Pd/C-catalyzed synthesis of oxamates by oxidative cross double carbonylation of alcohols and tertiary amines through C-N bond cleavage

Article abstract of DOI:10.1039/c9nj04156g

An efficient synthesis of oxamates by Pd/C-catalyzed oxidative cross double carbonylation of alcohols and unactivated tertiary amines has been demonstrated. The in situ oxidative C-N bond cleavage of tertiary amines was achieved using molecular oxygen as an environmentally benign oxidant providing a user-friendly approach to the synthesis of oxamates. The developed protocol showed excellent activity towards the cyclic as well as aliphatic tertiary amines and long-chain alcohols. This developed protocol system is advantageous, as it is phosphine ligand free, base free, and copper-free and the Pd/C catalyst is easily recyclable. The Pd/C catalyst was recycled up to five consecutive cycles.

Full text of DOI:10.1039/c9nj04156g

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Pd/C-catalyzed synthesis of oxamates by
oxidative cross double carbonylation of alcohols
and tertiary amines through C–N bond cleavage†
Cite this: New J. Chem., 2019,
43, 18072
Yuvraj A. Kolekar and Bhalchandra M. Bhanage
*
An efficient synthesis of oxamates by Pd/C-catalyzed oxidative cross double carbonylation of alcohols
and unactivated tertiary amines has been demonstrated. The in situ oxidative C–N bond cleavage of
tertiary amines was achieved using molecular oxygen as an environmentally benign oxidant providing a
user-friendly approach to the synthesis of oxamates. The developed protocol showed excellent activity
towards the cyclic as well as aliphatic tertiary amines and long-chain alcohols. This developed protocol
system is advantageous, as it is phosphine ligand free, base free, and copper-free and the Pd/C catalyst
is easily recyclable. The Pd/C catalyst was recycled up to five consecutive cycles.
Received 10th August 2019,
Accepted 16th October 2019
DOI: 10.1039/c9nj04156g
rsc.li/njc
Introduction
Oxamates and their derivatives are significant building blocks
in synthetic chemistry and their structural motifs have a wide-
spread application in pharmaceuticals, natural products and
agricultural chemicals (Scheme 1a).1–6 They also play a crucial
role in biologically active drugs, such as orally active antiallergic
agents and an antimalarial agent.7–10 The oxamates and their
derivatives are also used as N- and O-functional ligands in
organic transformations.11–13 Recently, the synthesis of glycol
derivatives has been demonstrated by the catalytic hydrogenation
of oxamates.14,15 Typically, those compounds are synthesized by
the acylation of amines with diesters of oxalic acid or monoester
of oxalyl chloride (Scheme 1b).16–19 However, these methods are
used stoichiometric reagents such as as phosgene and isocyanates,
which are highly toxic and corrosive chemicals, and have serious
effects on the environment as well as on human health. On the
other hand, they need equimolar quantity of oxalyl chloride and
alcohol and need a separate distillation assembly for the separation
of the products from the reaction mixture. However, in the past two
decades, several modifications have been made for the broadening
of the area of carbonylation for the synthesis of oxamates and their
derivatives. Recently, oxidative carbonylation has gained consider-
able attention from the economic point of view. Carbon monoxide
Scheme 1 Comparison of previous protocol with current protocol.
(CO) is one of the most resourceful and cost-efficient C1-source that processes becomes significant due to the fast decline of petroleum
is easily available from natural gas, biomass and coal. The direct and coal resources.20–23 Previous literature showed that there
conversion of CO into the high value-added chemicals by catalytic are several reports on oxidative carbonylation reactions, especially
palladium-catalyzed carbonylative cross-coupling of primary/
secondary amines with alcohols, for the synthesis of oxamates
Department of Chemistry, Institute of Chemical Technology, Mumbai-400019, India.
