Direct methyl C(sp3)-H azolation of thioanisoles: Via oxidative radical coupling

Article abstract of DOI:10.1039/c9ob01530b

A method for metal-free, 1,3-dibromo-5,5-dimethylhydantoin mediated methyl C(sp³)-H bond azolation of thioanisoles has been developed, affording a facile route for the construction of nitrogen-functionalized thioanisoles, possibly via a nitrogen-centered radical process. This reaction represents an important addition to the limited number of existing methods for the methyl C(sp³)-H bond functionalization of thioanisoles, and may find practical application in the synthesis of nitrogen-alkylated azoles.

Full text of DOI:10.1039/c9ob01530b

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DOI: 10.1039/C9OB01530B
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COMMUNICATION
Direct Methyl C(sp3)−H Azolation of Thioanisoles via Oxidative
Radical Coupling
Xin Wang,ab Changhao Li,a Yixiao Zhang,a Bing Zhan,b and Kai Suna*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
method for metal-free, 1,3-dibromo-5,5-dimethylhydantoin
path a:
mediated methyl C(sp3)−H bond azolation of thioanisoles has been
X
Z
Ar SCH2
X
N
Y
N
N
N
N
Y
Z
Br reagent
Y
Z
1
N
thioanisoles
- HBr
X
X
S
Ar
developed, affording a facile route for the construction of
N
N
Br
potential nitrogen
H
Y
Z
nitrogen-functionalized thioanisoles, possibly via
a nitrogen-
3
2
azoles
A
radical precursor
centered radical process. This reaction represents an important
addition to the limited number of existing methods for the methyl
C(sp3)−H bond functionalization of thioanisoles, and may be find
practical application in the synthesis of nitrogen-alkylated azoles.
path b:
X
N
Br reagent
- HBr
Br reagent
- Br
N
2
azoles
+
Y
Ar SCH2
Ar SCH2
1
thioanisoles
Z
S
Ar
B
B
3
Methods for the construction of C−N bonds are of considerable
interest in synthetic chemistry because nitrogen-containing
compounds are of importance in medicinal and agricultural fields.1
To date, substantial progress has been made regarding C(sp3)−H
amidations, imidations, and azolations at allylic2 and arylmethylic
positions3 and at the C−H bond adjacent to nitrogen or oxygen
atoms.4-5 However, there has been limited work disclosed on the
direct methyl C(sp3)−H amidation of the more challenging
thioanisoles6 and anisoles.7 Thioanisoles are prevalent structural
components in agrochemicals and in compounds involved in
material science. The corresponding sulfoxides and sulfones are
important compounds in medicinal and synthetic chemistry.8 Direct
C−N bond construction at the methyl C(sp3)−H bond of thioanisoles
using N-nucleophiles is a highly appealing goal. However, challenges
still exist because thioanisoles can be easily oxidized during the
amidation step. Therefore, it is desirable to develop a convenient
procedure for the N-functionalization of the methyl C(sp3)−H bond
of thioanisoles under mild reaction conditions.
Scheme 1. Azolation of thioanisoles.
electron oxidant or amino radical initiator, would allow for an
accelerated nitrogen radical-based methyl C(sp3)−H azolation step.
Thus, this method would overcome the problematic competing
oxidation reaction. As shown in Scheme 1, the N-nucleophile react
with a Br reagent to form the relatively active nitrogen radical
precursor A. Subsequently, H-abstraction with the Br radical
generated from homolysis of the N-Br bond of A would generate
the active amino radical and sulfide methyl radical B. Finally,
coupling between amino radical and B would lead to the final
product and regeneration of the Br radical to participate in the next
catalytic cycle (path a). Alternatively, the active sulfide methyl
radical B would genereted through a hydrogen atom capture
pathway with Br reagent. Subsequently, single electron oxidation of
B would genereted a sulfide methyl cation C, which would react
with N-nucleophile to deliver the final product (path b).
Considering that imidazoles, triazoles, and tetrazoles are the
largest chemical family that exists in medicinal and pesticide
chemistry,10 the direct N-functionalization of imidazoles, triazoles,
and tetrazoles has also attracted considerable interest.11 Therefore,
we herein report a 1,3-dibromo-5,5-dimethylhydantoin (DBDMH)-
mediated oxidative radical azolation that enables the installation of
biologically attractive imidazole, triazole, and tetrazole skeletons
into the methyl C(sp3)−H bonds of thioanisoles.
