Fischer indole synthesis in bronsted acidic ionic liquids: A green, mild, and regiospecific reaction system

Article abstract of DOI:10.1002/ejoc.200600886

A novel one-pot Fischer indole synthesis approach has been developed by using Bronsted acidic ionic liquids as dual solvent-catalysts. Yields of 83-97 % were obtained after reaction in BMImHSO₄ at 70-110°C in 0.5-6 h, and exclusive formation of 2,3-disubstituted indoles was observed in the reaction of alkyl methyl unsymmetrical ketones. The indoles produced could be conveniently separated from the reaction mixture without any volatile organic solvents, and the BMImHSO₄ could be readily reused without efficiency loss after simple treatment involving only 1 equiv. of HCl for neutralization followed by filtration. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejoc.200600886

FULL PAPER
DOI: 10.1002/ejoc.200600886
Fischer Indole Synthesis in Brønsted Acidic Ionic Liquids: A Green, Mild, and
Regiospecific Reaction System
Dan-Qian Xu,[a] Wen-Long Yang,[a] Shu-Ping Luo,[a] Bing-Tao Wang,[a] Jian Wu,[a] and
Zhen-Yuan Xu*[a]
Keywords: Ionic liquids / Indoles / Regiospecificity / Recycling
A novel one-pot Fischer indole synthesis approach has been
developed by using Brønsted acidic ionic liquids as dual sol-
vent-catalysts. Yields of 83–97% were obtained after reaction
mixture without any volatile organic solvents, and the
BMImHSO4 could be readily reused without efficiency loss
after simple treatment involving only 1 equiv. of HCl for neu-
in BMImHSO4 at 70–110 °C in 0.5–6 h, and exclusive forma- tralization followed by filtration.
tion of 2,3-disubstituted indoles was observed in the reaction
of alkyl methyl unsymmetrical ketones. The indoles pro-
duced could be conveniently separated from the reaction
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
per area. In addition, traditional organic solvents are still
used which can have a considerable environmental impact.
Noncatalytic Fischer indole syntheses have also been
studied in different solvents such as ethylene glycol, diethyl-
ene glycol, and tetralin; however, high temperatures above
200 °C are required for such a cyclization.[7]
1. Introduction
The indole ring system is probably the most ubiquitous
heterocycle that represents an important structural compo-
nent in many pharmacologically active compounds. Al-
though many methods have been developed for the synthe-
sis of indoles,[1] Fischer indole synthesis is still one of the
most versatile and widely employed methodologies for the
preparation of indole intermediates and biologically active
compounds.[2] It can be regarded as the elimination of am-
monia from the N-arylhydrazone of a ketone through a
[3,3] sigmatropic rearrangement with an acid catalyst
(Scheme 1).[3] Since its discovery in the 1880s by Emil
Fischer, various catalysts have been used to effect the cycli-
zation of arylhydrazones derived from ketones. Alternative
catalysts, including Brønsted acids (H2SO4, HCl, PPA,
AcOH),[4] Lewis acids (ZnCl2, TiCl4, PCl3),[5] and solid ac-
ids (zeolite, montmorillonite clay),[6] have been reported for
the synthesis of the indole nucleus, but the search for new
catalysts is still being actively pursued because the reported
Brønsted and Lewis acids are environmentally unfriendly,
hazardous, or difficult to reuse, and they are usually re-
quired in large amounts. For example, PCl3 is needed in
equimolar quantity, PPA in 8–9-fold excess by weight, and
ZnCl2 is used in as large as a 3-fold excess. Although utiliza-
tion of solid acids may solve the above problems, solid ac-
idsЈ own shortcomings limit their application to some de-
gree such as restricted accessibility of the matrix-bound
acidic sites, rapid deactivation, and decrease of activity site
Scheme 1. The mechanism for Fischer indole synthesis with an acid
catalyst.
