Selective oxidation of thiols and alcohols by physically encapsulated nickel schiff-base complex prepared via sol-gel method as nano-catalyst

Article abstract of DOI:10.1080/15533174.2012.740719

A heterogeneous catalyst was prepared by direct physical encapsulation of nickel Schiff-base complex into the silicon dioxide matrix via sol-gel method. The compounds were characterized by FT-IR, SEM, TEM, XRD, CHNS, and AAS. The catalytic performance in the alcohols and thiols oxidation was investigated in the presence of H₂O₂ (10 mmol, 30%) as oxidant. After five recycle runs, the catalyst showed 13% reduction in activity with slightly Ni(II)-complex leached to the solvent. Varieties of primary and secondary alcohols were efficiently oxidized to the corresponding ketones. Thiols were oxidized in coupling reaction to disulfides in excellent yields using nano-catalyst with H₂O₂.

Full text of DOI:10.1080/15533174.2012.740719

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Synthesis and Reactivity in Inorganic, Metal-Organic,
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Selective oxidation of thiols and alcohols by physically
encapsulated Nickel Schiff-base complex prepared via
sol-gel method as Nano-Catalyst
Hossein Eshtiagh-Hosseini a , Taymaz Tabari a , Reza Takjoo a & Hossein Eshghi a
a Department of Chemistry, Faculty of Sciences, Ferdowsi University of Mashhad, Mashhad,
91779, Iran
Accepted author version posted online: 19 Nov 2012.
To cite this article: Hossein Eshtiagh-Hosseini, Taymaz Tabari, Reza Takjoo & Hossein Eshghi (2012): Selective oxidation of
thiols and alcohols by physically encapsulated Nickel Schiff-base complex prepared via sol-gel method as Nano-Catalyst,
Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, DOI:10.1080/15533174.2012.740719
To link to this article: http://dx.doi.org/10.1080/15533174.2012.740719
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Selective oxidation of thiols and alcohols by physically encapsulated Nickel Schiff-base
complex prepared via sol-gel method as Nano-Catalyst
Hossein Eshtiagh-Hosseini*, Taymaz Tabari, Reza Takjoo, Hossein Eshghi
Department of Chemistry, Faculty of Sciences, Ferdowsi University of Mashhad, Mashhad
91779, Iran
Taymaz Tabari email address: Taymaz_Tabari@yahoo.com
Reza Takjoo email address: rezatakjoo@yahoo.com
Hosseinn Eshghi email address: heshghi@um.ac.ir
Corresponding author: Tel.: +98 511 8797022; fax: +98 511 8796416; E-mail address:
heshtiagh@ferdowsi.um.ac.ir (Hossein Eshtiagh-Hosseini).
Abstract
A heterogeneous catalyst was prepared by direct physical encapsulation of nickel Schiff-base
complex into the silicon dioxide matrix via sol-gel method. The compounds were characterized
by FT-IR, SEM, TEM, XRD, CHNS and AAS. The catalytic performance in the alcohols and
thiols oxidation was investigated in the presence of H2O2 (10 mmol, 30%) as oxidant. After five
recycle runs, the catalyst showed 13% reduction in activity with slightly Ni(II)-complex leached
to the solvent. Varieties of primary and secondary alcohols were efficiently oxidized to the
corresponding ketones. Thiols were oxidized in coupling reaction to disulfides in excellent yields
using nano-catalyst with H2O2.
Keywords: Heterogeneous Catalyst, Selective Oxidation, Disulfides, Aldehyde, Schiff-base.
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Introduction
The oxidation of alcohols to the corresponding carbonyl compounds is fundamental and
important in organic chemistry. Carbonyl groups are valuable synthetic intermediates; and many
oxidation methods have been reported for this conversion [1]. In addition, the selective oxidation
of thiols to the corresponding disulfides is of great interest from biological and chemical point of
view [2-5]. The development of new oxidation processes continues to attract attention in spite of
the availability of numerous oxidizing agents [6,7].