As part of our continued interest in C−N bond construction,9 we
have previously described the oxidative imidation of the methyl
C(sp3)−H bond with anisoles.9b For the methyl C(sp3)−H
functionalization of thioanisoles, rather than anisoles, we
envisioned that the use of a mild Br reagent as a potential single
The initial reaction screening focused on bromide reagents,
which can act as efficient amino radical initiators. Hence, N-
bromosuccinimide (NBS) was initially screened to stimulate the
model reaction with methyl(phenyl)sulfane 1a and 2-chloro-1H-
benzo[d]imidazole 2a in DCE at 90 C. To our delight, the coupling
product 3a was formed in 23% yield (Table 1, entry 1). Encouraged
a.College of Chemistry and Chemical Engineering, Anyang Normal University,
Anyang 455000, P. R. China E-mail: sunk468@nenu.edu.cn
b. College of chemistry and energy, Zhengzhou University, Zhengzhou 450001, P. R.
China.
o
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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Table 1. Optimization of the reaction conditions.[a]
Table 2. Direct C(sp3)−H azolation of thioanisoles withVaiezwolAertsic.[lae,bO]nline
DOI: 10.1039/C9OB01530B
N
R
N
conditions
R
Cl
DBDMH (1.0 equiv)
+
N
R'
S
N
+
S
CH3CN, 90 o
C
N
N
N
S
N
H
S
H
N
R'
1a
2a
3a Cl
1
2
3
Entry
1
Initiator
NBS
Br2
DBDMH
I2O5
DIB
DBDMH
DBDMH
DBDMH
DBDMH
DBDMH
DBDMH
DBDMH
DBDMH
DBDMH
no
Solvent
DCE
DCE
DCE
DCE
Temp. (oC)
90
yieldof 3a (%)[b]
23
0
58
0
0
Cl
N
Cl
Cl
OMe
Cl
N
N
N
S
N
2
3
4
5
6
7
8
9
10
11
12
13
14
90
90
90
90
90
90
90
90
90
90
90
120
60
S
S
3a
3b
3c
, 76%
Cl
, 78%
, 64%
Cl
Cl
N
DCE
Cl
N
N
N
N
EtOAc
toluene
CH3CN
CHCl3
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
0
N
N
S
MeO
S
EtO
Br
S
26
51
47
55[c]
76[d]
74[e]
34[d]
61[d]
0
3d
3e
3f
, 60%
, 74%
, 73%
Cl
Cl
Cl
N
N
N
N
N
F
S
S
Cl
S
3g
3j
3h
3i
, 64%
, 43%
, 70%
F3C
Cl
N
F3C
N
N
N
N
N
N
15
[a]
90
S
S
MeO
S
Reactions were carried out with methyl(phenyl)sulfane 1a (0.3
mmol), 2-chloro-1H-benzo[d]imidazole 2a (0.3 mmol), initiator (1.0
3k
3l
, 73%
, 52%
, 68%
F3C
N
F3C
N
Br
[b]
equiv.) in solvent (1.0 mL) at 90 °C for 12 h. Yield of the isolated
N
N
product. [c] 3 equiv. of methyl(phenyl)sulfane 1a was added instead
N
F
S
Br
S
S
[d]
of solvent.
6 equiv. of methyl(phenyl)sulfane 1a was added
[e]
instead of solvent.
added instead of solvent.
10 equiv. of methyl(phenyl)sulfane 1a was
3m
3n
3o
, 69%
, 41%
, 59%
Br
Br
N
N
N
N
Br
S
MeO
S
by this result, Br2 and 1,3-dibromo-5,5-dimethylhydantoin (DBDMH)
were tested as initiators, and DBDMH proved to be an efficient
amino radical initiator, giving the desired product 3a in 58% yield
(entries 2−3). Previous investigations have indicated that
hypervalent iodine reagents can act as efficient single-electron (1.0 equiv) at 90 °C in CH3CN (1 mL). [b] Yield of isolated product.
