Hydrogenation versus hydrogenolysis with a safe, selective and reusable catalyst: Palladium black on Teflon

Article abstract of DOI:10.1039/b501096a

Palladium black deposit is obtained by reduction and metallization of the Teflon polymer surface of magnetic stirring bars. These stirring bars can be used to perform selective hydrogenation of olefins and acetylenic compounds whilst hydrogenolysis is not observed. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2005.

Full text of DOI:10.1039/b501096a

View Online / Journal Homepage / Table of Contents for this issue
L E T T E R
Hydrogenation versus hydrogenolysis with a safe, selective and
s
reusable catalyst: palladium black on Teflon
a
ab
b
a
Damien Belotti, Guillaume Cantagrel, Catherine Combellas, Janine Cossy,*
b
b
Frederic Kanoufi and Sandra Nunige
´ ´
a
b
Laboratoire de Chimie Organique associe´ au CNRS, ESPCI, 10 rue Vauquelin, 75231 Paris
Cedex 05 - France. E-mail: janine.cossy@espci.fr; Fax: þ33 1 40 79 46 60;
Tel: þ33 1 40 79 44 29
Laboratoire Environnement et Chimie Analytique associe´ au CNRS, ESPCI, 10 rue Vauquelin,
7
5231 Paris Cedex 05 - France
Received (in Montpellier, France) 21st January 2005, Accepted 18th March 2005
First published as an Advance Article on the web 27th April 2005
1
3
Palladium black deposit is obtained by reduction and metalliza-
s
tion of the Teflon polymer surface of magnetic stirring bars.
These stirring bars can be used to perform selective hydrogena-
tion of olefins and acetylenic compounds whilst hydrogenolysis
is not observed.
sufficiently negative potential. Then, the reduced zones were
reacted with a diazonium or a metallic salt. This led to the
localized grafting of the organic moiety derived from the
diazonium salt or to localized electroless metallization. This
process takes advantage of the n-doped character of polymeric
s
14
carbon obtained when Teflon is reduced.
We have performed metallization with palladium according
to a two-step procedure deduced from that used for Au, Ag
The classical heterogeneous catalysts for carbon–carbon
multiple bond hydrogenation involve supported precious me-
tals, activated base metal catalysts and Ni supported on oxides,
all being able to activate hydrogen under mild conditions. Due
to its chemoselectivity, Pd is the most widely used catalyst
compared to other Group VIII transition metals, Pt, Rh, Ru
and Ni. Several excellent reviews dealing with the catalytic
hydrogenation of carbon–carbon multiple bonds have ap-
peared in the last few years, providing comprehensive informa-
1
2b
s
A Teflon coated stirring bar was reduced
and Cu plates.
0
upon contact with a stainless steel wire in the presence of 2,2 -
dipyridyl (P) in DMF. The wire was biased at a more negative
0
potential than the reduction potential of 2,2 -dipyridyl (E1 ¼
ꢀ2.10 V vs. saturated calomel electrode) in order to generate
0
ꢀ
d
ꢀ
the 2,2 -dipyridyl radical-anion (P þ e - P ). This proce-
s
dure led to the carbonization and the n-doping of Teflon
