A facile synthesis of 2-arylindenes by Pd-catalyzed direct arylation of indene with aryl iodides

Article abstract of DOI:10.1016/S0040-4039(02)00499-9

A series of 2-arylindenes were prepared by the reaction of aryl iodides with indene in the presence of a catalytic amount of Pd(OAc)₂.

Full text of DOI:10.1016/S0040-4039(02)00499-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 3213–3215
A facile synthesis of 2-arylindenes by Pd-catalyzed direct
arylation of indene with aryl iodides
Ilya E. Nifant’ev,* Alexander A. Sitnikov, Nonna V. Andriukhova, Ilya P. Laishevtsev and
Yuri N. Luzikov
Department of Chemistry, Moscow State University, Moscow 119899, Russia
Received 7 January 2002; revised 28 February 2002; accepted 7 March 2002
Abstract—A series of 2-arylindenes were prepared by the reaction of aryl iodides with indene in the presence of a catalytic amount
of Pd(OAc)2. © 2002 Elsevier Science Ltd. All rights reserved.
Zirconocenes derived from 2-arylindenes 1 demonstrate
an excellent catalytic performance in the polymerization
of a-olefins.1,2 It is known that the nature of sub-
stituents in the aryl fragment strongly affects the cata-
lytic properties of such zirconocenes.1 Unfortunately,
no general and simple procedures for the synthesis of
2-arylindenes 1 have been developed as yet. These
compounds are usually obtained by dehydration of
arylindanols which can be synthesized either by the
reaction of indan-2-one with arylmagnesium halides,3
or by the reaction of a dimagnesium derivative of
o-xylene with ethyl benzoate.4 Both approaches have
some drawbacks such as moderate yields, relatively
expensive reagents, the impossibility of introducing sub-
stituents incompatible with organomagnesium com-
pounds, etc. In this work, we describe a new synthetic
approach to 2-arylindenes. We found that these com-
pounds could be readily obtained by direct arylation of
indenes catalyzed by palladium salts.
Previously,5–7 it has been shown that several 2-arylinde-
nes could be obtained in low to moderate yields by a
stoichiometric reaction between indene and the corre-
sponding arylpalladium derivative. It would be
Scheme 1.
* Corresponding author. Fax: +7(095)9394523; e-mail: inif@org.chem.msu.ru
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00499-9

