Facile hydrogenation of N-heteroarenes by magnetic nanoparticle-supported sub-nanometric Rh catalysts in aqueous medium

Article abstract of DOI:10.1039/c8cy00936h

The hydrogenation of nitrogen-containing heterocyclic precursors in aqueous medium at low temperature without imposing molecular hydrogen pressure is quite challenging. Herein, we report the synthesis and performance of a novel catalyst capable of facile hydrogenation (employing tetrahydroxydiboron (THDB) as the reductant) of N-heteroarenes in water at 80 °C with good recyclability. Rhodium particles in the sub-nano range (<1 nm) were produced by in situ reduction of a Rh precursor on freshly prepared superparamagnetic iron oxide nanoparticles (SPIONs, Fe₃O₄), using aqueous ammonia as a reducing agent at 50 °C. HRTEM and elemental mapping images reveal a homogeneous distribution of <1 nm Rh particles within the matrix of Fe₃O₄ nanoparticles having an average size within a narrow range of 7-9 nm. The superparamagnetic nature of the composite was confirmed by VSM analysis. The Rh@Fe₃O₄ catalyst was found to be highly efficient in the heterogeneous hydrogenation of nitrogen-containing heterocyclic compounds with quantitative conversion. It showed selectivity towards the hydrogenation of 1,2,3,4-tetrahydroquinoline (py-THQ) in water using THDB with a high TOF of 1632 h^-1. These results are compared with the conversion and selectivity data obtained from reduction with molecular hydrogen gas pressure. The catalytic activity is extended to the successful hydrogenation of simple aromatics like benzene, toluene etc. Isotopic labelling studies were performed to determine the source of hydrogen in quinoline hydrogenation in the presence of THDB. It was found that it could be used for 16 consecutive cycles with gaseous hydrogen, without any undesired by-products; it also retained its original crystallinity.

Full text of DOI:10.1039/c8cy00936h

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PAPER
Qingzhu Zhang et al.
Catalytic mechanism of C–F bond cleavage: insights from QM/MM
analysis of fluoroacetate dehalogenase
rsc.li/catalysis

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ARTICLE
Facile hydrogenation of N-heteroarenes by magnetic
nanoparticles supported sub-nanometric Rh catalysts in aqueous
medium
Received 00th January 20xx,
Accepted 00th January 20xx
,
a
a
b
a
DOI: 10.1039/x0xx00000x
M. Nasiruzzaman Shaikh,* Md. Abdul Aziz, Abdul Nasar Kalanthoden, Aasif Helal, Abbas S.
a
c
Hakeem, and Mohamed Bououdina
www.rsc.org/
The hydrogenation of nitrogen-containing heterocyclic precursors in aqueous medium at low temperature
without imposing molecular hydrogen pressure, is quite challenging. Herein, we report the synthesis and
performance of a novel catalyst capable of facile hydrogenation (employing tetrahydroxydiboron (THDB) as
the reductant) of N-heteroarenes in water at 80 °C with good recyclability. Rhodium particles in the sub-
nano region (< 1 nm) were produced by in-situ reduction of a Rh precursor on freshly prepared
superparamagnetic iron oxide nanoparticles (SPIONs, Fe
0 °C. HRTEM images and elemental mapping reveal homogenous distribution of < 1 nm Rh particles within
the matrix of Fe nanoparticles having average size was within a narrow range of 7-9 nm. The
superparamagnetic nature of the composite was confirmed by VSM analysis. The Rh@Fe catalyst was
3 4
O ), using aqueous ammonia as reducing agent at
5
3 4
O
3
O
4
found to be highly efficient in the heterogeneous hydrogenation of nitrogen-containing heterocyclic
compounds with quantitative conversion. It showed selectivity towards the hydrogenation of 1,2,3,4-
-1
tetrahydroquinoline (py-THQ) in water using THDB with high TOF of 1632 h . These results are compared
with the conversion and selectivity data obtained from the reduction by molecular hydrogen gas pressure.
The catalytic activity is extended to the successful hydrogenation of simple aromatics like benzene, toluene
etc. Isotopic labelling studies were performed to know the source of hydrogen in quinoline hydrogenation in
presence of THDB. It was found that it could be used for 16 consecutive cycles with gaseous hydrogen,
without
any
undesired
byproducts;
it
also
retained
its
original
crystallinity.