The development of one-pot approaches to the indole
scaffold is attracting considerable attention because of their
significance from both economical and ecological points of
view. These methods usually directly start from a single,
commercially available precursor, which obviates the prepa-
ration or isolation of the unstable arylhydrazones
(Scheme 2). Some reported examples within this context are
listed below: (1) one-pot synthesis of 5-substituted N,N-di-
methyltryptamine with the use of 4% H2SO4;[8] (2) one-pot
synthesis of 1,2,3,4-tetrahydrocarbazole, which involves the
use of trifluoroacetic anhydride and the recycling of tri-
fluoroacetic acid;[9] (3) one-pot synthesis of the indole scaf-
fold by using K-10 montmorillonite as a catalyst in
MeOH.[6c] Clearly, in spite of the diverse synthetic ap-
proaches developed so far, it is hoped that environmentally
[a] State Key Laboratory Breeding Base of Green Chemistry-Syn-
thesis Technology, Zhejiang University of Technology,
Hangzhou 310032, P. R. China
Fax: +86-571-88320066
E-mail: greenchem@zjut.edu.cn
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FULL PAPER
friendly one-pot options will be adopted and it seems pos- these BAILs is easy and the remaining BAILs can be read-
sible. Recently, with the development of ionic liquids, there ily recycled. Here, we first report a green, mild, and regio-
have been reports concerning the one-pot Fischer indole specific reaction system for the one-pot Fischer indole syn-
synthesis in Lewis acidic ionic liquids such as 1-butyl- thesis by using BAILs. In the reaction process, no other
pyridinium chloride·3AlCl3 or choline chloride·2ZnCl2 organic solvents or catalysts are used and the BAILs act
(Scheme 3).[10]
as dual solvent-catalysts. The BAILs used are depicted in
Figure 1.
Scheme 2. The Fischer indole synthesis with (a) conventional two-
step route and (b) one-pot method.
Figure 1. The chemical structures of the BAILs used.
2. Results and Discussion
Initially the one-pot Fischer indole synthesis of phenyl-
hydrazine with cyclohexanone was carried out as a model
reaction in various BAILs. As shown in Table 1 all of the
BAILs tested proved catalytically active in the indole syn-
thesis under mild reaction conditions with yields in the
range of 81–92%. The best result was obtained in
BMImHSO4, in which the reaction was performed at 70 °C
and 92% yield of 1,2,3,4-tetrahydrocarbazole was obtained
after 1 h (Entry 1). Even at room temperature, yields of
89% and 82% could also be obtained in BMImHSO4 and
BMImH2PO4, respectively (Entries 2 and 5). The results
indicate that BAILs have a high catalytic activity for the
Fischer indole synthesis. On the other hand, the catalytic
performance of BAILs is dependent on the character of the
Scheme 3. The Fischer indole synthesis with the use of Lewis acidic
ionic liquids.
With the approach to minimize the use of hazardous ma-
terials or volatile solvents in reactions, ionic liquids have
attracted much attention because of their advantageous
properties, which include negligible vapor pressure, high
thermal stability, large electrochemical window, and easy of
recycling.[11] In recent years, ionic liquids have been widely
researched in many organic reactions for the replacement of
traditional solvents or catalysts.[12] In particular, the task-
specific ionic liquids (TSILs), in which a functional group
is covalently tethered to the cation or anion, further en-
larges the application scope of ionic liquids in chemistry.[13]
Brønsted acidic ionic liquids (BAILs) are one kind of
TSILs, which contain a Brønsted acid functionalized com-
ponent in the cation or anion. Because of their unique fea-
tures, such as their nonvolatility, flexibility, noncorro-
siveness, and immiscibility with nonpolar organic solvents,
they have exhibited great potential as alternatives to con-
ventional acids in acid-catalyzed reactions, such as Fischer
esterification,[14] acetalization of carbonyls,[15] dimerization
of α-methylstyrene,[16] alkylation of phenol with TBA,[17]
and Mannich reactions.[18] In contrast to conventional acids
and Lewis acidic ionic liquids, the BAILs have their own
advantages as follows: (1) their “greenness” features make
them environmentally benign and safe; (2) they are air- and
moisture-stable; (3) their preparation procedures are rela-
tively simple and the acidity can be adjusted by the careful
choice of the cation or anion; (4) product isolation from
–
–
cation and anion. For BAILs with HSO4 and H2PO4 as
counteranions, the imidazolium cation with a n-butyl group
on the alkyl side chain was more advantageous to the reac-
Table 1. The one-pot Fischer indole synthesis of phenylhydrazine
with cyclohexanone to afford 1,2,3,4-tetrahydrocarbazole in vari-
ous BAILs.
Entry
BAILs
Time
[h]
T
[°C]
Yield
[%]
1
2
3
4
5
6
7
8
9
BMImHSO4
BMImHSO4
HMImHSO4
BMImH2PO4
BMImH2PO4
HMImH2PO4
HMImTA
HMImBF4
HMImNO3
HMImOTf
1
5
1
70
92
89
82
89
82
80
87
85
81
82
room temp.
70
1
70
room temp.
