Synthesis of tetralin and chromane derivatives via In-catalyzed intramolecular hydroarylation

Article abstract of DOI:10.1016/j.tetlet.2010.06.091

In(OTf)₃ was found to be an effective catalyst for the cyclization of ω-aryl-1-alkenes to form tetralin and chromane derivatives. Compared with the known Ru-catalyzed version, the In-catalyzed intramolecular cyclization was found to give higher yields and cleaner reactions in some cases. The use of cheaper indium salts instead of the expensive noble metal Ru can be advantageous when the reaction is run on a large scale.

Full text of DOI:10.1016/j.tetlet.2010.06.091

Tetrahedron Letters 51 (2010) 4466–4469
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of tetralin and chromane derivatives via In-catalyzed intramolecular
hydroarylation
*
Kai Xie, Sizhuo Wang, Ping Li, Xiujian Li, Zhiyong Yang, Xiangyu An, Can-Cheng Guo, Ze Tan
College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
In(OTf)3 was found to be an effective catalyst for the cyclization of
x-aryl-1-alkenes to form tetralin and
Received 24 April 2010
Revised 11 June 2010
Accepted 18 June 2010
Available online 22 June 2010
chromane derivatives. Compared with the known Ru-catalyzed version, the In-catalyzed intramolecular
cyclization was found to give higher yields and cleaner reactions in some cases. The use of cheaper
indium salts instead of the expensive noble metal Ru can be advantageous when the reaction is run on
a large scale.
Ó 2010 Elsevier Ltd. All rights reserved.
Tetralins and chromanes are important structural motifs and
they are present in a wide variety of natural products and pharma-
ceuticals.1 To construct the tetralin and chromane structural
frameworks, conceptually, one of the simplest ways would involve
the intramolecular cyclization of an alkene which is tethered to an
arene ring (Scheme 1, Eq. 1). This type of transformation can be for-
mally regarded as a hydroarylation reaction. When the reaction is
mediated with sulfuric acid, the reaction is called Dazens tetralin
synthesis.2 Though the Dazens reaction is very simple to run, the
need to use a strong acid severely limits its synthetic utility since
the acid is not compatible with many different functional groups.
As a result, chemists have strived to develop new versions of this
useful transformation, aiming to make the reaction milder so that
the synthetic scope of the reaction can be greatly expanded.3 To-
ward this end, Sames recently developed a RuCl3/AgOTf catalyzed
intramolecular hydroarylation of olefins (Scheme 1, Eq. 2),4 which
was based on their earlier works on the Pt-catalyzed intramolecu-
lar hydroarylation of alkynes,5 allowing the facile synthesis of
tetralin and chromane derivatives. As simple and straightforward
as this reaction is, however, this transformation is not problem-
free. In some cases, some isomerization products in addition to
the desired intramolecular hydroarylation product were produced.
Since the polarities of the isomerization products are similar to the
cyclization product, they can complicate the isolation of the final
product through column chromatography separation. Indeed,
when we treated 5-phenyl-1-pentene with 5 mol % of RuCl3 and
10 mol % of AgOTf in dichloroethane at 80 °C, the desired cycliza-
tion product 1-methyl-tetralin (A) was produced in 82% yield,
which is consistent with the result reported by Sames. However,
GC analysis of the crude reaction mixture also indicated the
formation of several isomerization products in 8% combined yield
(Table 1, entry 1). From these results, it is clear that a similar reac-
tion without producing any of the isomerization products is highly
desirable.
We recently became interested in the development of reactions
catalyzed by indium and iron salts because of the recent huge in-
flux of reports on In-,6,7 and Fe-catalysis8 and their extensive use
as Lewis acids. They have attracted much attention lately because
of the relatively cheap prices of indium and iron salts, compared to
some of the noble metals, and their environmental friendliness due
to their low toxicities. Our success of developing an In-catalyzed
one-pot synthesis of tetrasubstituted furans from propargyl alco-
hols and 1,3-diketones9 prompted us to examine the possibility
of using indium or iron salts to catalyze the cyclization of
x-phe-
nyl-1-alkenes to form tetralin and chromane derivatives. Much to
our delight, when we treated 5-phenyl-1-pentene with 10 mol %
of In(OTf)3 in refluxing anhydrous dichloroethane, the desired
product 1-methyl-tetralin A was produced cleanly in greater than
97% yield and more importantly, no isomerization product was ob-
served as evidenced by GLC analysis of the crude reaction mixture
(Table 1, entry 3). Instead of careful column chromatography, a
simple filtration of the reaction mixture through a plug of silica
R
R
cat.
eq. 1
eq. 2
FG
FG
X
X
X
X = C, NR1, O
R
R
RuCl3 (1-5 mol%)
AgOTf (2-10 mol%)
FG
FG
X
(CH2)2Cl2
50-92%
X = C, NR1, O
* Corresponding author. Tel./fax: +86 0731 88822484.
E-mail address: ztanze@gmail.com (Z. Tan).
Scheme 1. Synthesis of tetralins and chromanes via intramolecular hydroarylation.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.06.091