THQ) and decahydroquinoline (DHQ) in different or the same
ratio. The underlying challenge, therefore, is to generate the
hydrogenated pyridine ring compound (py-THQ) selectively as
Introduction
Catalytic hydrogenation of organic functional groups, such as
13
14
15
16
the prime product. In this context, Ru , Rh , Ir and Pd
1
-3
4
5
6
7-9
alkene , alkyne , carbonyl , imine , nitro etc., has recently
become very crucial in organic synthesis. For example, the
direct hydrogenation of N-bearing heterocyclic compounds,
based homogeneous catalysts have been applied for the direct
hydrogenation of quinoline with excellent results. For
example, Fish and co-worker reported Rh-based catalyst (η -
5
1
0
such as quinoline, to 1,2,3,4- tetrahydroquinoline (py-THQ)
pentamethylcyclopentadienyl) rhodium dicationic complex,
for the regioselective reduction of
has attracted lot of interest due to their great relevance as
major building blocks in the synthesis of agrochemicals,
pharmaceuticals, dyes, alkaloids and numerous other fine
2+
]
[
Cp*Rh(CH
3
CN)
2
17-19
nitrogen-containing model compounds such as quinoline.
Recently, an iridium complex of BINOL-derived
phosphoramidite PipPhos was used for the hydrogenation of a
1
1
products.
However, the heteroarene hydrogenation
encounters serious challenges owing to their rather high
stability; the lability of the lone-pair of electrons on the
heteroatom (nitrogen) often deactivates the catalysts.
Hydrogenation of quinoline may produce three possible
products, such as, py-THQ, 5,6,7,8- tetrahydroquinoline (bz-
20
series of quinolines.
Unfortunately, these homogeneous
catalytic procedures suffer from serious drawbacks of
employing harsh reaction conditions accompanied by highly
tedious and cumbersome purification process. Their rather
1
2
20
low efficiency limits and restricted reusability options pose
impediments to wide commercialization endeavours.
Recent focus on the development of recyclable and more
efficient catalysts has drawn attention to their production in a
greener and safer environment. Therefore, alternative to
homogeneous catalysis is required where separation of the
product is expected to be effortless with high product purity.
Hence, the path of heterogeneous catalysis becomes an
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Journal Name
2
1-23
obvious choice.
Accordingly, numerous heterogeneous As a part of our sustained efforts towards developing highly
2
4
25
26-28
29-32
33
34
45-46
catalytic systems based on Mn , Ni , Ru
, Rh
have been developed for quinoline in this study we describe a new construct consisting of sub-
hydrogenation. Although, in some cases complete conversion nanometer rhodium nanoparticles supported on ultra-small
of quinoline with good selectivity was observed with 50 bar or magnetic nanoparticles. Magnetite (Fe ) was chosen as
, Ir , Pt , active and reusable noble-metal supported solid catalysts,
3
5-37
38,39
Pd
and Au
3
O
4
support because of its simple, easy and cost-effective synthesis
from inexpensive precursors and its proven magnetic
properties. Moreover, the reaction could be carried out in
water – a benign and green solvent. For the first time,
tetrahydroxydiboron was employed for reduction of quinoline
thus obviating the need for using compressed hydrogen gas.
more of H2 pressure at about 200 °C, the reaction,
nevertheless, progressed at much slower pace, taking tens of
hours to reach to completion; it also involved the use of toxic
organic solvents. Thus, there is a need to seek alternative
material paths, where the reactions could be conducted in low
temperature regimes without employing hydrogen gas at high
pressure in a green medium such as water. In this context,
judicious choice of the solid support is crucial to obtain high
conversion, greater selectivity and reusability. A number of
precious metal-impregnated solid supports such as zeolite,
polymer, silica and cellulose have been investigated as
3 4
The availability of basic sites of the solid-support (Fe O ) to
attract the substrate and interaction with Rh nanoparticles
appear favourable for high catalytic activity and exceptional
reusability without deactivation.
Experimental
2
1-39
catalytic systems.
