Rhodium-catalyzed [2+2+1+1] cyclocarbonylative coupling of alkynes with carbon monoxide affording tetrasubstituted p-benzoquinones

Article abstract of DOI:10.1002/chem.200700839

In this strategy, the tetrasubstituted benzoquinones have been prepared directly by a [2+2+1+1] cyclocarbonylative coupling reaction of internal alkynes with CO in the presence of [RhCl(CO)₂]₂. The low concentration of CO in the reaction is the crucial point for the chemoselective formation of tetrasubstituted benzoquinones in good to high yields. Functional groups such as chloro, methoxy, cyano, vinyl, fluoro, and carboxylate are tolerated under the reaction conditions.

Full text of DOI:10.1002/chem.200700839

FULL PAPER
DOI: 10.1002/chem.200700839
Rhodium-Catalyzed [2+2+1+1] Cyclocarbonylative Coupling of Alkynes
with Carbon Monoxide Affording Tetrasubstituted p-Benzoquinones
Qiufeng Huang and Ruimao Hua*[a]
Abstract: In this strategy, the tetrasubstituted benzoquinones have been prepared
directly by a [2+2+1+1] cyclocarbonylative coupling reaction of internal alkynes
with CO in the presence of [RhCl(CO)2]2. The low concentration of CO in the re-
action is the crucial point for the chemoselective formation of tetrasubstituted ben-
zoquinones in good to high yields. Functional groups such as chloro, methoxy,
cyano, vinyl, fluoro, and carboxylate are tolerated under the reaction conditions.
Keywords: alkynes · benzoquinone ·
carbonylation
rhodium
·
cycloaddition
·
Introduction
structures of unsaturated cyclic ketones can be selectively
obtained depending on the use of different substrates, cata-
lysts, and reaction conditions. For example, as shown in
Scheme 1, furan-2(5H)ones could be selectively formed
from the reductive cyclocarbonylation of alkynes in the
Transition-metal-catalyzed carbonylation is one of the most
important and useful reactions for the synthesis of carbonyl
compounds in organic synthesis.[1] The intermolecular cyclo-
carbonylation from an alkyne,
alkene, and CO as well as intra-
molecular
cyclocarbonylation
from an enyne or diyne and CO
have been proven to be ex-
tremely valuable protocols for
the synthesis of the unsaturated
cyclic ketones, which are widely
applied as intermediates for or-
ganic synthesis, fine chemicals,
and pharmaceutical synthesis.
There has been extraordinary
progress in these atom-econom-
ical cyclization reactions during
the last two decades, and it has
been disclosed that various
Scheme 1.
presence of water catalyzed by a variety of catalyst sys-
tems,[2] 2-cyclopenten-1-ones could be obtained by the
[2+2+1] cyclocarbonylative coupling of an alkyne, alkene,
and CO (Pauson–Khand-type reaction),[3] and 2-cyclohex-
ene-1,4-diones formed through the [2+2+1+1] cyclocarbo-
nylative coupling of an alkyne, alkene, and two molecules of
CO; these compounds are intermediates for the synthesis of
functionalized hydroquinones.[4]
[a] Q. Huang, Prof. R. Hua
Department of Chemistry, Tsinghua University
Innovative Catalysis Program
Key Laboratory of Organic Optoelectronics
& Molecular Engineering of Ministry of Education
Beijing 100084 (China)
Fax : (+86)10-6279-2596
E-mail: ruimao@mail.tsinghua.edu.cn
Moreover, the synthesis of bicyclopentenones[3] and the
formation of bicyclopentadienones[5] were reported by the
direct intramolecular cyclocarbonylation of an enyne or
diyne with CO (Scheme 2).
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author: A general experi-
mental procedure, characterization data, and copies of H, 13C NMR
1
spectra for all products.
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ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8333

