Solvent free selective oxidation of alcohols catalyzed by a trinuclear complex with a dicopper(ii)-monozinc(ii) centre using hydrogen peroxide as an oxidant

Article abstract of DOI:10.1039/c3ra44970j

The oxidation of alcohols to carbonyl compounds was investigated by an efficient catalytic system comprising of a new heterotrinuclear complex containing a dicopper(ii)-monozinc(ii) centre, [ZnCu₂(slsch)(NO ₃)₂(H₂O)₈]·2H₂O with hydrogen peroxide under solvent free conditions. Primary and secondary alcohols were oxidized to the corresponding aldehydes and ketones in good to excellent yield. The newly developed catalyst system can be reused in four consecutive runs without any significant loss of activity.

Full text of DOI:10.1039/c3ra44970j

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Solvent free selective oxidation of alcohols catalyzed by
a trinuclear complex with a dicopper(II)–monozinc(II)
centre using hydrogen peroxide as an oxidant†
Cite this: RSC Adv., 2013, 3, 22957
Received 9th September 2013
Accepted 30th September 2013
a
b
c
a
Rosmita Borthakur, Mrityunjaya Asthana, Arvind Kumar, Angira Koch
and R. A. Lal*a
DOI: 10.1039/c3ra44970j
www.rsc.org/advances
The oxidation of alcohols to carbonyl compounds was investigated copper enzymes like particulate methane monooxygenase, lac-
by an efficient catalytic system comprising of a new heterotrinuclear case, ascorbate oxidase, human ceruloplasmin, fungal laccase,
1
2,13
complex
containing
(H O)
a
dicopper(II)–monozinc(II)
centre, FET 3 and phenoxazinase.
As copper salt alone is an ineffi-
[
ZnCu (slsch)(NO
2
3
)
2
2
8
]$2H
2
O with hydrogen peroxide under cient catalyst, a variety of ligands are used with copper salts to
solvent free conditions. Primary and secondary alcohols were increase the catalytic activity. Unfortunately, most of these
oxidized to the corresponding aldehydes and ketones in good to copper salt complexes need an external base for efficient
9
excellent yield. The newly developed catalyst system can be reused in oxidation.
four consecutive runs without any significant loss of activity.
While there has been no lack of pioneering work on alcohol
oxidation with aqueous hydrogen peroxide promoted by a
homogeneous catalyst, the heterogeneous switching of catalytic
oxidation to meet green chemical requirements still remains
major challenge. Heterogeneous catalytic systems for the
oxidation of alcohols with hydrogen peroxide would realize
environmentally benign, secure processes as well as facilitated
methods for product purication and catalyst recycling.
Inspired by natural metal containing enzymes, we are interested
in studying the catalytic activity of the metal organic framework
Introduction
The selective oxidation of alcohols is one of the most widely
1
employed transformations in organic synthesis. Although
14
numerous protocols are available, many employ stoichiometric
3
or superstoichiometric heavy metal compounds like CrO ,
2
4 2 2
KMnO , MnO , SeO etc. Safety hazards associated with these
2 2 2
(MOFs) whose catalytic activity using O or H O as a terminal
oxidants and their toxic by-products and the difficulty to work
up the reaction mixture are the main problems of such
processes. One way to alleviate this problem is to rationally
design catalytic systems that employ transition metals that
oxidant is hardly explored. One advantage of MOFs is that they
do not require the addition of an external base to form the metal
complexes to act as catalyst which have the potential to function
as heterogeneous catalyst. Moreover, the use of heterogeneous
catalyst in the liquid phase offers several advantages over
homogeneous ones such as recovery and recycling, atom utility
and enhanced stability. This is a clean technology process
where compounds are formed free from contaminated by-
products. It is quite surprising that only a few examples using
3,4
readily activate oxidants like oxygen or hydrogen peroxide.
Consequently, a number of catalytic methods using complexes
5
6
7
8
9
containing transition metals, such as Pd, Ru, Rh, Mo, Cu,
10
11
Co,
V
and clean oxidants such as dioxygen or hydrogen
peroxide have been developed. From an economical and envi-
ronmental view point, catalytic oxidation processes employing
9
cheap and green copper catalyst and hydrogen peroxide are
H O are particularly attractive.
2
2
known so far. To the best of our knowledge, although reports
are available on the use of MOFs as sole catalyst/heterogeneous
catalysts in aerobic oxidation, no reports is available on their
Among the above mentioned metal salts, Cu is more rele-
vant, as it is biomimetic functional model of the multinuclear
2 2
use as catalyst using H O as terminal oxidant.
a
Department of Chemistry, North Eastern Hill University, Shillong-793022,
In the present work, we report the oxidation of benzylic
alcohols to the corresponding carbonyl compounds in good
yields using a new heterometallic trinuclear complex with
Meghalaya, India. E-mail: profralal@gmail.com
b
Department of Chemistry, Banaras Hindu University, Varanasi-221005, UP, India
c
Department of Chemistry, Faculty of Science and Agriculture, The University of West-
15
di-Cu(II) and mono Zn(II) centre (MOF)
2
[ZnCu (slsch)-
Indies, St. Augustine, Trinidad and Tobago, West Indies
(
NO (H O) ]$2H O (Fig. 1) derived from polyfunctional dis-
3
)
2
2
8
2
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c3ra44970j
16
alicylaldehyde succinoyldihydrazone (H slsch) (Fig. 2) as a
1
4
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2 3 2 2 8 2
Fig. 1 Structure of the catalyst [ZnCu (slsch)(NO ) (H O) ]$2H O.
