Recyclable zinc (II) ionic liquid catalyzed synthesis of azides by direct azidation of alcohols using trimethylsilylazide at room temperature

Article abstract of DOI:10.1016/j.tetlet.2017.05.046

A new efficient method has been reported for the synthesis of azides by direct azidation of alcohols with TMSN₃ in presence of recyclable task specific ionic liquid (TSIL) [bmim]ZnCl₃ as a catalyst in DCM at room temperature. Ionic liquid [bmim]ZnCl₃ was synthesized under solvent free conditions and characterized by IR, ¹H NMR, ¹³C NMR and HRMS. The Lewis acidity of catalyst was also examined using IR spectroscopy. The main features of this new methodology are high yields of products, recyclability of catalyst, scalability of reaction to gram scale and short reaction time.

Full text of DOI:10.1016/j.tetlet.2017.05.046

Accepted Manuscript
Recyclable Zinc (II) ionic liquid catalyzed synthesis of azides by direct azidation
of alcohols using trimethylsilylazide at room temperature
Ashima Singh, Harjinder Singh, J.M. Khurana
PII:
S0040-4039(17)30627-5
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.05.046
TETL 48939
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
18 March 2017
12 May 2017
13 May 2017
Please cite this article as: Singh, A., Singh, H., Khurana, J.M., Recyclable Zinc (II) ionic liquid catalyzed synthesis
of azides by direct azidation of alcohols using trimethylsilylazide at room temperature, Tetrahedron Letters (2017),
doi: http://dx.doi.org/10.1016/j.tetlet.2017.05.046
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2
Graphical Abstract
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Recyclable Zinc (II) ionic liquid catalyzed
synthesis of azides by direct azidation of
alcohols using trimethylsilylazide at room
temperature
Ashima Singha, Harjinder Singha,b, J.M. Khuranaa*
[bmim]ZnCl3 (5 mol%)
DCM, RT
RR'R"COH
TMSN3
RR'R"CN3
+
R = Aryl; R' = alkyl, aryl;
R" = H, aryl, alkyl
High yields of products upto 94%
Short reaction time (20-25 min)
Recyclability of catalyst
Scalability of reaction to gram scale

