A novel reduction of azides to amines with tellurium metal in near-critical water

Article abstract of DOI:10.1080/10426500307826

Tellurium metal in near-critical water (275°C) provides a novel and efficient method for the reduction of azides, producing amines in good yields.

Full text of DOI:10.1080/10426500307826

Phosphorus, Sulfur, and Silicon, 178:1807–1813, 2003
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Copyright Taylor & Francis Inc.
ISSN: 1042-6507 print
DOI: 10.1080/10426500390221465
A NOVEL REDUCTION OF AZIDES TO AMINES
WITH TELLURIUM METAL
IN NEAR-CRITICAL WATER
Lei Wang, Pinhua Li, Jianhui Chen, and Jincan Yan
Huaibei Coal Teachers College, Huaibei, Anhui, P. R. China
(Received January 8, 2003; accepted March 13, 2003)
Tellurium metal in near-critical water (275◦C) provides a novel and
efficient method for the reduction of azides, producing amines in good
yields.
Keywords: Amines; azides; near-critical water (NCW); reduction;
tellurium metal
Organic reactions carried out in water have received much more at-
tention in the last decade.1 Unfortunately most organic compounds
are poorly soluble in water at ambient temperature. Nonetheless, the
unique properties of water near its critical point (T = 374◦C, P =
c
c
221 bar) have encouraged researchers to use it instead of organic sol-
vents or ambient temperature water in organic synthesis. As water is
heated toward its critical point, it changes from a polar liquid to an
almost nonpolar fluid. Its dielectronic constant, ε, decrease from 78.5
at room temperature to 20 at 275◦C, favoring the solubility of organic
molecules and ions. Its dissociation constant, Kw, increases several or-
ders of magnitude from ambient to near-critical conditions (Kw = 10−11
at 275◦C), providing hydronium and hydroxide ions that can act as a
modest acid or base in chemical reactions.2 Although much of supercrit-
ical water research has been focused on the total oxidation of organic
We wish to thank the National Natural Science Foundation of China (No. 20172018),
the Excellent Scientist Foundation of Anhui Province (No. 2001040), the Natural Sci-
ence Foundation of the Education Department of Anhui Province (No. 2002kj254ZD), the
Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Edu-
cation Ministry (No. 2002247), and State Key Laboratory of Organometallic Chemistry,
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences for support of
this research.
Address correspondence to Lei Wang, Department of Chemistry, Huaibei Coal Teach-
ers College, Huaibei, Anhui 235000, P. R. China. E-mail: leiwang88@hotmail.com
1807

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L. Wang et al.
compounds and geochemical modeling,3 there is an increasing num-
ber of papers that suggest near-critical water (250∼325◦C) can be used
as an excellent solvent for organic reactions because organic reactions
in near-critical water offer many advantages over those in traditional
organic solvents or in supercritical water. For example, it is environ-
mentally benign and the separation of products from reaction mixtures
is simplified.4
Amines, widely used as important intermediates in the synthesis of
dyes, antioxidants and photographic, pharmaceutical and agricultural
chemicals can be obtained easily from the reduction of azides with good
regio, stereo, and enantioselectivity using catalytic hydrogenation5 and
a variety of other reducing agents. For example, zinc borohydride,6
iodotrimethylsilane,7 samarium-iodine,8 diboranes,9 benzyltriethyla-
minonium tetrathiomolybdate,10 and hexamethyldisilathiane11 have
been recommended for this transformation. However, most of them are
carried out in organic solvents, such as CH3CN, DME, CH3OH, THF,
HMPA, which pose waste handling problems. Recently, Poliakoff re-
ported the selective reduction of nitroarenes to anilines using metallic
zinc in near-critical water (250◦C).12 To our knowledge, there is no re-
port so far on the reduction of azides to amines with tellurium metal in
water. Here we report a novel reduction of azides by using metallic tel-
lurium powder in near-critical water (NCW) at 275◦C, which generate
amines in good yields.
Te/H2O
R-N3 −→ R-NH2
275◦C
RESULTS AND DISCUSSION
Our initial studies were directed toward exploring the reaction condi-
tions for the reduction of azides with metallic tellurium powder in hot
water. The results are summarized in Table I. p-Methylphenyl azide
was chosen as the model compound for this investigation.
As seen from Table I, the reaction temperature has a strong effect
on the reduction of p-methylphenyl azide with metallic tellurium in
hot water. It is evident that p-methylphenyl azide could not be reduced
to p-toluidine with tellurium metal in water at less than 200◦C with-
out any additive. Meanwhile, only a smaller yield of reduction product
was generated at temperatures between 200 and 250◦C. When the re-
action temperature has reached ca. 275◦C, a good yield of reduction
product was obtained. The ratio of tellurium to p-methylphenyl azide
also was examined. The results show that when the ratio of tellurium to

