Carbon-coated magnetic palladium: Applications in partial oxidation of alcohols and coupling reactions

Article abstract of DOI:10.1039/c4gc00748d

A carbon-coated magnetic Pd catalyst has been synthesized via in situ generation of nanoferrites and incorporation of carbon from renewable cellulose via calcination; the catalyst can be used for oxidation of alcohols, amination reaction and arylation of aryl halides (cross-coupling reaction). the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00748d

Green Chemistry
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Carbon-coated magnetic palladium: applications
in partial oxidation of alcohols and coupling
reactions†
Cite this: Green Chem., 2014, 16,
4333
R. B. Nasir Baig, Mallikarjuna N. Nadagouda and Rajender S. Varma*
Received 25th April 2014,
Accepted 8th July 2014
A carbon-coated magnetic Pd catalyst has been synthesized via in situ generation of nanoferrites and
incorporation of carbon from renewable cellulose via calcination; the catalyst can be used for oxidation of
alcohols, amination reaction and arylation of aryl halides (cross-coupling reaction).
DOI: 10.1039/c4gc00748d
www.rsc.org/greenchem
of a sustainable and active catalyst system, we now report a ver-
satile carbon-coated magnetic Pd catalyst for the common oxi-
Introduction
The oxidation of alcohols and olefins to carbonyl compounds dation and coupling reactions.
and coupling of aryl halides are some of the most important
The first step in the accomplishment of this goal was the
transformations in synthetic organic chemistry because of straightforward synthesis of the carbon-coated magnetic Pd
their use in a wide range of pharmaceutically important pro- catalyst (Scheme 1) by sequential addition of reagents in one
ducts, agro-chemicals, and fragrances.1,2 Conventionally, oxi- pot. The magnetic nanoferrite (Fe3O4) was generated in situ via
dation is carried out using stoichiometric amounts of oxidants the hydrolysis method by stirring the solution of FeSO4·7H2O
such as chromium and manganese compounds which gen- and Fe2(SO4)3 in water in 1 : 1 ratio at pH 10 (adjusted using
erate copious amounts of hazardous metal waste.3–6 Due to 25% ammonium hydroxide solution) followed by heating in a
the increasing environmental consciousness, persistent efforts water bath at 50 °C for 1 h (TEM image, ESI, Fig. 1†). The reac-
are being made to develop environmentally benign catalysts tion mixture was cooled down to room temperature and cellu-
for common oxidation and coupling reactions; various hetero- lose was added with vigorous stirring which was continued for
geneous catalyzed protocols are now available.7–13 There have 6 h under ambient conditions. To this solution, PdCl2 was
been many heterogeneous supports utilized for the immobili- added and stirring was continued for another 8 h (Scheme 1).
zation of metal complexes and metal nanoparticles such as Magnetic cellulose supported Pd materials were separated
metal oxides, silica, natural and unnatural polymers, silicates using an external magnet (TEM image, ESI, Fig. 2†), washed
and mesoporous carbon.7–13 Among these, Ru hydroxide and with water and calcinated at 450 °C for 3 hours. The crystal
Pd supported on magnetic nanoferrite (Fe3O4), and Pd nano- structure was determined by X-ray diffraction (XRD) (Fig. 2).
particles supported on SBA-15 have shown promise.14,15 The major phases were Fe3O4, Pd and C which were compared
However, most systems can be useful for the oxidations of only with JCPDS patterns 01-075-0033, 01-088-2335 and 01-075-0444,
activated substrates or require larger quantities of additives respectively. Scanning electron microscopy (SEM) and trans-
such as bases and electron transfer mediators thus limiting mission electron microscopy (TEM) (Fig. 1a and b) were con-
their applications.6–15 Recently, the use of nano-γ-Fe2O3 for the ducted to determine the surface morphology. The SEM images
oxidation of alcohols with high selectivity has been reported.16 showed the formation of spherical particles along with big aggre-
The activity of magnetic catalyst was further improved by gates. The TEM images indicated the particle size of Fe3O4@CPd
immobilization of Pd over dopamine-modified magnetic fer- ranging from 20 to 50 nm. The weight percentage of Pd was
rites;17 however, this effort requires several synthetic manipu- found to be 4.81% by inductively coupled plasma-atomic emis-
lations. In continuation of our efforts18–27 in the development sion spectroscopy (ICP-AES) analysis. Surface analysis of the cata-
lyst showed a BET surface area of 175.71 m2 g−1
.