However, ligand-free metal supported
Materials. All chemicals were purchased from Sigma-Aldrich
and were used as-received unless otherwise stated. Standard
procedures were followed for the dry and deoxygenated
solvents. Schlenk line techniques were used to carry out
reactions under inert atmosphere wherever needed. Deionized
on SPIONs has rarely been explored. It should be stated that
solid-supported catalyst
(DI) water (specific conductivity: 18.2 mΩ) was used in all the
experiments.
FTIR. Furrier-Transform Infrared (FTIR) spectroscopic data
were obtained on a Nicolet 720 in the wave number range of
-
1
5
00 to 4000 cm , using KBr as the IR transparent window
material.
XRD. X-ray diffraction data were collected on Rigaku model
Ultima-IV diffractometer employing Cu-Kα radiation (λ =
1
9
.5405 Å) at 40 kV and 25 mA over a 2
0°.
θ range between 20 and
Fig.1 Schematic representation of quinoline hydrogenation using TEM. The Transmission Electron Microscopy images were
acquired at the Instituto de Nanociencia de Aragón (LMA-INA),
University of Zaragoza, Spain, on a TEM (JEOL, JEM 2011
operated at 200 kV with 4k × 4k CCD camera (Ultra Scan
00SP, Gatan). The TEM samples were prepared by dropping
3 4
catalyst, Rh@Fe O , and THDB as reductant in water.
)
require post-reaction separation from the products; this is
generally achieved either by filtration, centrifugation or
precipitation, each of which severely affects the prospects of
reusability as a result of gradual but monotonous loss of
4
on a copper grid from an ethanolic suspension and drying at
room temperature.
4
0
material in every cycle.
DLS: The particle size was determined by the Microtrac nanosize
analyzer (Model S3500/Turbotrac, USA). Powder samples were
taken in ethanol and sonicated using ultrasonic probe sonicator
Superparamagnetic iron oxide nanoparticles (SPIONs) have
attracted
a
great deal of attention because of their (Model VC 50, Sonics, USA) for
1 h to have maximum
straightforward synthesis procedure from low cost precursors. deagglomeration. The particle size was determined on average
based on five measurements.
Their insolubility in water (and other common solvents) and
distinct magnetic attributes render their separation from the
ICP-OES. The amount of Rh in the catalyst was determined by
Inductively Coupled Optical Emission Spectrometry (ICP-OES;
product stream by an external magnet effortless. Magnetically
separable catalysts have been exploited to synthesize
4
PlasmaQuant PO 9000 - Analytik Jena). The samples were first
asymmetric hydrogenated digested in a dilute HNO and HCl mixture. Calibration curves
1
42
biphenyl,
higher olefins,
3
4
ketones and transesterified biodiesel. No magnetic catalyst were prepared for Rh and Fe using standard solutions (ICP
3
44
Element Standard solutions, Merck).
has, however, been developed for agile and highly selective
hydrogenation of N-heterocyles in water without applying
molecular hydrogen pressure.
2
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SEM-EDS. Samples for Scanning Electron Microscopy (SEM) autoclave fitted with a pressure gauge, mechanical stirring and
were prepared from ethanolic suspensions on alumina stubs
and coated with gold in an automatic gold coater (Quorum,
Q150T E). For the elemental analysis and mapping, the energy
dispersive X-ray spectra (EDS) were collected on a Lyra 3
attachment to the SEM.
automatic temperature controller. The reactor was flushed
repeatedly with H , filled with H2 up to the 40 bar, and the
2
reaction initiated with heating and stirring and continued for
1
0 h. After the completion of reaction, the reactor was allowed
to cool down to room temperature, depressurized and
VSM. The magnetic susceptibilities were measured at room
temperature using a vibrating sample magnetometer (VSM, opened. The degree of conversion measured in a GC and the
model PMC Micromag 3900) equipped with a 1 tesla magnet.
product was identified by the NMR or GC-MS.
Autoclave. Catalytic reactions were performed in Teflon lined
autoclaves from HiTech, USA (model M010SSG0010-E129A-
00022-1D1101), fitted with a pressure gauge and a mechanical
stirrer.
Aromatic hydrogenation with gaseous hydrogen. The similar
method was followed for the hydrogenation of aromatic
compounds like benzene, toluene and xylene.
