Nanoporous gold-catalyzed [4+2] benzannulation between ortho- alkynylbenzaldehydes and alkynes

Article abstract of DOI:10.1055/s-0031-1289527

Nanoporous gold material exhibits a catalytic activity on the benzannulation reaction between ortho-alkynylbenzaldehydes and alkynes. The catalyst is easily recoverable and can be reused several times without leaching and loss of activity. Georg Thieme Verlag Stuttgart. New York.

Full text of DOI:10.1055/s-0031-1289527

66
CLUSTER
Nanoporous Gold-Catalyzed [4+2] Benzannulation between
ortho-Alkynylbenzaldehydes and Alkynes
Nanoporous
G
old-
a
Catalyzed B
o
enzannulati
k
on i Asao,*a Menggenbateer,b Yohei Seya,c Yoshinori Yamamoto,*a Mingwei Chen,a Wei Zhang,d
Akihisa Inoued
a
WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
Fax +81(22)2176165; E-mail: asao@m.tohoku.ac.jp; E-mail: yoshi@m.tohoku.ac.jp
b
Center for Integrated NanoTechnology Support, Tohoku University, Sendai 980-8577, Japan
Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
c
d
Received 1 August 2011
The nanoporous gold was readily prepared by selective
leaching of silver from an alloy foil consisting of
Au30Ag70 (in at%) with thickness of 40 mm in HNO3 (70
wt%). Since the ligament size and the resulting pore size
are highly dependent on the temperature and the time of
the dealloying process,14 we fabricated several porous
structures having pore sizes from 25 nm to 100 nm. The
compositions of these catalysts were found to be around
Au97Ag3 (see Supporting Information). With these cata-
lysts, we conducted the benzannulation reaction with
ortho-phenylethynylbenzaldehyde (1a) and phenylacet-
ylene (2a), and the results are shown in Table 1.
Abstract: Nanoporous gold material exhibits a catalytic activity on
the benzannulation reaction between ortho-alkynylbenzaldehydes
and alkynes. The catalyst is easily recoverable and can be reused
several times without leaching and loss of activity.
Key words: nanoporous gold, benzannulation, nanostructures, ca-
talysis, cycloaddition
Nanoporous gold (AuNPore) consists of a three-dimen-
sional network of gold ligaments and contains pores on
the order of a few nanometers to a few hundreds of na-
nometers in size.1 Due to its intriguing material proper-
ties,2 it has many applications such as actuation,3 sensing,4
etc. This material has been also used as a catalyst for some
molecular transformations, such as carbon monoxide oxi-
dation,5 gas-phase oxidative coupling of methanol,6 gas-
phase oxidation of benzyl alcohol,7 aerobic oxidation of
D-glucose,8 and electrocatalytic reactions.9 Recently, we
reported that this material was an effective catalyst for the
oxidation of organosilanes with water, leading to organic
silanols.10 However, to our knowledge, there is no report
about the carbon–carbon bond formation catalyzed by this
nanoporous material.11 We have previously reported the
homogeneous gold-catalyzed formal [4+2] benzannula-
tion reaction between ortho-alkynylbenzaldehydes and
alkynes, leading to naphthalene derivatives.12,13 There-
fore, we examined the catalytic activity of the nanoporous
gold in this transformation and found that it exhibited a
high catalytic activity (Equation 1). In this paper, we re-
port the detail of this reaction as well as the effect of the
porous size on the catalytic activity.
Table 1 AuNPore-Catalyzed Benzannulation of 1a with 2a
CHO
H
AuNPore (20 mol%)
+
o-C6H4Cl2, 150 °C, 2.5 h
Ph
Ph
1a
2a
+
Ph
Ph
O
3a
Ph
4a
Entry
Average pore size
(nm)
Yield of
3a (%)
Yield of Recoveryof
4a (%) 1a (%)
1
2
3
4
5
6
7
AuNPore-1 (25)
reuse 1
62
61
60
61
12
trace
0
14
15
17
13
trace
0
0
0
reuse 2
0
AuNPore-2 (30)
AuNPore-3 (40)
AuNPore-4 (60)
AuNPore-5 (100)
0
CHO
H
66
83
93
AuNPore (cat.)
heat
R2
+
R1
R2
O
R1
1
2
3
0
Equation 1 Nanoporous-gold-catalyzed benzannulation reaction
We were delighted to see that the reaction proceeded with
20 mol% of AuNPore-1, having around 25 nm pore size,
at 150 °C for 2.5 hours, leading to naphthyl ketone prod-
uct 3a in 62% yield together with decarbonylated naph-
thalene 4a in 14% (Table 1, entry 1). On the other hand,
SYNLETT 2012, 23, 66–69
Advanced online publication: 11.11.2011
DOI: 10.1055/s-0031-1289527; Art ID: Y04111ST
x
x
.x
x
.2
0
1
1
© Georg Thieme Verlag Stuttgart · New York

