Visible-light promoted dithioacetalization of aldehydes with thiols under aerobic and photocatalyst-free conditions

Article abstract of DOI:10.1039/c8gc02237b

A novel photocatalyst-free visible-light-mediated dithioacetalization of aldehydes and thiols has been developed. This protocol is operationally simple, mild and atom-economical, which provides an environmental benign access to dithioacetals at room temperature under aerobic conditions.

Full text of DOI:10.1039/c8gc02237b

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Visible-light promoted dithioacetalization of
aldehydes with thiols under aerobic and
photocatalyst-free conditions†
Cite this: Green Chem., 2018, 20,
5117
Received 19th July 2018,
Zhimin Xing,a Mingyang Yang,a Haiyu Sun,a Zemin Wang,a Peng Chen,a Lin Liu,a
Accepted 14th October 2018
a,b
Xiaolei Wang,*a Xingang Xie *a and Xuegong She
DOI: 10.1039/c8gc02237b
rsc.li/greenchem
A novel photocatalyst-free visible-light-mediated dithioacetaliza- benign synthesis methods and reaction conditions to replace
tion of aldehydes and thiols has been developed. This protocol is the original unfriendly ones. Over the past decade, visible-light
operationally simple, mild and atom-economical, which provides photo(redox) catalysis has emerged as a vital and powerful
an environmental benign access to dithioacetals at room tempera- platform for the development of novel “green” organic reac-
ture under aerobic conditions.
tions with high synthesis efficiency and functional group toler-
ance.12 Very recently, Lei et al. reported that the aldehyde acti-
vation could be achieved via visible light irradiation via an
energy transfer process.13 In this context, they described a
simple and general approach for acetalization of aldehydes
with a catalytic amount of Eosin Y as the optimal photocatalyst
at room temperature under low-energy visible light irradiation.
Inspired by Lei’s work, we speculated that the dithioacetaliza-
tion reaction of aldehydes and thiols could also be promoted
by visible-light irradiation. And due to thiols’ weak acidity,
stronger nucleophilicity and ease of radical generation by
aerobic oxidation, their condensation with aldehydes assisted
by visible light irradiation might be more facile and possibly
Thioacetals are versatile building blocks widely used in the
synthesis of bioactive natural products and pharmaceuticals.1,2
Moreover, thioacetals serve not only as masked acyl anions3
but also as masked methylene functions4 in the construction
of C–C bonds. Hence, considerable efforts have been made
to prepare thioacetals.5 Among the various established
approaches, the condensation reaction between thiols and
aldehydes or ketones is unarguably the most direct one.
However, this type of condensation reaction typically involves
the use of strong Brønsted acids,6 Lewis acids7–9 or high
energy microwave irradiation10 and UV light irradiation11 to
activate the carbonyl group. The harsh reaction conditions not
only affect the functional group tolerance but also make the
work up and product purification process tedious and
difficult. All these disadvantages seriously limited the use of
thioacetals in organic synthesis. Accordingly, it is still in great
urgency and highly desirable nowadays to find mild, cost-
effective condensation reaction conditions between aldehydes
and thiols to prepare thioacetals.
exhibit
a different reaction mechanism. So we recently
initiated a program towards the direct synthesis of 2-aryl-1,3-
diathians from aldehydes and 1,3-dithiol assisted by visible
light irradiation. Herein, we wish to report a facile and mild
protocol for the dithioacetalization reaction of aldehydes
under aerobic and photocatalyst-free conditions through a
visible light photoredox catalysis way (Scheme 1).
