The synthesis of β-enaminones using trialkylamines and a Pd/DNA catalyst

Article abstract of DOI:10.1016/j.mcat.2020.111365

A new one-pot procedure leading to β-enaminones has been developed. The elaborated methodology involves the cascade sequencing of carbonylative Sonogashira coupling of aryl iodide with a terminal alkyne and the oxidative dealkylation of tertiary amines. The efficient syntheses of β-enaminones were performed with a Pd/DNA catalyst, with NEt₃, NPr₃ and NBu₃ used as amine sources. The catalyst was successfully recovered and used in several subsequent tests with the same results. The reaction mechanism, summarizing the activity of immobilized Pd NPs (palladium nanoparticles), was proposed.

Full text of DOI:10.1016/j.mcat.2020.111365

Molecular Catalysis 502 (2021) 111365
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
The synthesis of β-enaminones using trialkylamines and a Pd/DNA catalyst
M. Mart, A.M. Trzeciak*
University of Wrocław, Faculty of Chemistry, 14 F. Joliot-Curie, 50-383, Wrocław, Poland
A R T I C L E I N F O
A B S T R A C T
Keywords:
A new one-pot procedure leading to β-enaminones has been developed. The elaborated methodology involves the
cascade sequencing of carbonylative Sonogashira coupling of aryl iodide with a terminal alkyne and the
oxidative dealkylation of tertiary amines. The efficient syntheses of β-enaminones were performed with a Pd/
DNA catalyst, with NEt3, NPr3 and NBu3 used as amine sources. The catalyst was successfully recovered and used
in several subsequent tests with the same results. The reaction mechanism, summarizing the activity of immo-
bilized Pd NPs (palladium nanoparticles), was proposed.
Palladium nanoparticles
Carbonylative coupling
Enaminones
Oxidative amine dealkylation
1. Introduction
syntheses of enaminones [25]. However, there is no example of a
palladium-catalyzed synthesis of β-enaminones with tertiary amines.
Palladium-catalyzed carbonylation reactions present a very impor-
tant methodology leading to carbonyl compounds broadly applied in the
pharmaceutical and agrochemical industries [1–7]. Carbonylative
coupling can be used for the synthesis of ketones, amides, esters, car-
boxylic acids and other valuable derivatives, starting with aryl halides as
substrates. The Pd catalyzed double carbonylation is a one-step pathway
Typical palladium catalysts used in β-enaminone synthesis contain
nitrogen ligands (bipyridine, phenantroline, imidazole), phosphorus li-
gands (triphenylphosphine, bis(diphenylphosphino)ferrocene, Xant-
phos) or N-heterocyclic carbene (NHC) [26]. The mechanism of
enaminone formation has been scarcely discussed, however, a homo-
geneous pathway with a soluble palladium species as a catalytically
active form seems to be most likely. In contrast, immobilized or het-
erogeneous palladium catalysts have not yet been tested in the synthesis
of β-enaminones. Therefore, we decided to use our experience with a
Pd/DNA catalyst and apply it to this reaction. The peculiar structure,
non-toxic properties and low price of DNA motivate its use as a support
for palladium catalysts [27–34].
to α-ketoamides and oxamates. The oxidative carbonylation performed
in the presence of CO and O2 produced carbamates [8,9].
β-Enaminones belong to biologically active compounds which have
anti-inflammatory activity [10]. They are also attractive substrates for
the synthesis of various heterocyclic compounds [11–13]. Enaminones
can be obtained by the nucleophilic addition of secondary amines to 1,2,
3-triazine [14] or by the amination of 1,3-dicarbonyl compounds with
primary amines or ammonium salts [15,16]. Moreover, these com-
In this paper, we report the first example of a highly efficient four-
component synthesis of β-enaminones using trialkyl amines and an
immobilized Pd/DNA catalyst (Scheme 1).
pounds can be obtained through a reaction of
αꢀ keto acids with
iodoalkyne [17] and by the oxyaminalization of alkenes using amines,
oxygen and Togni’s reagent [18]. Iron catalyzed synthesis using ketones
and amines was also reported [19].
