Br2 as a novel Lewis acid catalyst for Friedel-Crafts alkylation of indoles with α,β-unsaturated ketones

Article abstract of DOI:10.1016/j.tetlet.2016.01.078

The inexpensive Br₂ can serve as a novel Lewis acid catalyst for Friedel-Crafts alkylation of indoles with α,β-unsaturated ketones. Under the catalysis of only 3 mol % of Br₂, this Michael addition proceeded smoothly with high effici

Full text of DOI:10.1016/j.tetlet.2016.01.078

Accepted Manuscript
Br2 as a novel Lewis acid catalyst for Friedel–Crafts alkylation of indoles with
α,β-unsaturated ketones
Deqiang Liang, Xiangguang Li, Wanshun Zhang, Yanni Li, Mi Zhang, Ping
Cheng
PII:
S0040-4039(16)30077-6
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.01.078
TETL 47242
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
22 November 2015
20 January 2016
21 January 2016
Please cite this article as: Liang, D., Li, X., Zhang, W., Li, Y., Zhang, M., Cheng, P., Br2 as a novel Lewis acid
catalyst for Friedel–Crafts alkylation of indoles with α,β-unsaturated ketones, Tetrahedron Letters (2016), doi:
http://dx.doi.org/10.1016/j.tetlet.2016.01.078
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Graphical Abstract
Br2 as a novel Lewis acid catalyst for
Friedel–Crafts alkylation of indoles with α,-
unsaturated ketones
Leave this area blank for abstract info.
Deqiang Liang *, Xiangguang Li, Wanshun Zhang, Yanni Li *, Mi Zhang, Ping Cheng

1
Tetrahedron Letters
Br2 as a novel Lewis acid catalyst for Friedel–Crafts alkylation of indoles with
α,-unsaturated ketones
Deqiang Liang a,*, Xiangguang Li a, Wanshun Zhang a, Yanni Li a,*, Mi Zhang a, Ping Cheng b
a Department of Chemistry, Kunming University, Kunming 650214, China
b School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, China
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
The inexpensive Br2 can serve as a novel Lewis acid catalyst for Friedel–Crafts alkylation of
indoles with α,-unsaturated ketones. Under the catalysis of only 3 mol% of Br2, this Michael
addition proceeded smoothly with high efficiency and broad substrate scope. Moreover,
theoretical calculations suggested that Br2 possesses only the modest power to activate chalcones
and is inferior to most tested acids, indicating that acidity might not be the primary cause for the
unique activity of Br2 in the current communication.
Available online
Keywords:
Homogeneous catalysis
C–C coupling
2009 Elsevier Ltd. All rights reserved.
Alkylation
Bromine effect
Indole functionalization
———
* Corresponding author. Tel.: +86 871 65098482; e-mail: ldq5871@126.com (D. Liang), suliyn@163.com (Y. Li)

