A polymer supported palladium(II) β-ketoesterate complex as active and recyclable pre-catalyst for selective reduction of quinolines in water with sodium borohydride Dedicated to the memory of Professor Benedetto Corain.

Article abstract of DOI:10.1016/j.molcata.2015.03.013

A polymer supported palladium catalyst, obtained by copolymerization of Pd(AAEMA)₂ [AAEMA^- = deprotonated form of 2-(acetoacetoxy) ethyl methacrylate] with ethyl methacrylate (co-monomer) and ethylene glycol dimethacrylate (cross-linker), exhibited excellent activity and selectivity for the hydrogenation of quinolines to 1,2,3,4-tetrahydroquinolines in the presence of NaBH₄ as hydrogen donor in water. Both the activity and selectivity could be maintained for at least seven reaction runs. No metal leaching into solution occurred during recycles. TEM analyzes carried out on the catalyst showed that the active species were supported palladium nanoparticles having a mean size of 3 nm, which did not aggregate with the recycles.

Full text of DOI:10.1016/j.molcata.2015.03.013

Journal of Molecular Catalysis A: Chemical 402 (2015) 83–91
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
A polymer supported palladium(II) ␤-ketoesterate complex as active
and recyclable pre-catalyst for selective reduction of quinolines in
water with sodium borohydride
Maria Michela Dell’Annaa,∗, Giuseppe Romanazzia, Simona Intinia, Antonino Rizzutia,
Cristina Leonellib, Alberto Ferruccio Piccinnia, Piero Mastrorillia,c
a DICATECh, Politecnico di Bari, via Orabona 4, 70125 Bari, Italy
b Dipartimento di Ingegneria “Enzo Ferrari”, Università di Modena e Reggio Emilia, via Pietro Vivarelli 10, 41125 Modena, Italy
c Consiglio Nazionale delle Ricerche, Istituto di Chimica dei Composti Organometallici (CNR-ICCOM), via Orabona 4, 70125 Bari, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
A
polymer supported palladium catalyst, obtained by copolymerization of Pd(AAEMA)2
[AAEMA− = deprotonated form of 2-(acetoacetoxy) ethyl methacrylate] with ethyl methacrylate
(co-monomer) and ethylene glycol dimethacrylate (cross-linker), exhibited excellent activity and
selectivity for the hydrogenation of quinolines to 1,2,3,4-tetrahydroquinolines in the presence of NaBH4
as hydrogen donor in water. Both the activity and selectivity could be maintained for at least seven
reaction runs. No metal leaching into solution occurred during recycles. TEM analyzes carried out on the
catalyst showed that the active species were supported palladium nanoparticles having a mean size of
3 nm, which did not aggregate with the recycles.
Received 17 December 2014
Received in revised form 17 March 2015
Accepted 19 March 2015
Available online 21 March 2015
Dedicated to the memory of Professor
Benedetto Corain.
© 2015 Elsevier B.V. All rights reserved.
Keywords:
Water solvent
Quinoline transfer hydrogenation
Polymer supported Pd nanoparticles
Recyclable catalyst
1. Introduction
the presence of a co-catalyst. Also heterogeneous catalytic systems
based on noble metals [18], such as Ru [19,20], Rh [21], Au [22]
The hydrogenation of quinoline and its derivatives is an impor-
tant area of research, since its products are valuable intermediates
for the synthesis of drugs, agrochemicals, dyes, alkaloids, and many
other biological active molecules [1–3]. In fact, the direct catalytic
hydrogenation of readily available quinolines in order to selec-
tively obtain 1,2,3,4-tetrahydroquinolines is preferable in terms of
atom economy compared to other synthetic approaches, such as
the catalytic cyclization [4,5] and the Beckman rearrangement [6].
However, the hydrogenation [7] of these N-heterocycles is a diffi-
cult task [8] due to the resonance stabilized aromatic nucleus and
the fact that the product secondary amines could act as poison to
most industrially common heterogeneous metal catalysts [9,10].
