Cascade Synthesis of Pyrroles from Nitroarenes with Benign Reductants Using a Heterogeneous Cobalt Catalyst

Article abstract of DOI:10.1002/anie.202007613

A bifunctional 3d-metal catalyst for the cascade synthesis of diverse pyrroles from nitroarenes is presented. The optimal catalytic system Co/NGr-C@SiO₂-L is obtained by pyrolysis of a cobalt-impregnated composite followed by subsequent selective leaching. In the presence of this material, (transfer) hydrogenation of easily available nitroarenes and subsequent Paal–Knorr/Clauson-Kass condensation provides >40 pyrroles in good to high yields using dihydrogen, formic acid, or a CO/H₂O mixture (WGSR conditions) as reductant. In addition to the favorable step economy, this straightforward domino process does not require any solvents or external co-catalysts. The general synthetic utility of this methodology was demonstrated on a variety of functionalized substrates including the preparation of biologically active and pharmaceutically relevant compounds, for example, (+)-Isamoltane.

Full text of DOI:10.1002/anie.202007613

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Accepted Article
Title: Cascade Synthesis of Pyrroles from Nitroarenes with Benign
Reductants Using a Heterogeneous Cobalt Catalyst
Authors: Pavel Ryabchuk, Thomas Leischner, Anke Spannenberg,
Kathrin Junge, and Matthias Beller
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Direct and Selective Synthesis of Adipic and Other Dicarboxylic
Acids by Palladium-Catalyzed Carbonylation of Allylic Alcohols
Ji Yang, Jiawang Liu, Yao Ge, Weiheng Huang, Helfried Neumann, Ralf Jackstell, and Matthias Beller*
Dr. J. Yang, Dr. J. Liu, Dr. Y. Ge, W. Huang, Dr. H. Neumann, Dr. R. Jackstell, Prof. Dr. M. Beller
Leibniz-Institut für Katalyse e. V.
Albert-Einstein-Straße 29a,18059 Rostock (Germany)
E-mail: matthias.beller@catalysis.de

These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the document.
Abstract: A general and direct synthesis of dicarboxylic acids
including industrially important adipic acid by palladium-catalyzed
dicarbonylation of allylic alcohol is reported. Specifically, the
combination of PdCl2 and a bisphosphine ligand (HeMaRaphos)
promotes two different carbonylation reactions with high activity and
excellent selectivity.
from the biosynthesis of terpenes. Therefore, the
(mono)carbonylation of allylic alcohols has attracted significant
attention from both academic and industrial researchers.[5]
Despite these interesting works, to the best of our knowledge,
there is no report on direct and selective synthesis of linear
dicarboxylic acids through dicarbonylation of allylic alcohols.
Aliphatic dicarboxylic acids constitute important intermediates in
the chemical industry.[1] Specifically, they are used on large scale
in the preparation of copolymers such as polyamides and
polyesters. In this respect, the most prominent example is adipic
acid (AA), which is applied for the synthesis of Nylon-6,6. In 2018,
the global production of adipic acid was more than 3.0 billion kg´s
with a market value estimated at approximate $6.0 billion.[2] In
addition, other dicarboxylic acids including naturally occurring
aspartic acid and glutamic acid find use for the production of
polyester and polyurethane resins, plasticizer, and additives.
Apart from these industrial applications, dicarboxylic acids have
been widely used as versatile building blocks in organic chemistry.
The main routes for the synthesis of dicarboxylic acids either
involve selective oxidation reactions, e.g. of alcohols (Scheme
1A) or depend on multiple carbonylations of dienes (Scheme 1C).
