CuBr-catalyzed selective oxidation of N-azomethine: Highly efficient synthesis of methine-bridged bis-indole compounds

Article abstract of DOI:10.1039/c004213g

An efficient CuBr catalyzed cleavage of C-N bonds in the oxidative cross-dehydrogenative-coupling (CDC) of N-benzyl amines with indoles mediated by tert-butyl hydroperoxide (TBHP) was reported. A series of methine-bridged bis-indole derivatives were successfully synthesized under the optimized reaction conditions.

Full text of DOI:10.1039/c004213g

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www.rsc.org/obc | Organic & Biomolecular Chemistry
CuBr-catalyzed selective oxidation of N-azomethine: highly efficient synthesis
of methine-bridged bis-indole compounds†
Jianguo Yang,*a Zhijing Wang,b Fuyou Pan,a Yongmin Lia and Weiliang Bao*b
Received 16th March 2010, Accepted 20th April 2010
First published as an Advance Article on the web 11th May 2010
DOI: 10.1039/c004213g
An efficient CuBr catalyzed cleavage of C–N bonds in the oxidative cross-dehydrogenative-coupling
(CDC) of N-benzyl amines with indoles mediated by tert-butyl hydroperoxide (TBHP) was reported. A
series of methine-bridged bis-indole derivatives were successfully synthesized under the optimized
reaction conditions.
Table 1 Optimization of the reaction conditions.a
Introduction
Formation of new bonds (such as C–C, C–O and C–N) by
direct and selective activation of C–H bonds is one of the
most challenging research areas in organic chemistry, because it
is unique organic transformations that are hard to achieve by
classical methods.1 It has attracted great interest in recent years
since it avoids the preparation of functionalized starting materials
and makes synthetic schemes shorter and more efficient.2 The C–H
activation process can be completed by several methods. Oxidative
C–H functionalization catalyzed with3,5a–e,6 or without4 metallic
catalysts is one of the most effective and versatile methodologies.
Activation of the C–H bond adjacent to a heteroatom (such
as N, O or S) has been widely investigated and a number of
excellent results have been obtained.5,6 In the published works,
functionalizations of sp3 C–H adjacent to nitrogen are most
common. Many pioneering works have been done.6
The C–N bond is one of the most abundant and unreactive
bonds in organic molecules.7–10 The cleavage of the C–N bond
in the oxidative C–H bond activation is rare. However, an
interesting example of the cleavage of the C–N bond after the
functionalization of the adjacent carbon was reported by Zhi-
ping Li.11 In their report, methylene-bridged bis-1,3-dicarbonyl
derivatives were formed by the coupling of N,N-dimethyl aniline
with 1,3-dicarbonyl compounds and thereafter cleavage of the
C–N bond.
Entry Catalyst
T ◦
C
1a (equiv.) Oxidant/mmol
yield b(%)
1
2
3
4
5
6
7
8
CuBr
CuBr
CuBr
CuBr
CuBr
CuCl2
CuCl
CuBr2
CuI
50
70
80
80
80
80
80
80
80
1.0
1.0
1.0
0.6
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
t-BuOOH (0.8)
t-BuOOH (0.8)
t-BuOOH (0.8)
t-BuOOH (0.8)
t-BuOOH (0.6)
t-BuOOH (0.6)
t-BuOOH (0.6)
t-BuOOH (0.6)
t-BuOOH (0.6)
t-BuOOH (0.6)
t-BuOOH (0.6)
t-BuOOH (0.6)
t-BuOOH (0.6)
t-BuOOH (0.6)
t-BuOOH (0.6)
(t-BuO)2 (0.6)
32
48
52
42
62
34
38
34
46
41
41
47
58c
<5
50d
26e
9
10
11
12
13
14
15
16
Cu(OTf)2 80
Cu(OAc)2 80
CuBr
CuBr
no
CuBr
CuBr
90
80
80
80
80
a 0.5 mmol indole, 0.025 mmol [Cu], t-BuOOH (5~6 M in decane).
b Isolated yields and based on indole. c Reaction time: 26 h. d 10% CuBr
was used. e Di-tert-butyl peroxide in 0.5 mL toluene was used as oxidant.
