Scandium triflate-catalysed synthesis of N-substituted pyrroles from amine and 2,5-dimethoxytetrahydrofuran

Article abstract of DOI:10.3184/030823409X393628

The Clauson-Kass pyrrole synthesis catalysed by scandium triflate afforded N-substituted 2- and 3-unsubstituted pyrroles with yields ranged from good to excellent. Aromatic amines, heteroaomatic amines, 4-methylbenzenesulfonamide and 2-chlorobenzamide are good substrates in this transformation.

Full text of DOI:10.3184/030823409X393628

14 RESEARCH PAPER
JANUARY, 14–16
JOURNAL OF CHEMICAL RESEARCH 2009
Scandium triflate-catalysed synthesis of N-substituted pyrroles from
amine and 2,5-dimethoxytetrahydrofuran
Bing Zuoa, Jiuxi Chena, Miaochang Liua, Jinchang Dinga, Huayue Wua* and Weike Sua,b
*
aCollege of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325027, P.R. China
bCollege of Pharmaceutical Sciences, Zhejiang University of Technology, Zhejiang Key Laboratory of Pharmaceutical Engineering,
Hangzhou 310014, P.R. China
The Clauson–Kass pyrrole synthesis catalysed by scandium triflate afforded N-substituted 2- and 3-unsubstituted pyrroles
with yields ranged from good to excellent. Aromatic amines, heteroaomatic amines, 4-methylbenzenesulfonamide and
2-chlorobenzamide are good substrates in this transformation.
Keywords: NꢀVXEVWLWXHGꢁS\UUROHVꢂꢁꢃꢂꢄꢀGLPHWKR[\WHWUDK\GURIXUDQꢂꢁVFDQGLXPꢁWULÀDWHꢂꢁ&ODXVRQ±.DDV
The pyrrole skeleton is prevalent in a wide range of natural
and pharmacologically active compounds.1-3 In recent
years, great interest has been paid to the synthesis of
pyrrole derivatives. Among the existing methods, the most
commonly used methods are the Knorr pyrrole synthesis,
the Hantzch pyrrole synthesis, the Paal–Knorr synthesis and
the Clauson–Kass synthesis.4-7 The Clauson–Kass pyrrole
synthesis involves the condensation reaction of the amine
with 2,5-dimethoxytetrahydrofuran for the synthesis of the
N-substituted, 2- and 3-unsubstituted pyrroles, where the
2- and 3-positions can be reacted for further functionalisation.
By far, a number of synthetic strategies have been developed
for the condensation of Clauson–Kass pyrrole synthesis.
Various acidic materials, such as glacial acetic acid,8
TfOH,9 P2O5,10 montmorillonite K-10,11 4-chloropyridine
hydrochloride,12 as well as the microwave technology11 have
been used to promote these condensations. However, some
of these methods often suffer from certain drawbacks such as
strong acidic reaction conditions,9 long reaction time,11 high
reaction temperature,13 strictly anhydrous condition,10 tedious
workup11 and large amounts of solid catalysts.11 Hence, an
HI¿FLHQWꢁDQGꢁPLOGꢁ&ODXVRQ±.DVVꢁFRQGHQVDWLRQꢁLVꢁQHHGHGꢁIRUꢁ
contemporary chemical synthesis.
Catalyst
Solvent
+
NH2
N
OMe
MeO
O
Scheme 1 Clauson–Kass pyrrole synthesis.
Table 1 Effect of catalysts for the synthesis of pyrrolesa
Entry
Catalyst
Yield/%b
1
2
None
No reaction
FeCl3
20
43
18
79
70
78
80
81
83
82
84
3
CuCl2
4
InCl3
5
Cu(OTf)2
Mg(OTf)2
Zn(OTf)2
Yb(OTf)3
Y(OTf)3
Bi(OTf)3
La(OTf)3
Sc(OTf)3
6
7
8
9
10
11
12
aReaction conditions: aniline (1 mmol), 2,5-dimethoxy-tetra-
hydrofuran (1 mmol), catalyst (5 mol%) and toluene (2 ml),
110°C, 40 min.
bIsolated yields.
