Palladium-catalyzed oxidative carbonylation of 1-alkynes into 2-alkynoates with molecular oxygen as oxidant

Article abstract of DOI:10.1246/bcsj.77.2033

A new preparative method to produce alkyl 2-alkynoates from 1-alkynes in alcohol under atmospheric pressure of CO at room temperature was developed with palladium-phosphine catalysts, using molecular oxygen as an oxidant. On the basis of the behavior of model complexes such as methoxycarbonylpalladium and alkynylpalladium complexes, we propose a mechanism accounting for the catalytic carbonylation of alkynes through an intermediate having the both methoxycarbonyl and alkynyl ligands that liberates methyl 2-alkynoates and a Pd(0) species on reductive elimination. The oxidation of Pd(0) to Pd(II) species in the presence of a halide ion was confirmed to proceed cleanly with molecular oxygen as the oxidant. On the basis of the findings on homogeneous catalysts, a heterogeneous catalytic system using Pd/C has also been developed.

Full text of DOI:10.1246/bcsj.77.2033

Ó 2004 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 77, 2033–2045 (2004) 2033
Palladium-Catalyzed Oxidative Carbonylation of 1-Alkynes
into 2-Alkynoates with Molecular Oxygen as Oxidant
ꢀ
ꢀ
Yusuke Izawa, Isao Shimizu, and Akio Yamamoto
Department of Applied Chemistry, Graduate School of Science and Engineering, Advanced Research Institute
for Science and Engineering, Waseda University, 3-4-1, Ohkubo, Shinjuku-ku, Tokyo 169-8555
Received March 24, 2004; E-mail: akioyama@kurenai.waseda.jp
A new preparative method to produce alkyl 2-alkynoates from 1-alkynes in alcohol under atmospheric pressure of
CO at room temperature was developed with palladium–phosphine catalysts, using molecular oxygen as an oxidant. On
the basis of the behavior of model complexes such as methoxycarbonylpalladium and alkynylpalladium complexes, we
propose a mechanism accounting for the catalytic carbonylation of alkynes through an intermediate having the both me-
thoxycarbonyl and alkynyl ligands that liberates methyl 2-alkynoates and a Pd(0) species on reductive elimination. The
oxidation of Pd(0) to Pd(II) species in the presence of a halide ion was confirmed to proceed cleanly with molecular
oxygen as the oxidant. On the basis of the findings on homogeneous catalysts, a heterogeneous catalytic system using
Pd/C has also been developed.
2-Alkynoates1 constitute an important class of compounds
as biologically active substances and as versatile intermediates
in organic synthesis of biologically important compounds such
as butenolides,2 macrolides,3 and carbapenem intermediates.4
Lithiation of 1-alkynes, followed by quenching with chlorofor-
mate, is the generally employed method for preparation of al-
kynecarboxylates. However, the method is stoichiometric and
entails the use of a strong base; this excludes the method’s ap-
plication to base-sensitive compounds. Thus, finding a conven-
ient method for catalytically synthesizing these alkynoates will
provide an important tool in organic synthesis. In view of the
versatile activity of palladium complexes to promote various
catalytic processes5–7 we have been examining applicability
of palladium-catalyzed carbonylation processes.8
In the palladium-catalyzed alkoxycarbonylation of 1-al-
kynes, two principal routes to unsaturated carboxylic esters
are known. One is hydroesterification of alkynes to give
ꢀ,ꢁ-unsaturated esters9 and the other is oxidative carbonyla-
tion of alkynes in alcohol to give alkyl 2-alkynoates with re-
tention of the triple bond. In the oxidative carbonylation of al-
kynes, addition of an oxidant is necessary. The use of a Cu(II)
salt as an oxidant in combination with NaOAc to carbonylate
1-alkynes in the presence of a palladium catalyst was first re-
ported by Tsuji (Eq. 1).10
and addition of these extra oxidizing reagents is not desirable.
We have been interested in synthesizing alkynecarboxylates
from alkynes and CO by a clean method without using extra
oxidizing agents.
Here we report a new palladium-catalyzed oxidative carbon-
ylation of 1-alkynes to 2-alkynoates performed under atmo-
spheric pressure of CO and oxygen at room temperature in
DMF–alcohol (Eq. 2).13 The method provides a simple, inex-
pensive, and clean catalytic carbonylation process of alkynes
with atmospheric oxygen, the cheapest oxidant, without using
any other added oxidizing agent such as copper salts, as de-
scribed below in the first part of the paper.
CO/O2
(1 atm)
+
MeOH
Ph
C C H +
(50 mol. amounts)
PPh3 (0.2 mol. amounts)
(0.1 mol. amounts)
ð2Þ
Pd(OAc)2
DMF, rt, 48 h
Ph C C CO2Me
83 %
The second part of the paper is concerned with the results of
mechanistic studies. Despite the usefulness of alkynecarboxyl-
ates, very few mechanistic studies have been made to allow us
to get some clues for improving the yields and the selectivity
of the process. We report here the results of fundamental stud-
ies on the behavior of organopalladium complexes assumed as
models of intermediates involved in the catalytic processes.
MeOH
CO
+
+
C C H
Ph
(1 atm)
ð1Þ
cat. PdCl2
CuCl2, AcONa
Results
Ph
C
C CO2Me
74 %
1. Catalytic Oxidative Carbonylation of Alkynes with
CO and Molecular Oxygen. 1.1 Examination of Experi-
mental Conditions: Following our finding that oxygen inad-
vertently admitted into the catalyst system improved the yield
of methyl 2-alkynoates, we examined the experimental condi-
tions to improve the oxidative carbonylation process. Table 1
rt, 2 h
Other oxidizing agents such as quinones in combination
with transition metal complexes have been used to perform
the carbonylation of phenylacetylene.11,12 Use of these addi-
tives, however, causes complication in separation processes
Published on the web November 11, 2004; DOI 10.1246/bcsj.77.2033

2034 Bull. Chem. Soc. Jpn., 77, No. 11 (2004)
Pd-Catalyzed Carbonylation of Alkynes
Table 1. Catalytic Oxidative Carbonylation of Phenylacety-
lene with Molecular Oxygen in Various Solvents
Table 2. Effect of Ligands on Oxidative Carbonylation of
Phenylacetylene Using Various Palladium Catalysts under
Various Conditions
+
+
CO/O2
Ph C C H
MeOH
C
H
Ph
C
(1/1 atm)
(50 mol. amounts)
CO/O2 (1 atm)
PPh3 (0.2 mol. amount)
Pd compound (0.1 mol. amount), PPh3
MeOH (50 mol. amounts)
Pd(OAc)2 (0.1 mol. amount)
Ph
C CO2Me
C
CO
2Me
Ph
C
C
solvent, 48 h
DMF, rt, 48 h
YieldaÞ/%
YieldaÞ/%
Run
Temp/ꢁC
Solvent
Run
Catalyst (0.1 mol. amount)
Ph–CꢂC–CO2Me
Ph–CꢂC–CO2Me
1
2
3
4
5
6
7
8
9
rt
rt
rt
(CH3)2CO
CH3CN
CH2Cl2
DMF
48
53
20
83
45
31
68
43
27
79
32
1
2
3
4
5
6
7
8
9
Pd(OAc)2
Pd(dba)2
Pd(OAc)2 + 2 PPh3
Pd(OAc)2 + 3 PPh3
Pd(OAc)2 + 4 PPh3
Pd(PPh3)4
trace
0
83
69
47
39
40
23
20
46
0
rt
50
80
rt
rt
rt
DMF
DMF
DMSO
1,4-Dioxane
MeOH
Pd(OAc)2 + dppb
Pd(OAc)2 + dppf
Pd(OAc)2 + 2 P(2-furyl)3
Pd(OAc)2 + 2 P(p-methoxyphenyl)3
PdCl2
10
11
rt
rt
THF
C6H5CH3
10
11
12bÞ
82cÞ
a) Determined by GC.
PdCl2 + 2PPh3
a) Carbonylation was carried out under the balloon pressure of
CO and O2 (1:1 ratio). The yield of the product was deter-
mined by GC. b) [Cat]=0.05 mol. amount. c) NaOAc was
added. d) Addition of PtBu3, PnBu3, P(o-tolyl)3, P(PyPh2)3,
P(OPh)3, P(O)Ph3, and 1,10-phenanthroline inhibitied the re-
action.
shows the influences of solvents and temperatures.
Among the solvents examined, DMF gave the highest yield
at room temperature, followed by THF and DMSO. Raising
the temperature in DMF caused decreases in the yield of the
2-alkynoate (runs 4, 5, and 6).
Examination of the effect of the CO pressure increase, while
keeping the O2 pressure at 1 atm showed that higher CO pres-
sure caused a decrease in the yield from 83% at 1 atm to 58%
(10 atm), 51% (30 atm), and 46% (50 atm). Increase of the
oxygen pressure from 1 to 10 atm at 30 atm of CO showed
little effect on the yields.
Employment of other transition metal catalysts such as
Ni(cod)2 with 2 mol. amounts of PPh3, as well as use of
other transition metal complexes such as IrCl(CO)(PPh3)2,
RhCl(CO)(PPh3)2, proved ineffective in causing the carbon-
ylation of phenylacetylene.
When Pd(OAc)2 was used in combination with PPh3, the
catalytic carbonylation reaction proceeded without addition
of any extra base (see below), whereas PdCl2 combined with
two molar amounts of PPh3 afforded the carbonylation prod-
ucts only in the presence of a base such as NaOAc (run 12).
The oxidative carbonylation of 1-alkynes can be catalyzed
not only by homogeneous catalysts but also by heterogeneous
systems such as palladium on carbon. The reaction of phenyl-
acetylene with Pd/C (0.05 mol. amount) used in combination
with PPh3 (0.1 mol. amounts), NEt3, and NEt4Cl in DMF at
room temperature under the pressure of CO (50 atm) and O2
(7.5 atm) in 50 molar amounts of methanol gave methyl 3-
phenyl-2-propynoate in 75% yield in 48 h. The yield of the
carbonylation product with the heterogeneous catalyst was
quite low when the reaction was carried out at atmospheric
pressure of CO and of O2.
1.2 Scope of the Reaction: The oxidative carbonylation
with molecular oxygen is applicable to various alkynes with
aryl and alkyl substituents, as shown in Table 3. The results
with other alcohols than methanol and the result with diethyl-
amine as a nucleophile are also included in the table.
Substitution of the phenyl group in phenylacetylene at the
para position with methyl group as well as with bromide group
gave the carbonylation product in good yields. Simple aliphat-
ic alkynes such as 1-heptyne were carbonylated similarly. A
hydroxy group attached to the alkyne somewhat hindered the
reaction, but protection with tert-butyldimethylsilyl group
gave the carbonylation product in a good yield. In the reaction
The effects of palladium catalysts and of ligands used in
combination with the palladium compound are summarized
in Table 2.
Palladium compounds such as PdCl2, Pd(OAc)2, and
Pd(dba)2 (dba=dibenzylideneacetone), proved inactive (runs
1, 2, and 11) in catalyzing the carbonylation process when they
were used without ligand such as PPh3. Such results suggest
the necessity of a proper ligand to generate and maintain the
activity of a palladium catalyst. However, addition of more
than two molar amounts of triphenylphosphine per mole of
palladium had some detrimental effect in giving the carbon-
ylation product (runs 3, 4, 5, and 6). Ligands such as bidentate
diphosphines that tend to coordinate strongly to palladium
proved less suitable for the carbonylation process (runs 7
and 8). Basic trialkylphosphine ligands and sterically demand-
ing ligands such as PnBu3, PtBu3, and P(o-tolyl)3 inhibited the
reaction, and phosphite and triphenylphosphine oxide sup-
pressed the reaction.