Extended efforts have been devoted to the selective hydrogena-
tion of these compounds using hydrogen gas and soluble metal
catalysts based on Os [11], Ir [8,12] Ru [13–15] and Rh [16,17],
although they were difficult to be re-used and very often needed
and Pd [23–25] have been developed, but they presented clear
drawbacks, such as the need of harsh conditions (30 ÷ 60 bar of
H2, 100 ÷ 150 ◦C) and/or the presence of toxic organic solvents (for
example: toluene or n-hexane). Only a limited number of heteroge-
neous catalysts have been found active in promoting the reduction
of quinolines in water under hydrogen gas. They are: Ru nanopar-
ticles intercalated in hectorite [26], Ir complex anchored onto a
solid support [27] and Pd nanoparticles stabilized by black wattle
tannin [28,29], only the latter being recyclable. Recently, with the
growth of green and sustainable chemistry concepts [30], consid-
ering water not only as a possible solvent [31] but also a reagent
has attracted great attention in organic synthesis [32]. In the field of
reduction, water has extensively been used for both hydrogenation
and transfer hydrogenation reactions [33]. Examples of transfer
hydrogenation catalytic systems active in water for promoting the
selective reduction of quinolines and quinoxalines are those based
on: a Ni–Al alloy [34], an Ir complex with HCOONa as the hydrogen
source [35], a Pd/C and Zn mixture [36], being the majority of trans-
fer hydrogenation catalysts active and selective only in commonly
used organic solvents (alcohols, toluene, etc.) [37–44].
∗
Corresponding author. Tel.: +39 80 5963695, fax: +39 80 5963611.
E-mail address: mariamichela.dellanna@poliba.it (M.M. Dell’Anna).
http://dx.doi.org/10.1016/j.molcata.2015.03.013
1381-1169/© 2015 Elsevier B.V. All rights reserved.

84
M.M. Dell’Anna et al. / Journal of Molecular Catalysis A: Chemical 402 (2015) 83–91
Scheme 1. Synthesis of Pd-pol.
However, the major problem related to the recyclability of
sources and used as received. Pd-pol (Pd%w = 5.0) was synthesized
according to literature procedure [53]. Palladium content in Pd-
pol was assessed after sample mineralization by atomic absorption
spectrometry using a PerkinElmer 3110 instrument. Catalyst min-
eralization prior to Pd analyzes was carried out by microwave
irradiation with an ETHOS E-TOUCH Milestone applicator, after
addition of 12 mL HCl/HNO3 [3:1, v/v] solution to each weighted
sample.
these transfer hydrogenation catalytic systems in water remains
unsolved and the development of more effective, recyclable, and
handle-convenient catalysts for this transformation is still highly
desirable.
With the aim to set up new environmentally friendly synthetic
methods to be performed under mild and sustainable conditions
with high efficiency, we decided to evaluate the catalytic activ-
ity of a polymer supported palladium catalyst (in the following
Pd-pol) for the partial hydrogenation of quinolines in water with
NaBH4. In order to obtain a material with a uniform distribution of
the active sites, the catalyst was synthetized in an uncommon way
[45], i.e., by co-polymerization of the metal-containing monomer
Pd(AAEMA)2 [AAEMA− = deprotonated form of 2-(acetoacetoxy)
ethyl methacrylate] with suitable co-monomer (ethyl methacry-
late) and cross-linker (ethylene glycol dimethacrylate) [46,47]
(Scheme 1).
Pd-pol was successfully tested as active and recyclable cata-
lyst in several palladium promoted reactions [48–53]. The reticular
and macro porous polymeric support of Pd-pol is able to immobi-
lize and stabilize palladium nanoparticles formed under reaction
conditions by reduction of the pristine Pd(II) anchored complex.
Furthermore, its good swellability in water renders Pd-pol an ideal
potential catalyst for reactions carried out in water, since the migra-
tion of the reagents to the active sites would not be hampered by
the solid support. In fact, Pd-pol was active and recyclable in the
Suzuki cross coupling of arylhalides with arylboronic acids [54], the
aerobic selective oxidation of benzyl alcohols [55] and the reduc-
tion of nitroarenes [56], all of them carried out in water. Moreover,
Pd-pol was already tested in aqueous medium as catalyst for the
hydrogenation of quinolines into 1,2,3,4-tetrahydroquinolines at
80 ◦C under 10 bar H2, resulting uncommonly recyclable at least
for nine consecutive runs with high yields [57]. To the best of our
knowledge, Pd-pol is still the second example reported to date
of active palladium catalyst recyclable in aqueous solvent for the
selective hydrogenation of quinolines and heteroaromatic nitro-
gen compounds under mild conditions using hydrogen gas, being
the first reported example Pd nanoparticles on black wattle tannin
[28,29]. For this reason, we decided to evaluate the catalytic activity
of Pd-pol also in the transfer hydrogenation of quinolines in water.
Following our studies, herein we report on the ability of Pd-
pol to efficiently catalyze the selective reduction of quinolines
into tetrahydroquinolines in neat water using NaBH4 as hydro-
gen source, thus using “safer” conditions compared to the ones
previously proposed by us [57].
