An easily recoverable and recyclable homogeneous polyester-based Pd catalytic system for the hydrogenation of α,β-unsaturated carbonyl compounds

Article abstract of DOI:10.1016/j.catcom.2015.07.002

Abstract Homogeneous catalysis is an efficient tool to carry out hydrogenation processes but the major drawback is represented by the separation of the expensive catalyst from the product mixture. In this view we prepared a polyester-based Pd catalytic system that offers the advantages of both homogenous and heterogeneous catalyses: efficacy, selectivity and recyclability. Here its application in the hydrogenation of selected α,β-unsaturated carbonyl compounds is described.

Full text of DOI:10.1016/j.catcom.2015.07.002

Catalysis Communications 69 (2015) 228–233
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
An easily recoverable and recyclable homogeneous polyester-based Pd
catalytic system for the hydrogenation of α,β-unsaturated
carbonyl compounds
a
a,b
a
c
d
a
Mattia Bartoli , Luca Rosi , Giorgio Petrucci , Lidia Armelao , Werner Oberhauser , Marco Frediani ,
e,f
e
e,
Oreste Piccolo , Vikas Damu Rathod , Stefano Paganelli ⁎
a
Dipartimento di Chimica “Ugo Schiff”, Università degli Studi di Firenze, via della Lastruccia 3-13, Sesto Fiorentino, FI, Italy
Consorzio Interuniversitario Reattività Chimica e Catalisi (CIRCC), via Celso Ulpiani 27, 70126 Bari, Italy
IENI-CNR and INSTM, Dipartimento di Scienze Chimiche, Università di Padova, Via Marzolo 1, 35131 Padova, Italy
Istituto di Chimica dei Composti OrganoMetallici (ICCOM) CNR, via Madonna del Piano 10, 50019 Sesto Fiorentino, FI, Italy
Dipartimento di Scienze Molecolari e Nanosistemi, Università Ca' Foscari Venezia, via Torino 155, 30170 Venezia Mestre, Italy
SCSOP, Via Bornò 5, 23896 Sirtori, LC, Italy
b
c
d
e
f
a r t i c l e i n f o
a b s t r a c t
Article history:
Homogeneous catalysis is an efficient tool to carry out hydrogenation processes but the major drawback is
represented by the separation of the expensive catalyst from the product mixture. In this view we prepared a
polyester-based Pd catalytic system that offers the advantages of both homogenous and heterogeneous catalyses:
efficacy, selectivity and recyclability. Here its application in the hydrogenation of selected α,β-unsaturated
carbonyl compounds is described.
Received 23 March 2015
Received in revised form 1 July 2015
Accepted 2 July 2015
Available online 6 July 2015
©
2015 Elsevier B.V. All rights reserved.
Keywords:
Hydrogenation
Homogeneous catalysis
Palladium
Macrocomplexes
Polyester
1
. Introduction
results have been achieved with polymer-bound palladium complexes
in heterogeneous reactions [8–10]. Recently, an innovative approach
to recycle a catalytic system has been proposed. It is based on a homo-
geneous system consisting in a polyester-based Pd-macrocomplex
that can be recovered by its precipitation and easy separation from
The selective hydrogenation of α,β-unsaturated carbonyl com-
pounds is an important process in the manufacture of some useful fine
chemicals as intermediates for the synthesis of pharmaceuticals, addi-
tives for food flavors and valuable building blocks for fragrances [1].
The selective reduction of α,β-unsaturated carbonyl compounds by
means of molecular hydrogen in the presence of catalysts still remains
an intriguing challenge in catalysis [2]. The selectivity, including
enantioselectivity, strongly depends on the nature of the active metal
catalyst [3] and on the reaction conditions [4]. Homogeneous catalysts
often present good activity and selectivity, but the major drawback of
homogeneous processes is the separation of the precious catalyst from
the product mixture that requires an energy intensive process such as
distillation. Hence several attempts have been accomplished in order
to facilitate the separation of the catalyst from the reaction medium
like the use of biphasic systems [5] or supported metal complexes [6].
In the realm of Pd-based catalysts the hydrogenation reaction has
been carried out in the presence of numerous systems [7]. Interesting
the reaction mixture. The catalyst is prepared by reaction of Pd(OAc)
2
with 4-pyridinemethylene-end-capped poly(L-lactide) (macroligand
L), as shown in Scheme 1 [11].
