Conversion of Arylboronic Acids to Tetrazoles Catalyzed by ONO Pincer-Type Palladium Complex

Article abstract of DOI:10.1021/acs.joc.6b02277

A convenient synthesis of a library of tetrazoles through a novel and operationally simple protocol effecting the direct conversion of arylboronic acids catalyzed by a new ONO pincer-type Pd(II) complex under mild reaction conditions using the readily available reagents is reported. The palladium complex was reused up to four cycles in an open-flask condition.

Full text of DOI:10.1021/acs.joc.6b02277

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Article
Conversion of Arylboronic Acids to Tetrazoles
Catalyzed by ONO Pincer Type Palladium Complex
Arumugam Vignesh, Nattamai S. P. Bhuvanesh, and Nallasamy Dharmaraj
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b02277 • Publication Date (Web): 29 Nov 2016
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Conversion of Arylboronic Acids to Tetrazoles Catalyzed by ONO
Pincer Type Palladium Complex
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Arumugam Vignesh, † Nattamai S. P. Bhuvanesh, ‡ and Nallasamy Dharmaraj*, †
† Inorganic & Nanomaterials Research Laboratory, Department of Chemistry,
Bharathiar University, Coimbatoreꢀ641 046, India. Eꢀmail: dharmaraj@buc.edu.in;
http://ndharmaraj.wix.com/inrl ; Fax: +91 4222422387; Tel: +91 4222428316.
‡ Department of Chemistry, Texas A & M University, College Station, TX 77843, USA.
ABSTRACT: A convenient synthesis of a library of tetrazoles through a novel and
operationally simple protocol effecting the direct conversion of arylboronic acids
catalyzed by a new ONO pincer type Pd(II) complex under mild reaction
conditions using the readily available reagents is reported. The palladium complex
was reꢀused up to four cycles in an openꢀflask condition.
TOC
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INTRODUCTION
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Direct access to target molecules in oneꢀpot operation using readily available
reagents is an attractive strategy in organic synthesis.1 Chemistry of nitrogenꢀrich
heterocycles gains immense importance2 due to their diverse application.2
Specifically, tetrazoles are an ubiquitous structural motif very often utilized in
pharmaceuticals3 (Figure 1), as ligands in coordination chemistry,4 in
organocatalysis,5 as synthons in organic synthesis6 as well as in various materials
science applications including polymers, photosensitive agents, energy materials,
and specialty explosives.7 In the literature, a plethora of methods were reported for
the synthesis of 5ꢀsubstituted 1Hꢀtetrazole derivatives.8,9 Nevertheless, those
protocols have some drawbacks such as, (i) high catalyst loading, (ii) expensive
reagents, (iii) harsh reaction conditions; water sensitivity and the involvement of
dangerous hydrazoic acid, and (iv) prolonged reaction time and elevated
temperature.8,9 Hence, development of an efficient, economical and ecoꢀfriendly9g
oneꢀpot methodology to construct the titled heterocyclic frameꢀworks with wide
substrate scope is a challenging scientific mission.