The application of new types of Lewis acid catalysts to
replace traditional, harmful, and corrosive mineral acids
is a rapidly growing area in organic synthesis. Among the
QHZꢁ YDULRXVꢁ NLQGVꢁ RIꢁ /HZLVꢁ DFLGꢂꢁ UDUHꢀHDUWKꢁ PHWDOꢁ WULÀDWHVꢁ
enjoy great prestige and are frequently used as potent
environmentally-benign Lewis acids and have the advantages
of low toxicity, high stability, easy handling and recovery
from water, and high reactivity in green media such as water,
ionic liquids, super critical CO2, solvent-free conditions as
well as solid supported synthesis.14-16
5HFHQWO\ꢂꢁZHꢁKDYHꢁDOVRꢁVXFFHVVIXOO\ꢁDSSOLHGꢁPHWDOꢁWULÀDWHVꢁLQꢁ
several organic synthesis reactions.18-23 To deepen our interest
in Lewis acid-catalysed organic reactions, we herein reported
a practical method for the synthesis of the N-substituted, 2-
and 3-unsubstituted pyrroles.
Initially, we investigated the effect of catalysts for the
formation of pyrrole from aniline (Scheme 1), and the results
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VFDQGLXPꢁWULÀDWHꢁLVꢁVKRZQꢁWRꢁEHꢁWKHꢁPRVWꢁHIIHFWLYHꢆꢁ,QꢁFRQWUDVWꢂꢁ
Lewis acids such as FeCl3, CuCl2 and InCl3 gave much lower
yields. Nevertheless, no product was obtained in the absence
RIꢁFDWDO\VWVꢂꢁZKLFKꢁDOVRꢁIXUWKHUꢁSURYHGꢁWKDWꢁVFDQGLXPꢁWULÀDWHꢁ
does play an important role in this reaction.
In order to optimise the reaction parameters, we carried out
VHYHUDOꢁUHDFWLRQVꢁXVLQJꢁVFDQGLXPꢁWULÀDWHꢁDVꢁWKHꢁFDWDO\VWꢁWRꢁVWXG\ꢁ
the effect of solvents and reaction temperature. The results
are summarised in Table 2. In addition, we further studied the
LQÀXHQFHꢁ RIꢁ WKHꢁ DPRXQWꢁ RIꢁ VFDQGLXPꢁ WULÀDWHꢁ RQꢁ WKHꢁ UHDFWLRQꢁ
\LHOGVꢆꢁ,QꢁWKHꢁSUHVHQFHꢁRIꢁꢄꢂꢁꢇꢂꢁDQGꢁꢅꢁPROꢈꢁVFDQGLXPꢁWULÀDWHꢂꢁWKHꢁ
corresponding yields were 92%, 91%, and 74%, respectively.
7KHꢁUHVXOWVꢁVKRZꢁWKDWꢁꢇꢁPROꢈꢁRIꢁVFDQGLXPꢁWULÀDWHꢁZDVꢁVXI¿FLHQWꢁ
to catalyse the reaction and excessive amount of catalyst did not
increase the yields. In the light of this, subsequent studies were
carried out under the following optimised conditions, that is,
3 mol% Sc(OTf)3, dioxane at 100°C.
7RꢁHYDOXDWHꢁWKHꢁVFRSHꢁRIꢁWKHꢁSURWRFROꢉVꢁDSSOLFDWLRQꢂꢁZHꢁ¿UVWꢁ
applied the above-mentioned optimised conditions to a variety
of aromatic amines and the results are summarised in Table 3.
Table 2 Sc(OTf)3-catalysed synthesis of pyrrole from aniline
under different reaction conditionsa
Entry
Solvent
Temp./°C
Yield/%b
1
2
3
4
5
6
7
8c
CH2Cl2
110
110
110
110
110
110
90
81
CHCl3
67
CH3CN
71
86
Dioxane
CH3NO2
n-Hexane
Dioxane
Dioxane
69
14
60
100
92, 91,c 74d
aReaction conditions: aniline (1 mmol), 2,5-dimethoxy-
tetrahydrofuran (1 mmol) and Sc(OTf)3 (5 mol%) were run for
40 min.
bIsolated yields.
cSc(OTf)3 (3 mol%).
dSc(OTf)3 (1 mol%).