according to eqns. (1) and (2) respectively, in which NBu
the cation of the electrolyte:
1
4
is
1
–6
tion on the subject.
Palladium catalysts, such as palladium black or palladium
on carbon (Pd/C), are mainly used to reduce alkenes and
ð1Þ
ð2Þ
7
alkynes to their corresponding hydrocarbons, they can also
8
be used to achieve the hydrogenolysis of halides, benzyl
9
ethers, benzylamines and to convert cyclopropanes to gem-
ꢀ
1
0
In eqn. (1), the electron (e ) is either provided by the electrode
0
dimethyl compounds. Even if the hydrogenation process is a
clean and easily implementable process, the palladium catalyst
is flammable and dry Pd/C catalyst must be handled carefully.
The danger of ignition is always present due to the electrical
discharge caused by friction when palladium is placed in a
reaction mixture. Even if the hydrogenation and the hydro-
genolysis are different reactions, the differentiation between
these two reactions is difficult as the experimental conditions
are the same. If different methods for selective hydrogenation
without hydrogenolysis have been reported, catalysts with
or the radical-anion of 2,2 -dipyridyl. The latter acts as a
reducing agent, especially in the places where there is bad
s
contact between the wire and the Teflon magnetic stirring
bar. The solution was electrolyzed under potentiostatic condi-
tions (Vref ꢀ V
¼ 2.5 V) for 400 s. The reduction resulted
s
stirrer
in the development of a black carbonized zone (2–3 mm wide)
around the stainless steel wire. Secondly, the modified Teflon
bar was reacted with a palladium salt. The latter was reduced
s
spontaneously by the negatively charged Teflon [eqn. (3)],
s
which resulted in the metallization of the Teflon bar:
1
1
selective activity based on their preparation are very rare.
s
Here, we would like to report that a Teflon magnetic
stirring bar with a deposit of palladium black (Pd/TMSB) is
an easily handled, non-flammable catalyst, easy to remove
from the reaction media. This catalyst can be used in the
hydrogenation of olefins and alkynes and is highly chemo-
selective as it can not perform the hydrogenolysis of cyclopro-
panes, benzyl ethers and allylamines. Furthermore, no trace of
the catalyst remains in the solution when Pd/TMSB is used
ð3Þ
The complete process is presented in Scheme 2. Using X-ray
fluorescence, metallization was confirmed by the appearance of
the characteristic system of Pd at about 3 ꢁ 10 eV (Fig. 1).
3
(
Scheme 1).
Chemical modification of Teflon
s
[poly(tetrafluoro-
ethylene), PTFE] by organic moieties or by metals such as
Au, Ag and Cu has already been reported in the case of
s
12
Teflon plates. At first the plates were reduced locally in
the vicinity or at the contact of a disc electrode biased at a
Scheme 1 Hydrogenation of olefins and alkynes using Pd/TMSB.
T h i s j o u r n a l i s & T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 5
N e w J . C h e m . , 2 0 0 5 , 2 9 , 7 6 1 – 7 6 4
761

