Iron-catalyzed cross dehydrogenative coupling (CDC) of indoles and benzylic C-H bonds

Article abstract of DOI:10.1002/adsc.201400938

An efficient method for the synthesis of 3-alkylated indoles is described. The oxidative cross-coupling reaction of diphenylmethane derivatives with indoles at the C-3 position was achieved employing the N,N-dimethylcarbamoyl moiety as an auxiliary group

Full text of DOI:10.1002/adsc.201400938

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DOI: 10.1002/adsc.201400938
Iron-Catalyzed Cross Dehydrogenative Coupling (CDC) of
À
Indoles and Benzylic C H Bonds
Shuaibo Guo,a Yaxuan Li,a Yu Wang,a Xin Guo,a Xu Meng,b and Baohua Chena,*
a
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Gansu, and Key Laboratory of Nonferrous
Metal Chemistry and Resources Utilization of Gansu Province, Lanzhou 730000, Peopleꢀs Republic of China
Fax: (+86)-931-891-2582; e-mail: chbh@lzu.edu.cn
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese
b
Academy of Sciences, Lanzhou 730000, Peopleꢀs Republic China
Received: September 25, 2014; Revised: December 22, 2014; Published online: March 13, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400938.
Abstract: An efficient method for the synthesis of
3-alkylated indoles is described. The oxidative
cross-coupling reaction of diphenylmethane deriva-
tives with indoles at the C-3 position was achieved
employing the N,N-dimethylcarbamoyl moiety as
an auxiliary group in the presence of iron(II) chlo-
ride (FeCl2) and 2,3-dichloro-5,6-dicyanoquinone
(DDQ).
DDQ as oxidant. And then Liꢀs group[7] developed
the system furthere to achieve the alkylation of meth-
ylene and aromatic compound.
Indoles and their derivatives have attracted consid-
erable attention owing to their cardiovascular, anti-in-
flammatory, and phosphodiesterase inhibitory activi-
ties.[8] Moreover, the structural motif is a versatile
precursor or intermediate for the preparation of other
important pharmaceuticals and natural products.[9] As
a matter of fact, an efficient approach to C-3 alkylat-
À
À
Keywords: C C coupling; C H activation; cross-
dehydrogenative-coupling (CDC) reaction; diphe-
nylmethanes; indole synthesis; iron catalysis
ed indoles with low-cost, regioselective direct func-
3
À
tionalization of C(sp ) H bonds to substrates is usual-
ly a challenging task.[10] Traditionally, the means to
achieve functionalization of indoles at the C-3 posi-
tion usually required cumbersome and costly sub-
À
À
À
À
strates in C X/C M or C H/C X coupling reac-
Selective catalytic dehydrogenation of alkanes with tions.[11] To alleviate this problem, a large number of
À
alkenes to construct C C bonds might become one of research achievements based on the catalytic systems
the most efficient and straightforward approaches in involving Pd,[12] Pt,[13] Au,[14] Ir,[15] Ru,[16] Rh,[17] and
chemical synthesis in the following decades due to the Cu[18] have been reported widely. Remarkably, Liꢀs
wealth of alkanes and olefins as raw materials.[1] Al- group[7d] developed an efficient way to obtain the C-3
À
though several versatile methods to construct func- alkylated indoles via iron-catalyzed C C bond activa-
À
À
tionalized compounds through direct C H or C C tion using 1,3-dicarbonyl units as leaving group. How-
bond activation have attracted considerable attention ever, to the best of our knowledge, an iron-catalyzed
in various studies over the past decades, these pro- alkylation of indoles at the C-3 position using a direct
cesses still suffer from drawbacks such as the need for C<C->H activation strategy has not been devel-
complex starting materials or expensive catalysts.[2,3] oped. Herein, we would like to report a direct FeCl2-
À
Notably, iron has found widespread catalytic applica- catalyzed oxidative activation of benzylic C H bonds
tions in organic synthesis, especially for the forma- to form C C bonds with indoles assisted by the N,N-
À
À
À
À
tions of C C and C heteroatom bonds via C H bond dimethylcarbamoyl moiety in the presence of DDQ
activation because of its numerous advantages includ- without a leaving group.
ing low price, non-toxicity, and availability.[4] Recently,
The cross dehydrogenative coupling reaction be-
Li and co-workers[5] reported a series of efficient and tween N,N-dimethyl-1H-indole-1-carboxamide (1a)
À
general iron-catalyzed C C bond formations using and diphenylmethane (2a) was initially chosen as
benzylic compounds as substrates which reacted with a model reaction to optimize the reaction conditions
heteroatomic compounds and alkanes. In addition, (Table 1).With 10 mol% FeCl2 as catalyst, the reaction
Shi[6] described an iron-catalyzed cross dehydrogena- of 1a with diphenylmethane (6.0 equiv.) in DCE
3
À
tive arylation of a benzylic C(sp ) H bond with com- failed to provide the desired product 3a at 1008C
pounds containing aromatic C(sp ) H bonds using (Table 1, entry 1). To our delight, a 62% isolated
2
À
950
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Adv. Synth. Catal. 2015, 357, 950 – 954

