Palladium-catalyzed cleavage reaction of carbon-carbon triple bond with molecular oxygen promoted by lewis acid

Article abstract of DOI:10.1021/ja8002217

A new Lewis acid promoted and palladium-catalyzed cleavage reaction of carbon-carbon triple bond with molecular oxygen was reported, in which alkyne or 1.3-diynes is split into carboxylic ester in various alcohols. Copyright

Full text of DOI:10.1021/ja8002217

Published on Web 03/22/2008
Palladium-Catalyzed Cleavage Reaction of Carbon-Carbon
Triple Bond with Molecular Oxygen Promoted by Lewis Acid
Azhong Wang and Huanfeng Jiang*
The College of Chemistry, South China UniVersity of Technology, Guangzhou 510640, P. R. China
Received January 10, 2008; E-mail: jianghf@scut.edu.cn
Transition-metal-catalyzed cleavage reactions of carbon-carbon
Table 1. Optimization of Reaction Conditions for the Palladium-
a
Catalyzed Triple Bond Cleavage Reaction
bonds have been recognized as powerful tools in organic trans-
1
formations. Various unique and useful catalytic processes involv-
ing C-C single and double bonds cleavage have recently been
2
developed. What has remained unexplored is the cleavage of C-C
b
yield (%)
triple bond, which is one of the most challenging subjects in modern
entry
O
2
(atm)
catalyst
Lewis acid
temp (°C)
3
c
synthetic organic chemistry. Most studies on alkyne cleavage have
1
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
100
100
100
100
100
100
100
100
60
100
100
100
100
0
3
2
3
4
5
6
7
8
7.5
focused on stoichiometric organometallic reactions, such as alkyne-
4
5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
ZnCl2 ·2H2O
CuCl2 ·2H2O
FeCl ·6H O
90(88)
43
72
77
73
0
43
80
83
77
89
ligand scission on metal complexes and oxidative cleavage.
Examples for the metal-catalyzed C-C triple bond cleavage
3
2
6
MnCl2 ·4H2O
AlCl3 ·6H2O
ZnCl2 ·2H2O
ZnCl2 ·2H2O
ZnCl2 ·2H2O
ZnCl2 ·2H2O
ZnCl2 ·2H2O
ZnCl2
reactions are rare, except for metathesis of alkynes. Jun et al.
reported catalytic C-C triple bonds cleavage through the rhodium-
7
catalyzed hydroimino-acylation. Yamamoto reported the cleavage
9
10
Pd(OAc)2
PdCl2
Pd2(dba)3
Pd(OAc)2
Pd(OAc)2
8
of diynes via ruthenium-catalyzed hydroamination. Liu et al.
1
1
1
1
2
3
reported catalytic cleavage of alkynes in which ethynyl alcohol is
d
9
split into alkene and CO by ruthenium complex. Very recently,
Liu et al. reported the gold-catalyzed cleavage of C-C triple bonds
10
a
in (Z)-enynols with molecular oxygen. However, there have been
Reaction conditions: All reactions were performed with 1,2-diphenyl-
ethyne (1 mmol), palladium catalyst (2 mol %) and Lewis acid (20 mol %)
in 2 mL of MeOH for 24 h. Determined by GC. Number in parentheses is
isolated yield. Under N2 atmosphere. With 10 mol % of ZnCl2 ·2H2O.
no reports in the literature concerning palladium-catalyzed cleavage
b
11,12
of C-C triple bond using molecular oxygen.
Herein, we
c
d
describe a new Lewis acid promoted and palladium-catalyzed
cleavage reaction with molecular oxygen in which alkyne is split
into carboxylic ester in various alcohols.
Scheme 1. Cleavage Reaction of 1,3-Diynesx
Initially, we employed palladium-catalyzed cleavage reaction of
1,2-diphenylethyne (1a) in MeOH at 100 °C as a model for
discovery of optimized conditions. As summarized in Table 1, it
was found that without oxygen the reaction did not occur (entry
1). A small amount of benzoate 2a, an oxidative cleavage product,
was observed when 7.5 atmospheric pressure of O2 was used as
the oxidant (entry 2). Although the conversion was quite low, it
was promising since the result indicated the possibility of catalyzing
C-C triple bonds cleavage via palladium-mediated reactions.
Further exploration led to a discovery that 88% isolated yield of
reaction. 1,2-Di-p-tolylethyne provided the desired product 2b in
excellent yield (Table 2, entry 1). The change of methanol to
ethanol or n-propyl alcohol did not alter the course of the reaction
2a was afforded from 1a if ZnCl2 ·2H2O is employed as the additive
(entries 2, 3). A notable exception was observed when using oct-
and O2 as the oxidant in MeOH (entry 3). We examined several
different Lewis acids for the cleavage reaction. ZnCl2 ·2H2O was
found to be the most effective one (entry 3). Other Lewis acids
such as CuCl2 ·2H2O, FeCl3 ·6H2O, MnCl2 ·4H2O, and AlCl3 ·
4
-yne as substrate (entry 4); only 18% yield of the cleavage product
was obtained, along with the byproduct from cyclotrimerization
of oct-4-yne. Unsymmetrical internal alkynes were also included
in this study. Compared to the symmetrical internal alkynes, the
unsymmetrical internal alkynes were cleaved to two different
products (entries 5–9), except for ethyl 3-phenylpropiolate (entry
6
H2O, were substantially less effective (entries 4–7). The reaction
provided no conversion without palladium catalyst (entry 8). Upon
decreasing the temperature to 60 °C, lower yield was obtained
10). The cleavage reaction of terminal alkynes afforded the
(entry 9). Different palladium species were also tested. Both PdCl2
corresponding products in good yields (entries 11, 12). Finally,
we carried out the cleavage reaction from 1,3-diynes. Under the
optimized conditions, the cleavage reaction of 1,4-diphenylbuta-
and Pd2(dba)3 had a comparable catalytic reactivity for this
transformation (entries 10, 11). When the dosage of ZnCl2 ·2H2O
was reduced to 10 mol %, lower yield was obtained (entry 12).
Anhydrous ZnCl2 was also found effective for the cleavage reaction
1
2
,3-diyne and hexadeca-7,9-diyne provided the desired products
a, and 3e in 83 and 77% yields, respectively (Scheme 1).
Although we are unable to determine the detailed pathway of
(entry 13).
Encouraged by the ease of this reaction, we next focused on
expanding the scope of the methodology. Table 2 highlights the
broad range of the various alkynes that can be used in the cleavage
the oxidative cleavage at present, on the basis of 1-methoxy-1,2-
diphenylethene A detected from GC-MS, we do propose A as
the key intermediate which undergoes the carbon-carbon cleavage
5
030 9 J. AM. CHEM. SOC. 2008, 130, 5030–5031
10.1021/ja8002217 CCC: $40.75  2008 American Chemical Society