2
Tetrahedron Letters
2
NBS (5)
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
24
24
3.5
11
2.0
24
2.0
24
24
24
24
24
24
24
24
24
24
24
24
24
24
7.0
24
nr
3
I2 (5)
82 (12)c
Elemental bromine is essential in chemistry, and as an
important member of the family of halogens, it seems to be well
studied.1 However, unlike solid molecular iodine, which has
found wide applications as a catalyst in modern synthetic
chemistry but is expensive,1,2 cheap liquid bromine is underrated
and found repulsive, for it is more volatile and corrosive.
Elemental bromine itself has rarely been used in a catalytic
manner except as an oxidant,3 though there is an increasing
tendency to explore the utility of N-bromosuccinimide (NBS)4
and other solid alternatives,5 which are associated with higher
costs and/or cumbersome procedures and yet no match for
molecular bromine in many cases.
4
HBr (5)
CuBr2 (5)
Br2 (5)
90
5
87
6d
7e
8f
91
HBr (5)
Br2 (5)
37 (59)c
93
9
HI (5)
78 (19)c
77 (18)c
75 (19)c
14 (81)c
73 (21)c
50 (46)c
83 (11)c
86 (11)c (32)g
88 (8)c
86 (9)c
nr
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
H2SO4 (5)
TsOH (5)
TFA (5)
We take leave to argue that chemists' distaste for bromine
would only put sand in the wheels of halogen chemistry. Some
time ago, HBr6 and CuBr27 were brought to the notice of Wang et
al. for their abnormal catalytic performances in some C–C bond-
forming processes, and they were claimed to depend on the
nature of the counterion. On the other hand, Jamison and co-
workers developed an NBS- or Br2-catalyzed synthesis of cyclic
carbonates from epoxides and CO2, and it was pointed out that
the epoxides were activated by electrophilic bromine.8 Though in
this seminal report the catalytic activity of Br2 was not unique, it
suggested that Br2 might serve as a novel and robust Lewis acid,
which was subsequently confirmed by the extremely high
efficiency of Br2 in activating aldehydes and ketones.9 And yet
for all that, the curious bromine effect is far from well
understood, more and further inquiries are still needed.
BF3·Et2O (5) MeCN
FeCl3 (5)
TiCl4 (5)
SnCl4 (5)
Br2 (5)
Br2 (5)
Br2 (5)
Br2 (5)
Br2 (5)
Br2 (5)
Br2 (3)
Br2 (2)
MeCN
MeCN
MeCN
CH2Cl2
THF
DMF
EtOH
80 (13)c
45 (51)c
82 (12)c
93
Toluene 50
MeCN
MeCN
MeCN
rt
50
50
88 (6)c
Referred to as “The Lord of the Rings” of aromatic
compounds, the privileged scaffold of indole continues to
fascinate chemists.10 In this regard, -indolylketones, generally
accessed by Michael-type Friedel–Crafts (F–C) reactions of
indoles with chalcones,11,12 are versatile intermediates in organic
synthesis11b,12a,13 and building blocks for biologically active
molecules.13c In spite of the great advances in asymmetric F–C
alkylations of indoles with chalcone derivatives, which were
catalyzed by dear metal complexes or other elaborated chiral
acids,11 in most cases the synthesis of -indolylketones required
rather high catalyst loading.11,12 Hence, there is still an urgent
need to develop cheaper and more powerful catalyst for this
important transformation.
Inspired by the aforementioned curious bromine effect,6-9 and
in an effort to gain further insights into it, we evaluated the
catalytic power of Br2 in this F–C alkylation. Here, we present an
efficient Br2-catalyzed Michael addition reaction of indoles with
simple α,-unsaturated ketones, wherein only 3 mol% of catalyst
was required, and theoretical calculations proved that Br2
possesses only the modest power to activate chalcone, suggesting
that acidity might not be the primary cause of the unique
performance of Br2.
a Reaction conditions: 1a (0.5 mmol), 2a (0.55 mmol), solvent (2.5 mL).
b Isolated yields.
c Recovery of 1a.
d 0.003 mL H2O was added.
e 200 mg 4 Å MS was added.
f 5 mol% of Bu4NBr was added.
g The reaction was run in a sealed tube with CH2Cl2 as the solvent.
occurred after 24 h with NBS as the catalyst (entry 2), the
employment of I2 resulted in incomplete F–C alkylation even
after 24 h (entry 3). HBr (40% aqueous) also proved to be an
excellent catalyst for this transformation, and 3a was delivered in
an excellent yield in a longer reaction time when it was used
(entry 4), whereas CuBr2 was less effective for the use of it led to
much longer reaction time (entry 5). It is worthy of notice that a
minute quantity of water (i.e., the same amount of water
contained in aqueous HBr in the same loading) hardly affected
the Br2-catalyzed addition (entry 6), and when 4 Å molecular
sieves (MS) were additionally added, the reaction catalyzed by
HBr was retarded (entry 7), probably due to the adsorption of the
catalyst. Tetrabutyl ammonium bromide (TBAB), an established
bromide ion source in Wang's work,6a did not show significant
counterion effect here (entry 8). Although the possibility that
more or less there might be HBr generated in situ by bromination
of indole in the Br2-catalyzed processes could not be ruled out,14
the apparently superior performance of Br2 clearly suggests that
they were two independent catalysts.
Our studies commenced with the F–C conjugate addition
between indole 1a and chalcone 2a (Table 1). Much to our
satisfaction, in the presence of only 5 mol% of Br2 in MeCN, the
addition was completed in
dihydrochalcone 3a in 92% yield (entry 1). While no reaction
2 h at 50 °C, affording
Table 1. Optimization of reaction conditionsa
Next, a survey of a range of typical Brønsted and Lewis acids
was conducted. It was found that none of the tested acids was
active enough to enable full conversion of 1a even after a
prolonged reaction time of 24 h, and while the employment of HI
(47% aqueous, entry 9), H2SO4 (entry 10), TsOH (entry 11),
BF3·Et2O (entry 13), TiCl4 (entry 15), or SnCl4 (entry 16) gave 3-
alkylindole 3a in relatively high yields, with trifluoroacetic acid
(TFA, entry 12) or FeCl3 (entry 14) as catalyst, 3a was yielded in
14% and 50% yields, respectively. The comparison of a series of
Entry Acid (mol%) Solvent
Br2 (5) MeCN
T (°C) t (h) Yield of 3ab (%)
50 2.0 92
1

