Pd(II) - dppb and syngas catalyze regioselective hydroesterification of terminal alkynes under neutral conditions

Article abstract of DOI:10.1016/S0040-4039(01)00152-6

Palladium(II) regioselectively catalyzes the hydroesterification of terminal alkynes under syngas forming α,β-unsaturated esters 3 and 4 in excellent chemical yields under neutral conditions. The high selectivity for the linear ester 4 was obtained with a catalytic system that includes Pd(II), 1,4-bis(diphenylphosphino)butane (dppb) and CO/H₂ in CH₂Cl₂ as solvent. The control of the regioselectivity depends strongly upon the type of ligand, the solvent and the use of the syngas mixture.

Full text of DOI:10.1016/S0040-4039(01)00152-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 2385–2387
Pd(II)—dppb and syngas catalyze regioselective
hydroesterification of terminal alkynes under neutral conditions
Bassam El Ali,* Jimoh Tijani and Abdel Moneim El-Ghanam
Chemistry Department, KFUPM, 31261 Dhahran, Saudi Arabia
Received 22 November 2000; revised 15 January 2001; accepted 25 January 2001
Abstract—Palladium(II) regioselectively catalyzes the hydroesterification of terminal alkynes under syngas forming a,b-unsatu-
rated esters 3 and 4 in excellent chemical yields under neutral conditions. The high selectivity for the linear ester 4 was obtained
with a catalytic system that includes Pd(II), 1,4-bis(diphenylphosphino)butane (dppb) and CO/H2 in CH2Cl2 as solvent. The
control of the regioselectivity depends strongly upon the type of ligand, the solvent and the use of the syngas mixture. © 2001
Elsevier Science Ltd. All rights reserved.
The hydroesterification of 1-nonyne (1b, R1=C7H15)
The catalytic hydroesterification and hydrocarboxyla-
with 1-butanol (2, R2=C4H9) was carried out under
tion of alkynes and other p-bonded compounds are
reactions with important industrial potential.1,2 The
hydroesterification of alkynes usually leads to excellent
selectivity for the branched a,b-unsaturated esters 3 or
to a mixture of esters 3 and 4. For example, the
carbonylation of alkynes has been recently carried out
using Pd(OAc)2, PPh3, p-TsOH, and semilabile anionic
bidentate ligands such as pyridine or piperidine car-
boxylic acids forming the branched a,b-unsaturated
esters 3 with excellent regioselectivity.3 We have previ-
ously reported the selective hydroesterification of l-
alkynes by Pd(0) or by Pd(II).4 The hydroesterification
of propyne catalyzed by Pd(OAc)2 in combination with
2-pyridyldiphenylphosphine (PPh2Py) and MeSO3H
gave ester 3 with excellent selectivity (99.9%).5 We have
recently reported a successful method for the regioselec-
tive control of the thiocarbonylation of terminal
acetylenes with thiophenols.6 In this communication we
wish to report a new catalytic system for the regioselec-
tive hydroesterification of terminal acetylenes catalyzed
by Pd(II) and diphosphine ligand to afford linear a,b-
unsaturated esters 4 as the major product under neutral
conditions (Eq. (1)).
syngas under pressure by changing the type of ligand,
catalyst, solvent, and other parameters.8 The effect of
varying the phosphine ligand in the presence of
Pd(OAc)2 as the catalyst in CH2Cl2 under 600 psi of
CO/H2 (1/1) has been carefully studied and the results
are summarized in Table 1. Only traces of esters were
observed with triphenylphosphine (PPh3), diphenyl-
phosphinomethane (dppm), or 1,2-bis(diphenylphos-
phino)ethane (dppe) as ligand (Table 1, entries 1–3).
However, the use of 1,3-bis(diphenylphosphino)-
propane (dppp) increased the catalytic activity of the
system but produced a 1:1 mixture of branched and
linear a,b-unsaturated esters 3b and 4b (Table 1, entry
4). Surprisingly, replacement of dppp with 1,4-bis-
(diphenylphosphino)butane (dppb) increased the total
yield of esters (98%) and the selectivity for the linear
a,b-unsaturated ester 4b (82%) (Table 1, entry 5).
Changing the CO/H2 ratio from 1/1 to either 5/1 or 1/5
only slightly affects the total yield of esters or the
selectivity for 4b (Table 1, entries 6 and 7). A decrease
in the temperature of the reaction from 110 to 80°C
significantly affected both the yield and the selectivity
(1)
Keywords: palladium(II); hydroesterification; carbonylation; alcohols; alkynes; unsaturated esters; syngas.
* Corresponding author.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00152-6

