Intramolecular redox reaction for the synthesis of N-aryl pyrroles catalyzed by Lewis acids

Article abstract of DOI:10.1039/c4ob02009j

An efficient approach to synthesize N-aryl pyrroles via Lewis acid-mediated 1,5-hydride shift and isomerization of 2-(3-pyrroline-1-yl)arylaldehydes has been achieved in up to 89% yield. This methodology is applicable to the synthesis of fluorazene deriva

Full text of DOI:10.1039/c4ob02009j

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Intramolecular redox reaction for the synthesis
of N-aryl pyrroles catalyzed by Lewis acids†
Cite this: DOI: 10.1039/c4ob02009j
Hong-Jin Du, Le Zhen, Xiaoan Wen, Qing-Long Xu* and Hongbin Sun*
Received 20th September 2014,
Accepted 10th October 2014
An efficient approach to synthesize N-aryl pyrroles via Lewis acid-mediated 1,5-hydride shift and isomeri-
zation of 2-(3-pyrroline-1-yl)arylaldehydes has been achieved in up to 89% yield. This methodology is
applicable to the synthesis of fluorazene derivatives as electron donor (D)/acceptor (A) molecules.
DOI: 10.1039/c4ob02009j
www.rsc.org/obc
Introduction
C–H bond functionalization strategies as a vibrant field have
made tremendous progress over the past decades due to
the intrinsic atom economy.1 The redox reaction as a novel
approach for C–H bond functionalization was revived by
Sames and co-workers.2 This type of reaction, mainly focusing
on a 1,5-relationship between the C–H bond and the electro-
philic site, is now widely studied.3 Until now, Michael
acceptors, imines and alkynes have been employed as electro-
positive fragments in this research area,2,4–6 but aldehydes are
reported rarely.7
N-aryl pyrrole containing compounds serve as an important
framework for the synthesis of various complex molecules and
exhibit a wide array of biological activities.8 Until now, the
most efficient method is to use metal-catalyzed cross-coupling
reactions that form N-aryl pyrroles by C–N bond formation.9
With this in mind, we turned our attention to the development
of an efficient method for the synthesis of N-aryl pyrroles
using a redox reaction.
Scheme 1 Strategies for the synthesis of N-substituted pyrroles.
Results and discussion
Compound 1a was chosen as a model substrate for the optim-
ization (Table 1). Firstly, different solvents were tested in the
reaction with Sc(OTf)3 as the catalyst (entries 1–6, Table 1). To
our delight, with DCE as the solvent, the reaction proceeded
smoothly with an excellent yield (88% yield, entry 5, Table 1).
Notably, the reaction could not occur when acetone was used
as a solvent (entry 3, Table 1).
In 2009, the Tunge group disclosed that redox isomeriza-
tion could be used to synthesize N-alkyl pyrroles via reductive
amination (eq 1, Scheme 1).10 Herein, we report a Lewis acid-
catalyzed intramolecular redox reaction that utilizes the
inherent reducing power of 3-pyrrolines (eq 2, Scheme 1),
With DCE as the solvent, different Brønsted and Lewis
acids were evaluated as the catalysts for the reaction. The
addition of AlCl3 (10 mol%) to the reaction at reflux conditions
led to product formation, but the reaction remained incom-
plete after 12 h (entry 8, Table 1). Using Zn(OTf)2 as the cata-
lyst did not lead to a shortened reaction time, but resulted in a
reduced product yield (76% yield, entry 10, Table 1). Other
Brønsted and Lewis acids were found to be viable catalysts for
this reaction, but proved less efficient in terms of reaction
time and/or yield. When the temperature was reduced to
50 °C, the product was obtained with only 60% yield (entry 14,
Table 1). This reaction proceeded well under N2 to afford the
product in 87% yield (entry 16, Table 1), excluding the effect of
O2 on the reaction. It was found that the catalyst loading of
Sc(OTf)3 could be reduced to 5 mol% without sacrificing the
chemical yields (entry 17, Table 1). The optimized conditions
used were DCE as the solvent and 5 mol% of Sc(OTf)3 as the
which leads to the formation of N-aryl pyrroles via
a
1,5-hydride shift and isomerization of 2-(3-pyrroline-1-yl)arylalde-
hydes. This reaction proceeded well under mild conditions
with an atom-economy and broad substrate scope.
