Iodine/Et3SiH: A novel reagent system for the synthesis of 3-aryl-1H-indenes from 1,3-diaryl propargyl alcohols

Article abstract of DOI:10.1016/j.tetlet.2010.08.061

1,3-Diaryl propargyl alcohols undergo smooth intramolecular Friedel-Crafts cyclization with triethylsilane in the presence of 10 mol % iodine 3-aryl-1H-indene derivatives in good yields in short reaction times. This is the first example on the synthesis of substituted indenes from 1,3-diaryl propargyl alcohols. The use of inexpensive and readily available molecular iodine makes this method quite simple, more convenient, and practical.

Full text of DOI:10.1016/j.tetlet.2010.08.061

Tetrahedron Letters 51 (2010) 5697–5700
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Iodine/Et3SiH: a novel reagent system for the synthesis of 3-aryl-1H-indenes
from 1,3-diaryl propargyl alcohols
⇑
B. V. Subba Reddy , B. Brahma Reddy, K. V. Raghavendra Rao, J. S. Yadav
Division of Organic Chemistry, Indian Institute of Chemical Technology, Hyderabad 500 007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
1,3-Diaryl propargyl alcohols undergo smooth intramolecular Friedel–Crafts cyclization with triethylsi-
lane in the presence of 10 mol % iodine 3-aryl-1H-indene derivatives in good yields in short reaction
times. This is the first example on the synthesis of substituted indenes from 1,3-diaryl propargyl alcohols.
The use of inexpensive and readily available molecular iodine makes this method quite simple, more
convenient, and practical.
Received 31 March 2010
Revised 16 August 2010
Accepted 18 August 2010
Available online 21 August 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Aryl propargyl alcohols
Molecular iodine
Triethylsilane
Cyclization
The chemistry of alcohols occupies a vital role in organic synthe-
sis. In particular, ‘ -activated’ alcohols are attracted as proelectro-
ple, convenient, and metal-free catalytic system for the synthesis of
indene scaffolds is highly desirable.
p
philes, capable of reacting with various nucleophiles, and their
Recently, molecular iodine has received considerable interest in
organic synthesis because of its low cost and ready availability. The
mild Lewis acidity associated with iodine has enhanced its use in
organic synthesis to perform several organic transformations using
stoichiometric levels to catalytic amounts.13 A catalytic amount of
iodine is able to activate the hydroxyl group and the elimination
processes are usually accompanied by rearrangements or intramo-
lecular cyclizations.
ability to undergo nucleophilic substitution reactions contributes
largely to their synthetic value.1 The use of ‘
p-activated’ alcohols
such as benzylic, propargylic, and allylic alcohols makes the C–OH
bond activation easier via the formation of stabilized positively
charged intermediates.2 Notably, indene scaffolds are attractive
targets for the synthesis of some biologically active molecules.3
Consequently, transition metal-catalyzed approaches such as Ni-
and Co-catalyzed carboannulation of alkynes with o-halophenyl
aldehydes or o-iodophenyl malonates, rhenium-,4 and palladium-5
catalyzed annulations and gold(I)-catalyzed intramolecular carbo-
alkoxylations6 have been introduced for the synthesis of indene
derivatives. In addition, substituted indene derivatives were pre-
pared via the Pd-catalyzed annulation of alkynes,7 and palladium-
catalyzed carboannulation of internal alkynes by substituted aryl
halides.8 Subsequently, Lewis acid-catalyzed ring expansion of
substituted cyclopropanes and cyclopropenes,9 and intramolecular
hydroarylation of phenyl-substituted alkenes10 have also been re-
ported for the preparation of indene scaffolds. Recently, FeCl3 has
also been utilized to accomplish the synthesis of indenes.11 Though
these methods are quite effective for the synthesis of simple ind-
enes; they have certain drawbacks in the preparation of highly
substituted indenes. Many of these methods involve multi-step
reaction sequences12 and often require expensive metal catalysts
and strong acidic conditions. Therefore, the development of a sim-
Following our interest in the catalytic uses of iodine,14 we here-
in, report a direct one-pot method for the synthesis of indenes via
an intramolecular Friedel–Crafts cyclization of aryl-substituted
propargylic alcohols. Initially, we attempted the deoxygenation
of diaryl-substituted propargyl alcohols with triethylsilane using
a catalytic amount of molecular iodine. Interestingly, indene deriv-
atives were formed instead of the expected deoxygenation. Thus,
treatment of 1,3-diphenyl-2-propyn-1-ol (1) with triethylsilane
(2) in the presence of 10 mol % molecular iodine in 1,2-dichloro-
ethane at 80 °C for 1 h gave the corresponding 3-phenyl-1H-indene
3a in 85% yield (Scheme 1).
OH
10 mol% I2
Et3SiH
+
º
DCE, 80 C
1
3a
2
⇑
Corresponding author. Tel.: +91 40 27193535; fax: +91 40 27160512.
E-mail address: basireddy@iict.res.in (B.V. Subba Reddy).
Scheme 1. Preparation of 3-phenyl-1H-indene.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.08.061

