Towards the development of a selective ruthenium-catalyzed hydroformylation of olefins

Article abstract of DOI:10.1002/chem.201300899

The ruthenium-catalyzed hydroformylation of 1- and 2-octene to give preferentially the corresponding linear aldehyde is reported. The catalyst system comprising of Ru₃(CO)₁₂ and an imidazole- substituted monophosphine ligand allows for high chemo- and regioselectivity. The hydroformylation proceeds with unprecedented rates for a ruthenium-based catalyst. Copyright

Full text of DOI:10.1002/chem.201300899

FULL PAPER
DOI: 10.1002/chem.201300899
Towards the Development of a Selective Ruthenium-Catalyzed
Hydroformylation of Olefins
Ivana Fleischer,[a] Lipeng Wu,[a] Irina Profir,[a] Ralf Jackstell,[a] Robert Franke,[b, c] and
Matthias Beller*[a]
Abstract: The ruthenium-catalyzed hydroformylation of 1- and 2-octene to give
preferentially the corresponding linear aldehyde is reported. The catalyst system
comprising of Ru3(CO)12 and an imidazole-substituted monophosphine ligand
allows for high chemo- and regioselectivity. The hydroformylation proceeds with
unprecedented rates for a ruthenium-based catalyst.
Keywords: aldehydes · carbonyla-
tion · hydroformylation · phosphine ·
ruthenium
Introduction
lyst activity. Although linear aldehydes are important inter-
mediates for production of bulk chemicals, selectively pro-
duced branched aldehydes bearing a stereogenic center are
of interest for the pharmaceutical industry, for instance.[4]
The early hydroformylation catalysts were based on
Homogeneously-catalyzed hydroformylation, also called
oxo-synthesis, is a powerful, atom-efficient method for the
synthesis of aldehydes from olefins with carbon monoxide
and hydrogen (Scheme 1).[1] Originally, this reaction was dis-
cobalt comACTHNUTRGNEUGpN lexes, but it was found in the 1970s that rhodi-
um-based systems are superior in terms of catalytic activity
and selectivity.[1,5] Since then, many efforts have been suc-
cessfully put into the development of efficient rhodium cata-
lysts. Other metals that are able to catalyze hydroformyla-
tion reactions display considerably lower activity than rhodi-
um. However, the rough order of their catalytic activity
(Rh> Co> Ir> Ru> Os> Pt> Pd> Fe > Ni) is simply
based on the use of non-modified metal carbonyl com-
Scheme 1. Hydroformylation of olefins to aldehydes.
covered by Otto Roelen, who can be considered as a “pio-
neer of industrial homogeneous catalysis”.[2,3] Nowadays,
more than 10 million tons of oxo products are industrially
produced by hydroformylation each year.
