Protonated DBU as catalyst for cascade addition-cyclization of 2-alkynylaniline and carbon disulfide

Article abstract of DOI:10.1016/j.tetlet.2013.02.074

Protonated 1,8-diazabicyclo[5,4,0]undec-7-ene as catalyst for cascade addition/cyclization of 2-alkynylaniline and carbon disulfide has been described. This process provides a convenient route for synthesis of a variety of benzo[d][1,3]thiazine-2(4H)-thio

Full text of DOI:10.1016/j.tetlet.2013.02.074

Tetrahedron Letters 54 (2013) 2357–2361
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Protonated DBU as catalyst for cascade addition–cyclization of 2-alkynylaniline
and carbon disulfide
⇑
Peng Zhao, Qian Liao, Hongxin Gao, Chanjuan Xi
Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology (Ministry of Education), Department of Chemistry, Tsinghua University, Beijing 100084, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Protonated 1,8-diazabicyclo[5,4,0]undec-7-ene as catalyst for cascade addition/cyclization of 2-alkyny-
laniline and carbon disulfide has been described. This process provides a convenient route for synthesis
of a variety of benzo[d][1,3]thiazine-2(4H)-thiones in high yields with high regio- and stereoselectivity at
room temperature without metal.
Received 10 January 2013
Revised 11 February 2013
Accepted 22 February 2013
Available online 7 March 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
1,8-Diazabicyclo[5,4,0]undec-7-ene
Protonated DBU
Carbon disulfide
Cascade reaction
Conjugated acid
Heterocyclic compounds are ubiquitous in natural products,
pharmaceuticals, and organic materials.1 As a consequence, the
ongoing interest for developing new versatile and efficient synthe-
ses of heterocycles has always been a thread in the synthetic com-
munity. Among them, cascade reaction is one of the most powerful
strategic tools for the rapid assembly of heterocyclic compounds.2
This process enables multiple bond-forming to occur in one
sequence, which greatly enhances the synthetic efficiency, while
producing less waste and minimizing handling. Recently, the
metallic Lewis acid-catalyzed cascade reaction provided a straight-
forward method for the construction of heterocyclic compounds.3
For example, Pd- or Cu-catalyzed amination/cyclization of
o-haloalkynylarenes or haloenynes afforded indoles, pyrroles, or
quinoline3h,i,4 and Ag-catalyzed addition/cyclization of 2-alkynyl-
benzenamines gave benzo[d][1,3]thiazine derivatives.3v,w None-
theless, all reactions required metallic reagents such as
palladium, silver, and copper and in many cases the corresponding
ligands were used as co-catalyst, which made these reactions more
expensive. Lately, great progress has been made in the develop-
ment of metal-free transformation in organic synthesis.5–7 In this
respect, Brønsted acid-catalyzed cascade reaction has attracted
considerable attention.8 In contrast to the metallic Lewis acid-cat-
alyzed reaction, Brønsted acid-catalyzed reaction is more economic
and has environmental benefits. With a point of these views, here-
in we would like to report protonated 1,8-diazabicy-
clo[5,4,0]undec-7-ene (DBU) as a catalyst for cascade addition/
cyclization of 2-alkynylaniline and carbon disulfide, in which
DBUH+ is a catalyst for both activation of CS2 and carbon–carbon
triple bond. This process provided a convenient synthetic route
to benzo[d][1,3]thiazine-2(4H)-thione for a variety of substrates
at room temperature without metal.
To optimize the reaction condition, several experiments were
performed using the reaction of p-methyl-2-(phenylethynyl)ani-
line 1a and carbon disulfide to obtain (Z)-4-benzylidene-6-
methyl-4H-benzo[d][1,3]thiazine-2-thiol 2a as model substrates
under different solvents and catalytic conditions (Table 1). Initially
we chose DBU as a catalyst for this reaction, based on our previous
work regarding DBU-promoted tandem reaction of o-haloanilines
and carbon disulfide.9a,b The reaction occurred and product 2a
was obtained in 40% yield (entry 1). We next changed the reaction
temperatures and the amounts of DBU, respectively, the yield of
product 2a did not improve (entries 2–5). It is known that Brønsted
acid or conjugated acid as a catalyst could activate carbon–carbon
triple bonds to make them much more electrophilic.10 Therefore,
5 mol % of H2SO4 was added into the reaction mixture. Gratifyingly,
the reaction proceeded smoothly and the yield of 2a increased to
80% (entry 6). It is noteworthy that H2SO4 was added as a single
catalyst and the reaction did not proceed (entry 7). When amount
and ratio of DBU/H2SO4 and solvents were screened (entries 8–16),
20 mol % DBU/10 mol % H2SO4 was found to be the best catalyst
system in acetonitrile as medium at room temperature (entry 9).
When other Brønstead acids such as p-toluenesulfonic acid (p-
TsOH), trifluoromethanesulfonic acid (CF3SO3H), trifluoroacetic
acid (CF3CO2H), and acetic acid (CH3CO2H) were employed, the
reaction performed smoothly and product 2a was also obtained
⇑
Corresponding author. Tel.: +86 10 62782695; fax: +86 10 62771149.
E-mail address: cjxi@tsinghua.edu.cn (C. Xi).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.02.074

