Gold-catalyzed direct indolation of tetrahydroisoquinolines

Article abstract of DOI:10.1002/cjoc.201201154

Nitrogen-containing heterocyclic compounds are important motifs of pharmaceuticals and functional materials, and there has been a growing interest in new synthetic methods for their preparations. In this paper, we report a direct cross-coupling reaction o

Full text of DOI:10.1002/cjoc.201201154

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DOI: 10.1002/cjoc. 201201154
Gold-Catalyzed Direct Indolation of Tetrahydroisoquinolines†
Zhang, Yana,b(张燕)
Luo, Shaa(罗莎)
Feng, Bainian*,a(冯柏年)
Zhu, Chengjianb(朱成建)
a School of Medicine and Pharmaceutics, Jiangnan University, Wuxi, Jiangsu 214122, China
b State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering,
Nanjing University, Nanjing, Jiangsu 210093, China
Nitrogen-containing heterocyclic compounds are important motifs of pharmaceuticals and functional materials,
and there has been a growing interest in new synthetic methods for their preparations. In this paper, we report a di-
rect cross-coupling reaction of indole with N-aryl tetrahydroisoquinolines in the presence of gold catalyst. The reac-
tion is compatible with a wide range of substituted indoles, to enable the formation of various alkylated heteroare-
nes under very mild reaction conditions.
Keywords C—H activation, sp3 bonds, indolation, gold, heterocyclic compounds
Introduction
amines, in particular N-phenyl tetrahydroisoquinoline,
with nucleophiles have received a lot of attention
(Scheme 1b).[3] A large number of different methods
have been developed since the pioneering studies of
Murahashi and Li.[1d,1e,4] Various nucleophiles have been
employed, including cyanides,[1f,1i,4d,5] nitroalkanes,[5i,6]
activated methylene compounds,[5f,6b,6d,6g,7] ketones,[8]
electron-rich arenas,[1e,9] alkynes,[4e,10] organometallic[11]
or siloxy compounds,[12] and heteroatom nucleo-
philes.[1j,4b,4c,6g,13] The oxidation protocols involve pre-
dominantly metal catalysts together with oxygen or
synthetic oxidants, but organocatalytic,[8c] photochemi-
cal,[5a,6f,6g,13d] electrochemical,[5b,14] and noncatalytic
methods[5d,5e,5h,9e,12d] are also known. Even a nonoxida-
tive method that releases hydrogen has been reported.[15]
The most notable exception is the combination of CuBr
as catalyst and tert-butyl hydroperoxide (TBHP) in
decane as oxidant, introduced by the Li group, which
has been used for a large number of nucleophiles.[1e,4e,4f]
Compounds containing heteroatoms, such as nitrogen,
widely exist in nature. The syntheses of these com-
pounds have attracted much attention in industrial and
academic researches due to their biological and phar-
maceutical properties. Tetrahydroisoquinolines and in-
doles are common substructures in natural products and
pharmaceuticals.[16] Moreover, 1-aryl tetrahydroisoqui-
noline analogues are potential neuroprotective agents[17]
and act as anti-HIV agents[18] in vitro. Methods for the
efficient preparations of such compounds are therefore
highly welcome. There are some gold-catalyzed reac-
tions building up tetrahydroisoquinolines.[19] To synthe-
Direct cross-coupling reactions where new carbon—
carbon bonds are generated via the oxidations of C—H
bonds are highly desirable transformations from the
atom-economic point of view. It is therefore not sur-
prising that these reactions, also called cross-dehydro-
genative coupling (CDC) reactions, have attracted sig-
nificant attention in the past few years (Scheme 1a).[1]
One of the major intents of the CDC reactions is the
construction of C—C bonds by catalytic activation of
the C—H bond. C—H bonds are ubiquitous in organic
molecules, and their functionalization enables the direct
synthesis of complicated products from simple starting
materials, without prior introduction of activating
groups. Under oxidative conditions, two C—H or het-
eroatom—H bonds can be coupled directly to form a
new bond.[2]
Scheme 1 (a) Oxidative coupling for functionalization of C—H
bonds. (b) Oxidative coupling of N-phenyl tetrahydroisoquinoline
with nucleophiles
[Catalyst]
R'
(a)
(b)
H
R
H
R
R'
[Oxidant]
[Catalyst]
[Oxidant]
Nu
N
N
Ph
Ph
Nu
Among these reactions, the oxidative couplings of
*
E-mail: zhangyan@jiangnan.edu.cn
Received November 21, 2012; accepted December 7, 2012; published online December 13, 2012.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201201154 or from the author.
