TEMPO visible light photocatalysis: The selective aerobic oxidation of thiols to disulfides

Article abstract of DOI:10.1016/j.cclet.2019.10.024

TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) is well-established in orangocatalysis that usually work in synergy with transition-metal catalysis or semiconductor photocatalysis. Here, TEMPO was turned into a visible light photocatalyst to conduct the selective aerobic oxidation of thiols into disulfides. With O₂ as an oxidant, a mild and efficient protocol for the selective oxidation of thiols into disulfides including symmetrical and unsymmetrical ones with 5 molpercent of TEPMO as a photocatalyst was developed at room temperature under the irradiation of 460 nm blue LEDs. It was found that a complex formed between TEMPO and thiols underpinned the visible light activity and disulfides were obtained in very high isolated yields. This work suggests that TEMPO takes diverse roles in for photocatalytic selective oxidative transformations with O₂ as the oxidant.

Full text of DOI:10.1016/j.cclet.2019.10.024

Journal Pre-proof
TEMPO visible light photocatalysis: The selective aerobic oxidation of
thiols to disulfides
Hui Xu, Yu-Fei Zhang, Xianjun Lang
PII:
S1001-8417(19)30628-X
https://doi.org/10.1016/j.cclet.2019.10.024
CCLET 5288
DOI:
Reference:
To appear in:
Chinese Chemical Letters
Accepted Date:
21 October 2019
Please cite this article as: Xu H, Zhang Y-Fei, Lang X, TEMPO visible light photocatalysis:
The selective aerobic oxidation of thiols to disulfides, Chinese Chemical Letters (2019),
doi: https://doi.org/10.1016/j.cclet.2019.10.024
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Communication
TEMPO visible light photocatalysis: The selective aerobic oxidation of thiols to
disulfides
a
a
a,
Hui Xu , Yu-Fei Zhang , Xianjun Lang
a
Sauvage Center for Molecular Sciences, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China
Graphical abstract
TEMPO, EtOH
SH + O2
S
S
R
H2O
2
+
R
460 nm blue LEDs
R
R
S
S
TEMPO, EtOH
SH
+
+ O2
+ H2O
R
HS
460 nm blue LEDs
Disulfides were obtained in a facile and efficient manner with very high isolated yields by TEMPO visible light photocatalysis in
ethanol. A complex formed between TEMPO and thiols during the reaction course underpins the visible light activity.
ARTICLE INFO
ABSTRACT
Article history:
TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) is well-established in orangocatalysis that
usually work in synergy with transition-metal catalysis or semiconductor photocatalysis.
Here, TEMPO was turned into a visible light photocatalyst to conduct the selective
Received 28 September 2019
Received in revised form 12 October 2019
Accepted 16 October 2019
Available online
2
aerobic oxidation of thiols into disulfides. With O as an oxidant, a mild and efficient
protocol for the selective oxidation of thiols into disulfides including symmetrical and
unsymmetrical ones with 5 mol% of TEPMO as the photocatalyst was developed at room
temperature under the irradiation of 460 nm blue LEDs. It was found that a complex
formed between TEMPO and thiols underpinned the visible light activity and disulfides
were obtained in very high isolated yields. This work suggests that TEMPO takes diverse
roles in opening up the avenue for photocatalytic selective oxidative transformations with
Keywords:
Radical
Visible light
TEMPO
Aerobic oxidation
Disulfides
2
O as the oxidant.
Disulfides are relatively stable products of thiol oxidation and play important roles in determining the secondary, tertiary, and
quaternary structures of proteins [1]. Moreover, disulfides are abundantly existed in the natural products and have wide applications in
the various industrial levels, which is mainly linked with their outstanding redox chemical characteristics [2]. The superiority on redox
sensitivity has made the disulfide bond bridge highlight its vital value in drug delivery program [3]. In the field of physiology, the
disulfide bonds are crucial building scaffolds for the stabilization of the folded form of proteins [4,5] and for the control of enzymes
activity [6]. Moreover, thiols exhibit serious negative effects on human health and natural environment because of their high toxicity and
pollution [7,8]. Thus, the oxidation of thiols is also of environmental significance.
Due to the importance of disulfides in both biological and chemical systems, the synthesis of them received much attention. For its
application on organic synthesis, on one hand, the disulfides are used as the available reagents to realize the sulfenylation of some organic
compounds [9-11] with few negative effects; on the other hand, as the protection group of thiols, disulfides are more stable in some
conversions [12], and they are not foul smelling just as the thiol ones. With the benefits from the above, various methods of synthesis of
disulfides have been reported in the past few decades, but the oxidation of thiols to disulfides is still the most common strategy [13]. In
order to alleviate the negative issue that the thiols were prone to be over oxidized [14,15], extensive types of oxidants, including H
16], halogens and derivatives [17], KMnO /CuSO [18] and sulfoxide [19], had been taken to implement the control oxidation tests. The
target disulfides could be obtained efficiently, but those methods commonly held the main drawback: Toxic oxidant input and waste
output all in stoichiometric amounts. Fortunately, O or aerial O was emerged as the optimal oxidant from the cost-efficient and eco-
friendly perspective and had gained great achievements on this type of transformation.
TiO is one of the most famous semiconductor photocatalysts [20]. However, it needs UV light rather than visible light illumination
2
O
2
[
4
4
2
2
2
to overcome its band gaps. Visible light, served as an easily available energy source to motivate and split organic molecules into highly
active radicals in reactions [21-25], has widely extended its application in laboratory. When it came to the issue of oxidation of thiols to
disulfides, this method has brought significant achievements and turned out to be one of the most promising strategies [26-28]. Apart
—