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Cl
N
O
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N NH
COOH
N NH
N NH
N
N
N
N
N
N
HO
N
N
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N
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N
Forasartan
Valsartan
Losartan
Figure 1. Tetrazole based sartans
Consequent to the discovery of transitionꢀmetalꢀcatalyzed cyanations,10 the
classical cyanation methods such as Rosenmund–von Braun reaction of aryl
halides11 and diazotization of anilines followed by Sandmeyer reaction are
practically neglected.12 On the other hand, directing group assisted C–H bond
cyanation with metal precursors of the type MCN (M = Na, K, Cu, and Zn) were
reported.13 Conversely, the toxicity of cyanide salts excluded the practicality of this
conversion to a larger extent in addition to the general problem associated with
high affinity of cyanide towards Pd, Ni, and Cu based catalysts, that often
deactivates the catalytic system.14 Hence, the development of new methodology for
the cyanation of arylboronic acids using a safe cyanide source is much desirable.14c
In this regard, we utilized DMSO and NH4HCO3 as harmless cyanide sources for
the titled conversion.14b, 14d
Recently, several reports were documented regarding the conversion of
arylboronic acids to various aryl functionalities15ꢀ26 such as halides (F,15 Cl,16 Br,17
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and I17), triazoles,18 sulfinates,19 amine,20 nitriles,21 nitro compounds,22 hydroxyls,23
azides,24 sulfones,25 and trifluoreomethylthiol.26 Arylboronic acids are considered
as source of aryl group owing to their stability, common availability, and often the
requirement of mild conditions with wide spectrum functional group tolerance.27 In
spite of the large number of publications those deal with organic transformations
using metal salts or coordination complexes, no report on the direct conversion of
arylboronic acids to corresponding tetrazoles is available in the literature.15ꢀ26
However, direct conversion of aryl halides to tetrazoles was reported earlier.28
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In our quest to design new palladium based catalysts for C–C and C–N bond
formation reactions,29 we herein describe the synthesis and catalytic activity of a
new Pd(II) complex incorporating ONO pincer type ligand for the direct
conversion of arylboronic acids to tetrazoles through a 3+2 cycloaddition of an
inꢀsitu generated aryl nitriles with sodium azide. To the best of our knowledge, this
is the maiden report on the use of Pd(II) complex catalyzed for the synthesis of
tetrazoles in a simple route utilizing arylboronic acids.
RESULTS AND DISCUSSION
From the reaction of an equimolar quantity of tridentate ONO pincer type
ligand benzoic acid [4ꢀ(1ꢀethylꢀpropyl)ꢀ2ꢀhydroxyꢀbenzylidene]ꢀhydrazide (H2L1)
and the precursor complex, [PdCl2(PPh3)2] in presence of Et3N, complex 1 of the
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molecular formula [Pd(L)(PPh3)] was obtained in 78 % yield as sketched in
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Scheme 1.
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Scheme 1 Synthesis of ONO Pincer Type Ligand and the New Pd(II) Complex.
The exact mode of coordination of the ligand to palladium ion in complex 1
was determined from the singleꢀcrystal Xꢀray structure presented in Figure S1 and
S2 in SI. The Pd(II) center in complex 1 adopted a distorted squareꢀplanar
geometry around the metal ion satisfied through the phenolate oxygen, azomethine
nitrogen and the imidolate oxygen atoms of ONO pincer type ligand with the
fourth coordination site occupied by the triphenylphosphine molecule. From the
XRD data, we inferred that the complex 1 was crystallized as monoclinic with the
space group of P 1 21/c1. Details on the data collection, structure refinements,
bond angles, and bond distances were furnished in Table S1 and S2 in SI.
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Optimization of reaction conditions were undertaken by selecting a
combination of phenylboronic acid (4a), NH4HCO3, and DMF as reagents for the
source of CN functionality in presence of an inorganic base in water for the
tetrazole formation. In view of atom economy and for economical reasons, NaN3
was chosen as an inexpensive azide source. As expected, the tetrazole was formed
in 42% yield. To achieve higher yield of the target product, we tested the
combinations of DMFꢀNH4HCO3, DMAcꢀNH4HCO3, and DMSOꢀNH4HCO3 as
safe sources of CN group (Table 1 entries 4, 9 and 10) and found that only the
combination of DMSOꢀNH4HCO3 was more effective for the inꢀsitu generation of
cyanide, as reported earlier.14b Further, among the bases tested for the catalytic
reaction, KOH served as an effective base. In our attempted tetrazole synthesis at
roomꢀtemperature, no progress was observed. From the literature,8, 9 it was clear
that a high yield of tetrazole synthesis required the reaction at high temperatures.
Hence, we screened the reaction through the temperature range of 60–120 °C
wherein the best yield of the tetrazole (92%) was achieved at 90 °C. The catalyst
loading test was performed by using 0.1 mol% to 0.001 mol% and realized that 0.1
mol % of catalyst afforded 92% of the desired product. Further addition of the
catalyst had not resulted in any improvement of the yield (Table 2).