In recent years, the structures of target molecules in organic syntheses are becoming more
complicated; hence better structure compatibility and higher selectivity have become
synonymous with an efficient oxidation. In this regard, conventional oxidants always satisfy the
requirements. As a result, the exploration of new oxidizing agents remains a challenging area of
research in organic syntheses [8,9].
Chromium(IV)-based oxidizing reagents such as Jones and Collins reagents are commonly used
in organic syntheses. However, toxic chromium residues, formed after the oxidation, cause
problem in the work-up and also in the disposal. To solve the problems, various catalytic systems
have been developed using environmentally benign oxidants like molecular Oxygen, hydrogen
peroxide and organic peroxides under homogeneous as well as heterogeneous conditions during
the last two decades [10]. Hydrogen peroxide as an oxidant has many advantages compared with
other oxidants because water is the only by-product. However, only a limited number of such
useful oxidation procedures have been reported [11-12]. Also a variety of different catalytic
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systems for oxidation of alcohols, using hydrogen peroxide, have been developed [13-18]. There
is still an ongoing interest in the search for new efficient metal catalysts for this system [19-24].
Use of some homogeneous catalysts such as metal complexes of Ru [25–31], Pd [32–34], Co
[35,36], Os [37] and Cu [38,39] has given rise to high catalytic activity and selectivity. Schiff-
base complexes of nickel, palladium and platinum have played an extremely important role in
the development of homogeneous catalysis [40]. In particular, nickel catalysis is of great interest
due to high performance and low cost. As the widespread use of traditional organic solvents in
many chemical processes is an issue of great environmental concern, the use of ionic liquids as
reaction media is a topic of much current interest [41].
The homogeneous catalysts have some difficulties in the catalyst recovery and product
separation (42-44). In order to overcome these problems, the heterogeneous catalysts with
similar activity to homogeneous catalysts have been used. For this purpose, immobilization of
metal Schiff-base complexes on organic and inorganic media has been widely reported (45-48).
In this work, we are going to report a very efficient and selective oxidation of alcohols and thiols
into corresponding products using hydrogen peroxide as oxidant catalyzed by the encapsulated
Ni(II) Schiff-base complex (Scheme 1).
Scheme 1
Experimental
Chemicals were purchased from Merck, Aldrich and Fluka Chemical Companies. All yields refer
to isolated products. The reactions were monitored by thin‐layer chromatography (TLC) carried
out on silica plates. The products were characterized by comparison of their physical data with
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those of known samples or by their spectral data. IR spectra were recorded on a Shimadzu-IR
1
470 spectrophotometer. H NMR spectra were recorded on a Bruker 100-MHz spectrometer in
CDCl3 as the solvent and TMS as internal standard.
Preparation of S-ethyl-β-N-(benzylidene)dithiocarbazate
Carbon disulfide (6.1 mL , 0.1 mol) added to a cold mixture of hydrazine hydrate (5 mL , 1 mol)
and KOH (5.6 g, 1 mol) in 20 mL of absolute ethanol making two phases. Then, ethyl bromide
(7.8 mL, 0.1 mol) is added slowly to the reaction mixture.
Benzaldehyde (10.6 mL , 0.1 mol) dissolved in absolute ethanol (15 mL) was mixed with the
above solution .The mixture was heated on a water bath for approximately 1 h and then left to
stand overnight. A yellow product was separated by filtration, washed with ethanol and dried in a
vacuum desiccator over silica gel. The compound was recrystallized from ethanol. Yield 74%.
Anal. calc. for C10H12N2S2 (224.35 g/mol): C, 53.54; H, 5.39; N, 12.49; S, 28.59 Found: C,
52.86; H, 5.41; N, 12.62; S, 29.11%.
Preparation of bis(S-ethyl-β-N-(benzylidene)dithiocarbazate)-nickel(II) complex
Nickel(II) nitrate hexahydrate (0.07 g; 0.22 mmol) in boiling ethanol (15 mL) was added to the
ethanolic solution of the ligand (0.1 g; 0.45 mmol) . The mixture was heated in a water bath for
30 min and then left to stand overnight. A brown product was filtered off, washed with ethanol,
dried in a vacuum over silica gel. Yield 48%; m.p. 349°C. Anal. Calc. for: C, 47.53; H, 4.39; N,
11.09; S, 25.38%. Found: C22H22N4NiS4 (505.37 g/mol): C, 47.39; H, 4.27; N, 10.98; S, 26.08%.