For example in case of AA, the industrial production mainly relies
on the catalytic oxidation of a mixture of cyclohexanol and
cyclohexanone (so-called KA oil) with an excess of HNO3 in the
presence of copper(II) and ammonium metavanadate catalysts. A
serious drawback of such oxidation processes is the formation of
the greenhouse gas nitrous oxide (N2O), the latter accounting for
approximately 5-8% of the worldwide anthropogenic N2O
emissions. Alternatively, oxidation of cyclic olefins in a biphasic
system using 30% H2O2 in the presence of Na2WO4 and [CH3(n-
C8H17)3N]HSO4 leads to the desired dicarboxylic acids.[3]
However, the industrial applicability of these processes is so far
limited. Another approach for the preparation of dicarboxylic acids
is based on the (di)carbonylation of the respective dienes. In fact,
several industrial companies tried to develop processes for AA
starting from 1,3-butadiene.[4] Due to the importance of aliphatic
dicarboxylic acids, there exists strong interest to find more general
and affordable routes for their preparation. Preferably, such
methodologies should be environmentally benign, atom-efficient
and based on renewable feedstock. In this respect, the
dicarbonylation of allylic alcohols is an ideal method. From a
practical perspective, a variety of substrates such as the parent
compound, crotyl alcohol, prenol, cinnamyl alcohol, and many
more including several vitamin intermediates are produced in
industry. Furthermore, natural derivatives are easily available
Scheme 1. Summary of the main synthesis methods of dicarboxylic acids
Based on our previous work of palladium-catalyzed
carbonylations with “built-in-base ligands”,[6] we became
interested in the ability of these catalysts for the cascade
synthesis of dicarboxylic acids from unsaturated alcohols.
Following this idea, herein we report a specific palladium catalyst
system for the synthesis of adipic and other (linear) dicarboxylic
acids with high yields and selectivities (Scheme 1D).
We started our investigations using the carbonylation of crotyl
alcohol 1a with water as a benchmark reaction. As shown in
Figure 1, via selective dicarbonylation of 1a it should be possible
to obtain directly adipic acid 3a. However, in this reaction a variety
of other carbonylated products will be formed more easily, e.g. the
mono-carbonylation product 5a and its various isomers as well as
isomeric dicarbonylation products 3aa and 3ab. Moreover,
isomerization of 1a to butyraldehyde has been reported in the
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presence of organometallic catalysts.[7] Clearly, to achieve the
desired target product 3a, the active palladium complex has to
catalyze efficiently a sequential carbonylation-isomerization-
carbonylation process, which was not realized yet.
butylphosphine units are replaced with more stable di-
phenylphosphine or di-adamantyl units did not give better results.
In order to improve the activity of the catalyst system towards the
formation of diacids, ligand L10 with 2-pyridyl-tert-butylphosphino
groups was applied,[12] the catalytic activity was significantly
improved and the yield of diacid products increased to 41%;
however, the selectivity for linear diacid decreased somehow
(72%). When testing our recently introduced ligand HeMaRaphos
(L11),[13] both the yield of di-acid products (56%), and the
regioselectivity for adipic acid (92%) are further improved. Among
all the ligands tested, HeMaRaphos exhibited best catalytic
activity for the conversion of the intermediate pentenoic acid 5a to
adipic acid.
To optimize the benchmark reaction further on, we evaluated the
influence of critical reaction parameters in the presence of L11.
By variation of catalyst precursor, acidic co-catalyst, temperature,
pressure, etc. the yield of C6-diacids reached 69% with a
selectivity of 93% for adipic acid (see SI; Tables S1-S8). In order
to verify the practicability of this method, we tested the reactivity
of the optimal catalyst system at lower catalyst loading (0.1 mol %
Pd). Using 1 mol of 3-penten-2-ol, the reaction proceeded
smoothly as expected, and we obtained 116 g of adipic acid
(>97% linear selectivity; 80% isolated yield) after recrystallization
(Scheme 2).
With good reaction conditions established for the synthesis of
adipic acid, we examined the generality and limitations of this
novel process applying different allylic alcohols. As shown in
Table 1, diverse aliphatic allylic alcohols gave the corresponding
carbonylative products in good to high yields with excellent
chemo- and regioselectivities. The parent compound allyl alcohol
1b gave glutaric acid 3b with 73% yield and 98% regioselectivity.
Using other representative allylic alcohols 1c-1o, the
corresponding linear dicarboxylic acids are obtained in 93-60%
yield. Notably, isomerization of the double bond is comparably
fast under these conditions. Even, 2-dodecen-1-ol 1h led to the
linear 1,14-tetradecanedioic acid 3h with 83% regioselectivity,
which requires a selective isomerization-functionalization process
involving 9 C-C bonds. Compared to primary allyl alcohols,
increased product yields are observed using secondary allyl
alcohols 1i-1n. Here, the catalytic system exhibits good reactivity
(about 90% yield) and selectivities (86-95%) at the same time.
