ZrCl 4-catalyzed C-O bond to C-N bond formation: Synthesis of 1,2,3-triazoles and their biological evaluation

Article abstract of DOI:10.1080/00397911.2014.910528

A simple and efficient protocol was developed for the synthesis of aryl azides directly from aryl carbinols using ZrCl₄ as a Lewis acid catalyst. The azides were converted to novel triazoles under click reaction conditions, which were evaluated

Full text of DOI:10.1080/00397911.2014.910528

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ZrCl4-Catalyzed C-O Bond to C-N Bond
Formation: Synthesis of 1,2,3-Triazoles
and Their Biological Evaluation
Gangavaram V. M. Sharma a , Kandikonda Suresh Kumar a b , Buddana
Sudheer Kumar b , Sheri Venkata Reddy a , Reddy Shetty Prakasham b
& Helmut Hugel c
a Organic and Biomolecular Chemistry Division, CSIR Indian Institute
of Chemical Technology , Hyderabad , India
b Biochemical and Environmental Engineering Sciences, CSIR Indian
Institute of Chemical Technology , Hyderabad , India
c Royal Melbourne Institute of Technology , Melbourne , Australia
Accepted author version posted online: 16 Jul 2014.Published
online: 04 Sep 2014.
To cite this article: Gangavaram V. M. Sharma , Kandikonda Suresh Kumar , Buddana Sudheer Kumar ,
Sheri Venkata Reddy , Reddy Shetty Prakasham & Helmut Hugel (2014) ZrCl4-Catalyzed C-O Bond
to C-N Bond Formation: Synthesis of 1,2,3-Triazoles and Their Biological Evaluation, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
44:21, 3156-3164, DOI: 10.1080/00397911.2014.910528
To link to this article: http://dx.doi.org/10.1080/00397911.2014.910528
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Synthetic Communications1, 44: 3156–3164, 2014
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2014.910528
ZrCl4-CATALYZED C-O BOND TO C-N BOND
FORMATION: SYNTHESIS OF 1,2,3-TRIAZOLES AND
THEIR BIOLOGICAL EVALUATION
Gangavaram V. M. Sharma,1 Kandikonda Suresh Kumar,1,2
Buddana Sudheer Kumar,2 Sheri Venkata Reddy,1
Reddy Shetty Prakasham,2 and Helmut Hugel3
1Organic and Biomolecular Chemistry Division, CSIR Indian Institute of
Chemical Technology, Hyderabad, India
2Biochemical and Environmental Engineering Sciences, CSIR Indian Institute
of Chemical Technology, Hyderabad, India
3Royal Melbourne Institute of Technology, Melbourne, Australia
GRAPHICAL ABSTRACT
Abstract A simple and efficient protocol was developed for the synthesis of aryl azides
directly from aryl carbinols using ZrCl4 as a Lewis acid catalyst. The azides were converted
to novel triazoles under click reaction conditions, which were evaluated for their antimicro-
bial activity against various strains.
Keywords Antimicrobial activity; click chemistry; 1,2,3-triazole; zirconium(IV) chloride
INTRODUCTION
Alcohol functionality, which is an attractive source of electrophile in view of
green chemistry principles,[1a] is not a good leaving group. Hence, the derivatization
becomes essential for its facile displacement.[1b] Though several methods were
reported[2–4] for the direct nucleophilic substitution of alcohols, many of them suffer
from elevated temperatures, longer reaction times, or stoichiometric use of the
reagents. Thus, the development of newer methods for direct conversion of a
C-OH bond into a C-N bond gains importance, because the amine group is very
Received March 5, 2014.
Address correspondence to Gangavaram V. M. Sharma, Organic and Biomolecular Chemistry
Division, CSIR Indian Institute of Chemical Technology, Hyderabad 500007, India. E-mail: esmvee@
iict.res.in
3156