Initially, our investigation started by using commercially
available benzaldehyde (1a) and 1,3-propanedithiol (2a) as
model substrates in the presence of photocatalysts under the
irradiation of blue LED light (Table 1). Since a different
mechanism was involved a possible sulphur radical was antici-
pated to work in the dithioacetalization reaction, and all the
reactions were conducted under air. As we expected, when the
reaction was performed in the presence of a series of common
photocatalysts such as Methylene blue, Rhodamine B, Acid
Red 87, Ru(bby)3Cl2, and Eosin Y, the desired product (3a) was
obtained in high yield in all cases (Table 1, entries 1–5). In
entries 2 and 3, a minor side product was also obtained at the
same time. It was deduced to be toluene by a later GC-MS ana-
lysis of the corresponding crude reaction products which
The sustainable development of human society requires
chemists to reduce the environmental impact of chemical
industrial process as much as possible which led to the birth
of green chemistry. And one of the most important tasks of
green chemistry is the development of new environmentally
aState Key Laboratory of Applied Organic Chemistry, Department of Chemistry,
Lanzhou University, Lanzhou 730000, People’s Republic of China.
E-mail: wangxiaolei@lzu.edu.cn, xiexg@lzu.edu.cn
bCollaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Tianjin, 30071, China
†Electronic supplementary information (ESI) available. CCDC 1850380. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c8gc02237b
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Table 2 Results for visible-light-induced dithioacetalization of 1,3-pro-
panedithiol with different aryl aldehydesa,b
Scheme 1 Visible-light-promoted direct synthesis of thioacetals from
aldehydes and thiols.
might be the reduction product of 1a by thiol 2a. To our
delight, in a subsequent control experiment to show the
essence of the photocatalyst (Table 1, entry 6), the conden-
sation reaction of 1a and 2a went smoothly to afford the
desired product 3a in 98% yield in the absence of any photo-
catalyst and no reduction side product was detected at this
time. It was then assumed that the slow generation of sulphur
radical without the photocatalyst increased the chemo-
selectivity of this reaction and decreased the amount of
reduced side products. So the yield is a little high without the
photocatalyst compared to that with the photocatalyst
(Table 1, entry 6 vs. 2 or 3). A further control experiment
without the photocatalyst and LED light irradiation only recov-
ered the two starting materials (Table 1, entry 7). With the opti-
mized conditions in hand, we sought to examine the scope
and generality of the method (Table 2). In general, both elec-
tron donating and electron-withdrawing substituent attached
aromatic aldehydes provided the desired 2-aryl substituted 1,3-
dithians in high yields. It should be noted that a wide range of
functional groups such as i-Pr (3d), OMe (3b, 3n, 3p), SMe (3c),
OH (3m, 3s, 3t), F (3f, 3q, 3r), Cl (3g), Br (3h, 3o–3r, 3u, 3v), I
(3i), CN (3k), CF3 (3j) and CO2Me (3l) were compatible with
a Reaction conditions: 1 (0.5 mmol), 2a (0.55 mmol) in MeCN (1.5 mL)
irradiation using 6 W blue LEDs under air at room temperature for
24 h. b Isolated yield.
Table 1 Optimization of the reaction conditionsa,b
this reaction, providing the possibility for further transform-
ation. Neither the electron-rich groups nor electron-poor ones
located at the para or ortho position of the aryl aldehyde had
significant deleterious effects (3b–3s). Heteroaryl aldehydes
such as 3-bromo-2-furaldehyde (1u), 4-bromo-2-thiophenyl
aldehyde (1v) and 1H-indole-3-carbaldehyde (1w) were well tol-
erated under the reaction conditions and readily underwent
dithioacetalization reactions to afford the corresponding 1,3-
dithians.
Next, we turned our attention to explore the reactivity of
other aldehydes and ketones in this process. To our delight,
aliphatic, α,β-unsaturated aldehydes and cyclohexanone were
all good substrates for this visible light promoted dithioacetali-
zation reaction. As shown in Table 3, cyclohexanecarbaldehyde
Entry
Photocatalyst
Yieldb (%)
1
2
3
4
Methylene blue
Rhodamine B
Acid Red 87
Ru(bby)3Cl2
Eosin Y
99
97
93
99
99
98
n.d.
5
6c
7d
—
—
a Reaction conditions: 1a (0.5 mmol), 2a (0.55 mmol), photocatalyst
(3 mol%) in MeCN (1.5 mL) irradiation using 6 W blue LEDs under air
at room temperature for 3 h. b Isolated yield. c Without photocatalyst,
24 h. d Without photocatalyst and LED irradiation, 24 h.
