G Model
CCLET 3123 1–4
L.-Y. Fan et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
3
Table 4
4. Conclusion
121
Effect of atmosphere and catalyst in the formation of 3a.a
Q3
In conclusion, we developed an efficient synthetic protocol for 122
the formation of benzothiazoles via YCl3-catalyzed aerobic 123
oxidation reaction between 2-aminothiophenol and benzalde- 124
hyde. This method provides a green and practical synthetic 125
approach to benzothiazoles that are important units of biologically 126
active molecules. The usage of molecular oxygen in air renders this 127
protocol very attractive and practical. Furthermore, our results 128
demonstrated that yttrium catalyst plays a decisive role in aerobic 129
S
CHO
N
SH
Catalyst
3a
+
+
.
EtOH, reflux
NH2
S
1a
2a
N
H
4a
oxidation.
A detailed study on the scope of our synthetic 130
methodology is ongoing in our laboratory.
131
132
Entry
Catalyst (mol%)
Atmosphere
Yield (%)
3a
4a
Acknowledgment
1
2
3
YCl3 (5%)
YCl3 (5%)
None
N2
Trace
99
98
Air
Air
Trace
68
We gratefully acknowledge the National Natural Science Q2133
17
Foundation of China (No. 20802052) for financial support.
134
a
o-Aminothiophenols (1.1 equiv.), aldehyde (1.0 equiv.), 5 h.
b
Isolated yield.
Appendix A. Supplementary data
135
94
95
96
97
98
99
During the reaction procedure (Table 1, entry 1), 2-phenyl-
Supplementary data associated with this article can be found, in 136
benzothiazoline 4a which further dehydrated to form of 3a was
detected by TLC. Obviously, oxidation is an indispensable process
during this reaction. It was suspected that oxygen in air may act as
an oxidant. In order to confirm our conjecture, three parallel
experiments were performed and the results were summarized in
Table 4.
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138
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100
101
102
103
104
105
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111
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Under nitrogen atmosphere, in the presence of YCl3, only a
trace amount of benzothiazole 3a was detected whereas the
intermediate product 4a was obtained in 98% yield (Table 4,
entry 1). When the same reaction was carried out in air, the yield
of 3a reached nearly 100% (Table 4, entry 2). It can therefore be
deduced that oxygen in air as an oxidant promotes the
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oxidation.
According to the aforementioned observations, a tentative
reaction mechanism is outlined in Scheme 2. The present reaction
consists of Schiff base formation, intramolecular cyclization,
dioxygen activation and heterocyclic compounds oxidation.
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desired product 3a.
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189
191
Scheme 2. Proposed reaction mechanism for the synthesis of benzothiazoles.
Please cite this article in press as: L.-Y. Fan, et al., Yttrium-catalyzed heterocyclic formation via aerobic oxygenation: A green approach