Highly regioselective Friedel-Crafts alkylation of indoles with α,β-unsaturated N-acylbenzotriazoles

Article abstract of DOI:10.1016/j.tetlet.2006.03.110

The Friedel-Crafts alkylation rather than acylation of indoles was realized with α,β-unsaturated acylbenzotriazoles catalyzed by samarium(III) iodide under reflux in anhydrous THF. The reaction was highly regioselective, and a series of new 3-substituted indole derivatives were obtained in moderate to good yields with the potential to be further transformed into various indole derivatives due to the presence of active acylbenzotriazole moiety.

Full text of DOI:10.1016/j.tetlet.2006.03.110

Tetrahedron Letters 47 (2006) 3767–3771
Highly regioselective Friedel–Crafts alkylation of indoles
with a,b-unsaturated N-acylbenzotriazoles
Xuefei Zou, Xiaoxia Wang,* Cungui Cheng, Lichun Kong and Hui Mao
Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, College of Chemistry and Life Sciences,
Zhejiang Normal University, Jinhua 321004, PR China
Received 26 December 2005; revised 16 March 2006; accepted 17 March 2006
Available online 12 April 2006
Abstract—The Friedel–Crafts alkylation rather than acylation of indoles was realized with a,b-unsaturated acylbenzotriazoles cata-
lyzed by samarium(III) iodide under reflux in anhydrous THF. The reaction was highly regioselective, and a series of new 3-substi-
tuted indole derivatives were obtained in moderate to good yields with the potential to be further transformed into various indole
derivatives due to the presence of active acylbenzotriazole moiety.
ꢀ 2006 Elsevier Ltd. All rights reserved.
a,b-Unsaturated N-acylbenzotriazoles, essentially a spe-
cial type of a,b-unsaturated amides with the –C@C–
CON– structure present therein, can be treated as a
new class of a,b-unsaturated carbonyl compounds (or
electron-deficient alkenes). They were found to be good
Michael acceptors as well as acylating agents depending
on the attacking electrophiles. For example, anhydrous
hydrazine were readily acylated by N-cinnamoylbenzo-
triazoles affording good to excellent yields of cinnamoyl
hydrazides;1a the lithium enolates of ketones were acyl-
ated with a,b-unsaturated acylbenzotriazoles to prepare
c,d-unsaturated b-diketones.1b We found that N-cinna-
moylbenzotriazoles were also good acylating agents for
thiolates thus providing an efficient synthesis of a,b-
unsaturated thioesters;2 besides being convenient and
mild N-, C- and S-acylating reagents as mentioned
above. Our recent investigation revealed3 that when ali-
phatic amines were the electrophiles, acylation reaction
took place exclusively to give cinnamoylamides, while
aromatic amines attacked the 4-position of N-cinna-
moylbenzotriazoles with high regioselectivity to produce
moderate to good yields of Michael adducts.
afford 3-acylindoles.5 The unique features of a,b-unsat-
urated acylbenzotriazoles prompted us to further inves-
tigate its reaction with indoles.
In continuation with our previous study of the aza-
Michael conjugate addition of aromatic amines to N-
cinnamoylbenzotriazoles,3 the reaction between indole
(1a) and a,b-unsaturated acylbenzotriazoles (2a) was
examined first with a base such as triethylamine as a
promoter to test the possibility of the N-alkylation of
indole (the Michael addition of indole with its 1-posi-
tion, the N atom to a,b-unsaturated acylbenzotriazoles),
but the reaction ended up with no expected product
formed. However, subsequent investigation found that
Lewis acid such as SmI3 could catalyze the Michael
addition of indole with its 3-position to 2a smoothly.
That was, in contrast to the fact that saturated N-acyl-
benzotriazoles were effective Friedel–Crafts acylating
agents for indoles, a,b-unsaturated acylbenzotriazoles
worked as good Friedel–Crafts alkylating agents for
indoles, showing excellent regioselectivity.
