3,3-bis(arylbenzofurans) via a gold-catalyzed domino process

Article abstract of DOI:10.1055/s-0030-1258566

A new heterogeneous gold-catalyzed system for the domino cyclization oxidative coupling of 2-alkynyl phenols for the formation of 3,3-bisbenzofurans was developed. The substrate and the catalyst scope as well as the reaction conditions were investigated and optimized. This method provides access to this novel structural theme in two steps starting from commercially available chemicals. The molecular structure of the 3,3-bisbenzofurans was confirmed by single-crystal X-ray analysis. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0030-1258566

LETTER
2443
3,3¢-Bis(arylbenzofurans) via a Gold-Catalyzed Domino Process
3
, 3
¢
-Bis(arylbenzofur
a
a
ns) via
a
G
o
t
ld-Cat
h
alyzed
D
omi
i
noPro
e
Department of Chemistry, University of Basel, St. Johanns-Ring 19, 4056 Basel, Switzerland
Fax +41(61)2670976; E-mail: hermann.wegner@unibas.ch
Received 27 July 2010
catalyzed coupling reaction: modified Castro reaction,11
copper-mediated palladium-catalyzed coupling7 or BuLi/
CuCl2 coupling.12 The quick and efficient access to bis-
benzofuran derivatives from easily available starting ma-
Abstract: A new heterogeneous gold-catalyzed system for the
domino cyclization oxidative coupling of 2-alkynyl phenols for the
formation of 3,3¢-bisbenzofurans was developed. The substrate and
the catalyst scope as well as the reaction conditions were investigat-
ed and optimized. This method provides access to this novel struc- terial is, therefore, a challenging field of investigations.
tural theme in two steps starting from commercially available
The gold-catalyzed formation of benzofurans from 2-
chemicals. The molecular structure of the 3,3¢-bisbenzofurans was
alkynyl phenols is well known since a decade and pro-
confirmed by single-crystal X-ray analysis.
ceeds through the gold activation of the alkyne
Key words: domino process, gold catalysis, 3,3¢-bisbenzofurans,
dimerization, oxidative coupling
(Scheme 1).13
OH
O
HAuCl4 (2 mol%)
Ph
Et2O, r.t., 7 h
Benzofurans are attractive compounds due to their inter-
esting properties.1 They are widespread in many mole-
3a (86%)
Ph
cules and natural products with important biological
1a
activities, including antitumor properties,2 inhibition of
Scheme 1 Gold-catalyzed 5-endo-dig cyclization of 2-alkynyl phe-
protein phosphatase 1B,3 5-HT2 and 5-HT3 antagonist ac-
nols to benzofurans reported by Belting and Krause13c
tivity,4 inhibition of 5-lipoxygenase,4b and antifungal
properties.5 Pharmaceutically, these properties are conve-
Besides the activation of p-systems, gold can also poten-
tially promote oxidative coupling.14 Recent examples
were developed for the gold-catalyzed ethynylation of
arenes,15 the gold-catalyzed diamination of alkenes,16 or
the oxidative coupling of nonactivated arenes.17 Thus, by
carefully controlling the reaction conditions, a domino
process can be set up to make the gold catalyst perform
multiple tasks successively.18 In our group, this approach
led us to develop the gold-catalyzed domino cyclization–
oxidative coupling reaction of aryl alkynyl esters for the
formation of dicoumarins.19 Such transition-metal-cata-
lyzed domino processes are relevant to open efficient
routes for C–C bond formation in complex molecules.20
nient in the treatment of cancers, cardiovascular diseases,
type 2 diabetes, migraines, dementia, and anxiety.2–4 Ad-
ditionally, benzofurans have been reported as fluorescent
markers for biomolecules.6
However, the bisbenzofuran scaffold is much less studied,
although it is present in a number of biological active mol-
ecules, for example, in kynapcin-24. This compound,
whose first total synthesis was achieved in 2009 by Yang
et al.,7 inhibits prolyl endopeptidase, an enzyme involved
in Alzheimer’s disease (Figure 1).8
HO
HO
O
O
During the 5-endo-dig cyclization of 2-alkynyl phenols,
the interaction of the gold catalyst with the alkyne, fol-
lowed by a nucleophile attack, leads to a gold complex–
benzofuran intermediate. Stabilization of this intermedi-
ate should enable an oxidative coupling to obtain 3,3¢-bis-
benzofurans. This hypothesis was corroborated by the
recent results of Hashmi.21 Taking into account our previ-
ous work on the gold-catalyzed cyclization–oxidative
coupling of aryl alkynylesters, we first examined the sim-
ple application of the reaction conditions of the dicou-
marin synthesis.19 Although tert-butylhydroperoxide
recently showed interesting results for re-oxidation of
gold in homogeneous catalysis,22 it appeared that such
transposition was not suitable for this system and led to
the formation of monobenzofuran only with 61% yield
(entry 1, Table 1).
OMe
OH
OH
MeO
O
O
Figure 1 Kynapcin-24, a prolyl endopeptidase inhibitor
Other dimers of benzofurans, such as 2,2¢-bisbenzofurans
exhibit also interesting properties like antiprotozoal
activities9 or antimicrobial effects.10 Yet, only few synthe-
ses of bisbenzofuran derivatives have been published.
Moreover, they are fastidious and multisteps demanding.
They mainly involve the building of two benzofuran units
which need to be activated prior to connection by metal-
SYNLETT 2010, No. 16, pp 2443–2448
x
x
.x
x
.2
0
1
0
Advanced online publication: 10.09.2010
DOI: 10.1055/s-0030-1258566; Art ID: G23210ST
© Georg Thieme Verlag Stuttgart · New York