Y. Izawa et al.
Bull. Chem. Soc. Jpn., 77, No. 11 (2004) 2035
Table 3. Catalytic Oxidative Carbonylation of Various Ter-
minal Alkynes with Methanol and Other Nucleophiles
with Molecular Oxygen
c) are conceivable for elucidating the catalytic formation of
methyl alkynoate from alkyne, CO and MeOH, and an oxidant
in the presence of a Pd(II) species and a base, as depicted in
Schemes 1–3.
+
NuH
CO/O2
R C C H
+
Mechanism a; In the mechanism a shown in Scheme 1,
formation of a methoxycarbonylpalladium species B from a
Pd(II) species A having one or more supporting ligand(s) such
as PPh3 and anionic ligands X (X=halide or acetate) is consid-
ered as the first step in the catalytic cycle. The methoxycarbo-
nylpalladium species B, produced from the Pd(II) complex A
with CO and methanol, reacts further with an alkyne to form
a Pd(II) intermediate that has both methoxycarbonyl and al-
kynyl groups bound with the palladium center C with removal
of HX. When PdCl2 was used in combination with PPh3, use
of an added base was necessary. On the other hand, with the
catalyst system using Pd(OAc)2 and PPh3, addition of the ex-
ternal base was not required. We will later address this prob-
lem in Discussion.
Reductive elimination from C liberates methyl alkynoate as
the product, generating a Pd(0) species D that is oxidized with
the aid of an oxidant to regenerate the Pd(II) species A to drive
the catalytic process.
Mechanism b; The second mechanism involves the initial
formation of an alkynylpalladium complex (E in Scheme 2) by
the reaction of the alkyne with the Pd(II) species A in the pres-
ence of a base. The alkynylpalladium species E further reacts
with CO, methanol, and a base to produce the Pd(II) species C
that has both the methoxycarbonyl and the alkynyl ligands.
The ensuing reductive elimination produces the methyl alky-
noate, similarly to Scheme 1.
(1/1 atm) (50 mol. amounts)
NaOAc (0.3 mol amount.)
PPh3 (0.2 mol. amount)
PdCl2 (0.1 mol. amount)
O
C
C
Nu
DMF, rt, 48 h
R
C
Run
R
NuH
YieldaÞ/%
1
2
3cÞ
4
5
6
Ph
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
82bÞ
74
79
75
42
83
4-MeC6H4
4-BrC6H4
n-C5H11
HO(CH2)4
TBSO(CH2)4
7
8
Ph
Ph
n-BuOH
HNEt2
86
60
a) Isolated yield. b) Determined by GC. c) Pd(OAc)2 (0.1 mol.
amount)/PPh3 (0.2 mol. amount) as a catalyst.
with phenylacetylene, butanol could be used to give butyl 3-
phenyl-2-propynoate; tert-butyl alcohol was not effective,
while diethylamine gave the amide in a moderate yield.
2. Mechanistic Studies. 2.1 Examination of the Feasibil-
ities of Possible Catalytic Cycles: Three mechanisms (a to
O2, X-
Oxidant
R C C CO2Me
Pd0Ln
R C C CO2Me
Pd0Ln
(D)
(O2, X-)
(D)
R
O
C OMe
C
C
PdIIX2Ln
PdIIX2Ln
(A)
LnPd
L2Pd
C
(C)
C OMe
O
(A)
C
(C)
HX.(Base)
R
HX.(Base)
CO, MeOH
(Base)
R C C H
(Base)
R
O
C
C
C
R
C H
(Base)
C OMe
HX.(Base)
HX.(Base)
CO, MeOH
(Base)
L2Pd
LnPd
X
X
(L = PPh3)
(L = PPh3)
(X = Anionic Ligand)
(E)
(B)
(X = Anionic Ligand)
Scheme 1. A proposed mechanism for catalytic carbonyl-
ation of a terminal alkyne using a palladium catalyst.
Scheme 2. A proposed mechanism for catalytic carbonyl-
ation of a terminal alkyne via alkynylpalladium complex.
R
C
O
Base
Pd0
MeOH
+
C
X
CO
C C C R
X
C CO
R C
2Me
LnPd
LnPd
HX.Base
+
Scheme 3. A conceivable mechanism involving CO insertion into Pd–alkynyl bond in catalytic carbonylation of an alkyne.