2358
P. Zhao et al. / Tetrahedron Letters 54 (2013) 2357–2361
Table 1
in high yield (entries 17–20). Out of concern of the cost, H2SO4 was
chosen as a Brønstead acid. Furthermore, we used other protonated
organic bases such as triethylamine (Et3N) and 1,4-diazabicy-
clo[2.2.2]octane (DABCO), the reaction proceeded in lower yields
(entries 21 and 22). When a protonated pyridine was used as a cat-
alyst the reaction did not proceed (entry 23). On the basis of these
results, the optimal condition involved the following parameters:
DBU 20 mol %/H2SO4 10 mol % as catalyst, acetonitrile as a solvent,
and reaction temperature at 25 °C.
Optimization between p-methyl-2-(phenylethynyl)-aniline 1a and carbon disulfidea
NH2
N
SH
Acid/base
S
C
S
S
solvent, 24 h
Me
Me
Ph
Ph
1a
2a
Under the optimized conditions, a study on the substrate scope
was carried out, and the results are summarized in Table 2. Firstly,
treatment of (phenylethynyl)aniline 1b and carbon disulfide affor-
ded product 2b in 82% yield (entry 2). To our delight, crystals of 2b
were suitable for single crystal analysis, and its structure was fully
characterized by X-ray diffraction analysis.11 The structure of 2b
clearly shows the formation of (Z)-4-benzylidene-4H-
benzo[d][1,3]thiazine-2-thiol, in which the substituent on double
bond is on the same side with the carbamodithioic group. Then
we used other 2-alkynylbenzenamine derivatives 1 to react with
carbon disulfide. Both electron-donating groups (such as methyl
and methoxyl group) and electron-withdrawing groups (such as
chloro and fluoro atom) on the benzene ring showed good perfor-
mance (entries 1, 3–5). It is noteworthy that 2-alkynylbenzen-
amine with stronger electron-withdrawing groups, such as CO2Et
or CN group, on the para-position of 2-alkynylbenzenamine failed
to generate any product and starting materials remained (entries
6 and 7). When 2,4-dimethyl-6-(phenylethynyl)aniline 1h was
used as a substrate, the desired product 2h was also formed in
71% yield (entry 8). Notably, in all cases, only one product was ob-
served in situ by 1H NMR, it is different from Ag-catalyzed reac-
tion.3v Next, different N-substituted 2-alkynylbenzenamine
derivatives were applied under the optimized conditions. For
example, when N-methyl-2-bromobenzamide 1i or N-ethyl-2-bro-
mobenzamide 1j was treated with carbon disulfide, the desired
product 2i or 2j was obtained in 72% or 54% isolated yield (entries
9 and 10), respectively. Benzyl substituted 2-alkynylbenzenamine
1k with carbon disulfide proceeded in 26% yield of product 2k
Entry
Acid/base (mol %/mol %)
T (°C)
Solvent
Yield of 2ab (%)
1
2
3
4
5
6
7
8
DBU/— (20/0)
DBU/— (20/0)
DBU/— (20/0)
DBU/— (50/0)
DBU/— (100/0)
DBU/H2S04 (20/5)
—/H2SO4 (0/5)
DBU/H2SO4 (10/5)
DBU/H2SO4 (20/10)
DBU/H2S04 (50/25)
DBU/H2SO4 (100/50)
DBU/H2SO4 (100/100)
DBU/H2S04 (20/10)
DBU/H2S04 (20/10)
DBU/H2S04 (20/10)
DBU/H2S04 (20/10)
DBU/p-TsOH (20/10)
DBU/CF3SO3H (20/10)
DBU/CF3C02H (20/10)
DBU/CH3C02H (20/10)
Et3N/H2S04 (20/10)
DABCO/H2S04 (20/10)
Pyridine/H2S04 (20/10)
25
50
80
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
40
48
27
33
25
80
NRc
53
9
94
91
10
11
12
13
14
15
16
17
18
19
20
21
22
23
73
NRc
79
p-Toluene
THF
21
CH2CI2
CH3OH
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
NRc
NRc
85
87
87
91
22
25
NRc
a
Unless otherwise noted the reactions were performed in a sealed tube with 4-
methyl-2-(phenylethynyl)aniline 1a (1 mmol), carbon disulfide (1.5 mmol) in sol-
vent (1.5 mL) for 24 h.
The yield were evaluated by 1H NMR with CH2Br2 as internal standard.
b
c
NR means no reaction.
Table 2
Reaction of various 2-alkynylbenzenamine 1 with carbon disulfidea
Entry
Substrate
Time (h)
24
Product
Yieldb (%)
NH2
N
SH
S
1
82
Me
Me
2a
1a
NH2
Ph
Ph
N
SH
S
2
3
4
5
24
24
32
24
80
84
74
86
2b
1b
Ph
NH2
Ph
N
SH
S
MeO
Cl
MeO
Cl
2c
1c
Ph
Ph
NH2
N
SH
S
2d
1d
Ph
Ph
SH
NH2
N
S
F
F
2e
1e
Ph
Ph