Preparation of physically encapsulated Ni(II) Schiff-base complex
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An amount of 0.25 g Ni(II) Schiff-base complex was dissolved in 30 mL of TEOS and 10 mL of
methanol at 60 ºC under stirring. Then 14.5 mL of water was added into the mixture to obtain a
homogeneous solution. Then, concentrated Hydrochloric acid was introduced to the solution to
adjust the pH to 4. The stirring was continued under 60 ºC until gelation was completed. Then,
the catalyst was dried at 80 ºC until a constant weight was obtained. The gel was washed with
water until the pH of the filtrate increased to 7, and then it was extracted by methanol in a
Soxhlet extractor for 6h. Then the content of Ni was determined by AAS.
Typical procedure for oxidation of alcohols
In a 25 mL round-bottomed flask, a solution of alcohol (1 mmol), in a solvent (5 mL) catalyst.1
(0.025 g) and hydrogen peroxide (10 mmol, 30%) were added to above solution. Then, the
reaction mixture stirred vigorously at room temperature.
Typical procedure for the oxidative coupling of thiols
In a 25 mL round-bottomed flask, a solution of thiol (1 mmol) in a solvent (2 mL) was prepared.
Catalyst.1 (0.02 g) was added to the solution, and the reaction mixture was stirred at room
temperature in the presence of hydrogen peroxide as oxidant. Scheme 2
The progress of the reaction was monitored by TLC (eluent: ethyl acetate/ n-hexane, 1:4). The
unreacted thiol was estimated by titrating against standard alcoholic iodine solution using
pyridine as a base. After the completion of the reaction, the reaction mixture was filtered and the
solid material was washed with 10 mL of the solvent used during the reaction. Evaporation of
solvent, using a rotatory evaporator, to dryness gave the product which was purified by
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recrystallization. Comparison of the melting points and 1H NMR spectra of the purified products
with those of the authentic disulfides proved the existence of pure disulfide with 85–98% yields
(Table 4). All reference disulfides were prepared from the corresponding thiols according to the
method described in Vogel [49]. The analysis of spectral and physical data revealed that only
disulfides were formed and no further oxidation occurred.
Results and discussion
Synthesis and characterization of the catalyst
First, the homogeneous catalyst Ni(II) Schiff-base complex with the structure shown in fig.1
prepared as described before.
The complex was then encapsulated physically into the SiO2 matrices by sol-gel method. This
method is simple and straightforward: the complex was added to polymerization mixture of
TEOS which is catalyzed by hydrochloric acid; when the polycondensation was completed, the
complex was entangled into the SiO2 polymeric network. The encapsulation conditions are
milled, to ensure the stability of the complex in the process. The FT-IR spectra of the neat
complex and catalyst.1 showed the bands at 1592 cm-1 and 673 cm-1, which were assigned to CN
and phenyl group. The reason for non-appearance of other bands of complex is the masking of
these bands by the presence of strong background bands of the SiO2 matrix. Furthermore,
frequencies of 1100 cm-1 and 800 cm-1 are assigned to stretching vibration of Si-O-Si and Si-OH
respectively.
The atomic adsorption was employed to confirm the entrapping of Schiff-base complex into SiO2
matrices. After the catalyst was washed thoroughly with methanol, less amount of impure
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complex was left the matrices. The washing process continued until the extracted methanol
didn’t show leached nickel complex. Thus, the investigation of the nickel content by AAS
revealed that the nickel complex was entrapped in the SiO2 matrices.
TEM, SEM test results
SEM and TEM images were used to evaluate the surface morphology and size distribution of the
Nickel complex deposited in the silica network. The prepared catalyst (1) was analyzed by SEM
(Fig 3). A clear change in the morphology of the catalyst occurred after the addition of nickel
complex to the support (Fig 2). SEM showed that the number of pores for the catalyst decreased
dramatically. It is deduced that Ni(II) Schiff-base complex was grafted in the silica support. The
TEM images of the Si/Ni complex composites showed that they are non-agglomerated, scattered
spherical particles of complex at a high concentration, and TEM image also determined particle
size of catalyst.1 to be in the range of 10 nm (Fig 4).
