LETTER
522
Hydrogenation and Hydrogenolysis with Pd/C in Poly(Ethylene Glycol)
(PEG): A Practical and Recyclable Medium
H
ydrogenation
.
and
H
ydrog
C
enolysis
w
ith Pd
h
-C in Poly(E
a
thylene
G
ly
n
col) drasekhar,* T. Shyamsunder, G. Chandrashekar, Ch. Narsihmulu
Division of Organic Chemistry-I, Indian Institute of Chemical Technology, Hyderabad 500 007, India
Fax +91(40)27160512; E-mail: srivaric@iict.res.in
Received 9 September 2003
led to precipitation of solid PEG and catalyst. This tech-
nique allowed us to decant the ether layer without loss of
PEG and catalyst. The method was repeated twice with 5
mL portions of diethyl ether to extract the product, which
was concentrated and purified by column chromatogra-
phy to give the required product 1b in 95% yield. Encour-
aged by this observation we verified the recyclability of
both catalyst and solvent. The residual PEG–catalyst
mixture obtained from the first run was brought to room
temperature and for the benzyl ether 1a added under the
hydrogen atmosphere. Work-up followed as described
above.
Abstract: Pd/C in PEG (400) has been found to be an efficient
reusable reaction medium for hydrogenation and hydrogenolysis.
Both the catalyst and PEG were recycled efficiently over four runs
without appreciable loss of activity.
Key words: hydrogenation, hydrogenolysis, Pd/C, poly (ethylene
glycol), recycling
Catalytic hydrogenation and hydrogenolysis occupy an
important status in organic synthesis1 especially as these
transformations are high yielding and more chemoselec-
tive. Palladium is arguably the most versatile and widely
applied metal catalyst. Many Pd catalysed reactions give
reliable results and are easy to perform in ordinary equip-
ments. The major drawback of this catalyst is its flamma-
bility and danger of ignition while adding the catalyst to
solvent or during filtration of the catalyst irrespective of
the volume and scale of the reaction. The reaction condi-
tions for hydrogenation and hydrogenolysis are identical
even though some modifications in the catalyst such as
poisoning may help to moderate or activate one over the
other. The most common hydrogenation reactions involve
reduction of alkynes and olefins to saturation.2,3 The
hydrogenolysis reactions involve cleavage of O-benzyl
ethers and N-benzyl compounds.4
Scheme 1
Scheme 2
En route to the recently liquid polymers or low melting
polymers have been used as solvents.5 We have developed This protocol was continued for third and fourth runs,
poly(ethylene glycol) (PEG) as an efficient recyclable re- which resulted in 93% and 90% yields respectively. Sugar
action medium for the Heck reaction6 and Sharpless derivative 2a underwent hydrogenation resulting the de-
asymmetric dihydroxylation.7 Herein we disclose PEG sired product 2b in 90% yield. Keto derivatives 3a and 4a
(400 Dalton) as the most efficient solvent system for com- debenzylated chemoselectively to give products in 92%
plete hydrogenation and hydrogenolysis of various func- and 90% yield respectively.
tionalities virtually at ambient temperature and
In addition, hydrogenation of alkenes, alkynes was
atmospheric pressure. The solvent (PEG) and catalyst (Pd/
achieved with Pd/C in PEG (Scheme 2). Simple alkynes
C) were recycled with retention of efficiency and activity
5a and 6a, alkene 7a, were reduced under the same condi-
tions yielding 5b, 6b and 7b in 97%, 93% and 92% yield
over four runs.
We initially examined the deprotection of benzyl ether respectively. Unsaturated esters and lactol ethers were
(Scheme 1, entry 1a in Table 1; 500 mg, 2 mmol), 2 g of also efficiently hydrogenated without any difficulty re-
PEG (400) and 10% Pd/C (20 mg, 0.02 mmol) were sulting to in 85% and 86% yield respectively.
stirred under hydrogen balloon for 6 hours. Anhydrous di-
In conclusion, this communication describes hydrogena-
ethyl ether (5 mL) was added, the mixture stirred for five
tion and hydrogenolysis in PEG as a recyclable solvent
minutes and the reaction was allowed to settle for 5 min-
medium. The Pd/C catalyst was found to be recycled effi-
utes. Cooling of the contents in an acetone–dry ice bath
ciently for 4 runs.
SYNLETT 2004, No. 3, pp 0522–0524
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Advanced online publication: 19.12.2003
DOI: 10.1055/s-2003-45007; Art ID: D22603ST
© Georg Thieme Verlag Stuttgart · New York