Dedicated to the 60th Anniversary of East China University of Science and Technology.
†
Chin. J. Chem. 2012, 30, 2741—2746
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Zhang et al.
COMMUNICATION
size and/or introduce these simple structures to complex
molecules would not only have significant potential
biological applications but also represent a significant
contribution to synthetic methodologies.
reaction, the mixture was filtered and the filtrate was
extracted with ethyl acetate (3—10 mL). The combined
organic layer was washed with brine, dried over anhy-
drous sodium sulfate and concentrated under reduced
pressure. The residue was purified by column chroma-
tography on silica gel and the fraction was collected and
concentrated to give the desired product.
Unlike other late transition metals, gold rarely un-
dergoes the necessary oxidation state changes required
to catalyze coupling reactions under homogeneous con-
ditions. However, when reacted in the presence of
strong external oxidants, AuI/AuIII redox cycles become
accessible leading to coupling reactions involving the
formations of new C—C bonds. Building on earlier
studies using stoichiometric gold(III), several impres-
sive gold-catalyzed C—C bond formation and cross-
coupling reactions using range of two-electron oxidants
have been reported in the last few years.[20] These redox
transformations complement the well-established re-
dox-neutral reactivity of homogeneous gold complexes
and represent a significant advance for the field. More-
over, when conventional gold-catalyzed processes de-
liver one or both organic fragments prior to coupling,
gold can catalyze novel C—C bond-forming method-
ologies of interest to the wider organic chemistry com-
munity. We have reported some gold-bipydine complex
catalyzed formations of C — C, C — O and C — N
bonds.[21] Oxidative couplings of tertiary amines with
nucleophiles including trimethylsilyl cyanide, nitroal-
kanes and ketones are catalyzed by gold complexes in
good to excellent yields. Inspired by the previous good
results, we are searching for a convenient, efficient, and
general gold catalyst/oxidant system to reach the oxida-
tive coupling of N-phenyl tetrahydroisoquinoline with
indole. Fortunately, an excellent NaAuCl4/tert-butyl
hydroperoxide (TBHP) system has been developed for
the oxidative coupling (Scheme 2).
Results and Discussion
As a starting point, we chosed indole (1a) and
2-phenyl-1,2,3,4-tetrahydroisoquinoline (2a) as standard
substrates to investigate suitable reaction conditions for
the desired reaction. The results are listed in Table 1.
The desired product 3a was isolated in only 28% yield
at room temperature with tert-butyl hydrogen peroxide
(TBHP) as the oxidant and Au-bipy complex as the
catalyst which is reported as the excellent catalyst for
the oxidative coupling (Table 1, Entry 1). Similar yields
were gained when other gold complexes were used (Ta-
ble 1, Entries 2, 3). To our surprise, the yield of 3a in-
creased to 80% when HAuCl4 was used as the catalyst
(Table 1, Entry 4). Further investigations showed that
NaAuCl4 was a more effective catalyst. Other gold salts
were either less effective than NaAuCl4 or inactive (Ta-
ble 1, Entries 5—9). The reaction also provided an ex-
cellent yield when the amount of NaAuCl4 was reduced
to 3 mol% (Table 1, Entries 17—19). The use of
tert-butyl peroxide instead of TBHP as an oxidant re-
sulted in lower yield (70%) of the product (Table 1, En-
try 10). Other oxidants, such as H2O2, O2 and air were
also examined (Table 1, Entries 11—13). However, no
product was detected. The effect of solvents (without
any previous procedure for the commercial available
solvents) was also investigated. Toluene was the most
effective solvent, although ethanol, methanol, and ace-
tonitrile could also be used (Table 1, Entries 14—16).