GC-MS: Catalytic products were identified by a Shimadzu 2010
Plus (Japan) gas chromatograph attached with
Results and discussion
Synthesis and characterization
a mass
spectrometer. The disappearance of the reactant and
sequential appearance of the product was recorded in real-
time, identifying the species in terms of their molecular ion The novel catalyst was prepared by the deposition of Rh
+
(
M ) by comparing and matching them with the available precursors on the surface of magnetite (Fe O )
3 4
Willey library of the mass spectrum database, in addition to
the identification of mass fragmentation.
support(average particle size
≈7-9 nm) followed by mild
reduction using ammonia to make sure no more unreacted Rh
precursors in the reaction system.
1 13
NMR: Solution H and C NMR experiments were performed
on a Bruker Ascend 400 spectrometer at Saudi International
Petrochemical Company (SIPCHEM) in Dhahran Techno Valley Structural characterization of the catalyst was carried out by a
(DTV) and chemical shifts (δ) values were referenced to
tetramethylsilane (TMS) as an internal standard.
number of analytical and spectroscopic techniques. The phase
analysis was investigated by X-ray diffractometry and
comparative XRD signatures of magnetite powder with and
without Rh are shown in Fig. 2. Well-defined broad peaks with
relatively high intensity are observed, indicating high
Synthesis of Rh@Fe
nanoparticles in the range of 7-9 nm were prepared according
3
O
4
3 4
Catalyst. Magnetite (Fe O )
4
7,48
to a procedure described elsewhere.
3 4
Nanoscale Fe O
powder (200 mg) was suspended in dry methanol (30 mL) and crystallinity with spinel structure of the freshly prepared Fe O
3
4
sonicated for 3 h. 200 μL of rhodium (III) nitrate solution (1.9
x10 mol, 0.019 g of Rh) was added to the homogeneous
suspension of magnetite and stirred at 50 °C for 3 h under
3 4
and Rh@Fe O nanopowders.
-4
4
argon atmosphere. Concentrated NH OH (27 M) was added to
achieve and maintain a pH >12, standing the mixture for 4 h.
Particles were settled down by means of an external magnet.
They were washed with a copious amount of water followed
by dichloromethane.
Quinoline hydrogenation with THDB. The catalytic
hydrogenation of quinoline was performed in a parallel 10-
place reaction tube reactor fitted with a magnetic stirrer and a
3 4
Teflon stopper. To the Rh@Fe O (2 mg) suspension in DI
water (2 mL), quinoline (0.5 mmol, 0.65 μL) was added and the
system flushed with argon gas 3 times. Tetrahydroxydiboron (4
mmol) was added, capped with Teflon stopper and heated to
80 °C. The reaction progress was monitored by TLC (thin layer
chromatography); the product was extracted with ethyl
acetate (EA) and dried with sodium sulphate. It was passed
through a short silica gel column using EA and hexane (8:2)
mixture as an eluent. Conversion was measured by GC and
identified by the GC-MS.
3 4 3 4
Fig. 2 XRD spectra of (a) pure Fe O and (b) Rh@Fe O .
Furthermore, the superimposition with the pure Fe
indicates the preservation of Fe
arrangement upon decoration of sub-nanosized Rh. XRD angle
at (2 ) 30.22°, 35.70°, 43.10°, 53.40°, 57.10° and 63.20°
3
O
4
peaks
3
O
4
structural lattice
θ
N-heteroarene hydrogenation with gaseous hydrogen. For
this, the catalyst (5 mg) and the selected reactant (for
example, pyridine, 0.5 mL) mixture was placed in a Teflon-lined
demonstrated the purity and formation of single phase
nanomaterials with cubic structure (JCPDS card No. 01-075-
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449) of magnetite (Fe
Journal Name
4
).
9
0
3
O
4
It is worthy to note that no (TEM), which revealed the formation of well-dispersed
additional recognizable peak(s) attributed to Rh was detected spherical particles, about 7-9 nm in size with narrow size
within the measured range of diffraction pattern. This may be distribution (Fig. 3a-e and 4a-g), which was confirmed by
attributed to the low concentration(s) as well as sub- measurement with
a particle size analyzer (Fig 4d).
nanometer sized Rh particles are dispersed over the spherical Furthermore, the surface of magnetite nanoparticles (7-9 nm;
solid surface of magnetite and hence it is hard to detect in Fig. 4a) is uniformly decorated with Rh particles that are < 1
XRD.
nm in size (Fig. 4b); as corroborated by the confined area of
elemental mapping (Fig 3c-d). It is speculated that during the
process of decoration on magnetite nanoparticles, increase in
surface potential led to the decrease in the size of Rh particles
5
0,51
to the sub-nano region.