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Iron-Catalyzed Cross Dehydrogenative Coupling (CDC) of Indoles
Table 1. Optimization of the reaction conditions.[a]
Table 2. Screening studies for protecting groups.[a]
Entry
Indole
Product [%]
Entry Catalyst
Oxidant
Solvent
Yield [%][b]
À
1
2
3
4
5
6
7
8
R= H
nd
nd
nd
nd
32
nd
nd
nd
À
1
2
3
FeCl2
FeCl2
FeCl3
none
DDQ
DDQ
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
–
62
35
12
5
–
R= CH3
À
R= COPh
À
R= COCH2=CH2
À
4
FeCl3·6H2O DDQ
R= COCH3
À
5
6
7
8
ferrocene
ZnCl2
CuCl
PdCl2
none
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
TBHP
DTBP
Cu(OAc)2 DCE
BQ
O2
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
R= Boc
À
R= CONHPh
À
trace
8
R= SO2C7H8
[a]
Reaction conditions: 1 (0.20 mmol), 2a (6.0 equiv.), cata-
lyst (10 mol%), oxidant (2.5 equiv.), solvent (1 mL), N2,
36 h, 1008C.
Isolated yields after column chromatography. nd=not
detected.
9
–
10
11
12
13
14
15
16
17
18
19
20
21
22
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
63[c]
34[d]/40[e]
[b]
43
10
–
–
–
Table 3. Reactions of various indoles (1) with phenylhydra-
zine (2).[a]
DCE
DCE
PhCl
dioxane 21
PhMe
DMF
DMSO
CH3CN
17
22
9
–
–
[a]
Reaction conditions: 1a (0.20 mmol), 2a (6.0 equiv.), cata-
lyst (10 mol%), oxidant (2.5 equiv.), solvent (1 mL), N2,
36 h, 1008C.
Entry
R
Product
Yield [%][b]
[b]
[c]
[d]
[e]
Isolated yields after column chromatography.
With 20 mol% FeCl2.
At 808C.
1
2
3
4
5
6
7
8
H (1a)
5-Cl (1b)
5-Br (1c)
5-F (1d)
5-CHO (1e)
5-CN (1f)
5-NO2 (1g)
6-Cl (1h)
3aa
3ba
3ca
3da
3ea
3fa
3ga
3ha
3ia
3ja
3ka
3la
3ma
3na
3oa
62
81
47
40
94
93
96
67
39
74
83
–
At 1108C.
yield was obtained when DDQ was used as oxidant
(Table 1, entry 2). Other iron salts such as FeCl3,
FeCl3·6H2O and ferrocene showed less catalytic activ-
ity under the same conditions(Table 1, entries 3–5).
When other catalytic metals including PdCl2, ZnCl2,
and CuCl were further explored, these reactions pro-
vided worse results (Table 1, entries 6–9). Moreover,
no improvement was observed with increasing the
catalytic amount to 20 mol% (Table 1, entry 10).
Meanwhile, there were no better results when the
temperature was changed (Table 1, entry 11). Com-
pared with other oxidants such as TBHP, BQ, O2,
DTBP and Cu(OAc)2, DDQ showed the best efficien-
cy (Table 1, entries 12–16). The transformation did
not proceed well under solvents such as DMF,
CH3CN, DMSO, PhCl, dioxane and toluene (Table 1,
entries 17–22).
9
6-F (1i)
10
11
12
13
14
15
4-COOCH3 (1j)
6-COOCH3 (1k)
3-Me (1l)
5-OMe (1m)
6-Me (1n)
2-Me (1o)
5
14[c]
23[c]
16
3pa
trace
[a]
Reaction conditions: 1 (0.20 mmol), 2a (6.0 equiv.), FeCl2
(10mol%), DDQ (2.5equiv.), DCE (1mL), N2, 36h, 1008C.
Isolated yields after column chromatography.
Reaction conditions: 1 (0.20 mmol), 2a (6.0 equiv.), FeCl2
(10mol%), TBHP (2.5equiv.), DCE (1mL), N2, 36h,
1008C.
[b]
[c]
Adv. Synth. Catal. 2015, 357, 950 – 954
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951