P. Zhao et al. / Tetrahedron Letters 54 (2013) 2357–2361
2359
Table 2 (continued)
Entry
Substrate
Time (h)
36
Product
Yieldb (%)
NH2
N
SH
S
6
7
0
0
EtO2C
EtO2C
NC
2f
1f
Ph
Ph
NH2
N
N
SH
S
36
24
NC
2g
1g
Me
Ph
Ph
Ph
Me
SH
NH2
8
71
72
54
26
S
Me
Me
2h
1h
Ph
NHMe
NHEt
NHBn
NH2
Me
N
S
S
9
24
24
48
S
1i
1j
2i
Ph
Ph
Et
N
10
11
S
2j
Ph
Ph
Bn
N
S
S
1k
Ph
2k
Ph
N
N
N
N
N
N
SH
S
12
13
14
15
16
17
18
24
24
24
24
24
24
24
78
71c
75
67
50
73
2l
1l
Tol-p
Tol-p
SH
NH2
S
2m
1m
PhOMe-p
PhF-p
PhOMe-p
SH
NH2
S
2n
1n
PhF-p
SH
NH2
S
2o
1o
PhCO2Me-p
PhNO2-p
PhCO2Me-p
SH
NH2
S
2p
1p
PhNO2-p
SH
NH2
S
2q
1q
Th-3
Th-3
N
SH
S
F
2r
81
F
1r
Th-3
Th-3
(continued on next page)

