Ruthenium catalysts for hydrogenation of aromatic and aliphatic esters: Make use of bidentate carbene ligands

Article abstract of DOI:10.1002/cssc.201200825

Committed carbenes: The convenient application of bidentate carbene ligands is described for the hydrogenation of carboxylic acid esters. The ligand precursors are easily synthesized through the dimerization of N-substituted imidazoles with diiodomethane. The catalyst is generated in situ and exhibits good activity and functional group tolerance for the hydrogenation of aromatic and aliphatic carboxylic acid esters. Copyright

Full text of DOI:10.1002/cssc.201200825

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DOI: 10.1002/cssc.201200825
Ruthenium Catalysts for Hydrogenation of Aromatic and
Aliphatic Esters: Make Use of Bidentate Carbene Ligands
Felix A. Westerhaus, Bianca Wendt, Andreas Dumrath, Gerrit Wienhçfer, Kathrin Junge, and
Matthias Beller*[a]
The generation of alcohols through the reduction of esters be-
longs to the key synthetic methodologies in organic chemis-
try.[1] In general, this transformation is still performed by using
stoichiometric reducing agents such as LiAlH4 or NaBH4.[2] The
disadvantages of such reagents are clear, that is, generation of
large amounts of waste, safety concerns, low functional group
tolerance, and sensitivity because of high reactivity of the
metal hydride. A more environmentally benign approach is to
use catalytic hydrogenations, which result in only the respec-
tive alcohol as the main product. Industrially, fatty acid ester
hydrogenations are accomplished by using heterogeneous
copper chromite catalysts. Unfortunately, this system requires
high temperatures (>2008C) and pressures (200 bar), while the
substrate scope and functional group tolerance are limited.[3]
Moreover, the rather harsh conditions are relatively energy
consuming. Because of these drawbacks, the development of
alternative and milder methods that omit these drawbacks and
limitations is desirable for both industry and academic
research.
easily oxidized and their electronic and steric properties allow
for easy fine-tuning. However, so far the use of solitary carbene
ligands for ester hydrogenation remains elusive.
Initially, two commercially available imidazolium salts, 1 and
2, were tested in combination with the ruthenium precursor
[Ru(p-cymene)Cl2]2 for the hydrogenation of methyl benzoate.
Despite numerous attempts, the yield of benzyl alcohol never
exceeded 7% (Table 1, entries 1 and 2). We then tested bi-
Table 1. Reactivity of different carbine–ruthenium catalysts.[a]
Entry
Ligand
Conversion [%][b]
Yield [%][c]
1
2
3
4
5
6[d]
7
8[f]
9[g]
10
11
12
13
14
1
2
3
4
5a
–
5a[e]
5a
5a
5b
5c
6
41
43
78
95
99
79
98
90
95
99
99
99
83
67
7
6
38
65
82
39
65
58
64
77
75
72
47
34
Several homogeneous catalyst systems for the hydrogena-
tion of esters, acids, or amides have been reported.[4] First at-
tempts of homogeneous hydrogenations of esters were ac-
complished by using hydridoruthenate salts or ruthenium–
carbonyl clusters.[5] Further improvements were achieved by
using an in situ-generated catalyst synthesized from [Ru(acetyl-
acetonate)3] and a tridentate phosphine ligand.[6] Recently,
more active systems for ester hydrogenation were disclosed by
Milstein[7] and Saudan.[8] In addition, other groups have also
contributed to this topic during recent years.[9] As an example,
our group reported a catalyst system for ester hydrogenation
based on imidazolyl–phosphine–ruthenium complexes.[10]
Although the existing methods exhibit high efficiencies, they
are still troubled by a number of limitations: a) functional
group tolerance is still restricted, and b) nearly all active ruthe-
nium catalysts require phosphorous ligands, which are some-
times expensive and/or difficult to modify and handle.
7
8
[a] Conditions: 0.5 mmol methyl benzoate, 0.5 mol% [Ru(p-cymene)Cl2]2,
2 mol% ligand 1–8, 30 mol% KOtBu, 2 mL 1,4-dioxane, 1008C, 50 bar H2,
6 h. [b] Conversion was determined by using GC with hexadecane as the
internal standard. [c] Yield was determined by using GC with hexadecane
as the internal standard. For ligands 1–3 between 4–5% benzylbenzoate
were formed. [d] Isolated complex [Ru(p-cymene)(5)Cl]Cl was used. [e] Iso-
lated complex [Ru(p-cymene)(5)Cl]Cl+5a was used. [f] Ru/Ligand=1:1.
[g] Ru/Ligand=1:4.
dentate imidazolium salts 3–8 because the corresponding bi-
dentate carbenes offered
(Figure 1).[12]
a
stronger binding mode
Based on our general interest in the selective reduction of
carboxylic acid derivatives,[11] we set a goal to develop im-
proved catalytic systems, which should not be based on tradi-
tional phosphines. More specifically, we proposed that car-
benes constitute promising alternative ligands for ester hydro-
genation. In comparison to phosphines, carbenes are less
The imidazolium salts were prepared through the dimeriza-
tion of commercially available N-substituted imidazoles with
dihalomethanes.[13] This straightforward synthetic pathway of-
fered numerous variations at the substituted nitrogen atoms in
the 4- and 5-positions of the imidazole backbone, which were
also used for the exchange of corresponding halides
(Scheme 1).
