Photocatalytic aerobic α-thiolation/annulation of carbonyls with mercaptobenzimidazoles

Article abstract of DOI:10.1039/c9ob00625g

A concise aerobic photocatalysis using a blue LED combined with a Lewis acid has been developed to enable α-thiolation/annulation of carbonyls. Inexpensive, nontoxic Rose Bengal was demonstrated as the best catalyst. Hence, this transition-metal-free protocol allows mild Csp³-S couplings with both ketones and aliphatic aldehydes with a range of compatible useful functionalities.

Full text of DOI:10.1039/c9ob00625g

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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
benzene
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DOI: 10.1039/C9OB00625G
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Photocatalytical aerobic α-thiolation/annulation of carbonyls with
mercaptobenzimidazoles
Received 00th January 20xx,
Accepted 00th January 20xx
Xiaochen Ji,a* Muyun Tan,a Mei Fu,a Guo-Jun Denga and Huawen Huanga*
DOI: 10.1039/x0xx00000x
www.rsc.org/chemcomm
A concise aerobic photocatalysis by blue LED combined
with Lewis acid has been developed to enable α-
thiolation/annulation of carbonyls. Inexpensive, nontoxic
Rose Bengal was demonstrated as the best catalyst. Hence,
this transition-metal-free protocol allows mild Csp3 − S
couplings with both ketones and aliphatic aldehydes with a
range of useful functionalities compatible.
formed by the initiation of single electron oxidation or pyrolysis of
X–S bonds (Figure 1d).10 Very recently, Wang and co-workers
described a one-pot, two-step synthesis of pharmacologically
important pyridine-2-ylmethyl thioethers using 2-picoline N-
oxides as an internal oxidant.11
O
R
I2 (10 mol%)
O
O
DTBP (3.0 equiv)
O
Ar
(a)
(b)
(c)
+
+
HSAr
HSAr
S
EtOAc, 120 oC, 12 h
R
R
Organosulfur compounds widely exist in pharmaceuticals,
naturally occurring products, and advanced functional materials.1
Hence, the development of C–S bond formation reactions has been
receiving much interest in molecule synthesis.2 Beyond the well-
established nucleophilic substitution of organohalides with
mercaptans and thiophenols, recently disclosed oxidative C–H/S–H
couplings represent a straightforward strategy to construct C–S
bonds, which therein shows advantages with respect to reaction
convergence and productivity. Furthermore, electrocatalysis3 and
photocatalysis4 found effective to this event, featuring easy
manipulation, mild reaction conditions, and environmental benign
byproducts. For example, Lei5 and Guo6 have independently
revealed dehydrogenative C–H/S–H cross-coupling of electron-rich
arenes with aryl/heteroaryl thiols through catalyst- and oxidant-free
electrochemistry and metal-free aerobic photocatalysis, respectively.
Regarding oxidative Csp3–S bond formation, in 2015, Lei et al.
R
H
NIS (30 mol%)
DTBP (4.0 equiv)
HO
HO
Ar
H
S
120 oC, 12 h
acridine red (2 mol%)
TBHP (4.0 equiv)
O
H
O
X
S
Ar
+
ArSSAr
green LED, rt
X
Y
R'
radical initiator
R
R'
(d)
+
XSR
S
R
difunctionalization
R
n
HO
H
N
PC, Lewis acid
blue LED
O2 (balloon)
S
O
H
N
N
or
(e)
+
SH
this work
H
N
N
N
n
S
O
reported
a iodine-catalyzed oxidative coupling between 1,3-
diketones and thiophenols to form β-dicarbonyl thioethers (Figure
1a).7 Soon later, the same group developed an oxidative β-Csp3–H
n
t
thiolation of BuOH (Figure 1b).8 In both cases, di-t-butyl peroxide
Figure 1 Oxidative Csp3–S bond formation.
(DTBP) was used as the oxidant. In 2016, Wang and co-workers
described a protocol to access α-arylthioethers via visible-light-
induced direct α-Csp3–H thiolation of ethers with diaryl disulfides
with acridine red as the efficient photocatalyst (Figure 1c).9 In
reaction complementary to these works, radical addition-involved
difunctionalization of alkenes also provides viable access to Csp3–S
bond formation, in which sulfur radicals were generally in-situ
In spite of these utilities, the mild and sustainable Csp3–S bond
formation and related applications are still highly desirable. Within
our own program on green chemistry with molecular oxygen,12
herein, we have revealed a photocatalytic aerobic Csp3–H/S–H
coupling of mercaptobenzimidazole with carbonyls triggered by
visible light (Figure 1e). Thus, salient features of this protocol
include (a) inexpensive, nontoxic Rose Bengal as the catalyst, (b)
easily available dioxygen as the green oxidant, (c) Csp3−S couplings
with both ketones and challenging aliphatic aldehydes, and (d) cyclic
a
Key Laboratory for Green Organic Synthesis and Application of Hunan Province, Key
Laboratory of Environmentally Friendly Chemistry and Application of Ministry of
Education, College of Chemistry, Xiangtan University, Xiangtan 411105, China. E-mail:
hwhuang@xtu.edu.cn; xcji@xtu.edu.cn.