The resulting Pd-pyridine poly(L-lactide) macrocomplex, trans-
[Pd(OAc)
oxidation of selected primary and secondary alcohols [12]. Here we
report on the capability of trans-[Pd(OAc) (L) ] to be used as catalyst
2 2
(L) ], has been applied by some of us to catalyze the aerobic
2
2
in the hydrogenation reaction of selected α,β-unsaturated carbonyl
compounds, precisely 2-cyclohexen-1-one (I), (3E)-4-phenylbut-3-
en-2-one (IV), (3E)-4-(6-methoxy-2-naphthyl)but-3-en-2-one (VII),
(2E)-3-phenylprop-2-enal (X) and (2E)-3-(1,3-benzodioxol-5-yil)-2-
methyl-prop-2-enal (XIII). The macrocomplex is soluble under the ap-
plied catalytic conditions but, upon addition of a hydrocarbon solvent
or preferably methanol, the polymer-anchored Pd catalyst precipitates
allowing the separation from the reaction solution and, thus, it is easily
recyclable. This system therefore combines the activity and selectivity of
a homogeneous catalyst with the easy recovery and recyclability of a
⁎
Corresponding author.
E-mail addresses: marco.frediani@unifi.it (M. Frediani), spag@unive.it (S. Paganelli).
http://dx.doi.org/10.1016/j.catcom.2015.07.002
566-7367/© 2015 Elsevier B.V. All rights reserved.
1

M. Bartoli et al. / Catalysis Communications 69 (2015) 228–233
229
2 2
Scheme 1. Synthesis of trans-[Pd(OAc) (L) ].
1
heterogeneous one without making any change to the catalyst
structure.
5937 (HP-5MS column 30 m × 0.25 mm × 0.25 μm). HNMR analyses
were performed by using a GEMINI 400 MHz using CDCl (Aldrich).
3
GPC analyses were carried out with Waters mod. binary HPLC 1525
equipped with three columns (Shodex KF802.5, KF-803, KF-804) and a
refractive index detector Wyatt T-REX optilab. Analyses were per-
formed with THF (Chromasolv, HPLC-grade ≥99.8%). ICP analysis was
performed by “Laboratorio di Microanalisi” of Chemistry Department
of University of Florence. XPS analysis was run on a Perkin-Elmer Φ
2
. Experimental
2
.1. Materials
2
-Cyclohexen-1-one, (3E)-4-phenylbut-3-en-2-one, (2E)-3-
phenylprop-2-enal, 4-pyridinemethanol, dry CH Cl , pyridine and
Pd(OAc) were Aldrich products. (3E)-4-(6-methoxy-2-naphthyl)but-
-en-2-one was a generous gift from Chemi SpA. (2E)-3-(1,3-
5600-ci spectrometer using non-monochromatized Al K
α
radiation
2
2
(1486.6 eV). The sample analysis area was 800 μm in diameter and
−
9
2
the working pressure was lower than 10
mbar. The spectrometer
3
was calibrated by assuming the binding energy (BE) of the Au 4f7/2
line at 83.9 eV with respect to the Fermi level. The standard deviation
for the BEs values was ± 0.2 eV. Survey scans were obtained in the
benzodioxol-5-yil)-2-methyl-prop-2-enal was synthesized as described
in the literature [13]. Starting materials for the synthesis of the
macroligand L and trans-[Pd(OAc)
pyridine poly(L-lactide) macrocrocomplex, trans-[Pd(OAc)
synthesized and characterized as described by Giachi et al. [11] (see
Supplementary content). GC analyses were carried out on an Agilent
−
1
−1
2
(L)
2
] were Purac products. Pd(II)
(L) ], was
0–1300 eV range (187.8 eV pass energy, 0.4 eV step , 0.05 s step ).
