Reusable cobalt-phthalocyanine in water: Efficient catalytic aerobic oxidative coupling of thiols to construct S-N/S-S bonds

Article abstract of DOI:10.1039/c7gc00401j

A new aerobic oxidative coupling of thiols in water to construct sulfenamides or disulfides was developed, utilizing cobalt(ii)phthalocyanine-tetra-sodium sulfonate as the catalyst and O₂ as the oxidant. The mother liquor could be recycled up to 20 times with negligible loss of activity and only a minor decrease of product yield.

Full text of DOI:10.1039/c7gc00401j

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Reusable Cobalt-Phthalocyanine in Water: Efficient Catalytic
Aerobic Oxidative Coupling of Thiols to Construct S-N/S-S Bonds
Yingchao Dou,a Xin Huang,a Hao Wang,a Liting Yang,a Heng Li,a Bingxin Yuan*a and Guanyu Yang a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A new aerobic oxidative coupling of thiols in water to construct
sulfenamides or disulfides was developed, utilizing
cobalt(II)phthalocyanine-tetra-sodium sulfonate as the catalyst
and O2 as the oxidant. The mother liquor could be recycled up
to 20 times with negligible loss of activity and only a minor
decrease of product yield.
including toxic waste, harsh reaction conditions (such as high
temperature, long reaction hours, and risk of over-oxidation),
extra additives or bases, and tedious work-up procedures.
Therefore, the development of a novel and green catalytic
method that can be carried out under mild conditions for
the synthesis of sulfenamides and disulfides remains a
challenge and is highly desired.
Organosulfur compounds have been vitally applied as
Over the past few decades, the sulfenamide accelerators
have been the most prevalent in the industry. Among those,
synthetic intermediates and reagents in organic synthesis,
pharmaceutical and biological science.1,
2
For instance,
tert-butyl-2-benzothiazole sulfenamide (TBBS) stands out for
its delayed action and faster curing rate.35,
36
organic compounds containing S-S bonds, defined as
disulfides, are key motifs to stabilize proteins structures and
responsible for their biological activities.3-6 Furthermore, the
unique structure and properties of S-N bonds in
sulfenamides have found their utilization as functional
groups in natural product synthesis and ubiquitous
intermediates in the preparation of fine chemical products
such as accelerators for rubber vulcanization in industry.7-10
Conventional methods to achieve S-S bonds involve metal-
catalyzed or metal-free oxidative coupling of thiols. There
have been many reports about metal-free oxidation using
hydrogen peroxide, azo reagents, halogen, sodium nitrite, or
Burgess Reagent.11-19 Metal-containing catalysts or oxidants,
such as Al,20 Cr,21, 22 Mn,23 Fe,24 Co,25 Cu,26 Pb,27 or Ce,28 are
typically used.29 However, when it comes to sulfenamides,
the transition-metal-catalyzed coupling of thiols with amines
has not been well developed up to date, due to the difficulty
to avoid disulfide as homocoupling product or the formation
of an inactive metal-thiolate complex.30-32 To solve these
problems, various metal catalysts have been studied, and it
turns out that the utilization of copper catalyst results in the
formation of sulfenamides with good yields and
selectivity.32-34 Nevertheless, the shortcomings are apparent,
From the
industrial, environmental, and biological prospects of view,
we sought to exploit a new catalytic system for the synthesis
of such compounds, by using molecular oxygen as the
oxidant and water as the solvent. For green and sustainable
chemistry, molecular oxygen has been considered as an ideal
oxidant in organic synthesis due to its natural, cheap and
environmentally abundant properties.37-39 While some
respectable achievements have been made, more efforts are
still required for its practical application in the industrial
production. Herein, we reported a reusable Co-catalyzed
aerobic oxidative coupling of thiols to construct sulfur-
nitrogen and sulfur-sulfur bonds in water.