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P. Zhao et al. / Tetrahedron Letters 54 (2013) 2357–2361
Table 2 (continued)
Entry
Substrate
NH2
Time (h)
48
Product
Yieldb (%)
N
SH
S
2s
19
20
68
Py-2
Py-2
N
1s
SH
S
F
2t
24
48
0
0
F
1t
Bun
Bun
SH
NH2
N
S
21
2u
TMS
1u
TMS
a
Unless otherwise noted the reactions were performed in a sealed tube with 1 (1.0 mmol), carbon disulfide (1.5 mmol), DBU (20 mol %)–H2SO4 (10 mol %) (0.134 M in
CH3CN, 1.5 mL).
b
Isolated yields.
Reaction temperature: 50 °C.
c
H2SO4 (10 mol%) + DBU (20 mol%)
C
S
S
R1
N
DBUH+
S
DBUH+
R2
R2
C
S
S
S
H
N
R1
R3
2
R3
1
R1
NH
R1
N
S
S
R2
R2
DBUH+
S
SH
R3
DBUH
R3
4
3
+
Scheme 1. Proposed route for DBUH+-catalyzed reaction of 2-alkynylaniline with carbon disulfide.
(entry 11). In addition, different substituent groups on the alkynes
In summary, we have developed a simple and practical method
for the synthesis of benzo[d][1,3]thiazine-2(4H)-thione derivatives
via protonated DBU as a dual functionalized catalyst in the reaction
of 2-alkynylaniline with carbon disulfide. The presented cascade
addition/cyclization represents a facile route for generation of het-
erocycles and avoids utilization of any metal under mild reaction.
Further reactions and mechanism of these small molecules are un-
der investigation in our laboratory, and the results will be reported
in due course.
of 2-alkynylbenzenamine were employed. Aromatic substituents
were on the end of alkynes, the reaction proceeded smoothly. For
instance, the aryl group attached on the triple bond with elec-
tron-donating groups (such as Me or OMe) (entries 12 and 13)
and electron-withdrawing groups (such as F, CO2Me, and NO2) (en-
tries 14–16). Both proceeded well and the desired products were
obtained in satisfying yields. Furthermore, thienyl and pyridyl
were on the end of alkynes, the desired products were formed in
73%, 81%, and 68% isolated yields, respectively (entries 17–19).
When alkyl and TMS groups were on the end of alkynes, the de-
sired product could not be observed and starting materials re-
mained (entries 20 and 21).
Based on the above results, the mechanism of this reaction is
proposed as shown in Scheme 1. First, the nucleophilic nitrogen
of 2-alkynylbenzenamine 1 attacks the carbon atom of carbon
disulfide,12,13 which might be activated by DBUH+ to form interme-
diate 3 and release DBUH+.9a,13 The intermediate 3 undergoes pro-
ton-transfer and complexation of carbon–carbon triple bond with
DBUH+ to form intermediate 4, which goes through intramolecular
nucleophilic addition to afford product 2 and release DBUH+
again.10 A single DBUH+ catalyst mediates both activities for CS2
molecule and alkyne species.
Acknowledgments
This work was supported by the National Natural Science Foun-
dation of China (21032004 and 21272132) and the national basic
research program of China (2012CB933402).
Supplementary data
Supplementary data (experimental procedures and full charac-
terization including 1H NMR, and 13C NMR data, and NMR spectra
for compounds compound 2a–2e and 2h–2s) associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.tetlet.2013.02.074.

P. Zhao et al. / Tetrahedron Letters 54 (2013) 2357–2361
2361
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11. X-ray data for 2b: empirical formula:
C
15H11NS2; unit cell parameters:
a = 8.144 Å, b = 8.670 Å, c = 10.028 Å,
group: P1.
a
= 83.40°, b = 76.08°, = 67.92°; space
c
ꢀ
12. Kruithof, A.; Ploeger, M. L.; Janssen, E.; Helliwell, M.; de Kanter, F. J. J. Molecules
2012, 17, 1675.
13. (a) Wang, F.; Wang, Y.; Cai, L.; Miao, Z.; Chen, R. Adv. Synth. Catal. 2008, 350,
2733; (b) Zhemg, D.; Li, S.; Luo, Y.; Wu, J. Org. Lett. 2011, 13, 6402.