Greener Paal-Knorr Pyrrole Synthesis by Mechanical Activation

Article abstract of DOI:10.1002/ejoc.201501223

A straightforward and solventless synthesis of pyrroles was developed by using mechanochemical activation and a biosourced organic acid as the catalyst. Relative to traditional Paal-Knorr methodologies, various N-substituted pyrroles were obtained in very short reaction times. By reaction with unreactive diketones, desymmetrized aliphatic and aromatic compounds were also synthesized.

Full text of DOI:10.1002/ejoc.201501223

DOI: 10.1002/ejoc.201501223
Communication
Mechanochemistry
Greener Paal–Knorr Pyrrole Synthesis by Mechanical Activation
Liudvikas Akelis,[a] Jolanta Rousseau,[b,c] Robertas Juskenas,[a] Jelena Dodonova,[a]
Cyril Rousseau,*[b,c] Stéphane Menuel,[b,c] Dominique Prevost,[b,c] Sigitas Tumkevičius,[a]
Eric Monflier,[b,c] and Frédéric Hapiot[b,c]
Abstract: A straightforward and solventless synthesis of pyrr- obtained in very short reaction times. By reaction with unreac-
oles was developed by using mechanochemical activation and
a biosourced organic acid as the catalyst. Relative to traditional
Paal–Knorr methodologies, various N-substituted pyrroles were
tive diketones, desymmetrized aliphatic and aromatic com-
pounds were also synthesized.
Introduction
focused on chemical transformations induced by mechanical
means, such as compression, shear, or friction. The mechanical
energy generated by grinding two solids or a solid and a liquid
substance breaks the order of the crystalline structure, which
leads to the formation of products. As such, mechanochemical
activation could be a substitute for thermal means to promote
chemical reactivity, as molecular transformations are generated
through mechanical forces.[21] For example, grinding and ball
milling have been widely used to powder inorganic materials
into fine particles in the preparation and modification of inor-
ganic solids.[22] Recently, mechanical activation was also consid-
erably reported in synthetic organic chemistry, in line with the
development of green methodologies.[23] Numerous organic re-
actions have been investigated with support of mechanical acti-
vation, including direct amidation,[24] peptide synthesis,[25] and
metal-catalyzed reactions such as Suzuki,[26] Heck,[27] and Sono-
gashira couplings.[28] Asymmetric organocatalytic reactions
such as aldol condensation have also been developed under
these conditions.[29] Using mechanochemistry, solvents are no
longer needed, and solid–solid reactions can be achieved in
good yields within short reaction times. Recently, a three-com-
ponent Hantzsch pyrrole synthesis promoted by mechano-
chemical activation was described.[30] Cerium(IV) ammonium
nitrate and silver nitrate were used as catalysts to afford poly-
substituted pyrroles within 1 h with yields ranging from moder-
ate to good. However, to the best of our knowledge, no me-
chanical activation has been described so far for the Paal–Knorr
pyrrole synthesis.