Jiangsu Key Laboratory of Drug Discovery for Metabolic Disease and Center of Drug
Discovery, China Pharmaceutical University, 24 Tongjia Xiang, Nanjing 210009,
China. E-mail: hbsun2000@yahoo.com, xuqing307@126.com;
Tel: +86-025-83271198
†Electronic supplementary information (ESI) available: The procedure for the
synthesis of substituted 2-(3-pyrroline-1-yl)arylaldehydes 1, and the copies of the
spectra. See DOI: 10.1039/c4ob02009j
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Table 1 Optimization of the reactiona
Entry
Solvent
Acid
X
Time
Yielde (%)
1
2
3
4
5
6
7
8
THF
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
FeCl3
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
5
1 h
1 h
12 h
80
44
Trace
80
88
72
73
36
44
76
Trace
44
58
60
44
87
87
76
MeCN
Acetone
Toluene
DCE
CHCl3
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
40 min
15 min
1 h
15 min
12 h
30 min
12 h
3 h
Scheme 3 Synthesis of fluorazene (4).
catalyst at refluxing, which led to product 2a in 87% yield
(entry 17, Table 1).
AlCl3
9
Cu(OTf)2
Zn(OTf)2
PhCOOH
TfOH
p-TsOH·H2O
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
10
11
12
13
14b
15c
16d
17
18
19
The substrate scope of this transformation is illustrated in
Scheme 2. Generally, good yields were obtained for the sub-
strates with various R1 substituents (e.g. 4-Me, 5-Cl, 4-Cl, 5-Br,
4-Br, and 4-CF3), except for the strong electron-withdrawing
group (5-NO2). When the phenyl group of 1a was replaced by a
pyridyl group, the desired product 2j was obtained in only
36% yield, even in the presence of 20 mol% catalyst. For
substrate 1 with an aryl group substitution (e.g. phenyl,
20 min
20 min
4 h
48 h
15 min
15 min
1 h
2
1
12 h
80
a Reactions were performed in a given solvent under reflux on a 50 mg 2-naphthyl, 4-methylphenyl, and 4-methoxyphenyl) on the
scale. b React at 50 °C. c React at room temperature. d React under N2.
3-pyrroline moiety, the desired products (e.g. 2k, 2l, 2m, and
2n) were obtained in 70–89% yields.
e Isolated yield.
This methodology could be applicable in the scalable
synthesis of electron donor (D)/acceptor (A) molecules,
9H-pyrrolo-[1,2-a]-indole (fluorazene, FPP),11 through intramo-
lecular Pd-catalyzed cross-coupling reaction (Scheme 3).12
Thus, 1a (500 mg) was readily converted to N-aryl pyrrole (3) in
61% yield over two steps, which was further converted to FPP
(4) in 94% yield.
Experimental section
General procedure for the synthesis of N-aryl pyrroles
catalyzed by Lewis acid
Approximately substituted 2-(3-pyrroline-1-yl)arylaldehydes 1
(50 mg) were added to a reaction flask and then dissolved in
methylene chloride. Sc(OTf)3 (5 mol%) was added to the reaction
flask and the reaction was stirred under reflux conditions. The
reaction was monitored by TLC until complete consumption.
The solvents were concentrated and purified by flash chromato-
graphy (PE/EA = 10/1, v/v) to get the desired products 2.
Spectral data
2a,12 brown oil, 87% yield. 1H NMR (300 MHz, CDCl3) δ
7.62–7.53 (m, 1H), 7.45–7.36 (m, 2H), 7.35–7.30 (m, 1H), 6.86
(t, J = 2.0 Hz, 2H), 6.35 (t, J = 2.0 Hz, 2H), 4.59 (s, 2H), 1.70
(br s, 1H).
2b, colorless oil, 84% yield. 1H NMR (300 MHz, CDCl3) δ
7.43 (d, J = 7.8 Hz, 1H), 7.20 (d, J = 7.8 Hz, 1H), 7.14 (s, 1H),
6.87 (t, J = 1.8 Hz, 2H), 6.33 (t, J = 2.1 Hz, 2H), 4.51 (s, 2H),
Scheme 2 The substrate scope. Reaction conditions:
Sc(OTf)3 (5 mol%), DCE, reflux. a Sc(OTf)3 (20 mol%).
1 (50 mg),
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2.40 (s, 3H), 1.92 (br s, 1H); 13C NMR (75 MHz, CDCl3) δ 139.2, 1H); 13C NMR (75 MHz, CDCl3) δ 139.6, 135.9, 135.4, 129.4,
138.2, 132.5, 128.9, 128.0, 126.7, 121.9, 108.7, 60.6, 20.5; 128.7, 128.6, 127.9, 126.4, 125.7, 125.1, 123.6, 119.1, 107.5,
HRMS (ESI): Exact mass calcd for C12H13NONa [M + Na]+ 61.1; HRMS (ESI): Exact mass calcd for C17H15NONa [M + Na]+
210.0895, found 210.0903.
