Pd/CaCO3 in liquid poly(ethylene glycol) (PEG): An easy and efficient recycle system for partial reduction of alkynes to cis-olefins under a hydrogen atmosphere

Article abstract of DOI:10.1016/j.tetlet.2004.01.097

Lindlar's catalyst (Pd/CaCO₃) in PEG (400) has been found to be the most reusable reaction medium for selective reduction of alkynes to cis-olefins. The catalyst and PEG were recycled five times without loss of activity.

Full text of DOI:10.1016/j.tetlet.2004.01.097

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 2421–2423
Pd/CaCO in liquid poly(ethylene glycol) (PEG): an easy and
3
efficient recycle system for partial reduction of alkynes to cis-olefins
under a hydrogen atmosphere
q
*
S. Chandrasekhar, Ch. Narsihmulu, G. Chandrashekar and T. Shyamsunder
Indian Institute of Chemical Technology, Hyderabad 500 007, India
Received 3 November 2003; revised 7 January 2004; accepted 16 January 2004
3
Abstract—LindlarÕs catalyst (Pd/CaCO ) in PEG (400) has been found to be the most reusable reaction medium for selective
reduction of alkynes to cis-olefins. The catalyst and PEG were recycled five times without loss of activity.
Ó 2004 Elsevier Ltd. All rights reserved.
ÔPollution controlÕ demands are a source of inspiration
to design and develop new catalysts and solvent media
for achieving high efficiency. Towards this goal, many
catalytic systems have been invented and demonstrated.
More recently, attention has been drawn to the devel-
opment of environmentally benign solvents viz., room
Many researchers worldwide have striven to achieve this
ideal situation. We conceived a protocol keeping in mind
all these facts. Catalytic partial hydrogenation of acetyl-
enes to olefins with Pd/CaCO is a perfect Ôatom econ-
3
omyÕ transformation. If one could develop a solvent,
which can prevent the catalyst from losing its activity
while allowing one to recycle the catalyst and solvent, the
whole operation would become very practical.
1
2
temperature ionic liquids (RTILs), scCO poly(ethyl-
2
3
4
ene glycol), and water are a few of the new additions.
It is customary to measure the efficiency of a catalyst by
the number of cycles for which it can be reused (turn
over number). Similarly, the value of a new solvent
medium primarily depends on its environmental impact,
the ease with which it can be disposed and the number of
times the solvent can be recycled. Secondary issues
concern a low vapor pressure, nonflammability, and
high polarity for solubilization. RTILs are one of the
recent additions, which meet several of the above
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Poly(ethylene glycol) (PEG), a biologically acceptable
polymer used extensively in drug delivery and in bio-
conjugates as tools for diagnostics, has hitherto not been
widely used as a solvent medium but has been used as a
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support for various transformations.
We have recently demonstrated that PEG is a useful
9
solvent medium for asymmetric dihydroxylation, and
10
5
requirements. However the high cost and unproven
disposal issues limit their use in industry.
2
Pd(OAc) catalyzed Heck reactions. Herein, we dis-
close a practical protocol for partial reduction of triple
11
bonds to cis-olefins using LindlarÕs catalyst in PEG
(400 Dal), (Scheme 1). The PEG and the catalyst can be
recycled in all the cases studied (3–5 times) without loss
in activity or yields.
The most ideal synthetic methodology could be defined
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as a system wherein 100% atom economy is preserved,
the solvent is recycled to the fullest extent and the cat-
alyst remains throughout in the solvent medium and
without losing activity for several runs.
In the first instance 2 g of PEG (400), 3-octyn-1-ol
(630 mg, 5 mmol), 5% Pd/CaCO (54 mg, 0.5 mol %), and
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Pd/CaCO (0.5 mol%)
3
R
R'
R,R' = alkyl,aryl
Scheme 1.
Keywords: Hydrogenation; LindlarÕs catalyst; PEG; Recycling.
IICT Communication No. 030701.
H , 1 atm, quinoline
2
R
R'
q
r.t., PEG (400)
*
Corresponding author. Tel.: +91-40-27193434; fax: +91-40-271605-
2; e-mail: srivaric@iict.ap.nic.in
1
0
040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.01.097