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B. El Ali et al. / Tetrahedron Letters 42 (2001) 2385–2387
Table 2. Hydroesterification of 1-nonyne 1b with
1-butanol in the presence of dppb and CO/H2 in CH2Cl2.
Effect of varying the type of palladium catalysta
Table 1. Hydroesterification of 1-nonyne 1b with
1-butanol catalyzed by Pd(OAc)2 and CO/H2 in CH2Cl2.
Effect of varying the type of liganda
Total yieldb (%)
Product distributionc (%)
Run Catalyst
Total yieldb (%) Product distributionc (%)
Run
Ligand
3b
4b
3b
4b
1
2
3
4
5
6
Pd(OAc)2
PdCl2(PPh3)2
PdCl2(PhCN)2 96
PdCl2
Pd/C
Pd(PPh3)4
98
95
18
18
19
27
20
58
82
82
81
73
80
42
1
2
3
4
PPh3
dppm
dppe
dppp
dppb
dppb
dppb
dppb
dppb
dpppt
dppf
Traces
Traces
Traces
71
98
87
91
6
Traces
77
94
–
–
–
47
18
18
21
86
–
–
–
–
53
82
82
79
14
–
90
97
72
5
6d
7e
8f
9g
10
11
a Reaction conditions: Pd (0.02 mmol), dppb (0.6 mmol), 1-nonyne
(2.0 mmol), 1-butanol (8.0 mmol), CH2Cl2 (5 ml), 600 psi (CO/H2=
1/1), 20 h.
71
81
29
19
b Isolated yield.
c Determined by GC and 1H NMR.
a Reaction conditions: Pd (0.02 mmol), ligand (0.06 mmol) except
PPh3 (0.12 mmol), 1-nonyne (2.0 mmol), 1-butanol (8.0 mmol),
CH2Cl2 (5 ml), 600 psi (CO/H2=1/1), 110°C, 20 h.
b Isolated yield.
1–4). However, palladium(0) complexes such as
Pd(PPh3)4 gave lower total yields (72%) and low selec-
tivity for 4b (42%). It seems that palladium(0) is proba-
bly not the active species in this process but a Pd(II)
complex is the intermediate throughout.
c Determined by GC and 1H NMR.
d CO/H2=1/5.
e CO/H2=5/1.
f 80°C.
g 1-Butanol (5 ml) was used as solvent.
The results of the hydroesterification of different termi-
nal alkynes by the catalytic system Pd(OAc)2/dppb/1-
butanol/CO/H2 in CH2Cl2 at 110°C are shown in Table
3. 1-Pentyne, 1a, also gave a high total yield (87%) and
selectivity for 4a (82%) (Table 3, entry 1). The regiose-
lectivity of the hydroesterification is sensitive to both
steric and electronic effects of the alkyne structure and
to steric hindrance of the alcohol. The presence of a
bulky tertiary group on alkyne favored only the forma-
tion of the linear unsaturated ester 4c (Table 3, entry 3).
The presence of a cyano, chloro or phenyl group
decreased the selectivity for 4 (Table 3, entries 4–7).
However, primary and secondary alcohols led to linear
unsaturated esters 4 as the major product (Table 3,
entries 1, 2, 8 and 9) but tertiary alcohols reacted with
lower regioselectivity (Table 3, entry 10). The results
with different alcohols confirm the possibility of the
formation of [PdCOOR2] in the first step in the cata-
lytic cycle, followed by the oxidative addition of the
alkyne.
of the process; only 6% of esters being obtained (Table
1, entry 8). In addition, the use of neat 1-butanol (in the
absence of CH2Cl2) inhibited the reaction giving only
traces of products (Table 1, entry 9). Other solvents
such as CH3CN, CHCl3, toluene, THF, and DMF gave
either low total yields or poor selectivity for 4b. Sur-
prisingly, an inversion of regioselectivity was observed
with 1,5-bis(diphenylphosphino)pentane (dpppt) and
1,1%-bis(diphenylphosphino)ferrocene (dppf) as ligands
(Table 1, entries 10 and 11). With these ligands, high
total yields (77–94%) of the esters were maintained but
good selectivity (71–81%) for the branched ester 3b was
achieved. The activity observed with dppb can be
explained by the fact that this ligand is probably biden-
dated to palladium metal throughout the process and
has the most suitable bite angle7 compared to other
bidentate phosphine ligands.
Table 2 shows the results of hydroesterification of
l-nonyne with 1-butanol with dppb and various palla-
dium complexes. It seems simply that all palladium(II)
complexes are very active in this process, where excel-
lent total yields (90–98%) and good to excellent selectiv-
ity for 4b (73–82%) were obtained (Table 2, entries
The hydroesterification of 1,7-octadiyne 5 (1 mmol)
with methanol (4 mmol) was also carried out in the
presence of the same catalytic system Pd(OAc)2/dppb/
CO/H2 in CH2Cl2 at 110°C for 20 h leading to the
formation of unsaturated diesters 6 (29%) and 7 (71%)
in 96% total yield (Eq. (2)).
(2)

