Synthesis of coumarins catalyzed by heterobimetallic Co/Rh nanoparticles

Article abstract of DOI:10.1055/s-2004-834826

Described here is the use of cobalt/rhodium (CO₂Rh₂) heterobimetallic nanoparticles in the catalytic synthesis of coumarins via cyclocarbonylation of alkynes with 2-iodophenol under 1 atm of carbon monoxide and reaction of phenol wit

Full text of DOI:10.1055/s-2004-834826

LETTER
2541
Synthesis of Coumarins Catalyzed by Heterobimetallic Co/Rh Nanoparticles
S
ynthesis of Coum
a
arins ng Hyun Park, Il Gu Jung, Young Keun Chung*
School of Chemistry, College of Natural Sciences, Seoul National University, Seoul 151-742, Korea
Fax +82(2)8890310; E-mail: ykchung@plaza.snu.ac.kr
Received 18 August 2004
and can be reused at least five times without loss of cata-
lytic activity.
Abstract: Described here is the use of cobalt/rhodium (Co2Rh2)
heterobimetallic nanoparticles in the catalytic synthesis of cou-
marins via cyclocarbonylation of alkynes with 2-iodophenol under
1 atm of carbon monoxide and reaction of phenol with propiolic
esters.
As mentioned above, the palladium-catalyzed cyclo-
carbonylation of alkynes with 2-iodophenol has been well
established. Thus, the use of Co2Rh2 nanoparticles was
considered to be somewhat unusual at first. However, our
preliminary experiment showed that Co2Rh2 nanoparti-
cles could be used to catalyze this reaction (Scheme 1).
Therefore, we carried out an investigation to find an opti-
mized set of reaction conditions. The reaction parameters
such as bases and CO sources were screened (Table 1).
Mo(CO)6 turned out to be a good CO source in the pres-
ence of pyridine and DBU (entry 2).15 However, a,b-un-
saturated aldehydes such as acrolein were not active as a
CO source.14b Pyridine was better than triethylamine (en-
tries 1 and 3). The final optimized reaction conditions for
the reaction of 2-iodophenol, diphenylacetylene, and car-
bon monoxide have thus been determined to be 1.0 mmol
of 2-iodophenol, 2.5 equivalents of diphenylacetylene, 3
mol% Co2Rh2, 1.2 equivalents of pyridine in 5 mL of
THF, the reaction carried our at 120 °C for 18 hours under
1 atm of CO. The best yield obtained was 87%. The reus-
ability of Co2Rh2 was also tested (entries 4–7). As shown
in Table 1, the catalyst could be reused at least five times
without loss of catalytic activity.
Key words: coumarins, nanoparticles, heterobimetallic, cyclocar-
bonylation, catalysis
Coumarin and its derivatives occur widely in nature, par-
ticularly in plants, with the majority of them showing a
wide range of biological activities.1 Recently, their syn-
thesis has attracted a lot of attention, not only because of
their pharmacological activity, but also because of their
applications as laser dyes, fluorescent labels, emission
layers in organic light-emitting diodes, and as optical
brighteners.2 As a result, many useful methods for the
synthesis of coumarins have been extensively developed,3
including the Perkin reaction,4 the Knoevenagel reaction,5
and the Pechmann reaction.6 Recently, several studies7 on
transition metal-catalyzed reactions have reported
methods for the synthesis of coumarins. In particular,
palladium-catalyzed reactions of phenols7d–f have been
widely used. They involve inter- and intramolecular reac-
tions of 2-iodo-phenol or its O-acyl derivatives in the
presence of CO, the intramolecular reaction of (2-
hydroxyphenyl)alkenes or -alkynes, reactions of phenols
and ethyl propioloates, or reactions of 2-iodophenol and
alkynes in the presence of CO.
Recently, the chemistry of transition-metal nanoparticles
has been developing rapidly8 and it has broadened to in-
clude many catalytic reactions, such as oxidation,9 hydro-
genation,10 coupling reactions,11 the Pauson–Khand
reaction,12 and some photocatalytic reactions.13 Very re-
cently, we found that cobalt–rhodium nanoparticles
(Co2Rh2) derived from Co2Rh2(CO)12 could be success-
fully utilized as catalysts in Pauson–Khand-type reactions
and silylcarbocyclization.14 While we were investigating
the use of Co2Rh2, we discovered a Co2Rh2-catalyzed
method of synthesis for coumarins. Here we report the use
of cobalt–rhodium heterobimetallic nanoparticles in the
catalytic synthesis of coumarins via the cyclocarbonyla-
tion of alkynes with 2-iodophenol under 1 atm of carbon
monoxide and the reaction of phenol with propiolic esters.
The cobalt–rhodium nanoparticles are easily recovered
Scheme 1
Table 1 Optimization of the Reaction Conditions
Entry Catalyst
CO source
Base
Yield (%)a
1
2
3
4
5
6
7
8
Pd nanoparticles 1 atm CO
Pyridine
Pyridine
Pyridine
Et3N
79
87
73
43
85
83
85
86
Co2Rh2
Co2Rh2
Co2Rh2
Co2Rh2
Co2Rh2
Co2Rh2
Co2Rh2
1 atm CO
Mo(CO)6
1 atm CO
1 atm CO
1 atm CO
1 atm CO
1 atm CO
b
c
d
e
Pyridine
Pyridine
Pyridine
Pyridine
a Isolated yields.
b Recovered from the reaction in entry 2.
c Recovered from the reaction in entry 5.
d Recovered from the reaction in entry 6.
e Recovered from the reaction in entry 7.
SYNLETT 2004, No. 14, pp 2541–2544
2
2
.1
1
.2
0
0
4
Advanced online publication: 20.10.2004
DOI: 10.1055/s-2004-834826; Art ID: U23304ST
© Georg Thieme Verlag Stuttgart · New York