X-ray Diffraction
X-ray Diffraction pattern of calcined catalyst.1 is shown in fig (3). The peaks at (111) and (200)
confirmed cubic structure of calcined catalyst.1 [50]. Furthermore, the XRD pattern of silicon
dioxide showed that the characteristic of Bragg reflection can be indexed to (100) planes of
hexagonal lattice of silicon dioxide [51]. The XRD pattern revealed the typical band of the nickel
phase, but it didn’t show the typical band of silicon dioxide phase because the silicon dioxide
peak was broadened by the presence of Ni(II)Schiff-base complex [52]. Thus, the XRD pattern
of physically entrapped nickel complex revealed that nickel Schiff-base complex was
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immobilized in the SiO2 matrices without any change. Also, the XRD demonstrated that the
particle size of catalyst is 19.8 nm at 2θ= 43.5° and 39.4 nm at 2θ= 38°.
Catalytic test
Optimization reaction of alcohols oxidation
Benzylalcohol (1 mmol), acetonitrile (5 mL), catalyst.1 (0.025 g) and H2O2 (10 mmol, 30%)
were taken in a 25 mL round-bottomed flask (scheme 1). We designed a reaction as a model in
order to find the optimum reaction variables such as: temperature, solvent, appropriate amount of
catalyst and length of the reaction time. The results are shown in Table 1. Among the various
solvents investigated, CH3CN showed better results compared with other solvents for oxidation
of alcohols under our experimental conditions (Table 1, entry 6), while toluene and ethanol
afforded lower conversion and selectivity (Table 1, entries 2 and 3). Moderate conversion and
selectivity were obtained when the reactions were carried out in CH2Cl2 and CH3OH (Table 1,
entries 1 and 5). Poor results were observed when the reactions were carried out in THF (Table
1, entry 4). The Oxidation took place to afford benzaldehyde in the presence of catalyst and H2O2
(10 mmol, 30%) in 50 minutes (Table1, entry6). Perhaps some of the hydrogen peroxides were
decomposed in the presence of catalyst.1, and O2 was released in the course of reaction. To
determine which of H2O2 or released O2 plays more essential role in the oxidation reaction in the
presence of our catalyst, we examined this reaction by simply bubbling molecular oxygen
through reaction mixture under similar conditions, and only a poor yield of product was obtained
(Table1, entry7). So decomposition played a little role in the oxidation of alcohols. When
concentration of catalyst increased in the presence of H2O2, the reaction took place in a shorter
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time but the selectivity decreased (Table1, entry8). Furthermore, when the oxidation of
benzylalcohol was carried out using less catalyst, the reaction was found to reach completion
over much longer time (Table1, entry9). Alternatively, the reaction was carried out in the
presence of neat complex as homogeneous catalyst using hydrogen peroxide as oxidant. The
results revealed that the oxidation was happened similar to Catalyst.1 under the same condition.