Our initial attempt at the catalytic synthesis of
sulfenamides explored the use of transition-metal
complexes as the catalyst (Table 1). A sustainable catalyst
should qualify two requirements: 1) slight solubility in the
water and 2) good tolerance against amines and thiols. The
latter was screened by slowly adding amine to the aqueous
solution of
a metal complex. Precipitation would be
observed if catalyst poisoning occurs. Taking clues from this,
a few transition metal complexes were suitable to screen the
reaction of thiol with amine (Table 1, Entry 1-9).
a.Department of Chemistry and Molecular Engineering, Zhengzhou University. Zhengzhou,
Henan, China. 450001
†The authors declares no competing financial interests.
Electronic Supplementary Information (ESI) available. See DOI: 10.1039/x0xx00000x
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Co(phcy)(SO3Na)4 was applied, the product was slightly blue
after filtration, indicating trace catalyst residue. Furthermore,
it has been reported that Co(phcy)(SO3Na)4 intends to form
oligomers in concentrated aqueous solution due to
intermolecular π-π stacking, reducing its solubility and
catalytic performance. In summary, even though higher yield
could be achieved with higher catalyst loading, an upper
limit exists. The control experiment, in which no catalyst was
added, resulted in no product, confirming the necessity of
Co(phcy)(SO3Na)4 in the catalytic cycle (Entry 14).
Table 1. Catalyst Screening.a
Entry
1
Catalyst
wt%
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
1.0
0.6
0.4
0.2
-
Yield (%)b
33
Na[Fe(EDTA)]
2
Na[Co(EDTA)]
59
3
Na2[Cu(EDTA)]
Na2[Mn(EDTA)]
Na2[Ni(EDTA)]
Co(phcy)(COONa)4
Fe(phcy)(COONa)4
Cu(phcy)(COONa)4
Co(phcy)(SO3Na)4
Co(phcy)(SO3Na)4
Co(phcy)(SO3Na)4
Co(phcy)(SO3Na)4
Co(phcy)(SO3Na)4
-
68
Table 2. Optimization of reaction conditions between 1a and 2a.a
4
7
5
trace
76
Entry
1
T / ℃
50
ΔP (O2)/MPa
Ratiob
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:4
1:3
1:2
1:1
Yield (%)c
52
6
0.3
0.3
0.3
0.3
0.3
0.3
-
7
trace
70
2
60
95
8
3
70
75
9
95
4
80
67
10
11
12
13
14
87
5
90
11
82
6
100
60
trace
26
68
7d
15
8
60
0.1
0.2
0.4
0.5
0.6
0.3
0.3
0.3
0.3
63
0
a
9
60
70
Reaction conditions: 1a (2 mmol), 2a (10 mmol), catalyst, 10 mL H2O, 0.3 MPa O2,
60 oC, 4h. b Isolated yields.
10
11
12
13
14
15
16
60
87
60
83
All tested reactions were conducted with 2-
60
78
mercaptobenzothiazole 1a and t-butylamine 2a in a reactor
under 0.3 MPa O2 at 60 oC for 4 hours. The desired product
3a, TBBS, was predicted to precipitate out due to its low
solubility in the solution. As expected, the reaction mixture
was filtered once completed, and the precipitation was
washed with water and collected as product. Initially, 0.8 wt%
Na[Fe(EDTA)] was tested and 3a was formed in 33% yield,
fortunately (Entry 1). More metal-EDTA complexes were
screened under the same catalyst loading. When
Na[Co(EDTA)] and Na2[Cu(EDTA)] were used as the catalysts,
3a was produced in 59% and 68% yield, respectively (Entry 2,
3). However, when it came to Na2[Mn(EDTA)] and
Na2[Ni(EDTA)], only trace amount of product was formed
(Entry 4, 5). Apparently, the metal-EDTA complex with a
copper or cobalt center had a higher catalytic ability, albeit
in modest yields. Then, we changed the ligand to
phthalocyanine-tetra-sodium carbonate. Compared with
NaM(EDTA) (M = Co, or Cu), M(phcy)(COONa)4(M = Cu or Co)
showed stronger catalytic ability and led to higher yields
(Entry 6, 8). Interestingly, only trace amount of product was
collected when Fe(phcy)(COONa)4 was adopted (Entry 7),
illustrating certain ligand effect on the catalytic performance.