This journal is © The Royal Society of Chemistry 20xx
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ChemComm
f
N2 atmosphere. e BPO (1 equiv) was used under N2 atmosphere. In
products via a formal oxidative [3+2] cycloaddition accessed in most
cases.
View Article Online
dark. g Performed at 60 °C in dark. h Under N atmosphere.
DOI: 10.1039/C9OB00625G
2
To commence our studies, 1H-benzo[d]imidazole-2-thiol (1a) and
cyclohexanone (2a) were utilized as the model substrates (Table 1).
While stoichiometric amounts of AlCl3 were used as a Lewis acid to
activate ketone 2a, photocatalysts such as Ru(bpy)3Cl2·6H2O, fac-
Ir(ppy)3, Eosin Y, and Rose Bengal (RB) combined with O2 found
all effective to trigger the oxidative Csp3−S bond formation (entries
1–4). Therein, cyclic product 3a via formal oxidative [3+2]
cycloaddition was generated in moderate yields, and the
diastereoselectivity of 3a was determined by comparison with
literature and nOe spectrum (See ESI for details). Among those
photocatalysts tested, Rose Bengal showed the most active for this
reaction (entry 4). Then, other Lewis acids such as Sc(OTf)3 and
Y(OTf)3 and Brønsted acid CF3COOH (TFA) instead of AlCl3
decreased the yield to some extent (entries 5–7). Solvent screening
revealed that 1,4-dioxane (entry 8) and THF (entry 9) slightly
lowered the yield to 48% and 42%, respectively, while DMF
completely quenched the transformation (entry 10). We also tested
some other oxidants including previously established peroxides and
found that among them only Na2S2O8 featured the same efficiency
(entries 11–13). Control experiments showed that photocatalyst,
light, Lewis acid, and O2 were all essential to this reaction (entries
14–18).
With the optimized reaction conditions in hand, the substrate
scope was explored (Table 2). Regarding 4-substituted
cyclohexanones, the corresponding cyclic products 3b-3f were
furnished in modest yields with a mixture of two diastereomers (dr =
1.5:1 to 4:1). Notably, most of those products with poor
diastereoselectivity could not be separated by column
chromatography. We only got pure diastereomers of 3b and the
structure of cis-3b was determined by nOe spectrum. Synthetically
useful ester group was well tolerated in this acidic system. To our
delight, the reaction with 2-methylcyclohexanone featured good
reactivity and excellent regioselectivity, in which 3g was exclusively
formed in 52% yield. For this formal oxidative [3+2] cycloaddition,
β-tetralone also gave cyclic product (3h) with exclusive
regioselectivity.
Other
effective
ketones
included
tetrahydropyranone and cyclopentanone, both of which afforded
moderate yields of products. The reaction of 5,6-dimethyl-1H-
benzo[d]imidazole-2-thiol and 2-methylcyclohexanone formed 3k in
40% yield. Both of 5-methyl- and 5-chloro-1H-benzo[d]imidazole-2-
thiol delivered a 1:1 mixture of two regio-isomers. For acyclic
aliphatic ketones, the corresponding products were generally formed
in lower yields. For example, 3-methylbutan-2-one afforded 3n as
the major product in only 29% yield.