Therefore, the reaction was first carried out under a CO
pressure of 5.0MPa (initial pressure at room temperature)
in the presence of [RhCl(CO)2]2 (2.5 mol%) in toluene at
1308C for 24 h.[12] The analyses of GC and GCMS of the re-
action mixture disclosed that a small amount of 2a was
Scheme 2.
Table 1. Catalytic cyclocarbonylation of 4-octyne with carbon monoxide.[a]
Development of synthetic
methods for substituted p-ben-
zoquinones is an important re-
search work in organic synthe-
sis,[6] as such types of com-
pounds are not only widely ap-
plied as versatile intermediates
in diverse organic synthesis, but
also widely exist in nature and
exhibit various important bio-
logical activities.[7] The tradi-
tional synthesis of p-benzoqui-
nones by oxidizing the substi-
tuted phenols has inevitable
drawbacks such as: 1) the use
of stoichiometric amounts of
oxidants which are either ex-
pensive or waste-forming pro-
cesses and 2) the limited availa-
bility of substituted phenols. On
the basis of retrosynthetic anal-
ysis, p-benzoquinones can be
obtained by the [2+2+1+1]
Entry Catalyst [mol%]
Solvent
[RhCl(CO)2]2 (2.5) toluene
[RhCl(CO)2]2 (2.5) toluene
Other conditions[b]
Yield of 2a [%][c]
1
A
CO (5.0MPa), 1308C for 24 h
CO (1.0MPa), 1308C for 24 h
<5
20
10
18
27
33
13
<5
0
0
2
ACHTREUNG
3
A
CHCl2CHCl2 CO (1.0MPa), 1208C for 24 h
toluene
CHCl2CHCl2 CO atmosphere, 1208C for 24 h
4
A
CO atmosphere, 1208C for 24 h
5
A
6
A
7
A
CO bubbling, 1208C for 24 h
CO bubbling, 1208C for 24 h
8
A
9
ACHTREUNG
10
11
12
13
14
C
72
71
94 (90)
22
CHCl2CHCl2 CO/N2 (ca. 1:1) bubbling, 1408C for 15 h
[a] Reactions were carried out by using 1.0 mmol of 1a in 1.0 mL of solvent. [b] Bubbling rate: ꢀ1.5–
2.0 mLminÀ1. [c] GC yield (isolated yield) based on the amount of 1a used.
chemoselective cyclocarbonylative coupling of two mole-
cules of an alkyne and two molecules of CO. Therefore, it is
an interesting and challenging research work to develop the
catalytic system for such cycloaddition reactions.[8,9]
formed and that without the formation of any other car-
bonylated products, 1a was recovered (entry 1). However,
unexpectedly, when the same reaction was repeated under a
mild initial CO pressure (1.0MPa) in toluene at 1308C and
in 1,1,2,2-tetrachloroethane (TCE) at 1208C for 24 h, 2a
was obtained in 20 and 10% GC yields, respectively (en-
tries 2 and 3). These results encouraged us to examine the
same experiment with a lower CO pressure.
As shown Table 1, under a CO atmosphere, the yield of
2a could be substantially increased (entries 4 and 5). Thus
we can ascertain that the low concentration of CO in the re-
action system favors the formation of 2a. As a matter of
fact, when bubbling CO gas (1.5–2.0 mLminÀ1) through the
reaction solution a higher yield of 2a resulted (33% GC
yield, entry 6). In order to further enhance the formation of
2a, the effects of solvents and additives were also briefly ex-
amined. It was found that the use of the coordinated sol-
vents (for example, BuOBu and DMSO), and ligands (for
example, PPh3 and Et3N) led to the decrease or complete
loss of the catalytic activity of [RhCl(CO)2]2 for the forma-
tion of 2a (entries 7–10).
Recently, we have been interested in the development of
the transition-metal-catalyzed carbonylation of alkynes. We
have developed the [Ru3(CO)12]-catalyzed reductive cyclo-
carbonylation of internal alkynes in the presence of water to
afford 3,4-disubstituted furan-2(5H)ones[10] and the rhodi-
um-catalyzed double hydroaminocarbonylation of terminal
alkynes with CO and amines to afford 1,4-diamides.[11] As
an extension of our efforts in this field, in this paper, we
wish to report a [2+2+1+1] cyclocarbonylative coupling
strategy of internal alkynes with CO to efficiently synthesize
p-benzoquinone
derivatives
in
the
presence
of
[RhCl(CO)2]2.
Results and Discussion
The reaction of 4-octyne (1a) with CO under different con-
ditions was examined to screen the optimal reaction condi-
tions for the chemoselective formation of 2,3,5,6-tetrapropyl
benzoquinone (2a) as summarized in Table 1. We consider
that in order to realize the “double” cyclocarbonylation of
1a, a high CO pressure might be favorable for 2a forma-
tion.
The catalytic activity of [RhCl(CO)2]2 in TCE could be
greatly improved by increasing the reaction temperature.
An increase of the reaction temperature up to 1408C result-
ed in the formation of 2a in 72% GC yield (entry 11) after
24 h. Surprisingly, an identical result was obtained by bub-
bling CO/N2 (CO/N2 =ca. 1:1, ꢀ1.5–2.0 mLminÀ1) at this
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Chem. Eur. J. 2007, 13, 8333 – 8337