The Friedel–Crafts alkylation of indole with activated
alkenes (also known as conjugate addition of indole to
electro-deficient olefins) has drawn much attention in
recent years due to the carbon–carbon bond formation
where useful 3-substituted indole derivatives as impor-
tant building blocks for natural product synthesis can
be produced.6 However, though the conjugate addition
of indole to enones and nitro alkenes7 promoted by var-
ious catalysts or under a variety of conditions including
N-acylbenzotriazoles derived from saturated carboxylic
acids have been well documented as highly efficient acyl-
ating agents4 and in the presence of titanium chloride
they undergo Friedel–Crafts acylation of indoles to
Keywords: Indoles; Friedel–Crafts alkylation; N-Cinnamoylbenzo-
triazoles; Samarium(III) iodide.
*
Corresponding author. E-mail: wangxiaoxia@zjnu.cn
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.03.110

3768
X. Zou et al. / Tetrahedron Letters 47 (2006) 3767–3771
ultrasonic irradiation7p and photoinduction7v tends to
be a subject of current interest and reports concerning
the enatioselective addition of indole to alkylidene mal-
onates can frequently be found,8 studies concerning
other electron-deficient olefins as the Michael acceptor
to realize the Friedel–Crafts alkylation of indole are less
known. Yadav et al. reported7o that InCl3 could effec-
tively catalyze the conjugate addition of indoles to en-
ones, nitro alkenes and benzylidene malononitriles, but
not to methyl acrylate and acrylonitrile. Catalyzed by
Bi(OTf)3, methyl acrylate and acrylonitrile were repor-
ted7n to undergo the Friedel–Crafts alkylation of indole
with moderate yields, but a systematic study concerning
a,b-unsaturated esters and nitriles was unknown to the
best of our knowledge. Herein, a,b-unsaturated acyl-
benzotriazoles as a new kind of Michael acceptors
(Friedel–Crafts alkylating agents) for indoles were
reported and the investigation resulted in a new class
of 3-substituted indole derivatives.
it was not so successful for promoting the Friedel–Crafts
alkylation of indoles with N-cinnamoylbenzotriazole
(Table 1, entry 4). Under reflux in anhydrous THF,
the presence of 50 mol % SmI3 afforded the Friedel–
Crafts alkylation products in 87% yield (Table 1, entry
6). When the reaction was further carried out with cata-
lytic amount (20 mol %) of SmI3, comparable yields
(Table 1, entry 7) could be obtained within reasonably
prolonged period of time. When the reaction catalyzed
by SmI3 was carried out in solvents other than THF,
such as dry CH2Cl2, toluene and CH3CN, no significant
improvement was observed both in reaction time and
product yields (Table 1, entries 8–10). Thus for the
Friedel–Crafts alkylation here, reflux in dry THF in
the presence of 20 mol % of SmI3 was developed for
the standard reaction conditions.
The general applicability of this reaction was illustrated
by the fact that a variety of N-cinnamoylbenzotriazoles
(2a–f) reacted well with both unsubstituted and substi-
tuted indoles affording the corresponding 3-alkylated
indole derivatives 3 in moderate to good yields (Scheme
2 and Table 2). Unsubstituted indole 1a undergoing
smooth 3-alkylation with 2a–g usually in 10 h afforded
3aa–ag in 59–92% yields (Table 2, entries 1–7, 17). An
introduction of a substituent such as methyl or phenyl
at the 2-position led to a prolonged period of time for
the completed alkylation. 2-Methylindole 1b required
13–16 h (Table 2, entries 8–10) while 2-phenylindole 1c
required even longer time as much as 20–26 h so as to
afford satisfactory yields (Table 2, entries 11, 12). N-sub-
stitution such as N-benzylindole (1d) was also investi-
gated and generally still longer times were necessary
(Table 2, entries 13–15). 4-Nitro-2-methyl indole 1e with
a strong electron-withdrawing group even located hete-
roannularly at the 4-position failed to produce the
expected alkylation product and in fact no reaction
occurred (Table 2, entry 16). The a,b-unsaturated acyl-
benzotriazoles derived from crotonic acid (2h) was also
used as the alkylating agent, which was reacted with
1a smoothly to give the corresponding alkylated indoles
(Table 2, entry 17).