100
14
0.75
1
1
7
70
70
100
100
10
1.5
1008
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Fischer Indole Synthesis in Brønsted Acidic Ionic Liquids
FULL PAPER
Table 2. One-pot synthesis of Fischer indoles from ketones/aldehydes and phenylhydrazines (substituted phenylhydrazines) in
BMImHSO4.[19-31]
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D.-Q. Xu, W.-L. Yang, S.-P. Luo, B.-T. Wang, J. Wu, Z.-Y. Xu
FULL PAPER
tion than the cation with a hydrogen on the N atom of the
imidazolium, 92%, 89% and 82%, 80% yields, respectively
(Entry 1 versus Entry 3 and Entry 4 versus Entry 6). For
BAILs with the same [HMIm]+ countercation, the catalytic
performance of HMImNO3 and HMImOTf were worse
than that of the others; phenylhydrazone was formed in the
first step, but the [3,3] sigmatropic rearrangement at 70 °C,
required for the completion of the one-pot Fischer indole
synthesis, did not occur. An elevated temperature of 100 °C
and a longer reaction time were required (Entries 9–10) to
complete the synthesis. On the basis of the obtained results,
BMImHSO4 was found to be superior in terms of activity
and yield.
The results of a series of Fischer indole syntheses in
BMImHSO4 are summarized in Table 2. The products were
characterized by IR, 1H NMR spectroscopic, and MS
analysis, which were consistent with the literature data. As
can been seen, BMImHSO4 proved to be highly catalyti-
cally active for the one-pot conversion of the ketones/alde-
hydes and hydrazines to their corresponding indoles at 70–
110 °C in 83–97% yields within 6 h. The results are better
than those previously reported in Lewis acidic ionic liquid
systems[10] and suggest that BMImHSO4 would be a more
suitable catalytic system for Fischer indole synthesis. Within
the operating conditions, the nature of the substrates was
taken into account for the temperature/time parameter.
When the temperature was increased, the maximum yield
was reached more quickly: for a 30 °C rise in temperature,
Scheme 4. Acid-catalyzed Fischer indole reaction from an unsym-
metrical ketone.
desired indoles are insoluble in water, the indole products
can be directly separated by adding water into the synthetic
system after the reaction is complete. The remaining
BMImHSO4 can then be recycled after removal of water
under vacuum, where some solid material was found in
BMImHSO4. According to the mechanism of Fischer in-
dole synthesis, 1 equiv. of NH3 is produced in the reaction,
–
which can coordinate with HSO4 to form ammonium hy-
drogen sulfate, which is insoluble in BMImHSO4. With the
repeated use of BMImHSO4 for a series of reactions, am-
monia will accumulate and reduce the efficiency of the reac-
tion as observed by a decrease in the yield. Alternatively,
the indole products were isolated by adding water and
1 equiv. HCl, followed by the similar operation as above; in
this case, the recycled BMImHSO4 could be reused with
Table 3. Recycling of BMImHSO4 in the synthesis of 1,2,3,4-tetra-
the reaction time was shortened from 3 h to 1 h (Entry 8 hydrocarbazole.
versus Entry 9), and from 6 h to 1 h (Entry 10 versus Entry
[a]
[b]
Run
BMImHSO4
BMImHSO4
11). Moreover, a relatively elevated temperature promoted
the completion of the one-pot conversion: the [3,3] sigma-
tropic rearrangement of the corresponding phenylhydraz-
one from phenylhydrazine and 4-methyl-2-pentanone oc-
curred rapidly at 110 °C, but did not occur to any extent at
70 °C (Entry 14).
t [h]
Yield [%]
t [h]
Yield [%]
1
2
3
4
5
1
92
89
82
74
trace
1
1
1
1
1
92
91
91
92
90
1.5
2.5
5.0
7
An important issue of the Fischer indole synthesis is the
selectivity of the cyclization of the phenylhydrazones de-
rived from unsymmetrical ketones. In previous studies on
the Fischer indole synthesis of the phenylhydrazone of iso-
propyl methyl ketone, a mixture of 2,3,3-trimethylindole
and 2-isopropylindole was obtained in different acidic con-
ditions (Scheme 4).[32a] In our studies, in all cases a single
more substituted indole was obtained (Entries 8–14), which
indicates that BMImHSO4 has a high selectivity as a cata-
lyst. Reports have shown that the direction of cyclization is
governed by the acidity of the reaction medium.[32,1a] Lower
acid concentrations or weaker acids promote cyclization
towards the more branched carbon and higher acid concen-
trations or stronger acids enhance the extent of cyclization
at the less branched position. Therefore, BMImHSO4
should provide a weak acid system for the Fischer indole
synthesis and result in a more branched carbon in the in-
dole ring.