2
2
Detailed scans were recorded for the C1s, O1s, N1s and Pd3d regions
−
1
−1
(23.5 eV pass energy, 0.1 eV step , 0.1 s step ). No further element
was detected. The BE shifts were corrected by assigning to the C1s
peak associated with adventitious hydrocarbons a value of 284.8 eV
[14]. Samples were mounted on steel holders and introduced directly
in the fast-entry lock system of the XPS analytical chamber. The analysis
involved Shirley-type background subtraction, non-linear least-squares
curve fitting adopting Gaussian–Lorentzian peak shapes, and peak area
determination by integration [15]. The atomic compositions were
evaluated from peak areas using sensitivity factors supplied by Perkin-
Elmer, taking into account the geometric configuration of the apparatus
6
850A gas chromatograph (HP1 column 30 m × 0.32 mm × 0.25 μm)
and GC–MS analyses were performed by using an Agilent MS Network
Table 1
Hydrogenation of 2-cyclohexen-1-one (I) catalyzed by Pd-catalysts.
Entry Pre-catalyst or catalyst t (h) Conversion (%)a II yield (%) III yield (%)
b
1
2
3
4
5
6
7
8
9
1
1
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
2
2
1
3
3
1
1
1
2
3
3
3
3
95
65
20
30
37
35
70
N99
N99
N99
N99
95
65
20
30
37
35
70
N99
N99
N99
N99
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
b
/Py (1/2)
/TBAB
[16]. Analysis was performed on different sample portions in order to
trans-[Pd(OAc)
2
(L)
2
]
check for chemical homogeneity.
c
c
[Pd](L)
[Pd](L)
2
2
2.2. General procedure for hydrogenation experiments
trans-[Pd(OAc)
trans-[Pd(OAc)
2
(L)
(L)
2
2
]
]
2
c
All the reactions were carried out following a procedure similar to
[Pd](L)
[Pd](L)
[Pd](L)
2
2
2
0c
that below described for the polyester-based Pd catalyzed hydrogena-
tion of (2E)-3-phenylprop-2-enal (X). Experimental details, different
for every substrate, are reported in Tables 1–5.
A 150-mL stainless steel reaction vessel was charged, under a nitro-
2 2
gen purge, with 396 mg (3 mmol) of X, 26.7 mg of trans-[Pd(OAc) (L) ]
(corresponding to 0.003 mmol of Pd) and 10 mL of toluene. The reactor
was then pressurized with 0.2 MPa of hydrogen and heated at 30 °C for
6 h (Table 4). The reactor was then cooled to room temperature, the
c
1
Reaction conditions: substrate = 10.5 mmol; substrate/Pd (molar ratio) = 1000/1;
toluene = 10 mL; p(H ) = 0.1 MPa; T = 30 °C; TBAB = tetrabutylammonium bromide;
Py = pyridine. nd = not detected in the reaction mixture.
2
a
Determined by GC (n-dodecane as internal standard).
Pd black was formed.
Reaction carried out by using the catalytic palladium species trans-[Pd(OAc)
b
c
2 2
(L) ], indi-
cated as [Pd](L)
2
, recovered from the previous run.
Table 3
Hydrogenation of (3E)-4-(6-methoxy-2-naphthyl)but-3-en-2-one (VII) catalyzed by
Table 2
Hydrogenation of (3E)-4-phenylbut-3-en-2-one (IV) catalyzed by trans-[Pd(OAc)
trans-[Pd(OAc)
2
(L)
2
].
2 2
(L) ].
Entry
P(H
2
) (MPa)
t (h)
Conversion (%)a
VIII yield (%)
IX yield (%)
Entry
t (h)
Conversion (%)a
V yield (%)
VI yield (%)
1
2
3
4
5
6
7
8
0.1
0.1
1
2
2
2
2
2
3
24
3
3
5
5
5
5
2
57
15
44
56
52
55
52
2
57
15
44
54
51
54
52
nd
nd
nd
nd
2
1
1
nd
1
2
3
4
5
1
3
3
3
3
3
20
20
nd
nd
nd
nd
nd
nd
N99
N99
N99
N99
39
N99
N99
N99
N99
39
b
b
b
b
c
b
6
b
Reaction conditions: substrate = 5.6 mmol; substrate/Pd (molar ratio) = 1000/1;
toluene = 10 mL; p(H ) = 0.1 MPa; T = 30 °C. nd = not detected in the reaction mixture.
2
Reaction conditions: substrate = 5.5 mmol; substrate/Pd (molar ratio) = 1000/1;
T = 30 °C; toluene = 25 mL. nd = not detected in the reaction mixture.
a
Determined by GC (n-dodecane as internal standard).