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Table 1 Optimization of Catalytic Reaction.a
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a = Reaction conditions: phenylboronic acid (5 mmol), base (6 mmol), NH4HCO3 (1.2 equiv.)
solvent (70:30%), and catalyst (0.1 mol %) stirred at 60ꢀ90 °C for 3ꢀ6 h. NR = No reaction.
Table 2 Effect of Catalyst Loading.
Entry
Mol %
Isolated
TON
yield (%)
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2
3
0.1
0.01
0.001
92
79
64
920
7900
64000
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Table 3 Screening of Pd(II) Catalysts.
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Catalyst (0.1 mol%)
Yield (%)
TON
920
640
720
680
410
590
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Complex
Complex
Complex
Complex
[PdCl2]
1
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4
92
64
72
68
41
59
[PdCl2(PPh3)2]
a = Reaction conditions: phenylboronic acid (5 mmol), base (6 mmol), NH4HCO3 (1.2
equiv.) solvent (70:30%), and catalyst (0.1 mol %) stirred at 90 °C for 6 h.
HO
HO
HO
N
N
N
N
N
N
Pd
O
Pd
O
Pd
O
O
O
MeO
O
PPh3
PPh3
PPh3
3
4
2
Complex
Complex
Complex
Figure 2. Structures of Previously Reported Pd(II) Complexes.29b
To ascertain, whether the present conversion of arylboronic acids to
tetrazoles could be effected only with complex 1 or also by other Pd sources, we
utilized [PdCl2], [PdCl2(PPh3)2], and few previously reported29b Pd(II) complexes
2, 3, and 4 (Figure 2) as catalysts for the same reaction under identical conditions.
Herein we noticed that the reaction proceeded with all the above mentioned
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catalysts, but the results showed in Table 3 highlighted that complex 1 is superior
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for this conversion.
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At this stage, the following optimized conditions were applied for the titled
conversion: arylboronic acid (5 mmol), KOH (6 mmol), NH4HCO3 (1.2 equiv.)
H2O/DMSO (50:50%), and complex 1 as the catalyst (0.1 mol%) at 90 °C under
openꢀflask conditions (Table 1).
Next, we turned our attention to explore the reaction scope of arylboronic
acids possessing both activating and deactivating groups. The results furnished in
Table 4 revealed that a smooth conversion of arylboronic acids occurred in all the
cases and delivered the target molecules 5a–5t in excellent yields. It is worth to
mention here that the electronic and steric effects due to the substituents of
arylboronic acids did not significantly influence the outcome of the conversion
process.
Arylboronic acids bearing electronꢀdonating groups like CH3 and OCH3 at
the sterically hindered ortho position afforded respectively 75 and 81% of products
5b and 5d. Moreover, sterically hindered arylboronic acids i.e., 2, 6ꢀdimethoxy and
2, 3ꢀdimethylphenylboronic acids underwent the conversion and provided 69 and
71% of the products 5f and 5g, respectively. Fabulously, arylboronic acids
featuring –CH3, –OCH3, tꢀbutyl, –OH, and –N(CH3)2 groups in para position too
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afforded the corresponding tetrazoles in 82, 86, 65, 70, and 68% yield. In addition,
arylboronic acids holding moderately deactivating chloro, bromo, formyl, and
acetyl functionalities also contributed well and yielded the tetrazole analogous 5k –
5o quantitatively. Biphenyl and naphthyl boronic acids were also converted into
desired tetrazoles (5r and 5s).
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Phenylboronic acids possessing deactivating –NO2 and –CF3 functionalities
at the para position also yielded the expected products in 83 and 80%,
respectively. Next, utilization of a heterocyclic boronic acid (4ꢀpyridineboronic
acid) as a reagent in the palladium catalysed conversion afforded 4ꢀtetrazolyl
pyridine (5t) in 76% yield. In all these experiments involving the conversion of a
series of arylboronic acids to tetrazoles catalyzed by the palladium complex 1, no
byꢀproducts were obtained. In order to realize the usefulness of the current protocol
for industrial application, we investigated gram scale synthesis of 5ꢀbiphenylꢀ4ꢀylꢀ
1Hꢀtetrazole (5r) as a representative example (Scheme 2) with 69% yield.
