A novel and highly regioselective route to construct methylthio-substituted pyridazines from aryl methyl ketones at room temperature

Article abstract of DOI:10.1055/s-0031-1290433

A novel and highly regioselective two-step homocoupling/cyclization procedure from easily available aryl methyl ketones and hydrazine hydrate was developed for the synthesis of methylthio-substituted pyridazines at room temperature. The method has remarka

Full text of DOI:10.1055/s-0031-1290433

LETTER
▌2137
LAettNer ovel and Highly Regioselective Route to Construct Methylthio-Substituted
Pyridazines from Aryl Methyl Ketones at Room Temperature
Methylthio-Substituted Pyridazines from Aryl Methyl Ketones
Liuming Wu, Yan Yang, Meng Gao, Dongxue Zhang, Wenming Shu, Yanping Zhu, Anxin Wu*
Key Laboratory of Pesticide and Chemical Biology, Ministry of Education, Central China Normal University, Wuhan 430079, P. R. of China
Fax +86(27)67867773; E-mail: chwuax@mail.ccnu.edu.cn
Received: 18.05.2012; Accepted after Revision: 12.06.2012
lyzed sulfination reaction to construct sulfur-containing
Abstract: A novel and highly regioselective two-step homocou-
pyridazines via heteroaryl halides and aryl sulfides.18d In
pling/cyclization procedure from easily available aryl methyl ke-
this paper, we attempt to search for suitable reaction con-
ditions with the aim of introducing methylthio groups into
tones and hydrazine hydrate was developed for the synthesis of
methylthio-substituted pyridazines at room temperature. The meth-
od has remarkable advantages owing to the simple substrates, mild pyridazine framework. Herein, we report a novel and
reaction conditions, ease of manipulation, good yields and high
regioselectivity.
highly regioselective method for the preparation of 4-me-
thylthio-substituted pyridazines from easily available aryl
Key words: regioselectivity, homocoupling/cyclization, aryl meth-
yl ketones, hydrazine hydrate, methylthio-substituted pyridazine
methyl ketones via a two-step homocoupling/cyclization
procedure (Scheme 2).
To initiate our study, we prepared a series of 2-methyl-
thio-substituted 1,4-diketones based on the homocoupling
of aryl methyl ketones in the presence of copper(II) oxide,
iodine, and dimethyl sulfoxide.20 The model reaction
of 2-(methylthio)-1,4-diphenylbut-2-ene-1,4-dione (1a)
with hydrazine hydrate (2) was then conducted to investi-
gate the optimum reaction conditions and the results are
summarized in Table 1. The reaction was first conducted
in ethanol at reflux. Although the substrate 1a was com-
pletely consumed, the desired product 3a was only ob-
tained in 60% yield (Scheme 3, route I) and the by-
product 5a was also obtained in 30% yield (Scheme 3,
route II). Thus, a series of reaction conditions were
screened for regioselective synthesis of the desired prod-
Pyridazine ring, a well-known six-membered nitrogen-
containing heterocyclic skeleton, is often encountered as
a structural component of compounds possessing biologi-
cal activities such as antibacterial,1 analgesic,2
antihypertensive3 and so on.4 Moreover, this heterocycle
is also useful for the preparation of several other
heterocycles5 and construction of some self-assembled su-
pramolecular architectures.6 Consequently, synthesis of
these compounds has attracted considerable interest, espe-
cially in the case of synthetic strategies from easily avail-
able substrates that allow the generation of a high level of
diversity.7
Up until now, various methods have been reported for the uct 3a.
synthesis of pyridazines (Scheme 1). In general, they
Gratifyingly, when the reaction was carried out in the
could be synthesized through the reaction of pyridazinone
with phosphoryl trichloride,8 4-chloro-1,2-diaza-1,3-bu-
tadienes with active methylene compounds,9 diazoalkane
with 1-bromocyclopropene,10 1,2,4,5-tetrazines with di-
enophiles,11 or 1,2,4-triazines with carbon nucleophiles,12
using either 1,4-dicarbonyl compounds,13 dihydrofuran,14
or 3-thiapentane-1,5-diones15 as one of the substrates, as
well as through other pathways.16
presence of KOH at room temperature, 3a was obtained in
a considerable yield (85%, Table 1, entry 2). Different
bases such as DABCO, DBU, t-BuOK, K2CO3 and so on
(entries 3–9) were then examined in this reaction, but the
desired product was not obtained in better yields. We also
screened other solvents, finding that isopropanol was the
solvent of choice (compare entries 2 and 10–16).
Table 1 Optimization of the Reaction Conditions
As reported in the literature, sulfur-containing pyridazines
are an important kind of organosulfur compounds and
they are endowed with high pharmacological properties
such as analgetical, antipyretical and antiphlogistical ac-
tivity.17 In recent years, some methods for the synthesis of
sulfur-containing pyridazines have been reported.18 For
example, Quéguiner and Plé tested Barbier-type
reaction19 with lithium metal under sonication condition,
allowing a very fast and smooth functionalization of pyr-
idazines.18a Organ and co-workers developed a Pd-cata-
catalyst,
solvent
O
SMe
N
N
Ph
Ph
SMe
N2H4⋅H2O
Ph
+
temp
Ph
O
2
1a
Entry
Catalyst
Solvent
Molar ratio Temp
Yielda
1a:2
b
1
2
3
4
–
EtOH
EtOH
EtOH
EtOH
1:1
1:1
1:1
1:1
reflux 60%
KOH
r.t.
r.t.
r.t.
85%
62%
65%
DABCO
DBU
SYNLETT 2012, 23, 2137–2141
Advanced online publication: 03.08.2012
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0031-1290433; Art ID: ST-2012-W0433-L
© Georg Thieme Verlag Stuttgart · New York