Table 1 Optimization of the reaction conditionsa
Table 2 Aerobic α-thiolation of ketones.
n
O
H
N
cat. (1 mol%),
additive (1 equiv)
HO
O
H
RB (1 mol%), AlCl3 (1 equiv)
R1
SH
+
HO
H
N
N
R1
S
+
H
N
N
SH
n
CH3CN, O2, rt, 48 h
blue LED
S
O2 (balloon)
solvent, rt, 48 h
blue LED
N
N
N
2
1
3
1a
2a
3a
R
HO
3b
(R=Me) 54% (dr 1.8:1)
(R=n-pentyl) 58% (dr 2.3:1)
(R=t-Bu) 55% (dr 4:1)
(R=Ph) 42% (dr 1.5:1)
(R=CO2Et) 49% (dr 1.5:1)
HO
entry catalyst
Lewis acid
solvent
yield
(%)b
42
45
41
58 (53)
19
40
25
48
42
0
22
55
33
0
0
0
HO
H
3c
3d
3e
3f
N
Me
N
H
N
S
S
S
N
N
N
1
2
3
4
5
6
7
8
9
10
[Ru]
[Ir]
Eosin Y
RB
RB
RB
RB
RB
RB
RB
AlCl3
AlCl3
AlCl3
AlCl3
TFA
Sc(OTf)3
Y(OTf)3
AlCl3
AlCl3
AlCl3
AlCl3
AlCl3
AlCl3
AlCl3
AlCl3
-
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
1,4-dioxane
THF
DMF
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
3a
53%
3g
52%
O
HO
HO
HO
OCH3
HO
H
H
N
Me
Me
N
Me
N
H
S
N
S
S
S
N
N
N
N
3h
49%
3i
40%
3k
40%
3j
45%
S
Me
HO
HO
HO
Me
Me
(Cl)H
(H)Cl
3l 3l'
Me
(Me)H
(H)Me
Me
N
N
S
N
S
N
N
11c RB
12d RB
13e RB
N
+
59% (1:1)
3m 3m'
+ 56% (1:1)
3n
29%
14f
15g RB
16 RB
17h RB
18
RB
Unexpectedly, two-fold cycloaddition was not happened when
[1,1'-bi(cyclohexane)]-4,4'-dione (2k) was subjected to present
aerobic photocatalysis even with excessive 1a. In this case, product
3o was furnished in 34% yield [Eq. (1)]. Furthermore, α-tetralone
AlCl3
AlCl3
trace
trace
-
a Reaction condition: 1a (0.3 mmol), 2a (0.3 mmol), cat. (1 mol%), [Eq. (2)] and cyclooctanone [Eq. (3)] gave only α-C-H thiolation
Lewis acid (1 equiv), solvent (3 mL), 7 W blue LED, O2 balloon, 48
products 3p and 3q, respectively, while propiophenone and
cycloheptanone generated a mixture of cyclic and acyclic products
and acetophenone did not work in present system.
b
h. [Ru] = Ru(bpy)3Cl2·6H2O; [Ir] = fac-Ir(ppy)3. Yields were
determined from the crude 1H NMR spectra using CH2Br2 as an
c
internal standard. Isolated yield in parentheses. TBHP (1 equiv)
was used under N2 atmosphere. d Na2S2O8 (1 equiv) was used under
2 | J. Name., 2013, 00, 1-3
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O
acyclic products were also observed in some cases. VHiewowAretivcleerO,ntlihnee
DOI: 10.1039/C9OB00625G
robust nature of the present aerobically photocatalytic system was
O
H
N
standard
conditions
further reflected by the effective gram-scale preparation, which gave
an acceptable productivity (46%, Figure 2a). Notably, the resultant
fused hydroxyl thiazoline 3a could be easily transformed to
tetrahydrobenzo[d]benzo[4,5] imidazo[2,1-b]thiazole 6a in an
almost quantitative yield in acidic systems (Figure 2b). Moreover,
hydroxyl thiazoline 5g was readily oxidized to amide product 6b by
pyridinium chlorochromate (PCC) (Figure 2c).
HO
SH
(1)
+
N
H
N
S
O
N
1a
2k
3o 34% (dr 1:1)
(3.0 equiv)
O
standard
conditions
H
N
H
N
SH
+
(2)
(3)
N
S
O
N
1a
2l
3p
47%
O
standard
conditions
H
N
H
N
O
HO
RB (1 mol%), AlCl3 (1 equiv)
H
N
Me
Me
SH
+
N
SH
(a)
+
S
N
N
CH3CN, O2, rt, 84 h
blue LED
S
O
N
N
1a
2m
3q
3f
46% (0.96 g)
48%
1a
(8 mmol)
Besides cyclic ketones, aliphatic aldehydes found also effective to
this aerobically photocatalytic Csp3-S coupling/cycloaddition (Table
3). For example, propionaldehyde generated 5a in 50% yield with
1.5:1 dr value, while decanal afforded 5b with similar reactivity and
selectivity. Alkylchloride was well accommodated with this system
when 5-chloropentanal was employed. Excellent stereoselectivity
was observed when 3-mercaptopropanal was used, in which only cis
isomer was detected probably due to the hydrogen bonding effect.