Pyrrole is one of the most important simple heterocycles; it is
found in broad structures of many bioactive natural products
such as porphyrins, alkaloids,[1] vitamins, heterocyclic com-
pounds of pharmacological interest,[2] and building blocks for
optical materials.[3] Classical methods for the synthesis of pyrr-
oles are Hantzsch, Paal–Knorr,[4] and Barton–Zard[5] syntheses,
but multistep approaches from preformed transition-metal in-
termediates,[6] reductive couplings,[7] aza-Wittig reactions,[8]
multicomponent reactions,[9] and other multistep operations[10]
are also known. Among these synthetic approaches, the Paal–
Knorr cyclization reaction between a 1,4-dicarbonyl compound
and a primary amine under slightly acidic conditions remains a
very attractive method for the synthesis of pyrroles. Since the
first Pall–Knorr synthesis was reported, many variations of the
synthetic procedure have been developed by using various
acidic reagents such as Brønsted acids[11] (e.g., H2SO4, HCl, p-
toluenesulfonic acid) and Lewis acids[12] [e.g., Sc(SO3CF3)3,
Bi(NO3)3·5H2O, RuCl3, InCl3, SnCl2·2H2O] to promote the con-
densations. Efforts have been made to optimize this methodol-
ogy through environmentally friendly processes involving the
use of surfactants in water,[13] modified or non-inorganic com-
pounds (e.g., clay, silica, alumina),[14] modified biopolymers
(e.g., cellulose, xanthan),[15] ionic liquids,[16] microwave heat-
ing,[17] ultrasound,[18] and microreactor under continuous-flow
conditions.[19] Concurrent to microwave irradiation and sono-
chemistry, mechanochemistry appears to be an attractive and
ecofriendly activation method, as it avoids the use of hazardous
solvents and is less energy-consuming.[20] Mechanochemistry is
In this article, we report a fast and easy method to prepare
N-substituted pyrroles through mechanical activation by ball
milling by using solid biosourced “green acids” such as citric
acid, pyroglutamic acid, succinic acid, ascorbic acid, camphor-
sulfonic acid, and oxalic acid.
[a] Department of Organic Chemistry, Vilnius University
Naugarduko g. 24, 03225 Vilnius, Lithuania
[b] Univ. Artois, UMR 8181, Unité de Catalyse et de Chimie du Solide (UCCS),
62300 Lens, France
E-mail: cyril.rousseau@univ-artois.fr
[c] CNRS, UMR 8181, Unité de Catalyse et de Chimie du Solide (UCCS),
Lille, 59000, France
Results and Discussion
To investigate the mechanochemical effects on the Paal–Knorr
pyrrole synthesis, we used a laboratory-scale ball mill (Retsch
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejoc.201501223.
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Communication
MM400) equipped with 10 mL zirconia grinding jars (that per- the acid (pKa, measured in solution) and the catalytic activity in
formed radial oscillations in a horizontal position) containing the solid state. We believe that the local concentration of the
one zirconia ball (diameter: 9 mm). The reagents in the solid acid functions in citric acid favor the catalytic process in the
state were shaken at a given ball-mill frequency (frequency of solid state.
the rocking back-and-forth motion conducted by the reaction
jar holder). The reaction between 4-iodoaniline and 2,5-hexane-
dione was used as a model system (Scheme 1), as both reagents
have already been proven to be good candidates for the assess-
ment of the reaction conditions and a relevant comparison with
previous results.[31]
Using citric acid as a catalyst, operating parameters (i.e., time,
ball-mill frequency, catalyst amount) were varied to optimize
the reaction yield. Reducing the reaction time from 15 to
10 min led to a significant decrease in the yield from 74 to 63 %
(Table 1, Entries 10 and 11). Conversely, increasing the reaction
time to 30 min yielded 87 % of pyrrole (Table 1, Entry 12). Sec-
ond, if the amount of citric acid was increased from 1 to 5 or
to 10 mol-%, the yield was very dependent on the ball-mill
frequency. Good conversions (from 58 to 74 %) were observed
by using 1 mol-% citric acid at 10, 20, or 30 Hz (Figure 1). A
regular increase in the yield with the frequency (from 69 to
96 %) was also noticed with the use of 5 mol-% citric acid. On
the other hand, high conversions were obtained by using
10 mol-% citric acid regardless of the frequency. The best result
(96 % conversion) was obtained with 10 mol-% citric acid at
30 Hz. Independent of the catalyst amount, these results
corroborate previous studies regarding the crucial role of the
ball-mill frequency on the reaction yield.[32,33] Actually, increas-
ing the frequency of the milling resulted in an increase in the
kinetic energy, which resulted in a transfer of more effective
energy to the reactants. Thus, higher conversions were ob-
tained at higher frequencies. In view of the above results, the
following study was implemented at 30 Hz.
Scheme 1. Paal–Knorr pyrrole synthesis with 4-iodoaniline and 2,5-hexanedi-
one as reagents.
To determine the best reaction conditions, we investigated
various solid organic acids as catalysts. The results are summa-
rized in Table 1.
