Highly chemoselective palladium-catalyzed cross-trimerization between alkyne and alkenes leading to 1,3,5-trienes or 1,2,4,5-tetrasubstituted benzenes with dioxygen

Article abstract of DOI:10.1021/jo101554r

A palladium-catalyzed 1:2 linear/cyclic cross-trimerization of alkyne with alkenes using molecular oxygen as the sole oxidant has been reported for the first time. A series of 1,3,5-trienes and 1,2,4,5-tetrasubstituted benzenes are obtained with high chemoselectivity respectively and mechanisms of these reactions are proposed based on some controlled examination.

Full text of DOI:10.1021/jo101554r

pubs.acs.org/joc
molecular oxygen as oxidant. However, it still remains a
Highly Chemoselective Palladium-Catalyzed
Cross-Trimerization between Alkyne and Alkenes
Leading to 1,3,5-Trienes or 1,2,4,5-Tetrasubstituted
Benzenes with Dioxygen
challenge to combine various factors of green chemistry with
Pd-catalyzed coupling reactions.
Chloropalladation of alkynes is well-recognized to be one
of the most efficient and atom economic methods for the
construction of both the carbon-carbon and carbon-halide
bonds.6 Among its application, a majority of research was
concerned about the capture of σ-vinylpalladium intermedi-
ate with alkynes, alkenes, allyl halides, and carbon monoxide
in which taking alkenes to carry out a tandem Heck cross-
coupling has been proven to be an extremely important
and convenient method for the synthesis of 1-chloro-1,3-
tetradiene.7 However, in all these systems, the activation of
the C-Cl σ-bond in vinyl chloride received little attention,
which were inert in the presence of the oxidant like cupric
salts, BQ, and etc., probably due to the efficient oxidation of
Pd(0) to Pd(II). However, a weaker oxidant may allow a
smaller TON of Pd(0) oxidation, thus sustaining Pd(0) for
another Heck catalytic cycle (Scheme 1). In this context, oxygen
was found to be a perfect candidate for its medium oxidation
ability toward Pd(0).8 Moreover, the use of oxygen may elim-
inate the byproducts from traditional oxidants in a Pd catalytic
system such as superfluous Cu(II) salts, Ag(I) salts, and
phenyliodine(III) diacetate. On the basis of our interest in oxygen
and chloropalladation chemistry on triple bonds,9 we herein
disclose a highly chemoselective palladium-catalyzed oxidative
cross-trimerization of one alkyne with two alkenes to 1,3,5-
trienes or 1,2,4,5-tetrasubstituted benzenes which both are very
useful in the field of pharmaceutical and materials chemistry.10
In an initial attempt, ethyl phenylpropiolate (1a) was used
as a model compound for the reaction with methyl acrylate
(2a) to screen the reaction conditions (see the Supporting
Information). Under the optimal conditions (10 mol %
PdCl2, 120 mol % LiCl, 150 mol % K2CO3 in acetonitrile
at 75 °C for 18 h under 1 atm O2) the cross-trimerization of 1a
Peng Zhou, Liangbin Huang, Huanfeng Jiang,*
Azhong Wang, and Xianwei Li
School of Chemistry and Chemical Engineering, South China
University of Technology, Guangzhou 510640,
People’s Republic of China
jianghf@scut.edu.cn
Received August 8, 2010
A palladium-catalyzed 1:2 linear/cyclic cross-trimerization
of alkyne with alkenes using molecular oxygen as the
sole oxidant has been reported for the first time. A series
of 1,3,5-trienes and 1,2,4,5-tetrasubstituted benzenes are
obtained with high chemoselectivity respectively and
mechanisms of these reactions are proposed based on
some controlled examination.