GC–MS data (EI, 70 eV) were acquired on
a HP 6890
instrument using a HP-5MS crosslinked 5% PH ME siloxane
(30.0 m × 0.25 mm × 0.25 mm) capillary column coupled with a
mass spectrometer HP 5973. The products were identified by com-
parison of their GC–MS or 1H NMR features with those of authentic
samples. Reactions were monitored by GLC or by GC–MS ana-
lyzes. GLC analysis of the products was performed using a HP 6890
instrument equipped with a FID detector and a HP-1 (Crosslinked
Methyl Siloxane) capillary column (60.0 m × 0.25 mm × 1.0 ␮m).
GLC yields were assessed by using biphenyl as internal standard.
Isolated yields were determined by flash column chromatography
using Merck® Kieselgel 60 (230–400 mesh) silica gel. 1H NMR spec-
tra were recorded on a Bruker Avance III 700 MHz spectrometer
and the signals are reported in ppm relative to tetramethylsilane
(Supporting information section).
Transmission electron micrographs were obtained using a JEM
2010 instrument operating at 200 kV (Jeol – Oxford Instruments)
equipped with a GIF100 GATAN and 1 k × 1 k pixel GATAN ERLANG-
SHEN ES 500 W CCD camera. Samples were finely ground and
suspended in bidistilled water with 15 min sonication. A few
droplets of the suspension were deposited on a copper grid cov-
ered with a Formvar® film and carbon sputtered to assure optimal
conductivity. The particle size distributions were obtained by
TEM image analysis using the ImageJ software (freeware soft-
ware: http://rsb.info.nih.gov/ij/). Micro-FTIR spectra (neat) were
recorded using a PerkinElmer Spectrum 2000 instrument.
2.2. General procedure for catalytic transfer hydrogenation of
quinolines
2.2.1. Procedure A
1.0 mmol of substrate, 37.2 mg of Pd-pol (Pd%w = 5.0,
0.0175 mmol of Pd) and 20.0 mmol of sodium borohydride
were stirred under nitrogen at 60 ◦C in 5 mL of double deionized
water for the appropriate amount of time, using a three-necked
flask equipped with a reflux condenser and a gas bubbler in order
to discharge the hydrogen excess produced during reaction. The
progress of the reaction was monitored by GLC. After completion
of the reaction, the reaction mixture was centrifuged to separate
the catalyst. The solid residue was first washed with deionized
water and then with acetone and diethyl ether to remove any
traces of organic material. The filtrate containing the reaction
mixture was extracted with ethyl acetate (3 × 5 mL) and then
dried over anhydrous Na2SO4. The solvent was evaporated under
2. Experimental
2.1. General considerations
Tap water was de-ionized by ionic exchange resins (Millipore)
before use. All other chemicals were purchased from commercial

M.M. Dell’Anna et al. / Journal of Molecular Catalysis A: Chemical 402 (2015) 83–91
85
Scheme 3. Metal catalyzed production of H2 by NaBH4.
substrate, leaving the reaction temperature at 60 ◦C and the sub-
strate/Pd molar ratio equal to 200 (entry 3). This test allowed us to
choose 20.0 mmol as the optimum amount of NaBH4 and T = 60 ◦C
as the best working temperature, since under these conditions and
using a Pd mol% equal to 0.5 (entry 3), the conversion was improved
up to 56%, even not yet satisfactory. At this stage, we decided to
increase the palladium/quinoline molar ratio up to 1/100 (entry 4)
and up to 1/50 (entry 5), registering the best results in the latter
case (t = 9 h, conv. = 99%, selectivity = 93%).
In all the above mentioned tests, the reactions were carried in a
flask equipped by a condenser and a bubbler in order to discharge
the excess hydrogen. Conversely, by carrying out the reduction
under the optimized conditions in a steel autoclave, the in situ
formed hydrogen gas (Scheme 3) reached a pressure up to 20 bar
shortening the reaction time to 4 h (entry 6).
The best conditions reported in entries 5 and 6 of Table 1 (with-
out and with the use of an autoclave, respectively) were applied
to two parallel reactions carried out in a flask and in an autoclave,
respectively, both in the absence of the palladium catalyst (entries
7 and 8). In both cases the reduction of the model substrate did
occur, being the conversion 56% (entry 7) and 58% (entry 8) after
9 h stirring, although the selectivity to 1,2,3,4-tetrahydroquinoline
was poor (46% and 54%, respectively), being the other products
dihydroquinolines and azo compounds, thus confirming that Pd-
pol catalyst was essential for a quantitative substrate conversion
with a high selectivity.