But, the purification of neat complex had some difficulties (Table1, entry10). The reaction was
carried out at 50ºC under the same conditions in which the oxidation completed after 30 min but
the selectivity decreased (Table 1, entry11). Furthermore, the reaction was carried out in the
same condition at 0 ºC, and the reaction completed with small conversion after 180 min. (Table
1, entry13). Oxidation of benzylalcohol took place in the absence of catalyst and just in the
presence of H2O2 (10 mmol, 30%). The main product of this reaction was benzoicacid with a few
amount of benzaldehyde (Table 1, entry12). Moreover, in the presence of heterogeneous Schiff-
base catalyst, the catalytic performance was effectively improved. The optimal conversion of
benzylalcohol reached to almost 100% and benzaldehyde was exclusively obtained over the
immobilized catalyst (Table 1, entry6). The reaction progress was monitored by TLC. After
completion of the reaction, the catalyst was removed by filtration. The obtained filtrate was
evaporated, and the residual was dissolved in a mixture of Ethylacetat/n-Hexan (1:4). Then, the
solvent was removed and the usual work-up gave corresponding products. One of the advantages
of heterogenized catalysts is their recyclability. The heterogenized catalyst in this work was used
in subsequent catalytic runs, and the results are given in Table 3. It clearly showed that the
activity of the encapsulated catalyst decreased by 13%. To prove the advantage of heterogeneous
catalyst, the reaction was catalyzed by homogeneous catalyst which worked only once because
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the homogeneous catalyst bore rapid degradation (Table 1, entry14). It should be mentioned that
the above results confirmed the AAS results of determination of the Ni content. Furthermore, the
results showed the content of nickel in fresh catalyst.1 to be 0.25%, and this amount was
decreased to 0.21% after the first run of reaction. Just like the test of stability and reusability of
catalyst.1, the reaction was carried out five times, and after these cycles, the nickel content
decreased to 0.19%. Therefore, the catalyst was obtained by sol-gel method not only was
recyclable, but also showed an appropriate catalytic performance in oxidation of primary and
secondary alcohols.
Oxidation of alcohols
The oxidation of alcohols was carried out to test the catalytic efficiency of the prepared catalyst.
To show the generality of this procedure, various alcohols such as primary, secondary and
aliphatic alcohols were selected, and the results are summarized in Table 2. Efficient oxidation
was observed on the benzylic alcohols, but aliphatic alcohols needed extra time (Table 2, entry
15-17). Moreover, the reaction of cyclohexanol (Table 2, entry17) required an extra 10 mmol%
of H2O2 after 240 min. The primary benzylic alcohols were found to be reactive and afforded
selectively their corresponding aldehydes compared to other aliphatic primary alcohols.
Secondary benzylic or aliphatic alcohols produced corresponding ketons in high yields. The
yields were confirmed after the products were isolated by filtration and purification with ethyl
acetate/ n-hexane. Although the conversion was observed by TLC, typically, 5-10% of products
were lost during the work-up procedure. The catalyst.1 affords selectively only Aldehyde (Table
2, Entry1-7) instead of the two usual products (Aldehyde and Carboxylic acid) in oxidation of
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primary alcohols. In case of the oxidation of secondary alcohols, the catalyst.1 afforded
selectively only ketone in lieu of two possible products (Ketone and Carboxylic ester). No pre-
treatment step was used; however, the recovered catalyst was washed with mixed solvent of
ethanol/n-hexane by 1:9 molar ratios to remove traces of the previous reaction mixture and dried
before the next cycle.
Oxidation of thiols
Oxidation of thiols to their expectable products (disulfides and sulfonic acid) occurred in the
presence of immobilized nickel schiff-base complex and the product obtained in excellent yields
in the presence of catalyst.1. 4-methylthiophenol was chosen as a model substrate for
optimization reaction. Among the various solvents, methanol had better results compared with
the other solvents (Table 3, entry 1-7). Then, the reaction was studied in the same condition and
different temperatures; the results showed that the conversion rate of thiols to disulfides
decreased at 0 °C. However, at higher temperatures, the rate of conversion increased and the
selectivity decreased (Table 3, entry 8-10). Furthermore, the coupling reaction of 4-
methylthiophenol was tested using various amounts of catalyst.1. The results showed that the
reaction rate increased in higher amounts of catalyst.1 (0.06 g), and the conversion decreased in
lower amount of catalyst.1 (0.01 g), and the conversion was not complete (Table 3, entry 11).
Moreover, two reactions were investigated in the absence of catalyst.1 and only in the presence
of silica support as catalyst respectively. The reaction was studied in the absence of any catalyst
and carried out only in presence of hydrogen peroxide. After 360 min, the reaction didn’t show
any conversion (Table 3, entry 12). Also, the reaction was done in presence of silica support as
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catalyst in the presence of hydrogen peroxide, and it didn’t show any conversion (Table 3, entry
13). Then, the reaction was studied in presence of catalyst.1 and molecular oxygen bubbling as
oxidant. In this situation the conversion happened with excellent yield and without any
selectivity (Table 3, entry 14). Also, without hydrogen peroxide and only catalyst.1, the reaction
didn’t show any conversion (Table 3, entry 15). The stability and recycling possibility of
prepared heterogeneous catalyst in oxidation of thiols to disulfides were further investigated.