View Online
Table 1 Recovery and reuse of Pd/TMSB
Run
1
2
3
4
5
Time (h)
Yield (%)
20
498
20
498
20
498
20
498
40
498
The hydrogenation of alkenes in the presence of the Pd/
TMSB catalyst is general and the results are reported in
Table 2. Terminal olefins without or with polar substituents
such as hydroxy groups can be hydrogenated in good yields
(
480%) (Table 2, Entries 1–3). However, 1,3-diol 5 was
recovered unchanged. The presence of the two hydroxy groups
is probably responsible for the deactivation of the Pd/TMSB
due to a chelating effect (Table 2, Entry 4).
The hydrogenation of the benzyl protected but-3-en-1-ol 6
0
led to benzyloxybutane 18 and to compound 18 in a ratio of
0
7
sponds to the migration of the double bond.
0 : 30 and with a global yield of 95%. Compound 18 corre-
16
s
Scheme 2 Carbonization and metallization of a Teflon coated
stirring bar and optical micrograph of Pd/TMSB.
Under
these conditions, the benzyl group was not cleaved (Table 2,
Entry 5).
The amount of deposited palladium, measured by inductive
coupling plasma-mass spectrometry (ICP-MS), was approxi-
mately 4.8 mg for 60 mm and corresponds to an average
In the case of N-Boc-N-benzylallylamine 7, the hydrogena-
tion did not lead to the corresponding deprotected amine but
to the saturated amine 19, still protected by the benzyl group,
in quantitative yield (Table 2, Entry 6). Disubstituted double
bonds were also hydrogenated as (E)-alkene 8 and (Z)-alkenes
9 and 10 were transformed easily and respectively to the
corresponding saturated products 2, 20 and 21 in good yield
(Table 2, Entries 7–9). As previously observed, benzyl protect-
ing groups were not cleaved (Table 2, Entry 9). Furthermore,
activated olefins, such as a,b-unsaturated esters, can be hydro-
genated easily as 11 was transformed to 22 in high yield (Table
2, Entry 10). However, disubstituted olefins with polar nucleo-
philic groups such as carboxylic groups were unreactive to
hydrogenation. For example, cinnamic acid 12 and hept-6-
enoic acid 13 were recovered unchanged due, maybe, to a
coordination of the carboxylic acid to the palladium surface,
which prevents the double bond from reaching the surface for
hydrometallation (Table 2, Entries 11 and 12). The hydrogeno-
lysis of cyclopropanes and hydrogenation of trisubstituted
olefins were not observed when Pd/TMSB was used under
these experimental conditions (Table 2, Entries 13 and 14). It is
worth noting that, with classical Pd/C, the hydrogenation of 10
and 11 leads to the reduction of the double bond, as expected,
but also to the deprotection of the alcohol.
2
deposit thickness of B7 nm.
The Pd/TMSBs were tested in the hydrogenation of olefins.
A typical hydrogenation was achieved as follows: an ethanolic
solution (5 mL) of 1 (1 mmol) containing a Pd/TMSB was
stirred by the Pd/TMSB itself under one atmosphere of hydro-
gen at room temperature. After 17 h, the Pd/TMSB was
removed with a magnetic stick and the solution was concen-
trated under vacuum to give 2 in quantitative yield with a
purity superior up to 98% without any additional purification.
The Pd/TMSB could be recycled after washing with EtOH and
drying under vacuum. The Pd/TMSB could be used four times
in the hydrogenation of 1 without any loss of activity as 2 was
obtained in yields superior to 98%. After the fifth run, the yield
in 2 was identical but the hydrogenation of 1 had to be
performed for 40 h (Table 1). It is worth noting that no trace
of palladium was detected in the hydrogenation solutions
1
5
(
by ICP-MS analysis).
The hydrogenation of acetylenic compounds was also
achieved and the results are summarized in Table 3. The
hydrogenation of 23, 24 and 25, in the presence of a Pd/TMSB,
under one atmosphere of hydrogen, led respectively to the
corresponding saturated compounds 2, 26 and 27 in quantita-
tive yields (Table 3, Entries 1–3).
s
In conclusion, palladium black deposit on a Teflon mag-
netic stirring bar (Pd/TMSB) promotes the hydrogenation of
terminal and disubstituted olefins as well as acetylenic com-
pounds in good yields when the reaction is performed under
one atmosphere of hydrogen. The Pd/TMSB is easy to use,
reusable and leaves no trace of residual palladium in the
solution. Moreover, the deposition of the catalyst on a stirring
bar allows the use of a high mass transfer rate, provided by the
stirring bar rotation rate, toward the catalyst. This should be
beneficial for the kinetics of classical Pd/solvent/H2 hydro-
genation processes. Furthermore, Pd/TMSB exhibits high
selectivity as trisubstituted olefins and a,b-unsaturated car-
boxylic acids are not hydrogenated and the hydrogenolysis of
benzyl ethers, benzylamines and deprotection of allylamines
s
Fig. 1 X-Ray fluorescence spectrum of a Teflon coated stirring bar
after carbonization and metallization.
7
62
N e w J . C h e m . , 2 0 0 5 , 2 9 , 7 6 1 – 7 6 4

