Base-Free Iron Catalyzed Transfer Hydrogenation of Esters Using EtOH as Hydrogen Source

Article abstract of DOI:10.1002/anie.201810605

Herein, we report on the use of the iron pincer complex Iron-MACHO-BH, in the base-free transfer hydrogenation of esters with EtOH as a hydrogen source. More than 20 substrates including aromatic and aliphatic esters and lactones were reduced affording the desired primary alcohols and diols with moderate to excellent isolated yields. It is also possible to reduce polyesters to the diols with this method, enabling a novel way of plastic recycling. Reduction of the renewable substrate methyl levulinate proceeds to form 1,4-pentanediol directly. The yields are largely governed by the equilibrium between the alcohol and the ethyl ester.

Full text of DOI:10.1002/anie.201810605

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Accepted Article
Title: Base-Free Iron Catalyzed Transfer Hydrogenation of Esters
Using EtOH as Hydrogen Source.
Authors: Ronald Farrar-Tobar, Bartosz Wozniak, Arianna Savini,
Sandra Hinze, Sergey Tin, and Johannes Gerardus de Vries
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201810605
Angew. Chem. 10.1002/ange.201810605
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10.1002/anie.201810605
Angewandte Chemie International Edition
COMMUNICATION
Base-Free Iron Catalyzed Transfer Hydrogenation of Esters Using
EtOH as Hydrogen Source
Ronald A. Farrar-Tobar,[a] Bartosz Wozniak,[a] Arianna Savini,[a] Sandra Hinze,[a] Sergey Tin,[a]
Johannes G. de Vries*[a]
Dedication ((optional))
Abstract: Herein, we report on the use of the iron pincer complex
Iron-MACHO-BH, in the base-free transfer hydrogenation of esters
with EtOH as a hydrogen source. More than 20 substrates including
aromatic and aliphatic esters and lactones were reduced affording
the desired primary alcohols and diols with moderate to excellent
isolated yields. It is also possible to reduce polyesters to the diols
with this method, enabling a novel way of plastic recycling.
Reduction of the renewable substrate methyl levulinate proceeds to
form 1,4-pentanediol directly. The yields are largely governed by the
equilibrium between the alcohol and the ethyl ester.
such a strong base can be detrimental in the case of base-
sensitive substrates. Relatively few transfer hydrogenation
catalysts are known that function without base.[5] Another aspect
that could be improved is the replacement of the precious metal
by a base metal.
Here, we report the use of a non-precious Fe-based catalyst
for the transfer hydrogenation of ester compounds in the
absence of base. Fe-MACHO-BH (3) is an excellent catalyst for
the dehydrogenation of alcohols for hydrogen production and for
the hydrogenation of a large number of other functionalities.[6]
For that reason, we envisioned the possibility of using this
catalyst in the transfer hydrogenation of esters. In a preliminary
experiment, methyl benzoate was stirred in the presence of 5
mol% of 3 in a large excess of EtOH at 100 ºC. After 24 hours,
benzyl alcohol was obtained in 88% GC-yield (Table 1, entry 1).
Additionally, 10% of ethyl benzoate was formed due to the trans-
esterification of the substrate with the solvent. Also, ethyl
acetate was detected by GC-MS as the by-product resulting
Transfer hydrogenation (TH) processes are an attractive
alternative to classical catalytic hydrogenations since the use of
expensive high pressure reactors can be avoided.[1] Most
transfer hydrogenations are used to reduce ketones to the
corresponding alcohols. Recent work on transfer hydrogenation
of more challenging substrates reported the reduction of lactams
to tertiary amines via transfer hydrogenation.[2] Surprisingly only
two examples are described in the literature of transfer
hydrogenation of esters, which are generally less easily reduced
than ketones.[3] Nikonov established proof of concept for the
transfer hydrogenation of ester compounds with isopropanol
Table 1. Screening of alcohols as the reducing agents for the base-free
transfer hydrogenation of esters.[a]
using the cationic ruthenium complex
1
as catalyst.[3a]
Transesterification was a serious problem as the isopropyl
esters are reduced much slower. Khaskin screened 9 different
ruthenium-based ester hydrogenation catalysts and found that
the Ru-NNS pincer catalyst 2 developed by Gusev was the most
effective. He also discovered that ethanol is a much more
effective reductant than isopropanol. He was able to reduce a
wide scope of esters including triglycerides and hetero-
substituted esters (Figure 1).[3b] It is obvious that this is an
interesting new methodology which can probably be improved
further. Both Nikonov and Khaskin have used KOtBu to activate
the catalyst. And indeed in many cases the base accelerates the
catalysis beyond this inactivation step.[4] However, the use of
Entry
Cat.Load.
[mol%]
Hydrogen
Source
Conv.
