Cobalt(II) chloride-catalyzed chemoselective sodium borohydride reduction of azides in water

Article abstract of DOI:10.1055/s-2000-6389

Reduction of azides to amines and amides was carried out with NaBH₄/CoCl₂·6H₂O in sole water at 25 °C under catalytic heterogeneous conditions. A broad spectrum of azides was reduced in a short time, chemoselectively in high yield and purity.

Full text of DOI:10.1055/s-2000-6389

646
PAPER
Cobalt(II) Chloride-Catalyzed Chemoselective Sodium Borohydride
Reduction of Azides in Water
Francesco Fringuelli, Ferdinando Pizzo,* Luigi Vaccaro
Università di Perugia, Dipartimento di Chimica, Via Elce di Sotto, 8-06123 Perugia, Italy
Fax +39(075)5855560
Received 4 October 1999; revised 7 February 2000
ter as a reaction medium should allow us to minimize the
excess of NaBH4, and to reuse the catalyst in further runs.
Abstract: Reduction of azides to amines and amides was carried
out with NaBH4/CoCl2•6H2O in sole water at 25 °C under catalytic
heterogeneous conditions. A broad spectrum of azides was reduced
in a short time, chemoselectively in high yield and purity.
We have been studying organic reactions in water and
have shown that an aqueous medium can be conveniently
Key words: azide, reduction, cobalt (II) chloride, sodium borohy- used in aldol-like condensations,28a epoxidations,28b-c and
Diels-Alder cycloadditions.28d Recently, we prepared
dride
b-azidoalcohols and a-hydroxy-b-azidocarboxylic acids
by a pH-controlled and metal-catalyzed azidolysis of
alkyl- and aryl-1,2-epoxides29a and 2,3-epoxycarboxylic
The reduction of the azido group to an amino group has
acids respectively.29b Our project concerns the synthesis
been extensively investigated because of a wide range of
of b-amino alcohols and b-hydroxy-a-amino acids in a
one-pot procedure starting from the corresponding ep-
oxides by azidolysis and subsequent reduction of the azi-
do group. Here, we report a mild and efficient NaBH4/
CoCl2-catalyzed reduction, in sole water, of azides to
amines and amides. The NaBH4/CoCl2 system has been
used to reduce several functional groups except azido, and
has never been used in water only.27
applications in organic synthesis, especially in nitrogen-
containing heterocycles1 and carbohydrate chemistry.2
Most of the reducing agents that have been used are of a
non-hydride type. Some have been known for a long time3
and others have been discovered in the last decade; the
best known are: SmI2,4 Sm/I2,5 Me3SiI,6 Fe/NiCl2·6H2O,7
AlI3,8 (PhCH2NEt2)2MoS4,9 Zn/NiCl2,10 In/NH4Cl,11 H2N-
NMe2/FeCl3·6H2O,12 and S(SiMe3)2.13
A variety of azides were reduced quickly at 25 °C and in
excellent yields with a catalytic amount of CoCl2•6H2O
(0.1 equivalents) in water, in the presence of NaBH4
(2.0 equivalents).30 The reaction was carried out in heter-
ogeneous phase by simply adding a water solution of
NaBH4 to a stirred mixture of azide and CoCl2•6H2O; this
addition was strongly exothermic. When the azide is high-
ly hydrophobic, the best results, in terms of yield and
time, are achieved by carrying out the reaction in the pres-
ence of 10% equivalents of cetyltrimethylammonium bro-
mide (CTABr). NaBH4 is unreactive towards the azido
group in both heterogeneous and homogeneous aqueous
medium in the presence of CTABr. The reduction is quan-
titative and the results are summarized in Table 1. The
azides 3a and 7a-10a and the products 3b-10b are new
compounds. Aliphatic, cycloaliphatic, aryl, aroyl, and
arysulphonyl azides are efficiently and quickly reduced
and the reducing agent tolerates functionalities such as
CO2H, CO2Me, OH, and OCH2O. Moreover, the reaction
is so fast that NO2, CMe2=CRH and CN functionalities are
not reduced.31
Hydride-reducing agents are widely employed, but some
of them have limitations. Diborane is not applicable to
compounds having double or triple bonds.3 LiAlH4 has to
be used in a large excess, requires a long reaction time and
is not tolerable to some functionalities (CO2R, NO2, CHO,
COR, epoxide).3,14,15 NaBH4 is a mild reducing agent and
except for arylsulphonyl azides does not usually reduce
azides to amines.3,14 Improvements have been obtained by
using NaBH4 under phase-transfer-catalysis conditions14
or in THF with stoichiometric amounts of methanol16 or
by using complex reducing agents such as NaBH4/
NiCl2,17 NaBH4/CuSO4,18 however, a complete chemose-
lectivity is not always achieved. Modified sodium borohy-
dride reducing agents such as borohydride exchange
20
resin-Ni(OAc)2,19 Zn(BH4)2,15 and LiMe2NBH3 have
also been used. Bu3SnH with a catalytic amount of azo-
bis(isobutyronitrile) has been employed to reduce azido-
nucleosides and azidodeoxysugars.21
In all cases, the organic solvent is the preferred reaction
medium and to our knowledge, sole water has never been
used.22
After removing the reaction product by ethyl ether extrac-
tion of the aqueous reaction mixture, the mother liquor
could be reused but its pH value is of crucial importance.