A modified Co catalyst, Co(phcy)(SO3Na)4, successfully
increased the yield to 95% (Entry 9). By varying the loading
of Co(phcy)(SO3Na)4 from 0.2 wt% to 1 wt%, 0.8 wt% turned
out to be the optimum amount (Entry 9-13). Notably, when
the catalyst loading rose from 0.8 wt% to 1 wt%, the yield
decreased. To understand this unanticipated reduction, we
60
87
60
trace
trace
trace
60
60
a Reaction conditions: 1a (2 mmol), 2a (10 mmol), Co(phcy)(SO3Na)4(0.8 wt%), H2O,
0.3 MPa O2, 4 h. b Ratio of amine against thiol. c Isolated yields. d Air atmosphere.
In the further optimization of the reaction conditions,
we evaluated temperature, O2 pressure, and the ratio of 2a
against 1a (Table 2). The temperature parameter study was
carried out in six various reaction temperatures (Entry 1-6).
o
As the maximum yield of 95% was obtained at 60 C, either
higher or lower temperature decreased the yield of 3a
.
Furthermore, the effect of O2 pressure was studied by
varying the pressure of O2 from 0.2 to 0.6 MPa. There was an
increase of the yield with the O2 pressure up to 0.3 MPa, and
after that, the yield decreased. When the reaction was
conducted in air, the product was obtained only in 26% yield
(Entry 7). It is known that the combination of O2 with the
catalytic metal center is affected by O2 pressure. Thus, it is
reasonable that the yield increased beyond 0.3 MPa O2
(Entry 2, 7-9). However, lower yield was observed once the
O2 pressure is further increased, illustrating lower selectivity
of the desired product (Entry 10-12). The ratio of 2a against
1a was examined in five different values (Entry 2, 13-16),
and the raising ratio of 2a increased the yield evidently.
When only one equivalent of t-butylamine was added to the
reaction mixture, the well-suspended 1a turned into a
set eyes on the work-up procedure. When
1 wt%
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glutinous mixture, which should be the neutral onium salt
tetramethylpiperidine 2l as a reaction partner, which led to
3l in as low as 19% yield. Aniline 2m and its derivatives 2n-o
bearing methyl group at para- or meta-position on the
benzene ring resulted in the desired sulfenamides 3m-o in
59-67% yields. 4-Chloroanilne and 4-bromoaniline furnished
products 3p and 3q in 47% and 53% yields, respectively.
Conclusively, aromatic amines bearing electron-withdrawing
substituents led to lower yields. Except for 1a, we tried other
thiols, for instance, substituted phenylthiol. Unfortunately,
none of those worked. Despite this defect, our work shows
great potential in industrial production of rubber
vulcanization accelerators since the work-up procedure is
operationally efficient.
and caused inefficient stirring. Moreover
，1a favored its
homocoupling with a low ratio of amine. Following its
increasing equivalents, 2a started to serve as an organic co-
solvent and promote the diffusion of 1a into water. Since we
were aiming at a catalytic reaction in water and 95% yield
was acceptable, it was unnecessary to increase the ratio
higher than 1:5.
Table 3. The scope of amines.a
Table 4. Synthesis of Symmetric Disulfides.
a Reaction conditions: 1 (2 mmol), Co(phcy)(SO3Na)4 (0.8 wt%), 10 mL H2O, 60oC, air.
b 0.3 MPa O2, 4 h.c t = 9 h.d t = 12 h.
a Reaction conditions: 1 (2 mmol), 2 (10 mmol), Co(phcy)(SO3Na)4 (0.8 wt%), 0.3 MPa
O2, 10 mL H2O, 60 ℃, 4 h. b 20 mmol amine; c 40 mmol amine.
Then we took an advantage of this novel and
environmentally friendly Co-catalyzed system for the
homocoupling of thiols to construct symmetric disulfides
(Table 4). The homocoupling was initially tested on 1a as
well. Instead of the optimal condition, a modified reaction
condition was used, in which the reaction time was
extended to 6 h and the O2 atmosphere was replaced by air.
Excitingly, 2,2-dibenzothiazole disulfide 4a was obtained in
99% yield. To screen the generality of this method, a series
of alkyl and aryl thiols were tested and well tolerated.
