C
Synlett
D. Chen et al.
Letter
benzoate (1n) and 2-nitrobenzamides 1o–q afforded the re-
duced products 2n–q in 75–86% yield. Next, the nitro
heteroaromatics such as 5-nitroquinoline (1r) and 8-nitro-
quinoline (1s) were investigated. Very interestingly, both
the nitro group and pyridine ring were reduced, affording
the amino-substituted 1,2,3,4-tetrahydroquinolines 2r and
In summary, we have developed a facile reduction of
nitro aromatics mediated by B (OH) in water under metal-
free conditions. A possible reaction pathway was also pro-
posed. Further investigations devoted to the mechanism
and synthetic application of this method are ongoing in our
laboratory.
2
4
10
2s in 76 and 73% yield, respectively. However, 6-nitro-
quinoline (1t) and 2-methyl-8-nitroquinoline (1u) had rela-
tively lower reactivity, and 6-aminoquinoline (2t) and 2-
methyl-8-aminoquinoline (2u) were isolated in 65 and 52%
yield, respectively, and the pyridine ring remained intact. In
addition, the nitro pyridine 1v was also applicable to this
reduction. The nitro group of 1v was reduced with the pyri-
dine ring remaining intact, giving the product 2v in 69%
yield. Next, the α,β-unsaturated nitro compound 1w and
aliphatic nitro compound 1x were also examined, but no
reaction occurred. Finally, enantiopure (S)-1-(4-nitro-
phenyl)ethan-1-ol (1y) with 83% ee was also reduced under
the optimized reaction conditions, but a racemic compound
Funding Information
National Natural Science Foundation of China (21202092). Startup
Foundation from the China Three Gorges University (KJ2012B080,
KJ2014H008).
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Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1610086.
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References and Notes
2y was obtained in 75% yield.
Inspired by the reaction mechanism in the reduction of
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2
2
6
(
Scheme 3). Initially, the nitro aromatic 1 coordinates with
B (OH) and H O to form a six-membered ring transition
2
4
2
state A, which eliminates B(OH) to afford intermediate B.
3
Then nitroso aromatic D is formed by rearrangement of B
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2
nitroso aromatic D coordinates with B (OH) and H O to
2
4
2
form a six-membered ring transition state E, which releases
B(OH) to afford intermediate F.
3
O–
OH
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O
B2(OH)4 Ar N+
HO
OH
Ar N+ O–
H
O
N+
H
B
Ar
O– H O
B
O
2
– B(OH)3
OH
B(OH)2
1
OH
A
B
(
3) Béchamp, A. Ann. Chim. Phys. 1854, 42, 186.
OH
O
N
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O
N
H
B (OH)4 Ar
N
H
2
O
H
HO
B
–
B(OH)3 Ar
B
O
Ar
OB(OH)2
H2O
OH
OH
C
D
E
OH
OH
N
H2O
– B(OH)3
B2(OH)4
N
Ar
B(OH)2
Ar
H
–
B(OH)3
– B(OH)3
F
G
(
5) Bae, J. W.; Cho, Y. J.; Lee, S. H.; Yoon, C. O. M.; Yoon, C. M. Chem.
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B(OH)2
2
H O
(
Ar NH2
N
Ar
H
– B(OH)3
2774.
2
H
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Scheme 3 A possible reaction pathway
(
(
(
10) (a) Xia, Y. T.; Sun, X. T.; Zhang, L.; Luo, K.; Wu, L. Chem. Eur. J.
The hydroxylamine G is generated after hydrolysis of F.
Then, G reacts with B (OH) to give intermediate H by re-
2016, 22, 17151. (b) Sharma, S.; Kumar, M.; Kumar, V.; Kumar,
2
4
N. J. Org. Chem. 2014, 79, 9433. (c) Orlandi, M.; Benaglia, M.;
Tosi, F.; Annunziata, R.; Cozzi, F. J. Org. Chem. 2016, 81, 3037. (d)
Kumar, M.; Sharma, U.; Sharma, S.; Kumar, V.; Singh, B.; Kumar,
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leasing B(OH) . The desired aromatic amine 2 is obtained
3
through hydrolysis of H.
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–D