After the optimization process of catalysts, oxidants,
solvents, the following coupling reactions were carried
out under the standard conditions: 3 mol% NaAuCl4 as
the catalyst, TBHP as the oxidant, and toluene as the
solvent. The cross dehydrogenative coupling of sp3 C—
H and sp2 C—H was maintained at room temperature
without exclusion of air. In contrast to the reaction
catalysed by Cu/TBHP, there was no peroxidation ob-
served at the benzylic position of N-phenyl tetrahydro-
isoquinolines.[9b]
Scheme 2 Gold catalyzed indolation of N-aryltetrahydroiso-
quinolines
NaAuCl4 (3 mol%)
r.t., TBHP
N
R1
+
N
H
R2
N
R2
NH
R1
The scope of this gold catalyzed CDC reaction under
optimized conditions was evaluated by employing a
variety of N-aryl tetrahydroisoquinoline derivatives and
substituted indole derivatives, and representative results
are listed in Table 2. In general, the desired coupling
products were successively formed in good to excellent
yields. As expected, the CDC reactions proceeded
smoothly without protection of NH of indole. The reac-
tions selectively occured at the C3 position of indoles if
both C2 and C3 positions of indoles were unoccupied.
Experimental
Gold-catalyzed indolation of N-aryl tetrahydro-
isoquinoline A 10 mL round-bottom flask equipped
with magnetic stirrer was charged with substituted in-
dole (0.1 mmol) and N-aryl tetrahydroisoquinoline (0.1
mmol), toluene (2 mL), TBHP (1.2 equiv.) and NaAuCl4
(3 mol%). The resulting mixture was continuously
stirred at room temperature for 5—8 h. At the end of the
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Chin. J. Chem. 2012, 30, 2741—2746

Gold-Catalyzed Direct Indolation of Tetrahydroisoquinolines
Table 1 Optimization of gold-catalyzed indolation of tetrahy-
droisoquinolinea
Continued
Entry Catalyst (X mol%) Oxidant Solvent Time/h Yielde/%
15 NaAuCl4 (10)
16 NaAuCl4 (10)
17 NaAuCl4 (5)
18 NaAuCl4 (3)
19 NaAuCl4 (1)
TBHP CH3OH
TBHP CH3CN
TBHP Toluene
TBHP Toluene
TBHP Toluene
6
6
6
6
6
81
84
91
91
80
N
H
N
1a
2a
Catalyst (X mol%)
r.t., Oxidant
a Unless otherwise indicated, reactions conditions were: 0.1 mmol
of indole and 2-phenyl-1,2,3,4-tetrahydroisoquinoline as sub-
strates, 10 mol% catalyst, 1.2 equiv. oxidant at room temperature
for 5—10 h. b TBHP (5—6 mol/L in decane). c DTBP (tert-butyl
NH
N
3a
Entry Catalyst (X mol%) Oxidant Solvent Time/h Yielde/%
d
e
peroxide). H2O2 (50 aqueous solution). Isolated yields for all
the products.
1
2
3
4
5
6
7
8
9
[AubpyCl2]Cl (10) TBHPb Toluene
8
8
8
5
5
5
5
8
8
8
28
25
22
80
69
83
92
0
[AuPy2Cl2]Cl (10) TBHP Toluene
More interestingly, the CDC reaction proceeded well
with a variety of indole derivatives with a range of sub-
stitutions at the 2- and 5-positions, indicating the sub-
stantial tolerance of steric hindrance of the reaction. The
electron-donating group or the electron-withdrawing
group at the 2- and 5-position of indole has no effect on
the outcome of the reaction. Furthermore, the electronic
properties of the N-aryl tetrahydroisoquinoline sub-
stituents influenced the efficiency of the reaction. The
electron-withdrawing substituented tetrahydroisoquino-
line exhibited very good reactivity to afford the products
in excellent yields, and in contrast, tetrahydroisoquino-
line with the electron-donating substituted methyl group
showed relatively lower yields.
PicAuCl2 (10)
HAuCl4 (10)
Bu4NAuCl4 (10)
KAuCl4 (10)
NaAuCl4 (10)
AuClPPh3 (10)
AuCl (10)
TBHP Toluene
TBHP Toluene
TBHP Toluene
TBHP Toluene
TBHP Toluene
TBHP Toluene
TBHP Toluene
0
10 NaAuCl4 (10)
11 NaAuCl4 (10)
12 NaAuCl4 (10)
13 NaAuCl4 (10)
14 NaAuCl4 (10)
DTBPc Toluene
70
0
d
H2O2
O2
Toluene 10
Toluene 10
Toluene 10
0
Air
0
TBHP Ethanol
5
79
Table 2 Gold-catalyzed CDC reactions of tetrahydroisoquinolines with indolesa
N
R1
NaAuCl4 (3 mol%)
N
R2
N
r.t., TBHP
H
1
2
NH
R2
R1
3
Entry
Indole
Amine
Product
Yieldb/%
NH
N
1
91
Ph
N
H
Ph
N
1a
2a
3a
NH
N
2
93
1a
4-ClPh
4-ClPh
N
2b
3b
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Zhang et al.