[a] F. A. Westerhaus, B. Wendt, A. Dumrath, G. Wienhçfer, Dr. K. Junge,
Prof. M. Beller
Until now, these chelating bisimidazolium salts have mainly
been reported for cross-coupling reactions[14] and CÀH activa-
tion processes;[15] however, they have scarcely been used in
catalytic hydrogenations.[16] Here, ruthenium catalysts were ap-
plied for the reduction of olefins, aldehydes, and ketones by
Leibniz-Institut fꢀr Katalyse e.V. an der Universitꢁt Rostock
Albert-Einstein-Straße 29a, 18059 Rostock (Germany)
Fax: (+49)381-1281-5000
E-mail: matthias.beller@catalysis.de
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Scheme 2. Model reaction for the catalytic hydrogenation of carboxylic acid
ester.
Scheme 2 and Table 1, entry 6). When a second equivalent of
ligand 5a was added to the [Ru(p-cymene)(5)Cl]Cl isolated
complex, the catalytic activity was improved (Table 1, entry 7).
Next, the most active catalyst system was used for different
types of aromatic esters. As shown in Table 2, good to excel-
lent yields of the corresponding benzylic alcohols were ob-
tained. Different alcohols in the ester moiety resulted in slight
to moderate changes in the yield (Table 2, entries 1 and 2;
Table 3, entries 6 and 7). Here, primary and secondary alcohols
were tolerated in the ester function (Table 2, entries 1 and 2;
Table 3, entries 1–3), as well as a number of diverse functional
groups, including halogens (Table 2, entries 7–9). Moreover,
diesters were easily reduced to the corresponding diols
(Table 2, entry 3). The reactivity of the catalyst was not signifi-
cantly affected by the presence of electron-donating (Table 2,
entries 4–6) or electron-withdrawing groups on the aromatic
ring (Table 2, entries 7 and 9).
Figure 1. Selection of carbene ligands that were used in this study.
Aliphatic esters are generally considered to be more chal-
lenging substrates for this transformation. Interestingly, our
catalytic system obtained moderate to good yields, even when
primary and secondary aliphatic esters were used as substrates
(Table 3, entry 10).
Scheme 1. Synthesis of imidazolium salts 3–8, which serve as precursors for
the carbene ligands.
Homobenzylic as well as aliphatic esters were converted into
the corresponding alcohols in good to excellent yield, in which
the presence of conjugated double bonds resulted in com-
plete hydrogenation (Table 3, entries 5 and 9). In addition, lac-
tones could also be reduced (Table 3, entries 11 and 13). Finally,
levulinic acid, which constitutes an interesting intermediate
that is easily available from biomass, was reduced and yielded
up to 80% g-valerolactone (Table 3, entry 12). Notably, this re-
action worked well at 15 mol% base concentration (Figure 2),
and ligand screening demonstrated that the best result was
using similar carbene ligands.[17] Bidentate carbenes have been
tested in combination with palladium, iridium, or rhodium for
catalytic transformations,[18] and a number of complexes bear-
ing different metals centers have also been characterized.[19]
With respect to ester reduction reactions, only catalysts con-
taining a carbene moiety in the ligand backbone have been re-
ported, but solitarily carbene ligands have not been successful
for this transformation.[20]
When we applied ligand precursors 3–8 to the hydrogena-
tion of methyl benzoate, improved reduction to the corre-
sponding alcohol was observed (Table 1, entries 3–14).
By using optimized conditions (1 mol% catalyst loading; ruthe-
nium-to-ligand ratio 1:2, 1008C, 6 h, 30 mol% base), the con-
version and yields were significantly higher than for monoden-
tate ligand precursors 1 and 2. Among the different bidentate
imidazolium salts, the benzyl-substituted derivative 5a pre-
sented the best results (Table 1, entry 5). Testing different
Ru/ligand ratios (Table 1; entries 5, 8, and 9) demonstrated an
optimum at 1:2. Furthermore, the defined ruthenium biscar-
bene complex [Ru(p-cymene)(5)Cl]Cl, bearing only one ligand
(5), was isolated. Positive ion ESIMS analysis of the isolated
complex showed a peak at m/z=599 for [Ru(p-cymene)(5)Cl]+
and the characteristic isotopic distribution for one ruthenium
atom.[21] This complex was used for the model reaction; how-
ever, only 39% yield of benzyl alcohol was obtained (see
Figure 2. Screening of different concentrations of KOtBu for the catalytic re-
duction of methyl benzoate with [Ru(p-cymene)Cl2]25a.
