
discovered the ruthenium-catalyzed formation of methyl pro-
pionate from ethylene with methyl formate,3 much effort has
been put into improving the efficiency and applicability of this
reaction. Hence, significantly milder reaction conditions have
been developed in the course of the past two decades. Beside
ruthenium, researchers are also striving to design hydroesteri-
fication catalysts based on other transition metals such as
rhodium,4 palladium5 or cobalt.6 Still, the restricted substrate
scope as well as the often irreversible decomposition of the
formates remain major challenges.
Scheme 1 Ruthenium-catalyzed olefin hydroesterification.
broader substrate scope but also allowed high conversion and
good yields of the desired products without supplementary gas
pressure derived from CO and N2 gas, respectively.9
In 2004, pyridylmethyl formate has been successfully uti-
lized in hydroesterification reactions as CO source.10 Chang
and co-workers investigated the positive ligand effects on the
reactions with this chelating formate which allowed the con-
version of a variety of alkenes in excellent yields using
different ruthenium sources as catalyst precursors.11 The
addition of catalytic amounts of Bu4NI made the reduction of
the working temperature to 70 °C possible.12 These conditions
were also successively applied in the formation of amides
based on formamide derivatives.13
More recently, efforts regarding a wider range of applicable
formates have been reported by Manabe and co-workers when
introducing imidazole derivatives as potent additives.14 With
respect to the most effective ligands they postulated bidentate
coordination to ruthenium forming a five membered metalla-
cycle that would activate the formate C–H bond and yield the
desired product without precedent decarbonylation. Despite
the improvements achieved by this approach, the amount of
ruthenium used for this conversion and related reactions
remained high with 15 mol%.
In the past, the most active catalyst systems for simple sub-
strates such as ethylene and methyl formate have been based
on ruthenium precursors containing PPN, halide and CO
ligands.7 Additionally, the coordination of polar solvents such
as DMF resulted in increased activity of the tested catalysts.8
Later, it was reported that ancillary PCy3 not only enabled a
aLeibniz Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Strasse 29a,
18059 Rostock, Germany
bInstitut für organische Chemie, Universität Regensburg, Universitätsstrasse 31,
93040 Regensburg, Germany. E-mail: ivana.fleischer@chemie.uni-regensburg.de
†Electronic supplementary information (ESI) available. CCDC 1003596.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4ob01246a
Herein, we present an improved catalyst system composed
of Ru3(CO)12 and a phosphine substituted imidazole derivative
(Scheme 1).15 The in situ generated ruthenium complex (2.5 mol%)
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successfully catalyzes the formation of esters in yields up to additive-free system, whereas PCy3 alone had a negligible influ-
95% at 135 °C. Although the formate undergoes decarbonyla- ence on the outcome of this conversion (Table 1, entry 4). The
tion, the reaction can be performed in glass pressure tubes combination of 4e and PCy3 seemed to inhibit the reaction
(3 mmol scale). Dimethyl formamide proved to be the most rather than promoting it (Table 1, entry 5) compared to the
suitable solvent with respect to both yield and purity of the results of utilizing 4a.
chromatographically isolated products.
While the diphenyl phosphine analogue 4b showed no
Employing Ru3(CO)12 and ligand 4a in our model reaction activity at all, the isopropylphosphine substituted imidazole 4c
(Table 1) at 135 °C yielded the ester in 89% with a linear to exhibited good activity and selectivity, but was rather difficult
branched ratio of 67 : 33. In comparison, the absence of any to handle due to its low melting point and severe air sensitivity
additive to Ru3(CO)12 yielded in only 2% and 10% product at (Table 1, entries 6 and 7). Ligand 4f, which was successfully
135 °C and 150 °C, respectively (Table 1, entries 1 and 2).