Palladium-catalyzed processes for carbon-carbon bond
formation have proven to be a powerful and useful tool for
the construction of natural products and complicated com-
pounds.1 Although classic reactions such as Sonogashira,2
Suzuki,3 Stille,4 and Heck cross-coupling5 have been devel-
oped and widely used in organic synthesis, from the view-
point of green chemistry, it is necessary to develop more
atom economic and environmental benign methodologies to
construct C-C bonds. To address this issue, a lot of effort
has been made to achieve the sequential reaction system with
(6) For a review on chloropalladation, see: Lu, X. Handbook of
Organopalladium Chemistry for Organic Synthesis; Negishi, E., Ed.; Wiley-
Interscience: New York, 2002; Vol. 2, pp 2267-2288.
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Tan, R.; Song, D.; Dong, V. M. Angew. Chem., Int. Ed. 2009, 48, 9690–9692.
(f ) Yin, G.; Liu, G. Angew. Chem., Int. Ed. 2008, 47, 5442–5445.
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2004, 43, 3400–3420. (b) Punniyamurthy, T.; Velusamy, S.; Lqbal, J. Chem.
Rev. 2005, 105, 2329. For selected recent palladium-catalyzed examples, see:
(c) Shi, Z. Z.; Zhang, C.; Li, S.; Pan, D. L.; Ding, S. T.; Cui, Y. X.; Jiao, N.
Angew. Chem., Int. Ed. 2009, 48, 4572–4576. (d) Aouf, C.; Thiery, E.; Bras,
J. L.; Muzart, J. Org. Lett. 2009, 11, 4096–4099. (e) Stahl, S. S. Science 2005,
309, 1824–1826. (f ) Li, B.; Tian, S.; Fang, Z.; Shi, Z. Angew. Chem., Int. Ed.
2008, 47, 1115–1118. (g) Wang, D.; Wasa, M.; Giri, R.; Yu, J. J. Am. Chem.
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(e) Shen, Y. X.; Jiang, H. F.; Chen, Z. W. J. Org. Chem. 2010, 75, 1321–1324.
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38, 674–677. (b) Horiguchi, H.; Hirano, K.; Satoh, T.; Miura, M. Adv. Synth.
Catal. 2009, 351, 1431–1436. (c) Saltiel, J.; Papadimitriou, D.; Krishna,
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(1) For selected reviews on C-C bond formation, see: (a) Corbet, J.;
Mignani, G. Chem. Rev. 2006, 106, 2651–2710. (b) Zeni, G.; Larock, R. C.
Chem. Rev. 2004, 104, 2285–2310.
(2) For reviews on Sonogashira cross-coupling, see: (a) Chinchilla, R.;
Najera, C. Chem. Rev. 2007, 107, 874–922. (b) Negishi, E.; Anastasia, L.
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A. Chem. Rev. 1995, 95, 2457–2483. (b) Kotha, S.; Lahiri, K.; Kashinath, D.
Tetrahedron 2002, 58, 9633–9695.
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Sevignon, M.; Gozzi, C.; Schulz, E.; Lemaire, M. Chem. Rev. 2002, 102,
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DOI: 10.1021/jo101554r
r
Published on Web 11/03/2010
J. Org. Chem. 2010, 75, 8279–8282 8279
2010 American Chemical Society

Zhou et al.
JOCNote
SCHEME 1. (a) Direct Chloropalladation of Alkyne and Heck
Cross-Coupling with One Alkene with Pd/Cu(II) and (b) Cross-
Trimerization of One Alkyne with Two Alkenes to 1,3,5-Triene or
1,2,4,5-Tetrasubstituted Benzene by One Pot with Pd/O2
TABLE 1. Pd-Catalyzed Linear Cross-Trimerization between Alkynes
and Alkenes to 1,3,5-Trienes with Oxygena
(0.5 mmol) with 2a (1.25 mmol) gave the corresponding
product linear E-3aa in 90% yield (Table 1, entry 1). The
1
structure of 3aa was determined by H NMR, 13C NMR,
MS, and NOE. Treated with the same conditions, 1a could
react with ethyl acrylate and water solvable 2-hydroxyethyl
acrylate very well (Table 1, entries 2 and 3). Only the E
isomer of 3ac was obtained as polar acrylates were used.