At the end of the reduction process, the aqueous medium
is strongly basic (pH>11) and if used as is, the reaction
proceeds with low yield. The best efficiency was obtained
by lowering the pH of the aqueous mixture to 8.0 by add-
ing some drops of concentrated HCl before adding the
This is quite surprising especially with reference to
NaBH4/metal salts as reducing systems. They are usually
used in methanol, although NaBH4 in this medium is con-
siderably less stable than in water at pH >7.25-27 In order
to perform the reduction in methanol26 a large excess (5-
10 equivalents) of hydride is required. The use of sole wa-
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Catalyzed Chemoselective Sodium Borohydride Reduction of Azides in Water
647
Table 1 Reduction of Azides with NaBH4/CoCl2◊6H2O in H2O at 25 °C
aYield of isolated product; b 10% mol equiv of CTABr.
azide and NaBH4. The yields of the 2nd, 3rd, 4th, and 5th for the reduction of azides, in sole water and under mild
runs for azides 1a and 11a, chosen as representative ex- conditions, reduces chemoselectively a broad spectrum of
amples, were excellent (Table 2).
azides to amines and amides with high yields. The easy
availability of the reducing system, the nature of the reac-
tion medium and the possibility to recycle the catalyst
make this procedure attractive and advantageous.
In conclusion, catalytic amount of the reducing system
NaBH4/CoCl2◊6H2O (catalyst), not previously employed
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648
F. Fringuelli et al.
PAPER
Table 2 Efficiency of the Reused Aqueous Mixture of Reducing
System at 25°Ca
2-(1¢, 3¢-Dioxan-2¢-yl)ethyl Azide (8a)
Oil.
IR (CCl4): 3674, 3504, 3005, 2961, 2859, 2508, 2114, 1705, 1470
cm-1.
1H NMR (CDCl3): d = 1.20-1.42 (s, 1H, H-5¢a), 1.86 (dt, J = 5.1,
6.8 Hz, 2H, H-2), 2.03-2.15 (m, 1H, H-5¢b), 3.39 (t, J = 6.8 Hz, 2H,
H-1), 3.81 (m, 2H, H-4¢a, H-6¢a), 4.10 (m, 2H, H-4¢b, H-6¢b), 4.65
(t, J = 5.0 Hz, 1H, H-2¢).
Run
1a → 1b
Yield (%)
11a → 11b
Yield (%)
1
2
3
4
5
95
97
98
96
96
97
96
95
95
97
13C NMR (CDCl3): d = 25.6, 34.4, 46.5, 66.8, 99.4.
Anal. Cald for C6H11N3O2: C, 45.85, H, 7.05, N, 26.74. Found: C,
45.91, H, 7.01, N, 26.78.
4-Azidobutanenitrile (9a)
Oil.
a After 10 min of reaction time.
IR (CCl4): 3254, 3014, 3941, 2872, 2504, 2254, 2105, 1731, 1705,
1290, 1258 cm-1.
1H NMR (CDCl3): d = 1.90 (q, J = 6.7 Hz, 2H, H-3), 2.40 (t, J = 6.7
Hz, 2H, H-2), 3.42 (t, J = 6.7, 2H, H-4).
13C NMR (CDCl3): d = 14.3, 24.7, 49.3, 118.5.