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Table 3 Results for visible-light-induced dithioacetalization of 1,3-pro-
panedithiol with different alkyl aldehydesa,b
Table 4 Results for visible-light-induced dithioacetalization of
different thiols with diverse aldehydesa,b,d
a Reaction conditions: 4 (0.5 mmol), 2a (0.55 mmol) in MeCN (1.5 mL)
irradiation using 6 W blue LEDs under air at room temperature for
24 h. b Isolated yield.
a Reaction conditions: 6 (0.5 mmol), 2 (0.55 mmol) in MeCN (1.5 mL)
irradiation using 6 W blue LEDs under air at room temperature for
24–48 h. b Isolated yield. c Overall yield. d The reaction was monitored
by TLC.
(5c), 3-phenylpropyl aldehyde (5e) and n-heptaldehyde (5f) all
afforded the desired dithiane products in 94% yields. Even
steric hindered aliphatic aldehydes which contained an i-Pr or
a t-Bu group at their α-carbons also gave the corresponding
products in modest yields (5g and 5h). Nevertheless, this reac-
tion could also be applied to phenylpropiolaldehyde and pro-
vided the desired dithiane (5a) in 34% yield. Adkins and
Hartung reported that extended conjugation results in a lower
reactivity for acetalization.14 Our experiment results indicated
that phenylpropiolaldehyde reacted significantly more slowly
than phenylpropyl aldehyde and it was in agreement with their
report. In addition, cyclic substituents attached at their α posi-
tion of the aldehydes did not significantly affect the reaction,
furnishing the desired products (5b–5d) in excellent yields. In
particular, dithioacetalization of cyclohexanone proceeded
smoothly to generate the corresponding product (5i) in 89%
yield; however cycloheptanone did not react at all and no 5j
was obtained. These results were in agreement with those
from the acid-catalyzed acetalization of carbonyl compounds
reported by Brown15 and Djerassi.16 In both studies it was
observed that cycloheptanone reacted more slowly than
cyclohexanone.
2,3-butanedithiol also reacted well to give the corresponding
2,4,5-tri-substituted 1,3-dithiolanes (7g–7k) in expedient yields
as a mixture of multiple diastereomers in which the 2,4,5-cis
product was the major ones and its structure was confirmed by
NMR spectroscopy data and X-ray single crystallographic
analysis.17
To demonstrate the applicable potency of our transform-
ation, a multi-gram scale preparation experiment was then per-
formed (Scheme 2). When a solution of 1a (1.00 g) and 2a
(1.09 g) in MeCN was irradiated by 6 W blue LEDs in an open
atmosphere for 24 h, the desired condensation product 3a was
obtained in 94% isolated yield (Scheme 2, a). Similarly, when
the reaction was scaled up to 61 mmol (6.47 g 1a was used), it
proceeded smoothly without a notable change in terms of the
isolated yield (Scheme 2, b). To our delight, when the visible-
light promoted dithioacetalization reaction was performed at
61 mmol scale without solvents (neat), 3a was obtained in 87%
Finally, various thios were screened for this visible light
promoted dithioacetalization reaction. As illustrated in
Table 4, 1,2-ethanedithiol was firstly proved to be another suit-
able sulphur source (7a–7f). 2-Aryl- (7a–7d), 2-thiophenyl- (7e)
and aliphatic- (7f) 1,3-dithiolanes were all obtained in modest
to high yields. And the commercial available diastereomeric Scheme 2 Multi-gram scale reaction.
