Eosin y catalyzed visible light mediated aerobic photo-oxidative cleavage of the C-C double bond of styrenes

Article abstract of DOI:10.1016/j.tetlet.2014.11.141

A mild, efficient, and metal-free oxidative cleavage of C-C double bond to aldehydes utilizing eosin Y as an organophotoredox catalyst has been developed. The green aspects of our work capitalize on the utilization of visible light and air (O₂) as inexpensive, readily-available, non-toxic, and eco-sustainable reagents with the removal of specialized glassware and a powerful UV light source. Moreover, a synthetic application of the photo-oxidative cleavage of C-C double bond is also demonstrated by the convenient synthesis of benzothiazoline.

Full text of DOI:10.1016/j.tetlet.2014.11.141

Accepted Manuscript
Eosin Y catalysed visible light mediated aerobic photo-oxidative cleavage of
the C–C double bond of styrenes
Atul K. Singh, Ruchi Chawla, Lal Dhar S. Yadav
PII:
S0040-4039(14)02181-9
DOI:
http://dx.doi.org/10.1016/j.tetlet.2014.11.141
TETL 45628
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
5 October 2014
17 November 2014
18 November 2014
Please cite this article as: Singh, A.K., Chawla, R., Yadav, L.D.S., Eosin Y catalysed visible light mediated aerobic
photo-oxidative cleavage of the C–C double bond of styrenes, Tetrahedron Letters (2014), doi: http://dx.doi.org/
1
0.1016/j.tetlet.2014.11.141
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Graphical Abstract
Eosin Y catalysed visible light mediated
aerobic photo-oxidative cleavage of the C–C
double bond of styrenes
Leave this area blank for abstract info.
Atul K. Singh, Ruchi Chawla, Lal Dhar S. Yadav

1
Tetrahedron Letters
journal homepage: www.elsevier.com
Eosin Y catalysed visible light mediated aerobic photo-oxidative cleavage of the C–C
double bond of styrenes
*
Atul K. Singh, Ruchi Chawla, Lal Dhar S. Yadav
Green Synthesis Lab, Department of Chemistry, University of Allahabad, Allahabad 211 002, India
*
Corresponding author. Tel.: +91 532 2500652; fax: +91 532 2460533; E-mail: ldsyadav@hotmail.com (L.D.S. Yadav).
ARTICLE INFO
ABSTRACT
Article history:
Received
A mild, efficient and metal-free oxidative cleavage of C-C double bond to aldehydes utilising
eosin Y as an organophotoredox catalyst has been developed. The green aspects of our work
Received in revised form
Accepted
Available online
2
capitalize on the utilisation of visible light and air (O ) as inexpensive, readily-available, non-
toxic and eco-sustainable reagents with the removal of specialized glassware and powerful UV
light source. Moreover, a synthetic application of the photo-oxidative cleavage of C-C double
bond is also demonstrated by the convenient synthesis of benzothiazoline.
Keywords:
Styrenes
Aldehydes
2009 Elsevier Ltd. All rights reserved.
C-C bond clevage
Eosin Y
Visble light
Photoredox catalysis
7
Nowadays, the field of visible light photoredox catalysis
experiences a veritable revival of activity in both academics and
industry. Research has been focused on harnessing visible light
with equivalent oxidants and oxygen. These processes generally
suffer from safety concern, the use of expensive and toxic metals
or the use of over stoichiometric amounts of oxidants, which are
also expensive and would produce a large amount of by-products.
Despite great progress has been made in the field of oxidative
cleavage of olefins over the past decades, there have been
increasing efforts to circumvent the requirement of different
metal catalysts for the oxidative cleavage of C-C double bonds.
The development of a safe, metal-free, green method is highly
desirable for these transformations obviating the need for harsh,
impractical, or environmentally hazardous reaction conditions.