(Scheme 1c).24–28
E-mail: bm.bhanage@ictmumbai.edu.in, bm.bhanage@gmail.com;
Initially, in 1987, Murahashi and co-workers revealed oxamate
Fax: +91 22 33611020; Tel: +91 22 33612603
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nj04156g synthesis by the carbonylative coupling of secondary amines with
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alcohols using homogeneous palladium complexes and excess we optimized several non-polar as well as polar solvents along
base.26 Further, they reported the synthesis of cyclic as well as with alcohols (Table 1). The influence of 10 mL (3 : 7) mixture
simple oxamates using a modified Pd–Cu catalyst with molecular of EtOH : toluene, EtOH : THF, EtOH : 1,4-dioxane and EtOH :
oxygen as an effective oxidant. However, they required high CO MeCN was studied, which provided 12%, 20%, 25% and 60%
pressure (80 kg cmꢀ2) and also trimethoxymethane as a dehydrating yields of the oxamate, respectively (Table 1, entries 2–5). However,
agent with triethylamine as a base.27 Subsequently, Alper and acetonitrile is a polar aprotic solvent, which is necessary for such
co-workers established a method for the synthesis of oxamates types of reactions. Polarity is required for the activation of the
using D-TBP (Di-tert-butyl Peroxide) as an oxidant in 1987.28 C–N bond and protic solvents affect the catalyst performance. We
However, they have faced several drawbacks including the use of found that the mixture solvent EtOH : MeCN with a ratio of 3 : 7
a hazardous oxidant, high pressure of CO, high catalyst and (mL) was highly effective compared to other solvents. To ensure a
ligand loading, and use of co-catalyst and base. This methodology higher yield, we further changed the EtOH : MeCN solvent ratio
produces a lot of side products, which leads to the decrease in such as 2 : 8 and 1 : 9 (10 mL) to obtain 81% and 95% yields of the
the atom economy. Our group also reported the synthesis of oxamate 3a, respectively (Table 1, entries 6 and 7). It is notable
oxamates using Pd/C and ionic liquid immobilized Pd as that the combination of EtOH: MeCN (1: 9) is the best solvent
heterogeneous catalysts without the need of a co-catalyst under and afforded 95% yield of the corresponding oxamate (Table 1,
base-free conditions.24,25 The cleavage of the highly stable C–N entry 7). Next, we investigated the influence of catalysts and
bond of tertiary amine is a challenging task.29–39
catalyst loading, and the results are explained in Table 1. It was
To the best of our knowledge, the synthesis of oxamates found that 10% Pd/C catalyst was a highly efficient catalyst
through the oxidative double carbonylation of tertiary amine compared to homogeneous PdCl2 and PdCl2(PPh3)2 complexes
with alcohol is not yet reported. This is due to the fact that the (Table 1, entries 8 and 9). Then, the effect of loading of the Pd/C
breaking of strong C–N bond of the highly stable tertiary amine catalyst was studied ranging from 3 to 10 mol% (Table 1, entries
and oxidative double carbonylation between the two nucleo- 10 and 11). The phosphine-free 10 mol% Pd/C provided an
philes are still a challenge. However, this method is routinely excellent yield of 3a. The additive plays an essential role in the
used in the laboratory due to its inherent features including not oxidative carbonylation reaction; in this regard, we further inves-
requiring an equimolar quantity of the reactants, not requiring tigated the effect of various additives on the model reaction.
the Lewis acid, low catalyst loading and wide substrate scope. When we used NaI as an additive, it was observed that 83% yield
However, a comprehensive literature survey reveals that there is of 3a was obtained, while TBAI provided 91% yield (Table 1,
no report on the synthesis of oxamates by the carbonylative entries 12 and 13). Next, to confirm the influence of the additive
cross-coupling of tertiary amines with alcohols.
in this transformation, we carried out the reaction without an
Herein, we report an efficient synthesis of oxamates through additive and failed to get the desired oxamate product (Table 1,
the oxidative cross double carbonylation of cyclic as well as aliphatic entry 14). Next, we moved our attention towards the effect of
tertiary amines with alcohols using Pd/C as a heterogeneous temperature and time on the model reaction. When we increased
recyclable catalyst and their carbon support provides a reasonable the reaction temperature from 100 1C to 120 1C, no significant
surface area and also showed redeposition ability.40 Reactions changes were observed in the yield of 3a (Table 1 entries 16 and 17).
were progressed without the use of the phosphine ligand co- However, when we decreased the temperature to 80 1C, only 62%
catalyst and base using O2 as an oxidant. Generally, phosphines yield of the product was obtained. Next, in the time study, it was
are not recommended under oxidative conditions because if they observed that on decreasing the reaction time from 24 to 20 h, the
dissociate, they are prone to oxidation forming phosphine oxides yield of the products also decreased to 77% (Table 1 entry 18).
and thus deactivating the catalyst. The catalyst was easily separable Hence, temperature and time study outcomes revealed that a
and could be recycled up to five runs.