Reaction carried out by using the catalyst recovered from the previous run.
Experiment carried out by using 25 ml of toluene instead of 10 ml.
b
c
a
Determined by GC (n-dodecane as internal standard).
Reaction carried out by using the catalyst recovered from the previous run.
b

230
M. Bartoli et al. / Catalysis Communications 69 (2015) 228–233
Table 4
Hydrogenation of (2E)-3-phenylprop-2-enal (X) catalyzed by trans-[Pd(OAc)
residual gases released and the reaction mixture analyzed by GC and
GC–MS. Methanol was then added to the reaction mixture until
complete precipitation of the polyester-based Pd-catalyst that was
recovered by filtration, dried under vacuum and reused in a recycling
experiment.
2
(L)
) (MPa) t (h) Conversion (%)a XI yield (%) XII yield (%)
2
2
].
Entry T (°C) P(H
1
2
3
4
5
6
7
30
30
60
30
30
30
30
0.1
0.1
0.1
2
2
2
6
24
6
6
6
9
96
70
9
90
35
91
95
95
95
nd
6
35
5
5
5
b
Compound II, GC–MS m/z: 98, 69, 55, 42.
96
c
c
c
N99
N99
N99
Compound V, GC–MS m/z: 148, 133, 105, 91, 77, 43.
Compound VIII, GC–MS m/z: 228, 171, 141, 115.
Compound IX, GC–MS m/z: 230, 212, 197, 172, 141, 115.
Compound XI, GC–MS m/z: 134, 105, 91, 78.
Compound XII, GC–MS m/z: 136, 118, 105, 91, 77.
Compound XIV, GC–MS m/z: 192, 135, 122, 105, 77, 51.
Compound XV, GC–MS m/z: 194, 176, 135, 77, 51.
6
6
2
5
Reaction conditions: substrate = 3 mmol; substrate/Pd (molar ratio) = 1000/1;
toluene = 10 mL. nd = not detected in the reaction mixture.
a
Determined by GC (n-dodecane as internal standard).
Pd black was formed.
Reaction carried out by using the catalyst recovered from the previous run.
b
c
3. Results and discussion
Table 5
Hydrogenation of (2E)-3-(1,3-benzodioxol-5-yil)-2-methyl-prop-2-enal (XIII) catalyzed
3.1. Catalyst stability
by trans-[Pd(OAc)
2
(L) ].
2
Entry
p(H
2
) (MPa)
t (h)
Conversion (%)a
XIV yield (%)
XV yield (%)
2 2
The behavior of the complex trans-[Pd(OAc) (L) ] has been studied
by 1HNMR spectroscopy, ICP and XRD analysis. HNMR was a useful
1
1b
0.2
0.2
0.5
0.5
0.5
0.5
3
2
7
7
7
7
5
28
N99
N99
96
5
13
33
27
36
41
nd
15
67
73
60
50
2
3
4
5
6
technique to study this system (Fig. 1). The formation of trans-
[Pd(OAc) (L) ] was confirmed by changing in chemical shift (δ) of
2 2
aromatic hydrogen signals of L (8.68 ortho-H and 7.46 meta-H ppm,
trace A) to different δ (8.79 ortho-H, 8.67 meta-H ppm, trace B). After
three catalytic runs no evidence of palladium nanoparticles (signals at
c
c
c
91
Reaction conditions: substrate = 1.05 mmol; substrate/Pd (molar ratio) = 100/1;
toluene = 10 mL; T = 30 °C. nd = not detected in the reaction mixture.
8
.82 ortho-H, 8.62 meta-H ppm, trace D) were detected. The pattern
a
of signals of trans-[Pd(OAc) (L) ] was changed (8.82 ortho-H, 7.58
Determined by GC (n-dodecane as internal standard).
Reaction carried out by using a substrate/Pd (molar ratio) = 1000/1.
Reaction carried out by using the catalyst recovered from the previous run.
2
2
b
meta-H ppm, trace C). Probably this was due to the formation, after
work up, of palladium(II) species, like palladium oxides, still bound
c
2 2 2 2 2 2
Fig. 1. Signals of aromatic hydrogens of: A) L, B) trans-[Pd(OAc) (L) ], C) trans-[Pd(OAc) (L) ] after three catalytic runs, D) [Pd(0)-L] obtained by reduction of trans-[Pd(OAc) (L) ] at 0.1
Mpa of H and 60 °C for 6 h.