The initial experiments were performed to test the catalytic
activity of the Fe3O4@CPd catalyst for oxidation of benzyl
alcohol (10 mmol) as a substrate (Scheme 2), using 1.2 equiva-
lents of hydrogen peroxide and 100 mg of Fe3O4@CPd; conver-
sion with very high TON (816), with >99% selectivity (Table 1,
entry 1) was observed.
Sustainable Technology Division, National Risk Management Research Laboratory,
U. S. Environmental Protection Agency, 26 West Martin Luther King Drive, MS 443,
Cincinnati, Ohio 45268, USA. E-mail: Varma.Rajender@epa.gov;
Fax: +1 513-569-7677; Tel: +1 513-487-2701
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4gc00748d
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Scheme 1 Synthesis of the carbon-coated magnetic Fe3O4@CPd catalyst.
fascinating molecules of biological importance;28–33 biaryls
and N-aryl functionality is widely distributed in several biologi-
cally important natural products and pharmaceuticals. The
Suzuki coupling and Buchwald–Hartwig amination using the
palladium catalyst has been well explored.34–38 However, many
of these methods involve the use of toxic and complex organic
ligands which are often air-sensitive, expensive and many reac-
tions require longer reaction times.27–40 In recent years, greater
emphasis is placed on the development of sustainable hetero-
geneous catalysts and the ease of recyclability, especially using
biodegradable polymers. We evaluated our iron-cellulose
derived Fe3O4@CPd catalyst for Suzuki and Buchwald–Hartwig
amination reactions; the arylation of aryl halides could be
easily performed using boronic acid within 30 min under
microwave (MW) irradiation conditions. The reaction outcome
did not alter due to the presence of electron donating and elec-
tron withdrawing groups; all the substrates (Table 2, entries
1–7) gave similar desirable results. The catalyst is also useful
for the amination of aryl halides (Table 3, entries 1–4); amin-
ation of 4-nitro-1-bromobenzene with various amines pro-
ceeded smoothly to deliver the corresponding aryl amines in
quantitative yields (Table 3, entries 1–4).
Separation and recovery of the catalyst is of paramount
importance in sustainable organic synthesis which is generally
achieved by tedious filtration or centrifugation with reduced
efficiency. In our catalytic system, due to the magnetic nature
of the nano-Fe3O4@CPd catalyst, it can be recovered easily
using an external magnet. For practical applications of hetero-
geneous systems, the life time of the catalyst and its level of
reusability are significantly important factors. To clarify this
issue, we established a set of experiments for the oxidation of
benzyl alcohol using the recycled nano-Fe3O4@CPd catalyst.
After the completion of the reaction, the catalyst was recovered
using an external magnet, washed with acetone, and dried at
60 °C for 20 min. A new reaction was then performed with
Fig. 1 (a) SEM image and (b) TEM image of the Fe3O4@CPd catalyst.
The catalyst was then evaluated for the general scope and a
variety of alcohols (Table 1) were investigated as substrates;
impressive turnover numbers were observed for aromatic alco- fresh reactants and H2O2, under the same reaction conditions.
hols (entries 1–7), heterocyclic alcohols (entry 6) as well as ali-
phatic alcohols (entry 7). However, in the case of hexanol,
The carbon coated magnetic Fe3O4@CPd catalyst could be
used at least 3 times without any change in activity (ESI,
relatively low TON may be attributed to its less reactivity. In all Table 1†). Metal leaching was studied by ICP-AES analysis of
the reactions, the salient and distinctive feature was high
selectivity using a very low amount of catalyst (0.01 mol%
the catalyst before and after the reaction; the Pd concentration
was found to be 4.81% before the reaction and 4.77% after the
of Pd). This method may find applications in large-scale reaction; no Pd metal was detected in the oxidation product
preparations where it is required that an alcoholic group is
selectively oxidized to an aldehyde without any over-oxidation.
which confirmed negligible Pd leaching. The trifling Pd leach-
ing may be due to the well defined magnetic carbon coated Pd
Arylation and amination of aryl halides is an important catalyst (Scheme 1). This non-covalent anchoring minimizes
transformation and a method of choice for the synthesis of
deterioration and metal leaching and allows efficient catalyst
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Fig. 2 XRD pattern of the Fe3O4@CPd catalyst.
