Carbon dioxide promoted palladium-catalyzed cyclotrimerization of alkynes in water

Article abstract of DOI:10.1081/SCC-120030761

In water, CO₂ was found to promote the palladium-catalyzed cyclotrimerization of alkynes. In the presence of PdCl₂, CuCl ₂, and CO₂, both aryl and alkylalkynes afforded the corresponding cyclotrimerization products regioselectively in high yields. However, tert-butylacetylene bearing a bulk group gave a dimerization product.

Full text of DOI:10.1081/SCC-120030761

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Carbon Dioxide Promoted Palladium‐Catalyzed
Cyclotrimerization of Alkynes in Water
Jin‐Heng Li a & Ye‐Xiang Xie a
a Institute of Fine Catalysis and Synthesis, College of Chemistry and Chemical
Engineering, Hunan Normal University, Changsha, 410081, China
Published online: 16 Aug 2006.
To cite this article: Jin‐Heng Li & Ye‐Xiang Xie (2004): Carbon Dioxide Promoted Palladium‐Catalyzed Cyclotrimerization
of Alkynes in Water, Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic
Chemistry, 34:9, 1737-1743
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SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 9, pp. 1737–1743, 2004
Carbon Dioxide Promoted
Palladium-Catalyzed Cyclotrimerization
of Alkynes in Water
*
Jin-Heng Li and Ye-Xiang Xie
Institute of Fine Catalysis and Synthesis, College of Chemistry and
Chemical Engineering, Hunan Normal University, Changsha, China
ABSTRACT
In water, CO2 was found to promote the palladium-catalyzed cyclotrimer-
ization of alkynes. In the presence of PdCl2, CuCl2, and CO2, both aryl
and alkylalkynes afforded the corresponding cyclotrimerization products
regioselectively in high yields. However, tert-butylacetylene bearing a
bulk group gave a dimerization product.
Key Words: Cyclotrimerization; Palladium-catalyzed; Alkynes;
Regioselectivity; Carbon dioxide.
*
Correspondence: Jin-Heng Li, Institute of Fine Catalysis and Synthesis, College of
Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081,
China; Fax: 86731-8872-226; E-mail: jinhengli@163.net.
1737
DOI: 10.1081/SCC-120030761
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

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Li and Xie
INTRODUCTION
Transition metal mediated [2 þ 2 þ 2] cyclotrimerization of alkynes has
been an area of extensive investigation, and has been utilized efficiently in
the synthesis of aromatic compounds and many natural products.[1–8]
Recently, we reported a novel palladium-catalyzed cyclotrimerization of
alkynes, which is useful for the regioselective synthesis of symmetric benzene
derivatives in C6H6/n-BuOH.[8] Unfortunately, phenylacetylene did not
undergo cyclotrimerization reaction and only red oils were obtained. On the
other hand, harmful solvents such as benzene or dichloromethane were used
in all case. Thus, development of new and environmentally benign methods
for palladium-catalyzed cyclotrimerization of alkynes is still interesting. In
this paper, we reported our new results of palladium-catalyzed cyclotrimeriza-
tion of alkynes, which were promoted by CO2 in water (Sch. 1 and Table 1).
Our recent studies[8–11] shown that the solvent effects played an import-
ant role in influencing selectivity (chemo-, stereo-, and regio-selectivity) of
the Pd(II)-catalyzed process. Indeed, we found that cyclotrimerization of
phenylacetylene occurred regioselectively by altering the reaction solvent
form C6H6/n-BuOH to water in the presence of PdCl2 and CuCl2, although
yield of the corresponding cyclotrimerization product 2a is low (1,3,5-
triphenylbenzene, 31%) and some red oils were still obtained (entry 2 in
Table 1). The cyclotrimerization could occur in the absence of CuCl2, but
the yield of 2a was decreased to 7% and a large amount of red oils were
obtained (entry 1). Very interestingly, we found that the presence of CO2
could enhance yields of the palladium-catalyzed cyclotrimerization of alkynes
(the effect of CO2 has already displayed in the application of CO2 as either a
reaction material[12–14] or reaction media.[15–17] In the 1.0 MPa of CO2, the
yield of 2a was increased sharply to 90% in the presence of PdCl2 and
CuCl2 in water (Entry 3).
Under the above optimum reaction conditions, cyclotrimerization of other
alkynes was carried out smoothly and excellent yields were obtained. The
;;
cyclotrimerization of p-MeC6H4C CH, C5H11C CH, C3H7C CC3H7
afforded symmetric benzenes 2b, 2c, and 2d, respectively (Entries 4, 6, and 8),
;;
;;
Scheme 1.