2

M. Bartoli et al. / Catalysis Communications 69 (2015) 228–233
231
to L. Moreover, ICP analysis showed only a 0.36% loss of palladium
mol/mol) after the first catalytic run (2-cyclohexen-1-one (I)
hydrogenation, 3 h, 30 °C and P(H ) = 0.1 MPa).
(
2
XPS investigation was performed on the fresh catalyst and after the
first catalytic run (2-cyclohexen-1-one hydrogenation, 3 h, 30 °C and
P(H ) = 0,1 MPa) in order to evaluate its chemical composition before
2
and after the hydrogenation reaction. In both cases, the expected
elements, i.e. palladium, nitrogen, carbon, and oxygen were found.
Fig. 2A shows the XPS spectra related to the Pd3d regions before
(
continuous line) and after (dotted line) the hydrogenation process
and work-up. Similar features for the peaks were observed, suggesting
an alike electronic environment around palladium in the fresh catalyst
and after the reaction and recovery. In particular, both spectra are com-
posed of a doublet corresponding to the emission from the spin-orbit
split 3d5/2 and 3d3/2 core levels. The corresponding binding energy
2
+
peaks positions at 337.1 and 342.2 eV are assigned to divalent Pd
ions in palladium compounds [17]. Therefore, the interpretation of the
Pd3d spectra indicates that palladium, which is reduced under the cat-
alytic conditions, is however reoxidized, during the work-up and recov-
ery, probably for the action of air [18,19]. Regarding the C1s line, a
complex shape profile is found with the presence of at least three differ-
ent carbon-related contributions, as shown in Fig. 2B. Peak fitting re-
vealed three components centered at binding energies (BE) 284.8 eV
(
C
I
), 286.8 eV (CII) and 288.7 eV (CIII). According to the chemical struc-
ture of the palladium macrocycle, the dominant lower energy compo-
nent (C ) can be ascribed to aliphatic carbon; a second component
II) at higher BE can be associated to oxygen-bound carbon; a third
I
(
C
component (CIII) at even higher BE that can be coherently assigned to
the electron-depleted carboxylic carbon.
3
.2. Catalytic experiments
Fig. 2. A) High resolution XPS spectra of the Pd3d regions before (continuous lines) and
after (dotted lines) the catalytic process. B) High resolution XPS spectrum of the C1s region
before the hydrogenation process. The fitting components of the peak are also reported.
The same profile is observed after reaction.
A first set of hydrogenation experiments was carried out on
-cyclohexen-1-one (I) as the substrate (Scheme 2), by using a
2
1
1
000/1 substrate to catalyst molar ratio and under 0.1 MPa H
2
: after
h at 30 °C cyclohexanone (II) was obtained with 30% yield (entry 4,
Table 1). Upon prolonging the reaction time up to 3 h, 100% of both con-
version and selectivity to II was achieved (Entry 8, Table 1). After each
experiment the catalytic system was precipitated by simply adding
methanol to the solution. The catalyst was then filtered, dried under
vacuum and reused in a consecutive experiment: noteworthy, the cata-
lytic activity and selectivity to cyclohexanone (II) remained unchanged
even after three recycling experiments (entries 9–11, Table 1). As trans-
Once aware of the capability of the catalyst to hydrogenate
selectively the C_C double bond, some hydrogenation experiments
on the α,β-unsaturated ketone VII, a valuable precursor of Nabumetone
(VIII), a nonsteroidal anti-inflammatory, analgesic and antipyretic drug
(Scheme 3b, Table 3) were carried out. Many synthetic procedures to
prepare Nabumetone are described in the literature and all of them
involve the selective hydrogenation of the carbonyl conjugated C_C
double bond [21]. Usually, heterogeneous catalysts as Pd/C, Ni Raney
[
Pd(OAc)
2 2 2
(L) ] was synthesized from Pd(OAc) and the polylactide