Next, a variety of aromatic allylic alcohols 1q-2b were tested.
Such compounds can be easily obtained by alkenylation of the
respective carbonyl compounds.[14] It should be noted that the
catalyst is compatible with several electron-withdrawing or
electron-donating groups including halogens (F, Cl, Br), -OMe, -
SMe and OPh. Interestingly, the pinacolborate 1y is
dicarbonylated while the C-B bond stayed intact. Furthermore,
annulated substrates are also compatible with this protocol.
To explore the reaction mechanism in more detail, the
dicarbonylation of 2d was monitored under standard conditions.
This substrate was used because of its convenient handling and
the characterization of the corresponding intermediates.
Figure 1. Pd-catalyzed dicarbonylation of crotyl alcohol 1a: Influence of
phosphine ligands.
Apart from the chemoselectivity, the regioselectivity of the
individual carbonylation steps and a fast isomerization process
under the reaction conditions are highly challenging. Obviously,
the choice of ligand is crucial for controlling the different selectivity
aspects and the activity of the catalyst. Thus, we compared
standard mono- and bidentate phosphine ligands (4 or 2 mol%,
respectively) with own previously developed phosphines for the
carbonylation of 1a in the presence of 1 mol% Pd(TFA)2 and 4-8
mol% PTSA·H2O as acid co-catalyst in THF at 120 oC. Using the
classic ligand triphenylphosphine L1, only small amounts (5%) of
isomeric monocarbonylation products 5a were obtained. Similarly,
for diphenyl(2-pyridyl)-phosphine L2,[8] which was introduced by
Drent et al. for highly efficient methoxycarbonylation of alkynes,
only a small amount of 5a (8%) was obtained. Xantphos L3[9]
exhibited enhanced activity, but the main products were isomers
of 5a. Other commonly used bisphosphine ligands, such as DPPP
L4, DPPB L5 and BINAP L6, did not exhibit any catalytic activity.
Interestingly, when testing 1,2-bis[(di-tert-butyl)phosphino-
methyl]benzene (L7, dtbpx),[10] which is used by Lucite
International to produce methyl methacrylate and was also
employed by Drent for carbonylation of 1,3-butadiene,[11] the
major product is still 5a; however, in addition 17% of C6-diacids
are achieved with high selectivity (95%) for linear adipic acid.
Unfortunately, using ligands L8 and L9 in which the di-tert-
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Table 1. Pd-catalyzed selective dicarbonylation of allylic alcohols with
water.[a]
Yield
[%]
Select.
n/iso
Entry
1
Allylic alcohol
Di-acid
69
73
93/7
98/2
14
63[b]
92/8
2
3
61
94/6
15
16
77
78
92/8
92/8
4
5
70
65
65
63
60
93/7
90/10
90/10
90/10
83/17
90
86/14
93
91
95/5
95/5
[a] Standard reaction conditions: allylic alcohols (1.0 mmol), PdCl2 (1.0
mol%), ligand (2.0 mol%), PTSA·H2O (4.0 mol%), H2O (0.2 mL, 11 equi.),
CO (40 atm), THF (2.0 mL), 24 h at 125 oC; [b] H2O (2 equi.), 90 oC.
6
7
As shown in Scheme 3A an immediate reaction to give a mixture
of various dienes is observed. Here, at least five different dienes
could be detected by GC (see SI; Figure S2). Already after half
an hour around 60% conversion is achieved and formation of the
monocarboxylic acid takes place achieving a maximum yield after
3 h. Notably, in the first 1,5 h no desired dicarboxylic acid is
observed. On the basis of these findings and previous
91
78
94/6
8
90/10
mechanistic
studies
of
Pd-catalyzed
alkoxy-
9
carbonylations,[5o,5q,5r,12] we propose the following main reaction
pathway. Initially, the stable Pd(II) salt is in situ reduced to give
Pd(0) phosphine complexes A1 in the presence of excess amount
of ligand.[12,15] After protonation, the active complex A2 is afforded.