2542
K. H. Park et al.
LETTER
The scope and limitations of this annulation reaction have et al. also reported3 a palladium-catalyzed synthesis of
been studied by allowing several internal alkynes to react coumarins from phenols and propiolic esters. Their results
with 2-halophenol under our optimized reaction condi- encouraged us to study the reaction between phenol and
tions. The results of this study are summarized in Table 2. alkynoates. Thus, we investigated the Co2Rh2-catalyzed
The yields were higher than those obtained by the synthesis of coumarins from the reaction of various
Pd(OAc)2-catalyzed carbonylative annulation of internal phenols with alkynoates under mild reaction conditions
alkynes.3 When the halide was a bromide (entry 2), the (Scheme 2 and Table 3).
yield was 65%. In the case of 2-chlorophenol (entry 3), the
yield was only 13%. The order of reactivity, ArI > ArBr >
ArCl, is consistent with the strength of the Ar-X bond.
Treatment of 2-iodophenol with a symmetric alkyne, 3-
hexyne and 4-octyne (entries 4 and 5), gave the product in
69% and 72% yields, respectively. When an unsymmetric
alkyne, 1-phenyl-1-propyne (entry 6), was used, two iso-
Scheme 2
meric products were obtained in 67% and 13% yields,
respectively. The regioselectivity (5.1:1) was higher than
that (3.8:1) obtained by the Pd(OAc)2-catalyzed carbonyl-
ative annulation of internal alkynes.3
Several years ago, Trost reported3 the palladium-cata-
lyzed synthesis of coumarins from phenols and alkynoates
under mild reaction conditions. Very recently, Kitamura
The reaction conditions were adopted from the previous
study.3 Table 3 indicates that coumarins were obtained in
high yields in all cases. This observation is somewhat
different from the results obtained in Pd-catalyzed reac-
tions.3 For the Pd-catalyzed reactions, electron-rich phe-
nols, such as 3-methoxyphenol, 4-methoxyphenol, 3,5-
Table 2 Synthesis of Coumarins by the Co2Rh2-Catalyzed Carbonylative Annulation of Internal Alkynesa
Entry
1
Phenol
Alkyne
Product
Yield (%)
87
2
3
4
5
6
65
13
69
72
67
13
a Isolated yields. Reaction conditions: o-Iodophenol (0.11 g, 0.97 mmol), alkyne (0.445 g, 2.50 mmol), pyridine (0.1 mL, 1.20 mmol), Co2Rh2
(0.01 g, 0.029 mmol), THF (5 mL), 1 atm CO, 120 °C, and 18 h.
Synlett 2004, No. 14, 2541–2544 © Thieme Stuttgart · New York