temperature of 100 1C and duration of 24 h were required for the
completion of the reaction. Next, we performed the same reaction
without an oxidant; unfortunately, the reaction did not proceed
(Table 1, entry 19). Hence, we conclude that the additives and
the oxidants played a crucial role in this transformation. Under
Results and discussion
We begin the optimization reaction with triethylamine and 5 : 1 (6 bar) ratio of CO : O2 pressure, the reaction proceeded well
ethanol for the synthesis of oxamates as a model reaction. toward the expected product. So finally, the efficiently optimized
For this purpose, here we have used triethylamine 1a (1 mmol) reaction parameters are as follows: 1a (1 mmol), 2a (1 mL), Pd/C
as an in situ aminal source and ethyl alcohol 2a (10 mL) as a (10 mol%), EtOH : MeCN (1 : 9), CO : O2 (1 : 5 atm) at 100 1C for
solvent as well as nucleophile with 10 mol% Pd/C catalyst, and 24 h. With the optimized reaction condition in hand, next, we
KI as an additive under the atmospheric pressure of CO : O2 explore whether the current protocol could be applicable for the
(5 : 1 atm, 6 bar) at 100 1C for 24 h, which afforded only 18% synthesis of a range of oxamate derivatives (Table 2). Here, we
yield of the desired oxidative cross double carbonylative product have used Pd/C as a heterogeneous, recyclable catalyst along
3a. Given the unsatisfactory yield, we optimized the reaction with KI as an additive using molecular oxygen as an environmen-
conditions to enhance the yield of the oxamate product. Next, tally benign oxidant. Initially, we used triethylamine, tripropylamine
we moved the attention towards the study on the effects of and tri-n-butyl amines that underwent double carbonylation with
various solvents on the model reaction. Conversely, at first, ethyl alcohol to provide corresponding oxamates 3a, 3b and 3c,
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Table 1 Optimization reaction parameters for oxamate synthesisa,b
Entry
Solvent
Solvent ratio
[Pd] (10 mol%)
Additive
Temp. (1C)
Time (h)
Yieldb (%)
Effect of solvent
1
2
3
4
5
6
7
EtOH
EtOH : Toluene
EtOH : THF
EtOH : dioxane
EtOH : MeCN
EtOH : MeCN
EtOH : MeCN
10 mL
3 : 7
3 : 7
3 : 7
3 : 7
2 : 8
1 : 9
10% Pd/C
10% Pd/C
10% Pd/C
10% Pd/C
10% Pd/C
10% Pd/C
10% Pd/C
KI
KI
KI
KI
KI
KI
KI
100
100
100
100
100
100
100
24
24
24
24
24
24
24
18
12
20
25
60
81
95
Effect of catalyst and catalyst loading
8
EtOH : MeCN
EtOH : MeCN
EtOH : MeCN
EtOH : MeCN
1 : 9
1 : 9
1 : 9
1 : 9
PdCl2
KI
KI
KI
KI
100
100
100
100
24
24
24
24
68
70
88
93
9
PdCl2(PPh3)2
10% Pd/C
10% Pd/C
10c
11d
Effect of additive
12
13
14
EtOH : MeCN
EtOH : MeCN
EtOH : MeCN
1 : 9
1 : 9
1 : 9
10% Pd/C
10% Pd/C
10% Pd/C
NaI
TBAI
—
100
100
100
24
24
24
83
91
—
Effect of temperature and time
15
16
17
18
19e
EtOH : MeCN
EtOH : MeCN
EtOH : MeCN
EtOH : MeCN
EtOH : MeCN
1 : 9
1 : 9
1 : 9
1 : 9
1 : 9
10% Pd/C
10% Pd/C
10% Pd/C
10% Pd/C
10% Pd/C
KI
KI
KI
KI
KI
80
24
24
24
20
24
62
95
95
77
—
110
120
100
100
a
b
Reaction condition: 1a (1 mmol), 2a (1 mL), solvent (9 mL), additive (0.5 mmol), CO : O2 (5 : 1 atm). Yield of the product determined by GC,
c
d
e
GC-MS. 5 mol% Pd/C. 8 mol% Pd/C. In the absence of O2.
respectively, in excellent yields (Table 2, entries 1–3). Cyclic tertiary Reaction mechanism
amines such as N-ethyl piperidine, N-methyl piperidine and N-ethyl
The mechanism of the Pd/C-catalyzed synthesis of oxamates by
morpholine also smoothly underwent cross double carbonylation
with alcohols to provide an excellent yield of oxamate derivatives
(Table 2, entries 5, 6 and 11–13). Here we observed that in the
symmetrical tertiary amine, the cleavage of one of the C–N bond did
not depend on the length of the alkyl chain. Next, we investigated
that the unsymmetrical tertiary amine could undergo carbony-
lative coupling under the optimized reaction conditions.
Remarkably, we observed that in unsymmetrical amines, the
cleavage of the C–N bond is at the less substituted side and it
gets carbonylatively coupled with alcohols, leading to oxamates
(Table 2, entries 4 and 10).
the oxidative carbonylation reaction was proposed according to the
previous reports, as shown in Scheme 2. We proposed two reaction
pathways for the cleavage of an inert tertiary amine. In path A, the
lone pair electron of nitrogen of the tertiary amine directly
coordinates with Pd(0) to give a Pd-iminium intermediate,36,41,42
which on hydrolysis affords a secondary amine and an aldehyde as
a byproduct.27,36,37,43 In path B, the reaction between iodine and
tertiary amine gives ammonium iodide as an intermediate.36,44,45
Next, HI could be eliminated from ammonium iodide to obtain
iminium iodide. Further, iminium iodide gets hydrolyzed to afford
an aldehyde and a secondary amine.