——
Corresponding author.
E-mail address: xianjunlang@whu.edu.cn (X. Lang).

from the classical approach which fulfils the transformation from thiols to disulfides (Scheme S1, Supporting information),
accompanying by the consumption of reduction species with extra sacrificial reagent, CdSe QDs (quantum dots) exhibited considerable
efficiency under visible light irradiation at room temperature without any additives [29], and the strategy highlighted on releasing
hydrogen as the exclusive byproduct. But actually, more substantial efforts were made toward the themes of exploiting green oxidant O
2
and metal-free catalysis in the photocatalysis category. Recently, rose bengal, an organic dye, had successfully induced the oxidation
coupling of thiols to disulfides with catalytic amounts under white light-emitting diodes (LEDs) at room temperature by using an efficient
micro-flow reaction device [30]. With the similar reaction conditions, the dye Eosin Y could also photocatalyze the transformation with
excellent product yields and scope, but equilmolar of additive tetramethylethylenediamine (TMEDA) was essential to short reaction time
[
31].
In this contribution, we would like to report a more facile and efficient test on the TEMPO-mediated selective oxidation of thiols into
disulfides including symmetrical and unsymmetrical ones. Employing O as the terminal oxidant and TEMPO as the photocatalyst, this
2
protocol was demonstrated with an extraordinarily high efficiency and disulfides could be obtained in high isolate yields (≥ 86%) at room
temperature without any additives, along with excellent reaction selectivity, substrate scope as well as functional groups compatibility.
This work demonstrates the versatile role of TEMPO that could potentially play in visible light-promoted selective organic
transformations.
This research was commenced with our previous two studies regarding the selective aerobic oxidation of sulfides into sulfoxides by
cooperative photocatalysis with TEMPO [32,33]. Beforehand, it was plausible because TEMPO is a very good hydrogen abstraction
agent which can be viewed as a simple extension of our previous studies. To this hypothesis, we employed 4-methylbenzenthiol (1b) as
the probe substrate under conditions quite similar to our previous studies. With alizarin red S-sensitized TiO
2
(ARS-TiO ) as the
2
photocatalyst, selective oxidation of 4-methylbenzenethiol into disulfides proceeded well (entry 1, Table 1). Adding TEMPO could
considerably promote the yield of disulfide (entry 2, Table 1). These results were very promising at the early stage which might validate
our hypothesis. Thereafter, we further our rigorous control experimental process. Quite unexpectedly, without the presence of ARS-TiO ,
2
the corresponding disulfide 2b could yield 44% (detected by gas chromatography) smoothly in methanol (MeOH) after dropping 5 mol%
of TEMPO into the reaction mixture excited by 460 nm blue LEDs (entry 3, Table 1), suggesting TEMPO itself can be a photocatalyst
for this specific reaction. Even though these results indicate that our previous cooperative photocatalysis with TEMPO does not reign for
selective oxidation of thiols, we were quite happy to uncover the hitherto unknown photocatalytic activity of TEMPO. Motivated by this
result, we were pleased to discover that ethanol (EtOH) acted as much better solvent than CH
3
OH and CH CN for this photocatalytic
3
reaction (entries 3-5, Table 1). Moreover, EtOH is a bio-compatible solvent which is also advantageous for the application of this protocol
in bio-related scenario. Metal-free photocatalysis is particularly suitable to bio-friendly conditions [34]. We did more investigations on
the photocatalytic selective oxidation reactions. Concluded from entries 6-8 of Table 1, we confirmed that the visible light, O
TEMPO made up the necessary components for the photocatalytic synthesis of disulfides. Low pressure O (0.1 MPa) for a closed Pyrex
reactor was selected to prevent the emitting of odor of thiols. Note that the reaction can proceed smoothly with aerial O
2
and
2
2
.
Table 1 Control experiments for the visible light photocatalytic selective aerobic oxidation of 4-methylbenzenethiol by TEMPO a.
b
b
Entry
Reaction conditions
ARS-TiO
ARS-TiO , TEMPO
Conv. (%)
Sel. (%)
>99
>99
> 99
> 99
> 99
> 99
> 99
> 99
1
2
3
4
5
6
7
8
2
46
93
44
95
99
19
12
16
2
TEMPO
CH
3
CN, TEMPO
EtOH, TEMPO
Dark, TEMPO
N
2
, TEMPO
No TEMPO
a
Reaction conditions: 4-methylbenzenethiol (0.6 mmol), TEMPO (0.030 mmol), 460 nm blue LEDs (3 W × 4), solvent (1 mL, MeOH for entries 1-3, CH
3
CN for
entry 4, EtOH for entries 5-8), O (0.1 MPa), 25 min.
2