2036 Bull. Chem. Soc. Jpn., 77, No. 11 (2004)
Pd-Catalyzed Carbonylation of Alkynes
Mechanism c; The third mechanism postulates the CO in-
sertion into an alkynylpalladium species, giving an alkynoyl-
palladium intermediate. Reaction of the alkynoylpalladium
species thus formed with methanol and a base liberates the
product methyl alkynoate, producing the Pd(0) intermediate
that carries the catalytic cycle upon oxidation.
in 1 with the alkyne, with 60% of 1 remaining unreacted
(Eq. 5).
Ph
O
C
- MeOH, CO
Ph3P
Cl
Ph3P
Cl
C OMe
C
+
Ph C C H
Pd
Pd
CH2Cl2, rt, 18 h
(20 mol. amt.)
PPh3
PPh3
In the following section we probe the feasibility of the three
possible mechanisms (a to c) by examining properties of mod-
el organopalladium complexes assumed in the catalytic cycles.
2.2 Examination of the Behavior of Methoxycarbonyl-
palladium Complexes in Relation to the Mechanism Repre-
sented by Scheme 1: Methoxycarbonylpalladium complexes
have been prepared and their properties have been examined
by various workers.14–16 However, the reactivities of the me-
thoxycarbonyl complexes in relation to catalytic carbonylation
have received less attention.17 We have prepared two types of
methoxycarbonylpalladium complexes, 1 and 2, having PPh3
and PMe3 ligands, respectively, by two different routes for ex-
amination of their reactivities toward phenylacetylene. Com-
plexes 1 and 2 have been characterized by spectroscopic meth-
ods with 1H, 13C, and 31P NMR and IR. In the reaction shown
in Eq. 3, methoxycarbonylpalladium(II) chloride having two
PPh3 ligands (1) was prepared by the reaction of CO and meth-
anol with PdCl2(PPh3)2 in the presence of a base such as trieth-
ylamine. The reaction is considered to proceed through CO co-
ordination to the cationic Pd(II) species, followed by attack of
methanol on the coordinated CO aided by a base to provide the
methoxycarbonyl complex 1. For comparison, a methoxycar-
bonylpalladium complex 2 having two trimethylphosphine li-
gands was prepared by oxidative addition of methyl chlorofor-
mate to a Pd(0) complex coordinated with trimethylphosphine
ligands and styrene with cleavage of the Cl–C bond in the
chloroformate (Eq. 4).
1
25%ð5Þ
3
A similar transformation of a methoxycarbonylpalladium
complex having Ph2Ppy ligand (Ph2Ppy=diphenyl(2-pyridyl)-
phosphine) into [Pd(Ph2Ppy)(CꢂCPh)Cl] quantitatively on
treatment with phenylacetylene was recently reported.18 The
transformation of the methoxycarbonylpalladium complex 1
into the alkynylpalladium complex suggests that phenylacety-
lene, being an acidic alkyne, reacts with the methoxycarbonyl
ligand to liberate methanol and CO to form the alkynylpalladi-
um complex 3.
When the reaction of 1 with phenylacetylene was carried
out in the presence of a base such as NaOAc, methyl phen-
yl-2-propynoate was produced in 73% yield at room tempera-
ture (Eq. 6).
O
Ph3P
Cl
NaOAc
C OMe
Ph C C CO2Me
+ Ph C C H
Pd
DMF, rt, 24 h
PPh3
73%
ð6Þ
1
The reactions of 1 with phenylacetylene using NEt3 in 1 to 4
molar amounts per mole of the methoxycarbonylpalladium
complex 1 gave 30 to 40% of the alkynoate. The atmosphere
employed for the reaction, either under argon or under CO,
had little effect. The effectiveness of the alkaline salt in com-
parison to the organic nitrogen base is in accord with the ex-
perimental results for the catalytic conversion of alkynes into
alkynoates with CO.
Another possible reaction course involving the methoxycar-
bonylpalladium species is the insertion of an alkyne into the
MeOCO–Pd bond. No insertion reaction took place, howevꢁer,
when 1 was treated with phenylacetylene in toluene at 130 C
for 2 h (Eq. 7).
O
CO (30 atm)
Et3N (excess)
MeOH, 50oC, 2 h
Ph3P C OMe
Pd
Cl PPh3
trans-[PdCl2(PPh3)2]
ð3Þ
1
70 %
+ styrene
- ethane
- ethylene
O
Ph
Ph3P
C OMe
Me3P
+
Ph C C H
trans-[PdEt2(PMe3)2]
Pd
Pd
reflux
Et2O
Me3P
Cl
PPh3
1
in situ
ð7Þ
ð4Þ
O
Ph
Ph3P
H
ClCO2Me
C OMe
Me3P
CO2Me
Pd
Pd
C6H5CH3, 130 oC, 2 h
1 h, rt
-styrene
Cl PMe3
Cl
PPh3
2
This result disfavors the mechanism involving insertion of
an alkyne into the methoxycarbonyl–Pd bond, followed by
elimination of the ꢁ-hydrogen in the alkenylpalladium inter-
mediate to liberate the alkynoate. Furthermore, the ꢁ-hydrogen
in the alkenylpalladium species is in an inappropriate position
for allowing the syn H abstraction without isomerization, mak-
ing the alkyne insertion mechanism even more unlikely.19
2.3 Examination of the Behavior of Alkynylpalladium
65%
No reaction took place on treatment of the PPh3-coordinated
methoxycarbonylpalladium complex 1 with an equimolar
amount of phenylacetylene in the absence of a base in CDCl3
at room temperature. Treatment of 1 with an excess of phenyl-
acetylene gave a phenylethynylpalladium complex 3 in a low
yield as an exchange product of the methoxycarbonyl entity

Y. Izawa et al.
Bull. Chem. Soc. Jpn., 77, No. 11 (2004) 2037
Complexes: Next we examined the properties of the alkynyl-
palladium species in relation to the mechanism shown in
Scheme 2. There are precedents of preparation of alkynylpal-
ladium complexes.20 The formation of alkynylpalladium com-
plexes from palladium dihalide and an alkyne usually requires
addition of a catalytic amount of a copper salt, which produces
an intermediate alkynylcopper compound whose alkynyl group
is transmetallated to palladium(II) on interaction with the pal-
ladium halide.21,22 Two types of alkynylpalladium complexes
3 and 4, having PPh3 and PMe3 ligands respectively, have
been prepared in the present study from palladium dichloride
and alkynes using copper iodide as a promoter in the presence
of diethylamine (Eqs. 8, 9).
The result suggests that the alkynylpalladium chloride com-
plex 3 reacts with CO and methanol in the presence of NaOAc
in DMF to give the assumed intermediate having both the me-
thoxycarbonyl and the alkynyl ligands, corresponding to com-
plex C in Scheme 2. The subsequent reductive elimination
from C liberates methyl 3-phenyl-2-propynoate. The effective-
ness of NaOAc over triethylamine, see below (Eq. 11), is in
agreement with the effectiveness of an alkaline salt in the
catalytic carbonylation processes.
Ph
C
Ph P
C
3
+
CO
+
MeOH
Pd
PPh
(50 mol. amounts)
Cl
(1 atm)
3
3
trans-[PdCl2(PPh3)2]
Ph C C H
(1.5 mol. amt.)
+
NEt Cl
4
NEt (2 mol. amounts)
3
ð11Þ
Ph
Ph C C CO Me
2
CuI (0.15 mol. amount)
C
ð8Þ
CH Cl , rt, 96 h
2
2
Ph3P
C
40 %
HNEt2 (1.5 mol. amounts)
CH2Cl2, rt, 4 h
Pd
O
Cl PPh3
C OMe
Ph P
3
3
Pd
Ph
C
C C C Ph
31 %
+
+
40 %
Cl PPh
1
3
24 %
cis-[PdCl2(PMe3)2]
+
Ar C C H
When NEt3 was used as a base in the presence of NEt4Cl in
the atmosphere of 1 atm of CO in MeOH in the reaction with
the phenylethynylpalladium complex 3, the reaction took a dif-
ferent course, as shown in Eq. 11.
CH3
Ar =
The yield of methyl 3-phenyl-2-propynoate was low (40%)
and the reaction was accompanied by formation of 1,4-diphen-
yl-1,3-butadiyne (31%) and of the methoxycarbonylpalladium
complex 1 in 24% yield.
These results show that the carbonylation mechanism as
shown in Scheme 2 is possible, once the alkynylpalladium in-
termediate corresponding to E is produced. However, in the
absence of a copper salt as in the present catalytic system,
the alkynylpalladium complex is not readily formed.
The formation of the dialkyne suggests occurrence of dis-
proportionation of the starting alkynylpalladium chloride com-
plex 3 into palladium dichloride complex and dialkynylpalla-
dium complex 5, as shown in Scheme 4.
The dialkynylpalladium complex 5 produced would release
the dialkyne on reductive elimination in the atmosphere of CO,
whereas the dichloropalladium complex further undergoes the
reaction with CO and methanol in the presence of a base to
give the methoxycarbonylpalladium chloride complex 1 ac-
cording to Eq. 3. Such a result accounts for the reaction course
to produce the diyne as a by-product in catalytic carbonylation
of alkynes, particularly when a copper salt is used in combina-
tion with alkynes.
For establishing further the reaction course to produce
diynes through dialkynylpalladium species, we have prepared
the dialkynylpalladium complex 5 from Pd(OAc)2, triphenyl-
phosphine, and phenylacetylene as shown in Eq. 12. The di-
alkynylpalladium complex 5 is stable in the absence of CO
and only a trace of the diyne was produced in the reaction to
produce 5 from Pd(OAc)2, PPh3, and phenylacetylene.
ð9Þ
Ar
HNEt2
C
Me3P
Cl
C
CuI (0.1 mol. amt.)
CH2Cl2, rt, 2 h
Pd
PMe3
4
60 %
For the preparation of an arylalkynylpalladium complex, we
employed p-tolylacetylene for facilitating the NMR character-
ization. The reason we used the cis-isomer for the preparation
of the PMe3-coordinated complex 4 as the starting product
is that treatment of trans-[PdCl2(PMe3)2] with an alkyne
under similar conditions gives a dialkynylpalladium complex
through a disproportionation process.23
Treatment of the phenylethynylpalladium chloride having
two PPh3 ligands (complex 3) with methanol and NaOAc in
the atmosphere of CO liberated methyl 3-phenyl-2-propynoate
in 75% isolated yield (Eq. 10).
Ph
C
Ph P
3
C
+
Pd
CO
MeOH
(50 mol. amounts)
+
Cl
PPh
3
(1 atm)
ð10Þ
3
NaOAc (4 mol. amounts)
DMF, rt, 32 h
Ph C C CO Me
2
75 %