2138
L. Wu et al.
LETTER
Table 1 Optimization of the Reaction Conditions (continued)
Moreover, the molar ratios of substrates 1a and 2 were
also tested. Elevation of the molar ratios to 1:1.5 could in-
crease the reaction yield (up to 93%, entry 17). Further el-
evation of the molar ratios could not obviously enhance
the reaction yield (entries 18 and 19). At length, the reac-
tion temperature was also taken into account. However, a
significant drop in yield was observed when the tempera-
ture increased (entries 2 and 20–22), mainly due to the
formation of 5a. Thus, the most efficient conditions were
found by using KOH in isopropanol at room temperature
furnishing 3a in excellent yield with high regioselectivity
(entry 17).
catalyst,
solvent
O
SMe
N
N
Ph
Ph
SMe
N2H4⋅H2O
Ph
+
temp
Ph
O
2
1a
Entry
Catalyst
Solvent
Molar ratio Temp
Yielda
1a:2
5
6
t-BuOK
K2CO3
NaHCO3
LiOH.H2O
NaOH
KOH
EtOH
1:1
r.t.
70%
75%
78%
81%
83%
83%
90%
81%
5%
EtOH
1:1
r.t.
7
EtOH
1:1
r.t.
Encouraged by the successful synthesis of 4-(methylthio)-
3,6-diphenylpyridazine, a series of 2-methylthio-substi-
8
EtOH
1:1
r.t.
9
EtOH
1:1
r.t.
KOH,
N2H4⋅H2O
O
SMe
N
N
O
10
11
12
13
14
15
16
17
18
19
20
21
22
MeOH
i-PrOH
t-BuOH
H2O
1:1
r.t.
CuO, I2
Ar
Ar
Ar
SMe
Ar
i-PrOH,
r.t.
DMSO,
65 °C
Ar
Me
KOH
1:1
r.t.
O
isomers
KOH
1:1
r.t.
Scheme 2 Synthesis of 4-methylthio-3,6-diarylpyridazine from aryl
methyl ketones
KOH
1:1
r.t.
KOH
DMF
1:1
r.t.
30%
35%
83%
93%
93%
93%
85%
75%
N
N
KOH
DMSO
MeCN
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
1:1
r.t.
route I
Ph
Ph
KOH
1:1
r.t.
SMe
3a 60%
desired product
KOH
1:1.5
1:2
r.t.
O
SMe
O
KOH
r.t.
N2H4⋅H2O
Ph
+
Ph
EtOH
KOH
1:3
r.t.
2
1a
KOH
1:1.5
1:1.5
1:1.5
40 °C
60 °C
Ph
Ph
KOH
route II
N
N
H
O
KOH
reflux 62%
5a 30%
a Isolated yield.
b No catalyst was employed.
Scheme 3 Regioselective synthesis of 3a
R1
O
O
Cl
O
R1
N
NH
R2
R1
S
N
N
R4
R
EWG
R3
N
N
1. DDQ, H2O
2
2
R1
R2
R1
Br
2. CHCl3 3. base
2. N2H4⋅H2O
R1
O
R3
R4
CN
R4
R1
O
N
N
N
N
R2
R1
R2
R1
N
N
O
Cl
N
Scheme 1 Various methods for the synthesis of pyridazines
Synlett 2012, 23, 2137–2141
© Georg Thieme Verlag Stuttgart · New York