applied in ruthenium-catalyzed hydroformylation of olefins by
A number of other additives has been tested under the opti- our group,16 was inactive in the model system. Also, the luti-
mized conditions (3.0 mmol benzyl formate, 4.5 mmol styrene, dine-based phosphine ligands17 5 that would supposedly form
0.83 mol% Ru3(CO)12, 2.5 mol% additive, 1.5 ml DMF, 135 °C, a five-membered ring with ruthenium as observed with 4a,
24 h). To confirm that the reactivity of the system derives from gave unsatisfactory yields up to 10% at 150 °C (Table 1, entries
the bidentate P,N-coordination of ligand 4a to the ruthenium 10–12). Using Bu4NI, Bu4NBr and LiCl as halide sources both
center, experiments investigating the individual impact of the in combination with and without ligand 4a had a decreasing
phosphine and imidazole moiety have been conducted. effect on the yield (Table 1, entries 16–20).
Notably, the presence of 1,2-dimethyl imidazole (4e) increased
With the exception of 4c, none of the ligands came close to
the yield of product (Table 1, entry 3) in comparison to the the activity of 4a. Analyzing the reaction mixtures via GC
usually revealed the presence of a small amount of benzyl
alcohol and a major amount of benzyl formate after 24 h. In
Table 1 Screening of additives on model reaction
most cases increasing the temperature to 150 °C did not
improve the conversion to a significant extent.
As far as formates are concerned, we were delighted to dis-
cover a broader applicability among both aliphatic and aro-
matic derivatives. Although undergoing decarbonylation,
yields up to 95% have been obtained. As depicted in Table 2,
steric properties of the utilized formates play a pivotal role
for successful product formation. While ethyl formate and
n-propyl formate showed excellent conversion resulting in 95%
and 94% of the corresponding esters, the yield dropped dra-
matically to 35% when isopropyl formate was applied under
Entry
Additive
Yield of 3a (%)
3aa : 3′aab
Table 2 Scope of formates
1
2
3
4
4a
—
4e
89 (82)c
2 (10)
21 (40)
6
10
Traces
87
28
Traces
7
Traces
10
Traces
No conversion
Traces
22
30
67 : 33 (69 : 31)c
65 : 35 (77 : 23)
76 : 24 (80 : 20)
67 : 33
72 : 28
—
78 : 22
77 : 23
—
76 : 24
—
82 : 18
—
PCy3
4e + PCy3
4b
4c
4d
4f
5a
5b
5c
Pyridine
TMEDA
PPh3
Bu4NI
Bu4NBr
4a + Bu4NBr
LiCle
4a + LiCle
5
6d
7
8
9d
10d
11d
12d
13
14
15
16
17
18
19
20
Entry
R (1x)
Yielda (%)
3xa : 3′xab
—
—
1
2
3
4
5c
6
7
Me
Et
1b
1c
1d
1e
1e
1f
71
95
94
35
50
—
56
52 : 48
66 : 34
64 : 36
83 : 17
81 : 19
—
72 : 28
83 : 17
62 : 38
75 : 25
51 : 49
nPr
iPr
iPr
tBu
Ph
61
57
41
1g
96 : 4
Conditions: 3.0 mmol benzyl formate, 4.5 mmol styrene, 0.83 mol%
Ru3(CO)12, 2.5 mol% additive, 1.5 ml DMF, 135 °C, 24 h. a Isolated Conditions: 3.0 mmol formate, 4.5 mmol styrene, 0.83 mol%
yield. b Determined by 1H NMR. c The data in parentheses represents Ru3(CO)12, 2.5 mol% ligand 4a, 1.5 ml DMF, 135 °C, 24 h. a Isolated
results obtained at 150 °C. d Reaction was carried out at 150 °C under yield. b Determined by 1H NMR. c Reaction was carried out at 150 °C
otherwise identical conditions. e 5.0 mol% of LiCl.
under otherwise identical conditions.
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identical conditions. tert-Butyl formate showed no product
generation at all (Table 2, entry 6). On the other hand, steric
hindrance in reactive formates had a positive impact on the
linear to branched product ratio, as is shown for isopropyl and
phenyl formate (Table 2, entries 4, 5 and 7).