1H NMR and 13C NMR spectra were recorded on a Bruker DRX -
ADVANCE 400 MHz spectrometer; chemical shifts are reported in
ppm and coupling constants are reported in Hz, using CDCl3 or
CD3OD as solvents with TMS as internal standard. IR spectra were
obtained on a FT-IR Bruker IFS 113v spectrophotometer. Mi-
croanalyses were performed on Carlo Erba Elemental Analyzer
mod. 1106. Mps were taken on a Büchi 510 melting point apparatus
and are uncorrected. Reaction progress was monitored by GC anal-
ysis, performed on a Hewlett Packard 5890 Series II equipped with
an SPB-5 fused silica capillary column (30 m, 0.25 mm diameter),
an “on column” injector system, a FID detector and H2 as carrier
gas.
Anal. Cald for C4H6N4: C, 43.63, H, 5.49, N, 50.88. Found: C,
43.60, H, 5.51, N, 50.87.
5-Azido-2-methyl-2-pentene (10a)
Oil.
IR (CCl4): n = 2972, 2930, 2917, 2876, 2088, 1718, 1675, 1451,
1295, 1265, 828 cm-1.
1H NMR (CDCl3): d = 1.63-1.65 (m, 3H, Me), 1.71-1.73 (m, 3H,
Me), 2.30 (br q, J = 7.1 Hz, 2H, H-4), 3.24 (t, J = 7.1 Hz, 2H, H-5),
5.11 (br t, J = 7.1 Hz, 1H, H-3).
13C NMR (CDCl3): d = 17.6, 25.6, 27.8, 51.0, 119.8, 134.8.
Compounds 1a-11a, and 14a were prepared by following known
procedures.29, 32 Compounds 12a, 13a, 1b, 2b, and 11b-14b are
commercial. Compounds 3a, 7a-10a, and 3b-10b are new com-
pounds. The azides 3a and 7a were prepared by azidolysis of the
corresponding epoxides under basic conditions29a and Cu(II) cata-
lyzed,29b respectively. The azides 8a, 9a, and 10a were prepared by
following the described procedure starting from the respective bro-
mide.32
Anal. Calcd for C6H11N3: C, 57.57, H, 8.86, N, 33.57. Found: C,
57.63, H, 8.83, N, 33.62.
Reduction of Azides with NaBH4/CoCl2◊6H2O in H2O; General
Procedure
To a mixture of azide (2.0 mmol) and CoCl2•6H2O (0.048 g,
0.2 mmol), and when necessary (Table 1) CTABr (0.07 g, 0.2
mmol), at 25 °C was added dropwise under stirring a solution of
NaBH4 (0.0152 g, 4.0 mmol) in H2O (4 mL). The formation of a
black precipitate indicated the formation of a cobalt boride species.
The mixture was stirred at 25 °C for more than 10 min when neces-
sary (Table 1). At the end of the reaction the mixture was extracted
with Et2O (5 ¥ 10 mL). The organic phase was dried (Na2SO4) and
concentrated under reduced pressure to give the pure amine or
amide, The yields are reported in Table 1.
1-Azido-3-(1¢,3¢-benzodioxol-5¢-yl)propan-2-ol (3a)
Oil.
IR (CCl4): n = 3421, 2916, 2901, 2106, 1503, 1489, 1443, 1249,
928, 810 cm-1.
1H NMR (CDCl3): d = 1.99 (br s, 1H, OH), 2.64-2.67 (m, 2H, H-
3), 3.20 (dd, J = 6.6, 12.5 Hz, 1H, H-1a), 3.31 (dd, J = 3.9, 12.5 Hz,
1H, H-1b), 3.81-3.94 (m, 1H, H-2), 5.87 (s, 2H, -OCH2O-), 6.55-
6.74 (m, 3H, H-Ar).
13C NMR (CDCl3): d = 40.2, 55.6, 71.5, 100.7, 108.1, 109.4, 122.0,
130.7, 146.1, 147.6.
Re-use of Reducing Agent
The pH of remaining mother liquor (approx. 4 mL) after extraction
with Et2O of amine or amide was adjusted to 8.0 by adding a few
drops of concd HCl. Azide (2.0 mmol) was then added, followed by
NaBH4 (0.0152 g, 4.0 mmol) in small doses. The mixture was
stirred at 25 °C for more than 10 min. when necessary (Table 1) and
then extracted with Et2O. The mother liquor can continue to be re-
used. Two examples are illustrated in Table 2.
Anal. Calcd for C10H11N3O3: C, 54.29, H, 5.01, N, 19.00. Found: C,
54.65, H, 4.98, N, 18.79.
Methyl trans-2-Azido-1-hydroxycyclohexanecarboxylate (7a)
Oil.
IR (CCl4): n = 3521, 2944, 2865, 2102, 1729, 1449, 1436, 1356,
1260, 869 cm-1.