LETTER
Methylthio-Substituted Pyridazines from Aryl Methyl Ketones
2139
tuted 1,4-enediones 1 were used to explore the scope of increase of diversity could be obtained through the me-
this synthetic strategy under optimal conditions. In gener- thylthio group in position 4. To the best of our knowledge,
al, this reaction showed broad tolerance for various aro- methylthio-substituted heterocycles are particularly bene-
matic substituents (Table 2, entries 1–16). For example, ficial since they are found as structural elements in many
substrates bearing electron-neutral (H, Me) and electron- bioactive compounds as well as electronic, magnetic, and
rich (OMe, OEt) substituents on the aromatic ring reacted optical materials.21 In addition, they are also versatile in-
efficiently to afford the desired products in excellent termediates for further transformations, such as oxida-
yields (entries 1–6). Much to our satisfaction, high yields tion,22 substitution,23 transition-metal-catalyzed cross-
were also obtained with halogenated substrates (entries 7– coupling reaction,24 addition–elimination,25 and electro-
10). Moreover, substrates with 1-naphthyl and 2-naphthyl philic cyclization.26 For example, we carried out a direct
substituents could also react smoothly to afford the de- oxidation of the methylthio group successfully. As shown
sired products in good yields (entries 11 and 12). Hetero- in Scheme 4, 4-(methylthio)-3,6-diphenylpyridazine (3a)
aryl groups such as 2-furyl, 2-benzofuryl, 2-thienyl, and in CH2Cl2 was treated with m-CPBA at 0 °C to afford 4-
3-thienyl groups were also investigated. To our delight, (methylsulfinyl)-3,6-diphenylpyridazine (4a) in an ac-
heterocyclic substrates for 2-furyl group and 3-thienyl ceptable yield (80%). This exemplification demonstrates
group could also work well under these conditions with an that a series of methylthio-substituted pyridazines could
excellent yield (entries 13 and 16) though substrates for 2- also be transformed into methylsulfinyl-substituted pyrid-
benzofuryl group and 2-thienyl group gave moderate azines, under these conditions.
yields (entries 14 and 15).
Table 2 Reaction Scope of 2-Methylthio-Substituted 1,4-Enediones 1
N
N
O
SMe
O
KOH, i-PrOH
Ar
Ar
SMe
N2H4⋅H2O
+
Ar
r.t.
Ar
2
1
3
Entry
Diketones Ar
Producta Yieldb
1
2
1a
1b
1c
1d
1e
1f
Ph
3a
3b
3c
3d
3e
3f
93%
93%
95%
94%
97%
91%
88%
89%
91%
83%
85%
87%
90%
73%
63%
92%
4-MeC6H4
4-MeOC6H4
4-EtOC6H4
Scheme 4 The oxidation of 4-methylthio-3,6-diarylpyridazine (3a)
3
4
In summary, we have developed a highly regioselective
synthetic route to 4-methylthio-substituted pyridazines di-
rectly from easily available aryl methyl ketones via a two-
step homocoupling/cyclization procedure. Compared
with previous methods,19 this protocol has remarkable ad-
vantages owing to the simple and readily available start-
ing materials, wide scope of substrates, mild reaction
conditions and excellent yields. More significantly, this
methodology can successfully introduce methylthio
groups into the pyridazine skeletons, retaining an area of
further transformations. Therefore, we believe this is a
promising route for the preparation of methylthio-substi-
tuted pyridazine derivatives that will be applied in many
fields. Further investigations on the applications of the
products are currently underway in our laboratory.
5
2-benzo[d][1,3]dioxyl
3- MeOC6H4
4-FC6H4
6
7
1g
1h
1i
3g
3h
3i
8
4-ClC6H4
9
4-BrC6H4
10
11
12
13
14
15
16
1j
3,4-Cl2C6H3
1-NaPh
3j
1k
1l
3k
3l
2-NaPh
1m
1n
10
1p
2-furyl
3m
3n
3o
3p
2-benzofuryl
2-thienyl
All the products were characterized by IR, 1H NMR and
13C NMR spectra, and by HRMS. The structures of 3a27
and 4a28 were further confirmed by X-ray diffraction anal-
ysis of single crystal and are shown in Scheme 4.
3-thienyl
a Reaction conditions: 2-methylthio-substituted 1,4-enediones 1 (1
mmol), hydrazine hydrate (2; 1.5 mmol), KOH (1 mmol), i-PrOH (5
Acknowledgment
mL), at r.t.
b Isolated yield.
This work was supported by the Central China Normal University
and the National Natural Science Foundation of China (Grant No.
21032001). We thank Dr. Xianggao Meng (Central China Normal
University) for performing X-ray crystallographic analysis and Dr.
The molecular diversity of the target products at positions
3 and 6 came from the aryl methyl ketones, while a further
© Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 2137–2141