2038 Bull. Chem. Soc. Jpn., 77, No. 11 (2004)
Pd-Catalyzed Carbonylation of Alkynes
O
Ph3P C OMe
Pd
Ph3P Cl
Pd
CO, MeOH
PPh3
Cl
Ph
C
PPh3
Cl
1
C
Ph3P
Pd
Ph
2
CO
Cl PPh3
C
reductive elimination
C
Ph3P
3
Ph
C C C C Ph
Pd
PPh3
C
C
Ph
5
Scheme 4. Reactions of alkynylpalladium complex 3.
+
Ar
Pd(OAc)
PPh
2
Ar
+
Ph C C H
+
3
C
C
AgBF (1 equiv.)
Me P
4
3
C
(4 mol. amounts)
-
(2 mol. amouts)
Me P
C
3
BF
4
o
Pd
Pd
CDCl , -30 C
3
Ph
(s)
Cl
PMe
PMe
3
3
C
C
4
ð12Þ
Ph P
3
NEt (8 mol. amounts)
3
Pd
C
C
PPh
3
DMF, rt, 48 h
CH
Ar =
3
Ph
5
61 %
ð14Þ
+
Ar
C
Treatment of the trans-dialkynylpalladium complex 5 in
DMF at room temperature with CO at atmospheric pressure
caused reductive elimination of the two alkynyl ligands, liber-
ating the diyne at room temperature without addition of an
extra base (Eq. 13).
CO (1 atm)
Me P
C
-
3
BF
4
CDCl
Pd
3
o
OC
PMe
3
-30 C
Ar
C
MeOLi (1 mol. amounts)
Me P
C
Ph
C
3
Pd
Ph P
CO (1 atm)
3
C
o
PMe
MeO C
O
Ph
C
C C C Ph
78%
3
Pd
CH Cl , -30 C, 3 h
2
2
DMF, rt, 48 h
C
C
PPh
3
Another approach preparing the presumed alkynyl(methox-
ycarbonyl)palladium complex starting from the methoxycarbo-
nylpalladium chloride 2, also gave palladium blꢁack on its treat-
ment with AgBF4 and tolylacetylene at ꢃ60 C and did not
produce the expected complex corresponding to C in
Schemes 1 and 2 (Eq. 15).
Ph
5
ð13Þ
The ready formation of the diyne by reductive elimination
from the dialkynylpalladium complex 5 on interaction with
CO indicates that the function of CO is to act as a ꢂ acid to
induce the reductive elimination.
+
O
O
Next we attempted the preparation of an alkynyl(me-
thoxycarbonyl)palladium complex corresponding to C in
Schemes 1 and 2. The reactivity of organopalladium com-
plexes is often enhanced when a cationic organopalladium
complex is generated in the reaction system from a neutral spe-
cies.24 For examining the properties of the cationic alkynylpal-
ladium complex, possibly involved in the carbonylation reac-
tions of alkynes, we removed the chloride ligand in the alky-
nylpalladium complex 4 with AgBF4 to generate a cationic al-
kynylpalladium complex having two PMe3 ligands. Treatment
of the solvent-coordinated cationic para-tolylethynylpalladium
complex with CO afforded the CO-coordinated alkynylpalladi-
um complex (Eq. 14). However, treatment of the CO-coordi-
natedꢁ cationic alkynylpalladium complex with LiOMe at
ꢃ30 C with expectation of getting the postulated trans-alky-
nyl(methoxycarbonyl)palladium complex C having two PMe3
ligands resulted in failure, with deposition of palladium black.
AgBF
4
C OMe
Me P C OMe
-
3
Me P
BF
3
4
o
Pd
CD Cl -60 C
Pd
2
2,
Cl PMe
3
PMe
3
(s)
2
ð15Þ
O
Ar
C
C H
Me P C OMe
3
Pd
o
C
PMe
3
CD Cl -60 C
2
2,
C
Ar
CH
Ar =
3
2.4 Examination of the Possibility of CO Insertion into
the Alkynyl–Palladium Bond: For examining the feasibility
of the third mechanism shown in Scheme 3, the reactivity of
the alkynylpalladium complex toward CO insertion was exam-
ined. The reaction of complex 3, the phenylethynylpalladium

Y. Izawa et al.
Bull. Chem. Soc. Jpn., 77, No. 11 (2004) 2039
complex having two PPh3 ligands, with CO did not give any
CO insertion product under the atmospheric pressure of CO
nor under 10 atm of CO at room temperature (Eq. 16).
ceivable mechanisms to account for the palladium-catalyzed
oxidative carbonylation (Schemes 1 to 3), the mechanism
involving CO insertion into the alkynylpalladium bond
(Scheme 3) is not supported by the mechanistic studies based
on the behavior of model organopalladium complexes. Al-
though such an insertion mechanism related to the carbonyla-
tion of aryl halides developed by Heck19 is sometimes pro-
posed for explaining the carbonylation of alkynes, there is
no solid experimental support.26
Ph
O
C
Ph P
C
Ph P
C
C CPh
3
3
CO
(1 atm)
+
Pd
Pd
Cl PPh
CH Cl rt, 18 h
Cl PPh
3
2
2,
3
3
ð16Þ
The possibility of operation of Scheme 2 can not be exclud-
ed when a Cu(II) salt is used, since the copper salt facilitates
the formation of alkynylpalladium species from alkynes. In
the absence of a copper salt as in the present case, however,
involvement of an alkynylpalladium species in the first step
of the catalytic process is not likely. In cases where diynes
are produced as the by-product, the involvement of the alky-
nylpalladium may be involved, since the dialkynylpalladium
can be produced by a disproportionation process as shown
by Scheme 4.
Thus on the basis of examination of various possibilities to
account for the mechanism, we propose the mechanism ex-
pressed by Scheme 1 as the most reasonable catalytic cycle,
the one compatible with experimental results. We have estab-
lished each elementary step in Scheme 1 (A to B, B to D with
formation of methyl 2-propynoate, and air oxidation of D to A)
and provided experimental support for the validity of the
Scheme.
2.5 Mechanisms of Oxidation of Pd(0) to Pd(II): Oxida-
tion of Pd(0) to Pd(II) Complex with Molecular Oxygen
without a Cu(II) Promoter; The result, that a palladium
complex in combination with molecular oxygen served as
the catalyst for oxidative carbonylation of alkynes without
use of a copper salt, was unexpected since a copper(II) salt
is generally believed to be required for oxidizing a Pd(0) com-
plex. Thus we investigated the course of oxidation of Pd(0)
into Pd(II) complex with O2.25
We found that treatment of [Pd(PPh3)4] with O2 at atmo-
spheric pressure in the presence of LiCl readily gave trans-
[PdCl2(PPh3)2] (Eq. 17). Formation of two molar amounts
of triphenylphosphine oxide was confirmed in the process of
oxidation.
O (1 atm)
2
LiCl (2.2 mol. amounts) trans-[PdCl (PPh ) ]
2
3 2
ð17Þ
[Pd(PPh ) ]
3 4
THF, rt, 6 h
92 %
The initial formation of methoxycarbonylpalladium species,
into which alkyne would undergo the insertion, is the remain-
ing conceivable course, but the difficulty of insertion of phen-
ylacetylene into the methoxy–Pd bond in 1 (Eq. 7) does not
support the alkyne insertion mechanism.
It was further confirmed that [Pd(dba)2] was converted into
trans-[PdCl2(PPh3)2] on treatment with O2 in the presence of
two molar amounts each of PPh3 and LiCl in THF at room
temperature for 3 h (Eq. 18).
We have also considered the possibility of operation of a
mechanism involving the initial formation of an alkynylpalla-
dium hydride by oxidative addition of an alkyne to a Pd(0)
species, since such an alkynyl hydride complex was reported
previously.27 However, the experimental results that CO inser-
tion into the alkynylpalladium bond was not observed and that
evolution of hydrogen, which is expected to arise from the pal-
ladium hydride, was not detected in the catalytic carbonylation
of alkynes, disfavor the operation of such a mechanism. We at-
tempted to isolate an alkynylpalladium hydride complex from
PPh3- and tricyclohexylphosphine-coordinated hydridopalladi-
um chlorides,28 but no evidence to suggest the formation of
such a complex was obtained. Since the catalytic carbonylation
is a slow process at room temperature, the operation of the
mechanism involving the alkynylpalladium hydride seems less
probable.
PPh (2 mol. amounts)
3
O (1 atm)
2
LiCl (2 mol. amounts)
ð18Þ
trans-[PdCl (PPh ) ]
[Pd(dba) ]
2
3 2
2
THF, rt, 3 h
77 %
The result indicates that the oxidation of a tertiary phos-
phine ligand to tertiary phosphine oxide is not a prerequisite
for driving the oxidation of a Pd(0) complex. We have further
found that a mixture of atmospheric pressure of oxygen and
CO converted [Pd(PPh3)4] into the methoxycarbonylpalladium
complex 1 in THF containing methanol, triethylamine, and
LiCl at room temperature in 5 h (Eq. 19).
[Pd(PPh ) ]
MeOH
+ CO/O (1/1 atm)
+
3 4
2
(50 mol. amounts)
We examined the process to convert [Pd(PPh3)4] into
[PdCl2(PPh3)2] under oxygen in the presence of LiCl
(Eq. 17); we found that the reaction proceeded readily in the
presence of water and confirmed that the solution became alka-
line after completion of the reaction. The result is in agreement
with formation of LiOH in the oxidation process. Examination
of the 7Li NMR did not provide conclusive evidence regarding
the identity of the lithium species formed, because of the prox-
imity of the 7Li signals of Li2O, LiOH, and the hydrate of
LiOH.
Et N (2 mol. amounts)
O
3
ð19Þ
OMe
C
LiCl (1.1 mol. amounts) Ph P
3
Pd
Cl PPh
THF, rt, 5 h
3
1
78 %
Discussion
Mechanism of the Palladium-Catalyzed Oxidative Car-
bonylation of Alkynes into Alkynoates. Of the three con-
On the basis of these results, the mechanism of oxidation of