When the reaction was run with aromatic ynones, the
corresponding 1,3,5-trienes 3bd-3fd were also obtained in
high yields (Table 1, entries 4-8). Interestingly, the elec-
tronic effect of the substituted group on the aromatic ring did
not affect the reactivity and selectivity of the reaction. When
ynone 1g was employed, the reaction system was complex due
to the byproduct derived from the Heck cross-coupling of
4-bromobenzene with the acrylate (Table 1, entry 9). Addi-
tionally, ynone 1d could react with 2c in good yield (Table 1,
entry 10). Surprisingly, to the alkynes with double electron-
withdrawing groups such as diethyl but-2-ynedioate (1h) and
dimethyl but-2-ynedioate (1i), few linear products were ob-
tained but with good yields of tetrasubstituted benzenes.
Stimulated by the cycloproducts of alkynes with double
electron-withdrawing groups and in view of the special
structure of these trienes, we speculated that other 1,3,5-
trienes could be cyclized too. To our delight, the benzene
ring (4aa) under consideration was obtained when 3aa
was treated at higher temperature under typical conditions.
Encouraged by this result and the comparability of the linear
cross-trimerization’s conditions, we tried to carry out the
cyclotrimerization in one pot. After screening different cat-
alysts, additives, solvents, and temperature, the reaction of
1a with 2a afforded 4aa in high yield in the presence of PdCl2
(10 mol %) and K2CO3 (40 mol %) in CH3CN (2 mL) and
CH3COOH (0.3 mL) under 1 atm of O2. With the optimum
reaction condition in hand, we then investigated the scope of
this transformationby differentalkynes and alkenes(Table2).
In the scope of alkynes, substrates 1h-i bearing two electron-
withdrawing groups (Table 2, entries 2 and 3) are more effi-
cient than their analogues with only one electron-withdrawing
group (Table 2, entry 1) in this cyclization upon comparison
of their respective product yields and reaction temperature.
This cyclization is also very suitable for other acrylates with
alterations of their substituents with ethyl, n-hexyl, water
solvable 2-hydroxyethyl, and long-chain alkyl (Table 2, en-
tries 5-8). Interestingly, styrene which was usually inert in
chloropalladation was also efficient in this system (Table 2,
entry 9). However, in the case of 1-hexylene (2h), the con-
sidered benzene ring, even the linear 1,3,5-triene was not
aAll the reactions of 1 (0.5 mmol) with 2 (2.5 equiv) were carried out
in the presence of PdCl2 (10 mol %), LiCl (1.2 equiv), and K2CO3
(1.5 equiv) in CH3CN (4 mL) under 1 atm of oxygen at 75 °C for 18 h.
bIsolated yield. And the ratio of E/Z was determined by 1H NMR.
cWithout designed product.
detected because of the easy trimerization of 2h itself
(Table 2, entry 10).11
To better understand this methodology, we executed some
other examinations for the potential mechanism (Scheme 2).
(11) Jiang, H. F.; Shen, Y. X.; Wang, Z. Y. Tetrahedron Lett. 2007, 48,
7542–7545.
8280 J. Org. Chem. Vol. 75, No. 23, 2010

Zhou et al.