2444
M. G. Auzias et al.
LETTER
Table 1 Optimization of the Gold-Catalyzed Formation of 3,3¢-Bisbenzofuran 2a
O
Ph
O
OH
conditions
+
Ph
Et2O, overnight
Ph
Ph
O
2a
1a
3a
Entry
1
Catalyst (mol%)
HAuCl4 (10)
HAuCl4 (10)
HAuCl4 (10)
HAuCl4 (10)
HAuCl4 (10)
HAuCl4 (5)
HAuCl4 (1)
HAuCl4 (10)
HAuCl4 (10)
HAuCl4 (10)
AuCl3 (10)
Solvent
1,2-DCE
1,2-DCE
MeCN
Me2CO
Et2O
Additives (equiv) Oxidant (equiv)
Temp
60 °C
60 °C
r.t.
Results
–
t-BuOOH (5)
DDQ (5)
3a 61%
2
–
2a 4%, 3a 29%
no reaction
3
–
Selectfluor® (5)
Selectfluor® (5)
PhI(OAc)2 (5)
PhI(OAc)2 (5)
PhI(OAc)2 (5)
PhI(OAc)2 (5)
PhI(OAc)2 (5)
PhIO (5)
4
–
r.t.
ratio 2a/3a 1:10a
2a 37%
5
–
r.t.
6
Et2O
–
r.t.
2a 37%, 3a 3%
2a 3%, 3a 16%
2a 26%
7
Et2O
–
r.t.
8
Et2O
K2CO3 (1.2)
r.t.
9
Et2O
Et3N (1.2)
r.t.
2a 33%, 3a 15%
2a 32%, 3a 24%
2a 11%, 3a 22%
2a 35%
10
11
12
Et2O
–
–
–
r.t.
Et2O
PhI(OAc)2 (5)
PhI(OAc)2 (5)
r.t.
AuBr3 (10)
Et2O
r.t.
a Determined by crude NMR.
Therefore, reaction conditions have been screened using tion between the 2-alkynyl phenol substrates and
1a as substrate. Different gold sources have been tested: (diacetoxyiodo)benzene leading to the formation of ben-
gold(I) and gold(III), in combination with several solvents zoquinone derivatives to be responsible for lowering the
(1,2-dichloroethane, dichloromethane, tetrahydrofuran, dimer yield. Unfortunately, PhI(OAc)2 was the only oxi-
acetone, acetonitrile, diethyl ether, dioxane). The regener- dant suitable to re-oxidize the gold catalyst and achieve
ation of the gold catalyst by oxidation, which is a crucial the turnover of the catalytic domino process.
point, was also a main part of the screening process:
It is noteworthy that (diacetoxyiodo)benzene itself does
Selectfluor® (‘F+’ donor reagents are well known for gold
not catalyze the dimerization nor the coupling of two
catalysts oxidation),14a,b,g,17b DDQ, NMO, chloranil,
MnO2, t-BuOOH, or Dess–Martin periodinane led only to
the formation of 3a with traces of dimer 2a (see selection
in Table 1, entries 1–4).
monobenzofuran units. Reaction of 1a and PhI(OAc)2 in
diethyl ether led to the formation of an uncharacterized or-
ange material which we assumed to be a polymer of
quinone derivatives.23 A control experiment between
Finally, using (diacetoxyiodo)benzene PhI(OAc)2 (5 monobenzofuran 3a and PhI(OAc)2 did not show any for-
equiv) as oxidant in anhydrous diethyl ether in presence of mation of 2a.
10 mol% of HAuCl4 (entry 5, Table 1) gave mainly dimer
Our attempts to avoid or reduce the side reaction (protec-
with small traces of the corresponding monomer. The cat-
tion of the phenol, low/high temperature reactions, diluted
alyst loading can be lowered to 5 mol% although this in-
conditions, slow addition of the substrate, reduced amount
creased the presence of monobenzofuran, generating
of oxidant) were not successful. Other hypervalent iodine
difficulties in the purification (entry 6, Table 1). A cata-
compounds have been tested: iodosobenzene (PhIO) led
lyst loading of 1 mol% led to the formation of monomer
to a mixture of monomer/dimer with a lower dimer yield
mainly with traces of dimerization (entry 7, Table 1). The
than (diacetoxyiodo)benzene (entry 10, Table 1); bis(tri-
use of additives such as organic or inorganic bases (Et3N
fluoroacetoxy)iodobenzene (PIFA) provided no forma-
or K2CO3), to catch the proton released during the process
tion of monomer or dimer. The use of PhI(OAc)2 with
and avoid the formation of monomer by protodeauration,
other gold catalysts did not improve the dimerization (en-
did not increase the dimerization yields (entries 8 and 9,
Table 1).
tries 11 and 12, Table 1). A crucial finding is the use of di-
ethyl ether as solvent. The gold catalyst and the 2-alkynyl
Yet, PhI(OAc)2 is also known to oxidize phenols to quino- phenols are soluble in diethyl ether while (diacetoxy-
ne derivatives.23 We assume the occurrence of a side reac- iodo)benzene is only slightly soluble resulting in a het-
Synlett 2010, No. 16, 2443–2448 © Thieme Stuttgart · New York