5698
B. V. Subba Reddy et al. / Tetrahedron Letters 51 (2010) 5697–5700
Table 1
Molecular iodine-catalyzed cyclization of propargyl alcohols with triethylsilane
Entry
a
Propargyl alcohol (1)
Triethylsilane (2)
Product (3)a
Time (h)
1.0
Yield (%)b
85
OH
Et3SiH
Ph
Ph
OH
b
c
d
e
f
Et3SiH
Et3SiH
Et3SiH
Et3SiH
Et3SiH
Et3SiH
0.5
1.5
0.5
1.5
2.0
1.5
86
80
88
82
78
80
Me
Ph
Ph
Ph
Me
Cl
OH
Cl
Ph
OH
MeO
Br
Ph
Ph
MeO
Br
OH
Ph
Ph
OH
O2N
Ph
Ph
O2N
OH
g
F
Ph
Ph
F
OH
Ph
h
Et3SiH
1.5
84
Ph
OH
OMe
OMe
MeO
i
Et3SiH
Et3SiH
Et3SiH
1.0
1.5
2.0
82
85
75
Ph
OMe
Ph
Ph
OH
OH
Ph
j
S
Ph
Ph
S
k
Me
OH
Me
Ph
l
Et3SiH
Et3SiH
1.5
2.0
82
80
Ph
MeO
OMe
OH
NO2
m
MeO
MeO
NO2
OBn
OMe
OMe
OH
OBn
n
o
Et3SiH
Et3SiH
1.5
2.5
85
OH
I
Me
( )5
80c
Me
( )5