Fig. 4 Optimization of solvent effect on the model reaction.
4
Fig. 2 Disalicylaldehyde succinoyldihydrazone (H slsch).
Scheme 1 Oxidation of alcohol using catalyst.
Fig. 5 Effect of catalyst loading on the model reaction.
catalyst with hydrogen peroxide under solvent free condition
(
Scheme 1). The present protocol is very simple, mild and spherical in shape with size in the range (12 ꢀ 10) nm (Fig. 3a).
environment friendly with water being the only by-product in The SAED shows the electron diffraction pattern of the selected
the reaction. No over oxidation of the alcohols to the acids area of nanoparticles. The SAED (Fig. 3c) shows moderately
occurred.
strong diffraction spots which conrm the formation of
moderately single crystalline cube of the trimetallic
17,18
complex.
TEM image of the catalyst was also recorded aer
Results and discussions
four consecutive runs. It shows that the morphology of the
The details of catalyst preparation and characterization have catalyst does not change aer the reaction which helps in the
15
been reported by us in our earlier publication.
reusability of the catalyst for several runs (Fig. 3b).
In order to know the nature of the particles TEM images of
Initial reaction to optimize the reaction conditions for
the catalyst was recorded which shows that the particles are oxidation of alcohols was carried out on benzyl alcohol. The
reaction of 1a was initially carried out using 5 mol% of catalyst,
2 2
30% H O as oxidant in dichloromethane, afforded the product
2a in 20% yield, characterized from spectral data as benzalde-
hyde (Fig. 4). The reaction was also performed using other
solvents (acetonitrile, water and toluene), poor yields of the
product 2a was obtained (Fig. 4). Interestingly, it was found
under solvent free condition, the catalyst converted benzyl
ꢁ
alcohol to benzaldehyde in 85% yield at 100 C (Fig. 4). On
ꢁ
increasing the temperature to 120 C the yield of 2a decreased.
Effect of other reaction parameters such as catalyst loading and
reaction time was examined under solvent free condition. Best
result was obtained using 8 mol% of catalyst (Fig. 5). On
lowering the catalyst loading to 2 and 5 mol%, low yields of the
product 2a were obtained. On increasing the catalyst loading to
Fig. 3 TEM images of [ZnCu
2
(slsch)(NO
3
)
2
(H
2
O)
8
]$2H
O)
2
O (a) before reaction; (b)
]$2H O.
after fourth run; (c) SAED of [ZnCu
2
(slsch)(NO
3
)
2
(H
2
8
2
Scheme 2 Oxidation of benzyl alcohol to benzaldehyde.
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a
Table 1 Hydrogen peroxide mediated oxidation of alcohols to aldehydes and ketones
b
Entry
Substrate
Product
Time (h)
10
Yield (%)
1
85
87
2
3
11
11
88
4
5
9
82
78
10
6
7
8
12
9
80
82
80
12
9
14
82
78
1
0
12
a
ꢁ
b
Reaction condition: substrate (5 mmol), catalyst (8 mol%), H
2
O
2
were heated at 100 C for appropriate time. Isolated yields.
10 and 12 mol%, the reaction still proceeded smoothly
The progress of formation of benzaldehyde from benzyl
accompanied by a drop in the isolated yield with longer reaction alcohol as a function of time using catalyst was studied
times. No conversion of benzyl alcohol to benzaldehyde was (Fig. S2†). It was observed that the conversion increased initially
observed in the absence of the catalyst. Hence, the catalyst with the progress of the reaction time, reached a maximum and
played an active role in the oxidation of alcohols.
then remained unchanged. A good isolated yield for the
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suppliers. The solvents were dried and distilled before use
19
following the standard procedures. The study of morphology
was performed by JEOL-JSM 6360CX with tungsten lament
using a Si(Li) detector with a resolution 3 nm. Transmission
electron microscopy images were measured on a JEOL-JEM-
2100CX electron microscope operated at 200 kV without the
addition of a contrast agent since the presence of the metal ion
provided enough contrast. Melting points were recorded in
open capillaries and are uncorrected. The FT-IR spectrum of the
samples was recorded on a Perkin-Elmer FT-IR system, Spec-
1
Fig. 6 Reusability chart of catalyst.
trum BX, infrared spectrophotometer using KBr pellets. H and
1
3
C NMR spectra were recorded in CDCl on a Bruker Avance-
3
400 spectrometer and Jeol-300 spectrometer.
formation of benzaldehyde was observed at the optimum reac-
tion time of 10 h (85%). Thus, the combination of catalyst (8
Typical procedure for one pot conversion of alcohols to
aldehydes and ketones catalyzed by [ZnCu (slsch)
NO (H O) ]$2H
All of the reactions were carried out at 100 C under reux in a
5 ml ask equipped with a magnetic stirrer. 30% H solution
10 mmol) was added to a mixture of alcohol (5 mmol) and the
ꢁ
mol%), 30% H
2
O
2
(10 mmol) at 100 C under solvent free
2
condition was found to be the most appropriate condition for
the oxidation of alcohols (Scheme 2).
