Rhodium-Catalyzed Reductive Esterification Using Carbon Monoxide as a Reducing Agent

Article abstract of DOI:10.1002/ejoc.202000438

Carbon monoxide used to have a limited number of applications in organic chemistry, but it gradually increases its role as a mild and selective reducing agent. It can be applied for the carbon–heteroatom single bond formation via the reductive addition of hydrogen-containing nucleophiles to carbonyl compounds. In this paper, rhodium-catalyzed reductive esterification is described, and a comparative study of the rhodium and ruthenium catalysis in the reductive addition reactions is provided. Rhodium performs better on highly nucleophilic substrates and ruthenium is better for compounds with less nucleophilicity.

Full text of DOI:10.1002/ejoc.202000438

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Rhodium-catalyzed reductive esterification using carbon
monoxide as a reducing agent
Authors: Vladimir S. Ostrovskii, Sofiya A. Runikhina, Oleg I. Afanasyev,
and Denis Chusov
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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to this Accepted Article as a result of editing. Readers should obtain
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202000438
Link to VoR: https://doi.org/10.1002/ejoc.202000438
01/2020

10.1002/ejoc.202000438
European Journal of Organic Chemistry
COMMUNICATION
Rhodium-catalyzed reductive esterification using carbon
monoxide as a reducing agent
Vladimir S. Ostrovskii,[a] Sofiya A. Runikhina,[a][‡] Oleg I. Afanasyev,[a][‡] and Denis Chusov *[a]
[a]
V.S. Ostrovskii, Dr. S.A. Runikhina, Dr. O.I. Afanasyev, Dr. D. Chusov
A.N.Nesmeyanov Institute of Organoelement Compounds of Russian Academy of Sciences
119991, Vavilova St. 28, Moscow, Russian Federation
E-mail: denis.chusov@gmail.com
[‡]
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the document.
Abstract: Carbon monoxide used to have a limited number of
applications in organic chemistry, but it gradually increases its role as
a mild and selective reducing agent. It can be applied for the carbon-
heteroatom single bond formation via the reductive addition of
hydrogen-containing nucleophiles to carbonyl compounds. In this
paper, rhodium-catalyzed reductive esterification is described, and a
comparative study of the rhodium and ruthenium catalysis in the
reductive addition reactions is provided. Rhodium performs better on
highly nucleophilic substrates and ruthenium is better for compounds
with less nucleophilicity.
special production, and the availability of this reagent is very high
on an industrial scale. Some groups are focused on the usage of
carbon monoxide as a reducing agent in organic transformations.
For example, prof. Ragaini group has a great experience in a nitro
group reduction with carbon monoxide [5] (Scheme 1). Nitro group
can be reduced to the nitrenoid intermediate which is highly
reactive and can interact with a great variety of nucleophiles and
electrophiles forming ureas, carbamates, different nitrogen-
containing heterocycles and so on.
Target formation of carbon-carbon and carbon-heteroatom bonds
has particular importance for the complex molecules preparation.
Multiple efforts of scientific groups all over the world are focused
on this challenge.[1] The ideal synthetic approach should allow
building the molecular skeleton by selective modification of the
target position in the molecule with high atom efficiency and
selectivity.[2] At the same time, reagents and catalysts that are
used to achieve the transformation should be cheap, stable, and
readily available.[3]
A
comprehensive part of organic reactions is reductive
transformations. The choice of the proper reducing agent plays a
crucial role in influencing the reaction outcome. However,
deciding on the most appropriate reducing agent for the particular
process, we should take into consideration not only the yield of
the target transformation but also the whole efficiency of the
process, including the production of the reducing agent. For
example, molecular hydrogen is a conventional reducing agent
that seems to be highly atom-economical. Usually, both hydrogen
atoms from the molecule of this gas are included in the structure
of the reaction product. However, molecular hydrogen requires
two-step high-temperature production from natural gas. Another
problem with molecular hydrogen is the low selectivity of reduction
using simple catalysts. For example, hydrogen with palladium on
carbon cannot tolerate even aryl chlorides at room
temperature.[4a,c] It was also demonstrated that hydrogen on
rhodium chloride can reduce aromatic ring to cycloalkane.[4b]
Besides, hydrogen is incompatible with some protecting groups
like Cbz. It is possible to achieve higher selectivity of reduction
using complex catalysts. However, this implies their synthesis,
often multistep, and it decreases the overall efficiency and
increases the price of the whole process.
Scheme 1. Selected examples of carbon monoxide
transformations of the nitro group.