LETTER
3,3¢-Bis(arylbenzofurans) via a Gold-Catalyzed Domino Process
2445
erogeneous system. These conditions led to the best dimer stituents are electron donating. Thus, the presence of elec-
yields by limiting the side reaction with the starting mate- tron-withdrawing group (entry 9, Table 2) prevented the
rial.24 In halogenated solvents, where (diacetoxyiodo)ben- dimerization process. Interestingly, the domino process
zene is soluble, the side reaction only occurs leading to took place on arylalkynyl phenols but did not occur with
decomposition of the starting material.
terminal alkynyl, silyl-alkynyl, alkyl-alkynyl phenols (en-
try 8, Table 2), or alkynyl cyclohexenol (entry 10,
Table 2).
We prepared the substrates 1a–l by known procedures
(see Supporting Information). Substitution on the para
and meta positions of the phenol were tolerated for the
dimerization (entries 2 and 4, Table 2) provided that sub-
Table 2 Substrate Scope for the Domino Process towards the Formation of 3,3¢-Bisbenzofurans 2
Entry
1
Starting material
Product
Yield (%)
37
O
OH
O
1a
2a
O
OH
2
3
20
25
O
1b
2b
O
OMe
OH
OMe
MeO
O
1c
2c
MeO
O
OH
4
24
MeO
O
1d
OMe
2d
2e
O
OH
5
6
23
25
O
1e
O
F
OH
F
F
O
1f
2f
Synlett 2010, No. 16, 2443–2448 © Thieme Stuttgart · New York

2446
M. G. Auzias et al.
LETTER
Table 2 Substrate Scope for the Domino Process towards the Formation of 3,3¢-Bisbenzofurans 2 (continued)
Entry
Starting material
Product
Yield (%)
OMe
O
OH
OMe
7
19
O
MeO
1g
2g
OH
R
8
no dimerization
–
1h (R = H)
1i (R = TMS)
1j (R = n-Pr)
OH
9
no dimerization
no dimerization
–
–
Cl
1k
OH
10
1l
The cylization–oxidative coupling sequence is not affect- catalyze the cyclization of another molecule of substrate
ed by the nature of the substituent on the aryl alkynyl to lead to complex D. Another possibility is the formation
moieties since the presence of electron-withdrawing and of D via a ligand exchange. Complex D undergoes an ox-
-donating groups is suitable (entries 1–7, Table 2).
idative coupling to the 3,3¢-bisbenzofuran and a gold spe-
cies which has to be re-oxidized to achieve the turnover of
the catalytic cycle.
Therefore, the transformation proceeds on a very fine line
of reactivity between the desired re-oxidation of the cata-
lyst and decomposition of the starting material. Thus, de- The structure of these novel 3,3¢-bis(arylbenzofurans) has
spite intense efforts of optimization, the yields for this been confirmed by a single-crystal X-ray analysis of com-
very economic transformation remained in the lower pounds 2a and 2b.25
range (around 20–37%) for the main part of the substrates.
We describe a new heterogeneous catalytic system for the
However, this method opens a quick and efficient way to
gold-catalyzed domino process to convert simple 2-(aryl-
this novel 3,3¢-bis(2-arylbenzofuran) scaffold from com-
alkynyl)phenols directly to novel 3,3¢-bis(arylbenzo-
mercially available starting materials. It is noteworthy,
furans) in one step and under mild conditions (taking
that such conditions did not lead to the formation of 3,3¢-
place in the presence of an appropriate oxidant only).
Although the transformation proceeds in low yields, such
bisindoles starting from 2-alkynyl anilines.
According to our preliminary results on the gold-cata- C–C bond-forming reactions from nonfunctionalized
lyzed domino cyclization and oxidative coupling towards starting materials are highly important for the synthesis of
the formation of dicoumarins,19 we propose a similar organic molecules. As benzofurans are interesting photo-
mechanism for the formation of 3,3¢-bisbenzofurans chromic compounds used as dyes,6 the photoluminescent
(Scheme 2). At the beginning of the reaction the starting properties of the p-system exhibited by the 3,3¢-bisbenzo-
material can take two different ways: the 2-alkynyl phe- furans will be investigated in the future.
nols can be oxidized by PhI(OAc)2 leading to the forma-
tion of quinone derivatives and lowering the yield of the
reaction by consuming the starting material; or it can enter
the domino process in which the first step is the coordina-
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
tion of the gold catalyst on the alkyne for activation of the
cyclization. Aromatization of intermediate B releases the
s-complex C, Hashmi described a quite similar gold(I) in-
termediate.21 Complex C can then either undergo a pro-
todeauration step and produce the monobenzofuran or
Acknowledgment
M.G.A. thanks the Roche Research Foundation and the Novartis
Foundation for financial support. H.A.W. is indebted to the Fonds
der Chemischen Industrie for a Liebig Fellowship. Umicore and
Synlett 2010, No. 16, 2443–2448 © Thieme Stuttgart · New York