2140
L. Wu et al.
LETTER
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Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnoIufproig
m
iotSrat
ungIifoop
r
t
Primary Data for this article are available online at
http://www.thieme-connect.com/ejournals/toc/synlett and can be
cited using the following DOI: PmriDyramPatrDiyar
1
at 0.4125/pd0033th.mPirayrDaZtahenl1
0
1Chemyrt
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Synlett 2012, 23, 2137–2141
© Georg Thieme Verlag Stuttgart · New York

LETTER
Methylthio-Substituted Pyridazines from Aryl Methyl Ketones
2141
goodness-of-fit on F2 1.386, final R indices [I >2σ(I)], R1 =
0.0912, wR2 = 0.1725, largest diff. peak and hole 0.320 and
–0.237 e.Å–3.
DC = 1.337 Mg/m3, μ = 0.221 mm–1, λ = 0.71073 Å, F(000)
616, crystal size 0.20 × 0.10 × 0.10 mm3, 3175 independent
reflections [R(int) = 0.0305], reflections collected 10377,
refinement method: Full-matrix least-squares on F2:
goodness-of-fit on F2 1.066, final R indices [I >2σ(I)], R1 =
0.0420, wR2 = 0.1189, largest diff. peak and hole 0.386 and
–0.227 e.Å–3.
(28) Crystal structure data for compound 4a: CCDC number
879830, C17H14N2OS, monoclinic, space group P2(1)/c, a =
9.899(2), b = 20.194(5), c = 7.881(2) Å, α = 90°, β =
111.832(4)°, γ = 90°, V = 1462.5(6) Å3, T = 296(2) K, Z = 4,
© Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 2137–2141

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