Herein, we report a highly efficient CuBr catalyzed cleavage
of C–N bonds in the oxidative cross-dehydrogenative-coupling
(CDC) of N-benzyl amines with indoles, mediated by tert-butyl
hydroperoxide (TBHP). A series of methine-bridged bis-indole
compounds were formed. The indole framework represents a
privileged structural motif of established value in biologically
natural products and pharmaceutical compounds.12 Introducing
or functionalizing indole derivatives to complex molecules13 is
important for not only the medical application but also the
development of synthetic methodologies.6d
Results and discussion
In the initial study, N-benzyl piperidine 1a and indole 2a were
subjected for the desired transformation at the presence of CuBr/t-
BuOOH. We were glad to find that the target methine-bridged
bis-indole 3a was formed and isolated in 32% yield after 18 h at
50 ◦C (Table 1, entry 1). This result encouraged us to optimize
the reaction parameters to enhance the yield of 3a on this
template reaction. Various reaction temperatures, the copper salts,
the reaction time and the amount of catalysts for the desired
CDC reaction were examined (Table 1). Among the reaction
temperature tested, 80 ◦C was the best one (Table 1, entries 1–
3 and 12). The amount of oxidant obviously affects the reaction
result. Decreasing the amount of tert-butyl hydroperoxide (TBHP)
from 0.8 to 0.6 mmol can improve the yield (Table 1, entries 3 and
5). Subsequently, various copper salts were tested, obviously, CuBr
was the most efficient one and the yield was 62% (Table 1, entry 5).
aSchool of pharmaceutical and Chemical Engineering, Taizhou University,
Taizhou 317000, China. E-mail: yjg@tzc.edu.cn
bDepartment of Chemistry, Xi Xi Campus, Zhejiang University, Hangzhou
310028, China. E-mail: wlbao@css.zju.edu.cn
† Electronic supplementary information (ESI) available: Experimental
details, NMR and mass spectra. See DOI: 10.1039/c004213g
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Table 3 Reaction of N,N-diethyl benzylamine with indolesa
Table 2 Reaction of benzylamine with indolesa
R2
Products
yield (%)b
Entry
n
R1
R2
Products
Yield (%)b
Entry
1
2
3
4
5
6
7
8
2
2
2
2
1
1
1
1
1
1
1
1
1
1
1
H
H
H
H
H
H
H
H
4-Cl
4-Cl
4-Cl
4-Cl
3-CH3
3-CH3
3-CH3
H
6-Cl
6-F
5-CH3
H
6-Cl
6-F
5-CH3
H
6-F
5-Br
5-CH3
5-Br
6-F
3a
3b
3c
3d
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
62
60
56
50
70
76
80
63
57
59
70
55
60
53
51
1
2
3
H
6-Cl
6-F
3a
3b
3c
52
65
66
a 0.5 mmol Indole, 0.5 mmol N,N-diethyl benzylamine, 0.025 mmol CuBr,
and 0.6 mmol of t-BuOOH (5~6 M in decane). b Isolated yields and based
on indole.
9
10
11
12
13
14
15
5-CH3
a 0.5 mmol Indole, 0.5 mmol benzylamine, 0.025 mmol CuBr, and 0.6 mmol
of t-BuOOH (5~6 M in decane). b Isolated yields and based on indole.
Scheme 1 Possible pathways for the formation of 3a.
As for the dosage of CuBr, increasing the amount of CuBr, the yield
was declined (Table 1, entry 15). Prolonged reaction time resulted
in slightly decreasing in yields (Table 1, entry 13). In the absence
of any metal catalyst, the desired product generated in a low yield
(<5%) (Table 1, entry 14). By replacing the tert-butyl peroxide with
di-tert-butyl peroxide, the reaction conducted smoothly, however,
the yield of the desired product was not high (Table 1, entry 16). So
the best reaction conditions is using 0.6 mmol TBHP and 5% mol
CuBr under 80 ◦C.
the intermediate 4. There are two possible paths for the transform
from 4 to product 3. Either an SN1 nucleophilic substitution
reaction or a tandem reaction of Cope elimination and Michael
addition was possible. However, both intermediate 4 and 5 were
found in the reaction solution after the reaction was carried out
for one hour. So the SN1 route was more possible.