A Novel Reduction of Azides to Amines
1809
TABLE I The Reaction Conditions for the Reduction of
p-Methylphenyl Azide with Tellurium Metal in Watera
Entry
Te:Azide
Temp. (◦C)
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
9
10
11
12
13
3:1
3:1
3:1
3:1
3:1
1:1
2:1
4:1
5:1
3:1
3:1
3:1
3:1
200
225
250
275
300
275
275
275
275
275
275
275
275
4
4
4
4
4
4
4
4
4
1
3
5
7
0c
19
52
90
89
43
71
90
89
34
77
88
85
aReaction conditions: p-methylphenyl azide (1.00 mmol), tap water
(10 mL) in a high T/p batch reactor system.
bIsolated yields.
cStarting material p-methylphenyl azide (97%) was recovered.
p-methylphenyl azide is less than 2:1, the reduction was not complete
(entries 6 and 7, Table I). As it equals or is higer than 3:1, satisfactory
results were achieved (entries 4, 8, and 9, Table I). We also investi-
gated the effect of reaction time on the reduction of p-methylphenyl
azide and the results indicated that the reaction was not finished when
reaction time is less than 3 h (entries 10 and 11, Table I). However,
no increase of yield was observed when reaction time is prolonged
(entry 13, Table I). The optimized reaction conditions for the reduction of
p-methylphenyl azide with tellurium metal in water were found to
be Te (Eq. 3), p-methylphenyl azide (Eq. 1), H2O (10 mL) at 275◦C
for 4 h.
We observed that tellurium metal in near-critical water (NCW) at
275◦C reduced aryl and alkyl azides efficiently to the corresponding
amines in good yields. The reaction was carried out by stirring azide
in water with metallic tellurium powder under a nitrogen atmosphere
without any organics or additive at 275◦C for 4 h. The results are sum-
marized in Table II. As is evident from Table II, the substituent effect for
aryl azides has been investigated. The results show that the electronic
characteristics of a general electron-donating group (such as CH3) or a
general electron-withdrawing group (such as CH3CO, Cl) and its loca-
tion on the aromatic ring were relatively insensitive to the reduction of
azides. However, bromo or iodo substituents on the aromatic ring un-
derwent reductive elimination of Br or I in a competitive process, but

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L. Wang et al.
TABLE II The Reduction of Azide with Tellurium Metal in
Near-Critical Watera
Entry
Azides
Amines
C6H5NH2
p-CH3C6H4NH2
m-CH3C6H4NH2
m-CH3COC6H4NH2
p-ClC6H4NH2
p-BrC6H4NH2
C6H5NH2
m-ClC6H4NH2
n-C12H25NH2
n-C16H33NH2
C6H5NH2
Yield (%)b
1
2
3
4
5
6
C6H5N3
82
88
86
89
90
32c
40c
80
87
91
70
72
92
p-CH3C6H4N3
m-CH3C6H4N3
m-CH3COC6H4N3
p-ClC6H4N3
p-BrC6H4N3
7
8
9
10
11
12
m-ClC6H4N3
n-C12H25N3
n-C16H33N3
p-IC6H4N3
o-HO2CC6H4N3
α-C10H7N3
C6H5NH2
α-C10H7NH2
aReaction conditions: azide (1.00 mmol), tellurium metal
(3.00 mmol), tap water (10 mL) in a high T/p batch reactor system
at 275◦C for 4 h.
bIsolated yields.
cDetermined by GC and NMR analysis of the reaction mixture.
a chloro substituent remained unchanged. The reactivity of halogen
atoms on the aromatic ring is I > Br > Cl, which is consistent with the
expected reactivity and Poliakoff’s experimental results.12 A carboxylic
group on the aromatic ring also underwent a decarboxylation reaction,
along with the reduction process.
In conclusion, a novel, reliable, and practical procedure for the prepa-
ration of amines has been developed which involves the use of tel-
lurium metal as reductive agent and azides as starting material in
near-critical water. The method has the advantages of simple opera-
tion, good chemoselectivity, and use of an inexpensive, nontoxic, non-
flammable solvent. It may be a new way for the application of tellurium
metal directly in organic synthesis.
EXPERIMENTAL
Melting points were recorded on a WRS-1A melting point apparatus
1
and are uncorrected. H NMR spectra were recorded on a 300 MHz
Bruker AZ 300 spectrometer. Chemical shifts are given as δ value with
reference to tetramethylsilane (TMS) as internal standard. IR spectra
were obtained by using a Nicolet NEXUS 470 spectrophotometer. The