1
as a promoter in organic transformations owing to its natural
abundance, renewability, ease of use and being an eco-friendly
energy source. The most commonly employed photocatalysts in
photoredox catalysis are tris(bipyridine) ruthenium and iridium
complexes which suffer from the disadvantages of being toxic,
highly expensive and difficult to remove, especially in the
pharmaceutical industry. Respites to these metal based
complexes are organic dyes, which are cheap, metal-free and
ecofriendly catalysts and have shown paramount potential for
high photocatalytic performances. Another aspect of photoredox
catalysis is the use of atmospheric oxygen as an oxidant which
again very-well complies with the green chemistry principles. So
the use of visible light to drive organic reactions catalyzed by an
organic dye and atmospheric oxygen as an oxidant provides an
economically and energetically beneficial pathway for green
synthesis.
To the best of our knowledge, there are only few
photochemical reports on the photo-oxidative cleavage of C-C
8
double bonds of styrene. These processes generally suffer from
safety concern, use of UV light, harsh, impractical reaction
condition lower yield and needs additional photosensitizer and
special implementation. However, the use of UV light in organic
synthesis is not very promising due to its detrimental effects and
the side reactions associated with it. Moreover, the generation of
high intensity UV light requires specialized photoreactor, which
limits their scalability for photochemical reactions. Control
(over-oxidation) and selectivity in oxidative processes also
remain to be a challenging task in target synthesis. Hence, the
discovery of new processes to achieve cleavage of C-C double
bonds under innocuous reaction conditions is still appealing to
address many of the challenges described above. We sought to
develop a new green visible light mediated photocatalytic method
for the aerobic C-C double bond cleavage. We hypothesised the
use of visible light photocatalysis for the aerobic C-C double
bond cleavage under much milder conditions. In continuation of
our work on simple starting materials and visible light
The oxidative cleavage of a C-C double bond belongs to that
elementary class of transformations that constitute the core of
synthetic chemistry. In particular, the cleavage of alkenes to the
corresponding carbonyl products find important applications in
the synthesis of natural products and biologically active
2
compounds. A large number of methods are available for this
transformation which can be broadly divided into chemical and
3
enzymatic methods. Chemical methods include, for instance,
ozonolysis and several metal-based variants (KMnO , OsO ,
4
4
4
5
RuO , etc.), use of transition metal as catalyst in combination
4
with peroxides, peracids, and other oxidising reagents,
6
electrochemical alternatives, organic molecules in combination

2
Tetrahedron
9
organophotoredox catalysis, herein we report what we have
Table 1
a
achieved on the realization of the strategy for the direct oxidative
cleavage of C-C double bond of styrenes to corresponding
aldehydes using visible light irradiation and air as the source of
the oxidant (Scheme 1). This new photocatalytic approach is
particularly attractive because the reaction is driven by visible
light at an ambient temperature in a flask open to air without the
need for specialized glassware and powerful UV light source and
is metal-free.
Photocatalytic oxidative aerobic cleavage of styrene.
Br
Br
HO
Br
O
O
eosin Y ( mo l %)
O
Br
air , sol vent, rt, 9 h
green LEDs
COOH
1a
2
a
eosin Y
b
Entry Visible light Photocatalyst Air
Solvent Yield (%)
1
2
+
+
+
+
+
N2
+
+
+
+
+
+
+
DMSO
DMSO
DMSO
DMSO
DMF
88
eosin Y (1 mol %)
O
_
+
-
air, DMSO, rt
R
R
_
h (2.5 W green LEDs)
3
+
+
+
+
+
+
+
+
+
traces
-
Visible light
4
+
+
+
+
+
+
+
+
5
80
Scheme 1. Photocatalytic oxidative aerobic cleavage of C-C
double bond.