A series of experiments were carried out to search for an
appropriate Lewis acid as the catalyst for the reaction
shown in Scheme 1 and the results are listed in Table
1. All runs were carried out for 8 h and among the 6
Lewis acids tested, SmI3 gave the best results (Table 1,
entries 1–6). Apart from the desired adducts 3aa, cin-
namic acid (the hydrolyzed product of 2a) as well as
the recovered 2a were also obtained after usual work-
up. The presence of Lewis acid facilitated the hydrolysis
of 2a greatly and therefore anhydrous reagent and sol-
vent were required for the success of the reaction.
Although TiCl4 was proved to be an effective promoter
for the acylation of indoles with N-acylbenzotriazoles,5
O
HN
Lewis acid
Ph
O
Bt
THF, reflux.
N
H
Ph
Bt
2a
1a
3aa
N
N
N
Bt =
The SmI3 catalyst was prepared in situ by reacting 1
equiv of samarium power with 1.5 equiv of iodine in
dry THF at room temperature with stirring for 0.5 h.
Scheme 1.
Table 1. Screening of Lewis acids and reaction conditions for the Friedel–Crafts alkylation of indole with a,b-unsaturated acylbenzotriazoles 2a
Entrya
Lewis acid
Catalyst (mol %)
Solvent
Time (h)
Yieldb (%) of 3aa
1
2
3
4
5
6
7
8
9
10
AlCl3
CuCl2
ZnCl2
TiCl4
BF3ÆEt2O
SmI3
SmI3
SmI3
SmI3
SmI3
50
50
50
50
50
50
20
20
20
20
THF
THF
THF
THF
THF
THF
THF
CH2Cl2
Toluenec
CH3CN
8
8
8
8
8
53
0
0
25
0
87
89
76
70
81
8
10
18
10
10
a All the runs were carried out under reflux conditions unless otherwise specified.
b Isolated yields.
c Reaction performed at 80 ꢁC.

X. Zou et al. / Tetrahedron Letters 47 (2006) 3767–3771
3769
R3
O
X
X
O
SmI3 (20 mol%)
Bt
R3
R2
Bt
THF, reflux.
R2
N
R1
N
R1
2
1
3
Scheme 2.
Table 2. Samarium(III) iodide-catalyzed Friedel–Crafts alkylation of indoles with a,b-unsaturated acylbenzotriazolesa,9
Entry
Indoles 1
R3 of 2
Productsb
Time (h)
Yield (%)c
1
3aa
10
89
92
86
2a
N
H
1a
1a
H3C
H3CO
Cl
2
3
3ab
3ac
10
10
N
H
2b
N
H
2c
1a
4
5
3ad
3ae
10
15
92
87
N
H
2d
1a
1a
N
H
Cl 2e
O2N
6
3af
3ag
3ba
3bc
3bf
3ca
3cb
3da
9
18
13
15
16
25
20
25
90
59
82
82
67
63
88
61
N
H
2f
1a
1a
7
N
H
O
2g
CH3
8
N
H
2a
1b
1b
1b
CH3
CH3
Ph
H3CO
O2N
9
N
H
2c
10
11
12
13
N
H
2f
N
H
2a
2a
1c
Ph
H3C
N
H
2b
1c
N
CH2Ph
1d
H3C
14
3db
26
83
N
2b
CH2Ph
1d
(continued on next page)

3770
X. Zou et al. / Tetrahedron Letters 47 (2006) 3767–3771
Table 2 (continued)
Entry
Indoles 1
R3 of 2
Productsb
Time (h)
22
Yield (%)c
88
Cl
15
3dd
2d
N
CH2Ph
1d
NO
2
d
16
17
—
20
14
—
CH
3
2a
N
H
1e
Me– 2h
3ah
82
N
H
1a
a All reactions were carried out in dry THF under reflux with 20 mol % of samarium(III) iodide.
b Alkylated products10 were characterized by 1H NMR, 13C NMR, IR and elemental analysis.
c Isolated yields.
d No reaction.