Table 1. Paal–Knorr pyrrole mechanosynthesis in the presence of various or-
ganic acids.[a]
Entry
Catalyst
Time [min]
Yield [%]
1
2
3
4
5
6
7
8
9
–
–
15
30
15
15
15
15
15
15
15
15
10
30
trace
trace
18
27
23
34
39
48
63
oxalic acid
succinic acid
ascorbic acid
tartaric acid
camphorsulfonic acid
pyroglutamic acid
malonic acid
citric acid
10
11
12
74
63
87
citric acid
citric acid
Figure 1. Yield variations with the catalyst amount and the ball-mill fre-
quency. Conditions: 2,5-hexanedione (308 mg, 2.7 mmol, 1 equiv.), amine
(592 mg, 2.7 mmol, 1.1 equiv.), and citric acid (1, 5, or 10 mol-%) in a ball
mill at 10, 20, or 30 Hz for 15 min.
[a] Reaction conditions: 2,5-hexanedione (2.7 mmol, 308 mg), 4-iodoaniline
(2.7 mmol, 592 mg), with or without the catalyst (1 mol-%) in a ball mill at
30 Hz.
To assess the importance of an acid catalyst to form the de-
sired pyrrole under mechanochemical conditions, blank tests
were performed. The amine and the diketone were shaken at
30 Hz for 15 or 30 min without any acid catalyst (Table 1, En-
tries 1 and 2). A trace amount of the desired pyrrole was ob-
served in each case, but the yields were estimated to be below
5 % on the basis of GC–MS analysis. These two tests confirmed
that even with mechanochemical activation an acid catalyst is
required to form the pyrrole from an amine and a diketone.
A wide range of substituted aromatic and aliphatic amines
were investigated with two different diketones, namely, 2,5-hex-
anedione (commonly used as a model reactant, as it reacts
easily) and 1-phenylpentane-1,4-dione (a more hindered ketone
with lower reactivity). The results are summarized in Table 2. To
fit green chemistry principles, the catalyst amount was chosen
to be as low as possible (1 mol-% citric acid) even if the above
results showed that moderate to good yields were obtained in
We next investigated several solid organic acids known for that case. Considering 2,5-hexanedione as a reagent, pyrroles
their low toxicity and their availability in biomass stocks. The 1a–9a were obtained in good to excellent yields. Conversely, if
catalyst amount was fixed at 1 mol-%. The conversion of 4- 1-phenylpentane-1,4-dione was used as the reagent, the yields
iodoaniline into the corresponding pyrrole was measured after of pyrroles 1b–9b dropped to moderate to good (Table 2, En-
a reaction time of 15 min. Moderate to good yields were ob- tries 1–9). The reactions having lower yields were then repeated
tained regardless of the solid organic acid (Table 1, Entries 3– with a reaction time of 30 min (Table 2, Entries 1, 2, 5, 8, and
10). Citric acid gave the best result (yield up to 74 % after 9). Acceptable yields were obtained (up to 55 %), except for the
15 min at 30 Hz; Table 1, Entry 10). In view of these results, it naphthylamine derivatives for which yields of ca. 15 % were
seems that there is no direct relation between the strength of obtained.
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Table 2. Citric acid catalyzed pyrrole synthesis in a ball mill.[a]
gave good reaction yields regardless of the diketone (Table 2,
Entries 1 and 7). Under shaking, these two amines are in a mol-
ten or liquid phase because of the existence of high thermal
energy regions (hot spots) inside the reactor that are generated
by the collision of the ball and the reagents.[34] Thus, diffusion
of the reactants and contact between them is greatly facilitated.
Accordingly, the Paal–Knorr reaction involving dodecylamine
and iodoaniline under mechanochemical conditions proceeds
through a classical acid-catalyzed process similar to that ob-
served in solution. Conversely, other amines such as naphthyl-
amine have high melting points and remain in the solid state
even under shaking. The solid-state reactions then give lower
yields, as the phenomenon is stochastic. Longer reaction times
are then required to obtain good yields.