Scheme 2 Gram Scale Synthesis of 5ꢀbiphenylꢀ4ꢀylꢀ1Hꢀtetrazole (5r).
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Table 4 Scope of Arylboronic Acids.a
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a = Reaction conditions: arylboronic acid (5 mmol), KOH (6 mmol), NH4HCO3 (1.2
equiv.) H2O/DMSO (50:50%), NaN3 (5 mmol) and catalyst (0.1 mol %) stirred at 90 °C for 6ꢀ9
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h. NR = No reaction, TON = turnover number = ratio of moles of the product formed to moles of
the catalyst used, TOF (hꢀ1) = turnover frequency = TON/h.
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After the completion of the reaction, the catalyst was recovered by
centrifugation upon the addition of ethyl acetate and washed thoroughly with water
(to remove inorganic salts) and dried under vacuum. The dried catalyst was
subjected to next reaction cycles under identical conditions with fresh portions of
reagents. The reusability study showed that the present palladium based catalytic
system remains active up to four consecutive runs with a gradual decrease in the
activity as summarized in Figure 3.
Figure 3. Reusability of the Catalyst.
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To understand the reaction pathway, the following control experiments were
carried out (Scheme 3). One such experiment was performed to effect the titled
conversion in the absence of catalyst, wherein no progress was observed indicating
the vital role of palladium pincer type complex. Another study conducted with
several solvents excluding DMF, DMSO or DMAc, failed to yield the product 5a.
On the other hand, use of ammonium salts such as NH4Cl, NH4OH or (NH4)2SO4
as the nitrogen source instead of NH4HCO3 (Table 1, entries 11, 12, and 13),
successfully gave the expected tetrazoles and thus proved that the carbon atom of
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13
CN moiety was not derived from NH4HCO3. Besides, use of Cꢀlabelled DMSO
as a reagent confirmed that the carbon of the cyano group was originated only from
DMSO but not from ammonium salts.14b Hg poisoning test done by adding
elemental mercury revealed that no catalyst poisoning occurred and the active
catalyst is likely to be a homogeneous species and not metallic palladium
nanoparticles.
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Scheme 3 Control Experiments for Mechanistic Investigation.
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Based on the above control experiments and literature reports, 8, 9, 29cꢀe, 30 the
probable reaction pathway is depicted in Scheme 4. Intermediate I (Pd(0)L) formed
through a twoꢀelectron reduction of the palladium(II) complex in presence of a
base30 underwent an .oxidative addition of phenylboronic acid to provide species
II. Next, the intermediate III was obtained through the coordination of inꢀsitu
generated –CN ion from DMSO and NH4HCO3 to intermediate II. Migratory
insertion of intermediate III yielded species IV which on cycloaddition with
sodium azide afforded intermediate IV. Cyclization of intermediate V afforded the
palladium coordinated compound VI which upon reductive elimination lead to the
release of the formed phenyl tetrazole from the palladium centre with regeneration
of active species I. The titled conversion reaction performed in the absence of air
also afforded the expected product.
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Scheme 4. Possible Mechanism for the Direct Conversion of Arylboronic Acids to
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Tetrazoles.
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N
B(OH)2
KOH
HN
N
NaOH
+
N
Ph
H2O
Na+
+
Pd(0)L
I
B(OH)3+ 2K+
Pd(II)L
N
N
N
Ph
N
Pd(II)L
VI
O
S
II
+ NH4HCO3
CN
N
Ph
Pd(II)L
N N N
Na+
V
NC
Pd(II)L
Pd(II)L
N
III
IV
NaN3
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CONCLUSION
In summary, a robust palladium based catalyst consisting of ONO pincer
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type ligand was synthesized, characterized, (viz., UVꢀvis., IR, H, C NMR, and
singleꢀcrystal XRD techniques) and successfully employed for the direct
conversion of arylboronic acids to tetrazoles under an openꢀatmosphere.
Mechanistic aspects of the titled reaction presented in this manuscript would
definitely attract the attention of synthetic chemists who are constantly moving
forward to design and develop new catalytic systems to synthesis tetrazoles from
inexpensive reagents. Low catalyst loading (0.1 mol%), safe cyanide source, nonꢀ
requirement of an oxidant/additives, openꢀflask conditions, wide substrate scope,
scalability, and reꢀusability are the key factors of the present methodology with a
definite scope for further explorations.