3214
I. E. Nifant’e6 et al. / Tetrahedron Letters 43 (2002) 3213–3215
expected that the corresponding catalytic process
known as the Heck reaction would not proceed in this
case. This reaction is known to proceed as a successive
cis-addition of PhPdHal and cis-elimination of HPdHal
(see reviews on the Heck reaction,8 for Density Func-
tional Theory study of the Heck reaction see Ref. 9)
(Scheme 1). Since cis-elimination of HPdHal from
intermediate 4 formed by cis-addition of PhPdHal to
the multiple bond of indene is impossible, the catalytic
cycle cannot proceed. Of course, 1- and 3-arylindenes
can also be formed via intermediate 5, however, this
reaction is complicated by steric hindrance encountered
upon attack of the aryl group at C-1 of indene. In this
case it would be expected that [1,3]-proton migration in
the 1-arylindene would result in the formation of the
thermally more stable 3-arylindene.
amine was added to absorb the liberated HI). At 100°C,
conversion of indene is at least 90% after 4 h. Pre-
sumably, the acceleration of the process is due to the
higher polarity of DMF compared to that of triethy-
lamine. It is important to note that the 1a/3a ratio in
this case remains almost the same as for the reaction in
triethylamine alone.
We also studied the reactions of several substituted aryl
iodides with indene. The results of our experiments are
presented in Table 1. It was found that all the reactions
proceeded smoothly with the reaction mixtures contain-
ing only isomeric 2- and 3-arylindenes as well as resid-
ual amounts of unreacted starting materials. The ratios
1
of 2- and 3-arylindene isomers were determined by H
1
NMR analysis. The H NMR spectra were analyzed
taking into account that for 3-arylindenes, 3, the signals
for the protons at C-2 were observed in the region l
5.5–6.5 with the vicinal coupling constant lying in the
range 1.5–3.5 Hz. For 2-arylindenes, 1, the signals of
the vinyl proton at C-3 appeared in the region l 7.0–8.0
and the corresponding coupling constant with the
methylene protons was less than 1.5 Hz. All the 2-
arylindenes obtained in this work were solids, their
yields were determined after recrystallization.
We found that no reaction between indene and
iodobenzene proceeded under typical Heck reaction
conditions. Assuming that the intermediate 4 may
undergo a thermal isomerization to give the trans-
product 6, we undertook a study of the reaction
between indene and iodobenzene in the presence of
palladium acetate with more extensive heating than is
usually required for the Heck-reaction. It was found
that the reaction in refluxing triethylamine did proceed
and that 80% of indene was consumed during 6 h. After
refluxing for 10 h, the indene conversion exceeded 90%
and a mixture of 2- and 3-phenylindenes in a ꢀ4/1
ratio was formed.
To sum up, it is more appropriate to use DMF rather
than triethylamine as the solvent since (i) the reaction
in DMF proceeds at a higher rate than in triethylamine
and (ii) sometimes a somewhat higher ratio of 2- and
3-arylindene isomers is observed in DMF. It was found
that the nature of the substituent in the aryl iodide
appreciably affects not only the rate of the reaction, but
also the ratio of 2- and 3-isomers. As a rule, aryl
iodides with acceptor substituents react faster than
those with donor substituents and give a larger propor-
tion of 2-arylindene.
2-Phenylindene 1a is a crystalline substance and can be
isolated by recrystallization in 63% yield. It should be
noted that it is impossible to isolate all the 2-phenylin-
dene by recrystallization since this compound forms
inseparable mixtures with oily 3-phenylindene 3a. The
reaction accelerates substantially if DMF is used as the
solvent (in this case, a stoichiometric amount of triethyl-
Table 1. Reaction of indene with substituted aryl iodides in Et3N (reflux, 10 h) or in DMF (100°C, 4 h)10
Run
Rn
Product
Solvent
Conversion (%)a
1/3b
Yield of 1 (%)c
1
2
3
4
5
6
7
8
H
–
1a3a
–
NEt3
DMF
NEt3
DMF
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
DMF
\90
\90
41
80/20
82/18
94/6
95/5
92/8
85/15
78/22
81/19
76/24
73/27
74/36
86/14
63
65
32
37
78
59
41
35
29
21
13
34
3,5-(CF3)2
–
4-COMe
4-COOEt
4-Me
4-Br
4-OMe
4-Cl
1b1b
–
49
1c3a
1dc,11
1e3a
1f3a
1g3a
1h3a
1jc,11
–
\90
\90
61
48
40
33
30
41
9
10
11
12
4-NHC(O)Me
–
a Determined by 1H NMR.
b Isolated yield.
c This work, analytical data for 1d and 1j are given in Ref. 11.

I. E. Nifant’e6 et al. / Tetrahedron Letters 43 (2002) 3213–3215
3215
References
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dried over Na2SO4. Dichloromethane was removed and
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another appropriate solvent. In DMF: 20 ml of DMF, 3
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(0.6 mmol) of Pd(OAc)2 and 20 mmol of aryl iodide were
stirred under reflux for 4 h. The reaction mixture was
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water (150 ml). The organic layer was separated, washed
twice with water (200 ml), filtered and dried over Na2SO4.
Dichloromethane was removed and the residue was
recrystallized from ethanol (20 ml) or another appropri-
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Ethyl 4-(1H-2-indenyl)benzoate, 1d: 1H NMR (CDCl3,
25°C) l: 8.08 (d, J=8.0 Hz, 2H), 7.70 (d, J=8.0 Hz, 2H)
(AA%BB%); 7.52 (d, J=7.2 Hz, 1H), 7.46 (d, J=7.2 Hz,
1H), 7.33 (t, J=7.2 Hz, 1H), 7.26 (t, J=7.2 Hz, 1H)
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126.6, 125.3, 125.2, 123.6, 121.3, 60.7, 38.8, 14.2. Anal.
calcd for C18H16O2: C, 81.79; H, 6.10. Found: C, 81.63;
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NMR (DMSO-d6, 25°C) l: 9.95 (br.s., 1H, NH); 7.64
(br.s, 4H) (AA%BB%); 7.46 (d, J=7.4 Hz, 1H), 7.38 (d,
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10. Typical procedure for the arylation of indene. In triethyl-
amine: 20 ml of triethylamine, 2.2 g (20 mmol) of indene,
0.134 g (0.6 mmol) of Pd(OAc)2 and 20 mmol of aryl
iodide were stirred under reflux for 10 h after which
triethylamine was removed under reduced pressure. The
residue was treated with a mixture of dichloromethane
(50 ml) and water (50 ml). The organic layer was sepa-