6
CH CN
73
3
7
THF
58
To prove the viability of the aerobic C-C double bond
cleavage involving visible light photoredox catalysis, a model
reaction was performed by stirring styrene (1a, 1 mmol) and
eosin Y (1 mol %) in DMSO under visible light irradiation (green
light emitting diodes (LEDs), λmax = 535 nm, 2.5 W) at rt for
overnight in an open-mouth round bottom flask. The
corresponding aldehyde 2a was obtained in 88% yield (Table1,
entry 1). After the formation of aldehyde, we focused our efforts
to know the minimum time required for completion of the
reaction and it was found to be 9 h. This initial result was
delighting and supported our hypothesis regarding the suitability
of photoredox catalysis to aerobic C-C double bond cleavage.
Encouraged by the result, we performed a series of control
experiments to confirm the necessary parameters of reaction such
as catalyst, air and visible light. There was no reaction in the
absence of eosin Y under the given conditions (Table 1, entry 2).
Similarly, when the reaction was conducted in dark, there was no
conversion of 1a (Table 1, entry 3). Thus, these results suggest
that both light and eosin Y are essential requirements for the
reaction. Upon performing the reaction under an atmosphere of
nitrogen, the reaction was considerably inhibited (Table 1, entry
8
dioxane
DCM
50
9
62
c
10
DMSO
DMSO
88
d
11
69
a
b
c
d
For experimental procedure, see Ref. 10.
Isolated yield after column chromatography.
catalyst (2 mol %).
catalyst (0.75 mol %) and reaction time 15 h.
nitro, and cyano, were also well tolerated under the new
photocatalytic conditions.
Table 2
a
Oxidative cleavage of substituted styrenes
eosin Y (1 mol %)
air, DMSO, rt, 9-12 h
green LEDs
O
R
R
4
). These control experiments strongly support that visible light,
1
2
photocatalyst, and air, are all essential (+) for the completion of
reaction and established the photocatalytic model of the reaction.
We next decided to try the reaction in different solvents. Use of
DMF as a solvent in place of DMSO afforded 2a in 80% yield
Table 1, entry 5). When other solvents such as acetonitrile, THF,
dioxane and DCM were used, 2a was obtained in lower yields
Table 1, entries 6-9). After choosing DMSO as the best solvent,
b
Entry
R
Product Time (h) Yield (%)
1
2
3
4
5
6
7
8
9
10
H
2a
2b
2c
2d
2e
2f
9
88
90
91
91
94
93
84
86
85
78
75
77
(
2-MeO
3-MeO
4-MeO
3-Me
4-Me
4-Cl
9
(
9
the quantitative optimization of catalyst was done. It was found
that the best yield of aldehyede (88%) was obtained with 1 mol
9
%
of catalyst (Table 1, entry 1). In the presence of 2 mol % of
9
catalyst, the yield of 2a did not change but the use of 0.75 mol %
of catalyst decreased the yield of β-ketosulfone to 69% even
when the reaction time was extended up to 15 h (Table 1, entries
9
2g
2h
2i
10
11
10
12
12
11
1
0,11).
We next performed a series of experiments to determine the
2-Br
scope of the olefinic component in this aerobic C-C double bond
cleavage protocol. As revealed in Table 2, a range of styrenes
bearing electron-donating substituents as well as electron-
withdrawing substituents on the aromatic ring also underwent the
desired reaction to provide the corresponding aldehydes in good
to excellent yields. It appears that the variation of position (o-, m-
and p-) of substituents on the phenyl ring of styrene does not
have considerable effect on the yield of the corresponding
aldehyde (Table 2, entries 2-4, 5-6, 8-9 and 10-11). Wide range
of functional groups including chloro, bromo, methyl, methoxy,
4-Br
3-NO2
4-NO2
4-CN
2j
1
1
2k
2l
1
2
a
b
Reaction conditions: olefin 1a (1.0 mmol), catalyst (1 mol
), DMSO (3 mL).
Isolated yield after column chromatography.
%

3
On the basis of our experimental results and the literature
References and notes
1
,8e-g
precedents,
a plausible mechanism for the aerobic C-C
double bond cleavage and formation of aldehyde 2 is depicted in
Scheme 2. A singlet oxygen reaction pathway can be excluded,
since it is known that photo-oxidative cleavage of aryl substituted
1. (a) Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Chem.