View Online
Table 2 Hydrogenation of alkenes with Pd/TMBS
Entry
1
Starting materials
Time (h)
17
Products
Yield (%)
498
2
3
4
5
16
20
40
40
498
82
—
—
95
6
7
8
9
30
20
42
22
24
24
20
20
20
498
95
91
78
10
11
12
13
14
498
—
—
—
—
—
—
—
—
Table 3 Hydrogenation of alkynes
Entry
1
Starting materials
Time (h)
20
Products
Yield (%)
498
2
3
20
20
498
498
N e w J . C h e m . , 2 0 0 5 , 2 9 , 7 6 1 – 7 6 4
763

View Online
ing solution was evaporated and the obtained product was
purified by flash chromatography if necessary.
are not observed. This effect does not result from any specific
s
chemical property of Teflon , which is well-known for its
1
4
inertness. It is likely due to the Pd deposition process and
should be related to the specific physico-chemical properties of
the n-doped carbonized PTFE: Pd deposition is achieved
according to a redox process at an n-doped material surface
of considerable roughness.
References
1
Houben-Weyl, Methoden der Organischen Chemie, ed. H. J. Rim-
bek, Thieme Verlag, Stuttgart, 1980, vol. IV/1c.
2
R. L. Augustine, Heterogeneous Catalysis for the Synthetic Che-
mist, Marcel Dekker, New York, 1996.
A. Molar, J. Mol. Catal. A: Chem, 2001, 17, 185.
`
A. O. King, R. D. Larsen and E.-I. Negishi, in Handbook
of Organopalladium Chemistry for Organic Synthesis, ed. E.-I.
Negishi, Wiley, New York, 2002, p. 2719.
3
4
Experimental
Preparation of the Pd/TMSB
5
6
H.-U. Blaser, C. Malan, B. Pugin, F. Spindler, H. Steiner and
M. Studer, Adv. Synth. Catal., 2003, 345, 103.
B. Chen, U. Dingendissen, J. G. E. Krauser, H. G. J. Lansink
Rotgerink, K. Mobus, D. J. Ostgard, P. Panster, T. H. Riermeier,
S. Seebald, T. Tacke and H. Trauthwein, Appl. Catal. A, 2005,
The magnetic stirring bars used are double cross-head cylinders
and before treatment they are rinsed with acetone under
sonication for 5 min and dried in an oven overnight (80 1C).
In the first step, the cylindrical part of the Teflon magnetic
stirring bar is surrounded with a stainless steel wire (diameter:
s
2
80, 17, and references therein.
7
8
R. L. Augustine, Catalytic Hydrogenation, Marcel Dekker, New
York, 1965.
1
mm) acting as a working electrode. The magnetic stirring bar
and the wire are immersed into a solution of DMF containing
tetrabutylammonium tetrafluoroborate (0.1 M, supporting
(a) W. H. Hartung and R. Simonoff, Org. React., 1953, 7, 263; (b)
P. N. Rylander, Catalytic Hydrogenation in Organic Synthesis,
Academic Press, New York, 1979; (c) P. N. Rylander, Hydrogena-
tion Methods, Academic Press, Orlando, Florida, 1985.
A. R. Pinder, Synthesis, 1980, 425.
0
electrolyte) and 2,2 -dipyridyl (10 mM, reducing agent precur-
sor). The solution also contains a platinum wire (diameter:
9
1
mm) as a counter-electrode and an Ag/AgCl reference
s
10 J. Newham, Chem. Rev., 1963, 63, 123.
1
electrode. After reduction, the solution used for the Teflon
reduction is immediately replaced by a 10 mM solution of
2
1
(a) S. Maki, Y. Harada, T. Hirano, H. Niwa, Y. Yoshida, S.
Ogata, S. Nakamatsu, H. Inoue and C. Iwakura, Synth. Commun.,
1
2ꢀ
Na PdCl
A Leo Stereoscan 440 scanning electron microscope (SEM
4
in DMF for 2 h.
2
000, 30, 3575; (b) H. Sajiki, K. Hattori and K. Hirota, J. Org.
Chem., 1998, 63, 7990; (c) H. Sajiki and K. Hirota, Tetrahedron,
1998, 54, 13981; (d) M. J. Gaunt, J. Yu and J. B. Spencer, J. Org.
Chem., 1998, 63, 4172; (e) S. Maki, M. Okawa, T. Makii, T.
Hirano and H. Niwa, Tetrahedron Lett., 2003, 44, 3717.
B), operating at 20 keV, was used (Princeton Gamma-Tech).
The coating system was a carbon coater Cressington 208
(
ELOISE).
1
2
3
(a) C. Combellas, F. Kanoufi, D. Mazouzi, A. Thie
Bertrand and N. Medard, Polymer, 2003, 44, 19; (b) C. Combellas,
F. Kanoufi, D. Mazouzi and A. Thiebault, J. Electroanal. Chem.,
003, 556, 43.
C. Combellas, F. Kanoufi and D. Mazouzi, J. Phys. Chem. B,
004, 108, 19260.
´
bault, P.
´
´
Hydrogenation using Pd/TMSB
2
1
A Pd/TMSB was introduced into a solution of an unsaturated
compound (1 mmol) in absolute ethanol (5 mL). The solution
was then stirred with the Pd/TMSB under one atmosphere
of hydrogen for around 20 h (GC/MS monitoring). The
Pd/TMSB was removed with a stick, washed with absolute
ethanol and dried in vacuo for further experiment. The result-
2
1
1
4
5
L. Kavan, Chem. Rev., 1997, 97, 3061.
Analysis by ICP-MS after solvent evaporation followed by treat-
ment of the residue with nitrohydrochloric acid (aqua regia).
16 B. M. Trost and T. Schmidt, J. Am. Chem. Soc., 1988, 110, 2301,
and references therein.
7
64
N e w J . C h e m . , 2 0 0 5 , 2 9 , 7 6 1 – 7 6 4