B. El Ali et al. / Tetrahedron Letters 42 (2001) 2385–2387
2387
a
Table 3. Hydroesterification of different alkynes in the presence of the catalytic system Pd(OAc)2/dppb/CO/H2 in CH2Cl2
Run
Alkyne, 1 R1
Alcohol, 2
Total yieldb (%)
Product distributionc (%)
3
4
1
2
3
4
5
6
7
8
9
CH3CH2CH2 1a
CH3(CH2)5CH2 1b
(CH3)3C 1c
CNCH2CH2CH2 1d
ClCH2CH2CH2 1e
PhCH2 1f
CH3CH2CH2CH2OH
CH3CH2CH2CH2OH
CH3CH2CH2CH2OH
CH3CH2CH2CH2OH
CH3CH2CH2CH2OH
CH3CH2CH2CH2OH
CH3CH2CH2CH2OH
CH3OH
87
98
85
70
67
98
93
91
95
96
18 3a
18 3b
0 3c
35 3d
27 3e
42 3f
67 3g
20 3b
25 3b
43 3b
82 4a
82 4b
100 4c
65 4d
73 4e
58 4f
33 4g
80 4b
75 4b
57 4b
Ph 1g
CH3(CH2)5CH2 1b
CH3(CH2)5CH2 1b
CH3(CH2)5CH2 1b
(CH3)2CHOH
(CH3)3COH
10
a Reaction conditions: Pd (0.02 mmol), dppb (0.6 mmol), alkyne (2.0 mmol), alcohol (8.0 mmol), CH2Cl2 (5 ml), 600 psi (CO/H2=1/1), 20 h.
b Isolated yield.
c Determined by GC and 1H NMR.
In conclusion, the regioselective hydroesterification of
terminal alkynes with various alcohols can be achieved
under neutral conditions by using the simple catalytic
system Pd(OAc)2/dppb/CO/H2. This method provides a
significant improvement over the present systems
described in the literature. Mechanistic studies and the
application of this catalytic system to other important
mono- and dialkynes with various alcohols and diols
are in progress.
2. Bolm, C.; Beller, M. In Transition Metals for Fine Chemi-
cals and Organic Synthesis; El Ali, B.; Alper, H., Eds.;
VCH: Weinheim-Germany, 1998; p. 49.
3. Jayasree, S.; Seayed, A.; Gupte, S. P.; Chaudhari, V. Cat.
Lett. 1999, 58, 213.
4. (a) El Ali, B.; Alper, H. J. Mol. Catal. 1991, 67, 29; (b) El
Ali, B.; Alper, H. J. Mol. Catal. 1995, 96, 197.
5. Drent, A.; Budzelaar, M. J. Organomet. Chem. 1993, 455,
247.
6. El Ali, B.; Tijani, J.; El-Ghanam, A.; Fettouhi, M. Tetra-
hedron Lett. 2001, 42, 1567.
Acknowledgements
7. Dierkes, P.; van Leeuwen, P. W. N. M. J. Chem. Soc.,
Dalton Trans. 1999, 1519.
8. Typical experimental procedure for the hydroesterification
of 1-alkynes with alcohols: A mixture of 2.0 mmol of
1-nonyne 1b, 2.0 mmol of 1-butanol, 0.020 mmol of
Pd(OAc)2, and 0.08 mmol of dppb was dissolved in 10 ml
of CH2Cl2 and placed in a 45 ml autoclave. The autoclave
was purged, pressurized (600 psi of CO+H2) and then
heated (110°C). After 20 h the reaction was cooled to
room temperature, filtered through Celite and concen-
trated by rotary evaporation. The esters 3b and 4b were
We thank King Fahd University of Petroleum & Min-
erals (KFUPM-Saudi Arabia) for providing the facility
and for financial support of this project.
References
1. (a) Tsuji, J. Palladium Reagents and Catalysts; Wiley: New
York, 1995; (b) Colquhoun, H. M.; Thompson, D. J.;
Twigg, M. V. Carbonylation; Plenum Press: New York,
1991.
1
identified by GC and H and 13C NMR. All esters 3, 4, 6
and 7 were clearly identified and characterized by GC–MS
1
and H and 13C NMR.
.