Scanning tunnelling electron
microscopy (STEM) was used to investigate the local physical
and electronic structure of surfaces, which led to the
identification of Rh in the sample (Fig 3e, 4e and see
supporting information, Fig S1). HRTEM image (Fig. 4f)
3 4
confirms the high crystallinity of Fe O nanoparticles, which
was also concluded from the powder XRD pattern shown
earlier in Fig. 2. Figure 4f and the Bragg reflections in the
corresponding SAED pattern (Fig. 4g) yielded a d-value of 0.249
nm which could be assigned to the <311> reflection of
magnetite in the specimen in cubic crystal structure. However,
ICP-OES data demonstrates that the ratio of Fe/Rh is found to
be 94:6 which is further confirmed by both EDS studies
3 4
coupled with both SEM and TEM. FTIR of Rh@Fe O showed a
-1
-1
vibrational red shift of 3 cm from 581 cm for the Fe-O bond
in the parent magnetite and these results demonstrated that
after incorporation of Rh nanoparticles on magnetite surface
did not significantly affect Fe-O vibration.
The XPS studies have been carried out to know the composition and
oxidation states of the element used in the process. The XPS data
0
revealed the formation of Rh on the surface of Fe O , as
3
4
corroborated by the peaks with binding energies of 307.1 eV (3d5/2
)
0
52
and 311.8 eV (3d3/2) associated with Rh species.
A small
shoulders at the photoelectron energy of 308.9 eV corresponding to
Rh was also seen, which could be an artifact of Rh oxidation by
ambient air.
2 3
O
5
2
Fig. 3 SEM image of (a) Fe
3
O
4
and (b) Rh@Fe
3 4
O ; Elemental mapping of
(
c) Fe
3
O
4
and (d) Rh@Fe
3
O
; (e) SEM-EDS of Rh@Fe O .
4 3 4
The size of magnetite crystallites was estimated to be less than
0 nm from Debye-Scherrer equation; this was also validated
from the FESEM and TEM images of the catalyst specimen (Fig.
and 4). The structural refinement performed by using the
1
3
Rietveld method (see supporting information, Table 1) which
confirmed the formation of the desired phase in high purity
with a goodness of fit close to the unity (GoF = 1.07-1.13). This
suggests that the incorporation of Rh did not alter the
structure nor dilate the lattice of the parent support,
suggesting that SPIONs remained stable during the Rh
decoration process.
The morphological and nanostructural features of Fe
3 4
O and
Rh@Fe O particles were investigated by scanning electron
3
4
microscope (SEM) and transmission electron microscopy
4
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Fig. 4 TEM images of: (a) Fe
3
O
4
, and (b) Rh@Fe
3
O
4
. Insets (c) shows To explore the catalytic hydrogenation activity, we selected
magnified image of RhNPs decorated on the Fe
particle size distribution curve of Rh@Fe
Rh@Fe particles is shown in (e); red circles denote RhNPs (size: < 1
nm). The HRTEM image of Rh@Fe
is shown in (f); inset (g) shows 1). Samples were taken out periodically and analysed by thin
the selected area electron diffraction (SAED) pattern of Fe on
Rh@Fe ; red circles denote RhNPs (size: < 1 nm).
3
O
4
particles and (d)
the benchmark substrate quinoline as a model reaction and its
3
4
O . The STEM image of
degree of sensitivity towards hydrogenation was tested (Table
3
O
4
3 4
O
3
O
4
layer chromatography (TLC), gas chromatography (GC) and gas
chromatography-mass spectrometry(GC-MS) to evaluate
conversion and selectivity; this assisted in optimizing the
3
O
4
The magnetization (M) vs. applied magnetic field (H) curves reaction process parameters such as, the solvent, time and
measurements were conducted by vibrating sample temperature, amount of catalyst and the reactant and,
magnetometery at room temperature. As can be seen in figure
hydrogen precursor. Since, the activity and selectivity was an
3 4 3 4
5a, both Fe O and Rh@Fe O show superparamagnetic
artefact of the solvent selection, the hydrogenation was
carried out in a number of solvents, such as, IPA, THF,
cyclohexane, methanol and 1,4-dioxane, as summarized in
Table 1. No hydrogenation occurred in methanol, dioxane,
cyclohexane or THF even after long reaction time (Entry # 3-6).