1-Amino-3-(1¢, 3¢-benzodioxol-5¢-yl)propan-2-ol (3b)
Oil.
1H NMR (CD3OD): d = 2.63-2.85 (m, 3H, H-1, H-3), 2.95 (m, 1H.
H-1), 3.85-4.00 (m, 1H, H-2), 5.88 (s, 2H, -OCH2O-), 6.63-6.78
(m, 3H, H-Ar).
1H NMR (CDCl3): d = 1.20-2.05 (m, 8H), 3.52 (dd, J = 4.9, 9.4 Hz,
1H, H-1), 3.61 (br s, 1H, OH), 3.82 (s, 3H, Me).
13C NMR (CDCl3): d = 21.4, 22.4, 27.3, 33.8, 52.8, 66.2, 76.3,
174.3.
Anal. Cald for C8H13N3O3: C, 48.23, H, 6.58, N, 21.09. Found: C,
48.25, H, 6.61, N, 21.08.
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Catalyzed Chemoselective Sodium Borohydride Reduction of Azides in Water
649
13C NMR (CD3OD): d = 42.1, 45.3, 70.0, 102.1, 107.5, 110.5,
123.4, 132.0, 147.7, 149.0.
4-Aminobutanenitrile (9b)
Oil.
Anal. Calcd for C10H13NO3: C, 61.53, H, 6.71, N, 7.17. Found: C,
61.60, H, 6.68, N, 7.21.
IR (CCl4): 3383, 3319, 2957, 2872, 2478, 2250, 2104, 1699, 1602,
1426, 1377, 1315, 1090, 885, 859 cm-1.
1H NMR (CDCl3): d = 1.95 (q, J = 6.9 Hz, 2H, H-3), 2.41 (t, J = 6.9
Hz, 2H, H-2), 2.81 (t, J = 6.9 Hz, 2H, H-4).
13C NMR (CDCl3): d = 14.5, 28.7, 40.4, 120.7.
(-)-3b-Amino-4a-hydroxy-trans-carane (4b)
Mp 88-89 °C (n-hexane/Et2O); [a]D 25-8° (c 0.98, CHCl3)
1H NMR (CDCl3): d = 0.59-0.71 (m, 2H, H-1, H-6), 0.90 (s, 3H, H-
8), 0.93 (s, 3H, H-9), 1.02 (br s, 3H, H-10), 0.97-1.06 (m, 1H, Hb-
2), 1.59 (ddd, J = 7.8, 10.2, 14.4 Hz, 1H, Hb-5), 1.80 (dd, J = 9.4,
14.4 Hz, 1H, Ha-2), 1.98 (dd, J = 7.2, 14.4 Hz, 1H, Ha-5), 2.15 (br
s, 3H, NH2, OH), 3.09 (dd, J = 7.3, 10.2 Hz, 1H, H-4).
13C NMR (CDCl3): d = 15.8, 17.5, 19.5, 19.6, 21.4, 27.2, 28.8, 35.7,
52.3, 74.2.
Anal. Calcd for C4H8N2: C, 57.11, H, 9.59, N, 33.30. Found: C,
57.13, H, 9.64, N, 33.31.
4-Methylpent-3-enylamine (10b)
IR (CCl4): n = 2970, 2910, 2903, 2856, 1718, 1681, 1429, 1291,
1210, 821 cm-1.
1H NMR (CDCl3): d = 1.60 (s, 3H, Me), 1.68 (s, 3H, Me), 1.78 (br
s, 2H, NH2), 2.10 (dd, J = 6.8, 13.7 Hz, 2H, H-2), 2.70 (dd, J = 6.6,
6.7, Hz, 2H, H-1), 5.06 (m, 1H, H-3).
Anal. Calcd for C10H19NO: C, 70.96, H, 11.31, N, 8.27. Found: C,
70.85, H, 11.45, N, 8.25.
13C NMR (CDCl3): d = 17.7, 25.7, 32.1, 41.9, 121.6, 132.7.
(+)-4b-Amino-3a-hydroxy-cis-carane (5b)
Mp 113-114 °C (n-hexane/Et2O); [a]D25+35° (c 1.80, CHCl3)
Anal. Calcd for C6H13N: C, 72.66, H, 13.21, N, 14.12. Found: C,
72.58, H, 13.25, N, 14.13.