Scheme 2. Reduction of quinoline catalyzed by Pd-pol.
reduced pressure to yield the crude product, which was then
purified by flash column chromatography using silica gel and an
appropriate eluent (Supporting information) to afford the pure
product. The products were characterized by GC–MS or 1H NMR
by comparison with authentic samples. For the assessment of
the chromatographic yields, biphenyl (50.0 mg) was used as the
internal standard.
2.2.2. Procedure B
A 50 mL stainless steel autoclave equipped with a transducer
for online pressure monitoring was charged, under air, of Pd-pol
(37.2 mg, Pd%w = 5.0, 0.0175 mmol of Pd), the substrate (1.0 mmol),
sodium borohydride (20.0 mmol) and water (5.0 mL). The autoclave
was then closed, set on a magnetic stirrer and heated to 60 ◦C. After
the minimum time needed to reach reaction completion, the auto-
clave was let to reach room temperature, the formed hydrogen was
vented and the autoclave opened. The catalyst was recovered by fil-
tration while the organic product was extracted with ethyl acetate
(3 mL), the water phase was washed with ethyl acetate (3 × 5 mL)
and the organic layers were collected and dried over anhydrous
Na2SO4. The solvent was evaporated under reduced pressure to
yield the crude product, which was then purified by flash column
chromatography using silica gel and an appropriate eluent (Sup-
porting information) to afford the pure product. The products were
characterized by GC–MS or 1H NMR by comparison with authen-
tic samples. For the assessment of the chromatographic yields,
biphenyl (50.0 mg) was used as the internal standard.
3.2. Scope of the Pd-pol catalyzed reaction
Using the optimized reaction conditions, the activity and the
scope of the catalyst was explored in the transfer hydrogenation
reaction of some quinolines and quinoxalines (Table 2).
2.3. Recycling experiments
Differently from the reduction of quinoline (Table 1 and entry 1
of Table 2), the transfer hydrogenation of 8-methylquinoline was
a tough reaction, probably due to the hyper conjugation of the
methyl group in 8 position [58]. In fact, by carrying out the reaction
under the optimized conditions in a flask equipped by a bubbler
(procedure A of the Section 2), the reduction stopped after 9 h at
33% conversion of the substrate (entry 2, Table 2). The conversion
did not increase by rising the temperature up to 100 ◦C (entry 3,
Table 2), or by replacing water for methanol (in which the sub-
strate is soluble, being insoluble in water) (entry 4, Table 2), or by
adding a weak acid, such as HCOOH (1.0 mmol, entry 5, Table 2), in
order to favor the pyridine nitrogen protonation [37]. The course
of the reaction was even worse by replacing NaBH4 for a base such
as N2H4·H2O (entry 6, Table 2) or by adding KOH (entry 7, Table 2),
aiming at promoting the hydrogen transfer from the solvent to the
substrate [59]. Finally, the 8-methylquinoline reduction gave good
results (86% of isolated yield) after 5 h when it was carried out
at 60 ◦C in autoclave (entry 8, Table 2), where the in situ formed
H2 pressure grew up to 20 bar. Remarkably, the latter reduction
stopped at 38% of conversion after 9 h stirring in the autoclave,
when it was carried out at room temperature (entry 9, Table 2).
Unlike the analogous reduction of quinoline in the absence of
palladium (entry 6 of Table 1), no reaction at all occurred when 8-
methylquinoline and NaBH4 were put together in water in a steel
autoclave, letting the mixture under stirring at 60 ◦C for 9 h in the
absence of metal catalyst (entry 10, Table 2).
The catalyst recovered by filtration was washed with water, ace-
tone, and diethyl ether and dried under high vacuum. The recovered
catalyst was thus weighed and reused for a new cycle employing
appropriate amounts of organic substrate, sodium borohydride and
solvent, assuming that the palladium content remained unchanged
with the recycles. Iteration of this procedure was continued for
seven reuses of the catalyst.
3. Results and discussion
3.1. Catalytic hydrogenation of quinoline
To determine the optimum amount of sodium borohydride and
catalyst required for the reduction of quinolines, different catalytic
tests were carried out varying the temperature and amounts of
NaBH4 and the catalyst, using quinoline as the representative sub-
strate (Scheme 2). The relevant results are reported in Table 1.
