Polyethylene glycol clicked Co(ii) Schiff base and its catalytic activity for the oxidative dehydrogenation of secondary amines

Article abstract of DOI:10.1039/c0ob01163k

Copper catalyzed [3+2] cycloaddition "click method" provided an efficient and high yielding immobilization of cobalt(ii) Schiff base to the azido-functionalized MeOPEG₅₀₀₀; whereas the direct reaction between MeOPEG₅₀₀₀ and Co(ii) Schiff base did not produce any immobilization. The prepared catalyst was tested for the oxidative dehydrogenation of various secondary amines using TBHP as oxidant and could be easily recovered by precipitation with diethyl ether at the end of the reaction.

Full text of DOI:10.1039/c0ob01163k

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Organic &
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PAPER
Polyethylene glycol clicked Co(II) Schiff base and its catalytic activity for the
oxidative dehydrogenation of secondary amines†
Praveen K. Khatri, Suman L. Jain,* L. N. Sivakumar K. and Bir Sain*
Received 13th December 2010, Accepted 22nd February 2011
DOI: 10.1039/c0ob01163k
Copper catalyzed [3+2] cycloaddition “click method” provided an efficient and high yielding
immobilization of cobalt(II) Schiff base to the azido-functionalized MeOPEG5000; whereas the direct
reaction between MeOPEG5000 and Co(II) Schiff base did not produce any immobilization. The prepared
catalyst was tested for the oxidative dehydrogenation of various secondary amines using TBHP as
oxidant and could be easily recovered by precipitation with diethyl ether at the end of the reaction.
they are expensive, generate huge amounts of waste and have lower
1.0 Introduction
efficiency, which makes their utility limited from an environmental
Immobilization of homogeneous metal complexes onto polymeric
viewpoint. Thus, catalytic methodologies using transition metal
supports is an area of growing commercial and academic interest
based catalysts with t-butyl hydroperoxides are of significant
interest, and in this context a variety of metals such as ruthenium,12
cobalt,13 rhodium14 and mixed metal oxides15 have hitherto been
used for this transformation. Maruyama et al.13 have reported the
use of a cobalt Schiff base complex for the oxidative dehydro-
genation of secondary amines, but the homogeneous nature of the
catalyst limited the scope of this method for practical applications.
To overcome these limitations we thought it would be worthwhile
to develop an efficient yet recyclable heterogenized cobalt(II) Schiff
base by using PEG as a promising support. For this purpose, the
required cobalt(II) Schiff base 2 was prepared by the reaction of
salicylidine-N-(methyl 3-(4¢-propargyloxy phenyl) propionate and
CoCl2 (Scheme 1) according to the literature procedure.16 At first
we tried to prepare PEG-immobilized Co(II) Schiff base by the
direct reaction of MeOPEG5000 with Co(II) Schiff base complex 2.
It was surprising to note that no reaction occurred and we could
recover MeOPEG5000 as such at the end.
Copper catalyzed17 [3+2] azide-alkyne cycloaddition
(CuAAC)18 is termed a “click reaction” because of its potential
advantages such as simplicity, high efficiency, yield and better
catalyst loading and has recently been recognized as one of the
powerful tools for the ligating molecule fragments in developing
immobilized catalysts/reagents.19,20 Therefore, we thought it
would be worthwhile to use the click method for the covalent
immobilization of cobalt(II) Schiff to azidofunctionalized
MeOPEG5000 support 1, the synthetic strategy for the preparation
of PEG-immobilized cobalt(II) Schiff base 4 is shown in Scheme 1.
as they can combine the advantages of both heterogeneous and
homogeneous catalysts. In addition, they are easy to separate
from the reaction mixtures, recyclable and are easier to tailor/or
fine tune than heterogeneous catalysts.1 Cobalt(II) Schiff base
complexes, due to their high oxidation potential, have gained
significant interest in developing highly effective methods in
synthetic and industrial chemistry.2 In recent years, the high
costs and tedious synthetic routes has led to an increased
interest in immobilizing these catalysts onto a support. These
immobilized/supported catalysts can provide the facile recovery
and recycling of the catalyst, therefore make the processes cleaner
and economically viable. In this regard, a number of solid supports
such as organic polymers,3 silica,4 clay5 and mesoporous hybrid
materials6 have been used to immobilize these homogeneous
catalysts. Polyethylene glycols,7 which are well known to be
inexpensive and biodegradable, are promising polymeric supports
possessing a unique property of being soluble in most organic
solvents and being insoluble in some of them, such as in diethyl
ether. Thus during the reaction, the PEG-immobilized catalyst
behaves like a homogeneous catalyst and at the end of the reaction
it can readily be recovered by precipitation with diethyl ether.