2040 Bull. Chem. Soc. Jpn., 77, No. 11 (2004)
Pd-Catalyzed Carbonylation of Alkynes
H+
[Pd(PPh3)4]
O2
[PdCl2(PPh3)2] PPh O + 2LiOH*
+ 2 LiCl +
+
3
PPh3
2 H+OH-
O2
[Pd(PPh3)4]
O
[Pd(PPh3)4]
(Ph3P)2Pd
O
PPh3O + PPh3
2+
H
O
(OH-)2
4 LiCl
(Ph3P)2Pd
Pd(PPh3)2
2 trans-[PdCl2(PPh3)2]
O
H
4 LiOH
Scheme 5. A proposed mechanism for oxidation of Pd(PPh3)4 with O2.
C
R
C CO2Me
(CO)
O
0.5
2
Pd0Ln
O
O
0.5
LnPd
(D)
MeOH
O
2+
H
O
C OMe
LnPd
0.5 LnPd
PdLn MeO-
2
(C)
C
O
H
C
R
(X-)
CO,
HX.(Base)
O
C OMe
R C C H
(Base)
LnPd
+
(L = PPh3)
(X = Anionic Ligand)
X
(B)
Scheme 6. A proposed mechanism for catalytic carbonylation of a terminal alkyne with a palladium catalyst and O2 in methanol.
Pd(0) species by molecular oxygen into Pd(II) species in the
catalytic process may be accounted for by Scheme 5.
anism presented here to account for the oxidation course of
Pd(0) to Pd(II) fills the remaining gap connecting D and A
in Scheme 1 to complete the catalytic cycle. Taking the body
of experimental results into consideration, we propose a cata-
lytic cycle to account for the whole sequence of catalytic trans-
formation of an alkyne into methyl 2-alkynoate by oxygen in
methanol, as represented by Scheme 6.
The left part in the catalytic cycle in Scheme 6 is the same
as in Scheme 1, which was supported by the reaction of the
methoxycarbonylpalladium species B with an alkyne in the
presence of a base to liberate methyl 2-alkynoate and Pd(0)
species D through C. The right part in Scheme 6 is slightly
modified from Scheme 5, where only the effect of water was
considered. In the actual catalytic process where methanol is
used, it is likely that methanol is involved in the oxidation
process of Pd(0) species. Thus transformation of a Pd(0) spe-
cies with O2 is assumed to give first an (ꢃ2-O2)Pd complex,
which further reacts with another Pd(0) species and methanol
to give the OH-bridged dimer. In Scheme 6, the intermediate
dimeric complex is tentatively presented as the OH-bridged
Formation of a dioxygen adduct with the formula of
[(PPh3)2Pd(ꢃ2-O2)] is probably the first step in the oxidation
of a Pd(0) complex with molecular oxygen. In the case of
the reaction of Pd(0) complexes with molecular oxygen, for-
mation of such an oxygen adduct was previously reported29
and the catalytic conversion of PPh3 into OPPh3 with the
PPh3-coordinated palladium complexes has been established.30
The dioxygen complex may further react with another Pd(0)
species to form an oxygen-bridged dinuclear complex. It is
possible that the ꢄ-oxo bridged complex reacts with a trace
of water to produce a dimeric species with OH bridges and
OHꢃ anions. The OHꢃ ligands are replaced with LiCl and
the system collapses into a palladium chloride complex,
[PdCl2(PPh3)2]. A related dimeric OH-bridged complex has
been characterized from structural studies as well as by chemi-
cal methods by Grushin and Alper.31 The reaction sequence
shown in Scheme 5 is in agreement with experimental results
on the properties of palladium complexes. The proposed mech-

Y. Izawa et al.
Bull. Chem. Soc. Jpn., 77, No. 11 (2004) 2041
tral Laboratory of Waseda University. The pH values were meas-
ured on a D21 pH meter (Horiba).
complex with the methoxide anions, but the possibility of for-
mation of a methoxy-bridged complex having OH anions can
not be excluded. Through either species, the Pd(II) species
formed by oxidation with O2 is then transformed into the me-
thoxycarbonylpalladium species B on interaction with CO and
methanol.
Reagents. All alkyne compounds except for 6-t-butyldimeth-
ylsiloxy-1-hexyne were commercial products (Aldrich: 4-bromo-
ethynylbenzene, Tokyo Kasei Kogyo Co. for the other alkynes)
and they were used without further purification. The 6-t-butyldi-
methylsiloxy-1-hexyne was synthesized by the reaction of 5-hex-
yn-1-ol with TBS–Cl in the presence of imidazole. Methanol
(Kanto Kagaku), 1-butanol (Kanto Kagaku), diethylamine (Tokyo
Kasei Kogyo Co.), sodium acetate (Kokusan Chemical Co.), tri-
ethylamine (Kanto Kagaku), lithium chloride (Kanto Kagaku), tet-
raethylammonium chloride (Tokyo Kasei Kogyo Co.), and CuI
(Kanto Kagaku) were commercial products, and were used direct-
ly. [Pd(PPh3)4]33 was prepared by the reported procedure.
[Pd(dba)2]34 and trans-[PdCl2(PPh3)2]35 were prepared according
to the references and were used without recrystallization of the
crude precipitate obtained at the end of the synthesis. [Pd(OAc)2]
(Aldrich), 10% Pd/C (Kojima Chemicals Co.), and [PdCl2]
(Tanaka Kikinzoku) were used as received from commercial
suppliers. All tertiary phosphines were commercial products and
were used without further purification.
As we described in the Results section in the present paper,
addition of a base such as NaOAc was required for making the
catalytic carbonylation process operative when PdCl2 was
used, whereas no base was required when Pd(OAc)2 was em-
ployed in combination with two molar amounts of PPh3. The
reason behind this is not clear at the moment. The acetate
anion may be playing a special role to abstract a proton be-
cause of its greater affinity with protons than the Clꢃ anion.
Another possibility is generation of a base in the course of ox-
idation of a Pd(0) species with O2 into a Pd(II) species, as we
discussed regarding Scheme 5. As we assumed the interaction
of water to make the system alkaline in Scheme 5, water might
participate in transformation of the [(PPh3)2Pd(ꢃ2-O2)] into
Pd(II) species with liberation of an OHꢃ anion. Since the cat-
alytic conversion of alkynes into methyl 1-alkynoates proceed-
ed smoothly with the Pd(OAc)2/2PPh3 catalyst system with
oxygen as the oxidant, we did not address the problem further.
The overall catalytic cycle represented in Scheme 6 is in
agreement with the experimental results obtained in the pres-
ent study. A similar mechanism involving the hydroxy-bridged
palladium intermediate has been proposed recently by Sheldon
to account for the aerobic oxidation of alcohols by atmospheric
oxygen.32
Synthesis of Model Organopalladium Complexes and Their
Reactions. Synthesis of trans-[PdCl(COOMe)(PPh3)2] (1):15,16
A mixture of trans-[PdCl2(PPh3)2] and triethylamine in 20 cm3 of
methanol was placed in an autoclave. The autoclave was purged
with carbon monoxide and pressurized with 30 atm of CO at room
temperature. After heating at 50 ꢁC for 2 h, the autoclave was
cooled to room temperature and depressurized. The orange sus-
pension obtained was transferred to a 50 cm3 Schlenk tube and
was filtered to obtain a solid, which was washed with methanol
(five times) and was dissolved in 15 cm3 of CH2Cl2. Addition
of diethyl ether (25 cm3) at ꢃ30 ꢁC yielded yellow crystals.
The crystals were collected by filtration, washed with ether (5
cm3, twice), and were dried at room temperature under vacuum
to give 1.04 g (72%) of an off-white solid. It was identified by
comparison with the NMR and IR spectra of the known sample:
1H NMR (CDCl3, r. t., 400 MHz) ꢅ 7.2–7.8 (m, 30H, Ph), 2.38
(s, 3H, COOCH3); 13C{1H} NMR (CDCl3, r. t., 125 MHz) ꢅ
184.6 (s, C=O) 51.3 (s, COOCH3); 31P{1H} NMR (CDCl3, r. t.,
Conclusion
A new process of converting alkynes into 2-alkynecarboxyl-
ates using molecular oxygen as the sole oxidant in the absence
of any other oxidizing agent has been discovered and the
mechanisms that reasonably account for the catalytic cycles
have been presented. On the basis of studies on the properties
of model organopalladium complexes, a methoxycarbonylpal-
ladium intermediate is proposed to play the most critical role.
The O2-promoted oxidation course of a Pd(0) into Pd(II) spe-
cies in the presence of LiCl without assistance of any external
oxidant was examined and a mechanism to account for the
reaction course is presented.
161 MHz) ꢅ 19.0 (s); IR (KBr disc), ꢆC=O 1673 cmꢃ1
.
Synthesis of trans-[PdCl(COOMe)(PMe3)2] (2): In a 50 cm3
Schlenk tube filled with argon and cooled to ꢃ30 ꢁC were placed
515 mg (1.63 mmol) of trans-[PdEt2(PMe3)2]36 and 15 cm3 of di-
ethyl ether. Styrene (0.279 cm3, 2.44 mmol) was added to the sys-
ꢁ
tem and the mixture was heated at 50 C for 3 h to give a yellow
Experimental
homogeneous solution. To the solution was added methyl chloro-
formate (0.125 cm3, 1.63 mmol) and the mixture was stirred at the
room temperature for 45 min. To the dark orange homogeneous
solution was added hexane (25 cm3) at ꢃ30 ꢁC to allow white
needles to precipitate. After separation of the liquid phase by fil-
tration, the solid was washed with hexane (5 cm3 ꢄ 3), and dried
General Procedures. All manipulations except for palladium-
catalyzed carbonylation of terminal alkynes were performed under
argon atmosphere using Schlenk tube techniques. Solvents were
purified by the usual methods under argon. NMR spectra were re-
corded on a JEOL Lamda 500 or a JEOL AL-400 spectrometer for
1H (referenced to SiMe4 via residual solvent protons), 13C{1H}
(referenced to SiMe4 via the solvent resonance) and 31P{1H} (ref-
erenced to 85% H3PO4 as an external standard). NMR Coupling
constants (J values) are given in hertz (Hz), and spin multiplicities
are indicated as follows: s (singlet), d (doublet), t (triplet), m (mul-
tiplet), vt (virtual triplet), and br (broad). Gas chromatographic
analyses (GC) were carried out on a GC-353 equipped with
TC1 column (0.25 mm I.D. ꢄ 30 m), using N2 as carrier gas.
Low-resolution mass spectra were obtained with a JEOL JMS-Au-
tomass 150 that is coupled with a gas chromatograph. Elemental
analyses were performed by the Materials Characterization Cen-
1
under vacuum to give 374 mg (65%) of 2. H NMR (400 MHz,
CDCl3, r. t.) ꢅ 3.62 (3H, s, COOCH3), 1.44 (18H, vt, JPH
¼
3:66 Hz, P(CH3)3); 13C{1H} NMR (125 MHz, CDCl3, r. t.) ꢅ
186.1 (s, C=O), 51.4 (s, COOCH3), 14.4 (m, P(CH3)3);
31P{1H} NMR (161 MHz, CDCl3, r. t.) ꢅ ꢃ14:71 (s); IR (KBr
disc) 1650 cmꢃ1 (ꢆC=O).
Synthesis of trans-[PdCl(CꢀCPh)(PPh3)2] (3): A mixture of
trans-[PdCl2(PPh3)2] (705 mg, 1.00 mmol) and phenylacetylene
(0.165 cm3, 1.50 mmol) in dichloromethane (50 cm3) was allowed
to react in the presence of diethylamine (0.156 cm3, 1.50 mmol)
and CuI (28.2 mg, 0.150 mmol) at room temperature for 4 h to