EXPERIMENTAL SECTION
General Experimental Considerations. Elemental analyses (C, H, and N)
were performed on Elemental analyzer instrument. IR spectra (4000–400 cm−1) of
the compounds were recorded on FTꢀIR spectrophotometer. Melting points were
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determined by melting point apparatus and are uncorrected. H and C NMR
spectra were recorded in CDCl3 as solvent on 400 and 100 MHZ instruments,
respectively. The following abbreviations were used for multiplicity: s = singlet, d
= doublet, t = triplet, m = multiplet, and dd = doublet of doublets. Mass spectra
were recorded in mass spectrometer. [PdCl2(PPh3)2] (97%), and arylboronic acids
(95ꢀ98%) were purchased from commercial suppliers and used as received.
Reagent grade solvents purchased from standard suppliers were purified and dried
according to standard procedures.31
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Synthesis of the Ligand H2L1:
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Pincer type ligand H2L1 was synthesized by condensing equimolar quantity
of 4ꢀ(1ꢀethylꢀpropyl)ꢀ2ꢀhydroxyꢀbenzaldehyde with benzhydrazide in ethanol
according to literature method.32 The reaction mixture was refluxed on a waterꢀbath
for 10 h and poured into crushed ice. The corresponding pincer type hydrazone
formed as colourless solid was filtered, washed repeatedly with distilled water and
recrystallized from ethanol with 80% (249 mg) yield.
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Synthesis of Palladium Complex 1(catalyst):
To a warm methanolic solution (20–30 mL) of pincer type ligand (H2L1) (1
equiv.), a chloroform solution of [PdCl2(PPh3)2] (1 equiv.) followed by two drops
of triethylamine were added, refluxed for 8 h and kept at room temperature for
crystallization. Needle like reddish brown crystals suitable for Xꢀray study were
obtained on slow evaporation over 30–40 days.
[Pd(L1)(PPh3)] (complex 1) Yield: 528 mg, 78%. M.p. 197ꢀ199 °C. Elemental
analysis (%) calculated C37H35N2O2PPd; C, 65.63; H, 5.21; N, 4.14. Found (%) C,
65.58; H, 5.11; N, 4.17. UVꢀvisible (solvent: DMSO, nm): 361, 399, 401, 436.
Selected IR bands (KBr, ν in cm−1): 1585 (C−N=N−C), 1512 (C=N), 1433 (PPh3),
1253 (imidolate −N=C–O), 1182 (phenolate C–O). 1H NMR (CDCl3, δ ppm) 10.30
(s, 1H), 7.45 (t, J = 4.2 Hz, 4H), 7.40 (d, J = 7.6 Hz, 2H), 7.31 (s, 3H), 7.23 (t, J =
6.4 Hz, 9H), 6.68 (d, J = 8.0 Hz, 2H), 6.30 – 6.37 (m, 3H), 3.99 (dd, J = 2.4, 9.2
Hz, 1H), 3.69 (dd, J = 5.6, 6.0 Hz, 4H), 3.06 (t, J = 2.6 Hz, 6H); 13C NMR (CDCl3,
δ ppm) 162.8, 149.5, 148.4, 147.5, 146.0, 131.4, 125.0, 122.1, 114.4, 114.3, 110.8,
109.6 108.1, 100.7, 55.7, 39.4, 20.9.
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General Procedure for the Catalytic Reaction:
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To a solvent mixture of H2O–DMSO (50:50%), complex 1 (0.1 mol %),
phenylboronic acid (4.0 mmol), KOH (6 mmol) and NH4HCO3 (1.2 mmol, 6
equiv.) were added and stirred for 2 hours. Later, NaN3( 4.0 mmol) was added and
continuously stirred at 90 °C under openꢀflask condition. The progress of the
reaction was monitored by thin layer chromatography (TLC). After the completion
of the reaction, the reaction mixture was cooled to room temperature and the
catalyst was precipitated by adding ethyl acetate, centrifugated, and washed
thoroughly with water (to remove inorganic salts) and dried under vacuum. The
identity of the products was confirmed by 1H and 13C NMR data.