Despite the potential advantages of PEG, to the best of our
knowledge there is no literature report using PEG as a support
for the immobilization of cobalt(II) Schiff base complexes.
Oxidative dehydrogenation of secondary amines is an important
synthetic transformation8 and extensively used in the multistep
synthesis of bioactive molecules. Stoichiometric reagents such
as hypervalent iodine,9 phenylselecinic anhydride,10 and N-tert-
butylphenylsulfinimidoyl chloride11 are often used, even though
2.0 Results and discussion
2.1 Synthesis of catalyst
Chemical Sciences Division, Indian Institute of Petroleum, Mohkampur,
Dehradun, 248005, (India). E-mail: suman@iip.res.in, birsain@iip.res.in
† Electronic supplementary information (ESI) available: See DOI:
10.1039/c0ob01163k
The required azido functionalized MeO-PEG5000-N3 1 was pre-
pared by the reaction of MeOPEG5000 with sodium azide in
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Scheme 2 Oxidative dehydrogenation of secondary amines.
At first, the oxidation of dibenzylamine was studied in
various solvents such as methanol, ethanol, acetonitrile and
dichloroethane at 60 ◦C using TBHP as oxidant and PEG-
immobilized cobalt Schiff base 4 as catalyst (2 mol%). Interest-
ingly, selective formation of benzaldehyde was obtained in ethanol
and methanol; whereas, in acetonitrile N-benzylidinebenzylamine
was obtained as a major product, these results are presented in
Table 1. The reaction was found to be very slow in dichloroethane
and gave poor yield of N-benzylidinebenzylamine along with trace
amounts of benzaldehyde. Next, the reaction was generalized for
a variety of secondary amines and the results of these experiments
are summarized in Table 1. Although all the substrates (except
entries 4 and 7) could be converted completely and afforded
corresponding imines 6 along with its hydrolysis product 7 as
shown in Scheme 2. The conversion of the substrates and the
selectivity (ratio) of the products was determined by GCMS. We
also compared the catalytic efficiency of the heterogeneous catalyst
4 with its analogous homogeneous catalyst 2 under identical
conditions. These results are presented in Table 1. The catalytic
efficiency of both the catalysts was found to be comparable,
establishing the advantages of the PEG-immobilized catalysts, as
they provide similar catalytic reactivity to the homogeneous ones,
with the added benefits of facile recovery and recycling of the
catalyst.
Next, to check the recycling and reusability of the catalyst, the
recovery of MeOPEG-bound catalyst 4 from the reaction mixture
was achieved by precipitation with diethyl ether. The recovered
catalyst was reused for 5 runs, and only a very slight decrease
in activity of the catalyst was observed in terms of conversion,
selectivity and reaction time after each recycling and reuse. The
results of these recycling experiments are presented in Table 2.
Furthermore, no leaching of the metal/ligand could be observed
during the reaction as the metal content (analyzed by ICP-AES)
of the recovered catalyst was found to be similar as to the fresh
catalyst 4.
Scheme 1 Synthetic pathway for the PEG-immobilized catalyst 4.
ethanol under reflux for 12 h. Cobalt(II) Schiff base 2 was treated
with propargyl bromide to give propargylated Co(II) Schiff base
complex 3, which on subsequent reaction with 1 in the presence of
a catalytic amount of CuI (5 mol%) resulted in the formation
of PEG-immobilized Co(II) Schiff base 4. The MeOPEG5000
immobilized complex 4 was separated from the reaction mixture by
precipitation with diethyl ether, washed with 2-propanol, diethyl
ether and dried under vacuum to give dark brown colored solid
in virtually quantitative yield (94%). To confirm the removal of
all copper contents from the prepared PEGylated Co(II) Schiff
base, we analyzed the catalyst by ICP-AES analysis. We did not
find any copper content in this analysis, confirming the absence
of copper in the prepared material. The covalent attachment
of the complex 3 to the PEG support was confirmed by the
disappearance of the typical –N3 band of azido-functionalized
MeOPEG5000 and appearance of the new bands at u 1720 and
1605 cm-1 corresponding to the C O of the ester and C N of
the Schiff base respectively. Furthermore, the value of nitrogen
content as determined by elemental analysis (0.8%) revealed the
loading of the Schiff base to the support to be 0.07 mmol g-1,
which is equivalent to 5.6% coverage of the PEG support. The
cobalt content per gram of the support was determined by the
complexometric titration with EDTA, which was found to be
0.06 mmol g-1.