A Novel Reduction of Azides to Amines
1811
reagents were used as received from a commercial supply without pu-
rification prior to use.
General procedure for the reduction of azides with metallic tellurium
in near-critical water:
Azide (1.00 mmol) and metallic tellurium powder (383 mg, 3.00
mmol) were added to a high temperature and pressure stainless steel
reactor charged with tap water (10 mL) under a nitrogen atmosphere,
with stirring. The reactor was heated at 275◦C for 4 h. After cooling,
diethyl ether (10 mL × 2) was added to extract the products. After the
organic layer was dried with anhydrous sodium sulfate, the solvents
were evaporated under reduced pressure. The product was purified by
flash chromatography to yield amine.
Aniline. Oil.13 1H NMR (CDCl3): δ 7.23 (t, 2H, J = 8.6 Hz), 6.78 (m,
3H), 3.58 (s, 2H). IR (film): νmax 3460, 3362, 1600, 1495 cm−1. (Found:
C, 77.09; H, 7.79; N, 15.12; C6H7N requires: C, 77.38; H, 7.58; N, 15.04).
1
p-Methylaniline. m.p. 43–45◦C (lit.14 44–46◦C). H NMR (CDCl3):
δ 6.98 (d, 2H, J = 8.2 Hz), 6.62 (d, 2H, J = 8.3 Hz), 3.42 (s, 2H), 2.28 (s,
3H). IR (KBr): νmax 3426, 3325, 1609, 1508 cm−1. (Found: C, 78.63; H,
8.58; N, 12.79; C7H9N requires: C, 78.46; H, 8.47; N, 13.07).
p-Chloroaniline. m.p. 67–69◦C (lit.15 68–70◦C). 1H NMR (CDCl3): δ
7.04 (d, 2H, J = 8.5 Hz), 6.72 (d, 2H, J = 8.4 Hz), 3.40 (s, 2H). IR (KBr):
νmax 3470, 3412, 1600, 1504 cm−1. (Found: C, 56.23; H, 4.86; N, 11.21;
C6H6ClN requires: C, 56.49; H, 4.74; N, 10.98).
p-Bromoaniline. m.p. 62–64◦C (lit.16 62–64◦C). 1H NMR (CDCl3): δ
7.23 (d, 2H, J = 8.6 Hz), 6.57 (d, 2H, J = 8.5 Hz), 3.65 (s, 2H). IR (KBr):
νmax 3456, 3365, 1614, 1488 cm−1. (Found: C, 42.03; H, 3.25; N, 8.29;
C6H6BrN requires: C, 41.89; H, 3.52; N, 8.14).
1
m-Methylaniline. Oil. b.p. 199–201◦C (lit.17 200–202◦C). H NMR
(CDCl3): δ 7.02 (m, 1H), 6.56 (t, 1H, J = 8.1 Hz), 6.42 (m, 2H), 3.57 (s,
2H), 2.32 (s, 3H). IR (film): νmax 3416, 3335, 1605, 1503 cm−1. (Found:
C, 78.18; H, 8.59; N, 13.23; C7H9N requires: C, 78.46; H, 8.47; N, 13.07).
m-Chloroaniline. Oil. b.p. 220–222◦C (lit.18 223◦C). 1H NMR
(CDCl3): δ 7.04 (m, 1H), 6.72 (t, 1H, J = 7.2 Hz), 6.64 (s, 1H), 6.48 (m,
1H), 3.42 (s, 2H). IR (film): νmax 3456, 3405, 1612, 1514 cm−1. (Found: C,
56.72; H, 4.61; N, 10.74; C6H6ClN requires: C, 56.49; H, 4.74; N, 10.98).
m-Acetyaniline. m.p. 92–94◦C (lit.19 92–93◦C). 1H NMR (CDCl3): δ
7.24 (t, 1H, J = 7.7 Hz), 7.08 (m, 2H), 6.68 (m, 1H), 3.62 (s, 2H), 2.59
(s, 3H). IR (KBr): νmax 3446, 3354, 1659, 1612, 1489 cm−1. (Found: C,
70.86; H, 6.87; N, 10.55; C8H9NO requires: C, 71.09; H, 6.71; N, 10.36).
1-Naphthalenamine. m.p. 47–48◦C (lit.20 48–49◦C). 1H NMR
(CDCl3): δ 7.82 (m, 2H), 7.38 (m, 2H), 7.15 (m, 2H), 6.67 (d, 1H, J = 8.0
Hz), 3.55 (s, 2H). IR (KBr): νmax 3458, 3365, 1607, 1502 cm−1. (Found:
C, 83.59; H, 6.47; N, 9.94; C10H9N requires: C, 83.88; H, 6.34; N, 9.78).

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L. Wang et al.
1
1-Dodecylamine. m.p. 28–29◦C (lit.21 28◦C). H NMR (CDCl3): δ
2.65 (t, J = 2.06 Hz, 2H), 2.35 (s, 2H), 1.55-1.30 (m, 20H), 0.89 (t, J =
6.80 Hz, 3H). IR (film): νmax 3458, 3365, 1380 cm−1. (Found: C, 77.85;
H, 14.75; N, 7.40; C12H27N requires: C, 77.76; H, 14.68; N, 7.56).
1
1-Hexadecanamine. m.p. 45–47◦C (lit.22 47◦C). H NMR (CDCl3):
δ 2.68 (t, J = 2.10 Hz, 2H), 2.40 (s, 2H), 1.57-1.29 (m, 28H), 0.90 (t,
J = 6.90 Hz, 3H). IR (KBr): νmax 3462, 3360, 1380 cm−1. (Found: C,
79.82; H, 14.47; N, 5.71; C16H35N requires: C, 79.59; H, 14.61; N, 5.80).
Safety Warning: This procedure involves a high temperature and
pressure and only must be carried out in an apparatus with the appro-
priate pressure rating at the reaction temperature. It also should be
performed in a safe place.
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