Substituted thiophenols bearing electron-donating groups
With the optimal reaction conditions in hand, the
substrate scopes of 1a were explored with a large variety of
amines (Table 3). In general, all alkyl amines and aryl amines
were well tolerated and furnished the desired sulfenamides
in modest to excellent yields. Primary and secondary alkyl
amines gave target products in relatively higher yields as
compared to aryl amines. The use of primary alkyl amines as
a reaction partner provided 3a-d in 81-95% yield. Secondary
alkyl amines were able to give sulfenamides 3e-l in good to
excellent yields as well. Moreover, cyclic secondary amines
afforded sulfenamides 3g-k in higher yields, comparing with
other secondary alkyl amines (2e, f). This might be due to
higher steric hindrance caused by the two flexible alkyl
chains on the amines (2e, f). Higher equivalents of those
substrates (2e, f) are also required. This hypothesis was
further confirmed by the employment of 2,2,6,6-
(
1b-g), such as amino, methoxyl, hydroxyl, or methyl group,
and electron withdrawing groups (1h-i), such as chloro or
methoxycarbonyl group, gave all desired disulfides 4b-i in
excellent yields. In spite of the electronic effect, thiophenols
with methyl group in the ortho-, meta- or para-position
showed no significant difference on the yields, either.
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Comparing with the 98% yield of 4e, the slightly lower yield
of 4g might be due to the steric effect of the methyl group
on ortho-position. Heteroaromatic thiols, such as 2-
mercaptopyridine 1j and 2-thiophenethiol 1k, were oxidized
to disulfides 4j and 4k in 97% and 91% yields, respectively.
Naphthyldisulfide 4l with extended π-framework was
synthesized in 94% yield. Aliphatic thiols led to desired
products, 4m and 4n, in 94% and 93% yield, respectively.
Conclusively, this oxidative coupling of thiols in the aqueous
phase is highly versatile and efficient for the synthesis of a
large variety of symmetric disulfides.
N
N
0.8 wt% Co(phcy)(SO3Na)4
H O, 0.3 MPa O , 60
℃, 4 h
S
HN
SH
+
H2N
2a
S
S
2
2
3a
1a
10 g scale
We were curious to continue our investigation with the
synthesis of asymmetric disulfides. It came to the
determination that the crosscoupling favors electron-
efficient aromatic thiols (Table 5). Reactions with mixture of
two aromatic thiols led to the formation of target
asymmetric disulfides (5a and 5b) in relatively high yields (91%
to 94%). Reactions between aromatic and aliphatic thiols led
to the formation of the target disulfides (5c and 5d) in
excellent yields (96% to 97%).
Figure 1. The circulation of the mother liquor.
Scheme 1. Experimental Evidence and Proposed Mechanism.
A
N
Table 5. Synthesis of Asymmetric Disulfides.
0.8 wt% [Co]
SH
+
no product
(1)
H2N
H2O, 60
℃, N2, 4 h
S
0.8 wt% [Co], 0.3 MPa O2
R1
S
R1 SH
1A
R2 SH
1B
+
S
R2
H2O, 60 oC, 12 h,
0.8 wt% [Co],
0.3 MPa O2.
N
N
S
5
S
S
N
(2)
+
H2N
10 equiv.
S
S
S
HN
H2O, 60
℃, 4 h
Yield: 76%
H2N
S
HO
S
1 equiv.
S
S
N
S
N
S
0.8 wt% [Co]
H2O, N2, 60 , 4 h
5a
: 91%
5b: 94%
S
S
S
N
(3)
+
H2N
℃
S
HN
10 equiv.
1 equiv.
Yield: 26%
H2N
S
S
H2N
S
S
4
5d
: 97%
5c
: 96%
B
a Reaction conditions: 1A (1mmol), 1B (5mmol), Co(phcy)(SO3Na)4(0.8 wt%,
calculated with 1A), 5 mL H2O, 60 oC, 12 h, 0.3 MPa O2.