Table 3 Scope of alkenes (a selection)
Utilizing different alkenes illustrates the scope and limit-
ations of the presented system (Table 3). In this context, benzyl
formate was chosen as substrate for analytical reasons. The degree
of decarbonylation and the conversion of the resulting alcohol to
the desired product could be observed via GC analysis of the
reaction mixtures. Additionally, the combination of yield and
regioselectivity obtained with this formate fulfilled the require-
ments the best in our opinion. While styrene derivatives have
been converted successfully at 135 °C (Table 3, entries 1–5), ali-
phatic alkenes proved to be more challenging. Notably, allylben-
zene (2g) and β-methyl styrene (2′g) gave mixtures of the same
products 3ag, 3′ag and 3″ag (Table 3, entries 7 and 8) and the
linear product 3ag was the main component. α-Substituted sty-
renes, such as 2h (Table 3, entry 8), could only be converted to a
low amount of product and α-phenyl styrene did not react at all.
Aliphatic alkenes react slower than aromatic alkenes, even
at 150 °C. This is shown exemplarily when using 3,3-dimethyl
1-butene (2j) which yielded the product in 11% after 24 h and
72% after 65 h (Table 3, entry 10). Similar to β-methyl styrene,
2-octene underwent isomerization of the double bond and
yielded the same products 3al and 3′al as 1-octene (Table 3,
entries 12 and 13). At this point, we expected more isomers to
be in the product mixture but identifying them by 1H NMR
was impossible due to their small concentration and overlap
of the signals with those of the main products.
Yieldb (%)
Entry Alkene
1
Producta
(3ay : 3′ay)c
88 (76 : 24)
2
3
85 (51 : 49)
72 (64 : 36)
4
86 (56 : 44)
5
57 (51 : 49)
6d,e
42 (59 : 22 : 18)
When applying isoprene (2m) under the milder conditions
(135 °C), the unsubstituted double bond was converted selec-
tively to the linear product. The sterically hindered double
bond was isomerized without further conversion resulting in
the β,γ- and α,β-unsaturated carbonyl compounds 3am and
3′am, respectively, at a ratio of 82 : 18 and an overall yield of
73% (Table 3, entry 14).
7d,e
8d,e
31 (51 : 29 : 20)
10 (>99 : 1)
9d
35
In our attempt to identify the active catalytic species, we
observed the coordination of ligand 4a to the ruthenium cluster
by applying different ratios and analyzing the resulting solu-
tions by NMR spectroscopy (Fig. 1). The metal to ligand ratios
[Ru]/4a 1 : 1 and 1 : 2 show one main phosphorous species each,
indicating complete coordination of the ligand to the metal in
both cases. In contrast, applying the metal in excess gives
another compound in addition to the 1 : 1 moiety. We assume
that the coordination of the ligand initiates the stepwise dissolu-
tion of the [Ru3] cluster, resulting in complexes [Ru3]-4a and
[Ru2]-4a at 66 ppm and 53 ppm, respectively, whereas single
[Ru] units are coordinated by two ligands at first (59 ppm).
Thus, these intermediate species can be observed as minor
signals in the 1 : 1 spectrum. When applying [Ru]/4a 1 : 3,
double coordination of 4a to ruthenium is observed along with
uncoordinated ligand. Transferring these ratios to our model
system, [Ru]/4a 2 : 1 gave lower yield but increased linear to
branched selectivity compared to 1 : 1, whereas [Ru]/4a 1 : 2
showed no conversion of the formate at all.
10d
11d
11 (>99 : 1)
72f (>99 : 1)
17 (76 : 24)
12d,e
13d
21 (76 : 24)
44f (70 : 30)
40f (67 : 33)
14
73 (82 : 18)
Conditions: 3.0 mmol benzyl formate, 4.5 mmol alkene, 0.83 mol%
Ru3(CO)12, 2.5 mol% ligand 4a, 1.5 ml DMF, 135 °C, 24 h. a Only linear
products are displayed in the table. For the structures of the branched
products please check the electronic supplementary information (ESI).
b Isolated yield. c Ratio determined by 1H NMR. d Reaction was carried
out at 150 °C. e 1.7 mol% Ru3(CO)12, 5.0 mol% ligand 4a. f Extended
reaction time of 65 h.