2. Results and discussions
β-Enaminones are formed at high yields and regioselectivity during
the Michael addition of an amine to an alkynone [20–22]. In this
context, an important step of this procedure consists of the preparation
of alkynones by the palladium-catalyzed carbonylative Sonogashira
coupling of aryl iodide and the alkyne [23]. Alternatively, an acid
chloride and a terminal alkyne can be coupled under Sonogashira con-
ditions to form an alkynone [24]. On the basis of these results, secondary
amines were employed in a four-component or three-component one-pot
2.1. Pd/DNA catalyzed four-component synthesis of β-enaminones
The formation of a trace amount of enaminone 1a was first observed
in the carbonylative Sonogashira reaction performed with NEt3 as a base
(Scheme 2) [34]. After identification of 1a, attempts were undertaken to
change the reaction selectivity and increase its yield. Variations of sol-
vent, temperature and reaction time allowed 1a to be obtained with a 67
% yield in DMF, at a temperature of 120 ◦C. Moreover, analysis of the
* Corresponding author.
E-mail address: anna.trzeciak@chem.uni.wroc.pl (A.M. Trzeciak).
https://doi.org/10.1016/j.mcat.2020.111365
Received 16 September 2020; Received in revised form 9 November 2020; Accepted 16 December 2020
Available online 19 January 2021
2468-8231/© 2021 Elsevier B.V. All rights reserved.

M. Mart and A.M. Trzeciak
Molecular Catalysis 502 (2021) 111365
Scheme 1. Palladium-catalyzed four-component one-pot synthesis of enaminones.
Scheme 2. Pd/DNA catalyzed four-component synthesis of β-enaminone 1a.
Table 1
Optimization of the reaction conditionsa (Scheme 2).
Entry
T [oC]
Solvent
Amount of NEt3 [equiv.]
Time [h]
Conv. [%]b
Yield 1a [%]b
Yield 2a [%]b
Yield 3a [%]b
1c
100
100
100
100
120
120
120
120
120
120
120
DMF
3
3
3
4
3
3
3
3
4
4
4
16
24
16
24
3
72
99
76
80
60
68
83
97
99
82
99
8
27
56
47
35
38
24
10
28
12
n.d.
n.d.
22
1
2
DMF
39
3
CH3CN
6
21
4
4
Dioxane
DMF
30
5
15
3
6
DMF
6
19
8
7
DMF
16
24
24
24
16
62
2
8
DMF
57
3
9
DMF
67 (60)d
24
6
10
DMF/Dioxane (1/1)
11
44
11
Ethylene Glycol
n.d.
a[Pd] (2 mol %), solvent (3 mL), iodobenzene (1 mmol), phenylacetylene (1.2 mmol), CO (balloon pressure).
b
Conversions and yields were determined by GC using mesitylene as an internal standard.
c
15 mmol of H2O was added.
d
Isolated yield.
product composition at different periods of time indicated that 1a was
formed from alkynone 2a, the product of carbonylative Sonogashira
coupling (Table 1).
reaction selectivity significantly decreased. The positive effect of the
DNA polymer on the reaction course was therefore clearly evident. In
addition, the activity of Pd/DNA visibly decreased after pyrolysis at 800
◦C, probably due to damage of the DNA structure. The pyrolyzed cata-
lyst, C2-P, formed only 28 % of 1a at 120 ◦C. The comparison of the two
Pd/DNA catalysts indicated the impact of the Pd content, and better
results were obtained for the C2 catalyst containing less palladium.
While the size of the Pd nanoparticles in both catalysts was similar at
10–21 nm vs. 11–13 nm, it could be assumed that better separation of
the palladium active centers in the DNA support in the C2 catalyst was
more favorable for yielding more 1a.