ligand (L), a reaction was carried out in the presence of Pd(OAc)
2
alone without any external ligand and in the same experimental condi-
tions, in order to make a comparison of the two systems: cyclohexanone
2 3 2
[22], Rh/Al O [23] and Cu/SiO [24] are employed, and in some cases,
when the catalyst is Pd, its pre-activation or the use of a base is neces-
(
II) was obtained with a 95% yield already after 1 h, but the catalyst
sary to obtain good selectivity [21]. The hydrogenation of VII was stud-
decomposed with formation of an unsoluble and catalytically inactive
Pd black, not suitable for any recycle (entry 1, Table 1). Also carrying
out the reaction at the same reaction conditions but in the presence of
pyridine (Py) as external ligand (Pd/Py = 1/2 molar ratio), 65% of
cyclohexanone (II) was formed but again Pd black was observed in
the mixture (entry 2, Table 1). Furthermore, in an experiment carried
ied at different H
at 30 °C the temperature and with a substrate to catalyst molar ratio of
1000/1. A first experiment carried out at 0.1 MPa of H for 3 h gave a
very low conversion to VIII. However, on increasing both pressure and
reaction time (2 Mpa, 5 h) a 56% conversion was obtained; in this case
also 2% of alcohol IX was formed (entry 5, Table 3). By comparing
these results with those obtained with the structurally related IV, a
sharp decrease of the reaction rate can be observed. The lower activity
2
pressures and reaction times, while keeping constant
2
2
out in the presence of Pd(OAc) and tetrabutylammonium bromide as
the ligand [20], the conversion was only 20% (run 3, Table 1) after 3 h,
so evidencing the good performance of the polyester-based Pd catalyst.
The polyester-based Pd catalyst showed good activity also in the
hydrogenation of (3E)-4-phenylbut-3-en-2-one (IV) (Scheme 3a,
Table 2). In this case V was quantitatively obtained, as the sole reaction
2
product, after 3 h at 30 °C, under 0.1 MPa of H and by using a 1000/1
substrate/Pd molar ratio. The catalyst, recovered by adding methanol
to the reaction mixture, was used in three successive experiments
in which it showed unchanged activity and selectivity (entries 2–5,
Table 2).
Scheme 2. Hydrogenation of 2-cyclohexen-1-one (I).

232
M. Bartoli et al. / Catalysis Communications 69 (2015) 228–233
of the catalyst with respect to the reaction on IV should be ascribed to
the high dilution of the reaction, due to the low solubility of the sub-
strate in the solvent employed. As a matter of fact, when the reaction
was carried out on ketone IV by using 25 ml of toluene instead of
selectivity was still very unsatisfactory as both XIV and the correspond-
ing alcohol XV were formed (entry 2, Table 5). A complete conversion
was obtained at 30 °C and 0.5 MPa of H for 7 h: XIV was formed but
2
the major product was XV (67%) (entry 3, Table 5). The catalyst was
recycled in three consecutive experiments but even if conversions
were always very high a certain deactivation of the catalyst occurred,
so showing a decreased ability to hydrogenate the carbonyl group
(entries 4–6, Table 5). Noteworthy, the catalyst showed a lower activity
with respect to X: surely, the presence of the methyl group in α-position
to the carbonyl group makes more difficult the coordination of Pd to the
sterically hindered trisubstituted C_C double bond. Moreover, the high
dilution could strongly influence the reaction rate analogously to that
hypothesized for the hydrogenation of ketone VII.
1
0 ml, conversion, after 3 h, was only 39% instead of 100% (entry 6,
Table 2). The catalyst, recovered as above described, was reused
in three consecutive recycling experiments and its activity remained
practically unchanged. Noteworthy the selectivity to VIII was always
very high, ranging from 96 to 100%.