1
Tetrahedron Letters
journal homepage: www.els evier.com
Recyclable Zinc (II) ionic liquid catalyzed synthesis of azides by direct azidation
of alcohols using trimethylsilylazide at room temperature
Ashima Singha, Harjinder Singha,b, J.M. Khuranaa*
a Department of Chemistry, University of Delhi, Delhi-110007, India,
b Department of Chemistry, M.M. Modi College, Patiala-147001, Punjab, India.
Email: jmkhurana@chemistry.du.ac.in, jmkhurana1@yahoo.co.in
* Corresponding author. Tel. +91 11 27667725-1384; fax: +91 11 27666605
ARTICLE INFO
ABSTRACT
Article history:
Received
A new efficient method has been reported for the synthesis of azides by direct azidation of
alcohols with TMSN3 in presence of recyclable task specific ionic liquid (TSIL) [bmim]ZnCl3
as a catalyst in DCM at room temperature. Ionic liquid [bmim]ZnCl3 was synthesized under
solvent free conditions and characterized by IR, 1H NMR,13C NMR and HRMS. The Lewis
acidity of catalyst was also examined using IR spectroscopy. The main features of this new
methodology are high yields of products, recyclability of catalyst, scalability of reaction to
gram scale and short reaction time.
Received in revised form
Accepted
Available online
Keywords:
2009 Elsevier Ltd. All rights reserved
.
Ionic liquid
Lewis acid
Azides
Green methodology
Organic azides are energy-rich, flexible nitrogenous
compounds which have numerous applications in organic
synthesis.1,2 Azides are used as starting material for the copper-
catalyzed azide–alkyne cycloaddition (CuAAC) reactions for the
synthesis of biologically and industrially important 1,2,3-
triazoles3,4 and in areas of material science.5,6 The azide group
itself can act as a pharamacophere and is a key component of the
HIV/AIDS drug zidovudine.7 Azides have been employed for the
synthesis of tetrazoles,8 isoquinolines,9 benzylamines,10
N-arylmethylarenes,11 quinolines,12 phenanthridines13 for Schmidt
reaction14 and Schmidt–Mannich type rearrangement reaction.15
They act as 1,3-dipoles and thus undergo 1,3-dipolar
found applications in synthetic organic chemistry as a catalyst20
but is much less explored compared to other Lewis acid ionic
liquids. Thus, in view of the importance of organic azides and
limitations of their methods of synthesis, we decided to explore
the application of environmentally benign [bmim]ZnCl3 as a
recyclable catalyst for the syntheses of various organic azides by
direct azidation of alcohols using trimethylsilylazide.
In this paper, we have reported an efficient and simple
protocol for the synthesis of organic azides directly from alcohols
using stable, non-explosive azidotrimethylsilane (TMSN3) in
presence of 5 mol % of Lewis acid ionic liquid [bmim]ZnCl3 as a
catalyst in DCM at room temperature.
cycloaddition reactions to give
heterocyclic compounds.
a variety of nitrogenous
The Lewis acidic ionic liquid [bmim]ZnCl3 was prepared
according to Scheme 1. In the first step, ionic liquid 1-methyl-3-
butylimidazolium chloride [bmim]Cl was obtained by the
reaction of 1-methylimidazolium (1.0 mmol) and butyl chloride
The synthesis of azides from alcohols is generally a two step
reaction involving conversion of alcohol to the corresponding
halide or sulfonate, followed by nucleophilic substitution by
azide anion. One of the disadvantages of using sodium azide,
besides its high toxicity, is its sensitivity to external factors such
as heat, light or pressure, which can cause them to explode when
handled in large quantity.16 Therefore methods using safer azide
substitutes such as diphenylazidophosphate17 and di-4-
nitrophenyl azidophosphate18 have been reported. The conversion
of alcohols directly to azides in one step reaction has been
reported by a combination of a Lewis acid such as copper
triflate,19a bismuth triflate,19b povidone-phophotungstic acid,19c
silver triflate19d and trimethylsilylazide. The reported methods
have few disadvantages such as long reaction time, poor yield,
and use of toxic organic solvents and catalysts. [bmim]ZnCl3 has
o
(1.0 mmol) at 80 C for 72 h.21 The desired Lewis acid catalyst
was then prepared by reaction of equimolar amounts of [bmim]Cl
and ZnCl2 under solvent-free conditions at 80oC for 10 h unlike
the reported procedure in solution and subsequent removal of
solvent and vacuum drying20 (Scheme 1).
80 o
C
N
N
+
ZnCl2
N
N
Cl
10 h
ZnCl3
[bmim]Cl
[bmim]ZnCl3, Yield = 83%
Scheme 1. Synthesis of Lewis acid ionic liquid [bmim]ZnCl