[%]
Yield of
4b [%]
Yield of
4c [%]
1
2[b]
3
5
5
5
5
3
3
EtOH
MeOH
>99
7
88
0
10
0
iPrOH
95
>99
98
41
56
40
61
1
39
60
35
36
4
BuOH
5
EtOH/MeOH[c]
EtOH/MeOH[d]
6
[a] Reaction conditions: Substrate (0.5 mmol), alcohol (96 equiv), cat. 3 (5
mol%), 100 ºC, 24 hours, conversion and yield determined by GC using
t
dodecane as an internal standard. [b] 25 mol% of BuOK was added. [c] 2.5
equiv of MeOH with respect to the substrate. [d] 96 eq of MeOH with respect
to the substrate.
from the dehydrogenation of the hemiacetal obtained from the
formed acetaldehyde and another molecule of ethanol.
Examples where EtOH is used as the hydrogen source are rare
in the literature.[3b, 7] This could be due to its known ability to
poison catalysts via decarbonylation of the acetaldehyde,
forming less reactive carbonyl complexes. In addition,
acetaldehyde can undergo condensation reactions with the
starting materials or products before it is scavenged as ethyl
acetate. Furthermore, its dehydrogenation is both kinetically and
Figure 1. Catalysts used in the transfer hydrogenation of esters
[a]
R.A. Farrar-Tobar, B. Wozniak, Dr. A. Savini, Dr. S. Hinze, Dr. S.
Tin, Prof. Dr. J.G. de Vries
Leibniz Institut für Katalyse e. V. an der Universität Rostock
Albert-Einstein-Straße 29a, 18055 Rostock (Germany)
E-mail: johannes.devries@catalysis.de
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201810605
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COMMUNICATION
thermodynamically less favored than that of secondary alcohols
like iPrOH.[7a, 7f] Thus, other sources of hydrogen were also tried.
In line with the findings of Nikonov, use of iPrOH resulted in only
56% of the desired product (Table 1, entry 3). Use of n-Butanol
also led to a slower reaction and poorer selectivity to benzyl
alcohol. The use of MeOH led to very low conversions even
when a base was added. (Table 1, entry 2) Similar results were
monitorings were then performed with the Ru and Mn analogues
of the Fe complex 3 (ESI, S3). Monitoring the Ru-catalyzed
reaction with the defined protocol, very similar results were
obtained (ESI3, Figure 2). Interestingly, the final ratio between
benzyl alcohol (4b) and ethyl benzoate (4c) was 3:1 in both
cases (ESI, S3, Figure1). These results confirm that the transfer
hydrogenation process is governed by the equilibrium between
obtained with an equimolar mixture of MeOH and EtOH (Table 1, EtOH, benzyl alcohol (4b) and ethyl benzoate (4c). On the other
entry 5). When the amount of MeOH in the mixture was reduced
to 2.5 equiv, the benzylic alcohol yield increased to 61% yield
(Table 1, entries 4). This result clearly suggests catalyst
poisoning by MeOH. Next, the reaction conditions were
optimized and the results are summarized in Table 2.
hand, with the Mn-PNP-BH complex 3c only ethyl benzoate (4c)
was obtained. (ESI, S3, Figure2).
Next, the scope and limitations of the transfer hydrogenation
with Fe-MACHO-BH were explored using 5 mol% of catalyst at
100 °C. Under these conditions a wide variety of aromatic esters
were reduced. Results are summarized in Scheme 1. Non-
substituted aromatic esters were smoothly reduced to the
desired alcohols with isolated yields up to 86 % (Scheme 1, 4-6).
Ester 7a, which also contains a ketone group, was completely
reduced to the corresponding diol with 80% isolated yield
(Scheme 1, 7b). High yields were obtained when p-fluoro (8a)
and p-chloro (9a) substituted benzoic acid methyl esters were
used as the substrates. However, the transfer hydrogenation of
methyl p-bromobenzoate(10a) resulted in only 14% yield of the
alcohol. Likewise, p-nitrobenzoate 11a only afforded the desired
product in 5% yield (Scheme 1, 11a). A possible explanation for
this behaviour is that the alcohol formed is too acidic, and
therefore binds to the metal thus deactivating the catalyst. p-
amine-substituted methyl esters 12a and 13a were completely
Table 2. Optimization of the reaction conditions.[a]
Entry
Cat.Load.
[%mol]
EtOH
[equiv]
Temp.
[ºC]
Conv.
[%]
Yield of
4b [%]
Yield o
4c [%]
1
2
3
4
5
6
5
4
5
5
5
5
96
96
100
100
100
100
80
>99
>99
>99
>99
>99
>99
88
64
74
78
87
40
10
29
21
21
13
59
120
45
96
96
60
[a] Reaction conditions: substrate (0.5 mmol), cat. 3, 100 ºC, 24 hours,
conversion and yield determined by GC using dodecane as an internal
standard.
