Preferential catalytic hydrogenation of aromatic compounds versus ketones with a palladium substituted polyoxometalate as pre-catalyst

Article abstract of DOI:10.1039/b110937p

A palladium-substituted polyoxometalate having a Keggin structure, supported on γ-alumina or active carbon, was used as a catalyst precursor for catalytic hydrogenation. The catalyst system enabled fast hydrogenation of arenes at 30 bar H₂ and 230°C. Most interesting was the finding that arenes could be selectively reduced in the presence of distal ketone groups under similar conditions, 30 bar H₂ and 200°C. For example, 1-phenyl-2-propanone yielded 1-cyclohexyl-2-propanone with no reduction of the ketone moiety. Additionally, aromatic compounds with vicinal (conjugated) ketone moieties underwent complete hydrogenation to saturated hydrocarbons and catalytic McMurry coupling was observed for aliphatic aldehydes.

Full text of DOI:10.1039/b110937p

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Preferential catalytic hydrogenation of aromatic compounds versus
ketones with a palladium substituted polyoxometalate as pre-catalyst
Vladimir Kogan,a Zeev Aizenshtata and Ronny Neumann*b
a
Department of Organic Chemistry, The Hebrew University, Jerusalem 91904, Israel
Department of Organic Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel.
E-mail: Ronny.Neumann@weizmann.ac.il
b
Received (in Montpellier, France) 29th November 2001, Accepted 15th January 2002
First published as an Advance Article on the web
an impurity of the starting material, [PW11O39]7À. The IR
spectrum showed peaks at 1100, 1046, 950, 851, 805 cmÀ1
A palladium-substituted polyoxometalate having a Keggin struc-
ture, supported on c-alumina or active carbon, was used as a cata-
lyst precursor for catalytic hydrogenation. The catalyst system
enabled fast hydrogenation of arenes at 30 bar H2 and 230 ꢀC.
Most interesting was the finding that arenes could be selectively
reduced in the presence of distal ketone groups under similar con-
ditions, 30 bar H2 and 200 ꢀC. For example, 1-phenyl-2-propa-
none yielded 1-cyclohexyl-2-propanone with no reduction of the
ketone moiety. Additionally, aromatic compounds with vicinal
(conjugated) ketone moieties underwent complete hydrogenation
to saturated hydrocarbons and catalytic McMurry coupling was
observed for aliphatic aldehydes.
.
Together, the elemental analysis and 31P NMR reveal the
formation of a rather pure compound as also previously
published.7 However, the IR spectrum, which is similar to the
lacunary Na7PW11O39ÁxH2O compound (1101, 1043, 951, 858,
807 and 741 cmÀ1), would seem to indicate that the Pd position
is somewhat removed from the polyoxometalate framework
but near the lacunary position.9 This is not especially sur-
prising since true framework substitution of Pd(II) into the
polyoxometalate would require a less or non-preferred octa-
hedral coordination for Pd(II), which generally adopts a square
planar coordination. The lability of the palladium center
was also verified by addition of cyclooctadiene to the
[PPdW11O39]5À compound, which resulted in the demetalation
The use of polyoxometalates as catalysts has become an
important area of research over the past two decades. Most of
the interest has been centered around the use of heteropoly
acids in acid-catalyzed reactions1 and the use of poly-
oxomolybdates and transition metal substituted poly-
oxometalates as catalysts for oxidation.2 The use of
polyoxometalates as catalysts in reductive transformations has
been less explored. Some important examples do exist and
include the use of Rh0 and Ir0 clusters stabilized by poly-
oxometalates for the hydrogenation of alkenes,3 the reduction
