Synthesis of fused and linked benzofurans from 2-alkynylphenol derivatives through rhodium(i)-catalyzed domino-type addition reactions

Article abstract of DOI:10.2533/chimia.2018.888

A rhodium(i)-catalyzed domino-type sequential 5-endo/5-exo cyclization reaction of [(2-acylphenyl) ethynyl]phenols produces indene/benzofuran-fused alcohols. A moderate asymmetric induction is observed when chiral diphosphine ligands are used for rhodium.

Full text of DOI:10.2533/chimia.2018.888

888 CHIMIA 2018, 72, No. 12
doi:10.2533/chimia.2018.888
Chemistry ties Linking Japan to switzerLand
Chimia 72 (2018) 888–891 © Swiss Chemical Society
Synthesis of Fused and Linked
Benzofurans from 2-Alkynylphenol
Derivatives through Rhodium(i)-catalyzed
Domino-type Addition Reactions
Takanori Matsuda*a, Shinya Abea, Hirotaka Itoa, Tomoya Tsuboib, Haruki Kirikaea, and Masahiro
Murakami*b
With the best tribute to the memory of my mentor, late Professor Teruaki Mukaiyama who passed
away on November 17, 2018 at the age of 91.
Abstract: A rhodium(i)-catalyzed domino-type sequential 5-endo/5-exo cyclization reaction of [(2-acylphenyl)
ethynyl]phenols produces indene/benzofuran-fused alcohols. A moderate asymmetric induction is observed
when chiral diphosphine ligands are used for rhodium. Indene/indole-fused compounds are synthesized by a
similar reaction of [(2-acetylphenyl)ethynyl]anilines. The domino-type 5-endo/5-exo cyclization reaction is ex-
tended to substrates having two phenolic hydroxy groups. A linearly-fused array of five- and six-membered rings
is constructed. Fused and linked benzofurans possessing 2-cyanoethyl side chains are also synthesized through
sequential formation of C–O and C–C bonds.
Keywords: Asymmetric reaction · Benzofuran · Cyclization · Domino reaction · Rhodium
Masahiro
Murakami
of photo-energy for organic synthesis is reported to effect the 5-endo cyclization
currently the major research interest in his reaction.[3,4] Among them, the cyclization
is
currently
a
group.
reaction catalyzed by a hydroxorhodium(i)
complex, developed by Lautens,[4] is par-
ticularly attractive since (benzofuran-
3-yl)-rhodium(i) species arising from the
cyclization are relatively stable but active
Chemistry Professor
at Kyoto University.
He grew up in a rice Introduction
farmer’s family in
Toyama, Japan and
then trained at the motif prevalently found in a variety of nat- alkenes such as acrylonitrile, furnishing
University of Tokyo, ural products and biologically active com- 2,3-disubstituted benzofurans in one-pot
receiving his DSc in 1984 under the su- pounds.[1] Benzofuran derivatives have reactions. The domino-type addition reac-
Benzo[b]furan is a privileged structural enough to further add to electron-deficient
pervision of Prof. Teruaki Mukaiyama. He
also received considerable attention as or- tion has a mechanistic distinction that the
started his career with an assistant position ganic electronic materials in recent years.[2] intermediate organorhodium(i) species is
to Prof. Mukaiyama at the University of 2-Alkynylphenols are readily accessible generated from a non-organometallic com-
Tokyo (1984–1987). He moved to Kyoto
University taking an assistant position to cursors of benzofurans. They readily un- precursory organometallic compounds
Professor Yoshihiko Ito in 1987, and was dergo, upon treatment with an appropriate (Scheme 1).[5]
compounds that are suitable synthetic pre- pound dispensing with pre-preparation of
promoted to Associate Professor in 1993. promoter, a 5-endo cyclization reaction to
The reaction merited further investiga-
In the meantime, he took leave to spend ten construct benzofuran skeletons in an ex- tion, targeting fused and linked compounds
months from May 1991 till March 1992 at peditious and atom-economical manner. containing benzofuran cores, which are of
the ETH Zürich as a post-doctoral fellow
There have been a number of promoters much interest from the viewpoint of or-
with Prof. Albert Eschenmoser. He was
appointed to Professor in 2002 at Kyoto
University. His research interests focus on
the development of new and interesting
Scheme 1.
Generation and
reaction of (benzo-
furan-3-yl)rhodium(i)
species
R
R
Rh(I)OH
organic transformations. The utilization
– H2O
OH
ORh(I)
*Correspondence: Prof. Dr. M. Murakamib,
Dr. T. Matsudaa
Rh(I)
E
E-mail: murakami@sbchem.kyoto-u.ac.jp
mtd@rs.tus.ac.jp
E+
aDepartment of Applied Chemistry
Tokyo University of Science
R
R
5-endo
cyclization
O
O
1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
bDepartment of Synthetic Chemistry and Biological
Chemistry
benzofuran-3-yl-
rhodium(I)
Kyoto University
Katsura, Kyoto 615-8510, Japan