LETTER
Synthesis of Coumarins
2543
dimethoxyphenol, and 2-naphthol gave much higher particles as efficient catalysts. The reaction does not need
yields than those of moderately activated phenols, such as any additives or special conditions. The reusability of the
3,4- and 3,5-dimethylphenol. In addition to the carbonyla- catalyst and the simplicity of the experimental conditions
tive cycloaddition reaction, hydroarylation of coumarins are especially attractive and should encourage the use of
was also observed for the Pd-catalyzed reaction under Ki- this catalyst system among synthetic chemists and in
tamura’s reaction conditions.3 In the case of m-cresol, no industrial applications.
reaction was observed under Trost’s reaction conditions.3
However, no hydroarylation was observed in the case of
our reaction conditions either. But, under our reaction
conditions, m-cresols produced a high yield.
General Remarks:
All the reactions were conducted under nitrogen using standard
Schlenk-type flasks. Workup procedures were done in air. All the
solvents were dried and distilled according to standard methods be-
fore use. The reagents were purchased from Aldrich Chemical Co.
and Strem Chemical Co. and were used as received. 1H NMR spec-
tra were obtained with a Bruker 300 MHz or 500 MHz spectrome-
Trost et al.3 proposed that it is a Pd(0) species that acts as
the actual catalyst. The reaction, they believed, was initi-
ated by a hydropalladation, namely the formation of HP-
dX. However, Kitamura et al.3 reported that a Pd(II)
species catalyzed the hydroarylation of 2-alkynoates in ter. The elemental analyses were done at the National Center for
Inter-University Research Facilities, Seoul National University.
High resolution MS was done at Korea Basic Science Institute
(Daegu).
TFA. They proposed the ortho-palladation of a phenol by
a Pd(II) species, followed by hydroarylation of an
alkynoate. This difference might originate in the differing
reaction conditions. The precise mechanism of Co2Rh2
catalysis has not yet been revealed, but it is likely to be
distinct from that of the palladium-catalyzed process.
Preparation of Co2Rh2 Nanoparticles:
To a two-neck flask were added o-dichlorobenzene (24 mL), oleic
acid (0.2 mL), and trioctylphosphine oxide (0.4 g). While the solu-
tion was heated at 180 °C, a solution of Co2Rh2(CO)12 (1.0 g) in 6
mL of o-dichlorobenzene was injected into the flask. The resulting
solution was heated at 180 ºC for 2 h. After the solution had cooled,
the solution was concentrated to ca 10 mL by distillation. To the
concentrated solution were added 200 mL of THF.
In conclusion, we have demonstrated a simple method of
synthesis for coumarins from a reaction of alkynes with 2-
iodophenol under 1 atm of CO, and from a reaction of
phenols with propiolic esters by using Co2Rh2 nano-
Immobilization of Co2Rh2 Nanoparticles on Charcoal:
To the concentrated solution of Co2Rh2 (ca 10 mL), obtained by the
above reaction, was added 25 mL of THF. After the solution had
been well stirred for 10 min, flame-dried charcoal (2.0 g) was added
to the solution. After the resulting solution was refluxed for 12 h, the
precipitates were filtered and washed with Et2O (20 mL), CH2Cl2
(20 mL), acetone (20 mL), and MeOH. Vacuum drying gave a black
solid.
Table 3 Co2Rh2-Catalyzed Reaction of But-2-ynoic Acid Ethyl
Ester with Phenolsa
Entry Phenol
1
Product
Yield
(%)
87
89
A Typical Procedure for the Synthesis of Coumarins by the
Co2Rh2-Catalyzed Carbonylative Annulation of Internal
Alkynes (Table 2):
2
o-Iodophenol (0.110 g, 0.97 mmol), diphenylacetylene (0.445 g,
2.50 mmol), pyridine (0.1 mL, 1.20 mmol), Co2Rh2 (0.010 g, 0.029
mmol), and THF (5 mL) were put into a stainless-steel bomb and
charged with 1 atm CO. The bomb was heated at 120 °C for 18 h.
After the solution had cooled, the solution was filtered and concen-
trated under reduced pressure. The residue was purified by column
chromatography on a silica gel (Et2O–hexane = 20:80) to give a
87% yield of coumarins.
3
4
85
79
A Typical Procedure for the Synthesis of Coumarins by the
Co2Rh2-Catalyzed Reaction of But-2-ynoic Acid Ethyl Ester
with Various Phenols (Table 3):
To a cold mixture of Co2Rh2 (0.010 g, 0.029 mmol), 3-methoxyphe-
nol (0.158 mL, 1.430 mmol), TFA (0.50 mL), and CH2Cl2 (5 mL)
at 0 °C was added tetrolic acid ethyl ester (0.21 mL, 1.820 mmol).
The mixture was stirred at r.t. for 12 h. The reaction mixture was fil-
tered and the filtrate was then neutralized with an aq NaHCO3 solu-
tion, extracted with CH2Cl2, dried over anhyd MgSO4, and
concentrated. Flash column chromatography on silica gel gave an
89% yield of the product.
5
83
a Isolated yields. Reaction conditions: phenol (0.158 mL, 1.430
mmol), Co2Rh2 (0.01 g, 0.029 mmol), TFA (0.5 mL), CH2Cl2 (5 mL),
tetrolic acid ethyl ester (0.21 mL, 1.820 mmol), r.t., 12 h.
Recycling Experiment:
In order to recycle the catalyst, it was filtered from the reaction
mixture and dried in vacuum. It could then be reused for further
catalytic reactions.
Synlett 2004, No. 14, 2541–2544 © Thieme Stuttgart · New York

2544
K. H. Park et al.
LETTER
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Acknowledgment
This work was supported by the Ministry of Science and Technolo-
gy of Korea through Research Center for Nanocatalysis, one of the
National Science Programs for Key Nanotechnology and by grant
No. [R02-2004-000-10005-0(2004)] from the Basic Research
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Synlett 2004, No. 14, 2541–2544 © Thieme Stuttgart · New York