After one time running of the reaction, the catalyst washed with methanol and dried for the next
five runs under the same reaction conditions (Table 3, entry 16). It is clear that the obtained
thiols conversion was almost the same within the five reaction runs, verifying that catalyst.1 is
highly stable and can be reused. In continuation, the oxidation of different types of thiols was
happened in the presence of heterogeneous nickel Schiff-base complex as catalyst in methanol
and in the presence of hydrogen peroxide as the oxidant (Table 4). As can be seen from Table 4,
a variety of aromatic (entries 1-8) and aliphatic (entries 9-12) thiols were easily converted to
corresponding disulfides in appropriate times with excellent yields. From the data of column 2 in
Table 4, it is also obvious that aromatic thiols react faster than aliphatic thiols.
Conclusion
In summary, heterogeneous nickel schiff-base complex as catalyst.1 was prepared by the sol–gel
method. A peer study on the structural, morphological and catalytic properties of catalyst.1 was
performed by various analytical techniques. XRD and microscopic analysis indicate that the
catalyst.1 prepared by the sol-gel precursor, has a suitable crystallinity. In addition, the
investigations have shown that the prepared catalyst.1 can be a favorable material to selective
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oxidation of thiols to corresponding disulfide over a wide range of aromatic and aliphatic thiols.
The processing parameters such as the amount of catalyst, effect of various solvents,
temperature, and different oxidants can all affect the coupling reaction of thiols. The results
showed that the prepared heterogeneous catalyst could effectively oxidize thiols to disulfide in a
high yield. The percentage of conversion obtained in the 26-45 minute of stirring was 80-98%,
using 0.02 g/ 2 mL of catalyst. Moreover, this catalytic oxidation system is clean, safe and
operationally simple and the yields of the products are excellent, so the oxidation method meets
the needs of contemporary green chemistry and is suitable for practical syntheses.
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Table 1. Optimization of reaction conditions by oxidation of benzylalcohol in acetonitrile by
catalyst.1 at room temperature and in presence of hydrogen peroxide as oxidanta
Reaction
Temperature Catalyst
Conversion Selectivity
Entry Solvent
Time
(min)
50
ºC
(g)
%
%
1
2
Dichloromethane 25
Toluene 25
0.025
0.025
80
30
<60
50
Small
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3
Ethanol
25
25
25
25
25
25
25
0.025
0.025
0.025
0.025
0.025
0.05
50
60
Small
Small
<60
4
THF
50
45
5b
6
Methanol
acetonitrile
acetonitrile
acetonitrile
acetonitrile
50
100
100
<20
100
100
50
100
7c
8d
9
180
45
Small
<70
0.01
150
100
Neat
10e
11
acetonitrile
acetonitrile
acetonitrile
25
50
25
55
100
100
100
>85
<80
<20
complex
0.025
Without
catalyst
0.025
0.025
30
12f
240
13
acetonitrile
acetonitrile
0
180
50
<20
92
100
100
14g
25
a
General reaction condition: benzylalcohol 1mmol, room temperature, 0.025 g of catalyst.1,
aqueous 30% H2O2 10 mmol.
b The reaction was performed in methanol and the product obtained is a mixture of benzaldehyde
and benzoicacid.
c The reaction was performed in the presence of molecular oxygen as oxidant.
d The reaction was performed in the presence of 0.05 g of the catalyst.1 and the product obtained
is a mixture of benzaldehyde and benzoicacid.
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e
The reaction was performed in the presence of 0.01 mmol of neat complex as catalyst and the
product obtained is a mixture of benzaldehyde and benzoicacid.
f The reaction was performed in the presence of hydrogen peroxide and without catalyst. Yield is
very poor and the product obtained is a mixture of benzaldehyde and benzoicacid.
g The reaction carried out with the first recycled catalyst.1 and the catalyst used four times more.