Rhodium Catalysis
FULL PAPER
temperature in a shorter time (15 h, entry 12). Therefore, a
prolonged reaction time up to 24 h led to a satisfactory yield
of 2a (94% GC), which was isolated in 90% yield
zoquinones of two regioisomers, and the electronic effect of
the substituent on the aromatic ring is very significant. The
monophenyl (1g–j), monoelectron-rich aryl-substituted (1k–
n) internal alkynes seem to be good substrates for the cyclo-
carbonylative coupling reaction to afford the corresponding
p-benzoquinones in good to high yields (Table 2, entries 6–
14). The cyclocarbonylative coupling reaction of electron-
deficient monoaryl-substituted internal alkynes (1p–s) with
carbon monoxide also occurred, but afforded relatively low
yields of the desirable products (Table 2, entries 15–18). As
evident from Table 2, this catalytic system can tolerate vari-
ous functional groups, such as
(entry 13).
Furthermore,
under
similar
conditions,
RhCl3·3H2O also showed catalytic activity to give 2a in
22% GC yield (entry 14).
A number of tetrasubstituted p-benzoquinones could be
synthesized by the present catalytic procedure by the cyclo-
carbonylative coupling of a variety of internal alkynes with
CO under the reaction conditions as indicated in entry 13 of
Table 1. As summarized in Table 2, the yields of p-benzoqui-
methoxy, cyano, vinyl, chloro,
fluoro, and ester.
Table 2. Cyclic carbonylation of internal alkyne.[a]
In the present [2+2+1+1] cy-
clocarbonylative coupling reac-
tion, when an unsymmetrical in-
ternal alkyne was employed,
the formation of two possible
Entry
Starting
material
R
R’
Product
Yield [%][b]
(2+2’)
2/2’
regioisomers was observed as
expected. The ratio of two re-
gioisomers could be determined
AHCTREUNG
1
2
3
4
5
6
7
8
1b
1c
1d
1e
1 f
1g
1h
1i
1j
1k
1l
1m
1n
C2H5
n-C4H9
n-C5H11
PhCH2
p-ClC6H4CH2
Ph
Ph
Ph
Ph
p-CH3C6H4
p-CH3OC6H4
p-CH3OC6H4
p-CH3OC6H4
C2H5
n-C4H9
CH3
n-C5H11
n-C5H11
CH3
2b
2c
2d
2e
2 f
2g
2h
2i
2j
2k
2l
2m
2n
87
96
28
35
40
89
85
1
48:52[c]
by either H NMR spectroscopy
53:47[c] (Table 2, entries 5–7, 13, and
51:49[d]
17) or GC analysis (Table 2, en-
tries 3–4, 15–16, and 18). In the
80:20[d]
C2H5
84:16[d]
cases of 1i–m and 1o used, two
regioisomers could be separated
by preparative TLC isolation.[13]
Interestingly, when 1-benzo-
yl-1-heptyne (1t) was subjected
to the same reaction conditions,
the corresponding p-benzoqui-
none derivative was not formed
at all. Instead, 2-n-butyl-5-phe-
nylfuran (4) was isolated in
95% yield (Scheme 3).[14]
As described above, monoar-
yl-substituted internal alkynes
undergo the [2+2+1+1] cyclo-
carbonylative coupling reaction
to afford the corresponding p-
benzoquinones. Unfortunately,
the coupling reaction of alkynes
possessing two bulky groups,
such as 1,2-diarylated alkynes,
n-C4H9
n-C5H11
n-C5H11
n-C6H13
n-C5H11
65 (33+32)
59 (27+32)
87 (45+42)
80 (40+40)
86 (40+46)
53
9
10
11
12
13
NC
(CH2)3
49:51[d]
14
15
1o
1p
n-C5H11
n-C5H11
2o
2p
50 (26+24)
p-ClC6H4
40
59:41[c]
59:41[c]
16
17
18
1q
1r
1s
n-C5H11
n-C5H11
n-C5H11
2q
2r
2s
45
31
26
75:25[d]
61:39[c]
[a] Reactions were carried out by using 1.0 mmol of 1 in 1.0 mL of solvent under reflux for 24 h. [b] Isolated
yield. [c] Isomer ratio was determined by GC analysis. [d] Isomer ratio was determined by 1H NMR spectros-
copy.
nones are sensitive to the nature of alkyne substitution. The
reactions of 3-hexyne (1b) and 5-decyne (1c) with carbon
monoxide gave the cycloadducts 2b and 2c in 87 and 96%
isolated yields, respectively (Table 2, entries 1 and 2). The
yields are similar to that obtained from 1a. However, when
2-octyne (1d), 1-phenyl-2-octyne (1e), and 1-(4-ClC6H4)-2-
octyne (1 f) were employed, the two regioisomers of p-ben-
zoquinones were isolated in relatively low total yields
(Table 2, entries 3–5).
could not take place under the same reaction conditions.
The formation of other carbonylated products was not ob-
served either and diarylacetylene was recovered completely.
Aryl and alkyl-substituted asymmetrical internal alkynes
also reacted efficiently to produce the corresponding p-ben-
Scheme 3.
Chem. Eur. J. 2007, 13, 8333 – 8337
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www.chemeurj.org
8335

