Ferric(III) nitrate: An efficient catalyst for the regioselective friedel-crafts reactions of indoles and tert-enamides in water

Article abstract of DOI:10.1002/ejoc.201000994

The ferric nitrate catalyzed Friedel-Crafts reactions of various indoles and tertiary enamides, which proceed in water at room temperature, are demonstrated for an efficient, atom-economical and environmentally friendly route to produce indole derivatives

Full text of DOI:10.1002/ejoc.201000994

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201000994
Ferric(III) Nitrate: An Efficient Catalyst for the Regioselective Friedel–Crafts
Reactions of Indoles and tert-Enamides in Water
Ran Jiang,[a] Xiao-Jin Wu,[a] Xu Zhu,[a] Xiao-Ping Xu,*[a] and Shun-Jun Ji*[a]
Keywords: Iron / Alkylation / Nitrogen heterocycles / Water chemistry / Green chemistry
The ferric nitrate catalyzed Friedel–Crafts reactions of vari- indole derivatives under mild conditions. Based on experi-
ous indoles and tertiary enamides, which proceed in water
at room temperature, are demonstrated for an efficient, atom-
economical and environmentally friendly route to produce
mental data and mechanistic studies, the iron(III) ion was
found to activate the electron-rich C=C bond with collabora-
tion of the carbonyl group in the catalytic process.
in a lot of cases to be an ideal solvent for organic reactions
due to its advantages of nontoxicity, nonflammability,
abundant availability, and inexpensiveness.[9]
Introduction
Indole, as a basic backbone, is frequently found in many
natural bioactive products, marketed drugs, functional ma-
terials, and agrochemicals.[1] Much attention has been paid
to its synthesis and modification in the past several decades.
The Friedel–Crafts (F–C) alkylation reaction of indole was
one of the most powerful routes to synthesize indole deriva-
tives.[2] Among the various alkylation reagents (e.g.,
alcohols,[3] esters,[4] and olefins[5]), olefins are highly attract-
ive, as they undergo atom-economical processes. Great pro-
gress has been made in the F–C alkylation of indole with
electron-deficient olefins. However, the applications of elec-
tron-rich olefins in such a reaction still remain a challenge.
Enamide, as a good candidate in the family of electron-
rich olefins, has been widely used in organic reactions.[6]
Although much less attention has been focused on the elec-
trophilicity of enamides than its nucleophilicity, recent ex-
amples have revealed successfully on its electrophilic role
(Scheme 1).[7] The groups of Terada and Zhou reported the
enantioselective F–C reactions of indoles and enamides,
respectively, in which enamides had to isomerize to iminium
ion electrophiles first so that they could participate in the
next reactions (Scheme 1, Equation 1). According to the in-
termediate formed in the above reactions, only secondary
enamides worked well. Attempted reactions with tertiary
enamides failed. It should be noted that Brønsted acid type
Scheme 1. Catalyst-mediated nucleophilic and electrophilic path-
way of enamide.
We have been interested in the syntheses of indole deriva-
tives and green chemistry for some years.[10] As a continuing
work, we here report a series of novel and ferric nitrate
catalyzed regioselective F–C reactions between indoles and
tertiary enamides, which were carried out in water at room
temperature to offer the C3-alkylated indoles exclusively in
moderate to high yields (Scheme 2).
catalysts are the only compounds that have been discovered Scheme 2. Ferric nitrate catalyzed F–C reactions of indoles and
to promote these reactions.[8] Moreover, to the best of our
tert-enamide in water.
knowledge, the reaction of enamide in water as a solvent
has not been reported. However, water has been confirmed
Results and Discussion
[a] Key Laboratory of Organic Synthesis of Jiangsu Province,
College of Chemistry, Chemical Engineering and Materials
To initiate our study, 1-vinylpyrrolidin-2-one (1a) was se-
Science, Soochow University,
lected as a model substrate to react with indole (2a) in water
at room temperature. No reaction was observed in the ab-
sence of a catalyst even when the mixture was stirred at
60 °C or tetrabutylammonium bromide (TBAB) was added
Suzhou, 215123, People’s Republic of China
Fax: +86-512-65880307
E-mail: chemjsj@suda.edu.cn
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201000994.
View this journal online at
wileyonlinelibrary.com
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Eur. J. Org. Chem. 2010, 5946–5950