– driven reductive
Another example of the application of carbon monoxide as a
reducing agent is the water-gas shift reaction (WGSR). A review
of this field was made by prof. Denmark et al.[6] Generally
CO+H2O mixture is considered as
a hydrogen synthetic
equivalent, and carbon monoxide with water on a metal catalyst
is used as a convenient reducing agent, allowing to form single
bonds via reductive coupling reactions.[7] For example, carbon-
carbon bonds can be made as a result of the reductive
Knoevenagel condensation (Scheme 2).[8]
In this context, the use of carbon monoxide in organic chemistry
seems to be very attractive. It is a multiton byproduct of steel
manufacturing, so, in contrast to hydrogen, it does not require
1
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10.1002/ejoc.202000438
European Journal of Organic Chemistry
COMMUNICATION
Table 1. Comparison of rhodium and ruthenium catalysis in the reductive
addition reactions
Nucleophile
Average TON
Comment
Ref.
Scheme 2. Water-gas shift reaction and reductive Knoevenagel condensation
using WGSR power.
Rh-cat.
Ru-cat.
175
High
nucleophilicity
[Rh] > [Ru]
[9a,10b]
255
42
In 2014 reductive addition without an external hydrogen source
was discovered in our group (scheme 3).[9] The unique
deoxygenating ability of carbon monoxide in reactions between
H-nucleophiles and carbonyl compounds is utilized under this
approach. It does not require any external hydrogen source
including water additives: carbon monoxide acts as an oxygen
scavenger from alcoholic intermediates, forming in the reaction of
nucleophiles with carbonyl compounds (Scheme 3).
Low
nucleophilicity
[Rh] < [Ru]
[11a, 11b]
81
70
[12], this
work
?
The average TON for the ruthenium-catalyzed reductive
esterification is 70.[12] We can anticipate that carboxylic acids
behavior in such reactions should be similar to that of amines and
amides as the O-H bond is chemically similar to N-H. Up to this
moment, no information about rhodium catalysis in this process
was published. To understand the fundamental trends for such
transformations we studied the rhodium catalysis in the reductive
esterification thus expanding the understanding of the titled
reaction.
Based on our previous experience we chose different rhodium
complexes to check their catalytic activity and compared with it in
case of other known rhodium-catalyzed reductive additions (figure
1). The tested species include Rh(I), Rh(II) and Rh(III) compounds
with ligands with different types of coordination.
Scheme 3. Reductive addition without an external hydrogen source.
This approach was applied to different nucleophiles: reductive
amination,[10] reductive amidation,[11] reductive esterification[12]
reactions, and some other transformations with C-nucleophiles[13]
These reactions are very attractive from the atom- and step-
economical point of view as they allow the preparation of complex
amines, amides, and esters directly from aldehydes and ketones
.
Selected optimization results are provided in table 2, full data is
provided in SI. Rhodium (III) with strong ligands like Cp* (1a) is
inactive in this process possibly due to the high stability of the
ligand environment (table 2, entry 1). It corresponds to the data
for the reductive amination.[10a]
–
widespread industrially available starting molecules. For
example, a standard approach for ester preparation usually
implies classical esterification reaction between carboxylic acids
and alcohols. However, alcohols are often prepared from the
corresponding carbonyl compounds by reduction. So, this
approach allows saving at least one step in the synthesis.
Up to this moment, comprehensive knowledge about this type of
transformations was collected, and herein the general trends in
the catalyst structure-activity relationships were summed up. The
average TON for rhodium- and ruthenium-catalyzed reductive
amination and amidation reactions to compare the performance
of these two metals were calculated (Table 1). In these
calculations, we used an average TON for substrates of similar
types only for preparative yields of products (>60%). It is evident
from the comparison that rhodium is more active in the case of
the reductive amination, but it is twice less active than ruthenium
in the case of the reductive amidation. Therefore, rhodium
catalysts work well for substrates with high nucleophilicity, while
ruthenium performs better for the less nucleophilic compounds.
Figure 1. Rhodium catalysts, tested in the reductive esterification reaction.
2
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10.1002/ejoc.202000438
European Journal of Organic Chemistry
COMMUNICATION
Table 2. Optimization of the reaction conditions. Full data is provided in SI.
working, however, this complex has low catalytic activity (Table 2,
entry 2). Complexes 1c and 1d have weaker ligands compared to
1a, thus increasing the catalytic activity (entries 3 and 4).
Cyclobutadiene rhodium complex 1d can be considered as one of
the most active reductive amination catalysts.[10a] However, in the
case of the reductive esterification, its activity is not superior to 1c.