The desymmetrization of aliphatic and aromatic diamines
was also assessed under mechanochemical conditions. The re-
sults are reported in Table 3. Using 2,3-diaminopropane and
2,5-hexanedione as reagents (Table 3, Entries 1 and 2), no de-
symmetrization was observed regardless of the amount of
amine (0.5 or 1 equiv./diketone). The major product was the
corresponding bis(pyrrole). If the reaction was performed by
using more hindered and less reactive 1-phenylpentane-1,4-di-
one, a slight desymmetrization was observed upon using stoi-
chiometric proportions of the aliphatic amine and the diketone
(Table 3, Entries 3 and 4).
For aromatic diamines (Table 3, Entries 5–21), if only 1 mol-
% citric acid was used, the desymmetrization took place in
moderate to good yields, probably because of deactivation of
the aromatic amine upon formation of the first pyrrole ring, as
it makes formation of the second pyrrole more difficult, espe-
cially if the acid catalyst is used at a loading of only 1 mol-%. If
10 mol-% of the catalyst was used with 2,5-hexanedione as the
reactant (better catalyst amount according to Figure 1), an in-
version in the proportion of products was observed for the
para- and meta-diaminobenzenes. The corresponding bis(pyrr-
ole)s were then identified as the major products (Table 3, En-
tries 6 and 12). Conversely, by using the ortho isomer, the Paal–
Knorr synthesis mainly led to the corresponding monopyrrole
[and a small amount of bis(pyrrole)] because of steric hindrance
(Table 3, Entry 18). If less reactive 1-phenylpentane-1,4-dione
was used as the reactant in the presence of 10 mol-% citric acid
(Table 3, Entry 15), the conversion was greatly enhanced, and
the monopyrrole was the major product because of the com-
bined deactivation of the aromatic amine and the lower reactiv-
ity of the ketone.
Conclusions
[a] Unless otherwise noted, reactions were performed with dione (2.7 mmol),
amine (2.7 mmol), and citric acid (0.03 mmol) in a ball mill at 30 Hz for
15 min. [b] Yield after 30 min at 30 Hz. [c] Yield was determined by GC–MS
analysis.
We showed that mechanochemical activation was efficient for
Paal–Knorr pyrrole synthesis within very short reaction times at
room temperature. This method allowed us to synthesize pyrr-
oles with various ketones, hindered or not. Compared to previ-
ously reported methods, the main advantages of the present
The difference in yields between pyrroles synthesized from procedure lie in the absence of solvent and the use of a “green”
2,5-hexanedione and 1-phenylpentane-1,4-dione probably lies organic acid that is nontoxic and produced from biomass. The
in the physical state of the amine under mechanochemical con- mild reaction conditions also allowed us to synthesize pyrroles
ditions. Iodoaniline and dodecylamine, two solid amines at with functional groups such as amines and alcohols and gave
room temperature with low melting points (ca. 30 °C and 63 °C), direct access to unsymmetrical amines or bis(pyrrole)s.
Eur. J. Org. Chem. 2016, 31–35
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Communication
Table 3. Paal–Knorr pyrrole synthesis with the use of diamines as reactants.[a]
[a] Unless otherwise noted, the reactions were performed with diketone (2.7 mmol), diamine (0.5 or 1 equiv./diketone), and citric acid (1 or 10 mol-%) in a
ball mill at 30 Hz for 15 min. [b] Yield determined by GC–MS analysis. [c] Experiment was performed with 10 mol-% citric acid. [d] Yield after 30 min at 30 Hz.
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General Methods: All chemicals were purchased from Acros and
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supplied without further purification. Distilled water was used in all
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on Merck aluminum sheets coated with silica gel (Silica Gel F254).
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1
for H nuclei and at 75 MHz for 13C nuclei. CDCl3 (99.50 % isotopic
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realized by using a GC–MS Shimadzu QP-2010 SE
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Received: September 22, 2015
Published Online: December 9, 2015
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