2
alcohol (entry 18) did not undergo any reaction under these
conditions. Phenol gave trimethylsilyloxy benzene under these
conditions (entry 19).
The formation of [bmim]ZnCl3 was confirmed by
1
spectroscopic studies i.e. IR, H NMR, 13C NMR and HRMS.
The IR spectra of [bmim]ZnCl3 showed peaks at 3106 cm-1,
1567 cm-1, 1460 cm-1, 748 cm-1 and 651 cm-1. 1H NMR spectra of
[bmim]ZnCl3 showed a triplet at δH 0.83-0.86 ppm due to
terminal methyl protons, two multiplets at δH 1.18-1.23 ppm and
δH1.68-1.75 ppm due to two methylene protons of n-butyl chain.
The two methylene protons (NCH2) of butyl chain appeared as
triplet at δH 4.10-4.14 ppm, while three methyl protons (NCH3)
appeared as singlet at δH 3.80 ppm. Three aromatic protons of
imidazole ring appeared in aromatic region at δH 9.06, 7.71, 7.64
ppm. The 13C NMR spectra of [bmim]ZnCl3 gave 8 peaks
corresponding to eight non-equivalent carbon atoms. The
composition of ionic liquid was further confirmed by recording
ESI-HRMS in both positive mode as well as in negative mode.
The positive mode HRMS showed a peak at m/z = 139.1224
corresponding to the [bmim]+H+ (Calc. for [bmim]+H+
m/e 139.1235) cation of the ionic liquid, while negative mode
HRMS of [bmim]ZnCl3 showed peak at m/e 168.8349 for ZnCl3
anion (Calc. for ZnCl3 m/e 168.8357) which confirms the
composition as well as purity of ionic liquid [bmim]ZnCl3.
OH
N3
Lewis acid
Solvent
... (1)
+
TMSN3
1a
2a
Table 1 Optimization of reaction conditions for synthesis of
organic azides using [bmim]ZnCl3
Entry
1.
Lewis acid catalyst
Solvent
MeCN
DMSO
DCM
-
Temperature
Time
(min)
Yield
(%)
[bmim]ZnCl3
(5 mol%)
RT
RT
RT
RT
360
360
20
20a
35a
94
2.
[bmim]ZnCl3
(5 mol%)
3.
[bmim]ZnCl3
(5 mol%)
4.
[bmim]ZnCl3
(5 mol%)
240
72
The Lewis acidity of [bmim]ZnCl3 was examined by FT-IR
spectroscopy using pyridine as a probe molecule and examining
IR bands in the range of 1400–1700 cm−1 arising from its ring
vibration modes. Pyridine showed a single absorption band at
1437 cm−1 while in presence of [bmim]ZnCl3 this absorption band
shifts to 1449 cm−1 which indicates that pyridine is coordinated to
Lewis acid sites and thus [bmim]ZnCl3 showed Lewis acidity and
hence can act as a acid catalyst. The solubility of [bmim]ZnCl3
was also examined. The ionic liquid is soluble in water,
methanol, acetone, chloroform and acetonitrile and insoluble in
ethyl acetate, toluene and petroleum ether.
5.
6.
-
DCM
DCM
RT
240
20
-
[bmim]ZnCl3
(5 mol%)
50 oC
93
7.
8.
[bmim]ZnCl3
(10 mol%)
DCM
DCM
DCM
DCM
DCM
DCM
RT
RT
RT
RT
RT
RT
20
60
92
81
72
70
83
ZnCl2
(10 mol%)
9.
InBr3
40
The application of [bmim]ZnCl3 as Lewis acid catalyst was
investigated for the synthesis of azides by direct azidation of
alcohols. The optimum reaction conditions were identified by
attempting model reaction of diphenylmethanol (1a, 1.0 mmol)
and trimethylsilylazide (1.1 mmol) (eq. 1) under different
reaction conditions by varying catalyst, catalyst loading, solvent
and temperatures as summarized in Table 1 (entries 1-12).
However, the best results were obtained when the reaction was
attempted using DCM as solvent in presence of [bmim]ZnCl3
(5 mol %) at room temperature as desired product
(azidomethylene)dibenzene (2a) was obtained in 94% yield in
20 min (Table 1, entry 3). The structure of
(10 mol%)
10.
11.
12.
CuCl
200
60
(10 mol%)
Bi(OTf)3
(10 mol%)
b
-
p-TSA
360
a Incomplete reaction, b No reaction,
A plausible reaction mechanism for the synthesis of organic
azides from alcohols is shown in Scheme 2. The first step
involves the interaction of Lewis acidic ionic liquid [bmim]ZnCl3
with alcohol to give complex A, which undergoes nucleophilic
substitution with TMSN3 to give the desired organic azides.
1
(azidomethylene)dibenzene (2a) was confirmed by IR H NMR
and 13C NMR spectra. Reaction attempted under other reaction
conditions gave inferior yields of product 2a and also required
longer reaction time as shown in Table 1 (entries 1-2, 4-12).
Thus, the reaction of diphenylmethanol and trimethylsilylazide
(1.1 mmol) using 5 mol% of [bmim]ZnCl3 in DCM at room
temperature was chosen as the optimum reaction condition to
further explore this reaction.
+
[bmi
m] ZnC
-
TMSN
l3
3
R
R''
+
-
R''
OH
[bmi
m] ZnC
l3
R
R''
N3
R
O
+
OH
R'
H
R'
R'
A
The scope of this reaction was further explored by attempting
reactions of different alcohols with trimethylsilylazide under
optimized conditions. All the reactions were complete in less
than 30 min and gave the corresponding azides in high yields.
The results have been summarized in Table 2. The reaction can
be generalized as in eq.2.
Scheme 2. Plausible reaction mechanism for synthesis of
azides from alcohols using TMSN3
In order to determine the type of nucleophilic substitution
(SN1 or SN2) mechanism, we performed a reaction of optically
pure S-(-)-1-phenylethanol with trimethylsilylazide using
[bmim]ZnCl3 in DCM at room temperature. The corresponding
product was obtained in low optical purity of 24% ee22 and the
remaining 76% product was consisted of racemic mixture.
Therefore, the reaction is taking place predominantly via (SN1)
pathway.
1
All the products were characterized by IR, H NMR and 13C
NMR spectra. It can be inferred from the results in Table 2 that
only secondary and tertiary benzylic alcohols could be converted
to corresponding azides (entries 1 to 12). Secondary alcohols
attached to heteroaryl group could also be converted to
corresponding azides (entries 13, 14). However, primary benzylic
alcohol (entry 15), aliphatic secondary alcohol (entry 16), allylic
secondary alcohol (entry 17) and α-keto secondary benzylic