Rev. 2013, 113, 5322; (b) Majek, M.; Filace, F.; von Wangelin J.
A. Beilstein J. Org. Chem. 2014, 10, 981; (c) Hari, D. P.; Konig,
B. Chem. Commun. 2014, 50, 6688; (d) Ischay, M. A.; Ament, M.
S.; Yoon, T. P. Chem. Sci. 2012, 3, 2807.
2. (a) Sheldon, R. A.; Kochi, J. K. Metal Catalyzed Oxidations of
Organic Compounds; Academic Press: New York, 1981. (b)
Stewart, R. Oxidation in Organic Chemistry; K. Wiberg, Ed.;
Academic Press: New York, 1965. (c) Hudlicky, M. Oxidations in
Organic Chemistry; American Chemical Society Monograph 186;
American Chemical Society:Washington D.C., 1990. (d) Larock,
R. C. Comprehensive Organic Transformations, 2nd ed.; Wiley-
VCH: New York, 1999; p 1234. (e) Lee, D. G.; Chen, T.
Comprehensive Organic Synthesis; Trost, B.M., Fleming, I., Eds.;
Pergamon Press: Oxford, 1991; Vol. 7, p 541.
8
e-g
ethylenes does not involve singlet oxygen.
Eosin Y is
photoexcited by visible light, and the resulting excited state of
eosin Y* participates in a one-electron oxidation of styrene 1 to
+
form the alkene radical cation (1˙ ) and the radical anion of eosin
Y, which then transfers an electron to oxygen present in the
reaction to form superoxide radical anion. The resulting alkene
+
radical cation (1˙ ) undergoes [2+2] cycloaddition with
superoxide radical anion to afford a dioxetane 3, which
ultimately undergoes oxidative cleavage to afford the aldehyde 2.
3
.
Rajagopalan, A.; Lara, M.; Kroutil, W. Adv. Synth. Catal. 2013,
55, 3321.
3
4
.
(a) Criegee, R. Angew. Chem., Int. Ed. 1975, 14, 745; (b) Bailey,
P. S. Ozonation in Organic Chemistry; Academic Press: New
York, 1978. (c) Schiaffo, C. E.; Dussault, P. H. J. Org. Chem.,
1
1
2
008, 73, 4688; (d) O’Brien, M.; Baxendale, I. R.; Ley, S. V. Org.
eosin Y*
Lett. 2010, 12, 1596; (e) Willand-Charnley, R.; Fisher, T. J.;
Johnson, B. M.; Dussault, P. H. Org. Lett. 2012, 14, 2242; (f)
Pappo, R.; Allen, D. S.; Lemieux, Jr. R. U.; Johnson, W. S. J. Org.
Chem. 1956, 21, 478.
eosin Y
O2
eosin Y
O2
O
5. (a) Qian, Y.; Nishino, H.; Kurosawa, K. J. Heterocycl. Chem.
1993, 30, 209; (b) Baucherel, X.; Uziel, J.; Juge, S. J. Org. Chem.
2001, 66, 4504; (c) Boer, J. W.; Brinksma, J.; Browne, W. R.;
Meetsma, A.; Alsters, P. L.; Hage, R.; Feringa, B. L. J. Am. Chem.
Soc. 2005, 127, 7990; (d) Liu, S.-T.; Reddy, V. K.; Lai, R.-Y.
Tetrahedron 2007, 63, 1821; (e) Biradar, A. V.; Sathe, B. R.;
Umbarkar, S. B.; Dongare, M. K. J. Mol. Catal. A: Chem. 2008,
285, 111; (f) Griffith, W. P.; Shoair, A. G.; Suriaatmaja, M. Synth.
Commun. 2000, 30, 3091; (g) Plietker, B. J. Org. Chem. 2003, 68,
O
oxidative
cleavage
O
3
2
Scheme 2. A plausible mechanism for the aerobic C-C double
bond cleavage and formation of aldehyde 2.