b
Determined by gas chromatography-flame ionization detector (GC-FID) using chlorobenzene as the internal standard, conversion of 4-methylbenzenethiol,
selectivity of the corresponding disulfide.
After the establishment of TEMPO as a photocatalyst for the selective aerobic oxidation of thiols, we subsequently did some additional
screening experiments to make more evident its importance. As such, we evaluated the influence of different kinds and amounts of
TEMPO on the photocatalytic aerobic oxidation reaction which were summarized in Table S1 and Table S2 (Supporting information).
Known from Table S1, TEMPO would be preferred as the photocatalyst from the views of efficiency and commercial cost. From the
results in Table S2, it was reasonable to employ 5 mol% of TEMPO (30 μmol) as the optimized initiator. Finally, we would like to
continue the reaction with optimized conditions: 0.6 mmol of thiol, 5 mol% of TEMPO photocatalyst, 460 nm blue LED irradiation,
EtOH solvent and 0.1 MPa of O .
2
The scope of thiols was further investigated under the optimal reaction conditions. As showed in Table 2, we first examined the aerobic
oxidation of benzenethiol and this transformation was accomplished in 45 min (detected by thin-layer chromatography) with an isolated
yield of 90% (entry 1, Table 2). When the substrates bearing electron donating groups on the para-substituted benzene ring (entries 2-5,
Table 2) were tested, it was found that the reaction rate, according to the reaction time, had a positive correlation with the ability of
electron donation of substrate. Additionally, the ortho-substituted thiol 1g (entry 7, Table 2) needed much longer irradiation time than
the para- and meta-substituted species (entries 5 and 6, Table 2) to accomplish the oxidation coupling. This sluggish performance could
ascribe mainly to its unique steric hindrance. Analogously, the disubstituted 1h (entry 8, Table 2) also suffered a long reaction time.
Subsequently, after some other attempts on substrates bearing electron withdrawing groups (entries 9-12, Table 2), we summarized that
they needed much longer irradiation time than those ones possessing electron donating groups. From a view of electronic effect, it could
be concluded that the electron donating group makes a positive contribution to stabilizing the radical intermediates generated in the
conversion. Conveniently, these attempts showed a tolerance to various functional groups (F, Cl, Br, CF
suitable for the hetero- and poly-aromatic thiols (entries 13 and 14, Table 2).
3
). The reaction strategy was also
Table 2 Visible light photocatalytic selective aerobic oxidative synthesis of disulfides by TEMPO a.
R
SH
S
TEMPO, EtOH
S
2
R
+ 1/2 O2
R
+ H O
2
4
60 nm blue LEDs
b
Entry
1
Substrate
Product
T (min) Yield (%)
SH
S
S
45
25
90
94
1
a
2
a
SH
S
2
3
S
1
b
2
b
Et
SH
S
S
25
30
90
93
Et
1c
Et
2c
t-Bu
SH
S
4
5
S
S
t-Bu
1d
t-Bu
2d
OMe
SH
1e
S
20
92
MeO
MeO
MeO
MeO
2e
SH
S
6
7
S
OMe
30
90
89
1
f
2
f
OMe
OMe
SH
S
S
150
1
g
2g
OMe
OMe
OMe
MeO
MeO
SH
MeO
MeO
S
8
9
S
S
150
30
90
91
1h
2
h
F
SH
S
F
1i
F
2i
Cl
SH
S
1
0
1
S
S
90
105
180
40
97
97
91
94
1
j
Cl
Br
Cl
2j
Br
SH
1k
SH
S
1
Br
2k
CF3
S
1
1
2
3
S
1
l
F3C
F3C
2l
N
SH
N
S
S
N
1
m
2
m