In addition, the reaction progress in IPA was quite slow and did
not reach completion even after prolonging the reaction time
to 5 h. On the other hand, the conversion was quantitative in
water. The effect of temperature was also briefly examined.
There was no conversion at room temperature, but the
conversion was appreciable at 50 and 80 °C (table 1, entry# 1).
In fact, it took only 10 min. at 80 °C to achieve > 99%
behaviour as no hysteresis loop was found in the recorded
curve. It is important to highlight that slight reduction of
saturation magnetization establishes the viability of the
surface decoration with non-magnetic element, Rh.
From the foregoing discussion, it is clear that the reported
preparation technique offers an excellent pathway to
designing catalysts with well-dispersed, sub-nanometre Rh
particles dispersed on nanostructured magnetic support; this
recipe could be easily extended to other solid surfaces as well,
for important organic transformations, including the
hydrogenation of aromatic precursors.
-
conversion with 100% selectivity with the TOF value of 1632 h
1
Catalysis
.
Nevertheless, on increasing the reaction temperature, no
Hydrogen transfer reaction of quinoline using
tetrahydroxydiboron (THDB) as reductant
deteriorating effect on selectivity was evident from the results
table 1, entry# 1). The effect of relative concentration of
(
THDB on quinoline hydrogenation in water as a function of on-
stream reaction time, is shown in Table 2. The amount of THDB
were varied systematically from 2 to 8 equivalent of quinoline.
The conversion was seen to increase steadily with increasing
THDB and 97% conversion into py-THQ was achieved with 6
equivalent THDB after 20 min. of reaction time. But the
To evaluate the selectivity and conversion, quinoline was used
a model benchmark substrate for the catalytic hydrogen
transfer reactions using Rh@Fe O as catalyst. Efficacy of the
3
4
magnetic catalyst towards hydrogenation was tested by using
THDB as the reductant in lieu of compressed H gas in
cylinders. No conversion of py-THQ was observed in presence
of the catalyst, Rh@Fe , in absence of THDB. In presence of
2
optimum conversion of
> 99% was achieved with 8 equivalent
3 4
O
of THDB (table 2, entry # 4) in just 10 min with very high TOF
THDB without catalyst, the reaction was proceeded in much
slower rate and after 10 min of the reaction, 27% conversion
was observed (table 1, entry# 1). Hence, a combination of
-1
value, 1632 h . Hence, quinoline can be smoothly
o
hydrogenated to py-THQ in water at 80 C by adding eight
equivalent of THDB as hydrogen source to selectively produce
py-THQ in 10 minutes. It should be also pointed out that in a
conventional scheme of quinoline hydrogenation, bz-THQ and
Rh@Fe
3
O
4
and THDB, as hydrogen source, is required to carry
summarizes the
out the reduction process. Table
1
experimental parameters and the results of quinoline
hydrogenation in various media.
3
1
DHQ have also been observed.
Table 2. Effect of the relative concentration of tetrahydroxydiboron
a
THDB) on quinoline hydrogenation in water as a function of on-
a
-mediated hydrogenation of quinoline using THDB
Table 1. Rh@Fe
3
O
4
(
-
--
--
---
---
---
---
-
nd
--
-
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stream reaction time
Hydrogenation of simple aromatics, like benzene, toluene,
The hydrogenation of heterocyclic rings in comparison to
carbocyclic rings depends on the nature of interaction of
1
7,18
quinoline with the metal center in the catalyst.
The
a
b
Conv .
(%)
Sel.c
(%)
Entry
1
Subst.
Prod.
Solv.
Time
(
Temp
( C)
Pres.