(
3
)
2
2
8
2
O
ꢁ
Encouraged by these results, the scope of reaction was
extended to a variety of alcohols including primary and
secondary aromatic alcohols using the above optimized reac-
tion conditions, the results are summarized in Table 1. It was
found that the alcohols were converted to the corresponding
carbonyl compounds in good yield and excellent selectivity.
Benzyl alcohols with substituents of varying electronic proper-
ties were all smoothly converted to the corresponding aldehydes
2
(
2 2
O
catalyst (8 mol%). The reaction solutions in all cases were
vigorously stirred using magnetic stirrers, and an oil bath was
used to achieve the desired reaction temperature. Aer
completion (TLC), the reaction mixture was cooled to room
temperature and ltered. Filtrate was extracted with ethyl
acetate (3 ꢂ 10 ml) and the combined organic extract was
washed with water (3 ꢂ 10 ml), brine (10 ml) and dried over
(entries 2–5) in excellent yields. Electron donating substituents
on benzyl alcohol gave slightly higher yields (entries 2 and 3).
On the other hand electron withdrawing substituent on benzyl
alcohol gave slightly lower yields (entries 4 and 5). The over
oxidation to acid was not observed at all under the present
reaction conditions with any of the alcohols. The conversion
also proceeded smoothly even with cinnamyl alcohol without
affecting the carbon–carbon double bond (entry 7). It was found
that benzil was obtained in good yield from benzoin (entry 6).
The secondary alcohols, 1-phenyl ethanol and substituted
anhydrous Na
product was puried by column chromatography over silica gel
60–120 mesh) using ethyl acetate and hexane as eluent to
2 4
SO . Aer removing the solvent the crude
(
afford the pure products 2a–j.
Spectral data of compounds 2a and 2h
ꢃ1
1
Benzaldehyde (2a). Ir (KBr): 2852, 2743, 1687 cm
. H
(
(
CDCl
d, J ¼ 6.9 Hz, 2H), 7.64–7.43 (m, 2H).
Acetophenone (2h). Ir (KBr): 2928, 1686 cm . H (CDCl
00 MHz): d ¼ 7.95 (d, J ¼ 7.2 Hz, 2H), 7.56–7.41 (m, 3H), 2.58 (s,
3
, 300 MHz): d ¼ 10.00 (s, 1H), 8.17 (d, J ¼ 7.5 Hz, 1H), 7.88
1-phenyl ethanol were equally reactive affording the corre-
ꢃ1
1
sponding ketones in about 78–82% yield (entries 8–10).
3
,
3
3
1
3
Recovery and reusability of the catalyst
H). C (CDCl , 75 MHz): d ¼ 197.9, 136.9, 133.0, 128.2, 26.5.
3
It is also important to verify the viability of reusing catalytic
system for different substrates. The basic advantage of hetero-
geneous catalyst is their reuse for consecutive catalytic cycles.
To evaluate the catalyst reusability, it was recovered by ltra-
tion, washed with DCM and dried in the oven for 30 min.
Catalyst was reused under the optimized condition with benzyl
alcohol to give benzaldehyde; the data obtained are shown in
Conclusions
In conclusion, we have developed a simple, efficient and envi-
ronmentally benign process for the oxidation of benzylic and
allylic alcohols to the corresponding carbonyl compounds using
[
ZnCu (slsch)(NO ) (H O) ]$2H O as a heterogeneous catalyst
2 3 2 2 8 2
under solvent free condition in good yields. The straightforward
recovery of catalyst, reusability until the fourth run without
substantial loss in activity and selectivity and use of hydrogen
peroxide as oxidant are some features of the present work.
(Fig. 6). No signicant loss of efficiency was observed aer four
runs. This was also conrmed from the TEM image of the
catalyst recorded aer the fourth run (Fig. 3b).
Experimental section
Acknowledgements
All the reagents used were chemically pure and were of analyt-
ical reagent grade. 4-Nitro benzyl alcohol and 4-methoxy benzyl Authors are thankful to Head, SAIF, North Eastern Hill
alcohol were purchased from Aldrich and the rest of the University, Shillong for elemental analyses, recording spectra
chemicals along with solvent were procured from local and TEM. R. B. and A. K. would like to thank U. G. C. for the
2
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award of RFSMS (Research Fellowship in Sciences for Merito-
rious Students) and M. A. to CSIR for the award of Senior
Research Fellowship.
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2962 | RSC Adv., 2013, 3, 22957–22962
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