* Correspondent. E-mail: hywu3@163.com

JOURNAL OF CHEMICAL RESEARCH 2009 15
Table 3 Formation of pyrroles from various aromatic aminesa
under the same conditions. The desired product of 3o was
obtained in excellent yield (95%) for a short time (30 min)
and the product of 3p was also obtained in good yield (85%)
for 300 min (Scheme 2). Moreover, we also examined the
reaction of aliphatic amines with 2. Unfortunately, scandium
WULÀDWHꢁIDLOHGꢁWRꢁSURPRWHꢁWKHꢁFRQGHQVDWLRQꢁRIꢁDOLSKDWLFꢁDPLQHVꢁ
and 2,5-dimethoxytetrahydrofuran.
3 mol % Sc(OTf)3
Dioxane, 100oC
+
Ar
N
ArNH2
MeO
OMe
O
1
2
3
Entry ArNH2
Product Time/min Yield/%b
,Qꢁ VXPPDU\ꢂꢁ DQꢁ HI¿FLHQWꢁ SURWRFROꢁ IRUꢁ WKHꢁ V\QWKHVLVꢁ RIꢁ
N-substituted, 2- and 3-unsubstituted pryyoles has been
achieved with good to excellent yields. Compared with
UHSRUWHGꢁ PHWKRGVꢂꢁ WKHꢁ UHDFWLRQꢁ XVLQJꢁ VFDQGLXPꢁ WULÀDWHꢁ DVꢁ
catalyst has exhibited the accelerated reaction time, mild
conditions, improved yields, simple work-up and avoidance
of strong acidic reaction conditions. In additional, this method
demonstrated good functional group tolerance and amines
such as aromatic amines, sulfonamide, and benzamide could
be good substrates.
1
2
C6H5NH2
3a
3b
3c
3d
3e
3f
3g
3h
3i
40
40
40
40
40
30
30
90
30
20
30
91
90
86
83
85
77
74
88
81
80
80
4-(CH3)C6H4NH2
2-(CH3)C6H4NH2
4-(Cl)C6H4NH2
3-(Cl)C6H4NH2
4-(NO2)C6H4NH2
2-(NO2)C6H4NH2
3
4
5
6
7
8
2-(OCH3)C6H4NH2
4-(I)C6H4NH2
2-(CN)C6H4NH2
2-(Cl)-5-(NO2)C6H3NH2
9
10
11
3j
3k
12
3l
30
94
Experimental
NH2
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the melting points were uncorrected. 1H NMR and 13C NMR spectra
were measured on a FT-Bruker AT-300 spectrometer (1H: 300 MHz,
13C: 75 MHz), using CDCl3 as the solvent with tetramethylsilane
(TMS) as an internal standard at room temperature. Chemical shifts
are given in G relative to TMS, the coupling constants J are given in
Hz. Mass spectra were measured in a Shimadzu GC-MS QP 2010.
Elemental analyses were carried out using a Carlo-Erba EA1112
instrument. Column chromatography was performed using EM Silica
gel 60 (300–400 mesh)
Cl
NH2
13
14
3m
3n
240
300
84
74
N
N
N
NH2
aReaction conditions: aromatic amines (1 mmol), 2,5-dimethoxy-
tetrahydrofuran (1 mmol), dioxane (2 ml), and Sc(OTf)3 (3
mol%) were heated at 100°C.
bIsolated yields.
In all cases, the corresponding N-substituted pyrroles are
formed in good to excellent yields.
General procedure for preparation of 3a–p
A Schlenk reaction tube was charged with amine (1 mmol), 2,5-
dimethoxytetrahydrofuran (1 mmol), Sc(OTf)3 (3 mol%), and dioxane
(2 ml). The mixture was stirred at 100°C for a certain period of time
as indicated in Table 1 and Scheme 2. The progress of the reaction
was monitored by TLC. The reaction mixture was concentrated in
vacuoꢁDQGꢁWKHꢁUHVLGXHꢁZDVꢁSXUL¿HGꢁE\ꢁÀDVKꢁFROXPQꢁFKURPDWRJUDSK\ꢁ
on a silica gel to give the desired, pure product. The physical and
spectra data of the compounds 3a–p are as follows.