(bar)
TO
F
(h )
---
---
54
observed exclusive selectivity could be explained on the
basis of quinoline interaction with or its absorption to the
active metal through nitrogen in the ring, which can impact
the instantaneous microenvironment generated around the
o
h)
-
1
Water
2
2
2
rt
40
50
20
20
20
---
>67
>99
---
100
100
2
3
Methan
ol
2
50
30
61
100
33
5
3
metal center. This results in strong interaction by virtue of
the basic nature of the support which plays a crucial role in
Toluene
2
50
30
36
100
19
5
4
directing the reaction.
d
None
IPA
10
14
115
130
40
30
>99
>99
100
100
217
93
4
Hydrogenation of N-heterocycles using gaseous H pressure
2
e
5
The catalytic performance of Rh@Fe
hydrogenation of a series of N-heterocyclic compounds with
gaseous H was evaluated in water, methanol, IPA, toluene
3 4
O catalyst towards the
6
7
THF
THF
24
24
140
150
40
40
70
12
100
100
3
2
and THF as well as without a medium. The reaction variables
and the results are summarized in Table 3. First, the
reactions were optimized using quinoline as model substrate
with the variation of solvent, pressure and temperatures.
Here, influence of solvent was clear. Quinoline (0.5 mmol) is
hydrogenated in 2 h to py-THQ in water under 20 bar
hydrogen pressure. While employing other solvents like
toluene and methanol, there were significant drop on
conversion (table 3, entry# 2 and 3). The same reaction was
0.5
f
8
Water
water
Water
Water
2
2
50
50
50
50
20
20
20
20
<4
<2
nd
nd
nd
nd
g
9
h
1
0
1.5
1
98
100
100
71
tried at room temperature and ended up with no conversion
o
table 3, entry#1). But on increasing temperature at 50 C,
(
the conversion was quantitative with maximum selectivity.
i
1
1
>99
108
The effect of pressure on the conversion and selectivity
were studied and summarized. In case of pyridine, the
complete hydrogenation occurred in 10 h at 115 °C without
solvent; a similar trend was seen for pyrazine hydrogenation
in IPA (table 3, entry# 4 and 5). The hydrogenation activity of
the catalyst was also evaluated for the five-membered ring
heterocycles (table 3, entry# 6-7). At 130 °C/40 bar, indole
hydrogenation attained 70% conversion with 100% selectivity
xylene etc, was performed with gaseous H pressure without
2
after 24 h in THF (table 3, entry# 6). However, only 12% any solvent. Here we realized that the reaction temperature is
conversion was achieved at 150 °C/40 bar, in the case of 2- one of the important determining factor to produce the
methyl indole, even after 24 h of reaction time (table 3, entry# desired hydrogenated compound. For instance, it was noticed
7
). This anomaly could be partly ascribed to the steric
that benzene could be rapidly hydrogenated to cyclohexane at
00 °C and 30 bar pressure (table 4, entry # 1). In case of
toluene, hydrogenation into methylcyclohexane, 51%
hindrance of the methyl group and the lone pair on the
nitrogen atom in the ring.
1
The influence of the basic support on quinoline hydrogenation conversion was achieved at 115 °C at 50 bar pressure and this
o
were studied using series of metal oxides such as MgO, CaO, as increased to 70% when temperature was raised to 130 C for
well as the support used in this work, Fe
that as the basic nature of the support increased (Fe
CaO) the rate of hydrogenation become faster and this was
3
O
4
. It was observed
3 4
O
< MgO
<
5
4
Entry
Substrate
Prod.
Time
Temp.
(°C)
Pres.
(bar)
b
c
TOF
-1
Conv.
%)
Sel.
reflected from its TOF values in table 3 (entries# 10, and 11).
This observation can be explained that the hydroxylated basic
supports, such as CaO, MgO and Fe have been shown to be
capable of engaging the N-bearing heterocyclic moieties via
(
h)
(
(%)
(h )
d
1
8
100
30
>99
100
297
3 4
O
e
2
24
24
115
130
50
50
51
70
100
100
21
29
5
5-57
hydrogen bonding.
e
3
2
4
150
50
22
100
8
Aromatic hydrogenation
a
b
c
measured by GC; identified by GC-
d
e
MS; used 1 mL benzene; used 0.5 mL toluene and p-xylene
6
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the same reaction time of 24 h. This activity enhancement also to produce an intermediate compound, 1,2-dihydroquinoline
implies catalyst stability at higher temperature. In the case of ). Same sequence of process occurs at positions C(3) and
(
7
other arenes, the hydrogenation propensity decreased steadily C(4), leading to the formation of the ultimate product, viz,
with the increase of methyl substitution (table 4, entry#3). For 1,2,3,4-tetrahydroquinoline. As can be readily seen from the
example, p-xylene could be hydrogenated to give 1,4-dimethyl proposed catalytic reaction scheme, at least four equivalent of
cyclohexane with 22% of conversion at 50 bar pressure.