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Sc(DS)3: (f) Manabe, K.; Aoyama, N.; Kobayashi, S. Adv.
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CeCl3Æ7H2O–NaI for enones: (k) Bartoli, G.; Bartolacci,
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Sambri, L.; Torregiani, E. J. Org. Chem. 2003, 68, 4594–
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toni, E.; Sambri, L.; Torregiani, E. J. Org. Chem. 2005, 70,
1941–1944; Bi(OTf)3: (m) Reddy, A. V.; Ravinder, K.;
Goud, T. V.; Krishnaiah, P.; Raju, T. V.; Venkateswarlu,
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Reddy, B. V. S.; Sabitha, G. Synthesis 2001, 2165–2169;
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Nitrogen protection was unnecessary and the simple
procedure made it promising for large-scale manipula-
tion.
In conclusion, we presented a first catalytic regio-
selective alkylation of indoles with a,b-unsaturated
acylbenzotriazoles. The synthesized new type of 3-alkyl-
ated indoles possess N-acylbenzotriazole moiety and
would be promising acylating agents capable of realizing
various N-, C-, S-, and O-acylation4 and could be
further transformed into versatile indole derivatives.
Acknowledgement
We are grateful to Zhejiang Provincial Natural Science
Foundation of China (Project No. Y405035) for finan-
cial support.
References and notes
1. (a) Katritzky, A. R.; Wang, M.; Zhang, S. ARKIVOC
2001, 19–23 (http://www.arkat-usa.org/ark/journal/2001/
I09_Voronkov/195/MV-195F.pdf); (b) Katritzky, A. R.;
Meher, N. K.; Singh, S. K. J. Org. Chem. 2005, 70, 7792–
7794.
2. Wang, X. X.; Zou, X. F.; Du, J. X. J. Chem. Res. (s) 2006,
64–66.
3. Wang, X. X.; Zou, X. F.; Li, J.; Hu, Q. H. Synlett 2005,
3042–3046.
4. For review: Katritzky, A. R.; Suzuki, K.; Wang, Z. Synlett
2005, 1656–1665.
5. Katritzky, A. R.; Suzuki, K.; Singh, S. K.; He, H.-Y.
J. Org. Chem. 2003, 68, 5720–5723.
6. (a) Gribble, G. W. J. Chem. Soc., Perkin Trans. 1 2000,
1045–1075; (b) Fukuyama, T.; Chen, X. J. Am. Chem. Soc.
1994, 116, 3125–3126; (c) Moore, R. E.; Cheuk, C.; Tang,
X. Q.; Patterson, G. M. L.; Bonjouklain, R.; Smita, T. A.;
Mynderse, J.; Foster, R. S.; Jones, N. D.; Skiartzendrub-
ber, J. K.; Deeter, J. B. J. Org. Chem. 1987, 52, 1036–1043;
(d) Moore, R. E.; Cheuk, C.; Patterson, G. M. L. J. Am.
Chem. Soc. 1984, 106, 6456–6457.
7. For example: Hf(OTf)4 and Sc(OTf)3: (a) Kawastura, M.;
Aburatani, S.; Uenishi, J. Synlett 2005, 2492–2494;

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3771
Garavelli, M.; Melloni, A.; Trigari, V.; Umani-Ronchi, A.
J. Org. Chem. 2004, 69, 7511–7518; Photoinduction: (v)
Moran, J.; Suen, T.; Beauchemin, A. M. J. Org. Chem.
2006, 71, 676–679.