It can be seen from Table 1 that the reduction success depends on
several parameters. In fact, following our procedure for the reduc-
tion of quinoline under 10 bar H2 in aqueous medium [57], we used
a Pd/substrate molar ratio equal to 1/200, registering a low conver-
sion [30%] after 9 h at 60 ◦C (entry 1). We tried to overcome this
inconvenience by carrying out the reaction under the same con-
ditions but increasing the temperature up to 80 ◦C (entry 2), still
obtaining unsuccessful results. On the base of our experience on
the nitroarenes reduction in water [56], we decided to increase
the molar amount of NaBH4 up to twentyfold compared to the
Also the transfer hydrogenation of 2,6-dimethylquinoline
occurred only when the reaction was carried out in a steel

86
M.M. Dell’Anna et al. / Journal of Molecular Catalysis A: Chemical 402 (2015) 83–91
Table 1
Optimization of different parameters for the reduction of quinoline.a
Entry
Amount of sodiumborohydride (mmol)
Substrate/Pd(mmol/mmol)
T (◦C)
t (h)b
Conv.c(%)
Selectivityd(%)
>99
>99
>99
>99
93
94
46
54
1
2
3
4
10
10
20
20
20
20
20
20
200
200
200
100
50
50
No cat
No cat
60
80
60
60
60
60
60
60
9
9
9
9
9
4
9
9
30
37
56
70
99
99
56
58
5
6e
7f
8e,f
a
Reaction conditions: 1.0 mmol of quinoline and given amounts of NaBH4 and Pd-pol (Pd%w = 5.0) were stirred under nitrogen in H2O (5.0 mL), following the procedure A
(see the Section 2).
b
Minimum time at which the reaction stopped.
Determined by GLC analysis.
Selectivity into 1,2,3,4-tetrahydroquinoline.
Reaction carried out in autoclave, following the procedure B (see the Section 2).
c
d
e
f
In the absence of Pd-pol.
3.4. Analysis of the mother liquors
In order to verify whether the catalysis observed was truly het-
erogeneous or not, the reaction mixture was filtered hot at 40% of
quinoline conversion, and fresh NaBH4 was added to this solution.
Further stirring of the filtrate under the above reaction conditions
resulted in the same reaction course already observed in the reduc-
tion carried out in the absence of the metal catalyst. In addition, the
samples of Pd-pol recovered at the end of the first run and at 40%
conversion, respectively, were mineralized and analyzed by atomic
absorption spectrometry showing the same palladium content of
the virgin catalyst. The same palladium amount was also found in
the catalyst recovered at the end of the seventh cycle of quino-
line reduction. These results rule out any possible contribution of
homogeneous catalysis by leached palladium species.
Fig. 1. Recyclability of Pd-pol (2.0 mol% of Pd) in the transfer hydrogenation of
quinoline with NaBH4 in water at 60 ◦C (t = 9 h). Procedure A of the Section 2. GLC
yields using biphenyl as the internal standard.
3.5. TEM analyzes
This study was completed with the TEM characterization of Pd-
pol before, after the first and the seventh cycle, respectively, with
the aim of ascertaining whether the reaction cycles affected the
morphology and the dispersion of the palladium active species on
the surface of the support.
autoclave (entry 11, Table 2) and needed longer time (15 h), being
the latter a challenging substrate for steric hindrance reasons [60].
The
water
soluble
5-methylquinoxaline
heteroaromatic
and
nitrogen
2-
methylquinoxaline,
important
compounds used as intermediates for the synthesis of dyes,
pharmaceutics and antibiotics, were smoothly converted into the
corresponding 1,2,3,4-tetrahydroquinoxaline (entries 12 and 13,
Table 2) under 1 atm pressure, without the use of a steel autoclave.
Finally, the reduction of harmaline, another biologically important
heteroaromatic nitrogen compound, successfully proceeded in 9 h
giving quantitative yield in leptaflorine (entry 14, Table 2).