ZrCl4-CATALYZED C-O BOND TO C-N BOND FORMATION
3157
Scheme 1. Conversion of aryl carbinols to azides.
common in many natural products as well as pharmaceutically important
compounds.[5] Thus, the direct displacement of an alcohol group with an azide group
gains prominence, because the azide group is a direct source of amines, in addition to
its stability and reactivity in click chemistry[6] and bioconjugation.[7] The most com-
mon method for the synthesis of azide is by the Mitsunobu displacement[8] with HN3
and its modifications.[9] The other C-N bond formation methods include a two-step
conversion of alcohol to azide through mesylate,[10a] palladium-catalyzed hydroazi-
dation of homoallyl alcohols,[10b] and gold-catalyzed direct amination of benzhydryl
alcohols.[10c] In continuation of our interest in the catalytic applications of ZrCl4 as a
Lewis acid for various organic transformations,[11] we herein disclose ZrCl4-
catalyzed conversion of the known carbinols 1–5[12] (Scheme 1), into the correspond-
ing azides 6–10, with TMSN3 in CH3CN at room temperature, click reaction of
azides, and biological evaluation of the derived 1,2,3-triazoles (6a–10d).
RESULTS AND DISCUSSION
Accordingly, carbinol 1 on reaction with TMSN3 in the presence of ZrCl4
(5 mol%) in anhydrous CH2Cl2 (Scheme 2) gave 6 in 38% yield. Further, reaction
of 1 in toluene or 1,4-dioxane afforded azide 6 in lower yields along with the respect-
ive tetramethylsilane (TMS) ether in 1a in greater yields. However, reaction of 1 in
acetonitrile or nitromethane was found to be good and gave 6 in excellent yields,
with no traces of 1a. Further, reaction of 1 in CH3CN with different mol% of ZrCl4
(10, 15, 20) revealed that 10 mol% is the optimum quantity.
A comparative study on the conversion of carbinol 1 to azide 6 was made using
NaN3 in the presence of ZrCl4(10 mol%) in different solvents. The results, as tabu-
lated in Scheme 2, evidently indicate that TMSN3 is superior in the conversion of
carbinol to azide.
Having established the reaction conditions, reaction of 1–5 in CH3CN with
ZrCl4 (10 mol%) at room temperature for 0.5 h afforded the respective azides 6
(90%), 7 (89%), 8 (90%), 9 (87%), and 10 (82%) respectively (Scheme 1).
Antimicrobial Activity of Azides
Azides are energy-rich molecules with many applications[13] and are known for
biocidal property. The synthesized azides 6–10 were evaluated for antibacterial

3158
G. V. M. SHARMA ET AL.
Scheme 2. Standardization of solvent=comparison of reagents.
activity by the agar well plate method against four bacterial cultures (viz.,
Escherichia coli, Klebsiella pneumoniae, Bacillus subtilis, and Micrococcus luteus)
and compared with the antibacterial activity of streptomycin sulfate (Himedia) at
37 ꢀC for 24 h. The data revealed that azides 7 and 8 were active against all the test
cultures (Table 1), whereas the remaining azides 6, 9, and 10 have showed no
activity. Analysis of the inhibition zone data for 7 and 8 revealed that antibiotic
activity is 50–55%, suggesting them to be only less potent with reference to strepto-
mycin sulfate.
The antibacterial activity was evaluated by measuring the inhibition zone
formed around the wells at a concentration of 100 mg. Wells containing sterile water
and solvent (DMSO) were used as controls.
Having found two of the five azides with antibacterial activity, it was proposed
to convert them in to 1,2,3-triazoles and evaluate their antibacterial activity.
Scheme 3. Conversion of aryl azides to 1,2,3-triazole.