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yield, which was only slightly less efficient than the standard
reaction conditions (Scheme 2, c vs. b). For the above multi-
gram preparation experiments, the obtained 2-phenyl-1,3-dia-
thine 3a could be easily purified by recrystallization from
ethanol which made the work up and purification process
greener and more environmentally friendly.18
In order to clarify the reaction mechanism, several control
experiments were conducted whose results are shown in
Scheme 3. When the common radical trapping reagent,
2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO, 3.0 equiv.) was
added into the reaction system, no 3a was obtained and a
cross coupling product containing TEMPO and 1,3-propane-
dithiol fragments was isolated, confirmed by HRMS
(Scheme 3, a). The above result indicated that a sulphur
radical intermediate might be involved in this transformation
which was different from Lei’s work.13 A previous experiment
in the absence of light (Table 1, entry 7) indicated that light
irradiation was essential. And a further experiment without air
(under N2, Scheme 3, b) suggested that O2 in air might react as
oxidants to generate the sulphur radical in this dithioacetaliza-
tion reaction. In addition, when the reaction was performed in
the presence of Na2CO3, none of 3a was detected which indi-
cated that the weak acidity of thiols also played a vital role in
the condensation reactions (Scheme 3, c).
Fig. 1 (a) EPR spectra of a mixture of 1a (0.2 mmol), 2a (0.22 mmol)
and DMPO (0.22 mmol) under air in MeCN (0.6 mL) with blue LED
irradiation at room temperature for 10 min. (b) EPR spectra of a solution
of 2a (0.2 mmol) and DMPO (0.22 mmol) in MeCN (0.6 mL) under air
with blue LED irradiation at room temperature for 10 min.
of 2a was formed independently by the single electron transfer
oxidation in air under optimized conditions.
On the basis of the above results and the related litera-
ture,20 a possible mechanism was thus proposed as shown in
Scheme 4. Initially, 1,3-propanedithiol (2a) was partially con-
verted to thiol anion Int 1 and H+ followed by the single elec-
tron oxidation of lnt 1 by O2 in air with the assistance of blue
LED light to form the thiyl radical lnt 2. Subsequently, the
addition of thiyl radical lnt 2 to the CvO bond of proton-acti-
vated aldehyde Int 3 would lead to radical cation lnt 4. There
are two possible ways to transform lnt 4 into the final product
3. As shown in path a, lnt 4 underwent an unusual 1,7-H shift
process to generate radical cation lnt 5, followed by an intra-
molecular cyclization to afford sulphur radical cation lnt 6.
Finally, Int 6 was reduced to 2-R-1,3-dithian 3 by radical anion
O2−. In path b, lnt 4 was directly reduced to hemiacetal Int 7
According to the related literature,19 the existence of sulfur
free radical in the system can be indirectly proved by the
addition of a free-radical spin-trapping agent DMPO (5,5-
dimethyl-1-pyrroline N-oxide). Subsequently, such EPR experi-
ments were performed to get some insights into the reaction
mechanism. To our delight, as shown in Fig. 1a, when DMPO
was added to the reaction of 1a and 2a in MeCN under air with
blue LED irradiation for 10 min, a similar distinct EPR signal
was identified. The parameters observed for the spin adduct
were g = 2.0075, aN = 13.43 G and aH = 13.72 G which were very
close to the values reported in the literature.19d So this radical
signal was an indirect proof of the existence of sulphur rad-
icals in our reaction system. When DMPO was added to the
solution of 2a in MeCN under air with blue LED irradiation for
10 min, the same strong signal peak was also observed
(Fig. 1b). The above result suggested that the sulphur radical
•−
by radical anion O2 which was then transformed to 2-R-1,3-
dithian 3 by the loss of a molecular H2O and a subsequent
cyclization reaction.
Scheme 3 Control experiments.
Scheme 4 Plausible mechanism.
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Conclusions
In conclusion, we have, for the first time developed a simple
and convenient visible-light-induced strategy for the dithioace-
talization of aldehydes with thiols using air as the environmen-
tally begin oxidant. The developed synthesis protocol proceeds
with the merits of mild conditions, broad substrate scope,
operation simplicity, high atom efficiency, eco-energy source,
green solvent, metal-free photocatalysis and ambient con-
ditions. Further studies into the detailed mechanism of this
process as well as the synthesis applications are still ongoing
in our laboratory.
Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
This work was supported by the National Science Foundation
of China (21472079, 21572088), and the Fundamental
Research Funds for the Central Universities (lzujbky-2017-91).
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