Concerning the solvent, it was found that DMF and dioxane produced
better results than acetonitrile, therefore, DMF was used for further
studies. The reaction carried out at 100 ◦C, according to Scheme 1,
provided a total conversion of PhI, however, 2a was the main product
while the yield of 1a was only 39 %. The transformation of 2a to 1a was
◦
more efficient at 120 C, when 1a became the predominant product.
Different amounts of NEt3 were tested, such as three, four and six
equivalents, and it was found that four equivalents was the optimal
amount for the formation of 1a.
The Pd loading experiments were carried out using 1 mol % and 0.5
% of palladium with the C2 catalyst (Table 2). When 1 mol % of palla-
dium was used, the yield of 1a (68 %) was slighly lower than by using 2
mol % (71 %). Further decrease in the yield, to 57 %, was observed with
0.5 mol % of palladium. Some decrease in selectivity to the desired
product 1a was observed at the lower Pd loading.
Next, the same reaction was performed using various sources of
palladium to compare their usefulness in the synthesis of enaminone 1a.
Thus, for this test, two Pd/DNA catalysts were selected, each containing
different amounts of Pd. For comparison, the composite obtained by
pyrolysis of Pd/DNA (C2-P) was also tested (Table 2).
The Pd/DNA was found to be superior to commercial catalysts Pd
(OAc)2 and Pd/C. Under the applied conditions, Pd(OAc)2 catalyzed
only Sonogashira coupling leading to diphenylacetylene 3a. With Pd/C,
the maximum yield of 1a was 21 % at 100 ◦C, whereas at 120 ◦C, the
2.2. Effect of the tertiary amine structure
Afterward, other tertiary amines were tested with the C2 catalyst
2

M. Mart and A.M. Trzeciak
Molecular Catalysis 502 (2021) 111365
selectively 90 % of 1c under the same conditions. These results showed
that the structure of tertiary amine had a high impact on the reaction
course. This is probably because in the studied system amine played a
dual role as the base in the formation of the product 2a and as a substrate
in the formation of the desired product 1a.
Table 2
Different palladium catalysts in the synthesis of β-enaminones[a,b]
.
Entry
Source
of Pd
Time
[h]
Temp.
Conv.
Yield
1a
Yield
2a
Yield
3a
[T oC]
[%]b
[%]b
[%]b
[%]b
1
Pd/DNA
(C1)
24
24
24
16
24
24
24
100
120
100
120
120
120
120
99
99
99
99
99
99
91
22
75
2
6
7
4
5
5
5
(68)c
12
2.3. The method applicability to different derivatives of iodobenzene and
phenylacetylene
2
Pd/DNA
(C1)
67
(60)c
31
3
Pd/DNA
(C2)
56
(50)c
20
NBu3 was used in the next series of experiments employing various
aryl iodides and substituted phenylacetylenes. In most cases, the rele-
vant enaminones were obtained with a good or very good yield
(Table 4). While good results were achieved with aromatic alkyne de-
rivatives, in the case of an aliphatic alkyne only trace amount of the
desired product (10c) was obtained. The reason for this is that activation
of the less nucleophilic substrate, 1-pentyne, was not efficient in the
applied catalytic conditions.
4
Pd/DNA
(C2)
57
(51)c
71
5
Pd/DNA
(C2)
6
(67)c
68
6d
7e
Pd/DNA
(C2)
9
Pd/DNA
(C2)
57
11
8
C2-P
24
24
24
100
120
100
38
80
99
8
25
3
9
C2-P
28
n.d.
46
1
2.4. Catalyst recycling
10
Pd
n.d.
38
(OAc)2
Pd
11
12
16
24
120
100
99
99
n.d.
21
n.d.
6
10
29
The C2 catalyst was separated from the reaction mixture and suc-
cessfully reused in five subsequent tests without any significant loss of
activity (Fig. 1).
(OAc)2
Pd/C
(10 %
wt)
The TEM analysis of the catalyst recovered after the recycling
experiment showed some aggregates of palladium and individual Pd
nanoparticles, 4–10 nm in diameter. The size of the nanoparticles was
similar to that before the reaction (Figs. 2 and 3).