CLUSTER
Nanoporous Gold-Catalyzed Benzannulation
67
any reactions did not take place in the absence of the cat- the current transformation was catalyzed by the AuNPore
alyst or in the presence of the nondealloyed Au30Ag70 foil, catalyst.
having no nanoporous structure. These results clearly in-
The benzannulation reactions with other substrates were
dicated that the nanoporous structure of the catalyst is
conducted, and the results are summarized in Table 2. The
necessary for this transformation. The catalyst was easily
reaction of 1b (R1 = 4-F3CC6H4) with 2a proceeded
smoothly to give the corresponding product 3b in 76%
recoverable by picking up with tweezers, and any cumber-
some separation procedures, such as filtration and centrif-
ugation, are not required. The catalyst was used at least
yield along with 4a in 8% yield (Table 2, entry 1). How-
ever, the reaction of 1c (R1 = 4-MeOC6H4) with 2a gave a
three times, and 3a was obtained in good yields in each
mixture of 3c and its regioisomer, bearing R2 group at the
case (Table 1, entries 1–3).15 According to the scanning
3-position on the naphthalene ring. The ratio was deter-
electron microscopy (SEM) analysis, no significant
changes were observed before and after three uses. We
next investigated the catalytic activity of the nanoporous
gold having other pore sizes. The catalyst, AuNPore-2,
1
mined to be 5:1 by H NMR analysis (Table 2, entry 2).
On the other hand, the decarbonylated naphthalene 4a was
obtained as a main product in the reaction of 1d, having
the butyl group at the terminus of alkyne. Not only phen-
having around 30 nm pore size, exhibited the similar ac-
ylacetylene but also pentyne is suitable for this transfor-
tivity with AuNPore-1, and 3a was obtained in 61% yield
mation (Table 2, entry 4).
(Table 1, entry 4). However, the chemical yields were dra-
We have previously proposed the reaction mechanism of
matically decreased when the reactions were carried out
this transformation catalyzed by the homogeneous gold
with catalysts having 40 nm and 60 nm pore sizes
catalysts. We believed that the formation of the benzo-
(Table 1, entries 5 and 6). Finally, the nanoporous gold,
pyrylium intermediate 6 was a key step, which was pro-
AuNPore-5, having 100 nm pore size did not show any
catalytic activity, and most of 1a was recovered (Table 1,
moted effectively by the gold catalyst due to its strong
alkynophilicity.12,13 The obtained results with the nanopo-
entry 7). These results clearly indicated that the catalytic
rous gold catalyst suggests that the nanoporous gold
activity is highly dependent on the pore size, and the size
would also behave as a Lewis acid catalyst as shown in
Scheme 1. After the formation of 6, the cycloaddition be-
should be less than 30 nm for this transformation. Hence,
we decided to use AuNPore-1 having 25 nm pore size for
tween 6 and 2 takes place to give the intermediate 8
through 7. The subsequent bond rearrangement, as shown
in 9 with arrows, affords the naphthyl ketone derivative 3
and regenerates the catalyst. On the other hand, protonoly-
sis of 8 by water, which might exist in the media, produces
10, which undergoes the retro Diels–Alder reaction lead-
ing to the formation of 4. The decarbonylated product 4a
was obtained predominantly in the reaction of 1d with 2a
as mentioned above, probably because the butyl group of
1d stabilized the intermediate 8 due to its electron-
donating property, which allows easier attack of water to
8.
the following studies.16
We next examined the reaction to clarify whether a dis-
solved gold species in solvents participate in the current
reaction or not. The reaction of 1a with 2a was carried out
for one hour under the typical reaction conditions. Then,
1
a half amount of the solution was picked up. H NMR
analysis of this solution indicated that 3a was formed in
43% together with 4a in 12%. While stirring of this solu-
tion was continued in the absence of the catalyst for 1.5
hours, further consumption of 1a was not detected at all.
On the other hand, stirring of the residual reaction mixture
having the catalyst for 1.5 hours gave 3a in 65% yield to-
gether with 4a in 19%. These results clearly indicated that
In conclusion, we have demonstrated that the nanoporous
gold material could be an effective catalyst for the benz-
Table 2 AuNPore-Catalyzed Benzannulation of 1 with 2
CHO
H
AuNPore-1 (20 mol%)
R2
+
+
R2
o-C6H4Cl2, 150 °C, 2.5 h
R1
R2
O
R1
1
2
3
4
Entry
1
R1
2
R2
3a
Yield (%)
76
4a
Yield (%)
1
2
3
4
1b
4-F3CC6H4
4-MeOC6H4
Bu
2a
2a
2a
2b
Ph
Ph
Ph
Pr
3b
3c
3d
3e
4a
4a
4a
4b
8
1c
1d
1a
51b
32
47
22
12c
Ph
68d
a Isolated yield.
b Ratio between 3c and its regioisomer is 5:1.
d Ratio between 3d and its regioisomer is 3:1.
c Ratio between 3e and its regioisomer is 6:1.
© Thieme Stuttgart · New York
Synlett 2012, 23, 66–69