Interesting results were obtained also in the hydrogenation of (2E)-
3
-phenylprop-2-enal (X) (Scheme 3c, Table 4). Initially X was hydroge-
nated in the presence of the polyester-based Pd catalyst by using a
000/1 substrate to Pd molar ratio, at 30 °C and 0.1 MPa of H for 6 h,
obtaining a very low conversion to 3-phenylpropanal (XI) (9%) (entry
, Table 4). When the reaction time was prolonged to 24 h, a 96% con-
1
2
1
3.2.1. Homogeneity test
version was achieved but also a small amount (6%) of alcohol XII was
recovered (entry 2, Table 4). When the reaction temperature was
increased to 60 °C the conversion rose to 70% in 6 h but in detriment
of selectivity, since XI and XII were also produced in equal amount
When transition-metal complexes are used as precatalysts in
hydrogenation processes it is important to establish the nature of the
active species. The true catalyst may be a homogeneous metal complex
or active metal particles present in solutions as colloids or metal
nanoclusters, formed under reaction conditions. As Hg(0) is able to
poison heterogeneous (colloidal or nanoparticulate) metal catalysts
while it does not deactivate true homogeneous metal complexes, we
carried out the hydrogenation of 2-cyclohexen-1-one (I) at 0.1 MPa of
(
entry 3, Table 4); moreover, in these conditions, the catalyst was not
stable and Pd black was formed. A very good conversion (96%) was ob-
tained at 30 °C and 2 MPa of H for 6 h, maintaining a substrate to Pd
2
molar ratio 1000/1: XI was the prevailing product being the alcohol
XII formed in small amount (5%) (entry 4, Table 4). The recovered
catalyst maintained a very good activity also in three recycling experi-
ments affording complete conversions and 95% selectivity to the
saturated aldehyde XI (entries 5–7, Table 4).
2
H and 30 °C for 3 h in the presence of Hg(0), according to a procedure
described in the literature [35]. The catalytic activity remained
unchanged, so indicating that it is due most likely to an homogeneous
metal complex.
As an application of the described catalytic system, the
hydrogenation of 3-(1,3-benzodioxol-5-yil)-2-methyl-propenal (XIII),
an α,β-unsaturated aldehyde precursor of Helional® (XIV), a valuable
fragrance used in both feminine and masculine perfumes [13,25–34],
was studied (Scheme 3d, Table 5). A first experiment carried out at
4. Conclusions
Pyridine poly(L-lactide)-based Pd macrocrocomplex, trans-
[Pd(OAc) (L) ], has showed an interesting activity and good
2 2
3
0 °C and 0.2 MPa of H
2
for 3 h and using a 1000/1 substrate to Pd
recyclability in the hydrogenation of some α,β-unsaturated carbonyl
compounds. Its selectivity may be strongly affected by the substrate
shape. In fact for substrates I, IV, VII and X the corresponding saturated
carbonyl compounds are formed with comparable or higher selectivity
molar ratio gave a very disappointing result (conversion 5%) (entry 1,
Table 5). Increasing the substrate/Pd ratio to 100/1, conversion rose
up to 28% after 2 h, but despite the improvement in conversion the
Scheme 3. Hydrogenation of ketones IV and VII and aldehydes X and XIII.

M. Bartoli et al. / Catalysis Communications 69 (2015) 228–233
233
[
[
10] D. Astruc, F. Lu, J. Ruiz Aranzaes, Angew. Chem. Int. Ed. 44 (2005) 7852–7872.
11] G. Giachi, M. Frediani, W. Oberhauser, E. Passaglia, Polym. Chem. 49 (2011)
than other palladium catalysts [35]; only by forcing the reaction condi-
tions the saturated alcohols IX and XII are produced, even if, in most
cases, in almost negligible amounts. On the contrary a mixture of satu-
rated carbonyl compound XIV and saturated alcohol XV is obtained by
hydrogenation of substrate XIII. This fact is justified, according to us,
by steric reasons, the methyl group in α position in this unsaturated al-
dehyde being able to affect negatively the interaction between C_C
bond with this hindered Pd species and positively its interaction with
C_O. Noteworthy, in all the hydrogenation experiments, whatever
the unsaturated carbonyl compound tested, the corresponding allylic
alcohol has never been detected in the reaction mixture. It is to point
out that this homogeneous catalyst is easily separated from the reaction
mixture by simple addition of methanol to the reaction mixture and the
recovered catalytic system maintains a good activity and selectivity also
in recycling experiments. Considering both the data collected and the
easy recyclability, this homogeneous catalytic species is a promising
and peculiar catalyst for the hydrogenation of α,β-carbonyl com-
pounds. Moreover, the catalyst was substantially unmodified after
4
708–4713.
[12] G. Giachi, M. Frediani, W. Oberhauser, E. Passaglia, Polym. Chem. 50 (2012)
725–2731.
[
[
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7
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[
1
catalytic runs and recovery as shown by ICP, XRPD, XPS and HNMR
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[
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