As shown in Table 3, a series of aromatic amines bearing
either electron-donating (Table 3, entries 2–3, 8) or electron-
withdrawing (Table 3, entries 4–7, 10–11) groups on the
aromatic ring was investigated. The reaction occurred
smoothly without showing a marked substituent effect in the
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aromatic ring have no obvious effect on the yield and the
reaction time under the above optimal conditions. Note that
the same substituent group at different positons of the phenyl
ULQJꢁDOVRꢁGLGꢁQRWꢁPDNHꢁDꢁVLJQL¿FDQWꢁGLIIHUHQFHꢁLQꢁWKLVꢁSURWRFROꢁ
(Table 3, entries 2–3, 4–5, 6–7).
1-phenyl-1H-pyrrole (3a): Pale yellow solid, m.p. 58–59°C (lit.24
62°C); 1H NMR: G 7.39–7.47 (m, 4H), 7.23–7.29 (m, 1H), 7.11 (t, J =
2.2 Hz, 2H), 6.37 (t, J = 2.2 Hz, 2H). 13C NMR: G 140.5, 129.2, 125.3,
120.2, 119.0, 110.0.
1-p-tolyl-1H-pyrrole (3b): White solid, m.p. 80–81°C (lit.25 81.5–
83°C); 1H NMR: G 7.20–7.30 (m, 4H), 7.06 (t, J = 1.7 Hz, 2H), 6.34
(t, J = 1.6 Hz, 2H), 2.38 (s, 3H). 13C NMR: G 138.2, 135.0, 129.7,
120.2, 119.1, 109.7, 20.5.
Furthermore, we examined the reactivity of heteroaromatic
amines with 2,5-dimethoxytetrahydrofuran (2) in the presence
RIꢁVFDQGLXPꢁWULÀDWHꢆꢁ+RZHYHUꢂꢁJRRGꢁ\LHOGVꢁZHUHꢁRQO\ꢁREWDLQHGꢁ
by prolonged reaction time (Table 3, entries 13–14). For
example, the reaction of 5-chloropyridin-2-amine with 2 in the
presence of VFDQGLXPꢁ WULÀDWHꢁ IRUꢁ ꢃꢊꢋꢁ PLQXWHVꢁ IXUQLVKHGꢁ ꢄꢀ
chloro-2-(1H-pyrrol-1-yl)pyridine (3m) in 84% yield (Table 3,
entry 13). Simiarly, when pyrimidin-2-amine was employed
for 300 minutes, 2-(1H-pyrrol-1-yl)pyrimidine (3n) was
obtained in 74% yield (Table 3, entry 14).
1-o-tolyl-1H-pyrrole (3c): Yellow oil, 1H NMR: G 7.23–7.29 (m,
4H), 6.79 (t, J = 2.1 Hz, 2H), 6.31–6.33 (m, 2H), 2.21 (s, 3H). 13C
NMR: 140.3, 133.5, 130.7, 127.2, 126.3, 126.2, 121.7, 108.4, 17.5.
EI-MS: m/z (%): 157 (95). Anal. Calcd for C11H11N: C, 84.04; H,
7.05; N, 8.91. Found: C, 84.09; H, 7.01; N, 8.90.
1-(4-chlorophenyl)-1H-pyrrole (3d):26 Yellow solid, m.p. 88°C;
1HNMR:G7.22–7.40(m,4H),7.03(t,J=2.2Hz,2H),6.35(t,J=2.1Hz,
2H). 13C NMR: G 139.3, 131.0, 129.6, 121.6, 119.2, 110.8.