Rhodium (II) trifluoroacetate 1e was shown to be less active than
rhodium acetate 1f, demonstrating the importance of electronic
properties of ligands. This trend is opposite to that in the case of
reductive amination, where 1e is more active than 1f. The best
performance was found in the case of simple rhodium (III) chloride
1g that was chosen for further optimization (entry 7).
Temperature optimization has shown that this reaction starts at
130°C, but the optimal temperature is 160°C (entries 7-10).
Increasing the temperature to 180°C leads to a drop of the yield.
This fact requires additional comment. The main constituents of
the reaction mixture are the product (target ester), starting
aldehyde (in case of low conversion) and the corresponding
toluene formed due to the deoxygenation of the carbonyl
compound (Scheme 5). Carbon monoxide in combination with
water can lead to the reduction of aldehyde to alcohol followed by
deoxygenation with the toluene formation. In the case of the low
temperature, the reaction mixture mainly consists of the starting
aldehyde. Too high temperature leads to the preferential
formation of the corresponding toluene. Some other factors also
influence this balance, e.g. donating properties of the substituents
in the aldehyde. They will be discussed below with the substrate
scope.
Entry[a]
1
Rh-cat
1a
Temperature
130°C
130°C
130°C
130°C
130°C
130°C
130°C
110°C
160°C
180°C
160°C
160°C
160°C
160°C
160°C
160°C
160°C
160°C
160°C
160°C
Solvent
THF
4aa Yield [b]
0%
2
1b
1c
THF
7%
3
THF
7%
4
1d
1e
THF
6%
5
THF
5%
6
1f
THF
15%
7
1g
1g
1g
1g
1g
1g
1g
1g
1g
1g
1g
1g
1g
1g
THF
31%
8
THF
Traces
66%
9
THF
10
11
12
13
14
15
16
17
18
19
20
THF
31%
EtOAc
PhMe
EtOH
MeCN
Neat
62%
62%
40%
0%
52%
PhMe
PhMe
PhMe
PhMe
PhMe
37%[c]
21%[e]
14%[e]
58%[f]
70%[f, g]
[a] 0.25 mmol scale. See SI. [b] NMR yields with dinitrobenzene
as internal standard [c] without water. [d] 1 eq. of water. [e] 20 eq.
of water. [f] 5 eq. of 3a. [g] 48 h.
Scheme 5. Main by-process of the reductive esterification
The solvent screening revealed that tetrahydrofuran, ethyl acetate,
and toluene fits this reaction almost equally (entries 9, 11, 12),
while protic solvents like ethanol decrease the yield (entry 13).
Besides, the reaction can proceed without any solvent (entry 15),
but the efficiency is lower. Water additive is essential for this
process and has a nonlinear effect on the reaction outcome.
Reaction without water leads to the 37% yield (entry 16); the
addition of 1 equivalent of water drops the yield almost twice
(entry 17). Further increasing of the water content in the reaction
mixture firstly leads to increasing of the yield to 62% (5 equiv.,
entry 12) followed by the reduction of the yield to 14% (20 equiv.,
entry 18). Without water, the reaction likely works via reductive
addition without an external hydrogen source mechanism
(scheme 3) that does not require any water. Five equivalents of
water seem to be enough to run this process via the WGSR
Scheme 4. η5-η3 isomerization of indenyl-rhodium complex
Indenyl complex 1b was found to be active in the reductive
amination reaction. It was accounted for η5-η3 isomerization,
which can open the coordination slot on rhodium thus increasing
the catalytic activity (scheme 4).[14] The reductive esterification
also might be catalyzed by the indenyl complex, the idea is
3
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10.1002/ejoc.202000438
European Journal of Organic Chemistry
COMMUNICATION
mechanism. One equivalent of water disturbs the reductive
addition without an external hydrogen source and does not
provide enough hydrogen via WGSR, so the yield drops
dramatically. However, in the case of the same substrate
synthesized using a ruthenium catalyst, no critical influence of the
water was detected.[12] So we can conclude that rhodium
facilitates WGSR more efficiently while ruthenium – reductive
addition without an external hydrogen source depicted on
Scheme 3. Decreasing the acetic acid amount slightly decreases
the yield, but it can be increased by prolongation of the reaction
time (entries 19, 20).
With the optimized reaction conditions in hands, we started the
substrate scope investigation (Scheme 6). We found that this
protocol leads to good yields in the case of para-substituted
benzaldehydes with weak electron-donating or electron-
withdrawing
substituents
(4ad,
4aa).