Finally, the α-branched aldehydes worked all well, giving the
corresponding products 5e–5i in moderate yields.
H+
HO
N
N
(b)
H
S
S
H2O, reflux, 3 h
N
N
3a
6a
99%
HO
N
O
N
Me
Me
Me
Me
PCC (5 equiv)
CH2Cl2, rt, 48 h
(c)
S
S
N
N
6b
5g
60%
Figure 2. Applications of the aerobic α-thiolation.
Table 3 Aerobic α-thiolation of aldehydes.
To determine the reaction mechanism for the visible-light
triggered aerobic C-S bond formation, some control experiments
were performed (Figure 3a). The addition of radical scavengers such
as 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), 3,5-di-tert-butyl-
4-hydroxytoluene (BHT), and 1,1-diphenylethene dramatically
reduced the product yields, which suggests a radical pathway is
taking place. Furthermore, when the singlet oxygen quencher 1,4-
diazabicyclo[2.2.2]octane (DABCO) was added to the reaction
mixture, the yield of 3a decreased to 23%. In addition, fluorescence
quenching experiments were also investigated to prove an energy
transfer process between mercaptobenzimidazole (1a) and Rose
Bengal (Stern–Volmer studies) (Figure 3b). Indeed, it was found that
the emission intensity of excited Rose Bengal (RB*) was
dramatically diminished along with the increasing of the amount of
1a. In contrast, no such effect was observed when cyclohexanone 2a
was added separately. Thus, the interaction of 1a and photocatalyst
should be the initial step.
R2
HO
H
N
R3
R2
RB (1 mol%), AlCl3 (1 equiv)
CHO
R1
N
N
SH
Me
+
S
R1
R3
N
CH3CN, O2, rt, 48 h
blue LED
1
4
5
HO
HO
HO
Cl
Me
N
N
N
N
N
S
S
S
N
5a
50%
(dr 1.5:1)
5b
45%
5c
41%
(dr 1.5:1)
(dr 1.2:1)
HS
HO
N
HO
HO
Me
Me
N
N
N
N
S
S
S
N
5e
5f
52%
5d
45%
70%
(dr >20:1)
(dr 1:1)
HO
HO
HO
Me
Me
Me
Me
Me
Me
F
F
Me
Me
N
N
N
N
N
N
S
S
S
5g
47%
5h
43%
5i
41%
As shown above, the oxidative annulation products 3 and 5 were
generally obtained in moderate yields. Actually,
mercaptobenzimidazole was not stable in present photocatalytic
system, and decomposition of it was observed. Minor amounts of
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(a)
couplings of Csp3–S bonds. This protocol provides a VnieowveAlrtieclnetOrynlintoe
DOI: 10.1039/C9OB00625G
oxidative formal [3+2] annulation, which allows smooth
participation of a broad range of carbonyls including cycloketones
and aliphatic aldehydes. The application of this method was further
demonstrated by the effective gram-scale reaction and readily viable
late-stage modification of the resultant adducts.
H
N
O
HO
RB (1 mol%), AlCl3 (1 equiv)
H
N
SH
+
S
N
CH3CN, O2, rt, 48 h
additive (2.0 equiv)
N
3a
1a
2a
TEMPO:
BHT:
12%
32%
0
1,1-diphenylethene:
DABCO:
23%
ACKNOWLEDGMENT
(b)
This work was supported by the National Natural Science
Foundation of China (21502161, 21602187), the Collaborative
Innovation Center of New Chemical Technologies for
Environmental Benignity and Efficient Resource Utilization,
Scientific Research Fund of Hunan Provincial Education Department
(16B251), and Hunan Provincial Natural Science Foundation of
China (2017JJ3299).
Notes and references
1
(a) A. Dondoni, Angew. Chem. Int. Ed., 2008, 47, 8995-8997; (b) D. A.
Boyd, Angew. Chem. Int. Ed., 2016, 55, 15486-15502.
Figure 3. Key mechanistic findings. (a) Radical trapping reactions;
(b) Fluorescence quenching experiments (Stern–Volmer studies).
2
3
(a) T. B. Nguyen, Adv. Synth. Catal. , 2017, 359, 1066–1130; (b) T.
Chivers and P. J. Elder, Chem. Soc. Rev., 2013, 42, 5996-6005; (c) H. Liu
and X. Jiang, Chem. Asian. J., 2013, 8, 2546-2563.
On the basis of the experimental results and previous reports,4g, 10b,
a plausible reaction mechanism was proposed (Figure 4).