K. Xie et al. / Tetrahedron Letters 51 (2010) 4466–4469
4467
Table 1
suppressed by switching the solvent to nitromethane (Table 2, en-
try 4). The test also showed that 1.1 equiv of FeCl3 was necessary
for the reaction to reach completion even in nitromethane. Though
the price of FeCl3 is very cheap, the need to use it in more than stoi-
chiometric amounts made the approach much less attractive. As a
result we chose running the reaction in the presence of 10 mol % of
In(OTf)3 in refluxing dichloroethane as our standard reaction
condition.10
Reaction condition optimization for the cyclization of 5-phenyl-1-pentene
cat.
+
solvent
A
B
Entry
Catalyst
Solvent
Temperature
Time
(h)
Yieldd (%)
(°C)
With a satisfactory protocol in hand, we next set out to examine
the scope of the reaction. As summarized in Table 3, a variety of
tetralins and chromanes could be synthesized smoothly from 5-
ary-1-pentenes and arylhomoallyl ethers. A methyl substituent at
the 2-position of the alkenes did not impede the reaction. For
example, when 5-phenyl-2-methyl-1-pentene was treated with
10 mol % of In(OTf)3 in refluxing dichloroethane, the desired prod-
uct was produced in 89% yield (Table 3, entry 1). As for the synthe-
sis of chromane derivatives, the yields are generally lower, ranging
from 52% to 80% (Table 3, entries 2–6). The lower yields might be
due to the deprotection of the ether substrate to a minor extent.
The reaction is also compatible with various functional groups
such as alkyl, methoxy, and chloride on the aryl ring. Replacing
the phenyl ring with a naphthyl ring also gave the tricyclic chro-
mane derivative in 61% yield (Table 3, entry 7). It is also important
1
2
3
4
5
6
RuCl3/AgOTf
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
80
90
90
90
90
90
12
20
12
12
12
12
A(85)B(8)
No reaction
A(97)B(0)
A(8)B(0)
A(81)B(0)
A(43)B(0)
a
InCl3
In(OTf)3
FeCl3
FeCl3
a
a
b
c
FeCl3
a
b
c
Using 10 mol % of the catalyst.
Using 1.1 equiv of the catalyst.
Using 0.5 equiv of the catalyst.
d
Yields are determined by GLC using decane as the internal standard.
gel to get rid of the catalyst and evaporation of the solvent afforded
the final product A that was sufficiently pure enough to give satis-
factory 1H NMR and 13C NMR spectra. The use of InCl3, a weaker Le-
wis acid, as catalyst was not successful (Table 1, entry 2). The
running of the reaction in a less polar solvent such as toluene slo-
wed down the reaction considerably and this might be due to the
low solubility of the indium salts in the nonpolar solvent. Surpris-
ingly, the use of FeCl3 (10 mol %), a much weaker Lewis acid, also
gave the desired product A in 8% yield in refluxing dichloroethane,
as determined by GLC analysis and the rest of the starting material
was left unchanged (Table 1, entry 4). A further increase of the
amount of FeCl3 to 50 mol % resulted in 50% reaction conversion
and the desired product A was produced in 43% yield (Table 1, en-
try 6). Finally, by using 110 mol % of FeCl3, a full reaction conver-
sion was realized and the desired product A was produced in
81% yield (Table 1, entry 5). The result obtained with FeCl3 was sur-
prising since no reaction was observed with FeCl36ÁH2O as reported
by Sames.4
When we try to cyclize 4-phenoxy-1-butene to produce a chro-
mane derivative C, a vastly different result was obtained with FeCl3
from indium salts. As in the case of cyclizing 5-phenyl-1-pentene,
InCl3 failed to give any of the desired product C in refluxing dry
dichloroethane (Table 2, entry 1), whereas the reaction catalyzed
by 10 mol % of In(OTf)3 proceeded smoothly to afford the desired
cyclization product C in 78% yield (Table 2, entry 2). On the other
hand, the use of 1.1 equiv of FeCl3 gave a sideproduct, tentatively
assigned as the chloro-substituted sideproduct D, in 31% yield in
addition to the desired chromane product C in 52% yield (Table
2, entry 3). However, the sideproduct formation can be completely
Table 3
Indium-catalyzed intramolecular hydroarylation
Entry
Substrate
Product
Yielda (%)
89
1
Me
Me
Me
O
O
2
3
4
5
6
62
58
70
80
52
Cl
Cl
Me
O
O
Me
Me
tBu
Me
O
O
tBu
Me
O
O
MeO
Me
MeO
Me
Me
O
O
Table 2
Reaction condition optimization for the cyclization of 4-phenoxy-1-butene
Me
Me Me
O
O
O
O
cat.
Me
+
solvent
O
7
8
61
Cl
D
C
Entry Catalyst
Solvent
Temperature (°C) Time (h) Yieldc (%)
Me
a
1
2
3
4
InCl3
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
CH3NO2
90
90
90
20
12
12
12
No reaction
C(78)D(0)
C(52)D(31)
C(73)D(0)
a
In(OTf)3
76
b
FeCl3
FeCl3
b
N
105
N
Tf
Tf
a
b
c
Using 10 mol % of the catalyst.
Using 1.1 equiv of the catalyst.
Isolated yields.
a
All reactions were carried out using In(OTf)3 (0.1 mmol),
(1.0 mol) in anhydrous dichloroethane (3 mL) at 90 °C for 12 h. Isolated yields.
x
-aryl-1-alkene