of nitroaromatics,4 and the reductive deoxygenation of alde-
hydes and ketones.5 Although the synthesis of various noble
metal substituted polyoxometalates has been described,6 sur-
prisingly the simple use of these compounds in reductive
transformations such as hydrogenation has not been reported.
We have now found that a palladium-substituted Keggin-type
polyoxometalate, supported on active carbon or g-alumina,
yields under hydrogen pressure a palladium(0) catalyst that
shows catalytic activity and selectivity significantly different
than those shown by the common supported Pd(0) catalyst
Pd=C. This interesting selectivity is exemplified by: (a) the
selective hydrogenation of aromatic rings to saturated cyclo-
alkanes in the presence of distal ketone groups that remain
unchanged under the reaction conditions; (b) the complete
hydrogenation of aromatic compounds with vicinal ketone
moieties to saturated hydrocarbons and (c) catalytic McMurry
coupling observable for aliphatic aldehydes.
of the polyoxometalate and formation of [PW11O39]7À
.
The supported pre-catalysts were prepared by wet impreg-
nation with water of K5PPdW11O39Á12H2O (Pd-POM) onto
active carbon and g-alumina, yielding 10wt % (Pd-POM) =C
and 10wt % (Pd-POM) =Al2O3 . These catalysts were first tested
in the hydrogenation of aromatic compounds, Table 1. As can
be seen from the table, the (Pd-POM)=C catalyst is active for
the complete hydrogenation of aromatic compounds to
cycloalkanes at 30bar H 2 and 230 ꢀC. In difficult cases a slight
increase in temperature has a positive effective on the reactivity.
The catalysts were recycled three times by filtration and wash-
ing with dichloromethane without noticeable loss of activity.
Significantly, under the conditions described in the table,
1,2-octanone was not reduced either in the presence or absence
of toluene, which itself was reduced to methylcyclohexane.
This led to the possibility that compounds containing both an
aromatic nucleus and ketone moiety could be hydrogenated
selectively at the aromatic nucleus.10 Therefore, a screening of
hydrogenation of aromatic ketones is presented in Table 2.
Under relatively harsh hydrogenation conditions (30bar,
200 ꢀC), the (Pd-POM)=C catalyst shows high selectivity for
reduction of aromatic rings in the presence of distal ketone
moieties, which remained unchanged, while simple aromatic
ketones such as acetophenone and benzophenone were com-
pletely reduced. However, for the latter substrates hydro-
genation under milder conditions (15 bar, 130 ꢀC) showed
initial reduction of the ketone to the methylene product and
then hydrogenation of the aromatic ring. Interestingly, the
commercial Pd=C catalyst shows significantly different reac-
tion selectivity, leading to reduction of the aromatic ring to the
saturated cycloalkane and the ketone moiety to the corres-
ponding alcohol. With Pd=C ketone moieties are always
reduced to alcohols while with (Pd-POM)=C distal or aliphatic
ketones are not reactive and aromatic ketones are reduced or
deoxygenated to the methylene moiety.
The palladium-substituted Keggin-type polyoxometalate
formulated by elemental analysis as K5PPd(H2O)W11
O39Á12H2O was prepared in a manner similar to the published
literature procedure for the preparation of [(C4H9)4N]5-
PPd(H2O)W11O39 .7 The 31P NMR spectrum (85% H3PO4 as
external standard) revealed a major peak at À15.17 ppm
(95%) attributable to [PPd(H2O)W11O39]5À and=or possibly its
dimer,8 [PW11O39Pd–O–PdPW11O39], and a small peak at
À13.32 ppm (5%) attributable, from an authentic sample, to
272
New J. Chem., 2002, 26, 272–274
DOI: 10.1039/b110937p
This journal is # The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2002