13
a
Pd/C
(10 %
wt)
16
120
99
7
2
29
[Pd] (2 mol %), DMF (3 mL), iodobenzene (1 mmol), phenylacetylene (1.2
mmol), NEt3 (4 mmol), CO (balloon pressure).
Promising results obtained in the recycling experiment indicated on
the good stability of the catalyst and possible heterogeneous reaction
pathway. In order to prove this assumption, a leaching test and a hot
filtration test were performed. According to the ICP analysis, 0.37 wt %
of Pd was leached to the solution. However, the solution obtained after
catalyst separation by hot filtration, did not show any catalytic activity.
Thus, the studied reaction proceeds according to the heterogenous
mechanism.
b
Conversions and yields were determined by GC using mesitylene as an in-
ternal standard.
c
Isolated yield.
d
1 mol % of Pd was used.
e
0.5 mol % of Pd was used.
Table 3
The yield of β-enaminones using different tertiary amines[a,b]
.
2.5. Studies on the reaction mechanism
Entry
Amine
Temp.
Conv.
Yield
1a-1d
[%]b
Yield
2a
Yield
3a
[T oC]
[%]b
According to the literature, alkynones formed in carbonylative
Sonogashira coupling reacted with diamine, according to the Michael
addition, forming enaminones [20–22]. It was checked that in our
conditions the same yield of 1a was formed in both the presence and
absence of a catalyst when HNEt2 was used (99 %) (Scheme 3).
Moreover, it was possible to obtain 75 % of 1a in six hours following
a four-component one-pot procedure. This result is similar to those ob-
tained with other palladium catalysts used with HNEt2 (Scheme 4).
However, the test reaction of alkynone 2a with NEt3 produced a
much lower yield of 1a, up to 18 % in the presence of the Pd/DNA
catalyst (Scheme 5).
derivative
[%]b
[%]b
1
2
3
4
5
6d
7
8
Triethylamine
Triethylamine
100
120
100
120
100
100
120
120
99
99
99
99
99
99
99
0
31 (1a)
56
7
(50)c
6
71 (1a)
(67)c
5
Tri-n-
93 (1b)
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
7
propylamine
Tri-n-
(88)c
80 (1b)
(72)c
propylamine
Tri-n-
90 (1c)
butylamine
Tri-n-
(85)c
71 (1c)
Therefore, it was assumed that dealkylation of NR3 is a key issue for
forming β-enaminones in the studied system. In the control experiment,
an NEtiPr2 amine was used to confirm the occurrence of the dealkylation
process (Scheme 6). Analysis of the reaction products revealed the
presence of similar amounts of two enaminones containing NEtiPr and
NiPr2 substituents. The formation of these products was slower than
with other NR3 amines, most likely due to the steric hindrance of the iPr
group. A similar observation was also made for amides formation with
these amines [43].
butylamine
Tri-n-
87 (1c)
(80)c
n.d.
n.d
n.d.
butylamine
Tribenzylamine
(1d)
a
Catalyst C2 [Pd] (2 mol %), DMF (3 mL), iodobenzene (1 mmol), phenyl-
acetylene (1.2 mmol), amine (4 mmol), CO (balloon pressure).
b
Conversions and yields were determined by GC using mesitylene as an in-
ternal standard.
c
Isolated yield.
d
Pd/C (10 wt%) was used instead of Pd/DNA.
2.5.1. Effect of O2
The literature examples of oxidative dealkylation of tertiary amines
reported the application of Cu salts or O2 as oxidative agents [35–37]. In
these reactions, dealkylation of NR3 amines enabled the formation of
amides with high yields [38–41].