Chemistry ties Linking Japan to switzerLand
CHIMIA 2018, 72, No. 12 889
ganic electronics and nanotechnology.[1,2]
Herein, we report the results of our study
on the synthesis of fused and linked ben-
zofurans on the basis of the rhodium-cat-
alyzed domino-type cyclization protocol.
Scheme 2.
Rhodium(i)-catalyzed
domino-type
5-endo/5-exo
O
O
H3C
5 mol% [Rh(OH)(cod)]2
H3C
10 mol% rac-BINAP
THF, 80 °C, 12 h
cyclization of 1a
ORh(I)
OH
1a
H3C
OH
CH3
Rh
O
Results and Discussion
5-exo
5-endo
O
O
cyclization
2-[(2-Acetylphenyl)ethynyl]phenol
(1a) was designed to synthesize indene/
cyclization
2a 89%
benzofuran-fused alcohol 2a through
Scheme 3.
domino-type sequential 5-endo/5-exo cy-
clization. An electrophilic acetyl group is
placed on an alkynylphenol backbone in a
way that an intermediate (benzofuran-3-yl)
rhodium(i) species further undergoes the
second cyclization onto the acetyl group
in a 5-exo mode, attaching a fused indene
skeleton. It was synthesized through the
Sonogashira coupling reaction of 1-iodo-
2-(methoxymethoxy)benzene with 2-ethy-
nylphenyl methyl ketone and the follow-
ing deprotection of the methoxymethoxy
Rhodium(i)-catalyzed
domino-type
O
R
OH
R
5-endo/5-exo cycli-
zation of 1a. aReaction
conditions: substrate
1 (0.10–0.37 mmol),
[Rh(OH)(cod)]2
(10 mol% in Rh), rac-
BINAP (10 mol%),
THF (0.1 M), 80 °C,
12 h. bIsolated yield.
Rh catalyst
O
OH
2
1
yield [%]b
75
entry
substrate (1)
product (2)
O
Et OH
Et
1
group under acidic conditions. When 1a
was treated with a catalytic amount of a
rhodium complex prepared in situ from
[Rh(OH)(cod)]2 and rac-BINAP, inde-
nobenzofuranol 2a was obtained in 89%
isolated yield (Scheme 2). Initially, a
rhodium(i) alkoxide is generated from 1a
by deprotonation of the phenolic hydroxy
O
OH
OH
2b
1b
O
i-Pr OH
i-Pr
71
70
2
3
O
2c
1c
O
O
group with the hydroxorhodium(i) com-
Ph OH
Ph
plex. The first cyclization takes place in a
5-endo mode.[6] The resulting (benzofuran-
O
3-yl)rhodium(i) species then undergoes the
OH
OH
2d
1d
second cyclization onto the pendent ace-
tyl group in a 5-exo mode to afford 2a.[7]
H3C OH
H3C
The resulting rhodium(i) alkoxide acts as
a base to release the alcohol 2a, promoting
the next catalytic cycle.
4
5
84
76
O
Other
2-[(2-acylphenyl)ethynyl]
2e
1e
phenols 1b, 1c, 1e, and 1f were pre-
pared in an analogous manner to 1a.
2-[(2-Benzoylphenyl)ethynyl]phenol (1d)
was prepared through the Sonogashira
O
H3C
H3C OH
coupling reaction of 2-iodobenzophenone
with 1-ethynyl-2-(methoxymethoxy)ben-
O
OH
1f
2f
zene and the following deprotection. They
successfully participated in the domino-
type 5-endo/5-exo cyclization reaction to
O
R
Table 1.
Asymmetric cyclization
of 1a
R
OH
chiral Rh
catalyst
afford indene/benzofuran-fused alcohols
2b–f in good yield (Scheme 3).[8]
O
OH
Induction of enantioselectivity in the
2
1
second 5-exo cyclization process producing
2a was investigated using representative
axially chiral biaryl diphosphine ligands
entry
1 (R)
diphosphine ligand
product
yield [%]b
ee [%]c
1
2
3
4
5
6
7
1a (Me)
1a
1a
(R)-BINAP
(R)-MeO-BIPHEP
2a
2a
94
80
51
79
for rhodium (Table 1). An enantioselectiv-
ity was modest when the most typical (R)-
(R)-C3-TunePhos 2a 82 84
BINAP was tried (entry 1). Among other
chiral ligands examined, (R)-SEGPHOS
gave the best result in terms of both chemi-
cal yield and enantioselectivity (entries
2–4). (R)-SEGPHOS induced good enanti-
oselectivities around 85% ee also with other
substrates 1b–d, although chemical yields
were moderate (entries 5–7).[9]
1a
(R)-SEGPHOS
(R)-SEGPHOS
(R)-SEGPHOS
(R)-SEGPHOS
2a
2b
2c
2d
84d
51d
38d
53d
88d
85d
83d
86d
1b (Et)
1c (i-Pr)
1d (Ph)
a Reaction conditions: substrate 4 (0.050 mmol), [Rh(OH)(cod)]2 (10 mol% in Rh), chiral
diphosphine ligand (13 mol%), THF (0.03 M), 80 °C, 12 h. b Isolated yield. c Determined by chiral
GC (Rt-βDEXm) or HPLC (CHIRALCEL OJ-H). d Average of two runs.