7
2
123; (h) Kogan, V.; Quintal, M. M.; Neumann, R. Org. Lett.
005, 7, 5039; (i) Tabatabaeian, K.; Mamaghani, M.; Mahmoodi,
The synthetic application of visible light photocatalytic C-C
double bond cleavage methodology is also demonstrated through
N. O.; Khorshidi, A. Catal. Commun. 2007, 9, 416; (j) Wang, A.;
Jiang, H. J. Org. Chem. 2010, 75, 2321; (k) Shing, T. K. M.; Tam,
E. K. W. V.; Tai, W. -F.; Chung, I. H. F.; Jiang, Q. Chem.;Eur. J.
sequential
one
pot
synthesis
of
2-phenyl-2,3-
dihydrobenzo[d]thiazole (benzothiazoline) by the reaction of in
situ generated benzaldehyde from styrene and 2-aminothiophenol
1
996, 2, 50; (l) Neisius, N. M.; Plietker, B. J. Org. Chem. 2008,
1
2
73, 3218; (m) Barton, D. H. R.; Wang, L. Tetrahedron Lett. 1994,
(
Scheme 3). It is noteworthy that benzothiazoline is an useful
3
1
6
5, 1519; (n) Barton, H. R.; Chavasiri, W. Tetrahedron 1994, 50,
9; (o) Dhakshinamoorthy, A.; Pitchumani, K. Tetrahedron 2006,
2, 9911; (p) Dhakshinamoorthy, A.; Alvaro, M.; Garcia, H. ACS
13
and convenient reducing agent.
Catal. 2011, 1, 836; (q) Xing, D.; Guan, B.; Cai, G.; Fang, Z.;
Yang, L.; Shi, Z. Org. Lett. 2006, 8, 693; (r) Travis, B. R.;
Narayan, R. S.; Borhan, B. J. J. Am. Chem. Soc. 2002, 124, 3824;
(s) Dhakshinamoorthy, A.; Pitchumani, K. Catal. Commun. 2009,
10, 872.
i. eosin Y (1 mol %)
air, DMSO, rt, 9 h
Green LEDs
H
N
NH2
SH
o
ii. 100 C, 4 h
S
6
7
.
.
(a) Wu, X.; Davis, A. P.; Fry, A. J. Org. Lett. 2007, 9, 5633; (b)
Bauemer, U.-St.; Schaefer, H. J. J. Appl. Electrochem. 2005, 35,
5
1
6
1
283.
(a) Wang, T.; Jiao, N. J. Am. Chem. Soc. 2013, 135, 11692; (b)
Lin, R.; Chen, F.; Jiao, N. Org. Lett. 2012, 14, 4158; (c)
Narasimhan, V.; Rathore, R.; Chandrasekaran, S. Synth. Commun.
Scheme 3. Sequential one pot synthesis of 2-phenyl-2,3
dihydrobenzo[d]thiazole.
1
985, 15, 769; (d) Singh, F. V.; Milagre, H. M. S.; Eberlin, M. N.;
In conclusion, we have developed an efficient and
environmentally benign photocatalytic method for the oxidative
C−C double bond cleavage of styrenes to aldehydes at ambient
Stefani, H. A. Tetrahedron Lett. 2009, 50, 2312; (e) Miyamoto,
K.; Tada, N.; Ochiai, M. J. Am. Chem. Soc. 2007, 129, 2772; (f)
Miyamoto, K.; Sei, Y.; Yamaguchi, K.; Ochiai, M. J. Am. Chem.
Soc. 2009, 131, 1382; (g) Thottumkara, P. P.; Vinod, T. K. Org.
Lett. 2010, 12, 5640.
temperature. The protocol utilizes eosin
Y
as an
organophotocatalyst along with visible light and air (O ) as the
8. (a) Clennan, E. L.; Pan, G.-l. Org. Lett. 2003, 5, 4979; (b)
Gollnick, K.; Schnatterer, A. Tetrahedron Lett. 1984, 25, 2735, (c)
Hirashima, S.-I.; Kudo, Y.; Nobuta, T.; Tada, N.; Itoh, A.