ZrCl4-CATALYZED C-O BOND TO C-N BOND FORMATION
3159
Table 1. Antimicrobial activity of azides
Entry
Aryl azides
R1
E. coli
M. luteus
K. pneumoniae
B. subtilis
1
2
3
4
5
6
Phenyl
00
14
15
00
00
00
32
00
16
17
00
00
00
31
00
16
15
00
00
00
30
00
15
14
00
00
00
33
7
Isovanillyl
Napthyl
Methyl
8
9
10
Isobutyl
Control (DMSO)
Streptomycin sulfate
1,2,3-Triazoles, prepared by Huisgen 1,3-dipolar cycloaddition reaction[14] and their
wide range of biological applications in research, made them very attractive targets.
Click chemistry reaction conditions, using Cu(I) as a catalyst, were adopted in the
present study to obtain the 1,4-regioisomers[15] of triazoles.
Accordingly, azide 6 was treated with phenyl acetylene 11a in the presence of
sodium ascorbate and CuSO4 ꢁ 5H2O in t-BuOH-H2O at room temperature for 8 h to
give 6a in 95% yield. Having established the reaction conditions for the conversion of
azide to 1,2,3-triazole to create diverse triazoles, each of the five azides 6–10 were
independently treated with four acetylenes (viz., 11a–d) to afford the triazoles
6a–d to 10a–d respectively. All the triazoles were characterized by spectral and
analytical methods.
Antimicrobial Activity of Triazoles
Because triazoles are known to be powerful antimicrobial agents,[16] the
synthetic triazoles were evaluated for their antibacterial behavior (Table 2) against
Gram-negative (E. coli and K. pneumoniae) and Gram-positive (B. subtilis and
M. luteus) microbial strains, by adopting the same methodology used for the azides.
All the synthetic triazoles showed significant antibacterial activity against the
tested cultures. Maximum activity was indicated for triazole 9b against B. subtilis
(25 mm) and M. luteus (24 mm), whereas minimum activity was indicated for
observed for triazole 8d. The SAR (structure–activity relationship) studies revealed
that the parent azide moieties (phenyl, methyl, and isobutyl) have no activity but
also have exhibited good antibacterial activity when they were converted into
triazoles (6a–d, 9a–d, and 10a–d). Further observations revealed that among the
triazoles, compounds with NHBoc and n-hexyl side chains (6b, 6d, 9b, 9d, 10b,
and 10d) showed good intensity of antibacterial activity compared with the other
two side chains (phenyl and -CH2OH). SAR studies on 7a–d and 8a–d prepared from
active azide moieties 7 and 8 showed an increase in the antibacterial activity of 7c
and 7d, whereas triazoles from 8 displayed diminished activity.
MIC by Tube Dilution Method
Out of the several synthetic triazoles, 10 triazoles (Table 3) were selected for
MIC (minimum inhibitory concentration) studies. From the studies, it is evident
that triazole 9b showed MIC of 15.625 mg=mL concentration to inhibit the growth
of the organism against B. subtilis and K. pneumoniae. All the new triazoles showed