As depicted in Scheme 1, the formylation can in general
occur at the terminal or internal carbon atom. Thus, mix-
tures of linear (n) and branched (iso) aldehydes can be ob-
tained. Moreover, the double bond can isomerize and the
following hydroformylation of internal olefins will increase
the portion of branched regioisomers. Therefore, specific
control of the regioselectivity is a very important issue for
any application along with chemoselectivity and high cata-
plexes.[1a] Recently, we and others have shown that there
ACHTUNGTRENNUNG
exists a huge potential to utilize modified reaction condi-
tions and ligands to improve the performance of so-called
“alternative” hydroformylation catalysts, which might substi-
tute the particularly expensive rhodium.[6,7]
Ever since the first ruthenium-catalyzed hydroformylation
was reported by Evans, Osborn, Jardine, and Wilkinson in
1965,[8] several catalysts have been developed, but they
could not compete with rhodium-based systems.[9] Notably,
Takahashi, Yamashita, Tanaka, and Nozaki reported in 2012
a [RuIICp*] complex with bidentate ligands for highly regio-
selective hydroformylation of propene and 1-decene.[10] The
observed chemo- and regioselectivity is comparable to
standard rhodium catalyst system, however, the catalytic ac-
tivity was still significantly lower. More recently, a modified
catalyst for domino hydroformylation/reduction was report-
ed.[11]
[a] Dr. I. Fleischer, L. Wu, I. Profir, Dr. R. Jackstell, Prof. M. Beller
Leibniz Institut fꢀr Katalyse e.V. an der Universitꢁt Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
Fax : (+49)381-1281-51113
E-mail: matthias.beller@catalysis.de
Our recent investigations in this field led to the develop-
ment of a new hydroformylation/reduction catalyst system
based on a Ru0 carbonyl complex and the imidazoyl-substi-
tuted monophosphine ligand L1 (Scheme 2).[12] Good activi-
ty, chemo- and regioselectivity were obtained in the domino
hydroformylation of alkenes and subsequent reduction of
the aldehydes to the corresponding alcohols. The scope of
the reaction was demonstrated on terminal and internal ali-
[b] Prof. Dr. R. Franke
Evonik Industries AG
Paul-Baumann-Strasse 1, 45772 Marl (Germany)
[c] Prof. Dr. R. Franke
Lehrstuhl fꢀr Theoretische Chemie
44780 Bochum (Germany)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300899.
Chem. Eur. J. 2013, 19, 10589 – 10594
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
10589

The following reactions were performed with a low cata-
lyst loading of 0.1 mol% Ru and analyzed by GC after
three hours. A minor yield of aldehyde and an undetectable
amount of alcohol were obtained in NMP and PC (propy-
lene carbonate) and their mixtures with water, respectively
(Table 1, entries 4–6). However, increased partial hydrogen
pressure led to a considerably higher aldehyde yield (79%,
Table 1, entry 7) in propylene carbonate. Notably, the trans-
formation proceeded with improved regioselectivity of 95:5,
because hydroformylation of the internal olefins does not
take place under these conditions. Applying the same pres-
sure and varying solvents, inferior results were achieved
(Table 1, entries 8–10). The catalyst system was active even
at 0.05 mol% loading, which is, to the best of our knowl-
edge, the lowest reported catalyst loading of any ruthenium
complex in hydroformylation reactions (Table 1, entry 11).
Furthermore, the reaction proceeded smoothly at 808C and
even at 608C applying a longer reaction time (Table 1, en-
tries 12 and 13). The contamination of the autoclave by
other metals (e.g., rhodium) was excluded by a blank test
(Table 1, entry 14).
Scheme 2. Ruthenium-catalyzed domino hydroformylation/reduction of
alkenes to alcohols.
phatic, cyclic and araliphatic alkenes, 1,3-dienes and elec-
tron-poor methacrylic acid derivatives. In addition, the cata-
lyst could be also applied to hydroaminomethylation reac-
tions of various olefins and amines.[13] These reactions
extend the scope of synthetically important Ru-catalyzed
[14,15]
À
C C bond forming reactions.
Herein, we would like to present further investigations of
the previously reported catalyst system and its application in
the selective hydroformylation of alkenes to aldehydes.
Results and Discussion
In the following, the ligand structure was modified.[16] De-
cisive results are summarized in Scheme 3 and depicted in
Figure 1. The alteration of the nitrogen substituent revealed
an ameliorative effect of aromatic groups, in particular of
the electron-rich ortho-anisyl substituent (L2). Further var-
iations were conducted on the phosphorus atom. The gas
consumption curves shown in Figure 1 provide a clear and
direct representation of activities of the respective catalyst
systems. Whereas the iso-propyl- and cyclohexyl- substituted
ligands L2 and L6 exhibit highest reactivity, bulkier tert-
butyl and adamantyl-derived ligands L5 and L7 were entire-
ly inactive. Slightly diminished aldehyde yield and lower re-
gioselectivity were obtained with the less basic L8. Benzimi-
dazol-based ligands L9–10 were
In the course of our studies on hydroformylation/reduction
of 1-octene that was carried out at 1308C in a mixture of
NMP (N-methyl-2-pyrrolidone) and H2O as solvents, the
formation of a mixture of alcohols and aldehydes was ob-
served in the absence of lithium chloride (Table 1, entries 1
and 2). The selectivity towards aldehydes increased at lower
reaction temperature demonstrating that the carbonyl re-
duction requires more drastic conditions (Table 1, entry 3).