(a) M. Yan, Y. Kawamata and P. S. Baran, Angew. Chem. Int. Ed., 2018,
57, 4149-4155; (b) S. Mohle, M. Zirbes, E. Rodrigo, T. Gieshoff, A.
Wiebe and S. R. Waldvogel, Angew. Chem. Int. Ed., 2018, 57, 6018-6041;
(c) S. Tang, Y. Liu and A. Lei, Chem, 2018, 4, 27-45; (d) S. R.
Waldvogel, S. Lips, M. Selt, B. Riehl and C. J. Kampf, Chem. Rev., 2018,
118, 6706-6765; (e) G. Zhang, C. Liu, H. Yi, Q. Meng, C. Bian, H. Chen,
J. X. Jian, L. Z. Wu and A. Lei, J. Am. Chem. Soc., 2015, 137, 9273-9280.
(a) T. P. Yoon, M. A. Ischay and J. Du, Nat. Chem., 2010, 2, 527-532; (b)
J. Xuan and W. J. Xiao, Angew. Chem., Int. Ed., 2012, 51, 6828-6838; (c)
C. K. Prier, D. A. Rankic and D. W. MacMillan, Chem. Rev., 2013, 113,
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Soc. Rev., 2016, 45, 6165-6212; (e) D. C. Fabry and M. Rueping, Acc.
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and A. Lei, Asian J. Org. Chem., 2017, 6, 386-396; (g) W. Guo, K. Tao,
W. Tan, M. Zhao, L. Zheng and X. Fan, Org. Chem. Front., 2019,
10.1039/c8qo01353e.
10d
Initially, Rose Bengal (RB) was converted into the excited RB*
under visible-light irradiation. Then, a single electron transfer (SET)
from thiol 1a to RB* afforded the thiyl radical ion A, which
underwent a deprotonation to form radical B. Subsequently, the
addition of thiyl radical B to enol C, in situ activated by Lewis acid,
led to the formation of alkyl radical D. Furthermore, a SET process
and deprotonation occurred again to give sulfide intermediate E,
which underwent nucleophilic annulations and workup to afford the
final product 3a. The reductive photocatalyst RB• would transfer to
catalytically active catalyst via SET with O2 and light stimulation.
4
LA
O
O
LA
- H+
2a
C
5
6
7
P. Wang, S. Tang, P. Huang and A. Lei, Angew. Chem. Int. Ed., 2017, 56,
3009-3013.
H
N
H
N
H
N
HO2
S
O
LA
S
O
O
LA
LA
S
SET
-H2O2
N
W. Guo, W. Tan, M. Zhao, K. Tao, L.-Y. Zheng, Y. Wu, D. Chen and X.-
L. Fan, RSC Adv., 2017, 7, 37739-37742.
N
N
E
D
B
HO2
O2
H+
H. Cao, J. Yuan, C. Liu, X. Hu and A. Lei, RSC Adv., 2015, 5, 41493-
41496.
H
N
H
N
SH
8
9
Y. Li, D. Liu, C. Liu and A. Lei, Chem. Asian J., 2016, 11, 2246-2249.
X. Zhu, X. Xie, P. Li, J. Guo and L. Wang, Org. Lett., 2016, 18, 1546-
1549.
S
hv
N
RB*
SET
RB
N
A
F
RB
- LA
H
O2
N
10 (a) Q. Lu, H. Wang, P. Peng, C. Liu, Z. Huang, Y. Luo and A. Lei, Org.
Chem. Front., 2015, 2, 908-912; (b) S. Tang, K. Liu, C. Liu and A. Lei,
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Yang, Y. Zhang, H. Zhao, L. Wang and H. Wang, Green Chem., 2017, 19,
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SH
3a
N
O2
1a
Figure 4 Possible reaction mechanism.
In summary, a photocatalytically aerobic reaction system with
visible light has been developed for the efficient dehydrogenative
4 | J. Name., 2013, 00, 1-3
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11 D. Wang, Z. Liu, Z. Wang, X. Ma and P. Yu, Green Chem., 2019, 21,
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Organic & Biomolecular Chemistry
Page 6 of 6
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DOI: 10.1039/C9OB00625G
H
N
N
N
S
S
n
SH
PC, Lewis acid
blue LED
N
HO
H
N
or
O
S
O
Me
Me
O2 (balloon)
N
N
N
n
A concise aerobic photocatalysis by blue LED combined with a Lewis acid has been
developed to enable α-thiolation/annulation of carbonyls.