Isotope labelling and mechanism elucidation
Based on the observed activity and selectivity of the catalyst, a
reaction route is proposed (Scheme 1). For this, isotope
2 2
labelling studies were performed by using D O instead of H O,
as the reaction medium and identifying the location of
additional hydrogen in the hydrogenated quinoline.
Scheme 1. Isotopic labelling experiment
Mass spectrometric and NMR spectroscopic studies revealed
the possible deuteration sites of quinoline which was
estimated to be deuterated at the C(2), C(3) and C(4) positions
of the N-bearing heterocyclic ring. Likewise, nitrogen should
also be deuterated. However, the possibility of rapid exchange
of deuterium with hydrogen coming from trace moisture
during the workup process, mitigates the deuteration of
5
8
nitrogen (see supporting information, Fig S8, S9 and S15).
water molecules are required for the hydrogenation of one
molecule of quinoline, producing py-THQ.
From the above discussion, it is evident that quinoline can be
quantitatively and selectively reduced to py-THQ in water by
THDB. The results also highlight the definite role of water
molecules in the hydrogenation process. On the basis of above
discussion, we propose the catalytic cycle for the
hydrogenation of quinoline in the following sequence (Scheme
Re-usability studies
To assess the reusability of the spent catalyst in subsequent
hydrogenation cycles of quinoline in aqueous medium under
optimized reaction condition, it was isolated from the product
by putting a magnet at the bottom of the flask and decanting
the liquid. The catalyst was thoroughly washed several times
with methanol and dichloromethane, and dried for use in the
next hydrogenation step without adding any fresh material.
2).
Firstly, Rh undergoes oxidative addition to give
the coordination of water molecule to boron atom to form the
adduct The Rh- hydride complex
elimination of boric acid (H BO ). This leads quinoline which is the catalyst could be used in 16 consecutive cycles, showing
1, followed by
2.
3 is produced by the This procedure was repeated several times. It was found that
3
3
in intimate interaction with the magnetite support through excellent catalytic propensity. However, the efficiency in terms
hydrogen bonding, to enter into the catalytic cycle and of catalytic activity declined slightly with increasing number of
th
generate an intermediate
4
. At this point, the C(2)-N bond is cycles; it was ~93% by the 16 cycle (Figure 5b), which could
be compensated by increasing the on-stream reaction time.
reduced by the transfer of hydrogen from Rh-hydride
3
in to
5
9
C(2) as it is the most electrophilic site in quinoline. Since
there could be an inherent difficulty for the nitrogen atom to
The spent catalyst was collected at the end of cumulative
cycles and its metal contents quantified by ICP-OES which
showed 6.1% loss of Rh as a result of leaching from the surface
of magnetic nanoparticles and may be this slight amount of
loss of Rh was associated with the exhaustion of the catalyst.
6
0
disrupt its aromaticity in the heterocyclic ring, this could be
the rate determining step. Coordination of another water
molecule with boron atom generates another Rh-hydride
complex
5, followed by another hydrogen transfer (and
liberating boric acid) gives
6. Reductive elimination then occurs The stability of the catalyst corroborated by the HRTEM and
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STEM investigations on the recycled catalyst, which showed no
severe damage to the magnetite surface; no sign of surface
agglomeration was observed either (see supporting
information, Fig S2).
Conflicts of interest
There are no conflicts to declare.
Notes and references
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Graphical abstract
Facile hydrogenation of N-heteroarenes by magnetic nanoparticles supported
sub-nanometric Rh catalysts in aqueous medium
This work describes the preparation and systematic characterization of reusable magnetic
heterogeneous nanocatalyst (Rh@Fe O ) for the hydrogenation of N-heterocycles and simple
3
4
aromatics. The activity, selectivity and mechanistic catalytic pathway were investigated.