Next, the reaction conditions were further optimized in
order to suppress the hydrogenation activity of the catalyst
system further on and to improve its activity and regioselec-
tivity.
Table 1. Reaction of 1-octene with synthesis gas: Optimization of reaction conditions.[a]
less active than their imidazole
analogs and required longer re-
action times. By comparison,
pyrrole-derived ligand L11 was
not active under the standard
reaction conditions. The same
T
[8C]
CO/H2
[bar]
Solvent
t
x
Yield [%][d]
result was obtained with
ligand-free system.
a
G
[h]
[mol%]
2a (n/i)
3a (n/i)
1[b,c]
2[b]
3
4
5
6
7
8
9
130
130
100
100
100
100
100
100
100
100
100
80
30/30
30/30
30/30
30/30
30/30
30/30
20/40
20/40
20/40
20/40
20/40
20/40
20/40
20/40
NMP/H2O
NMP/H2O
NMP/H2O
NMP/H2O
PC/H2O
PC
PC
NMP
PhMe
THF
PC
20
20
20
3
3
3
3
3
3
3
0.2
0.2
0.2
90 (88:12)
1[d]
Using Ru3(CO)12/L2, nonanal
was isolated in 59% yield by
extraction with heptane and
bulb-to-bulb distillation. Un-
fortunately, a small amount of
propylene carbonate was found
in the isolated material. The
lower isolated yield was caused
by the partial oxidation of the
aldehyde in the presence of
ruthenium catalyst.
37 (95:5)
6 (95:5)
0
0
0
<1
0
0
0
0
0
0
0
54 (85:15)
83 (89:11)
23 (92:8)
10 (94:6)
32 (94:6)
79 (95:5)
60 (95:5)
11 (95:5)
<1
74 (95:5)
79 (95:5)
80 (94:6)
0
0.033
0.033
0.033
0.033
0.033
0.033
0.033
0.0167
0.033
0.033
0
10
11
12
13
14
8
PC
PC
PC
15
72
20
60
100
In order to demonstrate the
unique activity of our rutheni-
um catalyst, the hydroformyla-
[a] 50.0 mmol 1-octene, Ru3(CO)12, L1, 25 mL solvent, CO/H2, 100 mL autoclave. [b] Performed in 25 mL au-
toclave with 20.0 mmol 1-octene. [c] With LiCl (25 mol%). [d] Determined by GC with isooctane as internal
standard.
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Chem. Eur. J. 2013, 19, 10589 – 10594

Ruthenium-Catalyzed Hydroformylation of Olefins
FULL PAPER
Figure 1. Gas consumption curves in the hydroformylation of 1-octene
with varying ligands.
suppress this sequential hydrogenation, the reaction was per-
formed with a 1:1 CO/H2 mixture (Table 2, entry 3). Indeed,
no alcohol was observed, however, the overall activity and
selectivity diminished. Further experiments were performed
with higher catalyst loading at 1008C under constant pres-
sure of carbon monoxide and hydrogen (5/20 bar). A higher
partial hydrogen pressure was applied in order to facilitate
the isomerization of the double bond. Indeed, these condi-
tions led to an aldehyde yield of 75% and n/iso selectivity
of 81:19 (Table 2, entry 4).
Scheme 3. Hydroformylation of 1-octenes: Ligand variation. [a] Deter-
mined by GC with isooctane as internal standard. [b] Isolated yield is
given in brackets. [c] 24 h.