TEM pictures of the pristine Pd-pol and of the catalyst recov-
ered after the first and the seventh runs, are reported in Fig. 2a–c,
respectively. The pristine catalyst was constituted mainly by
polymer-bound Pd(II) species (not visible in the TEM micrographs)
[53], together with very small Pd nanoparticles with a spherical
shape and a homogeneous size. Most of these Pd nanoparticles had
a diameter smaller than 5 nm and were hardly recognizable since
they were all grouped in clusters collecting from 6 to 10 primary
nanoparticles (see enlargement of Fig. 2a). Clusters were measured
in diameter and the distribution is reported in Fig. 2a. As it can
be observed, the highest number of nanoparticle clusters had a
size around 10 nm with a small number having larger size. This
nanostructure reflected the way of preparation of the pre-catalyst
[61]. In fact, these nanoparticles likely formed during the thermal
polymerization step of the catalyst synthetic procedure [53]. The
TEM image of Pd-pol recovered after the first run (Fig. 2b) showed
an evident increase in the number of primary particles homoge-
neously distributed throughout the polymer. Pd nanoparticles had
an average diameter from 2 nm to 4 nm, ascribable to the in situ
NaBH4 reduction of Pd(II) supported polymer. In fact, the morpho-
logical features of Pd-pol recovered after a run carried out under
typical reaction conditions, but without substrate, were similar to
those shown by the TEM image reported in Fig. 2b. In this case
the Pd nanoparticles were not grouped in clusters but are individ-
ually distributed (enlargement in Fig. 2b). The shape of these Pd
3.3. Reusability of the Pd-pol catalyst
The reusability of Pd-pol catalyst in the transfer hydrogenation
of quinoline with NaBH4 in a glass flask (procedure A of the Section
2) was investigated. After the first use, the supported catalyst was
recovered by centrifugation and reused in the next run after several
washings. The recovered catalyst was successfully employed in the
subsequent seven cycles with a high catalytic activity, giving the
product in good yields (87–95%, Fig. 1).
Aiming at comparing the activity and selectivity of Pd-pol with
a commercial system, we decided to investigate the performance
of the Pd/C catalyst in promoting the quinoline reduction under
the same conditions reported in the present work (NaBH4, water,
60 ◦C). Pd/C showed limited recyclability (Table 3), probably due to
the difficulty in recovering the finely powdery catalyst. In fact, the
quinoline conversion dropped to 47% at the fourth run with Pd/C.

M.M. Dell’Anna et al. / Journal of Molecular Catalysis A: Chemical 402 (2015) 83–91
87
Table 2
Pd-pol-catalyzed transfer hydrogenation of quinolines and heteroaromatic nitrogen compounds.a
Entry
Substrate
Product
Additive
None
t (h)
Yieldb (%)
1c
4
94(86)
N
H
N
N
2
None
9
9
9
9
9
9
33
N
H
3d
4e
5
None
30
13
31
2
N
H
N
N
N
N
N
None
N
H
HCOOH (1.0 mmol)
N2H4·H2O (20.0 mmol)
KOH (1.0 mmol)
N
H
6f
N
H
7
8
N
H
8c
None
5
(86)
N
N
H
9c,g
None
9
38
N
N
H

88
M.M. Dell’Anna et al. / Journal of Molecular Catalysis A: Chemical 402 (2015) 83–91
Table 2 (Continued)
Entry
Substrate
Product
Additive
None
t (h)
Yieldb (%)
10c,h
9
2
N
H
N
11c
None
None
15
(86)
(86)
N
H
N
H
N
N
N
12
9
N
H
H
N
N
13
None
None
9
9
(88)
(91)
N
N
H
NH
N
14
N
H
N
H
H3CO
H3CO
a
Reaction conditions: substrate (1.0 mmol), H2O (5 mL), NaBH4 (20.0 mmol), Pd-pol (Pd: 2 mol%), 60 ◦C, procedure A of the Section 2.
b
c
d
e
f
GLC yields using biphenyl as the internal standard. Isolated yield after column chromatography in parenthesis.
Reaction carried out in a steel autoclave, following the procedure B of the Section 2.
At 100 ◦C.
Using CH3OH (5 mL) as the solvent.
In the absence of NaBH4.
At room temperaturehIn the absence of metal catalyst.
g
60 ◦C with NaBH4 (entry 8, Table 2) needed 5 h stirring, while the
hydrogenation of the same substrate promoted by the same batch
of catalyst in H2O/CH3OH under 10 bar H2 at 80 ◦C took 9 h [57]. We
already noticed the effect of the reducing agent on the catalyst mor-
phology and performance also during our previous studies on the
nitroarenes reduction [56]. It is known that NaBH4 acts as a reduc-
ing agent of the metal in a dual way, by providing hydrides and by
generating hydrogen gas. In fact, the hydrides from sodium borohy-
dride can displace negatively charged ligands bound to Pd(II) (the
␤-ketoesterato moieties in the case of Pd-pol, Scheme 1), generat-
ing Pd-hydrides directly, which upon ␤-hydride elimination of H2,
give the reduced metal. In addition, the well-known in situ formed
H2 in water (Scheme 3) in the closeness of Pd(II) can reduce it to
Pd(0) with formation of small size metal nanoparticles.
On the contrary, in the case of Pd-pol stirred under 10 bar H2,
the reduction occurs only by hydrogen gas, which has to be trans-
ported into the liquid phase and then to the Pd-pol structure. Since
the metal reduction proceeds more slowly compared to the one
obtained in the presence of NaBH4, there is enough time to form
larger palladium crystallites.