3
solvents revealed that using CH2Cl2 (entry 17), THF (entry 18),
EtOH (entry 20) or Toluene (entry 21) as the solvent, this Br2-
catalyzed F–C alkylation did not reach completion even after 24
h, and that the use of N,N-dimethylformamide (DMF) proved
unfruitful (entry 19). A high yield of 82% was still achieved
when the reaction was performed at ambient temperature for 24 h
(entry 22). Finally, we were pleased to find that 3 mol% of Br2
was sufficient to make this F–C reaction complete within 7 h
(entry 23), and even further reducing the catalyst loading to 2
mol%, 88% yield of 3a was still obtained by prolonging the
reaction time to 24 h (entry 24).
exclusively furnished within 2 h. Attempt to achieve double
addition just by using 2 equivalents of indole 1a met with no
success, but resulted in much lower reaction rate (entry 11),
probably attributed to the complexation of Br2 by excessive 1a.9b
Optimized geometry at B3LYP16/6-31+G(d,p)17 level illustrated
that there is no substantial steric hindrance for further addition of
indole 1a to 3j (see Figure S1), though calculations at the same
level suggested that this monoadduct is thermodynamically more
stable. Nevertheless, with 10 mol% of Br2 at 80 °C, diadduct 4
was efficiently furnished in 87% yield within 5 h (Scheme 1). On
the other hand, chalcones 2l-p bearing either electron-deficient
(entries 12 and 13) or -rich β-aryl ring (entries 14-16) all worked
well in this F–C alkylation, affording expected products 3k-o in
high to excellent yields. In the case concerning 4-nitrochalcone
2m, the reaction should be performed at room temperature or
complex mixture would be yielded (entry 13). The addition of
indole 1a to 2-furfurylideneacetophenone 2q also proceeded
well, providing product 3p in 79% yield (entry 17). Notably,
vinyl ketone 2r reacted smoothly with 1a as well at 80 °C and the
catalyst loading of 10 mol%, giving adduct 3q in a high yield
after 24 h (entry 18).
Having developed optimized reaction conditions (entry 23,
Table 1), we subsequently explored the scope of this Michael
addition reaction with respect to the indoles 1 and α,β-
unsaturated ketones 2.15 As shown in Table 2, a broad range of
chalcones 2 reacted smoothly with indole 1a to provide the
corresponding adducts 3 in high to excellent yields. Both
electron-donating (entries 2 and 3) and -withdrawing substituents
(entries 4-7) at the para, meta, or ortho positions of the aroyl
group were compatible with this transformation. The use of 2-
furoyl (entry 8) or -thenoyl (entry 9) substrates 2h,i also provided
the desired products 3h,i in excellent yields. Unfortunately,
benzylideneacetone 2j proved to be a challenging substrate, and
the reaction of it with 1a did not yield the target adduct but an
unidentified compound, the NMR spectra of which were rather
complex and defied analysis (entry 10), probably due to the
competitive condensation at the carbonyl carbon.9b Surprisingly,
when dibenzylideneacetone 2k was used, monoadduct 3j was
Then, the F–C reaction was extended to various substituted
indoles 1. Whereas 2-methylindole 1b reacted rapidly with
chalcone 2a to furnish adduct 3r in 96% yield within 12 min
(entry 19), the reaction of electron-deficient ethyl indole-2-
carboxylate 1c needed to be carried out at 80 °C using 10 mol%
of catalyst, and still, after 24 h only a moderate yield of 3-
Table 2. Michael reaction leading to adduct 3a
Entry
1
1
R1
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
R2
2
R3
Ph
R4
Ph
t (h)
7.0
3
Yieldb (%)
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
H
2a
3a
3b
3c
3d
3e
3f
3g
3h
3i
93
2
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
2b
2c
2d
2e
2f
Ph
4-MeOPh
2-MePh
4-ClPh
9.0
14
91
3
Ph
90
4
Ph
10
92
5
Ph
4-NO2Ph
3-NO2Ph
2-NO2Ph
2-Furyl
2-Thienyl
Me
3.0
4.0
6.0
7.0
6.0
5.0
92
6
Ph
90
7
2g
2h
2i
Ph
82
8
Ph
91
9
Ph
90
10
11
12
13e
14
15
16
17
18f
19
20f
21
22
23
24
2j
Ph
nd
2k
2l
Ph
PhCH=CH 2.0
3j
3k
3l
81 (82)c (56)d
4-ClPh
Ph
Ph
Ph
Ph
7.0
24
93
70
2m 4-NO2Ph
2n
2o
2p
2q
2r
2a
2a
2a
2a
2a
2a
4-MePh
2-MePh
12
3m 84
8.0
6.0
9.0
24
3n
3o
3p
3q
3r
3s
89
75
79
72
96
44
86
95
93
3,4-(MeO)2Ph Ph
2-Furyl
H
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
1b 2-Me
1c 2-CO2Et
1d 5-MeO
1e 5-Br
Ph
0.2
24
Ph
Ph
7.0
1.0
16
3t
Ph
3u
3v
1f
5-NO2
Ph
1g 6-Cl
Ph
3.5
3w 88