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Table 2. Ruthenium-catalyzed hydrogenation of aromatic esters.[a]
Table 3. Catalytic hydrogenation of aliphatic esters.[a]
Entry
1
Ester
Alcohol
Yield [%][b]
Entry
Ester
Alcohol
Yield [%][b]
1
2
3
67
77
50
78
2
3
4
5
92
60
74
85
74
4
5
89
65
88
77[c]
6
7
8
9
90
89
75
6
7
8
77
40
10
11
83
59
72
62
9
63
70
12
13
75[d]
80[e]
10
32
54[f]
[a] Conditions: 0.5 mmol ester, 0.5 mol% [Ru(p-cymene)Cl2]2, 2 mol%
ligand 5a, 30 mol% KOtBu, 2 mL 1,4-dioxane, 1008C, 50 bar H2, 6 h.
[b] Yield was determined by using GC with hexadecane as the internal
standard.
[a] Conditions: 0.5 mmol ester, 0.5 mol% [Ru(p-cymene)Cl2]2, 2 mol%
ligand 5a, 30 mol% KOtBu, 2 mL 1,4-dioxane, 1008C, 50 bar H2, 6 h.
[b] Yield was determined by using GC with hexadecane as the internal
standard. [c] 20 mol% KOtBu. [d] 1 mol% Ru, 2 mol% ligand 5c, 15 mol%
KOtBu. [e] 1 mol% Ru, 2 mol% ligand 7, 15 mol% KOtBu. [f] 1 mol% Ru,
2 mol% ligand 5b, 15 mol% KOtBu. Most of the catalytic experiments
were realized by using 30 mol% of KOtBu to reach high yields and full
conversion within a reasonable time (Figure 2).
achieved with the cyclohexyl-substituted bisimidazol ligand 7.
Furthermore, the corresponding lactone was reduced to
1,4-pentanediol in good yield (Table 3, entry 13).
Conclusions
KOtBu (30 mol%), and hexadecane as an internal standard was
added into the reaction vial, sealed, and purged with argon. The
liquid ester (0.5 mmol) was then added together with 1,4-dioxane.
If a solid ester was used, it was placed directly into the reaction
vial. The reaction vials (up to 6) were placed into a 300 mL auto-
clave. The autoclave was flushed with hydrogen twice (ca. 20 bar)
and pressurized to 50 bar hydrogen. It was then placed into an alu-
minum block that had been preheated to 1008C, and stirred for
the indicated time (6 h). After the reaction was complete, the auto-
clave was cooled to room temperature and the hydrogen was re-
leased. The crude reaction mixture was filtered through silica gel
and analyzed by using GC. The yield was determined by using
hexadecane as an internal standard.
For the first time ruthenium biscarbene catalysts are described
for the straightforward homogeneous hydrogenation of esters.
Key to their success is the application of bidentate carbene
ligands. The ligand precursors are easily synthesized through
the dimerization of N-substituted imidazoles with diiodo-
methane. The synthesis is straightforward and highly modular,
and as such offers multiple positions at which the ligand can
be modified. The most active catalyst is generated in situ from
[Ru(p-cymene)Cl2]2 and biscarbene 5a, showing broad applica-
bility for the hydrogenation of aromatic as well as aliphatic
carboxylic acid esters.
Synthesis of [Ru(p-cymene)(5)Cl]Cl: 5a or 5b (300 mg, 0.75 mmol)
in THF (10 mL) was placed in a Schlenk flask at À788C and nBuLi
(600 mL, 2.5m, 2 eq.) was added dropwise. The mixture was
warmed to rt over 2 h and was then filtered through Celite. A sus-
pension of [Ru(p-cymene)Cl2]2 (0.5 eq.) in THF (5 mL) was added at
Experimental Section
General procedure for the catalytic hydrogenation of esters: A mix-
ture of [Ru(p-cymene)Cl2]2 (1 mol% of Ru), ligand 1–8 (2 mol%),
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Acknowledgements
The authors thank Dr. C. Fischer, S. Buchholz, S. Schareina, and
S. Rossmeisl (all at the Leibniz-Institut fꢀr Katalyse e.V.) for excel-
lent analytical and technical support.
Keywords: alcohols · carbenes · homogeneous catalysis ·
hydrogenation · ruthenium
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[21] The defined ruthenium complex [Ru(p-cymene)(5)Cl]Cl was prepared
from [Ru(p-cymene)Cl2] and the free carbene (5), which was generated
from 5a or 5b by addition of nBuLi.
Received: October 31, 2012
Revised: February 21, 2013
Published online on && &&, 0000
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F. A. Westerhaus, B. Wendt, A. Dumrath,
G. Wienhçfer, K. Junge, M. Beller*
&& – &&
Ruthenium Catalysts for
Hydrogenation of Aromatic and
Aliphatic Esters: Make Use of
Bidentate Carbene Ligands
Committed carbenes: The convenient
application of bidentate carbene ligands
is described for the hydrogenation of
carboxylic acid esters. The ligand pre-
cursors are easily synthesized through
the dimerization of N-substituted imida-
zoles with diiodomethane. The catalyst
is generated in situ and exhibits good
activity and functional group tolerance
for the hydrogenation of aromatic and
aliphatic carboxylic acid esters.
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These are not the final page numbers!