Next, we focused our attention on the reactions using different
alcohols. Then, triethylamine, tripropylamine and tributylamine
showed successful cross carbonylation with methanol to furnish
products with excellent yields (Table 2, entries 7–9). For further
exploration of the substrate scope, next, we carried out the reaction
between triethylamine and propanol under the optimized reaction
condition, which provided 90% yield of the respective product
(Table 2, entry 14). Encouraged with the abovementioned results,
next, we used aromatic tertiary amines as a substrate with ethanol.
Unfortunately, the reaction proceeded to 0% yield of the respective
oxamates. This result indicates that aromatic amines are not
suitable for the reaction (Table 2, entries 15–17).
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Table 2 Pd/C-catalyzed oxidative double carbonylation of tertiary amines and alcoholsa
Entry
1
31 Amine
Alcohol
Product
Yieldb (%)
93
2
3
2a
2a
89
87
4
5
6
7
2a
2a
2a
69
81
78
93
8
2b
91
9
2b
2b
76
70
10
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Table 2 (continued)
Entry
11
31 Amine
Alcohol
Product
Yieldb (%)
2b
84
12
2b
2b
76
13
14
75
88
15
16
NR
NR
2a
17
2a
NR
a
b
Reaction conditions: 1a (1 mmol), 2a (1 mL), MeCN (9 mL), KI (0.5 mmol), CO/O2 (5 : 1 atm), 100 1C, 24 h. Isolated yield of the product, NR = no
reaction.
In the presence of an oxidant and an additive, the species to (A) or (A0) complex and the consequent nucleophilic attack of
of Pd(0) gets converted into Pd(II), then oxidative addition amine or alcohol give the alkoxycarbonylcarbamoylpalladium
of amines and alcohols followed by carbonylation forms complex (C).27,28,39,46,47 Finally reductive elimination of complex
species such as amoylpalladium (A) and alkoxy (A0) complex, (C) to obtained oxamate (D) and recovered palladium is used for
respectively (Path I and Path II). Next, the co-ordination of CO next cycle.
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General
The Pd/C was commercially purchased from Sigma-Aldrich.
Products were monitored using TLC and GC-MS techniques
and purified by column chromatography on silica (100–200 mesh).
The 1H and 13C NMR spectra were recorded on 400, 500 MHz and
101, 126 MHz spectrometers, respectively, in CDCl3. The products
were confirmed by GC, GC-MS, and NMR spectroscopy with those
of reported data.
General experimental procedure for oxamate
We took tertiary amine (1 mmol), alcohol (1 mL) and 10 mol%
Pd/C as a catalyst in a 100 mL stainless steel autoclave. Then,
9 mL of acetonitrile was added as a solvent and the autoclave
was closed tightly and pressurized with oxygen (1 atm) and CO
(5 atm) at 100 1C for 24 h. The mixture was stirred with a
mechanical stirrer with 450 rpm. After the completion of the
reaction, the autoclave cooled down at room temperature and
the remaining pressure was removed carefully. The reactor
vessel was opened and the catalyst was separated using a
centrifuge tube. Then, the residue was concentrated by a rotary
evaporator. Finally, the crude product was purified by column
chromatography and the separated product was confirmed by
GC, GC-MS and NMR techniques.
Scheme 2 Tentative reaction mechanism for the cross double carbony-
lation of tertiary amines and alcohols.
Conclusions
Experimental
In summary, we have developed a process for the synthesis of
oxamate and their derivatives by oxidative N-dealkylation/double
carbonylation of highly stable tertiary amines with alcohols. This
transformation has been attained by using molecular oxygen as
an environmentally benign oxidant. This is the first report where
the synthesis of a wide range of oxamates via sp3 C–N bond
cleavage of a variety of aliphatic and cyclic tertiary amines as a
secondary amine resource has been achieved. Additionally, the
advantages of the developed protocol include the use of simple
and easily available starting materials, low pressure of CO/O2,
not requiring external ligand and Cu-catalyst-free reaction
conditions. The catalyst was easily separable and could be
recycled up to five consecutive runs with a marginal loss of
yield of the product.
Recyclability study of Pd/C
Next, we investigated the recyclability of the Pd/C catalyst in
oxidative double carbonylation using triethylamine and ethanol
as substrates under the pressure of CO : O2 (5 : 1 atm) at 100 1C
(Fig. 1).
After the completion of the reaction, the catalyst was separated
and washed first with distilled water and then with methanol for
the removal of trace impurities and was dried in an oven. For
the next run, we charged the reactor with the starting material
with the same catalyst. It was shown that the catalyst could be
reused up to five consecutive runs with a marginal loss in its
catalytic activity.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Y. A. Kolekar gratefully acknowledges the University Grants
Commission (UGC), New Delhi, India, for the award of Junior
Research Fellowship (JRF).
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