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S. Chandrasekhar et al. / Tetrahedron Letters 45 (2004) 2421–2423
Figure 3. After decantation.
Figure 1. During the reaction.
mixture was utilized in further runs by an addition of 3-
octyn-1-ol (630 mg) and two drops of quinoline (catalyst
poison) then exposure to hydrogen. To our pleasant
surprise even after four runs consistent partial reduction
with ꢀ90% yield of products was achieved (Figs. 1–3).
To generalize the process a few other substrates were
subjected to this protocol (Table 1). In all the cases
studied, even after a fourth run, the quantity of catalyst
and PEG was altered only to a minimal extent. Under
the above mentioned conditions a hydrogenation sensi-
tive benzyloxy group (entry 5) and an olefin (entry 6)
survived.
To further substantiate our claims, a cross-over experi-
ment was performed. After five consecutive runs of
partial reduction of diphenylacetylene to cis-stilbene, a
new substrate, 3-octyn-1-ol, was added and subjected to
hydrogenation without changing the catalyst and PEG.
After 5 h the corresponding reduced product (3Z)-octen-
1-ol was obtained in over 85% yield without contami-
nation by cis-stilbene. This demonstrates that the
extraction of products is complete and that the reaction
medium can be reused for different substrates.
Figure 2. After addition of diethyl ether.
two drops of quinoline were stirred under hydrogen for
5
h. Anhydrous ether (10 mL) was added, the mixture
stirred for 5 min and the reaction allowed to settle for
0 min. Cooling in an acetone-dry ice bath caused
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solidification of the PEG with the catalyst entrapped in
this solid. This technique allowed us to decant the ether
layer without loss of PEG or catalyst. The sequence was
repeated twice with 10 mL portions of ether to extract
the product. The residual PEG catalyst complex was
brought to room temperature for liquefacation. This
The efficiency of the sequence can be attributed to the
high solubility of hydrogen in PEG compared to con-
Table 1. Recycling of Pd/CaCO
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catalyzed partial reduction of alkynes to cis-olefins in PEG
a;b
a;b
a;b
a;b
a;b
Entry
1
Alkyne
1st (t/y)
1/97
2nd (t/y)
3rd (t/y)
4th (t/y)
5th (t/y)
Ph
Ph
1/97
1/95
1/91
1/90
2
3
2/96
3/92
2/94
3/92
2/94
3/90
3/90
4/89
3/90
4/90
OH
OTHP
4
4
5
2/98
1/91
2/98
1/90
2/95
1/89
3/94
3/91
HO
OH
c
c
BnO
N.D
N.D
OH
OH
6
4/89
4/87
4/8
5/80
5/80
a
b
c
Isolated yields of products after column chromatography.
t ¼ time in hours, y ¼ yield in %.
Not done.

S. Chandrasekhar et al. / Tetrahedron Letters 45 (2004) 2421–2423
2423
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ventionally used solvents, and efficient retention of the
catalyst by the PEG during extraction of products with
ether due to solidification.
3. (a) Santaniello, E.; Manzocchi, A.; Sozzani, P. Tetra-
hedron Lett. 1979, 47, 4581–4582; (b) Badone, D.; Jommi,
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(
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Overall, the goal of the study has been to evaluate PEG
as an inexpensive, nonvolatile, nonpyrophoric, eco-
friendly, and Ôthe appropriateÕ solvent for catalyzed
reductions. Other catalyzed reduction reactions are
presently being explored.
4. (a) Greico, P. A. Organic Synthesis in Water; Blackie
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. Ionic liquids cost 60–400 USD/50 g, as against PEG (400)
which costs 23 USD/500 g.
. (a) Trost, B. M. Acc. Chem. Res. 2002, 35, 695–705; (b)
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Acknowledgements
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Ch.N., G.C.S., and T.S.S. thank CSIR, New Delhi,
India for financial support.
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125, 4518–4526; (d) Trost, B. M.; Jonasson, C. Angew.
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