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Shuaibo Guo et al.
In order to evaluate the influence of the N-protect-
Subsequently, we turned our attention to examine
ing groups, a series of N-substituted indoles were sub- the generality of diphenylmethanes (Scheme 1). Nu-
jected to the reaction with diphenylmethane to pro- merous substrates bearing both electron-withdrawing
duce the corresponding C-3 alkylated indoles under and electron-donating groups were all successfully en-
the optimal conditions (Table 2). As a consequence, gaged in this transformation producing the corre-
the N,N-dimethylcarbamoyl moiety was found to be sponding products (3ab–3ai) in 28–83% yields. None-
the optimal activating group. It may result from the theless, an electronic effect could not be ignored as
efficient tuning of the reactivity and the relatively an important factor for those reactions. Those sub-
stable structure of N,N-dimethyl-1H-indole-1-carbox- strates bearing electron-withdrawing groups such as
amide (1a) whose indole nucleus is predestined to- Cl, F gave lower yields than those with the electron-
wards the synthesis of C-3 alkylated indoles.[19]
donating groups. It seems reasonable that electron-
Under the optimized conditions, we then set out to withdrawing groups weakened the SET process to
explore the scope and limitations of the CDC reaction generate the proposed methylenyl cation intermediate
with a wide range of substituted indoles and the re- by oxidation.[6] In addition, we could also draw a con-
sults are summarized in Table 3. As shown in the
Table 3, indoles substituted with electron-withdrawing
groups such as Cl, Br, NO2, CN and COOR reacted
smoothly with diphenylmethane and produced the
corresponding products (3aa–3ka) in 39–96% yields.
The molecular structure of complex 3ca was further
confirmed by X-ray diffraction analysis[20] (Figure 1).
Meanwhile, indoles substituted with electron-donating
groups, such as Me and OMe groups, showed low re-
activity (Table 3, entries 12–15). Thus, the presence of
electron-withdrawing groups on the indole clearly fa-
vored the reaction compared to that of electron-do-
nating groups which led to lower yields. In addition,
a heterocyclic indole (1p) was also tested under the
conditions, but only a trace of the desired product
was obtained (Table 3, entry 16).
Scheme 1. Reactions of N,N-dimethyl-1H-indole-1-carbox-
amide (1) with various diphenylmethanes (2). Reaction con-
ditions: 1a (0.20 mmol), 2a (6.0 equiv.), FeCl2 (10 mol%),
DDQ (2.5 equiv.), DCE (1 mL), N2, 36 h, 1008C. Isolated
Figure 1. X-ray crystallographic structure of compound 3ca.
yields after column chromatography are reported.
952
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Iron-Catalyzed Cross Dehydrogenative Coupling (CDC) of Indoles
ring bar under nitrogen in sequence, and then the DCE
(1 mL) was added and the reaction mixture was stirred at
1008C for 36 h. The solvent was evaporated and the residue
was purified by flash column chromatography on silica gel
to afford the desired product 3 with ethyl acetate and petro-
leum ether as the eluent.
Acknowledgements
We are grateful for sponsorship of this project by the Nation-
al Natural Science Foundation of China (Nos. 21372102 and
21403256).
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À
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