Table 3
commercial Pd/C catalyzed transfer hydrogenation of quinoline.a
Entry
Conv. (%)b
Selectivity (%)
1st Run
2nd Run
3rd Run
4th Run
95
92
71
47
98
98
97
98
a
Reaction conditions: quinoline (1.0 mmol), H2O (5 mL), NaBH4 (20.0 mmol), Pd/C
10%w (Pd: 2 mol%), 60 ◦C, 9 h.
b
GLC conversion using biphenyl as the internal standard.
nanoparticles still was spherical without any preferential growth
as already observed during the first run of the previously studied
hydrogenation of quinolines under 10 bar hydrogen at 80 ◦C [57],
where the Pd nanoparticles were slightly bigger (diameter rang-
ing from 1.5 to 8.5 nm) compared to the present ones and showed
morphology and size distribution typical for polymer stabilized Pd
nanoparticles formed by reduction under gaseous H2 [62].
The different nanoparticle diameter distributions observed in
the two Pd-pol samples, the first one reduced by NaBH4 and the sec-
ond one reduced by 10 bar H2, were presumably responsible for the
slightly different catalytic activities showed by the two catalysts,
being the latter less active. In fact, for example, the Pd-pol catalyzed
reduction of 8-methylquinoline carried out in autoclave in water at
On passing from the first to the seventh cycle (Fig. 2c), the
number of smallest size (2 nm) Pd(0) nanoparticles even increased,
although some of them grew up to 18–20 nm diameter. The primary
nanoparticles were still homogeneously distributed as shown in

M.M. Dell’Anna et al. / Journal of Molecular Catalysis A: Chemical 402 (2015) 83–91
89
Fig. 2. Transmission electron micrographs and associated size distribution of matrix polymer embedded Pd nanoparticles: (a) pristine Pd-pol; (b) Pd-pol recovered after the
first run of the reduction of quinoline with NaBH4; (c) Pd-pol recovered after the seventh run of the reduction of quinoline with NaBH4.
the enlargement in Fig. 2c. However, the nanostructure of the cat-
alyst was substantially maintained during the subsequent cycles
and the Pd nanoparticles remained embedded into the polymer
bulk. In fact, clusters of primary nanoparticles did not grow in
size indicating absence of particle mobility under the reaction con-
ditions. Interestingly, it was reported that palladium(II) centers
supported onto a methacrylic resin similar to Pd-pol pre-catalyst,
once reduced to Pd(0), gave nanoparticles, which in turn could
increase their size (growth) or/and agglomerate (aggregation) with
the time [62,63]. The aggregation process would decrease the cat-
alytically active superficial area much more than the growth of a
single nanoparticle. Remarkably, the Pd nanoparticles supported
onto Pd-pol remained isolated from each other and they did not
aggregate with the recycles, although a limited amount of them
slightly grew up. This explains why the catalytic activity of the sup-
ported catalyst recovered after seven cycles was comparable to that
of the catalyst re-used after the first run.
Numerous investigations concerning the development of the
incorporation technique of metal nanoparticles into the polymer
matrix have been published demonstrating the stabilization of
the metal nanoparticles encapsulated into the polymer matrix
[62–65]. ApolymerizablePd(II) ␤-ketoesteratecomplexis the start-
ing monomer for the synthesis of the metal-polymer composite
material Pd-pol [47]. This approach [45] is particularly attractive
from the synthetic point of view since both the components of
the composite material have ample scope for intimate contact. In
order to get further insights into the Pd nanoparticle stabilization
during the NaBH4 assisted Pd-pol reduction micro-IR spectroscopy
analyzes of the pristine Pd-pol and of the Pd–pol recovered after
7 reduction cycles were performed. Their FT-IR spectra (Fig. S1)
showed the disappearance of the weak ␤-ketoesterate combina-
tion band signals (1602 and 1509 cm−1) [46] on passing from the
pristine to the used catalyst. This phenomenon is ascribable both
to the loss of metal coordination by the protonated polymerized
ligand, and to the reduction of the oxidation state of palladium by
NaBH4 with formation of nanoparticles, likely stabilized by electro-
static interactions with the oxygen atoms of the polymeric support.
Similar stabilization behaviors were observed also in fluorinated
copolymers containing the AAEMA− ligand [66].
