Dioxygen-triggered oxidative cleavage of the C-S bond towards C-N bond formation

Article abstract of DOI:10.1039/c9cc05707b

Research on the cleavage of C-C bonds has been well developed. By comparison with this, the activation of C-S bonds remains challenging. Herein, dioxygen-triggered oxidative cleavage of C-S bonds has been achieved, delivering a series of N-containing heterocyclic compounds that are frequently found in pesticides and pharmaceuticals. Additionally, the potential utility of this protocol was further demonstrated by a gram-scale experiment. Mechanistically, dioxygen plays a key role in the cleavage of C-S bonds towards C-N bond formation.

Full text of DOI:10.1039/c9cc05707b

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Dioxygen-Triggered Oxidative Cleavage of C-S Bond towards C-N
Bond Formation
Zongchao Lv,a,† Huamin Wang,a,† Zhicong Quan,a Yuan Gao,a and Aiwen Lei*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The research on the cleavage of C-C bond has been well developed.
Thiazoles are important groups of heterocyclic compounds
By comparision with this, the activation of C-S bond remains due to their drug utility,9 and a number of biologically and
challenging. Herein, dioxygen-triggered oxidative cleavage of C-S physiologically active compounds with bactericidal, fungicidal,
bond has been achieved, delivering a series of N-containing tuberculostatic, and anti-inflammatory properties have been
heterocyclic compounds which are frequently found in pesticides synthesized based on them.10 Among, benzothiazoles with a
and pharmaceuticals. Additionally, the potential ultility of this nitrogen substituent have frequently been used as core
protocol was further demonstrated by gram-scale experiment. structures for the development of pharmaceutical reagents.11
Mechanistically, dioxygen plays a key role in the cleavage of C-S Considering the synthetic value, developing mild and
bond towards C-N bond formation.
sustainable strategies for the production of various nitrogen-
containing heterocyclic compounds is worthwhile. The
substitutions of 2-halobenzothiazoles or 2-thiobenzothiazoles
with amine have been successfully developed to provide 2-N-
substituted benzothiazoles, which usually require several steps
or have a limitation of substrate scope.12 Additionally, several
approaches based on metal catalysis have also been developed.
The highly discriminating activation and transformation of an
inert chemical bond are among the most important processes
throughout the chemistry. To date, transition-metal-catalyzed
the C−C bond cleavage have been well developed.1 In contrast,
the development of the cleavage of C–S bond, which are
important structural motifs in natural products, is relatively
late.2 Over the past several decades, with more and more
attention in the cross-coupling reaction, the C–S bond cleavage
and transformation have been explored from the standpoint of
diverse synthetic, catalytic, bioorganic, bioinorganic, and
theoretical chemistry.3 Thus, organosulfur compounds have
shown increasing importance in the construction of new
chemical bonds as coupling partners or building blocks.4
In this section of C–S bond activation, the most developed one
is the C−S bond insertion by a transition-metal, such as
palladium, ruthenium, rhodium, iridium, etc.5 Meanwhile, the
generation of a sulfur radical via the C−S bond homolyꢀc
cleavage, also has been developed.6 Although many methods
for the C-S bond cleavage are known,7 its efficient application is
still limited owing to the relative sluggishness of the C−S bond.8
Thus, seeking an efficient approach to facilitating activation of
C-S bond is valuable yet difficult.
Remarkable works have been focused on the use of copper11,13
,
palladium14, silver15, nickel16, cobalt and manganese17 as the
metal catalyst to offer the 2-N-substituted benzothiazoles with
the
requisite
substrates
of
2-halobenzothioureas,
benzothiazoles, N-aryl thioureas, isocyanide, 2-iodobenzamine,
and activated (hetero)aryl chlorides etc. Despite great success
had been made, an approach with broad functional group
tolerance to generating nitrogen-containing heterocyclic
compounds was necessary yet challenging.11-18
Fig. 1 The formation of N-substituted azoles.
Environmental concerns prompted us to seek more
environmentally friendly and practical method for the molecule
synthesis. As we all know, dioxygen, an attractive alternative of
oxidants, is environmentally friendly, non-toxic and economic.
Based on our previous research,19 we speculated that the N-
substituted aromatic heterocycle compounds might be
furnished through the C-S bond cleavage of aromatic
a. Institute for Advanced Studies (IAS), College of Chemistry and Molecular Sciences,
Wuhan University, Wuhan 430072, Hubei, P. R. China; E-mail:
aiwenlei@whu.edu.cn
b. National Research Center for Carbohydrate Synthesis, Jiangxi Normal University,
Nanchang 330022, Jiangxi, P. R. China
† These authors contributed equally to this work.
Electronic Supplementary Information (ESI) available: See DOI: 0.1039/x0xx00000x
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mercaptans by using dioxygen as a mediator. Herein, we Table 1. The scope of amines under dioxygen-triggered
describe a practical and sustainable protocol for the synthesis oxidative conditionsa
O2
S
S
N
of N-substituted aromatic heterocycle compounds through
dioxygen-triggered cleavage of C-S Bond process, which not
only offers a series of N-substituted aromatic heterocycle
compounds with good efficiency, but also provides an
interesting strategy for the C-S bond activation (Fig. 1).