SH
S
1
1
4
5
S
75
60
90
0
1
n
2
n
SH
S
S
1
o
2
o
a
2
Reaction conditions: thiol (0.6 mmol), 460 nm blue LEDs (3 W × 4), EtOH (1 mL), O (0.1 MPa).
Isolated yields of corresponding disulfides, see Supporting information for the NMR and HRMS data.
b
This visible light photocatalytic protocol was inadequate to convert the aliphatic thiol into disulfide (entry 15, Table 2), suggesting
aliphatic thiol should be stable under the standard condition. Opportunity arises for the synthesis of unsymmetrical disulfides. Different
from the symmetrical disulfide, the unsymmetrical one had its inherent challenge in synthesis with high selectivity. But the present
method could get around the drawback and achieve the unsymmetrical disulfide with excellent isolated yield in a more facile and efficient
manner when 3.0 equivalent of thiol 3 was added into the mixture at the initial stage (Scheme 1). Taking the synthesis of product 4b as
a model, this unsymmetrical synthesis occurred smoothly in 3 h irradiation and obtained a 92% isolated yield. This method was tolerated
to various functional groups and influenced slightly by electronic effect (4e, 4j and 4k). Hetero- (4m) and ploy-aromatic (4n) thiols were
also underwent this protocol to disulfides successfully.