(400 MHz, CDCl3): 8.19 (d, 1H, J = 8.0 Hz, ArH), 8.08
(d, 1H, J = 8.0 Hz, ArH), 7.76, (s, 1H, NH), 7.58–7.56 (m,
2H, ArH), 7.47–7.45 (m, 1H, ArH), 7.36 (d, 2H,
J = 8.0 Hz, ArH), 7.20 (d, 1H, J = 8.0 Hz, ArH), 6.97–
7.12 (m, 2H, ArH), 6.81 (d, 2H, J = 8.0 Hz, ArH), 5.16
(dd, 1H, J = 8.0, 8.0 Hz, CH), 4.38 (dd, 2H, J = 8.0,
8.0 Hz, CH), 3.75 (s, 3H, CH3), 2.43 (s, 3H, CH3). 13C
NMR (100 MHz, CDCl3): 171.1, 158.0, 146.1, 135.3,
131.7, 131.1, 130.2, 128.4, 127.3, 126.0, 121.0, 120.1, 119.5,
119.1, 114.5, 113.8, 113.0, 110.3, 55.2, 40.7, 36.5, 26.9,
12.3. Anal. C25H22N4O2. Calcd. C, 73.15; H, 5.40; N,
13.65. Found C, 73.39; H, 5.48; N, 13.59.
8. (a) Zhuang, W.; Hansen, T.; Jørgensen, K. A. Chem.
Commun. 2001, 347–348; (b) Zhou, J.; Ye, M.-C.; Huang,
Z.-Z.; Tang, Y. J. Org. Chem. 2004, 69, 1309–1320; (c)
Zhou, J.; Tang, Y. Chem. Commun. 2004, 432–433; (d)
Zhou, J.; Tang, Y. J. Am. Chem. Soc. 2002, 124, 9030–
9031.
9. General experimental procedure. To 10 mL of dry THF
were added Sm powder (0.03 g, 0.2 mmol) and I2 (0.077 g,
0.3 mmol), the mixture was stirred at rt for 0.5 h to obtain
the SmI3–THF solution. To the solution were added
indole (1 mmol) and N-cinnamoylbenzotriazole (1 mmol),
the resulting mixture was refluxed for indicated time
(Table 2) until the disappearance of N-cinnamoylbenzo-
triazole (monitored by TLC). Then it was quenched with
dilute hydrochloric acid (0.3 M) and extracted with diethyl
ether (3 · 20 mL). The combined organic layer was
washed with saturated Na2S2O3 and brine, dried with
anhydrous Na2SO4, and concentrated under reduced
pressure. The residue was purified by preparative TLC
on silica gel with ethyl acetate and cyclohexane (1:2) as
eluent to afford pure products 3.
Compound 3cb, 1-(1H-benzo[d][1,2,3]triazol-1-yl)-3-(2-
phenyl-1H-indol-3-yl)-3-p-tolylpropan-1-one, decomposed
beyond 95 ꢁC. mmax: 3293 (NH), 3058, 3022, 2924, 2850,
.
1742 (C@O), 1603 (Ar) cmÀ1 1H NMR (400 MHz,
CDCl3): 8.10 (d, 1H, J = 8.0 Hz, ArH), 8.02 (m, 2H,
ArH, NH), 7.66 (d, 1H, J = 8.0 Hz, ArH), 7.57–7.55 (m,
1H, ArH), 7.44–7.26 (m, 9H, ArH), 7.18–7.16 (m, 1H,
ArH), 7.10–7.08 (m, 3H, ArH), 5.36 (dd, 1H, J = 8.0,
8.0 Hz, CH), 4.45 (dd, 1H, J = 8.0, 8.0 Hz, CH), 4.22 (dd,
1H, J = 8.0, 8.0 Hz, CH), 2.30 (s, 3H, CH3). 13C NMR
(100 MHz, CDCl3): 170.8, 146.0, 140.2, 136.2, 135.9,
135.8, 132.6, 131.0, 130.0, 129.2, 128.7, 128.6, 128.0,
127.5, 127.4, 125.9, 122.2, 120.8, 120.0, 119.9, 114.5,
113.4, 111.0, 41.3, 37.4, 21.0. Anal. C30H24N4O. Calcd. C,
78.92; H, 5.30; N, 12.27. Found C, 79.17; H, 5.42; N,
12.15.