N
S
S
N
LxMn [H]
S
S
O2
S
N
S
Since it was our purpose to design a convenient and
clean method for industrial manufacturing, a large-scale
testing was performed under the optimal reaction condition
(Figure 1). Once the reaction completed, the reaction
mixture was filtered, and the solid was washed by H2O to
give pure product in 85% yield. Further extraction of the
aqueous layer furnished another portion in 6%. Such easily
scaled-up reaction guaranteed the promising application of
our methodology in chemical manufacturing and
commercialization. More importantly, the catalyst and
excess amine remain in the aqueous phase after the desired
product is easily collected through filtration. This strategy
was expected to accomplish the comprehensive circulation
of mother liquor. Thus, we performed the recovery of the
mother liquor. It’s worth noting that the mother liquor could
be recycled up to 20 times with almost comparable
reactivity and only a minor decrease of yield (Figure 1).
Hence, Co(phcy)(SO3Na)4 in aqueous phase is proved to be
an efficient catalytic system for the construction of S-N/S-S
bonds.
H
N
R1
R2
LxMn+1
H2O
N
S
N
S
SH
S
HN
To gain some insights into the reaction mechanism of S-
N bond formation, a few experiments were carried out. As
mentioned (Table 1, Entry 14), no reaction of thiol with
amine was observed in the absence of Co catalyst. The
elimination of O2 from the optimal condition led to no
product, either (Scheme 1A, Reaction 1). Preliminary results
are consistent with previous studies,40-42 suggesting the
formation of disulfide as homocoupling product is a key step
in the catalytic cycle. Then, we replaced 1,2-
bis(benzo[d]thiazol-2-yl)disulfane 4a with 1a to react with t-
butylamine 2a under the optimal reaction conditions
(Schem 1A, Reaction 2). TBBS 3a was produced in a
moderate yield (76%), which is lower than that of the
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reaction between 1a and 2a. Since amine was observed to
serve as a co-solvent and showed significant effect on the
reaction, the relatively lower solubility of 4a as compared to
1a should be responsible for the decreasing yield. To sum up,
4a serves as an intermediate in the catalytic cycle and is
significant to activate N-H bond of amines. A control
experiment of 4a with 2a under N2 was carried out to give
TBBS in 26% yield (Scheme 1A, Reaction 3), stating that O2
participates in the transformation of disulfide intermediate
into sulfenamide.
3
4
5
H. E. Swaisgood, Biotechnol. Adv. 2005, 23, 71-73.
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On the basis of above observations, the present Co-
catalyzed aerobic oxidative reaction mechanism to construct
S-N bond is proposed as shown in Scheme 1B. Initially, the
interaction of O2 with the Co(II) center results in the
formation of Co (III) complex.43-46 The reduction of Co(III)
complex by thiols regenerates the catalyst and gives the thiyl
radical. The homocoupling of two thiyl radicals leads to
disulfide as an intermediate. The reaction of disulfide with
amine furnishes the desired sulfenamide and releases a thiol
molecule to rejoin the catalytic cycle.
L. Craine and M. Raban, Chem. Rev. 1989, 89, 689-712.
10 G. Capozzi, G. Modena and L. Pasquato, in Sulfenic Acids
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Conclusions
In conclusion, we successfully developed a highly
efficient and versatile methodology to construct S-N/S-S
bond. The water media under O2 atmosphere, in which
cobalt(II)phthalocyanine-tetra-sodium sulfonate was adopt
as the catalyst, turned out to be a straightforward and
environment-friendly catalytic system for the oxidative
coupling of thiols under mild condition. After the reaction,
the catalyst within the solution can be easily recovered via
filtration and reused without any particular treatment.
Notably, the circulation of the mother liquor was conducted
up to 20 times while the yield remains commensurate,
providing a new prospect for commercial manufacturing of
disulfides and sulfenamines in a much greener and
economical way.
15 A. G. Choghamarani, M. Nikoorazm, H. Goudarziafshar, A.
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Acknowledgement
The authors would thank the National Natural Science
Foundation of China (No. 21502174 and J1210060), Beijing
National Laboratory for Molecular Sciences (No. 20140105),
Start-up Research Fund of Zhengzhou University (No.
1411316005), Research Fund of Henan Industry-University-
Research Institute Collaboration Association (No. 14210700
0006) and Scientific Research Fund of Henan Provincial
Education Department (No. 15A150080) for the funding
support.
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Reusable Co-catalyzed coupling of thiols to synthesize S-N/S-S bonds in
water