3.0 Conclusion
In summary, we have demonstrated the first successful synthesis
of PEG-immobilized Co(II) Schiff base by using click reaction
as a simple, efficient and high yielding approach under mild
reaction conditions. Interestingly, no immobilization could occur
by the direct reaction between MEOPEG5000 and Co(II) Schiff base,
establishing the superiority of the click method in developing the
immobilized catalysts/reagents. The prepared catalyst was tested
for the oxidative dehydrogenation of secondary amines with TBHP
and could be easily recovered by precipitation with diethyl ether
at the end of the reaction. Furthermore, the high product yields,
facile recovery/recycling of the catalyst, high catalytic efficiency
make the developed methodology superior than existing ones.
2.1 Catalytic activity
The catalytic activity of the prepared Co(II) Schiff bases 4 and
its comparison with the corresponding homogeneous Schiff base
2 was carried out for the oxidation of secondary amines using
tert-butylhydroperoxide (TBHP) as oxidant (Scheme 2).
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Table 1 Oxidation of secondary aminesa
Entry
1
Amine 5
Solvent
EtOH
Cat. (2 mol%)
Time (h)
12.0
Conv. (%)b
100
6 : 7 (%)b
4
0 : 100/97c
MeOH
4
12.0
3.0
12
3.0
12
100
96
30
98
—
4 : 96/92c
75 : 25
10 : 2
CH3CN
ClCH2CH2Cl
CH3CN
CH3CN
CH3CN
4
4
2
—
4
73 : 27
—
2
1.5
100
94 : 6/92c
2
4
1.0
1.0
100
100
96 : 4/93c
65 : 35
3
4
CH3CN
CH3CN
2
4
1.0
2.5
100
80
82 : 18
6 : 94
2
4
2.5
2.75
87
100
10 : 90
5
6
MeOH
0 : 100/96c
2
4
2.5
3.0
100
100
1 : 98
92 : 8
CH3CN
2
4
2.5
4.5
100
90
95 : 5
92c:0
7
CH3CN
2
4.5
94
98 : 2
a Reaction conditions: amine (1 mmol), TBHP (6.5 M in decane, 2.5 mmol), catalyst (2 mol %), acetonitrile (2 mL) at 60 ◦C. b Conversion and selectivity
determined by GCMS. c Isolated yields.
Table 2 Results of recycling experimentsa
recorded on Bruker 300 MHz spectrometer and the chemical shifts
are expressed in d parts per million relative to tetramethylsilane
Run
1
2
3
4
5
(TMS) as internal standard. The IR spectra were recorded on a
Perkin Elmer FTIR X 1760 instrument. Elemental analysis was
done by using ASTM D-3828 (Kjeldhal method). Analysis for
metal contents were carried out by using inductively coupled
plasma atomic emission spectrometer (ICP-AES, PS-3000UV)
by Leeman Labs. GCMS analysis were done on HP 5972 MSD
coupled with HP 5890 GC by using CP-Sil 5 column.
Time (h)
Conv.b
6 : 7b
3.0
96
75 : 25
3.0
96
78 : 22
3.5
94
76 : 24
3.5
92
74 : 26
3.5
92
78 : 22
a Conditions: dibenzylamine (1 mmol), TBHP (6.5 M in decane, 2.5 mmol),
catalyst (0.02 m mol), acetonitrile (2 ml) at 60 ◦C. b Conversion and ratio
of 6 : 7 was determined by GCMS.