890 CHIMIA 2018, 72, No. 12
Chemistry ties Linking Japan to switzerLand
Scheme 4. Domino-
type cyclization of 3.
Previous work by Lautens suggested
that it would be possible to extend the
domino-type 5-endo/5-exo cyclization
protocol for the synthesis of indene/ben-
zofuran-fused alcohols to the synthesis of
indene/indole-fused alcohols by replacing
the hydroxy group with a sulfonated amino
group. 2-[(Acetylphenyl)ethynyl]anilines
3a and 3b were prepared from 2-iodoace-
tophenone and 2-ethynylanilines by the
Sonogashira coupling reaction followed by
deprotection, and subjected to the identi-
O
H3C
OH
H3C
5 mol% [Rh(OH)(cod)]2
10 mol% rac-BINAP
THF, 80 °C, 12 h
N
R
NHR
3a (R = Ms)
3b (R = Ts)
4a (R = Ms) 82%
4b (R = Ts) 82%
Scheme 5. Synthesis
of heptacyclic pro-
duct 6.
O
HO
Me
10 mol% [Rh(OH)(cod)]2
20 mol% rac-BINAP
cal reaction conditions. They successfully
THF, 80 °C, 12 h
underwent an analogous domino-type
5-endo/5-exo cyclization reaction to afford
indene/indole-fused alcohols 4a and 4b
via sequential formation of C–N and C–C
bonds (Scheme 4).[10,11]
Me
OH
O
5
Me OH
O
We were intrigued also by the sub-
strates bearing two phenolic hydroxy
groups, each of which would trigger the
5-endo cyclization. It constructs more
conjugated benzofuran structures, which
would be more promising as the material
for organic electronics and nanotechnol-
ogy. When the hydroquinone derivative 5
was used, the domino-type 5-endo/5-exo
cyclization occurred on both sides to form
a linearly-fused array of five- and six-
membered rings in an expeditious manner,
affording 6 in 66% yield (Scheme 5).[12]
Another hydroquinone derivative,
2,5-bis(phenylethynyl)hydroquinone
(7a),[2d] was reacted with an excess amount
of acrylonitrile (8) in the presence of a
rhodium(i) catalyst. Each of the arising
(benzofuran-3-yl)rhodium(i) species un-
derwent intermolecular conjugate addition
to acrylonitrile (8), introducing two 2-cya-
noethyl side chains on the fused tricyclic
aromatic core to afford 9a (Scheme 6).[4]
An analogous intermolecular reaction
with acrylonitrile (8) was carried out us-
O
HO Me
6 66%
Scheme 6. Reaction
of 7a and 8.
HO
CN
+
Ph
Ph
8
OH
7a
(20 equiv)
CN
O
10 mol% [Rh(OH)(cod)]2
Ph
20 mol% rac-BINAP
THF, 80 °C, 12 h
Ph
O
NC
9a 42%
Scheme 7. Reaction
of 7b and 8.
HO
CN
+
8
OH
7b
(20 equiv)
ing
1,4-bis[(2-hydroxyphenyl)ethynyl]
CN
benzene (7b). 1,4-Di(benzofuran-2-yl)
benzene 9b was produced in 64% yield
(Scheme 7).[13] Similarly, the reaction of
the corresponding 1,3-disubstituted (meta-
disubstituted) derivative 7c with 8 afforded
1,3-di(benzofuran-2-yl)benzene 9c in 63%
yield (Scheme 8).
10 mol% [Rh(OH)(cod)]2
O
20 mol% rac-BINAP
O
THF, 80 °C, 12 h
NC
9b 64%
Scheme 8. Reaction
of 7c and 8.
Conclusion
HO
In summary, a rhodium(i)-catalyzed cy-
clization reaction was studied using [(2-ac-
ylphenyl)ethynyl]phenols and similar ani-
line derivatives. Domino-type sequential
5-endo/5-exo cyclization took place to pro-
duce indene/benzofuran-fused and indene/
CN
+
OH
8
7c
(20 equiv)
NC
10 mol% [Rh(OH)(cod)]2
indole-fused alcohols, respectively. The
20 mol% rac-BINAP
O
domino-type reaction was extended to the
substrates having two phenolic hydroxy
groups. A linearly-fused array of five- and
six-membered rings is constructed. Fused
and linked benzofurans possessing 2-cya-
THF, 80 °C, 12 h
O
CN
9c 63%

Chemistry ties Linking Japan to switzerLand
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