Reducing the catalyst loading to 4 mol%, a lower yield of the
desired product was obtained (Table 2, entry 2). Similarly,
increasing or reducing the number of equivalents of EtOH
(compared to optimal 96 equiv) causes
a decrease of
conversion towards benzyl alcohol (Table 2, entries 3-4). The
effect of temperature was also tested. At 80 ºC a similar result
o
was obtained (Table 2, entry 5). Running the reaction at 60 C
resulted in a significant drop of the yield (Table 2, entry 7).
Intrigued by the inability to achieve full conversions, a kinetic
profile of the reaction was performed using 3 at three different
concentrations as well as the analogous Ru-MACHO-BH (3b)[8]
and Mn-MACHO-BH (3c)[9] catalysts. (ESI, S3, Figure1 and 2).
As expected, with 3 transesterification of the starting material to
compound 4c was the fastest reaction. Then, the desired alcohol
4b was obtained via the reduction of both esters. Interestingly,
the reaction reaches the maximum yield after only 7 hours. This
suggests that an equlibrium has been reached. It seems that
under the reaction conditions the dehydrogenation of the benzyl
alcohol to the aldehyde and its further reaction with ethanol
followed by a second dehydrogenation gives the ethyl benzoate.
To check the feasibility of the reverse reaction, benzyl alcohol
(4b) was submitted to a reaction with ethyl acetate in ethanol. In
this experiment, 5% of ethyl benzoate and 3% of benzyl acetate
were the only observed products (ESI, S4). The other
Scheme 1. Transfer hydrogenation of aromatic esters. [a] Reaction
conditions: substrate (0.5 mmol), 96 equiv of EtOH (2800 µL), Fe cat. 3 (5
mol%, 9,7 mg), 100 ºC, 24 hours reaction time in a pressure tube covered
from light. All yields are isolated unless mentioned otherwise. [b] Yield
corresponds to saturated alcohol. [c] Mn cat. 3c (5 mol%) was used. The
product is ethyl 3-(p-methoxyphenyl)propanoate. [d] Yield is calculated by
dividing the amount of benzyl alcohol by 2 [e] Substrate (0.21 mmol), 120
equiv of EtOH (2100 µL). 7.5 mol% of Fe cat. 3. [f] Reaction conditions:
substrate 5 mmol, 96 equiv of EtOH (27 mL), Fe cat. 3 (5 mol%, 97.5 mg). [g]
Yield determined by GC.
inactive. This can possibly be explained by the high affinity of
amines for the metal center, which leads to deactivation of the
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10.1002/anie.201810605
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COMMUNICATION
of waste from polymers has become a significant issue in the
last decades. Despite recent advances in polymer recycling
catalyst.
-Unsaturated esters 14a and 15a were also
[16]
reduced to the corresponding saturated alcohols with high
isolated yields. Unfortunately, it is not possible to keep the
conjugated double bond intact as the saturated alcohols were
obtained as the products.
On the other hand, Mn-PNP-BH 3c was able to reduce only
the C=C double bond in compound 15a. Homogeneous base
metal-catalyzed reductions of alkenes and alkynes have been
reported before with iron-,[10] cobalt-,[11] and nickel-based[12]
catalysts and most of them use hydrogen sources other than
EtOH. This is the first example of the use of a manganese-
based pincer-catalyst for the TH of a C=C bond using EtOH as
hydrogen donor. The heteroaromatic ester 16a could be reduced
quantitatively.
many challenges are still unsolved in this field. One of these
challenges is the design of catalysts capable of
depolymerisation with sufficient rate and turnover number.[17]
Only three Ru based homogeneous catalytic systems have been
reported to be capable of polyester deconstruction under mild
conditions.[18] To the best of our knowledge, no examples exist in
the literature on the depolymerization of polyesters via transfer-
hydrogenation. Dynacol 7360 (23a) is made from adipic acid and
1,6-hexandiol. When this substance was submitted to our
protocol, 1,6-hexanediol was obtained in 87% isolated yield.
This result opens the possibility to recycle polyesters by
deconstructing them to their monomers using a complete new,
To test the robustness of the system, the reaction with 9a
was scaled up 10 times resulting in a 95% isolated yield of the
desired alcohol.
easy and green strategy.
Scheme 3. Transfer hydrogenation of lactones [a] Reaction conditions:
substrate (0.5 mmol), 96 equiv of EtOH (2800 µL), Fe cat. 3 (5 mol%, 9.7 mg),
100 ºC, 24 hours reaction time in a pressure tube covered from light. All yields
are isolated unless mentioned otherwise. [b] Product is γ-Valero lactone (GVL).
Yield is based on the single peaks observed by GC and products were
characterized by GC-MS. [c] 25 mol% of EtONa was used.