Conclusions
Under the optimized reaction conditions, various indoles were
used to react with N-benzyl piperidine and N-benzyl pyrrolidine
derivatives, and representative results are listed in Table 2. Both
N-benzyl piperidine and N-benzyl pyrrolidine derivatives are
effective for this CDC reaction. Indoles with electron-withdrawing
groups or electron-donating groups on C5 and C6 also worked well
with substrates under the present reaction conditions (Table 2,
entries 2–4, 6–8 and 10–15). The reactions selectively occur at the
C3 position of indoles if both C2 and C3 positions of indoles
are unoccupied. Interestingly, N-benzyl pyrrolidine reacted with
various indoles to give the desired products in 61–80% yields which
are higher than N-benzyl piperidine (Table 2, entries 1–8). N-
(3-methylbenzyl) pyrrolidine and N-(4-chlorobenzyl) pyrrolidine
could also react with various indoles, however, the yield were
slightly less than N-benzyl pyrrolidine (Table 2, entries 5–15). This
may be due to sterically hindered. N,N-diethyl benzyl amine could
also react with the indole derivatives and give the satisfactory
results (Table 3, entries 1–3). When N-ethyl pyrrolidine was used
as substrate, no corresponding product was found.
In summary, we developed a novel and efficient method for
the synthesis of bis(indolyl)methane derivatives via the reaction
between sp3 C–H bonds and sp2 C–H bonds catalyzed by
copper bromide. This novel methodology not only provides a
valid way to construct bisindoles derivatives but also opens a
new way to study and construct more complex molecules since
indole fragment widely featured in a variety of pharmacologically
and biologically active compounds. The scope, mechanism, and
synthetic application of this reaction are under investigation.
Experimental Section
Typical experimental procedure
To a mixture of CuBr (3.6 mg, 0.025 mmol) and indole (59 mg,
0.5 mmol), N-benzyl piperidine (88 mg, 0.5 mmol) was added.
Then tert-butyl hydroperoxide (0.6 mmol, 5–6 M in decane)
was added dropwise into the mixture under nitrogen at room
temperature. The resulting mixture was stirred at 80 ◦C for 18 h.
Then, the cooled reaction mixture was dissolved in water (5 mL)
and extracted with CH2Cl2 (3 ¥ 5 mL). The combined organic layer
was dried with anhydrous MgSO4, and the product was further
purified by silica gel column chromatography and eluted with
cyclohexane and petroleum ether mixture to afford the product.
Based on these results, a tentative mechanism for the formation
of methine-bridged bisindole compounds is proposed in Scheme 1.
Suggested by S.-I. Murahashi etc.,6f,14 the first step is most
likely the formation of an iminium ion via a copper coordinated
complex through hydrogen abstraction of the sp3 C–H adjacent to
nitrogen. Then a Friedel–Crafts-type reaction is followed to give
2976 | Org. Biomol. Chem., 2010, 8, 2975–2978
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Bis(5-bromoindol-3-yl)(4-chlorophenyl)methane (3g) 17
3,3¢-(Phenylmethylene)bis(1H-indole) (3a)
Pink solid; mp 122–124 ◦C; 1H NMR (400 MHz, CDCl3): d 7.91
(br s, 2H, NH), 7.34–7.39 (m, 6H), 7.25–7.29 (m, 2H), 7.14–7.17
(m, 3H), 7.00 (t, J = 6.8 Hz, 2H), 6.66 (s, 2H), 5.88 (s, 1H, Ar–
CH); 13C NMR (50 MHz, CDCl3): d 144.2, 136.9, 128.9, 128.4,
127.3, 126.3, 123.8, 122.1, 120.1, 120.0, 119.5, 111.2, 40.4; IR
(KBr) nmax/cm-1 3410, 3051, 2924, 2858, 1612; HRMS (El) calcd
for C23H18N2: 322.1470; found 322.1472.
Pink solid; mp 212–214 ◦C; 1H NMR (400 MHz, CDCl3) d
8.02 (br s, 2H), 7.44 (s, 2H), 7.18–7.24 (m, 8H), 6.59(d, J =
1.6 Hz, 2H), 5.71 (s, 1H) 13C NMR (50 MHz, CDCl3): d 141.8,
135.5, 132.4, 130.1, 128.8, 128.7, 125.3, 125.1, 122.4, 118.7, 113.0,
39.5. IR nmax/cm-1 3431, 3130, 2856, 1616; HRMS (El) calcd for
C23H15Br2ClN2: 511.9291; found 511.9297.