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Palladium Catalyzed Cyclotrimerization of Alkynes
1739
Table 1. Palladium-catalyzed cyclotrimerization of
alkynes under CO2 in water.a
Isolated
yield (%)
Entry
R
R0
1b
2c,d
3
Ph
Ph
Ph
H
7 (2a) þ c
31 (2a) þ c
90 (2a)
95 (2b)
91 (3)
H
H
4
p-MeC6H4
Ph
H
5
Me
H
6
C5H11
C5H11
C3H7
87 (2c)
c
99 (2d)
7e
8
H
C3H7
aReaction conditions: alkyne 1 (1 mmol), PdCl2 (5 mol%),
CuCl2 (2 mmol), CO2 (1.0 MPa), and H2O (5 mL) at room
temperature for 24 hr.
bIn the absence of CuCl2 and CO2.
cThe products were red oils and unidentified.
dIn the absence of CO2.
eIn the absence of CuCl2.
;;
but PhC CCH3 gave a unsymmetrical benzene 3 (1,3,5-triphenyl-2,4,6-
trimethylbenzene, Entry 5). CuCl2 is still crucial in cyclotrimerization of alkyl-
[8]
;;
acetylene (C5H11C CH). In its absence, only red oils were obtained
(entry 7). However, tert-butylacetylene, a bulky alkyne, was dimerized to
afford 4 (1,4-di(tert-butyl)but-1,3-diyne) in 98% yield (Sch. 2).
Based upon the above results, several points should be noteworthy: (1)
compared with our previous results,[8] the effect of solvents on the selectivity
of the reaction is obvious. In C6H6/alcohol, phenylacetylene did not cyclotri-
merize and tert-butylacetylene gave 1,3,5-tri(tert-butyl)benzene, whereas in
water, phenylacetylene was cyclotrimerized to 1,3,5-triphenylbenzene and
tert-butylacetylene was dimerized to 1,4-di(tert-butyl)but-1,3-diyne. (2) The
presence of CO2 can increase yields of the reaction. In its presence, the
yield of 1,3,5-triphenyl-2,4,6-trimethylbenzene was increased from 31% to
Scheme 2.

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1740
Li and Xie
90%. However, the effect of CO2 on the reaction is very difficult to under-
stand. (3) The structure of alkynes influences both the chemo- and regioselec-
tivity of the reaction to some extent. (4) CuCl2 can also affect the yields of the
reaction.[8]
A possible mechanism was proposed as shown in Sch. 3.[1–8] In water,
some of PdCl2 generally change to ClPd(OH) species (Eq. 1),[4] which
leads to decreasing complex of catalysts with alkynes to give the desired pro-
ducts 2. Trace of 2 was obtained in the absence of CuCl2 and CO2 due to
poor solubility of the catalysts in H2O. Addition of CuCl2 could not only
regenerate the palladium species in situ but also increase the solubility of cat-
alysts (Eq. 2). Both the reaction of CO2 with PdCl2 and H2O might be occurred
to form ClPdHCO3 (Eq. 3)[12] to decrease the complex of the catalysts with
alkynes resulting in shifting the chemoselectivity to the cyclotrimerization
reaction. Another increased yields of the reaction might be explained in
terms of acidity of H2CO3 from the reaction of CO2 with water. A dimeriza-
Scheme 3.

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Palladium Catalyzed Cyclotrimerization of Alkynes
1741
tion product was obtained in the cyclotrimerization of tert-butylacetylene due
to the steric hindrance.
Although further investigation is needed to settle the mechanism, the
present procedure provides a new general and environmentally benign route
to synthesize some benzene derivatives.
EXPERIMENTAL
1
All H spectra were recorded at 400 MHz with CDCl3 as solvent. TLC
was performed using commercially prepared 100–400 mesh silica gel plates
(HF254) and visualization was effected at 254 nm. All reagents were used
directly as obtained commercially.
A Typical Procedure for the Cyclotrimerization of Alkynes
Alkyne 1 (1 mmol) was added to a mixture of PdCl2 (5 mol%), CuCl2
(2 mmol), and H2O (5 mL) in HF-25 autoclave. Liquid CO2 was then trans-
ferred into the autoclave to 1.0 MPa. The reaction mixture was stirred at
room temperature for 24 hr. After the gas was vented, the residue was
extracted (ether), dried (anhydrous Na2SO4), and purified by preparative
TLC on silica gel (light petroleum–ethyl ether). Under the above reaction
conditions, alkyne 1 was consumed completely after 24 hr, which was detected
by GC analyses.
Caution: Be careful in working at high pressure.
ACKNOWLEDGMENTS
We thank the National Natural Science Foundation of China (No.
20202002) and Hunan Normal University for financial support.
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Received in Japan October 29, 2003

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