+
HN
SH
N
N
DMA (2 mL), 110 C, 24 h
1a
2a-2x
3aa-3ax
S
N
R
S
N
O
O
N
S
N
N
At the outset of this study, we attempted to investigate the
oxidative cleavage of C-S bond towards C-N bond formation by
using 2-mercaptobenzothiazole 1a and piperidine 2a as the
model substrates under the dioxygen atmosphere. When
employing iodine as an additive, the reaction of 1a (0.3 mmol)
and 2a (0.9 mmol) was carried out in N,N-dimethylacetamide
(DMA) (2.0 mL) at various temperatures for 12 h (Table S1,
entries 1-4). Pleasingly, the desired product 3aa can be obtained
in the higher GC yield of 55% at 110 oC (Table S1, entry 3). When
the other solvents such as dimethyl sulfoxide (DMSO), N,N-
dimethylformamide (DMF) and N-methyl pyrrolidone (NMP)
were used in the transformation, the yields of 3aa were
decreased, demonstrating that DMA was an optimal solvent for
the reaction (Table S1, entries 5-7). Compared with ammonium
chloride (NH4Cl), sodium acetate (NaOAc), and triethylamine
(Et3N), it was interesting that no additive was more suitable to
promote the formation (Table S1, entries 8-11). Then, the yield
wasn’t significantly improved while the reaction time was
lengthened to 24 h (Table S1, entry 12). However, it was
noteworthy that 3aa could be obtained in 94% GC yield by using
DMA as the solvent with 4 equivalent 2a at 110 oC under O2 for
24 h (Table S1, entry 13). Subsequently, the increase or
decrease of the reaction time were unfavorable (Table S1,
N
3aa, R = H, 86%
3ab, R = -OH, 76%
3aj, 72%
3ak, 56%
3ac, R = -OMe, 84%
3ad, R = -Me, 62%
3ae, R = -Me, 68%
3af, R = -Bn, 79%
3ag, R = -Ph, 89%
3ah, R = -4-Py, 81%
3ai, R = -CF3, 65%
S
N
N
N
N
S
3al, 73%
3am, 30%b
S
N
N
S
N
N
O
S
S
N
N R
3ar, 53%d
3as, 43%
N
3an, R = Me, 51%c
3ao, R = tBu, 60%
3ap, R = Ph, 78%
S
N
N
S
N
N
3at, 42%
3au, 33%
3aq, R = o-Me-Ph, 69%
O
O
HN
O
F
S
N
N
O
O
S
N
O
O
N
3av, 57%
3aw, 72%
a
Conditions: 1a (0.3 mmol), 2 (1.2 mmol) in DMA (2.0 mL) under a dioxygen
atmosphere at 110 oC for 24 h, isolated yields. b The reaction was conducted at 90
oC. c The reaction was conducted at 100 oC. d The reaction was conducted for 36 h.
entries 14-16). Moreover, the yields of 3aa fell sharply when the methylmethanamine, the alkyl secondary amine, did not hinder
reactions were carried out under air or N2 instead of O2 (Table the formation of product (3am). Unfortunately, we failed to
S1, entries 17-18). These results revealed that O2 played an extended the substrate scope to alkyl primary amine and
essential role in this transformation.
With the optimal condition in hand, various kinds of N- substitutions on both the aliphatic and aromatic portions,
substituted aromatic heterocycle compounds were furnished in moderate yields were achieved (3an
3ao 3ap and 3aq). In
arylamines. In the case of the N-alkyl piperazines with different
,
,
moderate to high yields by this oxidative cleavage of C-S bond addition, morpholine 2r and thiomorpholine 2s were also
towards C-N bond formation method under dioxygen (Table 1). tolerated as the substrates for the functionalization of C-N
First, we extended the scope of piperidines with respect to both bond, yielding the corresponding products in 53% and 43%,
electron-donating and electron-withdrawing substituents. Not respectively (3ar and 3as). For five-membered and seven-
only γ-hydroxyl-piperidine but also γ-methoxyl-piperidine were membered cyclic amines, such as pyrrolidine 2t and
the compatible substrates and reacted smoothly to give the hexamethyleneimine 2u, were all tolerated under the same
desired products (3ab and 3ac). As for γ- and β-methyl- condition, providing the chance for further chemical
substituted piperidines, the moderate yields of 62% and 68% transformation
were achieved, respectively 3ad and 3ae). Subsequently, pharmaceutical molecules containing N-atom such as troxipide
(3at and 3au). Last but not least, the
(
piperidines substituted on the γ-position of nitrogen atom, such and paroxetine delivered moderate to good yields (3av and
as γ-benzyl-piperidine and γ-phenyl-piperidine, showed the 3aw).