3
[bmim]ZnCl3 (5 mol%)
DCM, RT
RR'R"COH
1
RR'R"CN3
2
...(2)
TMSN3
+
R = Aryl; R' = alkyl, aryl; R" = H, aryl, alkyl
Table 2. Synthesis of organic azides from their corresponding alcohols (2a-2s)
S.No
Alcohol
Product
Time
(min)
Yield (2) (%)
OH
N3
1.
20
94
2a
N3
2b
1a
OH
2.
3.
20
25
20
20
20
25
25
20
20
90
82
80
92
92
94
85
85
92
1b
N3
OH
Cl
Cl
1c
2c
OH
N3
4.
Br
Br
1d
2d
N3
OH
5.
MeO
MeO
1e
2e
OH
N3
6.
Cl
Cl
1f
2f
OH
N3
7.
MeO
OMe
MeO
OMe
1g
2g
N3
OH
8.
1h
2h
2i
OH
N3
9.
1i
1j
OH
N3
10.
2j

4
OH
N3
11.
12.
20
20
88
94
1k
2k
OH
N3
1l
2l
OH
N3
S
S
13.
14.
15.
16.
17.
18.
20
20
85
84
1m
2m
OH
N3
N
N
1n
2n
_
100
100
100
100
25
NR
NR
NR
NR
90
OH
1o
OH
_
_
_
1p
O
OH
1q
OH
1r
OTMS
OH
19.
1s
2s
NR = No Reaction
The reaction of diphenylmethanol (1a) with [bmim]ZnCl3 in
absence of TMSN3 in dry DCM at room temperature, resulted in
the formation of oxybis(methanetriyl)tetrabenzene (Scheme 3).
The possibility of azide formation via corresponding ether was
discarded because oxybis(methanetriyl)-tetrabenzene did not
undergo any reaction with TMSN3 in DCM in presence or
absence of ionic liquid.
We have also attempted the gram scale reaction to show the
utility of this method on industrial scale. The reaction of 4 g
diphenylmethanol(1a) was attempted with TMSN3 using
[bmim]ZnCl3 (5 mol%) in DCM at room temperature
(Scheme 4). The reaction was complete in 35 min yielding
corresponding (azidomethylene)dibenzene (2a) in 92% (4.17 g)
yield. Thus, this procedure can be used on industrial scale also
for gram scale synthesis of azides.
OH
OH
N3
[bmim]ZnCl3 (5 mol%), DCM
RT
TMSN3
O
No Reaction
+
TMSN3
[bmim]ZnCl3 (5 mol%), DCM
RT
1a
1a
2a
Oxybis(methanetriyl)tetrabenzene
Yield = 55%
4.0 g
4.17 g, Yield = 92%
Scheme 3. Formation of oxybis(methanetriyl)tetrabenzene
Scheme 4. Gram scale synthesis of azide 2a