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Fig. 1 31P NMR spectra of different [Ru]/4a ratios in toluene-d8; (a) 2 : 1,
(b) 1 : 1, (c) 1 : 2, (d) 1 : 3, (e) ligand without ruthenium.
Scheme 2 Proposed mechanism for the hydroesterification of alkenes
with formates.18
only low conversion of the preliminary generated benzyl
alcohol and yields below 10% of the desired product.
Fig. 2 X-Ray structure of complex 6 generated from Ru3(CO)12 and
ligand 4a at a ratio of 1 : 1.
The recently proposed mechanism for the ruthenium-cata-
lyzed hydroesterification of alkenes with formates by Manabe
and co-workers is in agreement with our considerations
(Scheme 2).18 The reaction starts with the generation of
complex 6, though coordination of solvent molecules to the
ruthenium center in addition to or in lieu of CO is possible.
[Ru] then inserts into the C–H bond of the formate (I). The
path following the decarbonylation of the formate (II), coordi-
nation of the alkene (III) and carbonylation of the alkoxide
moiety (IV) seems more plausible in our case due to the
observed accumulation of benzyl alcohol and CO pressure in
the model system. On the other hand C–H activation and
direct coordination of the alkene to form species IV before the
product is released and the active ruthenium complex is regen-
erated may occur to a minor extent.
In addition we successfully isolated complex 6 from the
1 : 1 mixture of ruthenium and ligand 4a (Fig. 2) as dark
orange crystals. In contrast to the in situ catalyst system,
immediate gas evolution was observed in solution at ambient
temperature when applying the complex in the model reaction.
Analyzing the possible coordination of both the formate
and the alkene in the course of the reaction has also been
attempted in NMR tubes. There was no change in the NMR
shifts of the substrates or the complex at ambient temperature.
However, at 135 °C partial decomposition of the formate was
observed. These results led us to believe that the gas evolution
mentioned above derived from substitution of CO ligands by
solvent molecules in the complex rather than from decompo-
sition of benzyl formate. By application of only 1.5 mol% of 6
in model reaction 77% of product with a linear to branched
ratio of 69 : 31 were obtained after 24 h at 135 °C. For compari-
Conclusions
son, experiments have been carried out at 100 °C with both In summary, the employment of bidentate imidazole-based
the in situ system [Ru]/4a and the isolated complex 6, respect- phosphine ligands provided the most active Ru catalyst
ively. GC analysis of the reaction mixtures after 24 h revealed system for the alkoxycarbonylation of styrene derivatives with
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formates. Both aliphatic and aromatic formates were success-
fully converted to the corresponding esters. Steric hindrance
in both alkene and formate led to improved regioselectivity.
Aromatic olefins can be carbonylated under comparably
milder reaction conditions. In addition, for the first time we
were able to isolate and employ a well-defined Ru-complex in
this reaction, which provides opportunities for tuning of its
catalytic activity and further mechanistic studies.
6 Y. N. Cui, J. M. Yin, D. B. Gao, Y. P. Jia, G. Y. Zhou and
S. M. Li, Chin. Chem. Lett., 2007, 18, 17–20.
7 (a) G. Lavigne, N. Lugan, P. Kalck, J. M. Soulié, O. Lerouge,
J. Y. Saillard and J. F. Halet, J. Am. Chem. Soc., 1992, 114,
10669–10670; (b) S. Fabre, P. Kalck and G. Lavigne, Angew.
Chem., Int. Ed. Engl., 1997, 36, 1092–1095.
8 L. Noël, G. Lavigne, J. M. Soulié, S. Fabre and P. Kalck,
Organometallics, 1995, 14, 1712–1731.