4.3 MeOPEG5000-Immobilized Co(II) Schiff base 4
4.0 Experimental
A mixture containing azido functionalized MeO-PEG5000-N3 (20 g,
4 mmol), and propargylated Co(II) Schiff base 3 (1.10 g, 2.0 mmol)
in dry dichloromethane (30 mL) was added CuI (5 mol%) and
DIPEA (10 mmol), the resulting mixture was refluxed for 24
h. After being cooled at room temperature, the polymer was
separated from the reaction mixture by precipitation with diethyl
ether, washed with 2-propanol, diethyl ether and dried under
vacuum to yield dark brown colored PEG-supported catalyst 4 in
94% yield (19.41 g). IR (cm-1): 3260, 2879, 1720, 1605, 1466, 1154.
Analytical calc. for nitrogen (found; 0.8%). The loading of the
cobalt to the PEG support was determined by the complexometric
titration using EDTA. The loading of the cobalt was found to be
0.06 mmol g-1.
4.1 Materials
All the substrates were purchased from Aldrich and used as re-
ceived. L-(-) Tyrosine, salicylaldehyde and propargylated bromide
(80 wt% solution in toluene) were commercially available and used
as obtained. The Co(II) Schiff base 2 and bispropargylated Co(II)
Schiff base 3 were prepared according to the published method.3c
CoCl2 was heated at 110 ◦C for 3–4 h under vacuum prior to use.
4.2 Techniques used
The melting points were determined in open capillaries on a
Buchi apparatus and are uncorrected. The 1H NMR spectra were
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4.4 General experimental procedure for oxidative
dehydrogenation of secondary amines using TBHP as oxidant
Acknowledgements
Authors are thankful to the Director, IIP for his kind permission
to publish these results. Analytical Division of the Institute is
acknowledged for providing the analysis.
Into a stirred mixture of secondary amine (1 mmol), t-BuOOH
(6.5 M in decane, 2.5 mmol) in acetonitrile (2 mL) was added
polymer bound catalyst (2 mol%). The resulting mixture was
heated at 60 ◦C for the time reported in Table 1. Completion
of the reaction was analysed by TLC. After completion of the
reaction, the catalyst was recovered from the reaction mixture by
precipitation with diethyl ether and reused as such for subsequent
experiments. The filtrate so obtained was concentrated under
reduced pressure and dissolved in dichloromethane (10 mL). The
organic layer was washed with water (2 ¥ 15 mL), dried over
anhydrous MgSO4. The solvent was removed under vacuum to
give corresponding products. The conversion and selectivities
for the formation of imine and its hydrolyzed products were
determined by GCMS. The identity of the products was confirmed
by comparing their physical and spectral data with those of
literature compounds.21
Notes and references
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4.5 Product charaterization data
N-(Benzylidine)benzylamine (Table 1, entry 1): mp (51–52 ◦C);21a
IR (KBr): 3041, 1634, 1610, 1580, 1180, 976 cm-1; 1H NMR
(CDCl3, d ppm): 8.10 (s, 1H, CH N), 7.92–7.15 (m, 10 H, ArH),
4.82 (s, 2H).; 13C NMR (CDCl3): d = 165.1, 156.1, 142.1, 140.0,
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1
1697, 1204, 1168 cm-1. H NMR (CDCl3, d ppm): 10.02 (s, 1H,
CH), 7.87 (m, 2H, ArH), 7.64 (m, 2H, ArH), 7.52 (m, 2H, ArH).
13C NMR: (CDCl3): d = 192.8, 135.2, 134.8, 129.0, 127.7, 47.0 ppm.
N-Phenyl-N-(1-phenylmethylidine)amine (Table 1, entry 2): mp
(55–56 ◦C); IR (KBr): 3044, 1629, 1598, 1580, 1180, 970 cm-1; 1H
NMR (CDCl3, d ppm): 8.42 (s, 1H, CH N), 7.80 (m, 2H, ArH),
7.49–7.34 (m, 5H, ArH), 7.26–7.19 (m, 3H, ArH).
N-(Benzylidine)-t-butylamine (Table 1, entry 3): colorless oil,
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ppm) 8.19 (s, 1H, CH N), 7.61 (m, 2H, ArH), 7.15 (m, 2H, ArH),
2.38 (s, 3H, ArCH3), 1.26 (s, 9H, C(CH3)3). 13C NMR (CDCl3):
d = 160.1, 157.5, 136.1, 62.7, 23.5
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