Yields obtained excellent at all cases and the conversion is decreased after five time recycling by
13%.
Table 2. Oxidation of alcohols to corresponding carboxylic group by catalyst.1.a
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Reaction
Isolated
Entry Substrate
Product
Time (min)
Yields (%)
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O
OH
H
H
1
50
90
OH
O
H
H
2
45
35
70
99
99
90
MeO
MeO
OH
O
MeO
MeO
3
MeO
MeO
O
OH
4
O2N
Br
O2N
O
OH
Br
5
60
60
98
98
OH
O
Cl
6
Cl
O
OH
7
8
110
70
65
95
H
OH
O
OH
O
9
75
85
90
90
Cl
Cl
OH
O
Et
Et
10
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OH

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a
The reactions carried out with 1 mmol of substrate at room temperature, 0.025g of catalyst.1,
aqueous 30% H2O2 10 mmol.
Table 3. Optimization of reaction conditions by oxidation of 4-methylthiophenol in CH3OH at
room temperature in presence of hydrogen peroxide as oxidant.a
catalyst
(g)
Time
(min)
Conversion
%
Disulfide
Entry Solvent
Temperature
Selectivity%
1
methanol
0.02
25
25
25
25
25
25
25
0
30
100
10
100
90
2
dichloromethane 0.02
150
150
150
150
150
70
3
ethanol
toluene
0.02
0.02
60
100
90
4
10
5
tetrahydrofuran 0.02
20
90
6
acetonitril
0.05
0.02
0.02
0.02
0.02
40
95
7
100
20
100
100
95
DMF
8
methanol
100
20
9
methanol
methanol
50
75
100
100
10
15
90
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11b
12
methanol
methanol
methanol
methanol
methanol
methanol
0.01-0.06 25
55-20
360
360
35
100
0
100
None
Support
0.05
25
25
25
25
25
-
13
0
-
14c
15d
16f
100
0
0
0.02
180
30
0
0.02
100
100
a Isolated yield on the basis of the weight of the pure product obtained.
b
The reaction carried out with various amount of catalyst.1 (0.01-0.06 g). Results show the
conversion time is decreased by increasing catalyst amount. Yields obtained excellent at all
cases.
c the reaction carried out in the presence of molecular oxygen as oxidant.
d the reaction carried out by 0.02 g catalyst in the absence of oxidant.
f
The reaction carried out with the first recycled catalyst.1 and the catalyst used four times
more. Yields obtained excellent at all cases.
Table 4. Oxidative coupling of thiols to disulfides catalyzed by catalyst.1 a
Entry
1
Substrate
Time(min) Isolated Yields (%)
SH
32
98
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Cl
SH
2
3
4
5
27
40
30
25
94
95
97
90
Cl
SH
H3C
SH
NH2
SH
SH
N
6
7
26
35
85
85
O
SH
8
9
34
50
90
80
SH
CHOOH
SH-CH2-CH
NH2
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SH-CH2-COOH
10
11
12
25
45
40
90
80
90
O
SH-CH2-C
OEt
C8H17SH
a
The reactions carried out with 1 mmol of substrate in 2 mL of methanol at room temperature
with 0.02 g of catalyst.1, hydrogen peroxide (10 mmol, 30%) as oxidant.
Et
S
S
Ni
S
N
N
N
N
S
Et
Fig 1.the structure of bis[S-ethyl-β-N-(benzylidene)dithiocarbazato]-nickel(II)
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Fig 2. SEM image of silica prepared via sol- gel method without Schiff-base complex
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Fig 3. SEM image of heterogeneous catalyst
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Fig 4. TEM image of heterogeneous catalyst
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Fig 5. XRD diagram of heterogeneous catalyst
OH
C
O
C
catalyst (1), H2O2 30%
R1
R2
rt
R1
R2
H
R1,R2: H, Me, Et, Ph
Scheme 1. Oxidation of alcohols
catalyst (1), H2O230%
R-S-S-R
2RSH
rt
Scheme 2. Oxidation of thiols
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