a
Scheme 1. Visible light photocatalytic selective aerobic oxidative synthesis of unsymmetrical disulfides by TEMPO .
a
2
Reaction conditions: 0.6 mmol of thiol, 1.8 mmol of 3, 0.030 mmol of TEMPO, 3 W blue LEDs irradiation, 1 mL of EtOH, 0.1 MPa of O . The data below the
disulfide molecules are reaction time (h) and isolated yields respectively, see Supporting information for the NMR and HRMS data.
After gathering the optimized conditions and affirming the generality this visible light photocatalytic protocol, we wanted to elucidate
the underlying mechanistic insights regarding this TEMPO mediated process. For a photocatalytic reaction, it should be wavelength
dependent. Thus, we carried out the selective oxidation reaction with different emitting maximum wavelength (λmax). Table S3
(
Supporting information) shows that 460 nm blue LEDs (the light-emitting spectrum, Fig. S1 in Supporting information) give the best
results, indicating that the absorption of the visible light should be centered on this range.
In appearance, TEMPO is red-orange which means that it can directly absorb visible light around 460 nm range. However, absorption
of TEMPO disappears in EtOH due to solvatochromism phenomena. Therefore, we assumed that there should be other pivotal spices
involved to capture visible light. The trapping sulfur radical by TEMPO accounts for the photocatalytic oxidative synthesis of
symmetrical disulfide. We did some further investigations on the course. During the trial, the optimization of application amount of
TEMPO, we increased the TEMPO to 0.5 equivalent and 0.6 mmol of 4-methylbenzenethiol and 0.3 mmol of TEMPO were combined
in a 10 mL Pyrex vessel with 1 mL of EtOH dissolved. Subsequently, O was injected into the sealed vessel to reach 0.1 MPa of pressure.
2
Then the reaction mixture was magnetically stirred at 1500 rpm and simultaneously illuminated 25 min. We released the reaction pressure
and purified the crude product with column chromatograph (petroleum ether:ethyl acetate = 80:1, v:v), then the target products 2b and
5b (see photocatalytic experiments 3.7, Supporting information) were obtained. Complex 5b formed by 4-methylbenzenethiol and
TEMPO is reddish which can absorb visible light and can be excited by 460 nm blue LEDs for the activation of O . We successfully
2
trapped the radical intermediate 5b in 17% isolated yield, together with the product 2b in 81% yield (Eq. S1, Supporting information).
This observation indicated directly the sulfur-center radical was involved in the visible light-induced process.
Furthermore, kinetic studies were carried out in an effort to understand the mechanism. As for the unsymmetrical aspect, we recorded
the concentrations of main components (1b, 2b, and 4b) by GC during the synthesis of 4b, and summarized as follows (Fig. S2,
Supporting information): The product 2b was afforded sharply at the initial 20 min from 1b, then it suffered gentle consumption until it
was depleted. Whilst the product 4b began to increase dramatically after the mixture was irradiated 20 min, and received an excellent
yield at the end. From the observations, we concluded that: i) The substrate 1b was more prone to transform into 2b at the initial stage;
ii) the intermediate product 2b was not stable in the identical reaction conditions with thiol 3 presence, and it would be forced to transform
into the unsymmetrical disulfide 4b via a radical pathway motivated by visible light. This hypothesis was reinforced by an assistant test,
which implemented in dark and gave a 45% conversion of 1b, 34% yield of 2b, and 11% yield of 4b.
Based on the above evidences, we would like to propose a plausible mechanism for the visible light photocatalytic selective aerobic
oxidation of thiols into disulfides by TEMPO (Scheme 2). First, the key intermediate TEMPO-SR was obtained after the TEMPO attacked
•
•
the substrate thiol, along with the generation of H . The active H was prone to combine with TEMPO to give TEMPOH. For the newly
*
*
and stable TEMPO-SR, it was motivated by visible light to form TEMPO-SR in an excited state. Subsequently, the TEMPO-SR was
•−
treated by thiol and O
cycle could be finished after the TEMPOH combined with O
2
to afford the symmetrical disulfide, superoxide anion radical O
2
and TEMPOH. Then, the photocatalytic reaction
•−
2
to regenerate TEMPO, taking the H as the exclusive byproduct.
2
O
2
Moreover, the visible light photocatalytic synthesis of unsymmetrical disulfide includes two sequential reactions. The selective oxidation
of benzenethiol into symmetrical disulfide occurred beforehand and aliphatic thiol 3 was intact during this stage. Then, the formed
symmetrical disulfide underwent a homolytic cleavage in the presence of TEMPO to react with aliphatic thiol 3, affording the
corresponding unsymmetrical disulfides in the end. Meanwhile, O acts as acceptor for proton and electron.
2

Scheme 2. A plausible mechanism for the selective aerobic oxidation of thiols into disulfides by TEMPO visible light photocatalysis.
In summary, we have serendipitously disclosed a visible light photocatalytic protocol for the oxidation thiols into disulfides including
symmetrical and unsymmetrical ones by employing TEMPO as photocatalyst at room temperature. Besides, EtOH was selected the
solvent and O
process. A complex in situ formed between thiol and TEMPO has been identified to be central in implementing the photocatalytic
activation of O . This reaction strategy had been identified by a wide scope of thiols, and the results showed excellent yields, selectivity
2
was chosen as the terminal oxidant. Importantly, no additives and transition-metals were needed during the reaction
2
and functional groups compatibility.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence
the work reported in this paper.
The authors declare the following financial interests/personal relationships which may be considered as potential
competing interests:
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (Nos. 21773173, 21503086), the Fundamental
Research Funds for the Central Universities (No. 2042018kf0212) and the start-up fund of Wuhan University.
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