3160
G. V. M. SHARMA ET AL.
Table 2. Antimicrobial activity of triazole derivatives
Entry
Triazoles
R1
R2
E. coli
M. luteus
K. pneumoniae
B. subtilis
1
2
6a
6b
Phenyl
Phenyl
Phenyl
n-Hexyl
12
13
13
18
20
17
20
20
0
12
15
16
18
11
0
12
16
12
16
15
16
20
22
17
12
17
0
15
15
11
15
16
14
20
22
17
0
3
6c
6d
Phenyl
Phenyl
CH2OH
CH2NHBoc
Phenyl
4
5
7a
7b
Isovanillyl
Isovanillyl
Isovanillyl
Isovanillyl
Napthyl
Napthyl
Napthyl
Napthyl
Methyl
6
n-Hexyl
7
7c
7d
CH2OH
CH2NHBoc
Phenyl
20
15
0
8
9
8a
8b
10
11
12
13
14
15
16
17
18
19
20
n-Hexyl
11
17
0
14
15
10
24
24
15
19
15
17
11
18
0
8c
8d
CH2OH
CH2NHBoc
Phenyl
14
0
9a
9b
14
15
12
12
11
12
12
13
0
16
21
12
13
11
15
12
16
0
15
25
12
16
17
16
13
18
0
Methyl
Methyl
Methyl
n-Hexyl
9c
9d
CH2OH
CH2NHBoc
Phenyl
10a
10b
10c
10d
Control
standard
Isobutyl
Isobutyl
Isobutyl
Isobutyl
n-Hexyl
CH2OH
CH2NHBoc
32
31
30
33
Table 3. MIC studies
Entry
Compound
E. coli
B. subtilis
K. pneumoniae
M. luteus
P. putida
1
2
6b
6d
31.25
31.25
31.25
15.625
31.25
62.5
31.25
31.25
62.5
31.25
31.25
31.25
62.5
31.25
62.5
31.25
62.5
3
7a
7c
31.25
31.25
62.5
>125
62.5
62.5
4
62.5
62.5
5
7d
8c
62.5
31.25
62.5
6
31.25
62.5
31.25
31.25
>125
62.5
62.5
62.5
7
9a
31.25
8
9b
9d
>125
15.625
62.5
62.5
15.625
62.5
62.5
>125
62.5
125
9
10
62.5
31.25
6.25
10d
Streptomycin
62.5
6.25
6.25
6.25
150
moderate MIC values with a concentration of 31.25 and 62.5 mg=mL. Further, it
was also observed that the synthetic triazoles showed lesser MIC values than the
standard (streptomycin) against P. putida.
EXPERIMENTAL
Solvents were dried over standard drying agents and freshly distilled prior to
use. All commercially available chemicals were used without further purification.
All reactions were performed under nitrogen.1H NMR and 13C NMR spectra were
measured with Varian Gemini FT 200-MHz, Bruker Avance 300-MHz, Unity

ZrCl4-CATALYZED C-O BOND TO C-N BOND FORMATION
3161
400-MHz, and Inova 500-MHz spectrometers with TMS as internal standard for
solutions in CDCl3. J values are given in hertz. Chemical shifts were reported in
parts per million (ppm) relative to solvent signal. All column chromatographic
separations were performed using silica gel (Acme’s, 60–120 mesh). Organic solu-
tions were dried over anhydrous Na2SO4 and concentrated below 40 ꢀC in vacuo.
Infrared (IR) spectra were recorded on a fourier transform (FT) IR (Perkin-Elmer
IR-683) spectrophotometer with NaCl optics. Jasco DIP 300 digital polarimeter
was used for measurement of optical rotations at 25 ꢀC. Mass spectra were recorded
on direct inlet system or liquid chromatography (LC) by MSD trap SL (Agilent
Technologies). The high-resolution mass spectrometry (HRMS) data were obtained
using Q-TOF mass spectrometry.
1-(Azido(phenyl)methyl)-4-methoxybenzene (6)
A stirred solution of alcohol 1 (2.0 g, 9.35 mmol) in acetonitrile (10 mL) at 0 ꢀC
was sequentially treated with azidotrimethylsilane (3.0 mL, 23.38 mmol) and ZrCl4
(0.22 g, 0.93 mmol) and stirred for 30 min. The reaction mixture was diluted with
water (10 mL) and extracted with EtOAc (2 ꢂ 10 mL). The combined organic layers
were washed with brine (10 mL) and dried (Na2SO4). Solvent was evaporated and the
residue purified by column chromatography (60- to 120-mesh silica gel, 3% EtOAc in
petroleum ether) to afford 6 (2.02 g, 90%) as a yellow liquid; IR (CHCl3): 3030, 2091,
1609, 1510, 1243, 1174, 1031 cmꢃ1; 1H NMR (300 MHz, CDCl3): (7.37–733 (m, 2H,
ArH), 7.32 (m, 3H, ArH), 7.21 (d, 2H, J ¼ 8.7 Hz, ArH), 6.88 (dt, 2H, J ¼ 2.1, 8.8 Hz,
ArH), 5.67 (s, 1H, ArCH), 3.97 (s, 3H, OMe); 13C NMR (75 MHz, CDCl3): d159.2,
139.7, 131.6, 128.6, 128.5, 127.8, 127.1, 113.9, 67.9, 55.1.
1-((4-Methoxyphenyl)(phenyl)methyl)-4-phenyl-1H-1,2,3-triazole (6a)
A solution of azide 6 (0.20 g, 0.84 mmol) and phenyl acetylene 11a (0.09 mL,
0.84 mmol) in t-BuOH (1 mL) and water (1 mL) was treated with sodium ascorbate
(0.01 g, 0.04 mmol) followed by CuSO4 ꢁ 5H2O (0.02 g, 0.08 mmol) at room tempera-
ture and stirred for 8 h. Solvent was evaporated from the reaction mixture, and the
residue was diluted with water (5 mL) and extracted with EtOAc (2 ꢂ 10 mL). The
combined organic layer was washed with brine (5 mL) and dried (Na2SO4). Solvent
was evaporated and the residue purified by column chromatography (60- to
120-mesh silica gel, 15% EtOAc in petroleum ether) to afford 6a (0.27 g, 95%) as a
white solid; mp 157–159 ꢀC; IR (CHCl3): 3032, 2928, 1609, 1512, 1456, 1250,
1
1178, 1030, 763, 739 cmꢃ1; H NMR (500 MHz, CDCl3): (7.81 (d, 2H, J ¼ 7.2 Hz,
ArH), 7.60 (s, 1H, CH), 7.45–7.21 (m, 6H, ArH), 7.17–7.07 (m, 5H, ArH, ArCH),
6.91 (d, 2H, J ¼ 8.7 Hz, ArH), 3.82 (s, 3H, OMe); 13C NMR (75 MHz, CDCl3):
(129.6, 128.8, 128.7, 128.4, 128.1, 127.7, 125.6, 119.5, 114.3, 67.6, 55.3. HRMS
(ESIþ) m=z calcd. for C22H20N3O (Mþ þ H) 342.16009; found 342.16022.
Experimental Procedure for MIC Studies
All the test strains (E. coli, B. subtilis, K. pneumonia, M. luteus, and P. putida),
each 1 mL volume (OD equal to match the turbidity of a MacFarland 0.5 standard