4
Tetrahedron
25
1h 7-Me
H
2a
2a
Ph
Ph
Ph
Ph
3.0
0.5
24
3x
3y
97
96
nr
26e
27f
1i
1j
H
H
Me
Ph
Ph
PhSO2 2a
a Reaction conditions: 1 (0.5 mmol), 2 (0.55 mmol), MeCN (2.5 mL).
b Isolated yields.
c
The reaction time was prolonged to 24 h.
d The reaction was run with 2 equiv. of 1a for 24 h.
e The reaction was run at room temperature.
f 10 mol% of Br2 was used, and the reaction was run at 80 °C.
ones
of Br2 and I2 were basically unaffected
alkylindole 3s was generated (entry 20). It proved that indoles
1d-h bearing either electron-donating (entries 21 and 25) or -
withdrawing groups (entries 22-24) at C(5)-C(7) were all adept in
efficiently furnishing the corresponding products 3t-x in 86-97%
yields under the optimized reaction conditions. While desired
dihydrochalcone 3y was delivered rapidly from N-methylindole
1i in 96% yield within 30 min just at room temperature (entry
26), when the indolic nitrogen was protected by electron-
withdrawing phenylsulfonyl group, great sluggishness was
observed even under those enhanced conditions (entry 27).
(M062X/def2TZVP+MeCN20).
In an effort to gain insights into the extraordinary performance
Figure 2. NBO charge distributions on the β-carbon of chalcone
activated by positive ions
However, the comparison of free bromonium, iodonium, and
hydrogen ions revealed that bromonium ion is more robust as a
chalcone activator (Figure 2, for details see Table S1), which
might be related to the high electronegativity in combination with
high polarizability of the element of bromine. Thus, it appears
that the true catalyst in the above reactions is more likely to be
the bromonium ion. However, considering that the organic and
weak-polar solvent of MeCN was the reaction medium, the
electrophilic bromine was unlikely to be so free.1 This is also
supported by our calculations, which showed that in MeCN Br2
was not significantly polarized during activation of chalcone or
complexation by solvent molecules, judging by the bond lengths
and NBO charge distributions (see Figure S2). As a result, we
assume that in the present reaction acidity might not be the
primary cause of the unique activity of Br2, the origin of which
remains unclear at this time. Further exploration still needs to be
carried out.
Scheme 1. Double F–C alkylation of 2k
In conclusion, we have demonstrated that Br2 could act as a
novel Lewis acid catalyst for Friedel–Crafts alkylation of indoles
with α,-unsaturated ketones, and have developed a mild and
efficient synthesis of -indolylketone derivatives. Furthermore,
theoretical calculations revealed that in terms of the ability to
activate chalcones, Br2 possesses only the modest power and is
inferior to most tested Brønsted and Lewis acids, thus indicating
that acidity might not be the primary cause for the remarkable
activity of Br2 presented here. This work might inspire more
novel Br2-catalyzed reactions and provide further clues for the
understanding of the curious bromine effect, further studies
toward which are underway in our laboratory.
Figure 1. NBO charge distributions on the β-carbon of chalcone
activated by various acids
of Br2, theoretical calculations of the power of various
electrophilic species in activating the β-carbon of chalcone 2a
were performed. Upon coordination of the carbonyl oxygen atom
to them, the electron density of the β-carbon would be decreased.
First, a range of Brønsted and Lewis acids were compared
(Figure 1, for details see Table S1). Confusingly, natural bonding
orbital (NBO) charge distributions at various levels, including
Acknowledgments
B3LYP/6-31+G(d,p),
B3LYP/def2TZVP,18
and
M062X19/def2TZVP, all suggested that most tested acids were
superior to Br2 more or less, and while the powers of HBr, HI,
H2SO4, TsOH, BF3, and FeCl3 were further enhanced to varying
degrees when the solvent effect of MeCN was considered, the
This work was supported by the Applied Basic Research
Programs of Yunnan Science and Technology Department
(2014FD039), the Research Foundation for Introduced Talents of
Kunming University (YJL13001 and YJL15003), and the

5
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