(Scheme 2, Table 3). Enaminones (1a–1c) were formed at a high yield
when NPr3 and NBu3 were used instead of NEt3. In these reactions,
better results were obtained at 100 ◦C than at 120 ◦C. In contrast, tri-
benzylamine did not provide any products. In the reaction with NBu3
catalyst Pd/C produced 71 % of 1c and 7 % of 3a, while Pd/DNA formed
In the studied system, the most probable pathway of amine deal-
kylation is the process catalyzed by Pd/DNA with air/oxygen as an
3

M. Mart and A.M. Trzeciak
Molecular Catalysis 502 (2021) 111365
Table 4
The yield of β-enaminones obtained from different iodobenzenes and phenylacetylenes[a, b]
.
aCatalyst C2 [Pd] (2 mol %), DMF (3 mL), aryl iodide (1 mmol), terminal alkyne (1.2 mmol), NBu3 (4 mmol), 120 ◦C, CO (balloon pressure); b Yields were determined
by GC using mesitylene as an internal standard; c Isolated yield.
oxidant. In order to prove the effect of O2 on the reaction course, a two-
step experiment was performed. First, under air-free conditions, the
reaction was carried out for five hours. The reaction products contained
89 % alkynones and only 9 % 1a. Next, the balloon with CO was
removed and a balloon filled with O2 was connected to the Schlenk
vessel. After five hours, the yield of 1a increased to 59 % (Scheme 7). A
similar result was observed in reactions performed according to the
same procedure for three hours, where an increase of the yield of 1a
from 6 % to 31 % was obtained (ESI, Scheme 1).
Next, two reactions were performed for comparison. The reference
reaction was carried out with CO only, while in the second experiment,
CO and O2 were supplied to the Schlenk tube. In the first case, the ratio
of 1a to 2a was 31–56, while the presence of O2 changed this ratio to 55
to 27. Thus, the yield of 1a increased in the presence of O2 due to the
more efficient dealkylation of NEt3.
2.5.2. Influence of water
To check the influence of water on the reaction course, additional
experiments were undertaken using pre-dried reagents. The yield of
β-enaminones varied from 7 % for NiPr3 and NBu3 to 12 % for NEt3 and it
Fig. 1. Recycling of the Pd/DNA (C2) in the four-component synthesis of
β-enaminone 1c.
was significantly lower than the yield achieved with non-dried DMF and
amines (71–93 %, Tables 2 and 3).
However, the addition of a controlled amount of water, 2–4 mmol, to
the reaction with NBu3 resulted in an increase in the yield of
Fig. 2. Morphology and size distribution of C2 as prepared.
4

M. Mart and A.M. Trzeciak
Molecular Catalysis 502 (2021) 111365
Fig. 3. Morphology of the C2 recovered after recycling experiments.
Scheme 3. Formation of β-enaminones by the Michael addition.
Scheme 4. Four-component synthesis of β-enaminones with HNEt2.
Scheme 5. Reaction of alkynone with NEt3.
Scheme 6. Four-component synthesis of β-enaminones with a different alkyl bearing amine.
β-enaminone to 87 % (Scheme 8). On the other hand, it should be
pointed out that an increase in the amount of water to 10 mmol caused a
significant decrease in the yield of 1c to 59 %. Thus, the amount of water
in the reaction mixture should be precisely controlled to obtain a high
yield of the product.
formation of alkynones by the carbonylative coupling of iodobenzene
with phenylacetylene according to the Sonogashira path. Recently we
evidenced the high catalytic ability of Pd/DNA in the synthesis of
alkynones in the presence of NEt3 base [34]. The same catalytic system
was applied now to form β-enaminones by using a higher temperature
and longer reaction time. The original step of the presented procedure is
formation of secondary amines by the oxidative dealkylation of tertiary
amines on the same catalyst in the presence of water. We assume that
not only Pd, but also DNA, play an important role in this reaction. In
particular, the ability of DNA to form specific hydrogen bonds with
amines and water facilitated the activation of the substrates. This is in
agreement with the lower ability of Pd/C, as well as pyrolyzed Pd/DNA,
2.6. The plausible mechanism of β-enaminones formation in the presence
of Pd/DNA
Based on the obtained results, we propose that the catalytic reaction
occurs according to the heterogeneous pathway on the catalyst surface
(Fig. 4). The main steps of the proposed mechanism involve the
5

M. Mart and A.M. Trzeciak
Molecular Catalysis 502 (2021) 111365
Scheme 7. Effect of O2 on the synthesis of β-enaminone.