Bis(5-methylindol-3-yl)(4-chlorophenyl)methane (3h)
Pink solid; mp 148–150 ◦C; 1H NMR (400 MHz, CDCl3): d 7.78
(br, s, 2H, NH), 7.21–7.23 (m, 6H), 7.15 (s, 2H), 7.00 (d, J = 8.8 Hz,
2H), 6.54 (s, 2H), 5.78 (s, 1H, Ar–CH), 2.35 (s, 6H, CH3); 13C
NMR (50 MHz, CDCl3): d 143.0, 135.2, 131.9, 130.3, 128.8, 128.6,
127.3, 124.1, 123.9, 119.6, 119.0, 111.0, 39.7, 21.7. IR nmax/cm-1
3408, 3026, 2922, 2858, 1622; HRMS (El) calcd for C25H21N2Cl:
384.1393; found 384.1389.
Bis(6-chloroindol-3-yl)(phenyl)methane (3b)
Pink solid; mp 128–130 ◦C; 1H NMR (200 MHz, CDCl3): d 7.96
(br s, 2H, NH), 7.34 (s, 2H), 7.28–7.30 (m, 4H), 7.23–7.26 (m,
3H), 6.96 (d, J = 8.4 Hz, 2H), 6.61 (s, 2H), 5.78 (s, 1H, Ar–CH);
13C NMR (50 MHz, CDCl3): d 143.5, 137.2, 128.8, 128.6, 128.2,
126.7, 125.8, 124.3, 120.9, 120.3, 119.8, 111.2, 40.3. IR nmax/cm-1
3423, 3026, 2927, 2858, 1610; HRMS (El) calcd for C23H16N2Cl2:
390.0691; found 390.0694.
Bis(5-bromoindol-3-yl)(3-methylphenyl)methane (3i)
◦
1
Bis(6-fluoroindol-3-yl)(phenyl)methane (3c)
Pink solid; mp 155–157 C; H NMR (400 MHz, CDCl3): 7.97
(br, s, 2H, NH); 7.47 (s, 2H), 7.14–7.23 (m, 6H), 7.09 (s, 1H), 7.02–
7.05 (m, 3H), 6.58 (s, 2H), 5.68 (s, 1H, Ar–CH), 2.28 (s, 3H, CH3);
13C NMR (50 MHz, CDCl3): d 143.2, 138.2, 135.5, 129.5, 128.9,
128.5, 127.6, 125.8, 125.1, 125.0, 122.5, 119.3, 112.8, 40.0, 21.8.
IR nmax/cm-1 3421, 3024, 2922, 2864, 1602; HRMS (El) calcd for
C24H18N2Br2: 491.9837; found 491.9840.
Pink solid; mp 133–135 ◦C; 1H NMR (400 MHz, CDCl3): d 7.87
(br, s, 2H, NH), 7.19–7.31 (m, 7H), 6.98–7.00 (dd, J = 1.8, 9.8 Hz,
2H), 6.74(dt, J = 2.0, 9.2 Hz, 2H), 6.58 (s, 2H), 5.78 (s, 1H, Ar–
CH); 13C NMR (50 MHz, CDCl3): d 162.5, 157.8, 143.8, 136.9,
136.7, 128.8, 128.6, 126.6, 124.0, 123.9, 123.8, 120.9, 120.7, 119.8,
108.5, 108.0, 97.8, 97.3, 40.4. IR nmax/cm-1 3421, 3064, 3030, 2929,
1624; HRMS (El) calcd for C23H16N2F2: 358.1282; found 358.1284.
Bis(6-fluoroindol-3-yl)(3-methylphenyl)methane (3j)
Pink solid; mp 141–143 ◦C; 1H NMR (400 MHz, CDCl3): d 7.88
(br, s, 2H, NH), 7.23–7.27 (m, 2H), 7.14–7.18 (m, 2H), 7.09 (d,
J = 8.0 Hz, 1H), 6.98–7.03 (m, 3H), 6.75 (dt, J = 1.2, 9.1 Hz, 2H),
6.60 (s, 2H), 5.75 (s, 1H, Ar–CH), 2.28 (s, 3H, CH3); 13C NMR
(50 MHz, CDCl3): d 162.5, 157.8, 143.7, 138.1, 136.9, 136.6, 129.6,
128.4, 127.4, 125.8, 124.0, 123.8, 120.9, 120.7, 119.9, 108.5, 108.0,
97.8, 97.3, 40.3, 21.8. IR nmax/cm-1 3419, 3061, 2920, 2858, 1624;
HRMS (El) calcd for C24H18N2F2: 372.1438; found 372.1435.