2042 Bull. Chem. Soc. Jpn., 77, No. 11 (2004)
Pd-Catalyzed Carbonylation of Alkynes
give a turbid red-brown suspension. A saturated aqueous NaCl so-
lution was added to the resultant mixture and the aqueous layer
was extracted with dichloromethane. The combined dichloro-
methane solution was dried over MgSO4. After filtration to re-
ing sodium acetate (51.2 mg, 0.624 mmol). The mixture was stir-
red for 32 h under a balloon pressure of carbon monoxide at room
temperature. The GC and GC–MS analyses of the products were
performed with n-C12H26 as an internal standard. Formation of
methyl 3-phenyl-2-propynoate (75%) was confirmed by compari-
son with a sample prepared by carbonylation of phenylacetylene
(NMR and GC–MS). m=z (rel. intensity) 162 (9), 129 (73), 102
(94), 75 (100).
When a similar reaction of the alkynylpalladium complex 3 (77
mg, 0.10 mmol) dissolved in dichloromethane (9 cm3) with CO (1
atm) and methanol (0.200 cm3, 4.9 mmol) was carried out at room
temperature for 4 d in the presence of triethylamine (0.0289 cm3,
0.207 mmol) and tetraethylammonium chloride (17 mg, 0.10
mmol), GC analysis of the reaction mixture revealed the formation
of methyl 3-phenyl-2-propynoate (40%) and 1,4-diphenyl-1,3-bu-
tadiyne (31%). Formation of trans-[PdCl(COOMe)(PPh3)2] (1) in
24% yield was also confirmed.
ꢁ
move insoluble impurities, the solution was cooled to ꢃ30 C to
cause deposition of yellow crystals. The solid was subsequently
washed twice with pentane and dried under vacuum to give 305
mg (40%) of 3. It was identified as trans-[PdCl(CꢂCPh)(PPh3)2],
1
(3) by comparison of spectroscopic data. H NMR (CDCl3, r. t.,
400 MHz) ꢅ 7.3–7.8 (m, 30H, PPh3), 6.8–6.9 (m, 3H, m,p-C6H5),
6.10 (d, 2H, 3JHH ¼ 7:1 Hz, o-C6H5); 13C{1H} NMR (CDCl3, r. t.,
3
2
125 MHz) ꢅ 111.7 (t, JPC ¼ 7:2 Hz, CꢂC–Ph), 96.4 (t, JPC
¼
15:0 Hz, CꢂC–Ph); 31P{1H} NMR (CDCl3, r. t., 161 MHz) ꢅ
25.0 (s); IR (KBr disc) ꢆCꢂC 2121 cmꢃ1
Synthesis of trans-[PdCl(CꢀC–p-tol)(PMe3)2] (4):
.
cis-
[PdCl2(PMe3)2] (506 mg, 1.53 mmol) was dissolved in dichloro-
methane (100 cm3) in an Erlenmeyer flask at room temperature.
To the solution were added diethylamine (0.160 cm3), CuI (29.5
mg, 0.153 mmol), and p-tolylethyne; then the mixture was stirred
at room temperature for 2 h. Water was added to the dichloro-
methane solution; the organic layer was first extracted, then wash-
ed with aqueous solutions of ammonium chloride and NaCl. After
the organic layer was dried with magnesium sulfate, the organic
layer was separated by filtration and concentrated to dryness. Af-
ter the residue was dissolved in dichloromethane (3 cm3), hexane
(25 cm3) was added at ꢃ30 ꢁC to give yellow-needles, which were
separated and washed with hexane (8 cm3 ꢄ 4). Drying the crys-
tals at room temperature gave a yellowish white solid (378 mg,
Synthesis of trans-[Pd(CꢀCPh)2(PPh3)2] (5):
Palladium
acetate (56 mg, 0.250 mmol) was dissolved at room temperature
in dimethylformamide (20 cm3) together with triphenylphosphine
(131 mg, 0.500 mmol). Phenylacetylene (0.110 cm3, 1.00 mmol)
and triethylamine (0.270 cm3, 1.94 mmol) were added to the solu-
tion; this mixture was stirred at room temperature for 2 d to cause
formation of a white precipitate. The solvent was removed by fil-
tration and the residue was washed with hexane (5 cm3 ꢄ 3) and
dried to give a white powder (127 mg corresponding to 61% of 5).
1H NMR (400 MHz, CDCl3, r. t.) ꢅ 7.3–7.9 (30H, phenyl protons),
6.85–6.95 (6H, m, m,p-C6H5), 6.25–6.35 (4H, m, o-C6H5);
13C{1H} NMR (125 MHz, CDCl3, r. t.) ꢅ 124–136 (phenyl car-
1
60%). H NMR (CDCl3, r. t., 400 MHz) ꢅ 7.0–7.3 (4H, aromatic
3
2
protons), 2.31 (3H, s, C6H4-p-CH3), 1.58 (18H, vt, JPH ¼ 3:66 Hz,
bons), 114.8 (t, JPC ¼ 4:15 Hz, CꢂCPh), 113.6 (t, JPC ¼ 16:6
Hz, CꢂCPh); 31P{1H} NMR (161 MHz, CDCl3, r. t.) ꢅ 26.45
(s); IR (KBr disc) 2106 cmꢃ1 (ꢆCꢂC); Found: C, 74.81; H,
4.65%. Calcd for C52H40P2Pd: C, 74.69; H, 4.84%.
P(CH3)3); 13C{1H} NMR (125 MHz, CDCl3, r. t.) ꢅ 124–136
3
(aromatic carbons), 106.4 (t, JPC ¼ 5:2 Hz, CꢂC–p-tol), 94.2
2
(t, JPC ¼ 16:1 Hz, CꢂC–p-tol); 31P{1H} NMR (161 MHz,
CDCl3, r. t.) ꢅ ꢃ12:64 (s); IR (KBr disc) 2117 cmꢃ1 (ꢆCꢂC).
Reaction of trans-[PdCl(COOMe)(PPh3)2] (1) with Phenyl-
acetylene in the Presence of a Base: Phenylacetylene (0.017
cm3, 0.156 mmol) was added to a dimethylformamide solution
(5 cm3) containing trans-[PdCl(COOMe)(PPh3)2] (113 mg,
0.156 mmol) and sodium acetate (51.2 mg, 0.624 mmol) at room
temperature. The mixture was stirred for 24 h under balloon pres-
sure of carbon monoxide at room temperature. The GC analysis of
the reaction mixture with n-C12H26 as an internal standard re-
vealed the formation of methyl 3-phenyl-2-propynoate (73%),
which was identified by 1H NMR on comparison with an authentic
sample (Tokyo Kasei Kogyo Co.).
Reaction of trans-[Pd(CꢀCPh)2(PPh3)2] (5) with CO: The
dialkynylpalladium complex trans-[Pd(CꢂCPh)2(PPh3)2] (5) (123
mg, 0.48 mmol) was dissolved in dimethylformamide (7 cm3) in a
Schlenk tube; the mixture was stirred at room temperature for 24 h
under the atmosphere of carbon monoxide. The GC analysis of the
product with n-C10H22 as an internal standard revealed the forma-
tion of 1,4-diphenyl-1,3-butadiyne (78%).
Reaction of trans-[PdCl(CꢀCPh)(PPh3)2] (3) with CO: The
monoalkynylpalladium chloride trans-[PdCl(CꢂCPh)(PPh3)2] (3)
(29 mg) was dissolved in dichloromethane (5 cm3) and the mix-
ture was treated with CO (balloon pressure) at room temperature
for 18 h. Examination of the reaction product with NMR revealed
that the starting complex remained unreacted.
Reaction of trans-[PdCl(COOMe)(PPh3)2] (1) with Phenyl-
acetylene in the Absence of a Base: When the reaction of 1
(6.20 mg, 0.00900 mmol) dissolved in CDCl3 (0.4 cm3) with
phenylacetylene (0:930 ꢄ 10ꢃ3 cm3, 0.00900 mmol) was carried
out in an NMR tube for 16 h at room temperature, no change of
the NMR spectrum was observed. When a similar reaction of 1
(112 mg, 0.154 mmol) dissolved in dichloromethane (5 cm3) with
20 mol. amounts of phenylacetylene (0.339 cm3, 3.08 mmol) was
carried out at room temperature, an orange suspension was ob-
tained. After concentration of the solution in vacuo and addition
of hexane (20 cm3), an orange precipitate was produced. This was
collected and washed with pentane (5 cm3 ꢄ 3). Formation of
trans-[PdCl(CꢂCPh)(PPh3)2] (3) in 25% yield was confirmed.
Reaction of trans-[PdCl(CꢀCPh)(PPh3)2] (3) with Methanol
under CO: Methanol (0.208 cm3, 5.13 mmol) was added at
room temperature to a dimethylformamide solution (5 cm3) of
trans-[PdCl(CꢂCPh)(PPh3)2] (3) (113 mg, 0.156 mmol) contain-
Attempt at Preparation of an Alkynyl(methoxycarbonyl)-
palladium Complex from 4. The reaction of trans-
[PdCl(COOMe)(PMe3)2] (2) (11.7 mg, 0.0331 mmol) in CD2Cl2
(0.400 cm3) with phenylacetylene (0.0042 cm3, 0.0331 mmol),
ꢁ
carried out in an NMR tube at ꢃ60 C in the presence of AgBF4
(acetone-d6 solution, 0.0331 cm3, 1 mol dmꢃ3, 0.0331 mmol),
gave a complex mixture as indicated by observation of a compli-
cated spectrum. Formation of the expected alkynyl(methoxycar-
bonyl)palladium species could not be confirmed.
Another approach using trans-[PdCl(CꢂC–p-tolyl)(PMe3)2]
(4) by its first treatment with AgBF4 (1 molar amount) at ꢃ30
ꢁC in CDCl3 followed by reaction with CO and LiOMe in MeOH
gave a black reaction product. No indication of formation of
the expected alkynyl(methoxycarbonyl)palladium species was ob-
tained.
Reaction of [Pd(PPh3)4] with O2 (1 atm). A tetrahydrofuran