1-(3-chlorophenyl)-1H-pyrrole (3e): White solid, 1H NMR: G 7.20–
7.39 (m, 4H), 7.05 (t, J = 2.2 Hz, 2H), 6.35 (t, J = 2.2 Hz, 2H). 13C
NMR: 141.4, 134.9, 130.3, 125.3, 120.3, 118.9, 118.1, 110.7. EI-MS:
m/z (%): 77 (91). Anal. Calcd for C10H8ClN: C, 67.62; H, 4.54; N,
7.89. Found: C, 67.55; H, 4.56; N, 7.93.
7Rꢁ H[WHQGꢁ WKHꢁ VFRSHꢁ RIꢁ WKLVꢁ VFDQGLXPꢁ WULÀDWHꢀ
catalysed method of N-substituted pyrrole formation, 4-
methylbenzenesulfonamide and 2-chlorobenzamide were used
O
S
O
O
S
O
3 mol % Sc(OTf)3
+
NH2
H3C
H3C
N
H3CO
OCH3
40 min, 100 o
C
O
3o, yield 95%
O
C
O
C
3 mol % Sc(OTf)3
300 min, 100 oC
NH2
+
H3CO
N
OCH3
O
Cl
Cl
3p, yield 85%
Scheme 2 Reaction of 2,5-dimethoxytetrahydrofuranm with 4-methylbenzenesulfonamide or 2-chlorobenzamide.

16 JOURNAL OF CHEMICAL RESEARCH 2009
1-(4-nitrophenyl)-1H-pyrrole (3f):27 Yellow solid, m.p. 187°C;
1H NMR: G 8.29–8.34 (m, 2H), 7.49–7.55 (m, 2H), 7.16–7.19 (m,
2 H), 6.42–6.44 (m, 2H). 13C NMR: G 144.9, 144.4, 125.3, 119.1,
118.8, 112.2.
We are grateful to the National Natural Science Foundation
of China (No. 20676123) and Natural Science Foundation of
=KHMLDQJꢁ3URYLQFHꢁꢌ<ꢊꢋꢍꢋꢅꢋꢎꢏꢁIRUꢁ¿QDQFLDOꢁVXSSRUWꢆ
1
1-(2-nitrophenyl)-1H-pyrrole (3g):28 Yellow oil, H NMR: G 7.82
Received 20 September 2008; accepted 19 October 2008
Paper 08/0185 doi:10.3184/030823409X393628
Published online: 16 January 2009
(t, J = 0.7 Hz, 1H), 7.63–7.64 (d, J = 0.4 Hz, 1H), 7.43–7.48 (m, 2H),
6.79 (m, 2H), 6.35 (m, 2H). 13C NMR: G 145.0, 133.9, 132.9, 127.5,
127.3, 124.6, 121.0, 110.7.
1
1-(2-methoxyphenyl)-1H-pyrrole (3h): Dark yellow oil, H NMR:
G 7.03–7.12 (m, 2H), 6.79–6.85 (m, 4H), 6.17 (t, J = 2.2 Hz, 2H), 3.58
(s, 3H). 13C NMR: G 152.6, 130.2, 127.3, 125.6, 121.9, 120.8, 112.3,
108.7, 55.6. EI-MS: m/z (%): 173 (100). Anal. Calcd for C11H11NO:
C, 76.28; H, 6.40; N, 8.09. Found: C, 76.35; H, 6.32; N, 8.08.
1-(4-iodophenyl)-1H-pyrrole(3i):Whitecrystallinesolid,M.p.130–
131°C (lit.29 128–129°C); 1H NMR: G 7.68–7.72 (m, 2H), 7.10–7.14
(m, 2H), 7.01–7.04 (m, 2H), 6.34 (t, J = 2.0 Hz, 2H). 13C NMR:
G 140.1, 138.2, 121.9, 118.8, 110.6, 89.1.
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2-(1H-pyrrol-1-yl)benzonitrile (3j): Yellow oil, H NMR: G 7.64–
7.77 (m, 2H), 7.36–7.46 (m, 2H), 7.11–7.14 (m, 2H), 6.41–6.43 (m,
2H). 13C NMR: G 142.7, 134.2, 133.8, 126.3, 124.8, 120.8, 116.7,
110.9, 106.5. EI-MS: m/z (%): 168 (100). Anal. Calcd for C11H8N2:
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6
7
8
9
1
1-(2-chloro-5-nitrophenyl)-1H-pyrrole (3k): Yellow oil, H NMR:
G 8.13–8.25 (m, 2H), 7.72 (d, J = 8.8 Hz, 1H), 6.99 (t, J = 2.1 Hz,
2H), 6.41 (t, J = 2.1 Hz, 2H). 13C NMR: G 146.5, 139.2, 135.9, 131.4,
122.2, 122.1, 121.7, 110.3. EI-MS: m/z (%): 222 (71). Anal. Calcd
for C10H7ClN2O2: C, 53.95; H, 3.17; N, 12.58. Found: C, 53.91; H,
3.20; N, 12.55.