Bis(5-methylindol-3-yl)(phenyl)methane (3d)15
Red solid; mp 193–194 ◦C; 1H NMR (400 MHz, CDCl3): d 7.74
(br, s, 2H, NH), 7.32 (d, J = 7.2 Hz, 2H), 7.26 (t, J = 7.2 Hz,
2H), 7.20–7.22 (m, 3H), 7.18 (s, 2H), 6.98 (d, J = 8.0 Hz, 2H),
6.54 (s, 2H), 5.81 (s, 1H, Ar–CH); 13C NMR (50 MHz, CDCl3): d
144.4, 135.3, 128.9, 128.7, 128.4, 127.5, 126.3, 124.1, 123.8, 119.7,
119.5, 110.9, 40.3, 21.7. IR nmax/cm-1 3408, 3022, 2920, 2856, 1597;
HRMS (El) calcd for C25H22N2: 350.1783; found 350.1786.
Bis(5-methylindol-3-yl)(3-methylphenyl)methane (3k)
Bis(indol-3-yl)(4-chlorophenyl)methane (3e) 16
Pink solid; mp 140–142 ◦C; 1H NMR (400 MHz, CDCl3): d 7.70
(br, s, 2H, NH), 7.09–7.21 (m, 8H), 6.96–7.01 (m, 2H), 6.52 (s,
2H), 5.77 (s, 1H, Ar–CH), 2.34 (s, 6H, CH3), 2.27 (s, 3H, CH3);
13C NMR (50 MHz, CDCl3): d 144.3, 137.8, 135.2, 129.7, 128.6,
128.3, 127.6, 127.1, 125.9, 124.1, 123.7, 119.7, 110.9, 40.1, 21.8,
21.7. IR nmax/cm-1 3408, 3022, 2918, 2858, 1601; HRMS (El) calcd
for C26H24N2: 364.1940; found 364.1945.
◦
1
Pink solid; mp 76–77 C; H NMR (400 MHz, CDCl3): d 7.88
(br, s, 2H, NH), 7.35 (t, J = 7.2 Hz, 4H), 7.21–7.27 (m, 4H), 7.17
(t, J = 7.4 Hz, 2H), 7.01 (t, J = 7.4 Hz, 2H), 6.61 (s, 2H), 5.85 (s,
1H, Ar–CH); 13C NMR (50 MHz, CDCl3): d 142.8, 136.9, 132.0,
130.3, 128.6, 127.1, 123.8, 122.3, 120.1, 119.6, 119.4, 111.4, 39.8.
IR nmax/cm-1 3410, 3051, 2923, 2858, 1614; HRMS (El) calcd for
C23H17ClN2: 356.1080; found 356.1075.
Acknowledgements
Bis(6-fluoroindol-3-yl)(4-chlorophenyl)methane (3f)
This work was financially supported by the Natural Science
Foundation of Zhejiang Province for founding (No. Y406049).
◦
1
Pink solid; mp 98–100 C; H NMR (400 MHz, CDCl3): d 7.94
(br, s, 2H, NH), 7.21–7.24 (m, 6H), 7.02 (d, J = 9.6 Hz, 2H), 6.77
(dt, J = 1.1, 9.2 Hz, 2H), 6.60 (s, 2H), 5.77 (s, 1H, Ar–CH); 13C
NMR (50 MHz, CDCl3): d 162.6, 157.8, 142.3, 136.9, 136.7, 132.3,
130.2, 128.7, 124.1, 124.0, 123.6, 120.8, 120.6, 119.3, 108.7, 108.2,
98.0, 97.4, 39.8. IR nmax/cm-1 3427, 3066, 2864, 1624; HRMS (El)
calcd for C23H15F2ClN2: 392.0892; found 392.0892.
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