Table 1 Catalytic oxidation of alcohols using the carbon-coated mag-
netic Fe3O4@CPd catalysta
Entry
1
Substrate
Product
TON
923
Scheme 2 Fe3O4@CPd catalysed oxidation of benzyl alcohol.
2
3
4
5
942
932
913
903
recycling. The absence of metal deterioration in recycling
experiments for C–N coupling also confirms the effectiveness
and reusability of the catalyst.
The TEM and SEM images of the catalyst taken after the
third cycle of the reaction did not show significant change in
the morphology of the catalyst (ESI, Fig. 3 and 4†), which indi-
cates the retention of the catalytic activity after recycling. It is
always preferable to use a more accessible, economic Pd cata-
lyst, provided that the process works at high TON and that the
catalyst leaves no remnants of the metal in the final product,
since metal contamination is highly regulated by the pharma-
ceutical industry. All the above conditions were well satisfied
by our recyclable magnetic carbon supported Pd catalyst.
6
7
923
644
a Reactions were carried out with 10 mmol of alcohol, H2O2 (1.2
equiv.), 100 mg (0.0104 mol% of Pd) of Fe3O4@CPd nanocatalyst at
75 °C for 4 h.
Conclusion
(cellulose) has been utilized for the generation of magnetic
carbon. The active catalyst efficaciously catalyzed the oxidation
of alcohols, C–N coupling and arylation of aryl halides.
Because of the magnetic nature of the catalyst, it can be
A novel magnetic carbon-supported Pd catalyst has been syn-
thesized, which can be readily prepared in multi gram quan-
tities in aqueous media. A naturally abundant biopolymer
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Table 2 Fe3O4@CPd catalysed arylation of aryl halidesa
Entry
1
Substrate
Boronic acid
Time
Product
Yield
98%
30 min
2
3
30 min
30 min
98%
95%
4
5
6
30 min
30 min
30 min
96%
96%
98%
7
30 min
98%
a Reaction conditions: 1 mmol aryl halides, 1.2 mmol aryl boronic acids, 2 mmol K2CO3, 25 mg of Fe3O4@CPd catalyst, 4 mL water, MW 100 watts,
100 °C, 30 min.
Table 3 Fe3O4@CPd catalysed amination of aryl halidesa
Entry
1
Aryl halide
Amines
Time
Product
Yield
98%
60 min
2
3
60 min
60 min
97%
89%
4
60 min
98%
a Reaction conditions: 1 mmol aryl halides, 1.2 mmol amines, 2 mmol K2CO3, 25 mg of Fe3O4@CPd catalyst, 4 mL DMF, MW 100 watts, 100 °C,
60 min.
separated using an external magnet, which eliminates the (25%) was added slowly to adjust the pH of the solution to 10.
requirement of catalyst filtration after completion of the reaction. The reaction mixture was then continually stirred for 1 h at
50 °C. The reaction mixture was cooled down to room tempera-
ture and cellulose (10 g) was added to this solution with vigor-
ous stirring which was continued for 6 h under ambient
conditions. To this solution, PdCl2 was added and stirring was
continued for another 8 h (Scheme 1). Magnetic cellulose sup-
Experimental section
Synthesis of the carbon coated magnetic Fe3O4@CPd catalyst
ported Pd materials were separated using an external magnet,
washed with water and calcinated at 450 °C for 3 hours.
FeSO4·7H2O (2.78 g) and Fe2(SO4)3 (4.0 g) were dissolved in
150 mL water in a 500 mL beaker. Ammonium hydroxide
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Catalyst characterization by X-ray diffraction (XRD) and trans- Education through an interagency agreement between the U.S.
mission electron microscopy (TEM) confirms the formation of Department of Energy and the U.S. Environmental Protection
single-phase
carbon
coated
magnetic
nanoparticles Agency.
Fe3O4@CPd, with spherical morphology and a size range of
20–50 nm. The weight percentage of Pd was found to be 4.81%
by inductively coupled plasma-atomic emission spectroscopy
(ICP-AES) analysis.
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