Probably, this palladium hydride species is in equilibrium with the
N-protonated pyridinium complex.
86
83
91/9
86/14
10
63
86
98/2
95/5
Next, complex A2 is supposed to react with 2d to form the -
allyl-palladium intermediate B2, which can be carbonylated, but is
also prone to β-H elimination to form diene 6d. Notably, small
amounts of 6d can be observed by GC under acidic conditions
without palladium catalyst present (see SI; Scheme S4). Hence,
we cannot fully exclude an initial reaction of this diene to form B2,
although this seems unlikely due to the low initial concentration of
6d. As shown previously for 1,3-diene telomerization reactions,
B2 might be in a fast equilibrium with the corresponding -
palladium complexes.[16] Notably, further reaction of diene 6d and
palladium complex A2 forms the allyl-palladium intermediate B4.
In this way isomerization of the double bonds occurs and various
diene isomers can be formed. On the other hand, the allyl-
palladium complex B4 after CO coordination and insertion gives
the acyl Pd complex C. N-assisted nucleophilic attack of
intermediate C by water via transition state D provides the
11
12
83
94/6
13
88
63
88
89
86
83
86
92/8
92/8
91/9
85/15
92/8
92/8
92/8
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10.1002/anie.202007613
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monocarbonylated intermediate 5d. At this point, the
multifunctional role of the 2-pyridyl moiety in HeMaRaphos is
helpful. On the one hand, this integrated base acts as a proton
shuttle for the formation of the palladium hydride and the N-
coordination to the palladium center in the catalytic cycle. After
cycle II, the resulting intermediate 5d undergoes rapid
isomerization in the presence of the Pd-H catalyst A2.
Consistently, isomerization carbonylation of internal olefin has
been studied in detail by palladium catalyst.[17] Here, in a similar
catalytic process, the unsaturated alkenyl acids, which contain
terminal double bonds, are preferentially carbonylated according
to cycle III.
assisted hydrolysis. On the other hand, the nitrogen atom is
beneficial to improve the durability of the catalyst via hemilabile
transformation is highlighted by the synthesis of adipic acid
on >100 g-scale.
Acknowledgements
We thank Prof. Dr. Eite Drent for interesting discussions and
suggestions. We also thank Dr. Haoquan Li, Prof. Dr. Qiang Liu
and Dr. Fei Ye for their helpful suggestions. The author Ji Yang
thanks CSC Scholarship for financial support (201704910879).
According to this mechanistic proposal different isomers of allylic
alcohols (and even allylic alcohols and dienes) should give the
same acid product. Indeed, this is the case as shown be the
dicarbonylation of 1d, 1j, and 1k. Inspired by this observation, we
investigated the reaction of a mixture of seven isomeric C6 allylic
alcohols. To our delight, 1,8-octandioc acid (suberic acid) was
obtained with an 81% yield and 93% regioselectivity (Scheme 4).
Keywords: Carboxylic acids • adipic acid • palladium •
carbonylation • allylic alcohol • catalysis
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Scheme 4. The reaction of a mixture of seven isomeric C6 allylic alcohols.
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substrates.
Conclusion
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(linear) dicarboxylic acids from allylic alcohols by a palladium-
catalyzed carbonylation-isomerization-carbonylation cascade
process. Only in the presence of the Pd/HeMaRaphos catalyst
system efficient catalytic isomerization and selective
dicarbonylation of diverse allylic alcohols including several
functionalized derivatives are achieved. The utility of this novel
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Entry for the Table of Contents
Ji Yang, Jiawang Liu, Yao Ge, Weiheng
Huang, Helfried Neumann, Ralf Jackstell
and Matthias Beller*
Page No. – Page No.
Direct and Selective Synthesis of
Adipic and Other Dicarboxylic Acids
by Palladium-Catalyzed Carbonylation
of Allylic Alcohols
Green process for producing dicarboxylic acids: A general direct and selective synthesis of adipic acid and other dicarboxylic acids
by palladium-catalyzed dicarbonylation of allylic alcohol is reported. Specifically, the combination of PdCl2 and a bisphosphine ligand
(HeMaRaphos) promotes two different carbonylation reactions as well as olefin isomerization with high activity.
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