Tetrahedron Lett. 2009, 50, 4328; (d) Melone, L.; Gambarotti, C.;
Prosperini, S.; Pastori, N.; Recupero, F.; Punta, C. Adv. Synth.
Catal. 2011, 353, 147; (e) Lechner, R.; Kummel, S.; Konig, B.
Photochem. Photobiol. Sci., 2010, 9, 1367. (f) Eriksen J.; Foote,
C. S. J. Am. Chem. Soc., 1980, 102, 6083; (g) Eriksen, J.; Foote,
C. S.; Parker, T. L. J. Am. Chem.Soc.1977, 99, 6455.
2
greenest and cheapest reagents. The method addresses several
key limitations of prior work in the field, as it circumvents the
requirement for excess of expensive metal and non-metal
catalysts, reagents, oxidants as well as elevated temperatures. The
replacement of deleterious UV light with harmless visible light
and the controlled oxidative cleavage prove the present green
approach a superior alternative to the existing methods for the
C−C double bond cleavage of olefins.
9
.
(a) Singh, A. K.; Chawla, R.; Rai, A.; Yadav, L. D. S. Chem.
Commun. 2012, 48, 3766; (b) Chawla, R.; Kapoor, R.; Singh, A.
K.; Yadav, L. D. S. Green Chem. 2012, 14, 1308; (c) Chawla, R.;
Singh A. K.; Yadav, L. D. S. Tetrahedron 2013, 69, 1720; (d)
Chawla, R.; Singh, A. K.; Yadav, L. D. S. Synlett 2013, 24, 1558;
Acknowledgments
(
2
e) Singh, A. K.; Chawla, R.; Yadav, L. D. S. Tetrahedron Lett.
013, 54, 5099; (f) Yadav, A. K.; Srivastava, V. P.; Yadav, L. D.
We sincerely thank SAIF, Punjab University, Chandigarh, for
providing microanalyses and spectra. A. K. S. is thankful to
CSIR for the award of a Junior Research Fellowship (File No.
S. New J. Chem. 2013, 37, 4119; (g) Keshari, T.; Yadav, V. K.;
Srivastava, V. P.; Yadav, L. D. S. Green Chem. 2014, 16, 2986;
(h) Srivastava, V. P.; Yadav, A. K.; Yadav, L. D. S. Synlett 2014,
0
9/001(0379)/2013-EMR-I).

4
Tetrahedron
2
5, 665; (i) Singh, A. K.; Chawla, R.; Keshari, T.; Yadav, V. K.;
Yadav, L. D. S. Org. Biomol. Chem. 2014, 12, 8550.
1
0. General procedure for the the aerobic C-C double bond cleavage
and formation of aldehyde 2: A solution of olefin 1 (1 mmol) and
eosin Y (1 mol %) in DMSO (3 mL) was irradiated with visible
light (green light emitting diodes (LEDs), λmax = 535 nm, 2.5 W)
with stirring at rt for 9-12h (Table 2). After completion of the
reaction (monitered by TLC), water (5 mL) was added and the
mixture was extracted with ethyl acetate (3 x 5 mL). The
2 4
combined organic phase was dried over anhydrous Na SO ,
filtered and concentrated under reduced pressure. The resulting
crude product was purified by silica gel chromatography using a
mixture of hexane/ethyl acetate (49:1) as eluent to afford an
analytically pure sample of product 2.
Characterization data of
representative compounds
Compound 2a :Colorless liquid; 88% yield; H NMR (400 MHz,
5j
1
CDCl
3
): δ 10.05 (s, 1H), 7.89 (d, J = 7.6 Hz, 2H), 7.64 (t, J = 7.6
13
Hz, 1H), 7.56 (t, J = 7.6 Hz, 2H). C NMR (100 MHz, CDCl
3
): δ
+
1
92.62, 136.61, 134.74, 130.00, 129.21. EIMS (m/z): 106 (M ).