oxidants. However, no reaction occurred when iodine pentoxide
3p
3q
, 70%
, 65%
[a]
Reaction conditions: 1 (1.8 mmol), azoles 2 (0.3 mmol), DBDMH
and (diacetoxyiodo)benzene (DIB) were used as single-electron atoms, such as fluoro, chloro, and bromo, on the aromatic ring were
oxidants (entries 4−5). Solvents screening revealed that EtOAc and then investigated and found to be well tolerated under the present
toluene were not suitable for the reaction (entries 6−7), while reaction conditions, affording the corresponding products 3c−3i in
CH3CN and CHCl3 afforded 3a in 51% and 47% yields, respectively moderate to good yields (43%−74%), which should allow for further
(entries 8−9). Further experimental results showed that when 6 synthetic transformations. The steric effect had no obvious impact
equiv. of methyl(phenyl)sulfane 1a was added, the corresponding on this reaction, with o-, m-, and p-Cl substituted thioanisoles being
product 3a was isolated in good yield (entries 10−12). The yield of efficiently converted to the coupling products 3c, 3d, and 3h,
product 3a decreased when the temperature was increased to 120 respectively. In the case of substrates bearing a strong electron-
°C, and the corresponding sulfoxide was the main product (entry withdrawing nitro group, no coupling product was observed, and
13). A low yield was also obtained when the temperature was the corresponding oxidized sulfoxide was isolated. It is noteworthy
decreased to 60 °C (entry 14). In the absence of DBDMH, that methyl(p-tolyl)sulfane 1j gave the methylthio azolated product
compounds 1a and 2a were fully recovered, and no desired product 3j, with the methyl group remaining intact, which is inconsistent
3a was observed, suggesting that DBDMH is essential for this with our previously reported results.9b Next, the substituted 2-
reaction (entry 15).
(trifluoromethyl)
1H-benzo[d]imidazole
and
2-bromo-1H-
With the optimal conditions in hand, the generality of the benzo[d]imidazole were also reacted with thioanisoles, furnishing
method was explored, and the results are summarized in Table 2. the desired products 3k−3q in moderate to good yields (41%−73%).
Reactions between the thioanisoles
1
and 2-chloro-1H- Tetrazoles are of great importance because of their wide
benzo[d]imidazole 2a were tested, and thioanisoles 1 with electron- application in medicinal chemistry, organic chemistry, and material
donating groups (OMe, OEt) on the benzene ring were smoothly science; therefore, the development of practical synthetic routes
converted into the corresponding products 3b, 3e, and 3f in good for tetrazole-based compounds is attractive. To further explore the
yields (73%−78%). Substrates bearing electron-withdrawing halogen
2 | J. Name., 2012, 00, 1-3
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Table 3. Direct C(sp3)−H azolation of thioanisoles with triazole and tetramethylpiperidinooxy (TEMPO) was employed in tVhieewrAeraticclteioOnnlionef
tetrazoles.[a,b]
methyl(phenyl)sulfane 1a with 2-chloro-1H-benzo[d]imidazole 2a,
only a 23% yield of 3a was obtained (Scheme 3a). Upon addition of
2.0 equivent of 2,6-di-tert-butyl-4-methylphenol (BHT), the
formation of the desired 3a was completely suppressed, and the
benzylic C–H azolated product 5 was obtained in 61% yield (Scheme
3b). In addition, the azol-bromination product 6 could be isolated in
87% yield when 2.0 equivent of 1,1-diphenylethylene was added
(Scheme 3c). These results support the theory that the azolation
might be a radical reaction and that an N-radical is involved in the
azolation process. To provide further evidence regarding the nature
of the active intermediate, we prepared the N-Br reagent 7
according to a previous literature report.12 When the reaction
between 1a and 7 was performed at room temperature, 3x could
be isolated in high yield (Scheme 3d). This result supports our
previous assumption in Scheme 1 (path a) that the N-Br reagent
generated in situ may be the key active intermediate to stimulate
the methyl C(sp3)−H bond azolation of thioanisoles.