68
N. Asao et al.
CLUSTER
CHO
R1
1
O
R1
OH
R2
AuNPore
O
H
10
R1
PoreNAu
R2
5
H2O
O
R1
3
R2
O+
R2
4
R1
R1
O
+
O
PoreNAu –
R1
OH
R2
9
PoreNAu
11
O
8
R1
AuNPore
H
6
O
R1
R2
H
R2
AuNPore
2
7
Scheme 1 Plausible reaction mechanism of the AuNPore-catalyzed benzannulation
(4) (a) Biener, J.; Nyce, G. W.; Hodge, A. M.; Biener, M. M.;
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annulation reaction between ortho-alkynylbenzaldehydes
and alkynes. The catalyst was easily recoverable and
could be used at least three times with no significant loss
in activity. Further studies to elucidate the mechanism of
this reaction and to extend the scope of synthetic utility for
organic synthesis are in progress in our laboratory.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
Acknowledgment
This work was partially supported by the Grant-in-Aid for Scientific
Research on Innovative Areas ‘Reaction Integration’ (No. 2105)
and the Grant-in-Aid for Scientific Research on Priority Areas (No.
474) from the MEXT, Japan.
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Bäumer, M. Science 2010, 327, 319. (b) Xu, B.; Haubrich,
J.; Baker, T. A.; Kaxiras, E.; Friend, C. M. J. Phys. Chem. C
2011, 115, 3703.
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Synlett 2012, 23, 66–69
© Thieme Stuttgart · New York

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Nanoporous Gold-Catalyzed Benzannulation
69
(10) (a) Asao, N.; Ishikawa, Y.; Hatakeyama, N.;
Menggenbateer; Yamamoto, Y.; Chen, M.; Zhang, W.;
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reaction time tends to be longer for completion, probably
owing to the gradual coarsening of the porous structure.
(16) General Procedure
To a mixture of o-alkynylbenzaldehyde 1 (0.2 mmol) and
nanoporous gold (20 mol%) in o-C6H4Cl2 (0.5 mL) was
added alkyne compound 2 (1.2 mmol) at r.t. under Ar
atmosphere. The mixture was stirred for 2.5 h at 150 °C and
cooled down to r.t. The liquid moiety was separated by a
pipette and evaporated to leave the crude product, which was
purified by silica gel column chromatography using hexane–
EtOAc as eluent to give the pure product 3. The recovered
catalyst was washed with Et2O several times, dried under
reduced pressure, and reused without further purification.
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© Thieme Stuttgart · New York
Synlett 2012, 23, 66–69

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