3.6. Mechanistic considerations
Pioneering studies on the reduction of quinolines, isoquinolines
and quinoxalines with sodium borohydride have already been car-
ried out using methanol as the solvent in the absence of a metal
catalyst with the addition of acetic acid [67]. However, the reported
reductions occurred only in the presence of electron withdrawing
substituents on the aromatic ring, necessary to provide the activa-
tion of the C N bonds for the nucleophilic attack by the hydride
[68], and often led to the formation of N-alkylated product [69]. In
addition, the quinolines were reduced only to their correspond-
ing 1,2-dihydroquinolines, being this method unsuitable for the
formation of tetrahydroquinolines. These results are in agreement
with what we observed by carrying out the reduction of quino-
line with NaBH4 in water in the absence of Pd-pol (entries 7 and 8,
Table 1). Probably, the substrate was first converted into 1,2- and
1,4-dihydroquinolines, which are known to be instable [70], being
rapidly oxidized by air to quinoline or decomposing by dispro-
portionation into quinoline and tetrahydroquinoline. The proposed
mechanistic pathway in the absence of metal catalyst starts with
the formation of a cation (Scheme 4a) or, better, a species formed
by hydrogen bond with the water solvent and the heterocycle, thus
bearing a partial positive charge on nitrogen delocalized by res-
onance (Scheme 4b). Both species can easily be attacked by the
hydride in 2 or 4 carbon position to give the dihydroquinoline
products, which in turn would decompose.
The proposed mechanism explains the similar results obtained
by carrying out the reaction in the absence of catalyst in a glass

90
M.M. Dell’Anna et al. / Journal of Molecular Catalysis A: Chemical 402 (2015) 83–91
Scheme 4. (a) Formation of the pyridinium ion in the presence of trace acid; (b) interaction between water and quinoline.
4. Conclusions
In conclusion, we have demonstrated that polymer supported
palladium catalyst (Pd-pol) can promote the hydrogenation of
quinolines (a class of well-known poisons for the traditional noble
metal-based hydrogenation catalysts) in high yield and excellent
selectivity, by using sodium borohydride and water as the hydrogen
source under mild condition.
The catalyst did not change its nanostructure and could be
recovered at the end of the reaction and re-cycled for at least seven
times with the same activity and selectivity. TEM analyzes showed
that the active species are Pd nanoparticles with average size of
3 nm diameter, that do not aggregate with the re-uses and do not
leach out in solution during reaction. Pd-pol, which is still to the
best of our knowledge the second example reported to date of active
palladium catalyst recyclable in aqueous solvent for the selective
hydrogenation of quinolines and heteroaromatic nitrogen under
hydrogen gas, is also active, selective and recyclable for the trans-
fer hydrogenation of quinolines under safer conditions, i.e., by using
NaBH4 in water.
Scheme 5. Plausible transition state for the hydrogenation of 8-methylquinoline
with Pd-pol in water.
flask and in a steel autoclave, respectively (entries 7 and 8, Table 1),
because the hydrogen pressure does not affect the reaction course,
being dihydrogen not involved in the kinetic pathway. Indeed, the
reluctance of 8-methylquinoline to be reduced by NaBH4 in the
absence of metal catalyst can be due to the presence of the elec-
tron donor group on the benzene ring, which would prevent the
nucleophilic attack by the hydride. On the contrary, in the pres-
ence of Pd-pol, the dihydrogen (formed according to the reaction
reported in Scheme 3) may add to the C N double bond of the
8-methylquinoline, following an ionic pathway involving solvent
assistedheterolytichydrogenactivation promotedbythe basicsub-
strate (Scheme 5). As already substantiated in similar cases [22,71],
this would result in protonation of the 8-methylquinoline, followed
by H-transfer from the metal to the adjacent carbon atom, as also
suggested by Dobereiner et al. [8] and by Sánchez-Delgado and
co-workers [72,73].
Acknowledgements
The authors thank Dr. Mauro Zapparoli, CIGS, University of Mod-
ena and Reggio Emilia, Italy, for TEM observations and Italian MIUR
(project PRIN 2010–2011 n. 2010FPTBSH 001) for financial support.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.molcata.
2015.03.013.
When the substrate was 8-methylquinoline, the hydrogen
pressure was crucial, being dihydrogen the only reducing agent
involved in the kinetic pathway as the formation of 8-methyl-
1,2,3,4-tetrahydroquinoline occurred only when the reaction was
carried out in a steel autoclave retaining all the in situ formed
hydrogen gas. On the other hand, the reduction of the simple
quinoline in the presence of Pd-pol followed both mechanistic
routes depicted in Schemes 4 and 5, and the formation of 1,2,3,4-
tetrahydroquinoline occurred also when the reaction was carried
out in a flask discharging the produced hydrogen gas.
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