R. Hua and Q. Huang
Moreover, the present Rh(I)-catalyzed cyclocarbonylative
addition conditions are not effective for both terminal al-
kynes and propiolate derivatives. In the case of terminal al-
kynes employed, the reaction gave a mixture of (cyclo)trim-
ers. The reaction of propiolate derivatives resulted in the
formation of the cyclotrimer of alkynes as the major prod-
uct, accompanied with the formation of dimers confirmed
by GCMS analysis.
On the basis of the previous reports on the formation of
maleoylmetal complexes from alkynes and two molecules of
CO,[15] and p-benzoquinones from the reaction of maleoyl-
metal complexes with alkynes,[8] a proposed mechanism in-
volving the formation of rhodacycle intermediates is depict-
ed in Scheme 4. At first, the reaction of rhodium(I) dimer A
Conclusion
We have developed an efficient catalyst system to realize
the straightforward synthesis of tetrasubstituted benzoqui-
nones in modest to high yields by [2+2+1+1] cyclocarbony-
lative coupling of internal alkynes with CO in the presence
of [RhCl(CO)2]2. The present research work has extended
the utility of the cyclocarbonylative coupling reaction of al-
kynes with CO to provide a facile and atom-economic
method for synthesizing various tetrasubstituted p-benzoqui-
nones, some of which are not easily prepared by the tradi-
tional synthetic method.
Experimental Section
A representative procedure for the [2+2+1+1] cyclocarbonylative cou-
pling of 4-octyne (1a) with CO
Formation of 2,3,5,6-tetra-n-propyl-1,4-benzoquinone (2a): A mixture of
4-octyne (110.0 mg, 1.0 mmol) and [RhCl(CO)2]2 (9.8 mg, 0.025 mmol) in
1,1,2,2-tetrachloroethane (1.0 mL) was bubbled with carbon monoxide
and nitrogen (CO/N2 =ca. 1:1, ꢀ1.5–2.0 mLminÀ1) with stirring at room
temperature for 5 min. The resulting yellow solution was then refluxed
with continued bubbling of the gas mixture (oil bath temperature was
1408C) for 20 h. After GC and GCMS analyses of the reaction mixture,
volatiles were removed under a reduced pressure and the residue was
subjected to silica-gel column chromatography (eluting with cyclohexane
and then with a diethyl ether/hexane mixture (3ꢀ5:100)) to afford 2a
(124.1 mg, 90%) as a yellow viscous oil. GC analyses of the reaction mix-
ture disclosed the formation of 2a in 94% yield.
Acknowledgements
This project (20473043) was supported by the National Natural Science
Foundation of China.
Scheme 4. Proposed mechanism for [2+2+1+1] cyclocarbonylative cou-
pling of an internal alkyne and carbon monoxide.
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with an alkyne gives the alkyne-coordinating intermediate
B. The oxidative coupling of the coordinated alkyne and CO
to the rhodium generates a five-membered rhodacycle C,
followed by insertion of the alkyne into the rhodium–carbon
bond to form a seven-membered rhodacycle D. The carbon–
carbon bond formation by the reductive elimination fur-
nishes benzoquinone 2 and regenerates intermediate B.
From our present investigation, it is apparent that the cat-
alytic formation of p-benzoquinones is sensitive to the con-
centration of CO in the reaction, and the most important
factor for high chemoselective formation of p-benzoqui-
nones is to perform the cyclocarbonylation reaction under a
low concentration of CO. It is likely that over a certain
value of CO concentration, CO competes with the alkyne
for coordination to the rhodium center to restrain the for-
mation of intermediates B and/or D.
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Rhodium Catalysis
FULL PAPER
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(cod)(PPhMe2)] (cod=1,5-cyclooctadiene) did not give 2a at all.
A
ACHTREUNG
A
ACHTREUNG
1
[13] The structures of two regioisomers could be confirmed clearly by H
and 13C NMR spectroscopy, see the Supporting Information.
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Received: June 3, 2007
Published online: July 19, 2007
Chem. Eur. J. 2007, 13, 8333 – 8337
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8337