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Table
K5PPdW11O39=C and K5PPdW11O39=Al2O3
1
Hydrogenation of aromatic compounds catalyzed by
a
significantly hindered to prevent effective reduction of the
carbonyl and aryl moiety. (Pd-POM)=Al2O3 and (Pd-POM)=C
showed almost identical reactivity and selectivity. Also fas-
cinating is the reduction of aromatic aldehydes. While
hydrogenation of benzaldehyde yields methylcyclohexane,
hydrogenation of dihydrocinnamaldehyde yielded only a
catalytic McMurry coupling to the diol followed by some
reduction of the aromatic rings. This represents a very sig-
nificant change in the reaction pathway.
Finally, it is also worth noting that the (Pd-POM)=C cata-
lyst is superior to the analogous compounds (Ru-POM)=C and
(Rh-POM)=C, which were less reactive and less selective. Thus,
for the reduction of 4-phenyl-2-butanone under the reaction
conditions noted in Table 2, (Ru-POM)=C yielded 19% 4-
cyclohexyl-2-butanone and 28% 4-phenyl-2-butanol and
(Rh-POM)=C yielded 69% 4-cyclohexyl-2-butanone, 6%
butenylcyclohexane and butylcyclohexene isomers and 3%
cyclohexenyl-2-butanone and cyclohexadienyl-2-butanone.
Although we have no clear-cut understanding of the reaction
selectivity observed, it was useful to carry out some studies on
the supported catalyst after reaction or activation with hydro-
gen. Thus, an XPS measurement of the isolated ‘‘active’’ cat-
alyst from the (Pd-POM)=Al2O3 precursor revealed Pd in zero
oxidation state only.11 On the other hand trituration of the
active catalyst with water at 90 ꢀC showed by 31P NMR the
extraction of the lacunary species, PW11O397À, into solution
(peak at À13.25 ppm) and the absence of dissolved palladium
Substrate
Product, selectivity=mol%
Conversion=mol%
Toluene
Methylcyclohexane, 100
Methylcyclohexane, 100
Hexahydrodurene, 100
Perhydroanthracenes, 93
Octahydroanthracenes, 7
Dihydrophenanthrene, 31
Tetrahydrophenanthrene, 16
Octahydrophenananthrene, 5
Perhydrophenanthrenes, 99
Tetralin, 67
100
100
100
100
Tolueneb
Durene
Anthracene
Phenanthrene
52
Phenanthrenec
Naphthalene
100
100
Decalin, 33
Decalin, 100
Perhydroacridine, 100
Naphthalenec
Acridine
100
100
a
Reaction conditions: 1.75 mmol substrate, 0.3 g 10 wt% K5PPd-
b
W11O39=C, 1 mL pentadecane, 30bar H 2 , 230 ꢀC, 1 h. K5PPd-
W11O39=Al2O3 . 230 ꢀC, 4 h.
c
There are also significant steric effects in this reaction. For
instance, 3-methyl-3-phenyl-2-butanone and also 3-phenyl-2-
butanone (not shown in Table 2) are practically inert to the
hydrogenation reaction. Apparently, the key interaction
between the aryl nucleus and the palladium catalytic center is
Table 2 Hydrogenation of aromatic ketones catalyzed by 10wt% K 5PPdW11O39=Ca
Substrate
Products
b
a
ꢀ
Reaction conditions: 1.75 mmol substrate, 0.3 g 10 wt% K5PdW11O39=C, 1 mL pentadecane, 30bar H 2 , 20 0 C, 4 h. Yields in parentheses are
those found when using commercial 10% Pd=C as catalyst. 16 h.
b
New J. Chem., 2002, 26, 272–274
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2
3
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Therefore, we speculate that the active catalyst is possibly a Pd0
cluster stabilized by the lacunary polyoxometalate species.3
This combination of the Pd0 cluster and polyoxometalate pre-
sumably leads to the unique hydrogenation selectivity observed,
that is the significantly different reactivity observed for aliphatic
ketones and aldehydes versus aromatic ketone and aldehydes.
Notably, impregnation of the Pd=C catalyst with Na7PW11O39
does not lead to this selectivity.
4
B. Amadelli, G. Varani, A. Moldotti and V. Carassiti, J. Mol.
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5
6
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Experimental
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Chem.,Int. Ed. Engl. , 1995, 34, 1587; R. Neumann and A. M.
The Pd-POM formulated as K5PPdW11O39Á12H2O was pre-
pared by adding dropwise PdCl2 (1.1 mmol, 200 mg) dissolved
in 20mL deionized water to a solution of Na PW11O39ÁxH2O
7
(1.0mmol, 3.2 g) dissolved in 20mL hot deionized water. After
additional heating for 1 h at 90 ꢀC a saturated solution of KCl
(20mL) was added and the solution was cooled. The brown
precipitate was collected and recrystallized from water (yield
2.9 g, 90%). ICP elem. anal. exptal (calcd) K 5.86 (6.08), P 1.02
(0.96), Pd 3.12 (3.31), W 63.15 (62.94), H2O 7.37 (7.29)%.
Reactions were carried out in a 50mL Parr autoclave under
the conditions noted in the table captions. Analysis of the
reactions were carried out by GC and GC-MS along with use
of reference samples.
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9
C. Rocchiccioli-Deltcheff and R. Touvenot, J. Chem. Res. 1977,
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10Reports of such selectivity are very rare; one example is the re-
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274
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