10. Compound 3aa, 1-(1H-benzo[d][1,2,3]triazol-1-yl)-3-(1H-
indol-3-yl)-3-phenylpropan-1-one, mp 168–169 ꢁC. mmax
3416 (NH), 3063, 3027, 2924, 2883, 1721 (C@O), 1601
(Ar) cmÀ1 1H NMR (400 MHz, CDCl3): 8.22 (d, 1H,
:
.
Compound 3dd, 1-(1H-benzo[d][1,2,3]triazol-1-yl)-3-(1-
benzyl-1H-indol-3-yl)-3-(4-chlorophenyl)propan-1-one, mp
138–140 ꢁC. mmax: 3062, 3027, 2923, 2853, 1730 (C@O),
1614 (Ar) cmÀ1. 1H NMR (400 MHz, CDCl3): 8.21 (d, 1H,
J = 8.0 Hz, ArH), 8.10 (d, 1H, J = 8.0 Hz, ArH), 7.63–
7.61 (m, 1H, ArH), 7.49–7.47 (m, 2H, ArH), 7.39–7.36 (m,
2H, ArH), 7.25–7.20 (m, 5H, ArH), 7.14–7.01 (m, 6H,
ArH), 5.30 (s, 2H, CH2), 5.18 (dd, 1H, J = 8.0, 8.0 Hz,
CH), 4.26 (dd, 1H, J = 8.0, 8.0 Hz, CH), 4.13 (dd, 1H,
J = 8.0, 8.0 Hz, CH). 13C NMR (100 MHz, CDCl3):
170.5, 146.2, 141.7, 137.3, 137.0, 132.4, 131.0, 130.5,
129.2, 128.8, 128.7, 127.6, 127.0, 126.7, 126.6, 126.2, 125.6,
122.3, 120.2, 119.5, 117.0, 114.5, 109.8, 50.0, 41.9, 38.3.
Anal. C30H23ClN4O. Calcd. C, 73.39; H, 4.72; N, 11.41.
Found C, 73.16 H, 4.79; N, 11.35.
J = 8.0 Hz, ArH), 8.10 (d, 1H, J = 8.0 Hz, ArH), 7.99 (s,
1H, NH), 7.62–7.60 (m, 1H, ArH), 7.48–7.43 (m, 4H,
ArH), 7.31–7.27 (m, 3H, ArH), 7.17–7.15 (m, 3H, ArH),
7.05–7.03 (m, 1H, ArH), 5.20 (dd, 1H, J = 8.0, 8.0 Hz,
CH), 4.31 (dd, 1H, J = 8.0, 8.0 Hz, CH), 4.17 (dd, 1H,
J = 8.0, 8.0 Hz, CH). 13C NMR (100 MHz, CDCl3):
170.8, 146.2, 143.1, 136.6, 131.1, 130.3, 128.6, 127.8,
126.7, 126.6, 126.2, 122.4, 121.4, 120.1, 119.7, 119.4, 118.5,
114.6, 111.1, 42.0 38.8. Anal. C23H18N4O. Calcd. C, 75.39;
H, 4.95; N, 15.29. Found C, 75.20; H, 5.01; N, 15.23.
Compound 3bc, 1-(1H-benzo[d][1,2,3]triazol-1-yl)-3-(4-
methoxyphenyl)-3-(1-methyl-1H-indol-3-yl)propan-1-one,
decomposed beyond 76 ꢁC. mmax: 3411 (NH), 3099, 3058,
2925, 2848, 1736 (C@O), 1611 (Ar) cmÀ1 1H NMR
.