Y. Izawa et al.
Bull. Chem. Soc. Jpn., 77, No. 11 (2004) 2043
solution (10 cm3) of [Pd(PPh3)4] (593 mg, 0.513 mmol) and lithi-
um chloride (47.8 mg, 1.13 mmol), was treated with a balloon
pressure of oxygen at room temperature. After the solution was
stirred at room temperature for 6 h, the resultant mixture was
examined by GC with n-C12H26 as an internal standard to reveal
the formation of triphenylphosphine oxide. The transformation
of [Pd(PPh3)4] into trans-[PdCl2(PPh3)2] was also confirmed by
examination of NMR of the yellow solid (333 mg, 92%) recovered
from the system. Water (30 cm3) was added to the reaction mix-
ture and the pH of the filtrate was measured to be 12.2.
The autoclave was pressurized with CO (50 atm) and oxygen
(7.5 atm) and the contents were stirred at room temperature
for 48 h. The reaction mixture was filtered and separated into
water-soluble and ether-soluble fractions. The ether solution was
washed with aqueous solutions containing ammonium chloride
and NaCl; the resultant ether solution was dried over magnesium
sulfate. The product esters were analyzed with NMR using
(CHCl2)2 as an internal standard.
Characterization of Products.
Methyl 3-Phenyl-2-pro-
pynoate:37 Yield, 82%; 1H NMR (CDCl3, r. t., 400 MHz) ꢅ
7.2–7.6 (5H, aromatic H), 3.84 (3H, s, COOCH3). 13C{1H} NMR
(CDCl3, r. t., 125.6 MHz) ꢅ 154.3 (s, carbonyl C), 132.9 (s, aro-
matic C), 130.6 (s, aromatic C), 128.5 (s, aromatic C), 119.5 (s,
aromatic C), 86.5 (s, PhCꢂC), 80.3 (s, PhCꢂC), 52.8 (s, OCH3).
Reaction of Pd(dba)2, 2 molar amounts of PPh3, and LiCl with
molecular oxygen under similar conditions also gave trans-
[PdCl2(PPh3)2].
Reaction of [Pd(PPh3)4] with O2, LiCl, Triethylamine, and
CO in MeOH. Triethylamine (0.141 cm3, 1.01 mmol) dissolved
in 1 cm3 of methanol was added to a THF solution (12 cm3) con-
taining [Pd(PPh3)4] (584 mg, 0.505 mmol) and LiCl (23.4 mg,
0.552 mmol). The reaction mixture was stirred at room tempera-
ture under the balloon pressure of molecular oxygen and carbon
monoxide for 22 h. The light yellow suspension produced was
filtered to give a white solid, which was washed with methanol
(5 cm3 ꢄ 2) and pentane (5 cm3 ꢄ 2). Examination of the
NMR spectra of the white solid (287 mg, 78%) confirmed the
formation of trans-[PdCl(COOMe)(PPh3)2].
General Procedure in Catalytic Oxidative Carbonylation of
Alkynes (Method A). A typical procedure is as follows. Palla-
dium acetate (22.4 mg, 0.100 mmol) and triphenylphosphine
(52.4 mg, 0.200 mmol) were mixed in dimethylformamide (20
cm3) in a 100 cm3 two-necked round-bottomed flask under argon.
A rubber balloon filled with carbon monoxide was attached to the
flask. Methanol (2 cm3) and phenylacetylene (0.110 cm3, 1.00
mmol) were added, followed by attachment of a rubber balloon
of oxygen to the flask. The reaction mixture was allowed to react
with stirring at room temperature for 48 h. The mixture was then
treated with diethyl ether and water and the aqueous layer was ex-
tracted with diethyl ether. The combined ether solution was dried
over MgSO4 and the solvent was evaporated in vacuo. Purification
of the residue by column chromatography (hexane/ethyl acetate)
gave the corresponding product.
General Procedure (Method B). A typical procedure is as
follows. Palladium dichloride (8.7 mg, 0.050 mmol), triphenyl-
phosphine (26.3 mg, 0.100 mmol), and sodium acetate (26.4
mg, 0.300 mmol) were mixed in N,N-dimethylformamide (10
cm3) in a 50 cm3 two-necked round-bottomed flask under argon.
A rubber balloon filled with carbon monoxide was connected to
the flask. Methanol (2 cm3) and phenylacetylene (0.110 cm3,
1.00 mmol) were added to the solution and then a rubber balloon
of oxygen was attached to the flask. The reaction mixture was al-
lowed to react with stirring at room temperature for 48 h. The re-
action was quenched by adding diethyl ether and water; then the
aqueous layer was extracted with diethyl ether. The combined
ether solution was dried over MgSO4 and the solvent was evapo-
rated in vacuo. Purification of the residue by column chromatog-
raphy (hexane/ethyl acetate) gave the corresponding product.
Catalytic Carbonylation of Alkynes with Pd/C Catalyst.
Into a 100 cm3 stainless steel autoclave filled with argon were
added 10% Pd/carbon (53.4 mg, 0.0502 mmol), triphenylphos-
phine (26.7 mg, 0.102 mmol), tetraethylammonium chloride
(163 mg, 0.984 mmol), and N,N-dimethylformamide (20 cm3)
and the mixture was stirred at room temperature. To this system
were added methanol (2 cm3, 50 mmol), triethylamine (0.275
cm3, 2.00 mmol), and phenylacetylene (0.110 cm3, 1.0 mmol).
Butyl 3-Phenyl-2-propynoate:38
Yield, 86%; 1H NMR
3
(CDCl3, r. t., 500 MHz) ꢅ (d, 2H, JHH ¼ 7:32 Hz, aromatic H),
7.46–7.43 (m, 1H, aromatic H), 7.38–7.35 (m, 2H, aromatic H),
3
3
4.24 (t, 2H, JHH ¼ 6:96 Hz, OCH2), 1.70 (tt, 2H, JHH ¼ 7:51,
3
6.96 Hz, OCH2CH2), 1.44 (tq, 2H, JHH ¼ 7:51, 7.32 Hz,
CH2CH3), 0.96 (t, 3H, 3JHH ¼ 7:32 Hz, CH2CH3); 13C{1H} NMR
(CDCl3, r. t., 125.6 MHz) ꢅ 154.2 (s, carbonyl C), 133.0 (s, aro-
matic C), 130.5 (s, aromatic C), 128.5 (s, aromatic C), 119.7 (s,
aromatic C), 86.0 (s, PhCꢂC), 80.7 (s, PhCꢂC), 65.9 (s, OCH2),
30.5 (s, OCH2CH2), 19.0 (s, CH2CH3), 13.6 (s, CH2CH3). IR
(neat) ꢆ 2221, 1708 cmꢃ1
.
N,N-Diethyl-3-phenyl-2-propynamide:39
Yield, 60%;
1H NMR (CDCl3, r. t., 500 MHz) ꢅ 7.52–7.55 (m, 2H, aromatic
3
H), 7.34–7.43 (m, 3H, aromatic H), 3.67 (q, 2H, JHH ¼ 7:17
3
Hz, NCH2), 3.48 (q, 2H, JHH ¼ 7:17 Hz, NCH2), 1.28 (t, 3H,
3JHH ¼ 7:17 Hz, NCH2CH3), 1.18 (t, 3H, JHH ¼ 7:17 Hz,
3
NCH2CH3); 13C{1H} NMR (CDCl3, r. t., 125.6 MHz) ꢅ 153.9
(s, carbonyl C), 132.3 (s, aromatic C), 129.8 (s, aromatic C),
128.4 (s, aromatic C), 120.8 (s, aromatic C), 88.9 (s, PhCꢂC),
81.9 (s, PhCꢂC), 43.5 (s, NCH2), 39.3 (s, NCH2), 14.4 (s,
NCH2CH3), 12.8 (s, NCH2CH3). IR (neat) ꢆ 2220, 1626 cmꢃ1
.
Methyl 3-(4-Methylphenyl)-2-propynoate:11
Yield, 74%;
3
1H NMR (CDCl3, r. t., 500 MHz) ꢅ 7.48 (d, 2H, JHH ¼ 8:06
3
Hz, aromatic H), 7.18 (d, 2H, JHH ¼ 8:06 Hz, aromatic H),
3.83 (s, 3H, OCH3), 2.38 (s, 3H, PhCH3); 13C{1H} NMR (CDCl3,
r. t., 125.6 MHz) ꢅ 154.6 (s, carbonyl C), 141.3 (s, aromatic C),
133.0 (s, aromatic C), 129.4 (s, aromatic C), 116.4 (s, aromatic
C), 87.1 (s, PhCꢂC), 80.0 (s, PhCꢂC), 52.7 (s, OCH3), 21.7 (s,
–C6H4–CH3). IR (KBr disc) ꢆ 2221, 1713 cmꢃ1
.
Methyl 3-(4-Bromophenyl)-2-propynoate:40
Yield, 79%;
1H NMR (CDCl3, r. t., 500 MHz) ꢅ (d, 2H, JHH ¼ 7:32 Hz, aro-
3
3
matic H), 7.59 (d, 2H, JHH ¼ 7:32 Hz, aromatic H), 3.84 (s, 3H,
OCH3), 13C{1H} NMR (CDCl3, r. t., 125.6 MHz) ꢅ 154.2 (s, car-
bonyl C), 134.3 (s, aromatic C), 132.0 (s, aromatic C), 125.5 (s,
aromatic C), 118.5 (s, aromatic C), 85.2 (s, PhCꢂC), 81.3 (s,
PhCꢂC), 52.9 (s, OCH3). IR (KBr disc) ꢆ 2226, 1714 cmꢃ1
.
Found: C, 50.04, H, 2.87%. Calcd for C10H7O2Br: C, 50.24; H,
2.95%.
1
Methyl 2-Octynoate:38 Yield, 75%; H NMR (CDCl3, r. t.,
3
500 MHz) ꢅ 69 (s, 3H, OCH3), 2.26 (t, 2H, JHH ¼ 7:14 Hz,
3
CH2CꢂC), 1.52 (tt, 2H, JHH ¼ 7:51, 7.14 Hz, CH2CH2CꢂC),
3
1.18–1.35 (m, 4H, CH3(CH2)2), 0.83 (t, 3H, JHH ¼ 7:14 Hz,
CH3CH2); 13C{1H} NMR (CDCl3, r. t., 125.6 MHz) ꢅ 154.3 (s,
carbonyl C), 89.9 (s, CH2CꢂC), 72.8 (s, CH2CꢂC), 52.5 (s,
OCH3), 30.9 (s, CH3CH2CH2), 27.2 (s, CH2CH2CꢂC), 22.1 (s,
CH3CH2), 18.6 (s, CH2CꢂC), 13.8 (s, CH3CH2). IR (neat) ꢆ
2238, 1716 cmꢃ1
.
Methyl 7-Hydroxy-2-heptynoate:41 Yield, 42%; 1H NMR