Scheme 8. Influence of water on β-enaminone formation.
Sonogashira coupling and the oxidative N-dealkylation of tertiary
amines. Both stages are catalyzed by the Pd/DNA catalyst containing Pd
NPs as the catalytically active form. The catalyst is stable and it can be
easily recovered with the same activity. Advantageously, the catalytic
system is active without the addition of a phosphine ligand or any co-
catalyst. Reagents do not need to be dried and specially purified, how-
ever, precise control of the amount of water is necessary to obtain the
best results.
4. Experimental
4.1. Preparation of Pd/DNA catalysts
C1: Catalyst C1 was prepared according to references [32,33] using
fish sperm DNA (CAS: 438545-06-3) (Pd content estimated by ICP: 45
%).
C2: Catalyst C2 was prepared according to references [32,33] using
150 mg of low molecular weight fish sperm DNA (CAS:100403-24-5) in
10 ml of water and 100 mg of Pd(OAc)2 and 10 ml of ethanol (Pd content
estimated by ICP: 17.5 %).
Fig. 4. Postulated reaction mechanism.
to efficiently conduct the formation of β-enaminones with tertiary
amines.
The FT IR, TEM and XRD data are presented in the SI.
The proposed pathway of N-dealkylation of tertiary amines was
inspired by other studies [39,42,43].
4.2. Pyrolysis of C2
The catalytic cycle begins with an oxidative addition of ArI to Pd(0)
supported on DNA (Fig. 4). Insertion of CO forms a Pd-acyl intermediate
and in the same time iodide bonded to Pd reacts with coordinated amine
with elimination of HI. The HI reacts then with O2 to produce a water
molecule which contributes to the formation of diamine [35,40]. The
final steps involve formation of β-enaminone and its elimination from
the Pd/DNA catalyst.
100 mg of C2 was placed in a furnace in a ceramic crucible. The
furnace was heated up to 800 ◦C (5 ◦C per minute) under argon and kept
at 800 ◦C for one hour. Following this process, the heating was
completed and the furnace was cooled down to an ambient temperature
(Pd content estimated by ICP: 40 %).
4.3. General procedure for synthesis of β-enaminones
3. Conclusions
In the typical reaction, a 30 ml vial was filled with a catalyst (con-
taining 2 mol % of Pd), aryl iodide (1 mmol), the terminal alkyne (1.2
mmol) and the amine (4 mmol). Then, 3 ml of the DMF was poured into
the mixture. The vial was cooled in liquid nitrogen and, after the
evacuation of air, the vial was joined with CO balloon. The obtained
mixture was stirred under a CO atmosphere at 100–120 ◦C for 16–24 h.
We have presented for the first time a highly efficient one-pot four-
component methodology for the preparation of β-enaminones with ter-
tiary amines. The synthesis is based on the tandem reaction composed of
two processes, namely alkynone formation in copper-free carbonylative
6

M. Mart and A.M. Trzeciak
Molecular Catalysis 502 (2021) 111365
Next, the vial was cooled to room temperature. To extract the products,
5 ml of H2O and 3 × 7 ml of Et2O were added to the vial. After the
extraction, the sample was analyzed by GC-FID. For the GC-FID analysis,
0.076 ml of mesitylene was used as the internal standard. For purifica-
tion, the solvent was removed in vacuo. Afterwards, the residue was
purified by column chromatography, analyzed by GC–MS, 1H NMR and
13C NMR for characterization.
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Declaration of Competing Interest
The authors report no declarations of interest.
Appendix A. Supplementary data
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Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2020.111365.
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