JOCNote
TABLE 2. Pd-Catalyzed Cross-Cyclotrimerization between Alkynes
and Alkenes to 1,2,4,5-Tetrasubstituted Benzenes with Oxygena
SCHEME 2. Controlled Examinations for the Mechanism
SCHEME 3. The Possible Reaction Mechanism
treated with 2a under the terms of linear cross-trimerization
without oxygen to obtain 3aa in GC yield of 41%. As we
assume, 3aa was transferred into the corresponding benzene
ring 4aa under the condition of cross-cyclotrimerization. On
the basis of the above experimentations, we propose the
mechanism of these transformations may come by the combi-
nation of three steps: (1) trans-chloro-palladation of alkyne
and a Heck cross-coupling with alkene to chloro-substituted
1,3-diene, (2) another Heck cross-coupling with alkene
through a β-H elimination to 1,3,5-trienes,12 and (3) isomer-
ization of (1E,3E,5E)-trienes to (1E,3Z,5E)-trienes with
higher temperature13 and then palladium-catalyzed cycliza-
tion of trienes to afford benzenes. In the above three steps,
oxygen was needed by steps 1 and 3, which can oxidize Pd(0)
to Pd(II) (Scheme 3).
In summary, we have carried out a palladium-catalyzed
1:2 linear/cyclic cross-trimerization of alkyne with alkenes
taking molecular oxygen as the sole oxidant for the first time.
The reaction proceeds efficiently and a series of 1,3,5-trienes
and 1,2,4,5-tetrasubstituted benzenes have been synthesized
in good yields through this green and atom-economic route
by one pot. Otherwise, mechanisms of these reactions are
proposed based on some controlled examinations. Future
work will focus on expanding the scope, exploring mecha-
nisms all-around, and applying the method to the field of
natural products.
aAll reactions of 1 (0.5 mmol) with 2 (2.5 equiv) were performed in a
mixture of CH3CN (4 mL) and CH3COOH (0.3 mL) in the presence of
PdCl2 (10 mol %) and K2CO3 (40 mol %) under 1 atm of oxygen at 75 °C
for 24 h, unless otherwise noted. bIsolated yield. cAt 100 °C for 24 h.
By analyzing the results from GC-MS, we found as a trace
byproduct chloro-substituted 1,3-diene (A), which may be a
intermediate. Prompted by this detection, we carried out a
chloropalladation/Heck cross-coupling using 1a and 2a with
the system of PdCl2 (10 mol %), LiCl (1.2 equiv), CH3CN
(2 mL), and oxygen (8 atm). And chloro-substituted 1,3-diene
(A) was obtained in the yield of 83% after 12 h. Then, A was
(12) For selected examples on σ-vinylhalorine activation catalyzed by
palladium, see: (a) Hua, R.; Tanaka, M. New J. Chem. 2001, 25, 179–184.
(b) Larock, R. C.; Doty, M. J.; Han, X. J. Org. Chem. 1999, 64, 8770–8779.
(c) Larock, R. C.; Yum, E. K.; Doty, M. J.; Sham, K. K. C. J. Org. Chem.
1995, 60, 3270–3271.
(13) For some examples on the Z/E isomerization of alkene, see: (a) Yu,
J.; Gaunt, M. J.; Spencer, J. B. J. Org. Chem. 2002, 67, 4627–4629.
(b) Umeda, N.; Hirano, K.; Satoh, T.; Miura, M. J. Org. Chem. 2009, 74,
7094–7099.
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Zhou et al.
JOCNote
Experimental Section
purged with O2 three times, followed by the addition of methyl
acrylate (107 mg, 1.25 mmol), CH3CN (3 mL), and CH3COOH
(0.3 mL). The formed mixture was gradually heated from rt to
the corresponding temperature under O2 (1 atm kept with a
balloon) and stirred for 18 h as monitored by TLC. The solu-
tion was then cooled to rt, diluted with ethyl acetate (30 mL),
washed with H2O (3 ꢀ 10 mL), dried over MgSO4, filtered, and
evaporated under vacuum. The crude product was purified by
TLC on silica gel to afford colorless oil 4aa. 1H NMR (CDCl3,
400 MHz) δ 8.23 (s, 1H), 7.69 (s, 1H), 7.39-7.42 (m, 3H),
7.30-7.33 (m, 2H), 4.13 (q, J = 6.6 Hz, 2H), 3.93 (s, 3H), 3.95 (s,
3H), 1.02 (t, J = 5.3 Hz, 3H); 13C NMR (CDCl3, 100 MHz) δ
167.6, 167.1, 166.6, 145.4, 133.3, 130.9, 130.7, 129.7, 128.2,
128.2, 128.2, 128.1, 61.5, 52.9, 52.8, 13.6; MS (EI, 70 eV) m/z
(%) 342 (Mþ, 20), 311(30), 297 (35), 283 (20), 152 (35), 77 (5), 28
(100). Anal. Calcd for C19H18O6: C, 66.66; H, 5.30. Found: C,
66.71; H, 5.29.
