Preparation of 2-Aryl-2H-benzotriazoles by Zinc-Mediated Reductive Cyclization
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Scheme 1.
All of the o-nitrophenylazophenols and triazoles are
vents. In these procedures, benzotriazole N-oxides
were not detected. In the case of using chloro-substi-
tuted azo compounds 1i--k as substrates, no dechlori-
nated products (over-reduced products) could be de-
tected. Here, it is of benefit to employ zinc as reduc-
tant compared to the reported methods.
known compounds and were identified on the basis of their
1
melting points, H NMR and GC-MS data.
References
In conclusion, we have demonstrated that zinc
powder is an efficient reductant for the cyclization of
o-nitrophenylbenzenes in aqueous alkaline solutions.
The protocol is characterized by easy manipulation
and product separation, high selectivity and mild reac-
tion conditions. This procedure offers the practical
convenience of obviating the need of handling inflam-
mable and toxic organic solvents. From an environ-
mental point of view, this process also shows a poten-
tial advantage in reducing the adverse impact of sol-
vent disposal. Further work on the reductive cycliza-
tion of o-nitrophenylbenzenes with other metals in
aqueous alkaline solutions is currently in progress in
our laboratory and the results will be published in
due course.
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Experimental Section
Typical Procedure
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To a mixture of 2-nitro-2’-hydroxy-5’-methylazobenzene (1a;
2.57 g, 10 mmol) and zinc dust (1.32 g, 20 mmol, 2.0 equivs.)
in water (140 mL) was added dropwise a 25 wt% aqueous
NaOH solution (9.6 g, 60 mmol, 6.0 equivs.) within 0.5 h at
908C(Run 1). The reaction mixture was stirred at 90 8Cfor
1 h (monitored by HPLC). After the reaction was complete,
the reaction mixture was neutralized with concentrated HCl
to pH 2–3 to consume unreacted zinc and filtered. The solid
was recrystallized from MeOH, affording 2-(2-hydroxy-5-
methylphenyl)benzotriazole (2a); mp 124.2–125.88C(Lit.:
[3]
124.5–126.58C);
1H NMR (400 MHz, CDCl3): d=2.34
(3H, s, CH3), 6.92–7.18 (2H, m), 7.28–7.54 (2H, m), 7.82–
8.06 (2H, m), 8.22 (1H, d, J=2.0 Hz), 11.21(1H, s); GC-
E
MS: m/z=225 (M+), 210 (M+ꢀ15), 196, 182, 168, 154, 141,
93, 78, 66, 51.[11]
Adv. Synth. Catal. 2007, 349, 1637 – 1640
ꢁ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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