We are grateful to the State of Mecklenburg-Western Pomer-
ania, the Deutsche Forschungsgemeinschaft (Leibniz-price)
9 T. Kondo, T. Okada and T. Mitsudo, Organometallics, 1999,
18, 4123–4127.
and the Fonds der Chemischen Industrie (Liebig Fellowship 10 L. Wang and P. E. Floreancig, Org. Lett., 2004, 6, 4207–
for I.F.) for the financial support. We thank Dr K. Junge, 4210.
B. Wendt and Dr A. Pews-Davtyan for the provision of ligands 11 (a) S. Ko, Y. Na and S. Chang, J. Am. Chem. Soc., 2002, 124,
and Dr A. Spannenberg for crystallographic analysis of the
isolated complex. We further thank the analytical department,
750–751; (b) B. Li, S. Lee, K. Shin and S. Chang, Org. Lett.,
2014, 16, 2010–2013.
workshop and glassblower of the Leibniz Institute of Catalysis 12 (a) E. J. Park, J. M. Lee, H. Han and S. Chang, Org. Lett.,
for their support.
2006, 8, 4355–4358; (b) N. Armanino, M. Lafrance and
E. M. Carreira, Org. Lett., 2014, 16, 572–575.
13 (a) S. Ko, H. Han and S. Chang, Org. Lett., 2003, 5, 2687–
2690; (b) N. Armanino and E. M. Carreira, J. Am. Chem.
Soc., 2013, 135, 6814–6817; (c) B. Li, Y. Park and S. Chang,
J. Am. Chem. Soc., 2014, 136, 1125–1131.
14 H. Konishi, T. Ueda, T. Muto and K. Manabe, Org. Lett.,
2012, 14, 4722–4725.
15 K. Junge, B. Wendt, F. A. Westerhaus, A. Spannenberg,
H. Jiao and M. Beller, Chem. – Eur. J., 2012, 18, 9011–9018.
16 (a) I. Fleischer, K. M. Dyballa, R. Jennerjahn, R. Jackstell,
R. Franke, A. Spannenberg and M. Beller, Angew. Chem.,
Int. Ed., 2013, 52, 2949–2953; (b) I. Fleischer, L. Wu,
I. Profir, R. Jackstell, R. Franke and M. Beller, Chem. – Eur.
J., 2013, 32, 10589–10594.
17 A. Pews-Davtyan, X. Fang, R. Jackstell, A. Spannenberg,
W. Baumann, R. Franke and M. Beller, Chem. – Asian. J.,
2014, 9, 1168–1174.
Notes and references
1 For reviews see: (a) G. Kiss, Chem. Rev., 2001, 101, 3435–
3456; (b) B. El Ali, H. Alper, M. Beller and C. Bolm,
Transition Metals for Organic Synthesis, Wiley-VCH, Weinheim,
2008, 49–67; (c) A. Brennführer, H. Neumann and M. Beller,
ChemCatChem, 2009, 1, 28–41.
2 T. Morimoto and K. Kakiuchi, Angew. Chem., Int. Ed., 2004,
43, 5580–5588.
3 P. Isnard, B. Denise, R. P. A. Sneeden, J. M. Cognion and
P. Durual, J. Organomet. Chem., 1983, 256, 134–139.
4 J.-P. Simonato, J. Mol. Catal. A: Chem., 2003, 197, 61–64.
5 (a) J. Grévin and P. Kalck, J. Organomet. Chem., 1994, 476,
C23–C24; (b) I. Fleischer, R. Jennerjahn, D. Cozzula,
R. Jackstell, R. Franke and M. Beller, ChemSusChem, 2013,
6, 417–420; (c) H. Wang, B. Dong, Y. Wang, J. Li and Y. Shi,
Org. Lett., 2014, 16, 186–189.
18 H. Konishi and K. Manabe, Synlett, 2014, DOI: 10.1055/
s-0033-1339136.
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