4468
K. Xie et al. / Tetrahedron Letters 51 (2010) 4466–4469
pared to the Ru-catalyzed version, the use of In(OTf)3 gave cleaner
Me
Me
reactions in some cases. The use of cheaper indium salts instead of
the expensive noble metal Ru could be advantageous when the
reaction is run on a large scale. Though narrower in substrate
scope, the indium catalyzed reaction could potentially be useful
in selected cases and complementary to the Ru-catalyzed version.
In(OTf)3
messy
ClCH2CH2Cl
In(OTf)3
messy
Me
ClCH2CH2Cl
RuCl3/AgOTf
Me
Acknowledgments
Me
Me
Me
H
Support of this work by grants from the National Science Foun-
dation of China (No. 20702012), program for NCET in university
(NCET-09-0334) and the Chinese Ministry of Education is grate-
fully acknowledged.
E
F
99%
Scheme 2. Attempt to cyclize internal alkenes.
Supplementary data
In(OTf)3
Supplementary data (detailed experimental procedures and
characterization data; 1H and 13C NMR spectra for all new com-
pounds) associated with this article can be found, in the online ver-
sion, at doi:10.1016/j.tetlet.2010.06.091.
H
In(OTf)3
In(OTf)2
H
G
+ H+
- H+
References and notes
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In(OTf)2
H
Scheme 3. Possible mechanism for the In-catalyzed intromolecular hydroarylation.
to point out that an attempt to cyclize 4-phenyl-1-butene under
otherwise the same reaction conditions did not succeed. Though
most of the starting materials were gone, only several isomeriza-
tion products were produced in 20–30% combined yield (not
shown in Table 3). We suspect that the majority of the starting
materials underwent polymerization instead of the desired cycliza-
tion. In the case of cyclizing a substrate with a nitrogen tether (Ta-
ble 3, entry 8), we also found that the use of Tf group as the
protection group of the amine was essential for the reaction to suc-
ceed, as reported by Sames.4 The reaction of the free amine was
very sluggish. Compared with the excellent yields obtained with
terminal alkenes, rather disappointing results were obtained with
internal alkenes. For example, the reaction of cyclizing 2E-6-phe-
nyl-2-hexene, a simple internal alkene substrate, was quite messy
(Scheme 2). Attempts to cyclize substrate E to produce the tricyclic
product F also failed completely. In sharp contrast, cyclization
using RuCl3/AgOTf as the catalyst gave the desired product F in
99% yield. These results show that the scope of the In-catalyzed
reaction is narrower than the Ru-catalyzed version.
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Even though the exact mechanism of this reaction is unknown
at the present stage, we believe that the reaction is not based on
the C–H activation.11 A tentative mechanism is proposed and de-
picted in Scheme 3. First, the indium catalyst interacts with the
double bond to form an olefin–metal complex G. Then the complex
can undergo Friedel–Crafts reaction with the aromatic ring to form
intermediate H. After the loss of a proton, the aromatic ring is
regenerated and subsequent protonolysis of the carbon–indium
bond furnishes the final product. Though HOTf generated
in situ12 could also be responsible for the catalysis, this possibility
has been ruled out by Sames.4
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reactions, see: (a) Yasuda, M.; Somyo, T.; Baba, A. Angew. Chem., Int. Ed. 2006,
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In summary, we have developed a method for the synthesis of
tetralin and chromane derivatives via an intramolecular hydroary-
lation of
x-aryl-1-alkenes via In-catalysis. The use of In(OTf)3 was
found to be critical for the reaction to be successful while the use of
less Lewis acidic InCl3 was ineffective. While FeCl3 could also be
used to catalyze the cyclization, a stoichiometric amount of the
catalyst is necessary for the reaction to reach completion. Com-

K. Xie et al. / Tetrahedron Letters 51 (2010) 4466–4469
4469
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10. General procedure for the intramolecular cyclization of
x-phenyl-1-alkenes: In a
25-mL round bottomed flask fitted with a reflux condenser were placed 56 mg
of In(OTf)3 (0.1 mmol), 5-phenyl-1-pentene (146 mg, 1.0 mol), and 3 mL of
anhydrous dichloroethane. The mixture was refluxed for 12 h under N2 until
TLC indicated the complete consumption of the starting material. The reaction
mixture was cooled to rt and the mixture was filtered through a plug of silica
gel. The evaporation of the solvent gave the desired product 1-methyl-tetralin
A in >97% yield. The product was identified by NMR and MS analysis and the
data are consistent with the reported values.
11. For some recent reviews on C–H activation, please see: (a) Kakiuchi, F.; Kochi,
T. Synthesis 2008, 3013; (b) Kulkarni, A. A.; Daugulis, O. Synthesis 2009, 4087;
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12. The pH value of the reaction system, at the beginning of the reaction, was
measured to be around 6. However, it changed to 2 after one hour and
remained around 2 afterward.