B. V. Subba Reddy et al. / Tetrahedron Letters 51 (2010) 5697–5700
5699
Table 1 (continued)
Entry
Propargyl alcohol (1)
Triethylsilane (2)
Product (3)a
Time (h)
2.0
Yield (%)b
OH
I
Me
( )5
p
Et3SiH
78c
Me
( )5
MeO
MeO
a
All products were charecterized by NMR, IR, and mass spectroscopy.
Yield refers to pure products after chromatography.
Stoichiometric amount of iodine was used.
b
c
The starting secondary propargyl alcohols were prepared using
respect to various propargylic alcohols and the results are pre-
sented in Table 1.16
known procedures.15 Encouraged by this result; we turned our
attention to various substituted propargylic alcohols. Interestingly,
a wide range of substituted aryl propargyl alcohols participated
well in this reaction (entries b–g, Table 1). Notably, sterically hin-
dered propargyl alcohols such as 2,5-dimethoxyphenyl and 2-
naphthyl derivatives were also effective for this conversion (entries
h and i, Table 1). Furthermore, the propargyl alcohol derived from
thiophene-2-carboxaldehyde also gave the product in good yield
(entry j, Table 1). To know the effect of ortho-substituent, the reac-
tion was performed with the propargyl alcohol derived from o-tol-
ualdehyde and phenylacetylene (entry k, Table 1). It was observed
that the cycloannulation of o-tolyl propargyl alcohol requires a
long reaction time when compared to p-tolylpropargyl alcohol (en-
try b, Table 1). This may be due to the steric factors of the ortho-
substituent (entry k, Table 1).
In summary, we have developed a novel and efficient catalytic
process for the synthesis of substituted indenes from aryl-substi-
tuted propargylic alcohols by means of a tandem isomerization,
reduction followed by intramolecular Friedel–Crafts type cycliza-
tion.17 The remarkable features of this procedure are high conver-
sions, excellent control of geometry of olefins, operational
simplicity, and ready availability of reagents at low cost.
Acknowledgment
B.B.R. and K.V.R. thank the CSIR, New Delhi, for the award of
fellowships.
References and notes
To further evaluate the role of the substituents on this reaction,
several propargyl alcohols with different aryl substituents were
subjected to the present reaction conditions. The reactions with
substrates bearing electron- donating groups on the aromatic ring
are faster than those of electron-deficient substrates (Table 1). To
realize the regioselectivity, the reaction was performed with
m-methoxyphenyl propargyl alcohol (entries l, Table 1). Interest-
ingly, the cycloannulation was observed regioselectively at para
to methoxy group (entries l, Table 1). Next, we attempted the reac-
tions with secondary propargyl alcohols prepared from p-substi-
tuted phenyl acetylenes (entries m and n, Table 1). In the case of
p-benzyloxysusbtituted propargyl alcohol (entry n, Table 1), the
desired product was obtained in high yield in a shorter reaction
time compared to p-nitrosusbtituted one. Finally, we examined
the reaction of triethylsilane with alkyl-substituted propargyl alco-
hols. However, vinyl iodides were obtained exclusively instead of
the formation of indene derivatives when alkyl-substituted prop-
argyl alcohols were treated with 1 equiv of iodine and 2 equiv of
triethylsilane at 80 °C in dichloroethane (entries o and p, Table
1). The formation of indene was successful only with propargylic
alcohols derived from aromatic aldehydes and phenylacetylenes.
In all the cases, the yields are generally high and the reaction times
are quite reasonable.
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acids such as BiCl3, ZnCl2, and FeCl3Á6H2O was tested for this
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triethylsilane (2) in the presence of 10 mol % of FeCl3Á6H2O, BiCl3 or
ZnCl2 gave the 3-phenyl-1H-indene 3a in 50%, 45%, and 35% yields,
respectively. As a solvent, dichloroethane appeared to give the best
results. However, in the absence of either iodine or triethylsilane,
the reactions did not proceed even after 12 h. This clearly indicates
that both molecular iodine and triethylsilane are essential to facil-
itate the reaction. The products were characterized by NMR, IR, and
mass spectroscopy and also by comparison with authentic sam-
ples.11 The scope and generality of this process is illustrated with
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completion of the reaction as indicated by TLC, the mixture was diluted with
water (10 mL) and extracted with dichloromethane (2 Â 10 mL). The combined
organic layers were washed with saturated Na2S2O3 solution (2 Â 10 mL)
followed by brine (10 mL) solution and dried over anhydrous Na2SO4. Removal
of solvent followed by purification on silica gel column chromatography using
hexane as an eluent afforded the pure product. The spectral data of the
products were identical with the data reported in the literature.11 Spectral data
for selected products: Compound 3a: 3-phenyl-1H-indene: pale yellow liquid, IR
(neat): tmax 3028, 2906, 1602, 1491, 1443, 1215, 1041 cmÀ1 1H NMR
;
(300 MHz, CDCl3): d 7.43–7.38 (m, 2H), 7.28–7.11 (m, 7H), 6.01 (t, J = 6.7 Hz,
1H), 3.63 (d, J = 6.7 Hz, 2H); 13C NMR (75 MHz, CDCl3): d 142.8, 137.5, 135.8,
135.6, 129.2 (Â2), 128.5 (Â2), 128.3 (Â2), 128.1, 128.0 (Â2), 120.9, 43.6; ESI-
MS: m/z: 193 [M+H]. Compound 3b: 5-methyl-3-phenyl-1H-indene: colorless
liquid, IR (neat): tmax 3017, 2917, 1510, 1440, 1215, 1051 cmÀ1 1H NMR
;
(300 MHz, CDCl3): d 7.41–7.30 (m, 2H), 7.20–6.94 (m, 6H), 5.92 (t, J = 6.7 Hz,
1H), 3.51 (d, J = 6.7 Hz, 2H), 2.19 (s, 3H); 13C NMR (75 MHz, CDCl3): d 142.8,
137.5 (Â2), 135.8, 135.6, 129.2 (Â2), 128.5, 128.3, 128.0–128.2 (Â5), 43.6, 21.0;
ESI-MS: m/z: 224 [M+NH4]. Compound 3d: 5-methoxy-3-phenyl-1H-indene: pale
yellow liquid, IR (neat): tmax 2924, 2853, 1596, 1510, 1450, 1254, 1029 cmÀ1
;
1H NMR (300 MHz, CDCl3): d 7.42–7.08 (m, 6H), 6.81–6.75 (m, 2H), 5.97 (t,
J = 6.9 Hz, 1H), 3.71 (s, 3H), 3.54 (d, J = 6.9 Hz, 2H); 13C NMR (75 MHz, CDCl3): d
158.2, 144.6, 142.9, 137.7, 130.8, 130.2, 129.4 (Â2), 128.6, 128.2, 128.1 (Â2),
114.3, 114.0, 55.2, 43.1; ESI-MS: m/z: 222 [M+].
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16. Experimental procedure: A mixture of triethylsilane (2.0 mmol), aryl propargyl
alcohol (1.0 mmol) and iodine (10 mol %) in dichloroethane (5 mL) was stirred
in 1,2-dichloroethane at 80 °C for the appropriate time (Table 1). After