DOI: 10.1039/C9OB01530B
R
R
DBDMH (1.0 equiv)
CH3CN, 90 oC
N
+
N
H
S
S
1
2
3
Br
Cl
Ph
N
N
N
N
N
N
N
N
N
S
N
N
N
S
S
3r
3s
, 75%
3t
, 73%
, 82%
Ph
N
Ph
N
N
N
N
N
N
N
N
MeO
S
N
Cl
S
N
S
3u
, 75%
3v
, 74%
3w
, 79%
DBDMH (1.0 equiv)
N
Ph
TEMPO (2 equiv)
S
N
Cl
N
N
(a)
N
+
CH3CN, 90 o
C
N
N
N
N
N
N
N
Cl
S
Br
S
S
MeO
S
N
H
Cl
1a
1a
2a
3a
, 0.3 mmol
, 23%
3x
, 78%
3y
3z
, 77%
, 82%
N
Cl
DBDMH (1.0 equiv)
BHT (2 equiv)
[a]
N
Reaction conditions: 1 (1.8 mmol), azoles 2 (0.3 mmol), DBDMH
(1.0 equiv) at 90 °C in CH3CN (1mL). [b] Yield of isolated product.
N
Cl
+
(b)
OH
CH3CN, 90 o
C
N
H
S
2a
5
, 0.3 mmol
, 61%
potential of our methodology, 5-phenyl-1H-tetrazoles were also
Briefly investigated, and were found to react smoothly with
thioanisole 1a, providing the desired products 3r−3t in good to high
yields (73%−82%). In addition, reactions between 5-phenyl-1H-
tetrazole and substituted thioanisoles were performed, affording
the desired products 3u and 3v in good yields (75% and 74%).
Finally, benzotriazole was also found to be a good coupling partner
in this transformation, with the corresponding products 3w−3z
obtained in high yields (77%−82%).
Ph
Ph
Br
DBDMH (1.0 equiv)
diphenylethylene (2 equiv)
N
N
Cl
(c)
(d)
+
Cl
CH3CN, 90 o
C
N
H
S
S
N
1a
1a
2a
6
, 87%
, 0.3 mmol
S
N
N
N
no DBDMH was added
CH3CN, 90 o
N
Ph
+
N
C
N
Br
7
3x
, 74%
, 0.3 mmol
To demonstrate the synthetic utility of this protocol, we Scheme 3. Reactions determining the reaction mechanism.
investigated further manipulations of the azolated products 3.
When treating 3x with 1.2 equivent of m-chloroperoxybenzoic acid
Conclusions
(m-CPBA) at room temperature, the corresponding sulfoxide 4
could be obtained in 81% yield (Scheme 2a). Furthermore, the
In summary, we have developed a mild, economical, and practical
DBDMH-mediated azolation of thioanisoles, with azoles as nitrogen
nucleophiles, involving methyl C(sp3)−H bond oxidative radical
functionalization. The generality of this reaction was demonstrated
using various thioanisoles with benzimidazoles, 5-phenyl-1H-
tetrazoles, and benzotriazole, to produce a series of structurally
diverse N-alkyl substituted thioanisoles. Based on control
experiments, a possible radical mechanism was proposed to explain
the azolation reaction. Further study of the applications of this
reaction is currently underway in our laboratory.
gram-scale reaction with 1a and 5-phenyl-1H-tetrazole was
conducted, and found 59% of 3r could be isolated with 7.0 mmol 5-
phenyl-1H-tetrazole.
m-CPBA (1.2 equiv)
(a)
(b)
N
N
CH2Cl2, rt, 2 h
N
N
N
N
Ph
S
O
Ph
S
3x
4
, 0.5 mmol
, 81%
N
N
N
N
N
N
DBDMH (1.0 equiv)
CH3CN, 90 oC, 12 h
N
Ph
+
S
S
Conflicts of interest
Ph
Ph
N
H
Ph
, 59%
1a
3r
7.0 mmol
, 6 equiv
There are no conflicts to declare.
Scheme 2. Application investigation.
Acknowledgments
To gain insight into the reaction mechanism, several control
experiments were conducted. When 2.0 equivent of 2,2,6,6-
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This work was supported by the National Natural Science
Foundation of China (21801007), the Program for Innovative
Research Team of Science and Technology in the University of
Henan Province (18IRTSTHN004, 18HASTIT006), the Science and
Technology Plan Projects of Henan Province (19A150015) and the
Jilin Province Key Laboratory of Organic Functional Molecular
Design & Synthesis (130028911).
Singh, S. Sharma, S. Gudup, M. Bhogal, S. Kumar,VDie.wSAirntigclhe,OVnl.inKe.
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