2044 Bull. Chem. Soc. Jpn., 77, No. 11 (2004)
Pd-Catalyzed Carbonylation of Alkynes
(CDCl3, r. t., 500 MHz) ꢅ 3.76 (s, 3H, OCH3), 3.67 (m, 2H,
OCH2), 2.40 (m, 2H, CH2CꢂC), 1.69 (m, 4H, OCH2(CH2)2);
13C{1H} NMR (CDCl3, r. t., 125.6 MHz) ꢅ 154.2 (s, carbonyl
C), 89.4 (s, CH2CꢂC), 73.0 (s, CH2CꢂC), 62.0 (s, OCH2), 52.5
(s, OCH3), 31.5 (s, OCH2CH2), 23.8, CH2CH2CꢂC), 18.4 (s,
13 The first observation of palladium-catalyzed oxidative car-
bonylation without metal additive or organic oxidants under at-
mospheric pressure of carbon monoxide in a balloon was made
by Dr. Kazuo Nagasawa in the course of the synthetic study to-
ward 1ꢀ,25-dihydroxyvitamin D3 and its A ring synthon. (Kazuo
Nagasawa, Dissertation 1993 Waseda University). The dehydro-
genative carbonylation was found to proceed generally in the re-
action of 1-alkynes in DMF–alcohol under CO in a rubber balloon
in the presence of palladium acetate–PPh3 (1:2) catalyst giving 2-
alkynoates in moderate yields, accompanied by formation of 1,3-
dialkynes as oxidative coupling products. No hydrogen evolution
was detected in the reaction systems and further search for the
possible oxidizing agent revealed that oxygen inadvertently ad-
mitted into the catalyst system served as the oxidant (unpublished
results in Shimizu Lab, Waseda University).
CH2CꢂC). IR (neat) ꢆ 3399, 2236, 1718 cmꢃ1
.
Methyl 7-(t-Butyldimethylsiloxy)-2-heptynoate:42
83%; 1H NMR (CDCl3, r. t., 500 MHz) ꢅ 3.71 (s, 3H, OCH3),
Yield,
3
3
3.59 (t, H, JHH ¼ 5:86 Hz, OCH2), 2.33 (t, 2H, JHH ¼ 6:86
Hz, CH2CꢂC), 1.55–1.65 (m, 4H, OCH2(CH2)2), 0.84 (s, 9H,
SiC(CH3)3), 0.00 (s, 6H, Si(CH3)2); 13C{1H} NMR (CDCl3, r.
t., 125.6 MHz) ꢅ 154.2 (s, carbonyl C), 89.7 (s, CH2CꢂC), 73.0
(s, CH2CꢂC), 62.3 (s, OCH2), 52.5 (s, OCH3), 31.7 (s,
OCH2CH2), 25.9 (s, C(CH3)3), 24.1 (s, CH2CH2CꢂC), 18.5 (s,
CH2CꢂC), 18.3 (s, C(CH3)3), ꢃ5:4 (s, Si(CH3)2). IR (neat) ꢆ
2238, 1719 cmꢃ1
.
14 E. D. Dobrzynski and R. J. Angelici, Inorg. Chem., 14, 323
(1978).
15 a) M. Hidai, M. Kokura, and Y. Uchida, J. Organomet.
Chem., 52, 431 (1973). b) K. Kudo, M. Hidai, T. Murayama,
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The study was supported by grants from the Ministry of Ed-
ucation, Culture, Sports, Science and Technology and from
Nippon Zeon Co. Ltd. We thank the Material Characterization
Central Laboratory of Waseda University for elemental analy-
ses.
17 R. Bertani, G. Cavinato, L. Toniolo, and G. Vasapollo,
J. Mol. Catal., 84, 165 (1993).
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