3162
G. V. M. SHARMA ET AL.
tube) were inoculated in nutrient (1 mL) broth with final compound concentrations of
250 to 0 mg=mL and standard drug (streptomycin) concentration from 400 to 6.25 mg=
mL. All the tubes were incubated at 37 ꢀC for 12–16 h. The turbidity of each tube is
measured with respect to control tube. MIC values are defined as the lowest concen-
tration of compound at which growth is completely inhibited for at least for 8 h.
CONCLUSION
In conclusion, an efficient method for the conversion of C-O bond (carbinols)
to C-N bond (to azide) catalyzed by ZrCl4 has been developed. The method reported
is not only simple to operate but also affords the product azides in short duration of
time with good yields. The azides were converted into 1,2,3-triazoles by click chem-
istry. The azides and derived triazoles were evaluated for their antimicrobial activity
against Gram-positive and Gram-negative bacteria. The study revealed some of the
triazoles with interesting antibacterial activity. Further SAR studies to derive better
compounds with better activity are in progress.
FUNDING
B. S. is thankful to the Council of Scientific and Industrial Research, New
Delhi, for the senior research fellowship. K. S. is thankful to Indian Institute of
Chemical Technology–Royal Melbourne Institute of Technology (IICT-RMIT)
Research Programme (CLP-0092) for the financial support in the form of a research
fellowship. R. S. and G. V. M. S. are thankful to CSIR, New Delhi, for financial
support (CSC-0108; BSC-0116). H. H. is thankful to RMIT, Melbourne, Australia,
for a research grant.
SUPPORTING INFORMATION
1
Full experimental details, spectral data of the products, and H NMR, 13C
NMR, and HRMS spectra of all the new compounds can be accessed on the
publisher’s website.
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