2c, brown oil, 54% yield. H NMR (300 MHz, CDCl3) δ 7.43
272.1051, found 272.1056.
2l, brown oil, 80% yield. H NMR (300 MHz, CDCl3) δ 7.99
1
1
(d, J = 8.5 Hz, 3H), 6.90–6.86 (m, 4H), 6.33 (t, J = 2.1 Hz, 2H), (s, 1H), 7.82 (t, J = 6.9 Hz, 3H), 7.72 (d, J = 8.4 Hz, 1H),
4.49 (s, 2H), 3.83 (s, 3H), 1.70 (br s, 1H); 13C NMR (75 MHz, 7.65–7.56 (m, 1H), 7.49–7.41 (m, 5H), 7.31 (s, 1H), 6.96 (s, 1H),
CDCl3) δ 161.0, 142.3, 132.3, 129.4, 123.8, 114.8, 113.3, 110.7, 6.78 (s, 1H), 4.66 (s, 2H), 1.71 (br s, 1H); 13C NMR (75 MHz,
62.2, 56.9; HRMS (ESI): Exact mass calcd for C12H13NO2Na [M CDCl3) δ 139.4, 135.9, 134.0, 132.8, 132.0, 129.4, 128.6, 128.2,
− H]− 226.0844, found 226.0850.
128.0, 127.6, 126.4, 126.0, 124.9, 124.4, 123.8, 122.6, 119.5,
1
2d, brown oil, 86% yield. H NMR (300 MHz, CDCl3) δ 7.59 107.7, 61.3; HRMS (ESI): Exact mass calcd for C21H17NONa [M
(s, 1H), 7.35 (d, J = 8.4 Hz, 1H), 7.25 (t, J = 7.6 Hz, 1H), 6.82 (s, + Na]+ 322.1208, found 322.1216.
2H), 6.35 (s, 2H), 4.53 (s, 2H), 1.96 (br s, 1H); 13C NMR
2m, brown oil, 83% yield. 1H NMR (300 MHz, CDCl3) δ
(75 MHz, CDCl3) δ 137.4, 133.1, 130.1, 128.4, 127.9, 127.3, 7.60–7.57 (m, 1H), 7.47 (d, J = 8.0 Hz, 2H), 7.42–7.39 (m, 3H),
121.8, 109.2, 60.2; HRMS (ESI): Exact mass calcd for 7.18 (d, J = 7.9 Hz, 2H), 7.14 (t, J = 3.0 Hz, 1H), 6.89 (t, J =
C11H10NONaCl [M + Na]+ 230.0349, found 230.0351.
2.4 Hz, 1H), 6.63 (t, J = 3.0 Hz, 1H), 4.61 (s, 2H), 2.37 (s, 3H),
1
2e, brown oil, 82% yield. H NMR (300 MHz, CDCl3) δ 7.50 1.90 (br s, 1H); 13C NMR (75 MHz, CDCl3) δ 139.6, 135.9,
(d, J = 8.2 Hz, 1H), 7.36 (dd, J = 8.3, 1.8 Hz, 1H), 7.31 (d, J = 135.3, 132.5, 129.4, 128.6, 127.9, 126.4, 125.9, 125.1, 123.4,
1.7 Hz, 1H), 6.85–6.82 (m, 2H), 6.35–6.32 (m, 2H), 4.51 (s, 2H), 118.7, 107.5, 61.2, 21.1; HRMS (ESI): Exact mass calcd for
1.91 (br s, 1H); 13C NMR (75 MHz, CDCl3) δ 142.0, 135.8, C18H17NONa [M + Na]+ 286.1208, found 286.1204.
1
135.2, 131.8, 129.2, 128.0, 123.7, 111.2, 62.1; HRMS (ESI):
2n, brown oil, 89% yield. H NMR (300 MHz, CDCl3) δ 7.58
Exact mass calcd for C11H10NONaCl [M + Na]+ 230.0349, found (dd, J = 5.8, 3.1 Hz, 1H), 7.54–7.46 (m, 2H), 7.46–7.32 (m, 3H),
230.0353. 7.13–7.06 (m, 1H), 6.92 (d, J = 8.7 Hz, 2H), 6.89–6.86 (m, 1H),
2f, brown oil, 80% yield. 1H NMR (300 MHz, CDCl3) δ 6.63–6.52 (m, 1H), 4.62 (s, 2H), 3.83 (s, 3H), 1.96 (br s, 1H); 13
C
7.53–7.42 (m, 3H), 6.83 (s, 2H), 6.33 (s, 2H), 4.50 (s, 2H), 2.00 NMR (75 MHz, CDCl3) δ 159.3, 141.1, 137.3, 130.8, 130.0,
(br s, 1H); 13C NMR (75 MHz, CDCl3) δ 140.7, 134.9, 130.7, 129.6, 129.3, 127.8, 127.7, 127.1, 124.8, 119.8, 115.6, 108.8,
130.6, 129.4, 122.2, 121.5, 109.8, 60.6; HRMS (ESI): Exact mass 62.6, 56.7; HRMS (ESI): Exact mass calcd for C18H17NO2Na
calcd for C11H9NOBr [M − H]− 249.9868, found 249.9873.