Slightly higher regioselectivity was observed by employing
a more diluted reaction mixture (Table 2, entry 5). Low al-
dehyde yield (12%) was obtained, when non-isobaric condi-
tions were used (Table 2, entry 6). The variation of partial
pressures revealed that the CO/H2 5:20 mixture represents
the optimum ratio for this reaction (Table 2, entries 7–10).
However, the present catalyst system has to be further opti-
mized in order to achieve higher activity and improved che-
tion of 1-octene was carried out at 1308C in the presence of
ligands L1 and L2 and 0.017 mol% of the ruthenium precur-
sor (0.05 mol% in Ru). After 40 min, the aldehyde 3a was
formed in 60 (L1) and 68% yield (L2), which corresponds
to a turnover-frequency of 1800 and 2040 hÀ1, respectively.
With 1-butene as substrate and only 0.025 mol% of Ru, a
turnover-frequency of 2400 hÀ1 was achieved at
1308C. Notably, the obtained values are much
closer to the activities of rhodium catalysts as it was
ever expected for any ruthenium catalyst in hydro-
formylations.
Table 2. Reaction of 2-octene with synthesis gas: Optimization of reaction con-
AHCTUNGTRENNUNG
ditions.[a]
A considerably more challenging transformation
is the hydroformylation of internal olefins to linear
aldehydes, which proceeds through isomerization of
the internal double bond and the subsequent regio-
selective hydroformylation. Several rhodium-based
systems were developed in industry and academia
for this demanding reaction.[17] Thus, the rutheni-
um-catalyzed hydroformylation of 2-octene was
also investigated (Table 2). Unfortunately, no con-
version was obtained by applying the standard reac-
tion conditions developed for terminal alkenes
(1008C, 0.1 mol% Ru; Table 2, entry 1). However,
activity was observed at 1308C and 70% yield of
C9-aldehydes was achieved (Table 2, entry 2). At
this temperature, the isomerization of 2-octene oc-
curred and the obtained regioselectivity for alde-
hydes was 72:28. Though, the aldehyde was partial-
T
[8C]
CO/H2
[bar]
5
t
x
Yield [%][b]
2a (n/i) 3a (n/i)
G
A
[h]
ACHTUNGTRNE[NUNG mol%]
1
2
3
4
5
6
7
8
9
100
130
130
100
100
100
100
100
100
100
20/40
20/40
30/30
50
50
50
50
20
20
20
20
20
20
6
0.033
0.033
0.033
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0
0
40
40
20
20
20
20
20
20
20
13 (79:21)
0
5 (33:67)
5 (30:70)
<1
2
5 (30:70)
<1
70 (72:28)
37 (67:33)
75 (81:19)
75 (83:17)
12 (88:12)
54 (87:13)
67 (86:14)
36 (84:16)
29 (83:17)
5/20 const.[c]
5/20 const.[c]
5/20
2.5/22.5 const.[c]
3.5/21.5 const.[c]
8.5/16.5 const.[c]
12.5/12.5 const.[c]
10
0
[a] 2-Octene, Ru3(CO)12, L2, 25 mL PC, CO/H2, 100 mL autoclave. [b] Determined by
GC with isooctane as internal standard. [c] Performed under constant pressure in au-
ly reduced to the corresponding alcohol. In order to toclave connected to a burette filled with CO/H2 1:1.
Chem. Eur. J. 2013, 19, 10589 – 10594
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10591

M. Beller et al.
Figure 2. 31P NMR spectra of L1/Ru3(CO)12 mixtures: a) Ligand L1. b) L1/Ru3(CO)12 (1.1:1) (Table 2, entry 6) stirred at 1008C under argon atmosphere
for 1 h. c) L1/Ru3(CO)12 (1.1:1) stirred in autoclave at 1008C with CO/H2 (1:1, 60 bar) for 2 h. d) As c), with L1/Ru3(CO)12 (2:1). e) As c), with L1/
Ru3(CO)12 (0.95:1).
moselectivity for industrially important olefin mixtures.