LETTER
3,3¢-Bis(arylbenzofurans) via a Gold-Catalyzed Domino Process
2447
I(OAc)2
decomposition
OH
O
red
R
R
ox
OH
Au(III)
R
Au(III)
R
O
Au(I)
O
A
R
O
H
R
O+
+ H+
Au(III)
R
Au(III)
R
O
B
O
R
D
Au(III)
C
H+
OH
R
Scheme 2 Proposed mechanism for the domino process towards the formation of 3,3¢-bisbenzofurans
Johnson Matthey are gratefully acknowledged for generously pro-
viding auric acid.
(5) McAllister, G. D.; Hartley, R. C.; Dawson, M. J.; Knaggs,
A. R. J. Chem. Soc., Perkin Trans. 1 1998, 3453.
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(b) Piloto, A. M.; Costa, S. P. G.; Gonçalves, M. S. T.
Tetrahedron Lett. 2005, 46, 4757.
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Synlett 2010, No. 16, 2443–2448 © Thieme Stuttgart · New York

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M. G. Auzias et al.
LETTER
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(24) The procedure described for compound 2a was applied for
compounds 2b–g. HAuCl4 (17.5 mg, 10 mol%) was placed
into a 20 mL vial (well dried), equipped with a stir bar. Et2O
(10 mL) was added, and the mixture was stirred for 5 min at
r.t. Then, 2-alkynlphenol (100 mg, 1 equiv) was added first,
followed by PhI(OAc)2 (848 mg, 5 equiv) 5 min later. The
mixture was stirred at r.t. overnight. The reaction mixture
was filtered and concentrated. The crude product was
purified by flash column chromatography or on preparative
TLC. Compound 2a was isolated as a white powder with a
yield of 37% (37 mg); mp 179–181 °C. 1H NMR (400 MHz,
CDCl3): d = 7.80–7.74 (m, 4 H), 7.63 (d, J = 8.2 Hz, 2 H),
7.33 (m, 2 H), 7.29–7.20 (m, 6 H), 7.17–7.07 (m, 4 H).
13C NMR (101 MHz, CDCl3): d = 154.76 (2 C), 152.37 (2 C),
130.84 (2 C), 129.87 (2 C), 128.98 (4 C), 128.92 (2 C),
126.62 (4 C), 125.32 (2 C), 123.41 (2 C), 121.11 (2 C),
111.62 (2 C), 108.07 (2 C). ESI-HRMS: m/z calcd for
[C28H18O2Na]+: 409.1204 [M + Na]+; found: 409.1199.
MS (EI): m/z (%) = 386.1 (100) [M+], 308.1 (9), 281.1 (7).
IR: n = 1600, 1588, 1486, 1470, 1454, 1439, 1338, 1288,
1254, 1203, 1109, 1063, 1026, 1008, 920 cm–1.
(25) Crystallographic data for compounds 2a and 2b have been
deposited at the Cambridge Crystallographic Data Center,
the respective deposition numbers are 772967 and 772968.
(22) Hashmi, A. S. K.; Lothschuetz, C.; Ackermann, M.; Doepp,
R.; Anantharaman, S.; Marchetti, B.; Bertagnolli, H.;
Rominger, F. Chem. Eur. J. 2010, 16, 8012.
Synlett 2010, No. 16, 2443–2448 © Thieme Stuttgart · New York