1H NMR (CDCl3): d = 0.56-0.71 (m, 2H, H-1, H-6), 0.77-0.88 (m,
1H, Hb-5), 0.95 (s, 3H, H-8), 0.97 (s, 3H, H-9), 1.07 (s, 3H, H-10),
1.29 (dd, J = 6.1, 15.4 Hz, 1H, Hb-2), 1.81 (dd, J = 8.5, 15.4 Hz,
1H, Ha-2), 1.96-2.06 (m, 1H, Ha-5), 2.30 (br s, 3 H, NH2, OH),
2.78 (dd, J = 5.3, 10.5 Hz, 1H, H-4).
13C NMR (CDCl3): d = 15.8, 18.3, 20.8,, 24.7, 26.9, 28.6, 31.8,
34.2, 56.6, 72.5.
Acknowledgement
This work was supported by the Consiglio Nazionale delle Ricerche
(CNR), Ministero dell’Università e della Ricerca Scientifica e Tec-
nologica (MURST) and Università degli Studi di Perugia.
Anal. Calcd for C10H19NO: C, 70.96, H, 11.31, N, 8.27. Found: C,
70.80, H, 11.40, N, 8.20.
References and Notes
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1-Aminooctan-2-ol (6b)
1H NMR (CDCl3): d = 0.89 (t, J = 6.9 Hz, 3H, H-8), 1.55-1.75 (m,
10H), 1.55 (br s, 3H, NH2, OH), 2.51 (dd, J = 8.3, 12.6 Hz, 1H, H-
1a), 2.86 (dd, J = 3.3, 12.6 Hz, 1H, H-1b), 3.40-3.60 (m, 1H, H-2).
13C NMR (CDCl3): d = 14.4, 23.7, 26.7, 30.5, 33.0, 36.0, 48.0, 72.7.
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Anal. Calcd for C8H19NO: C, 66.16, H, 13.19, N, 9.64. Found: C,
66.23, H, 13.12, N, 9.63.
Methyl trans-2-amino-1-hydroxy-cyclohexanecarboxylate (7b)
Oil.
IR (CCl4): n = 3526, 2941, 2864, 2362, 1725, 1450, 1436, 1362,
1302, 1231 cm-1.
1H NMR (CDCl3): d = 1.20-2.08 (m, 8H), 2.23 (br s, 1H, OH), 2.73
(dd, J = 5.0, 10.2 Hz, 1H, H-2), 3.77 (s, 3H, Me).
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174.9.
Anal. Calcd for C8H15NO3: C, 55.47, H, 8.73, N, 8.09. Found: C,
55.48, H, 8.77, N, 8.10.
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2-(1¢, 3’-dioxan-2¢-yl)ethyl amine (8b)
Oil.
IR (CCl4): 3379, 2972, 2860, 2338, 1588, 1379, 1143, 1003, 804.3
cm-1.
1H NMR (CDCl3): d = 1.20-1.32 (m, 1H, H-5¢a), 1.68 (dt, J = 5.1,
6.6 Hz, 2H, H-2), 1.85 (br s, 2H, NH2), 1.94-2.12 (m, 1H, H-5¢b),
2.75 (t, J = 6.6 Hz, 2H, H-1), 3.60-3.78 (m, 2H, H.4¢a, H-6¢a),
3.95-4.10 (m, 2H, H-4¢b, H-6¢b), 4.58 (t, J = 5.1 Hz, 1H, H-2¢).
13C NMR (CDCl3): d = 22.2, 25.6, 37.1, 38.4, 66.7, 101.1.
Anal. Calcd for C6H13NO2: C, 54.94, H, 9.99, N, 10.68. Found: C,
54.99, H, 9.97, N, 10.71.
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F. Fringuelli et al.
PAPER
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(30) Cobalt (II) chloride with aqueous NaBH4 generates a black
precipitate of cobalt boride, which catalyzes the
decomposition of borohydride with hydrogen evolution. The
reduction is more like catalytic hydrogenation, but a cobalt
hydride species as active reducing agent cannot be excluded.27
(31) When the reaction time is extended to 4 hours, the double
bond of compound 10b is reduced.
(32) Curphey, T. J. Org. Prep. Proced. Int. 1981, 13, 12.
(27) Ganem, B.; Osby, J. O. Chem Rev. 1986, 86, 763.
(28) a) Fringuelli, F.; Pani, G.; Piermatti, O.; Pizzo, F. Tetrahedron
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Article Identifier:
b) Fringuelli, F.; Germani, R.; Pizzo, F.; Savelli, G.
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Synthesis 2000, No. 5, 646–650 ISSN 0039-7881 © Thieme Stuttgart · New York