Also, the conversion of challenging 4-octene was not suc-
cessful under the reaction conditions.
Comparable to our original work,[12] the formation of de-
fined complexes was studied by 31P NMR spectroscopy
(Figure 2). When a toluene solution of ligand L1 and triru-
thenium dodecacarbonyl was heated to 1008C under argon,
a new complex showing a signal at 44.6 ppm was mainly ob-
tained (Figure 2b). On the other hand, the same reaction
performed under synthesis gas pressure (60 bar, CO/H2 1:1)
gave an additional signal at 32.0 ppm (Figure 2c). The varia-
tion of the amount of the ligand and Ru revealed that these
two signals correspond to complexes with different Ru/L
ratio (Figure 2c–e). Signal at 44.6 ppm is dominant when
excess ligand is used, whereas only one complex with signal
at 32.0 ppm is formed with equimolar L/Ru ratio.
These results were adopted for the synthesis of the de-
fined complexes after careful optimization of the conditions
(Scheme 4). The complex with signal at 44.6 ppm was pre-
pared by heating of a toluene mixture of L1 and Ru3(CO)12
(L/Ru 1.8:1) to 1008C for 1 h and isolated in a pure form
after removal of the solvent by precipitation with heptane.
The particularly stable mononuclear complex A bearing two
ligands was characterized by NMR, HRMS, and X-ray anal-
Scheme 4. Synthesis and structures of ruthenium–ligand L1 complexes A
and B.
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Ruthenium-Catalyzed Hydroformylation of Olefins
FULL PAPER
ysis. On the other hand, complex B was obtained as a tol-
uene solution. It was synthesized in analogy to the prepara-
tion of complex A with different L/Ru ratio (0.95:1) under
synthesis gas pressure (60 bar, CO/H2 1:1). Due to its insta-
bility, an X-ray characterization was not successful. Howev-
er, NMR and HRMS analysis suggest formation of the mon-
onuclear complex B with one ligand that should bind in a bi-
dentate mode. This kind of structure is known in the litera-
ture.[16]
In contrast to the successful rhodium-based hydroformyla-
tion catalysts, the catalysis performed by the herein present-
ed system is suppressed in the presence of higher excess of
the ligand (>2 equivalents to Ru). Hence, we suggest com-
plex B to be responsible for the catalysis. It can be detected
in the crude mixture after the reaction as the major phos-
phorus-containing component. To test its catalytic perform-
ance, the hydroformylation of 1-octene was carried out with
the freshly prepared solution of B in toluene and compared
to the result of reaction catalyzed by the standard in situ
catalytic system and complex A (Table 3, Figure 3). As ex-
pected, the hydroformylation reaction catalyzed by A was
slower than the standard system, contrary to the perform-
ance of B, which was similar to the in situ catalyst.
Conclusion
In summary, we have developed a highly active ruthenium-
based catalyst for hydroformylation of aliphatic olefins. The
catalyst system consisting of Ru3(CO)12 and imidazoyl-sub-
stituted phosphine ligand exhibited excellent activity in the
n-selective hydroformylation of 1-octene. The transforma-
tion of challenging internal olefins was demonstrated with 2-
octene, although this tandem isomerization/hydroformyla-
tion sequence required more forcing reactions conditions. In
addition, a complex responsible for the catalysis was identi-
fied. We expect this catalyst to broaden the scope of hydro-
formylation reactions and offer an interesting alternative to
rhodium-based carbonylation chemistry.