5
The recycling of catalyst [bmim]ZnCl3 has also been
investigated for the synthesis of 2a. After completion of the
reaction as monitored by TLC, the solvent was removed under
vacuum and the product was removed by washing with ethyl
acetate (3 × 5 mL). The remaining ionic liquid was dried and
reused for subsequent reactions. No significant loss in the yield
of 2a was observed even after three cycles as 2a was obtained in
94%, 92% and 88% yield after first, second and third cycle,
respectively. The purity of recovered catalyst [bmim]ZnCl3 was
8. (a) Hansen SG, Jensen HH. Synlett. 2009; 3275-3278;
(b) Hiroki H, Ogata K, Fukuzawa SI, Synlett. 2013;
24:843-846; (c) Yamada YMA, Sarkar SM, Uozumi Y.
J Am Chem Soc. 2012; 134: 9285-9286.
9. (a) Fischer D, Tomeba H, Pahadi NK, Patil NT,
Yamamoto Y. Angew Chem Int Ed. 2007; 46(25): 4764-
4766; (b) Fischer D, Tomeba H, Pahadi NK, Patil NT,
Huo Z, Yamamoto Y. J Am Chem Soc. 2008; 130:
15720-15725.
1
confirmed by comparing its IR and H NMR spectra with that of
pure sample. The recovered catalyst was found to be pure for use
in subsequent reactions. Therefore, [bmim]ZnCl3 acts as an
excellent recyclable catalyst for synthesis of azides in good yield.
10. (a) Lee JG, Choi KI, Koh HY, Kim Y, Kang Y, Cho
YS. Synthesis. 2001; 1: 81-84; (b) Bosch I, Costa AM,
Martín M, Urpí F, Vilarrasa J. J. Org Lett. 2000; 2(3):
397-399.
In conclusion, we have developed a new efficient method for
the synthesis of azides by direct azidation of alcohols with
TMSN3 using recyclable 5 mol% Lewis acidic ionic liquid
[bmim]ZnCl3 as a catalyst in DCM at room temperature. The
catalyst [bmim]ZnCl3 was synthesized and characterized by IR,
1H NMR, 13C NMR and HRMS. The Lewis acidity of catalyst
was also examined using IR spectroscopy. The main features of
this new protocol are high yield of product, recyclability of
catalyst, scalability of reaction to gram scale and short reaction
times.
11. Tummatorn J, Thongsornkleeb C, Ruchirawat S.
Tetrahedron. 2012; 68(24): 4732-4739.
12. Tummatorn J, Thongsorn kleeb C, Ruchirawat S,
Gettongsong T.Org Biomol Chem. 2013;11: 1463-1467.
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Acknowledgments
AS thank UGC New Delhi for financial assistance (grant no-
2016-2017/PDFWM-2015-17-UTT-32744).
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Milligan GL, Aubé J. J Am Chem Soc. 2000; 122(30):
7226-7232.
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I, Costa AM, Martín M, Urpí F, Vilarrasa J. J. Org.
Lett. 2000; 2: 397-399.
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Y, Chen J, Cheng YE, Ding K. J Med Chem. 2012;
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−2153; (b) Thomas KD,
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Adhikari AV,
Shetty NS. Eur J Med Chem. 2010; 45(9): 3803–3810;
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Synthesis of Ionic Liquid [bmim]ZnCl3
5. Buckle D R, Rockell CJM. J Chem Soc Perkin Trans.
1982; 1: 627-630.
Firstly, Ionic liquid [bmim]Cl was prepared according to
reported procedure [21]. A mixture of equimolar amounts of
o
[bmim]Cl and ZnCl2 were stirred at 80 C for 10 h in round–
bottomed flask. The new ionic liquid was washed with ethyl
6. (a) Buckle D R, Outred DJ, Rockell CJM, Smith H,
Spicer BA. J Med Chem. 1983; 26 (2) 251-254; (b)
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acetate and dried at 70 C for 24 h to obtained pure task specific
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6
General procedure for the synthesis of azides from
alcohols (2)
A mixture of alcohol (1.0 mmol), trimethylsilylazide (1.1
mmol), [bmim]ZnCl3 (5 mol%) and dry DCM (3 mL) was placed
in a 50 mL round–bottomed flask under nitrogen atmosphere.
The reaction contents were stirred magnetically at room
temperature for appropriate time as mentioned in Table 2. The
progress of the reaction was monitored by TLC (eluent: ethyl
acetate: petroleum ether, 10:90, v/v). After completion of the
reaction as indicated by TLC, it was quenched with water (5 mL).
The reaction mixture was extracted with DCM (3 × 5 mL), The
combined organic layer was dried over anhydrous Na2SO4,
filtered, and the solvent removed under vacuum to get crude
product, which was purified by column chromatography using
(eluent: ethyl acetate: petroleum ether, 1:99, v/v) to give
corresponding product in good yield as mentioned in Table 2.
Procedure for recycling and reuse of TSIL [bmim]ZnCl3
The recycling of catalyst [bmim]ZnCl3 has been investigated
by using reaction for synthesis of compound 2a. After
completion of reaction between diphenylmethanol (1a, 1.0 mmol)
andtrimethylsilylazide (1.1 mmol) in presence of [bmim]ZnCl3 (5
mol%) in dry DCM (3 mL), the solvent (DCM) was removed
under vacuum. The product 2a was then removed by washing the
reaction mixture with ethyl acetate (3 × 5 mL) as [bmim]ZnCl3 is
insoluble in ethyl acetate. The remaining ionic liquid was dried
under vaccum and reused for subsequent.
Supplementary Material
Supplementary data (all experimental procedures, spectral
data for all the compounds, representative NMR, IR and HRMS
spectra) associated with this article can be found in
supplementary data files.
Highlights
•
The Lewis acid ionic liquid [bmim]ZnCl3 is
synthesized under solvent free conditions
and characterized.
•
•
•
TSIL is used as a catalyst for direct
azidation of alcohols at room temperature
Corresponding azides are obtained in high
yields and in short reaction time
The Lewis acidity and recyclability of
catalyst is also investigated