C O M M U N I C A T I O N S
a
Table 2. Palladium-Catalyzed Triple Bond Cleavage Reaction
nism, and its applications in organic synthesis is ongoing in our
laboratory and will be reported in due course.
Acknowledgment. We thank the National Natural Foundation
of China (Nos. 20332030, 20572027, 20625205, and 20772034)
and Guangdong Natural Science Foundation (No. 07118070)
for financial support.
Supporting Information Available: Experimental procedures for
a-k, A, and characterization of compound 2a-e, 3e-h. This material
is available free of charge via the Internet at http://pubs.acs.org.
1
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Reaction conditions: alkyne (1 mmol), Pd(OAc)2 (2 mol %), ZnCl2 ·
H2O (20 mol %), pressure of O2 (7.5 atm), alcohol (2 mL), 100 °C,
2
2
b
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4 h. Isolated yields. Determined by GC.
1
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3
involved by oxygen molecule. Furthermore, we rule out diphenyl
diketone C as a possible intermediate in this reaction, not only
because diphenyl diketone was not detected via GC-MS, but also
it did not yield any products in Pd(OAc)2-ZnCl2 ·2H2O-O2 catalytic
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1
(
14
system and is recovered quantitatively. A plausible mechanism
for this cleavage is shown in Scheme 2. In the first step, 1a is
hydroalkoxylated catalyzed by Pd(OAc)2 to form the intermediate
A, which is subsequently attacked by palladium activated
molecular oxygen to generate a cyclic peroxide intermediate B.
Fragmentation of B produces methyl benzoate and benzaldehyde,
and benzaldehyde could undergo further oxidation and esterification
to yield benzoate.
In summary, we have demonstrated that the palladium-catalyzed
cleavage of C-C triple bonds proceeds efficiently using oxygen
as a sole oxidant in various alcohols, affording carboxylic esters
in good yields. Further investigation of the reaction scope, mecha-
(
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H.-F.; Shen, Y.-X.; Wang, Z.-Y. Tetrahedron 2008, 64, 508.
15
(
13) Cleavage reaction of 1-methoxy-1,2-diphenylethene under the same conditions
afforded the corresponding product in good yield. See Supporting Information.
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JA8002217
J. AM. CHEM. SOC. 9 VOL. 130, NO. 15, 2008 5031