Meta-substituted
benzaldehydes lead to good yields in all cases (4ab, 4ae) except
for the strong electron-withdrawing substituents. Strong electron-
withdrawing groups provide a high extent of reduction of aldehyde
to the corresponding alcohol, thus decreasing the yield of the
target product (4ag). Strong electron-donating groups in the para-
position provide a decreased yield due to the high extent of the
carbonyl compound deoxygenation (4an) (deoxygenation is
depicted on the Scheme 5). The substituents, providing a sterical
hindrance near the carbonyl group, have a similar effect. Ortho-
substituted benzaldehydes react with lower yields in comparison
to the meta- and para-analogs (4ac vs. 4ab; 4af).
This effect was also noted for the naphthaldehydes esterification.
2-naphthaldehyde lead to the corresponding ester 4ak with 78%
yield, while 1-naphthaldehyde gave 4aj in a 66% yield. In the last
case a 1-methylnaphthalene, product of the reduction of the 1-
naphthaldehyde followed by deoxygenation, was detected.
Antracenecarbaldehyde did not react under these conditions –
only starting molecule and the product of deoxygenation were
detected in the reaction mixture. A strong electron-donating group
in 6-methoxynaphthaldehyde leads to the deoxygenation of the
starting molecule during the reaction, causing the formation of 4al
in a low yield. Aliphatic aldehydes are also applicable in this
reaction. Products 4ah and 4ai were prepared in good yields.
Different carboxylic acids were tested (4bk, 4ck, 4ba, 4ca, 4da)
and it was shown that even stearic acid could be used in this
reaction (4da) while with the low yield possibly due to the long
hydrophobic alkyl group together with the hydrophilic carboxylic
group. It could block the catalytically active specie inside micelles
thus stopping the reaction.
In conclusion, rhodium-catalyzed reductive esterification protocol
for esters synthesis directly from carboxylic acids and carbonyl
compounds was developed. Different types of aldehydes and
carboxylic acids were applied to show the scope of this approach.
General trends in the reactivity of aldehydes were noted: strong
electron-donating or bulky groups in the benzaldehydes lead to
deoxygenation of carbonyl compounds. The average TON for
rhodium-catalyzed reductive esterification (calculated for all
substrates in this paper with preparative yield) is 67, which is
almost the same as in the case of ruthenium (70). Therefore, in
contrast to the reductive amination and reductive amidation,
rhodium and ruthenium catalysts have similar activity in the
reductive esterification (Scheme 7). The ruthenium-catalyzed
esterification is better in the case of ortho-substituted aldehydes
and aldehydes with electron-withdrawing groups. In the case of
the rhodium catalysis, the reductive esterification utilizing the
WGSR power seems to be more efficient than the reductive
esterification without an external hydrogen source, while for
catalysis by ruthenium no big difference between this two types of
mechanisms was found.
Scheme 6. The substrate scope for the rhodium-catalyzed reductive
esterification. NMR yields (with internal standard). Isolated yields are provided
in parentheses. Details are provided in SI. [a] Compounds 4al, 4am, and 4an
were not isolated due to the low yield. [b] reaction without solvent.
4
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European Journal of Organic Chemistry
COMMUNICATION
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Experimental section
General procedure for esterification: A glass vial in a 10 mL
stainless steel autoclave was charged with 1 mol% of the catalyst,
the corresponding solvent, 500 mol% of the carboxylic acid, 500
mol% of water and 100 mol% of the aldehyde. The autoclave was
sealed, flushed with 10 atm of CO, and charged with 30 bar of CO.
The reactor was placed into a preheated oil bath. After the
indicated time, the reactor was cooled to room temperature and
depressurized. Metal residues were removed by flash
chromatography on silica gel using dichloromethane as an eluent
to get NMR yield. Isolation of pure product was achieved by
chromatography.
[8]
[9]
[10]
[11]
[12]
[13]
[14]
Acknowledgements
The work was financially supported by the Russian Science
Foundation (grant # 16-13-10393). NMR studies were performed
with the financial support from the Ministry of Science and Higher
Education of the Russian Federation using the equipment of the
Center for molecular composition studies of INEOS RAS.
Keywords: reductive esterification • carbon monoxide •
reduction • ester synthesis • reductive addition
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European Journal of Organic Chemistry
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Entry for the Table of Contents
Rhodium-catalyzed reductive esterification of carboxylic acids with carbonyl compounds was developed. This protocol fits a variety of
carboxylic acids and aldehydes, opening the short way from industrial bulk compounds to valuable esters.
6
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