Typical Procedure for the Synthesis of (1E,3E,5E)-3-Ethyl-
1,6-dimethyl 4-Phenylhexa-1,3,5-triene-1,3,6-tricarboxylate (3aa).
To a 25 mL round-bottomed flask with condensator were
added PdCl2 (8.8 mg, 10 mol %), LiCl (25 mg, 1.2 equiv), K2CO3
(105 mg, 1.5 equiv), and ethyl phenylpropiolate (1a) (87 mg,
0.5 mmol). Then, the flask was purged with O2 three times, followed
by addition of methyl acrylate (2a) (107 mg, 1.25 mmol) and
CH3CN (3 mL). The formed mixture was gradually heated from
rt to 75 °C under O2 (1 atm kept with a balloon) and stirred for
18 h as monitored by TLC. The solution was then cooled to rt,
diluted with ethyl acetate (30 mL), washed with H2O (3 ꢀ 10 mL),
dried over MgSO4, filtered, and evaporated under vacuum. The
crude product was purified by TLC on silica gel to afford
colorless oil 3aa, whose structure was determined by 1H
NMR, 13C NMR, MS, and NOE and compared with literature
data. 1H NMR (CDCl3, 400 MHz) δ 7.64 (d, J = 12 Hz, 1H),
7.40-7.42 (m, 3H), 7.12-7.14 (m, 2H), 7.06 (d, J = 16 Hz, 1H),
5.98 (d, J = 16 Hz, 1H), 5.61 (d, J = 16 Hz, 1H), 4.44 (q, J =
Acknowledgment. The authors are grateful to the National
Natural Science Foundation of China (Nos. 20625205,
20772034, and 20932002), National Basic Research Program
of China (973 Program) (No. 2011CB808603), Doctoral
Fund of Ministry of Education of China (20090172110014),
and Guangdong Natural Science Foundation (1035106410-
1000000) for financial support.
8 Hz, 2H), 3.67 (d, J = 16 Hz, 6H), 1.42 (t, J = 8 Hz, 3H); 13
C
NMR (CDCl3, 100 MHz) δ 166.4, 166, 144.6, 141.6, 137.8,
135.9, 133.8, 129.1, 128.5, 128.3, 126.4, 122.6, 61.6, 51.3, 51.2,
13.8; MS (EI, 70 eV) m/z (%) 344 (Mþ, 1), 285 (30), 253 (19), 239
(50), 180 (30), 151, (44), 77 (8), 58 (18), 27 (100). Anal. Calcd for
C19H20O6: C, 66.27; H, 5.85. Found: C, 66.23; H, 5.88.
Typical Procedure for the Synthesis of [1,10-Biphenyl]-2,4,5-
tricarboxylic Acid, 2-Ethyl-4,5-dimethyl Ester (4aa). To a 25 mL
round-bottomed flask with condensator were added PdCl2
(8.8 mg, 10 mol %), K2CO3 (28 mg, 40 mol %), and ethyl
phenylpropiolate (1a) (87 mg, 0.5 mmol). Then, the flask was
Supporting Information Available: Full experimental details
and copies of NMR spectral data. This material is available free
of charge via the Internet at http://pubs.acs.org.
8282 J. Org. Chem. Vol. 75, No. 23, 2010