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7.96 (m, 3H), 7.46–7.56 (m, 4H), 7.04 (t, J = 2.1 Hz, 2H), 6.46 (t,
J = 2.1 Hz, 2H). 13C NMR: G 137.9, 133.9, 129.5, 127.7, 127.5, 126.6,
126.2, 124.9, 122.94, 122.91, 122.8, 108.7.
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m.p. 68–69°C; 1H NMR: G 8.37 (d, J = 2.5 Hz, 1H), 7.66–7.70 (m, 1H),
7.46 (t, J = 2.3 Hz, 2H), 7.24 (d, J = 8.7 Hz, 1H), 6.37 (t, J = 2.3 Hz,
2H). 13C NMR: G 149.3, 147.0, 137.8, 127.3, 117.8, 111.6, 111.4.
EI-MS: m/z (%): 178 (100). Anal. Calcd for C9H7ClN2: C, 60.52; H,
3.95; N, 15.68. Found: C, 60.48; H, 4.03; N, 15.70.
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82°C; 1H NMR: G 8.59 (d, J = 4.8 Hz, 2H), 7.78 (t, J = 2.4 Hz, 2H),
6.99–7.03 (m, 1H), 6.34 (t, J = 2.3 Hz, 2H). 13C NMR: G 158.3,
156.3, 119.0, 117.0, 112.0. EI-MS: m/z (%): 145 (100). Anal. Calcd
for C8H7N3: C, 66.19; H, 4.86; N, 28.95. Found: C, 66.11; H, 4.50;
N, 29.39.
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22 W.K. Su; Z. Hong; W.G. Shan and X.X. Zhang; Eur. J. Org. Chem., 2006,
2723.
23 W.K. Su and C. Jin, Org. Lett., 2007, 9, 993.
24 H.J. Cristau, P.P. Cellier, J.F. Spindler and M. Taillefer, Chem. Eur. J.,
2004, 10, 5607.
1-tosyl-1H-pyrrole (3o): White solid, m.p. 101–102°C (lit.9 96.9–
1
97.5°C); H NMR: G 7.74 (d, J = 8.4 Hz, 2H), 7.25–7.29 (m, 2H),
25 J.F. Hartwig, M. Kawatsura, S.I. Hauck, K.H. Shaughnessy and L.M.
Alcazar-Roman, J. Org. Chem., 1999, 64, 5575.
7.15 (t, J = 2.3 Hz, 2H), 6.28 (t, J = 2.3 Hz, 2H), 2.39 (s, 3H). 13C
NMR: G 144.6, 135.8, 129.6, 126.5, 120.4, 113.2, 21.3.
26 H.C. Ma and X.Z. Jiang, J. Org. Chem., 2007, 72, 8943.
27 B.M. Choudary, C. Sridhar, M.L. Kantam, G.T. Venkanna and B. Sreedhar,
J. Am. Chem. Soc., 2005, 127, 9948.
(2-chlorophenyl)(1H-pyrrol-1-yl)methanone (3p): Pale yellow oil,
1H NMR: G 7.33–7.46 (m, 4H), 7.10 (s, 2H), 6.30 (t, J = 2.3 Hz, 2H).
13C NMR: G 164.9, 133.4, 131.5, 131.2, 129.8, 128.7, 126.5, 120.1,
113.6. EI-MS: m/z (%): 205 (16). Anal. Calcd for C11H8ClNO: C,
64.25; H, 3.92; N, 6.81. Found: C, 64.16; H, 3.82; N, 6.87.
28 Y. Harrak, S. Weber, A.B. Gómez, G. Rosell and M.D. Pujol, ARKIVOC,
2007 (iv), 251.
29 M.J. Plater and T. Jackson, Tetrahedron, 2003, 59, 4673.