[M + Na]+ 302.1157, found 302.1160.
1
2g, brown oil, 80% yield. H NMR (300 MHz, CDCl3) δ 7.73
(d, J = 1.6 Hz 1H), 7.48 (dd, J = 8.3, 1.9 Hz, 1H), 7.16 (d, J =
8.4 Hz, 1H), 6.80 (s, 2H), 6.33 (s, 2H), 4.52 (s, 2H), 1.96 (br s, Conclusions
1H); 13C NMR (75 MHz, CDCl3) δ 138.5, 138.1, 131.8, 131.3,
128.0, 122.2, 121.4, 109.7, 60.6; HRMS (ESI): Exact mass calcd
for C11H9NOBr [M − H]− 249.9868, found 249.9861.
In summary, Lewis acid-mediated 1,5-hydride shift and iso-
merization of 2-(3-pyrroline-1-yl)arylaldehydes have been rea-
lized to afford an efficient approach to synthesize N-aryl
pyrroles. The application of this methodology in the synthesis
of fluorazene derivatives as electron donor (D)/acceptor (A)
molecules has also been achieved. Further exploration of the
inherent reducing power of 3-pyrroline for the synthesis of
other N-aryl pyrroles is ongoing in our laboratory.
2h, colorless oil, 82% yield. 1H NMR (300 MHz, CDCl3) δ
7.74 (d, J = 8.1 Hz, 1H), 7.67 (d, J = 8.2 Hz, 1H), 7.59 (s, 1H),
6.85 (s, 2H), 6.38 (d, J = 1.8 Hz, 2H), 4.61 (s, 2H), 2.32 (br s,
1H); 13C NMR (75 MHz, CDCl3) δ 139.9, 139.8, 130.8, 129.4,
124.4 (q, J = 3.6 Hz), 123.6 (q, J = 270.0 Hz), 123.5 (q, J =
3.7 Hz), 122.3, 110.1, 60.5; HRMS (ESI): Exact mass calcd for
C12H9NOF3 [M − H]− 240.0636, found 240.0640.
1
2i, yellow oil, 32% yield. H NMR (300 MHz, CDCl3) δ 8.54
Acknowledgements
(d, J = 2.0 Hz, 1H), 8.24 (dd, J = 8.6, 2.3 Hz, 1H), 7.46 (d, J =
8.6 Hz, 1H), 6.92 (s, 2H), 6.41 (s, 2H), 4.73 (s, 2H), 2.06 (br s,
1H); 13C NMR (75 MHz, CDCl3) δ 146.0, 144.1, 136.3, 126.2,
124.2, 123.1, 121.6, 110.4, 60.1; HRMS (ESI): Exact mass calcd
for C11H9N2O3 [M − H]− 217.0613, found 217.0619.
Financial support from National Natural Science Foundation
of China (grants 81373303 and 81473080) is gratefully
acknowledged.
1
2j, brown oil, 36% yield. H NMR (300 MHz, CDCl3) δ 8.43
(d, J = 4.2 Hz, 1H), 7.97 (d, J = 7.5 Hz, 1H), 7.28 (t, J = 6.1 Hz,
1H), 7.19 (s, 2H), 6.35 (s, 2H), 4.73 (s, 2H), 2.20 (br s, 1H); 13C
NMR (75 MHz, CDCl3) δ 150.7, 147.9, 139.1, 128.4, 122.0,
121.2, 110.2, 60.7; HRMS (ESI): Exact mass calcd for
C10H10N2ONa [M + Na]+ 197.0691, found 197.0696.
Notes and references
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2k, brown oil, 70% yield. 1H NMR (300 MHz, CDCl3) δ
7.59–7.57 (m, 3H), 7.43–7.35 (m, 5H), 7.24–7.18 (m, 2H),
6.91–6.89 (m, 1H), 6.67–6.66 (m, 1H), 4.61 (s, 2H), 2.00 (br s,
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