Scheme 2. Transfer hydrogenation of aliphatic esters [a] Reaction
conditions: substrate (0.5 mmol), 96 equiv of EtOH (2800 µL), Fe cat. 3 (5
mol%, 9.7 mg), 100 ºC, 24 hours reaction time in a pressure tube covered
from light. All yields are isolated unless mentioned otherwise. [b] 7.5 mol% of
cat. 3. [c] Yield corresponds to unsaturated alcohol. [d] 5 mol% of cat. 3 per
equiv of ester moiety and 123 equiv of EtOH (3600 µL) were used. [e]
Substrate (0.25 mmol of basic unit of the polymer), 5 mol% of cat. 3 per equiv
of ester moiety is used.
It is also possible to reduce lactones using this method.
When 6- and 7-membered lactones were tested under our base
free protocol, the corresponding diols were obtained with very
high yields (Scheme 3, 24a, 25a). α-Angelica lactone (26a), a 5-
membered unsaturated lactone that can be derived from sugars,
underwent reduction of only the double bond whereas the ester
function remained intact (ESI, S6). However, it was possible to
obtain the corresponding diols from the 5 membered ring
lactones when 25 mol% of EtONa was added to the reaction
mixture. As representative examples, α-Angelica lactone (26a)
and Whiskey lactone (27a) afforded the desired diols in >99%
and 70% isolated yields respectively (Scheme, entries 4-5). We
assume that the function of the base is to enable the ring
opening of the lactone to the hydroxy-ester, which is the entity
that is reduced to the diol. Another possible explanation for the
role of the base is that hydrogenation of the ester enolate takes
place as was recently proposed by Milstein and co-workers in
the cobalt-pincer catalysed hydrogenation of esters.[19]
Gratifyingly, aliphatic esters could be reduced in good yields
in the same fashion (Scheme 2). For instance, methyl hexanoate
(17a) was reduced to 1-hexanol with an isolated yield of 77%.
The potential and relevance of adipic acid dimethyl ester (22a),
methyl levulinate (18a) and methyl oleate (19a) for renewable
chemistry is very well known.[13] For this reason, their
applications have been studied by many groups in both
academia and industry in the last decades. Few catalytic
systems in the literature are reported to reduce methyl levulinate
(18a) to 1,4 pentandiol.[14] We were glad to see how methyl
levulinate was reduced to the corresponding diol in almost
quantitative yield (Scheme 2, 18a). Adipic acid dimethyl ester
(22a) was converted to the diol in 99% yield. Methyl oleate (19a)
was converted to the unsaturated alcohol in 92% yield. Isolated
double bonds in the substrates remained untouched.
In conclusion, we have developed the very first Fe-catalyzed
base-free transfer hydrogenation of esters. The protocol uses
environmentally friendly and renewable EtOH as hydrogen
donor and mild conditions. The scope includes aromatic esters,
aliphatic esters and lactones. Important substrates stemming
from renewable resources like methyl oleate and α-Angelica
Another important topic for green chemistry is the recycling
of polymers via conversion to the monomers.[15] Interest on this
will keep growing in the following years because of accumulation
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COMMUNICATION
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pentanediol in a one pot reaction. Importantly, we were able to
depolymerize
a polyester to the diol using a transfer
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General procedure for transfer hydrogenations
In an overnight dried 5 mL glass pressure tube equipped with a stirring
bar, the ester (0.5 mmol) was dissolved in EtOH (2800 µL, 96 equiv)
under argon atmosphere. Dodecane as internal standard was added (60
µL) followed by Fe catalyst 3 (9,8 mg, 5mol%). The tube was sealed and
covered from light. The reaction was stirred vigorously for 24 hours at
100 ºC. After this time, the pressure tube was cooled to room
temperature, the reaction mixture was filtered over a pad of silica and
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cyclohexane:AcOEt, ratio depending on the substrate). For monitoring, a
stainless steel autoclave equipped with an auto sampler was used and
the reaction was scaled up 3 times. The rest of the procedure is identical.
Acknowledgements
We thank the State of Mecklenburg-Vorpommern for funding.
Conflict of Interest
The authors declare no conflict of interest
Keywords: base-free • transfer hydrogenation • Iron • ester
compounds • ethanol
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COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Even plastic can be reduced!
R.A. Farrar-Tobar, B. Wozniak, Dr. A.
Savini, Dr. S. Hinze, Dr. S. Tin, Prof. Dr.
J.G. de Vries*
Transfer hydrogenation of esters and
lactones with EtOH can be effected
using 5 mol% of Fe-MACHO-BH as
catalyst. This catalyst does not need
any base activation, thus resulting in
very high selectivities.
Page No. – Page No.
Base-Free Iron Catalyzed Transfer
Hydrogenation of Esters Using EtOH
as Hydrogen Source
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