satisfactory reactivity (3af and 3ag). Additionally, the good
Then we turned our attention toward the effect of various
yields were obtained when functional γ-substituted piperidines aromatic mercaptans on the oxidative cleavage of C-S bond
with electron-withdrawing groups were studied. For example, towards C-N bond formation (Table 2). The introductions of
γ-4-pyridyl-piperidine and γ-trifluoromethyl-piperidine were halogen atom such as Cl and Br on the phenyl ring of 2-
compliant substrates and afforded 81% and 65% yields, mercaptobenzothiazole were all tolerated well, providing the
respectively (3ah and 3ai). Furthermore, 4-piperidone ethylene desired products in 61% and 46% yield, respectively (3ba and
ketal 2j afforded 72% yield and decahydroquinoline 2k afforded 3ca). Upon using 2-mercapto-6-nitrobenzothiazole 1d, the
56% yield. Notably, 1,2,3,4-tetrahydroisoquinoline was the corresponding product could also be readily prepared (3da). 2-
compatible substrate and gave rise to the corresponding mercaptobenzoxazoles bearing not only electron-withdrawing
product in 73% yield (3al). Besides, 1-(anthracen-9-yl)-N-
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groups, such as F and Cl, but also electron-donating groups, (BHT, 2 equivalent) was added to the standard reaction system,
such as methyl, could react smoothly with 2a to give the desired the conversion was significantly suppressed and only 11% yield
was obtained (Scheme 2a), thus revealing that this process may
proceed through a radical pathway.
Subsequently, the intermediate experiments were also
Table 2. The scope of aromatic mercaptans under dioxygen-triggered
oxidative conditions a
performed (Scheme 2). 2,2'-Dithiobis(benzothiazole)
with 2a under the standard condition to give 3aa in 67% yield,
suggesting might be the intermediate in this transformation.
Furthermore, 2-(piperidin-1-ylthio)benzothiazole , which was
4 reacted
4
5
detected by GC-MS in the reaction, did not afford the desired
product 3aa under the standard condition without 2a. In
contrast,
bond formation, because of a 53% yield of 3aa was obtained
under the standard condition with 3 equivalent of 2a
5 might be the vital reaction intermediate for the C-N
.
a
Conditions: 1 (0.3 mmol), 2a (1.2 mmol) in DMA (2.0 mL) under a dioxygen
atmosphere at 110 oC for 24 h, isolated yields.
products in 79–96% yields (3ea, 3fa, 3ga and 3ha). Importantly,
1,3,4-oxadiazole-2-thiol derivative such as 5-phenyl-1,3,4-
oxadiazole-2-thiol proceeded in 72% yield as well (3ia).
Pleasingly, 4,6-dimethyl-2-mercaptopyrimidine was able as well
to give the expected product (3ja).
To demonstrate the utility of this C-N bond formation, the
efficiency of this method on the larger scale synthesis was
investigated. Delightfully, it was noteworthy that this procedure
could be scaled up to gram quantities of the desired 2-
piperidinobenzothiazole, 2-piperidinobenzooxazole and 2-
phenyl-5-piperidino-1,3,4-oxadiazole with the same level of
yields as that of the small scale reaction. For instance, 1.48 g of
3aa, 1.82 g of 3ea and 1.71 g of 3ia could be isolated in the
comparable yields of 68%, 90% and 74%, highlighting the
potential application of this method (Scheme 1).
Scheme 2. The controlled experiments.
According to the previous reports19,20 and the above results
of mechanistic studies, a plausible pathway for this reaction was
proposed (Fig. 2). Initially, the intermediate
from autoxidation of 1a. Then, a intermolecular substitution
reaction between and 2a afforded intermediate with the
release of 1a 2a was oxidized by O2, generating radical . Then,
radical addition of to afforded the desired product 3aa and
di(piperidin-1-yl)sulfane II which was detected by GC-MS.
4 was generated
4
5
.
I
I
5
Fig. 2 Proposed mechanism.
Scheme 1. The gram-scale reactions.
In summary, we have successfully developed a practical
strategy for the construction of C-N bond from readily available
starting materials through dioxygen-triggered oxidation
cleavage of C-S bond. This reaction features simple operation
and gram-scale synthesis. Crucially, neither metal catalyst nor
To know more details about the mechanism of this protocol,
a series of mechanistic experiments were performed. Given that
activation of O2 mostly involves a radical pathway, radical-
trapping experiment was firstly carried out. When the well-
known radical scavenger 2,6-di-tert-butyl-4-methylphenol
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Wang, J.-D. Xia, P.-C. Qian, R.-J. Song, M. Hu, L.-B. Gong and J.-H. Li,
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additional additive is necessary in these transformations.
Mechanistic investigations demonstrate that the S-N bond
compound might be the vital intermediate for the C-N bond
formation. Ongoing research, including further mechanistic
details and expanding the substrate scope, are currently
underway.
This work was supported by the National Natural Science
Foundation of China (21520102003) and the Hubei Province
Natural Science Foundation of China (2017CFA010). The
Program of Introducing Talents of Discipline to Universities of
China (111 Program) is also appreciated.
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Conflicts of interest
There are no conflicts to declare.
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