Experimental Section
General procedure, Ligand synthesis: In a three-necked 100 mL round-
bottomed flask equipped with a reflux condenser, the corresponding sub-
stituted 1H-imidazole (13.5 mmol) was dissolved in THF (30 mL) under
argon and cooled to À308C. nBuLi (1.6m in hexane, 8.4 mL, 13.5 mmol)
was added and the reaction mixture was stirred at À308C for 30 min. A
solution of the corresponding dialkylchlorophosphine (14.8 mmol in
10 mL of THF) was slowly added through a dropping funnel at À308C.
The reaction mixture was slowly warmed to 508C and stirred for 60 min.
After cooling with an ice bath, degassed aqueous NH4Cl-solution was
added, stirred for a few minutes, and the organic phase was separated.
The aqueous layer was extracted with toluene (2ꢃ20 mL) and the com-
bined organic layers were concentrated under vacuum. The product was
recrystallized from diethyl ether to give the pure ligand as a colorless
solid.
Table 3. Reaction of 1-octene with synthesis gas catalyzed by A and B.[a]
Hydroformylation of 1-octene: A 100 mL autoclave was charged with
Ru3(CO)12 (10.7 mg, 16.7 mmol), ligand L2 (20.4 mg, 55.0 mmol), 25 mL
propylene carbonate and 7.8 mL 1-octene (5.6 g, 50 mmol). Then 40 bar
of synthesis gas (CO/H2 1:1) and 20 bar H2 were introduced before the
autoclave was heated to 1008C. The reaction was stopped by cooling
down and releasing the pressure. Water (25 mL) was added and the
crude product was extracted with heptane (4ꢃ50 mL). The collected or-
ganic extracts were dried over MgSO4 and concentrated in vacuo. The
product was isolated by bulb-to-bulb distillation (75–808C, 20 mbar) as a
14:1:2 mixture of nonanal, 2-methyloctanal and propylene carbonate
(59% yield).
Catalyst
t [h]
3a Yield [%][b]
n/i[b]
1
2
3
L1/Ru3(CO)12 (L/Ru 1.1:1) in situ
3
20
10
79
75
77
95:5
95:5
95:5
A
B[c]
[a] 50.0 mmol 1-octene, catalyst (0.1 mol% of Ru), 25 mL solvent, CO/H2
(1:2, 60 bar), 1008C, 100 mL autoclave. [b] Determined by GC with isooc-
tane as internal standard. [c] Performed with a solution of B in toluene
(0.114m).
Nonanal: 1H NMR (300 MHz, CDCl3): d=9.71 (t, 3JHH =1.9 Hz, 1H,
CHO), 2.37 (dt, 3JHH =7.3 Hz, 3JHH =1.9 Hz, 2H, CH2CHO), 1.58 (p,
3JHH =7.3 Hz, 2H, CH2CH2CHO), 1.36–1.13 (m, 10H, CH2), 0.83 ppm (t,
3JHH =6.7 Hz, 3H, CH3); 13C NMR (75 MHz, CDCl3): d=202.7 (CHO),
43.8 (CH2CHO), 31.7 (CH2), 29.2 (CH2), 29.0 (CH2), 29.0 (CH2), 22.5
(CH2), 22.0 (CH2), 13.9 ppm (CH3); MS (EI): m/z (%): 141 [MÀH]+,
124, 114, 109, 98, 81, 70, 57, 55, 43, 41 (100), 29.
CCDC939937 contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
We are particularly grateful to the Bundesministerium fꢀr Bildung und
Forschung (the German Federal Ministry of Education and Research)
for financial support under the PROFORMING project (no. 03X3559).
We also thank the Deutsche Forschungsgemeinschaft (Leibniz-price),
Swiss National Science Foundation (grants for I.F.), Chinese Scholarship
Council (grants for L.W.) and the analytical department at LIKAT.
Figure 3. Gas consumption curves in the hydroformylation of 1-octene:
L1/Ru3(CO)12 (L/Ru 1.1:1).
Chem. Eur. J. 2013, 19, 10589 – 10594
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10593

M. Beller et al.
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À
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Received: May 13, 2013
Published online: July 4, 2013
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