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Continued
Yieldb/%
Entry
Indole
Amine
Product
NH
N
4-BrPh
2c
3
1a
4-BrPh
N
95
81
3c
NH
N
4
5
1a
1a
4-CH3Ph
4-CH3Ph
N
2d
3d
NH
N
80
92
93
75
86
90
4-OCH3Ph
4-OCH3Ph
N
2e
3e
NH
N
Cl
4-ClPh
Cl
6
N
N
H
4-ClPh
1b
2b
3f
NH
N
Cl
4-BrPh
N
7
1b
1b
4-BrPh
2c
3g
NH
Cl
4-CH3Ph
N
8
4-CH3Ph
N
2d
3h
NH
H3C
H3C
N
9
Ph
N
N
H
Ph
1c
2a
3i
NH
N
H3C
10
N
4-ClPh
1c
4-ClPh
2b
3j
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Chin. J. Chem. 2012, 30, 2741—2746

Gold-Catalyzed Direct Indolation of Tetrahydroisoquinolines
Continued
Yieldb/%
Entry
11
Indole
Amine
Product
NH
H3C
N
4-BrPh
93
79
1c
4-BrPh
N
2c
3k
NH
H3C
N
4-CH3Ph
12
13
1c
1c
4-CH3Ph
N
2d
3l
NH
H3C
4-OCH3Ph
N
80
4-OCH3Ph
N
2e
3m
a Reaction conditions: 0.1 mmol substituted indole, 0.1 mmol N-aryl-1,2,3,4-tetrahydroisoquinoline, 3 mol% NaAuCl4 catalyst, 1.2 equiv.
TBHP (5—6 mol/L in decane), room temperature. b Isolated yields for all the products.
To gain insight into the gold-catalyzed coupling re-
action,[22] some control experiments were carried out to
investigate the occurrence of potential intermediate to
elucidate the mechanism. Amine with 50 mol% of
NaAuCl4 was reacted in the presence of oxidant (TBHP)
but in the absence of nucleophile. LC-MS analysis of
the resulting mixture[23] did not show any signals corre-
sponding to amino peroxide. The only new species
identifiable was the iminium ion (Scheme 3).
Scheme 4 Proposed mechanism for the gold catalyzed indola-
tion of N-phenyltetrahydroisoquinoline
N
Ph
NH
3a
NaAuCl4
t-BuOH
N
Ph
2a
+
t-BuOOH
Scheme 3 Attempted observation of iminium ions using
NaAuCl4/TBHP
N+
N
Ph
H
NaAuCl4 (50 mol%)
1a
4
t-BuO-
NaAuCl4
N+
N
TBHP, r.t.
Br
Br
4
Based on the reported mechanistic study of oxidative
coupling of N-phenyltetrahydroisoquinoline,[24] a plau-
sible mechanism for the gold-catalyzed indolation of
tetraisoquinoline is shown in Scheme 4. Amine 2a re-
acts with TBHP in the presence of NaAuCl4 to iminium
ion 4. Trapping of 4 with indole 1a furnishes product 3a
and t-BuOH.
tetrahydroisoquinoline. The advantages of this method-
ology are larger substrate scope, high regioselectivity,
synthetic simplicity and moderate reaction conditions.
We believe that this strategy provides an elegant method
for the construction of biologically active natural prod-
ucts and alkaloids. Further study is underway in our
laboratory.
Conclusions
References
In conclusion, we have developed a mild, efficient
strategy for the oxidative cross dehydrogentative cou-
pling of indoles with N-aryl tetrahydroisoquinolines
catalyzed by gold. Most importantly, to the best of our
knowledge, this is the first report of indolation of N-aryl
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(Pan, B.; Qin, X.)
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