Anal. Calcd for C O: C, 79.22; H, 5.70. Found: C, 79.59; H,
7
H
6
j
5
1
5
.75.Compound 2e : Colorless liquid; 94% yield; H NMR (400
MHz, CDCl
3
): δ 9.98 (s, 1 H), 7.68 (d, J =6 Hz, 2H), 7.45-7.40
1
3
(m, 2 H), 2.43 (s, 3 H). C NMR (100 MHz, CDCl
3
): δ 192.65,
1
38.93, 136.49, 135.31, 130.03, 128.92, 127.28, 21.21. EIMS
+
(m/z): 120 (M ). Anal. Calcd for C
8
j
H
8
O: C, 79.97; H, 6.71. Found:
5
1
C, 79.62; H, 6.59.Compound 2j : Yellow solid; 78% yield;
NMR (400 MHz, CDCl
1
1
H
3
): δ 10.16 (s, 1H), 8.70 (s, 1H), 8.53 (s,
13
H), 8.26 (s, 1H), 7.79 (s, 1H). C NMR (100 MHz, CDCl
3
): δ
90.54, 149.05, 137.72, 135.33, 131.02, 129.01, 124.93. EIMS
+
(m/z): 151 (M ). Anal. Calcd for C
H
7 5
3
NO : C, 55.63; H, 3.33; N,
9
.27. Found: C, 55.59; H, 3.06; N, 8.99.
1
1. General procedure for the sequential one pot synthesis of 2-
phenyl-2,3 dihydrobenzo[d]thiazole 6. A solution of olefin 1 (1
mmol) and eosin Y (1 mol %) in DMSO (3 mL) was irradiated
with visible light (green light emitting diodes (LEDs), λmax = 535
nm, 2.5 W) with stirring at rt for 9 h. Then, aminothiophenol was
added and the reaction mixture was stirred at 100 °C for 4 h. After
completion of the reaction (monitered by TLC), water (5 mL) was
added and the mixture was extracted with ethyl acetate (3 x 5 mL).
2 4
The combined organic phase was dried over anhydrous Na SO ,
filtered and concentrated under reduced pressure. The resulting
crude product was purified by silica gel chromatography using a
mixture of hexane/ethyl acetate (49:1) as eluent to afford an
analytically pure sample of product 6.
Characterization data of representative compound
1
2a
1
Compound 6 : Solid; 83% yield; H NMR (400 MHz, CDCl
3
): δ
.50-7.56 (m, 2H), 7.31-7.39 (m, 3H), 7.02-7.06 (m, 1H), 6.91-
.98 (m, 1H), 6.73-6.79 (m, 1H), 6.62-6.68 (m, 1H), 6.36 (s, 1H),
7
6
4
1
7
1
3
.05 (brs, 1H). C NMR (100 MHz, CDCl
29.01, 128.88, 128.79, 126.63, 125.55, 121.61, 120.72, 109.80,
0.02. EIMS (m/z): 213 (M ). Anal. Calcd for C13H11NS: C, 73.20;
3
): δ 146.24, 141.61,
+
H, 5.20; N, 6.57. Found: C, 73.39; H, 5.34; N, 6.31.
1
2. (a) Chikashita, H.; Miyazaki, M.; ltoh, K. J. Chem. Soc. Perkin
Trans. I 1987, 699; (b) Chikashita, H.; Nishida, S.; Miyazaki, M.;
Itoh, K. Synth. Commun. 1983. 13, 1033; (c) Chikashita, H.;
Morita, Y.; Itoh, K. Synth. Commun. 1985, 15, 527; (d) Saito, K.;
Akiyama, T. Chem. Commun. 2012, 48, 4573; (e) Zhu, C.;
Akiyama, T. Org. Lett. 2009, 11, 4180.