Ferric(III) Nitrate Catalyzed Regioselective Friedel–Crafts Reactions
as a phase-transfer catalyst (PTC) (Table 1, Entries 1–3). Table 1. Screening of catalysts for the reaction of 1a with 2a.
Then, we investigated the feasibility of Lewis acid and
Brønsted acid catalysis. PTSA (p-toluenesulfonic acid) was
also unable to promote the reaction (Table 1, Entry 4).
Some well-documented catalysts[8b] were tested sub-
sequently, but no satisfactory results appeared in these reac-
tions (Table 1, Entries 5–8). With respect to some catalysts
Entry[a]
Catalyst
Time [h] Yield [%][b]
investigated, we fortunately found that the reaction could
take place in the presence of FeCl3 (Table 1, Entries 9–12)
for a 52% yield. When water-soluble catalysts were consid-
ered, it was surprising that the reaction performed well with
iron(III) salts (Table 1, Entries 12–14) but hardly with
iron(II) salts (Table 1, Entries 15 and 16). Among the
iron(III) salts investigated, Fe(NO3)3·9H2O was found to
offer the highest yield of 83% for product 3aa. In order to
explore the effect of the nitrate group on the reaction,
Bi(NO3)3·5H2O, Zn(NO3)2·6H2O, and Co(NO3)2·6H2O
were also employed. However, no reactions were observed
at all in the presence of Zn(NO3)2·6H2O or Co(NO3)2·
6H2O (Table 1, Entries 17–19), Bi(NO3)3·5H2O afforded the
product in comparable yield. From these experimental data,
it is clear that the metal ions play a more important role
in these catalytic transformations than their counterparts.
Furthermore, studies on the influence of the selected PTCs
indicate that tetrabutylammonium iodide (TBAI) was the
best one (Table 1, Entries 20–22). Although the addition of
TBAI did not give us a better result in screening conditions,
the expected adducts were isolated in very low yields with-
out TBAI (see Table 2) as other substituted indoles were
introduced.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
–
–
12
12
12
12
12
12
12
12
12
12
12
7
7
7
12
7
7
0
[c]
0
TBAB
0
0
0
0
0
0
0
0
0
52
77
83
0
PTSA[d]
Cu(OTf)2
Cu(OAc)2·H2O
CoCl2·6H2O
Yb(OTf)3
NiCl2
ZnCl2
CeCl3·7H2O
FeCl3
Fe2(SO4)3·7H2O
Fe(NO3)3·9H2O
(NH4)2Fe(SO4)2·6H2O
FeSO4·7H2O
Bi(NO3)3·5H2O
Zn(NO3)2·6H2O
Co(NO3)2·6H2O
Fe(NO3)3·9H2O + TBAB
Fe(NO3)3·9H2O + TBAI
Fe(NO3)3·9H2O + SDS[e]
18
74
0
12
12
7
7
7
0
82
87
71
[a] Reaction conditions: 1a (0.25 mmol), 2a (0.50 mmol), catalyst
(5 mol-%), H2O (1.5 mL), PTC (5 mol-%) at room temperature.
[b] Yield of isolated product after flash chromatography. [c] At
60 °C. [d] PTSA = p-toluenesulfonic acid. [e] SDS = sodium dode-
cyl sulfate.
From these optimized conditions [Fe(NO3)3·9H2O
(5 mol-%), H2O (1.5 mL), TBAI (5 mol-%), r.t.], the scope
of the reaction was explored by changing the structure of
the indoles, as summarized in Table 2. The reaction of 1a
with 2-methyl-1H-indole (2b) was able to perform smoothly
However, the protocol was limited when other N-vinyl
for a 79% yield of 3ab. When 2-phenyl-1H-indole (2c) was compounds were introduced. For example, N-vinylcarb-
applied, a longer reaction time was necessary, and neverthe- azole did not perform under the standard conditions. Other
less, a 58% yield of 3ac could be isolated after 24 h (Table 2, electron-rich aromatics that did also perform smoothly in
Entry 3). Meanwhile, 4-(benzyloxy)-1H-indole (2d) could this system are listed in Figure 2.
also react well in this catalytic system to afford a 73% yield
These data led us to explore the reaction mechanism. In
of desired product 3ad (Table 2, Entry 4). From these re- comparison with the structures of three electron-rich olefin
sults, we found that the yield decreased slightly with substit- substrates, we think that the carbonyl group in the enamide
uents at the 2- or 4-position of indole, possibly as a result of plays an important role in this kind of F–C reaction. We
steric effects. As for indoles bearing either electron-releasing tried to extract some valuable information from the IR
(Table 2, Entries 5, 7, 9) or electron-withdrawing groups stretching frequencies of the C=O group from 1-vinylpyr-
(Table 2, Entry 6), the reactions with 1a proceeded effec- rolid-in-2-one in the presence of Fe(NO3)3·9H2O. The ab-
tively to generate the C3-alkylated adducts (82–89%). Note- sorption peak of the carbonyl group in a mixture of 1a and
worthy is that 7-nitroindole underwent alkylation well with Fe(NO3)3·9H2O (1 equiv.) was redshifted from 1673 cm–1
1a under the same conditions for a 51% yield (Table 2, En- (for 1a) to 1640 cm–1 (intermediate A; see their spectra in
try 8). It was satisfactory that N-methylindole performed the Supporting Information). This is a convincing evidence
well in this system (Table 2, Entry 9), although it failed un- for the interaction between iron(III) and the carbonyl group
der Brønsted acid catalyzed conditions.[7b] Another tert-en- from enamide, which is consistent with the fact that elec-
amide within our accessibility was tested as well. It was ob- tron-rich alkenes without a carbonyl group did not react
served that the reaction of N-vinyl-ε-caprolactam (1b) gen- with indoles under the same conditions. Additionally, the
erally proceeded more slowly and gave a lower yield than wide and strong absorption at 1640 cm–1 implied the exis-
1-vinylpyrrolidin-2-one (Table 2, Entries 10–18). The struc- tence of a delocalized π bond in the [CCNCO] system. It
ture of representative compound 3bf was confirmed by X- also meant that the C=C bond might be activated by the
ray single crystal diffraction (Figure 1).[11]
iron(III) ion at the same time. Thus, hydration would read-
Eur. J. Org. Chem. 2010, 5946–5950
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5947