[a] Product isolation by adding H2O. [b] Product isolation by add-
ing H2O and 1 equiv. HCl.
Apart from the mild conditions of the process and its
excellent results, the simplicity of product isolation and the
possibility to recycle the BAILs offer a significant advan-
Figure 2. The results of Fischer indole synthesis with the use of
tage. Because BMImHSO4 is miscible with water and the recycled BMImHSO4.
1010
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Fischer Indole Synthesis in Brønsted Acidic Ionic Liquids
FULL PAPER
no appreciable decrease in the yield except for only small
mechanical loss (Table 3 and Figure 2). IR analysis with the
use of pyridine as a probe molecule[33] showed that the
wavenumber of the characteristic band corresponding to
coordination at Brønsted acid sites increased from
1540 cm–1 for pyridine/HCl to 1543 cm–1 for BMImHSO4/
pyridine/HCl, and 1551 cm–1 for fresh and recycled
BMImHSO4/pyridine (Figure 3), which demonstrated that
the added HCl acted just as an NH3 capture. Therefore, an
overall process for the one-pot Fischer indole synthesis can
be envisaged as shown in Figure 4.
3. Conclusion
In this paper, we describe a new method for the one-
pot synthesis of the Fischer indole scaffold with the use of
ecofriendly solvent BAILs which act as catalysts as well.
The products are obtained in a BMImHSO4 medium in
high yield, good purity, and without further purification.
In unsymmetrical cases, regiospecific formation of a single
product with more substitutes in the indole ring is obtained,
which indicates that the BMImHSO4 provides a weak acid
system for synthesis of indoles and has a high selectivity as
a catalyst. Compared with the conventional acid catalysts
and Lewis acidic ionic liquids, the present BMImHSO4-me-
diated Fischer indole synthesis conditions are quite mild
and efficient, the isolation–recycle process is convenient and
environmentally benign, and the BMImHSO4 can be reused
without any loss of efficiency. The developed method may
open new opportunities and alternatives of great interest
in the design and synthesis of naturally occurring indole
skeletons.
4. Experimental Section
General: Brønsted acidic ionic liquids were prepared according to
the reported procedures[14,18] including BMImHSO4, HMImHSO4,
BMImH2PO4, HMImH2PO4, HMImTA, HMImBF4, HMImNO3,
and HMImOTf. Melting points were measured with a Buchi Melt-
ing Point B-545. 1H NMR spectra were recorded with a Varian
PLUS 400 (TMS as internal standard). IR spectra were obtained
with a Bruker Equinox 55. GC–MS spectra were recorded with an
Agilent 6890N/5973N.
General Procedure for the Synthesis of Indoles: The Brønsted acidic
ionic liquid BMImHSO4 (5 mL) was mixed with cyclohexanone
(0.49 g, 5.0 mmol) in a 20 mL flask, and phenylhydrazine (0.54 g,
5.0 mmol) was added. The mixture was then stirred at 70 °C for
1 h. Reaction progress was monitored by GC–MS. After comple-
tion, the reaction mixture was cooled to room temperature and
HCl (0.5 , 10 mL) was added. The obtained 1,2,3,4-tetrahydrocar-
bazole was filtered off, washed with water, and dried. The remain-
ing BMImHSO4 phase was reused after removal of water under
Figure 3. FTIR spectra: (a) pure pyridine; (b) pyridine + HCl (2:1
by vol.); (c) BMImHSO4 + pyridine + HCl (2:1:1 by vol.); (d)
BMImHSO4 + pyridine (2:1 by vol.); (e) recycled BMImHSO4 (by
adding H2O and equiv.-HCl) + pyridine (2:1 by vol.).
1
vacuum and filtered off NH4Cl. H NMR (400 MHz, CDCl3): δ =
1.88–1.91 (m, 4 H), 2.71–2.73 (m, 4 H), 7.05–7.13 (m, 2 H, Harom.),
7.27 (d, J = 10.4 Hz, 1 H, Harom.), 7.46 (d, J = 7.2 Hz, 1 H, Harom.),
7.66 (br. s, 1 H, NH) ppm. IR (KBr): 3399 (NH), 3051 (ArH),
2926, 2848, 1446, 737 cm–1. MS (EI): m/z: 171[M]+.
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Received: October 10, 2006
Published Online: December 15, 2006
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Eur. J. Org. Chem. 2007, 1007–1012