R. Jiang, X.-J. Wu, X. Zhu, X.-P. Xu, S.-J. Ji
SHORT COMMUNICATION
Table 2. Ferric nitrate catalyzed F–C alkylation of indoles and tert- ily occur on the highly polarized C=C bond. The confirma-
tive evidence was achieved by 13C NMR spectroscopic
enamides.
analyses. The typical peaks of the sp3 carbon formed in situ
appeared at δ = 69 and 16 ppm in the presence of the cata-
lyst (1 equiv.), whereas the peaks of the sp2 carbon in 1a
appeared at 126 and 94 ppm (Figure 3).[12] To further verify
our assumed mechanism, we separated the product from
the reaction of indole and 1a in deuterated water. Just as
we expected (Scheme 3), the deuterium from the solvent ap-
peared in the product (characteristic data and spectra, see:
3aaЈ in the Supporting Information), which proved the hy-
dration of the highly polarized C=C bond.
Entry[a]
Substrates
R
Time Product
[h]
Yield
[%][b]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
1a
1b
H
2-Me
2-Ph
4-BnO
5-Me
5-Br
7-Me
7-NO2
N-Me
H
2-Me
4-Me
4-BnO
5-Me
5-Br
6-Me
7-Me
N-Me
2a
2b
2c
2d
2e
2f
2g
2h
2i
2a
2b
2j
2d
2e
2f
7
6
24
11
8
7
7
24
6
8
12
12
12
12
12
9
3aa
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
3ba
3bb
3bj
3bd
3be
3bf
3bk
3bg
3bi
87
79 (67)[c]
58
73
87
82 (21)[c]
89
51
88 (36)[c]
74
61
58
31
69
50
74
74
77
2k
2g
2i
12
10
[a] Reaction conditions:
1 (0.5 mmol), 2 (1.0 mmol), catalyst
[Fe(NO3)3·9H2O 5 mol-% + TBAI 5 mol-%], H2O (1.5 mL) at room
temperature. [b] Yield of isolated product after flash chromatog-
raphy. [c] Isolated yield without addition of TBAI.
Figure 3. 13C NMR spectra of 1a (top) and a mixture of 1a and
the catalyst (1 equiv.; bottom) in D2O.
Scheme 3.
Figure 1. Crystal structure of 3bf.
Here, we can understand the whole mechanism as follows
(Scheme 4):[13] With the coordination of the carbonyl
group, the iron(III) ion with a positive charge activates and
polarizes the C=C bond (intermediate A). Hydration of the
C=C bond subsequently occurs to form intermediate B.[14]
Under these circumstances, iron(III) activates the hydroxy
group[15] to form a stable carbocation, which is attacked
further by C3 of indole to form a new carbon–carbon bond.
Electron transfer and hydrogen elimination easily lead to
the formation of the final product.[16]
Figure 2. Ferric nitrate catalyzed F–C reaction of other aromatics
and tert-enamide.
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Eur. J. Org. Chem. 2010, 5946–5950

Ferric(III) Nitrate Catalyzed Regioselective Friedel–Crafts Reactions
3107, 2972, 2876, 1659, 1490, 1440, 1288, 1198, 751 cm–1. 1H NMR
(300 MHz, CDCl3): δ = 1.60 (d, J = 6.9 Hz, 3 H, CH3), 1.86–2.04
(m, 2 H, CH2), 2.38–2.46 (m, 2 H, CH2), 2.85–2.91 (m, 1 H), 3.23–
3.38 (m, 1 H), 5.78 (q, J = 6.6 Hz, 1 H, CH), 7.08–7.12 (m, 2 H,
ArH), 7.18–7.22 (m, 1 H, ArH), 7.37 (d, J = 8.1 Hz, 1 H, ArH),
7.62 (d, J = 7.8 Hz, 1 H, ArH), 8.32 (br., s, 1 H, NH) ppm. 13C
NMR (100 MHz, CDCl3): δ = 174.8, 137.0, 126.9, 122.8, 122.6,
120.3, 119.9, 116.4, 111.6, 43.1, 42.7, 32.2, 18.2, 17.1 ppm. HRMS:
calcd. for C14H16N2O 228.1263; found 228.1266.
Supporting Information (see footnote on the first page of this arti-
cle): Copies of the 1H and 13C NMR spectra of all products and
some information about the mechanism.
Acknowledgments
The work was partially supported by the National Natural Science
Foundation of China (No. 20672079, 20910102041), Natural Sci-
ence Key Basic Research of Jiangsu Province for Higher Education
(No. 06KJA15007), and Key Project in Science & Technology Inno-
vation Cultivation Program of Soochow University.
Scheme 4. Plausible mechanism for FeIII catalysis.
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Conclusions
In summary, we have described a ferric nitrate catalyzed
F–C alkylation of indoles with tert-enamides in water at
room temperature. This atom-economical and environmen-
tally friendly procedure uses an inexpensive catalyst with
low loadings and establishes a new Lewis acid catalyzed F–
C alkylation between indoles with electron-rich olefins. The
fact that the coordinative group promotes the activation of
the electron-rich C=C bond in the presence of a catalyst
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F–C alkylations. Applications of this methodology to other
reactions are currently under investigation in our labora-
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Experimental Section
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13C NMR spectra were recorded with a Varian INOVA 300 or 75
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Typical Experimental Procedure for the Reaction of Indole with En-
amide: Indole (0.50 mmol), Fe(NO3)3·9H2O (5 mol-%), TBAI
(5 mol-%), and enamide (1.0 mmol) were added into a flask. H2O
(1.50 mL) was then added, and the mixture was vigorously stirred
at room temperature until indole was completely consumed
(checked by TLC) or until an appropriate time had passed. After
completion of reaction, the product was extracted with ethyl acet-
ate, and the organic phase was dried with anhydrous Na2SO4. The
solvent was removed under the reduced pressure, and the residue
was purified by flash column chromatography (ethyl acetate/petro-
leum ether) to afford the pure product.
1-[1-(1H-Indol-3-yl)ethyl]pyrrolidin-2-one (3aa): Yield: 99 mg
(87%). White solid, m.p. 165–167 °C. IR (KBr): ν = 3243, 3165,
˜
Eur. J. Org. Chem. 2010, 5946–5950
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
5949

R. Jiang, X.-J. Wu, X. Zhu, X.-P. Xu, S.-J. Ji
SHORT COMMUNICATION
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this system. The catalyst may coordinate to the nitrogen atom
of 1-vinylimidazole in the absence of the C=O group, and the
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crystal size: 0.60ϫ0.60ϫ0.50 mm3. C16H21BrN2O2, Mr
=
353.26, monoclinic, space group P21/n, a = 10.3238(16) Å, b =
8.4856(13) Å, c = 18.447(3) Å, α = 90.00°, β = 93.119(4)°, γ =
90.00°, V = 1613.7(4) Å3, Z = 4, Dcalcd. = 1.454 gcm–3,
R[IϾ2σ(I)] = 0.0400, wR[IϾ2σ(I)] = 0.0919.
[12] Due to the paramagnetic properties of iron(III), no distin-
guished NMR spectra could be provided. Thus, in order to get
Received: July 15, 2010
Published Online: September 22, 2010
5950
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Eur. J. Org. Chem. 2010, 5946–5950