Analytical and Spectral Data of the Products Listed in Table 4:
Entry 5a: 5ꢀphenylꢀ1Hꢀtetrazole9h: Elemental analysis (%) calculated for C7H6N4 C,
57.53; H, 4.14; N, 38.34. Found (%) C, 57.56; H, 4.16; N, 38.40. 1H – NMR: 7.34
(d, J = 8.0 Hz, 2H), 7.18 (dd, J = 8.0 Hz, 1.2 Hz, 1H), 7.13 (dd, J = 6.4 Hz, 2.8 Hz,
2H). 13C– NMR: 161.4, 127.4, 127.3, 115.5, 115.3.
Entry 5b: 5ꢀoꢀtolylꢀ1Hꢀtetrazole9i: Elemental analysis (%) calculated for C8H8N4
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C, 59.99; H, 5.03; N, 34.98. Found (%) C, 59. 98; H, 5.07; N, 35.04. H – NMR:
7.40 (d, J = 8.0 Hz, 1H), 7.26 (s, 1H), 7.22 (dd, J = 6.4, 2.0 Hz 1H), 7.15 (d, J= 8.4
Hz, 1H), 2.48 (s, 3H). 13C – NMR: 153.2, 135.5, 133.7, 132.3, 130.9, 129.2, 127.5,
21.5.
Entry 5c: 5ꢀpꢀtolylꢀ1Hꢀtetrazole9i: Elemental analysis (%) calculated for C8H8N4
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C, 59.99; H, 5.03; N, 34.98. Found (%) C, 60.01; H, 5.05; N, 34.96. H – NMR:
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7.42 (d, J = 10.0 Hz, 1H), 7.21 (t, J = 4.2 Hz, 2H), 7.15 (d, J = 8.0 Hz, 1H), 2.51 (s,
3H). 13C – NMR: 160.6, 131.0, 128.9, 115.24, 19.9.
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Entry 5d: 5ꢀ(2ꢀmethoxyꢀphenyl)ꢀ1Hꢀtetrazole9i: Elemental analysis (%) calculated
for C8H8N4O C, 54.54; H, 4.58; N, 31.80. Found (%) C, 54.57; H, 4.60; N, 31.86.
1H – NMR: 6.76 (d, J = 8.0 Hz, 2H), 6.70 (d, J = 9.6 Hz, 1H), 6.63 – 6.65 (m, 1H),
4.21 (s, 3H). 13C – NMR: 147.8, 146.5, 126.4, 122.2, 109.6, 108.3, 100.9, 58.7.
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Entry 5e: 5ꢀ(4ꢀmethoxyꢀphenyl)ꢀ1Hꢀtetrazole9i: Elemental analysis (%) calculated
for C8H8N4O C, 54.54; H, 4.58; N, 31.80. Found (%) C, 54.57; H, 4.60; N, 31.86.
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1H – NMR: 7.35 (t, J = 7.0 Hz, 2H), 7.29 (t, J = 7.8 Hz, 2H), 4.26 (s, 3H). C –
NMR: 155.2, 136.4, 128.1, 127.9, 125.3, 58.0.
Entry 5f: 5ꢀ(2,6ꢀdimethoxyꢀphenyl)ꢀ1Hꢀtetrazole9i: Elemental analysis (%)
calculated for C9H10N4O2 C, 52.42; H, 4.89; N, 27.17. Found (%) C, 52.45; H,
4.93; N, 27.22. 1H – NMR: 7.32 (d, J = 8.4 Hz, 2H), 7.21 (d, J = 8.4 Hz, 1H), 4.25
(s, 6H). 13C – NMR: 161.4, 135.2, 134.0, 128.6, 127.0, 55.2.
Entry 5g: 5ꢀ(2,3ꢀdimethylꢀphenyl)ꢀ1Hꢀtetrazole9i: Elemental analysis (%)
calculated for C9H10N4 C, 62.05; H, 5.79; N, 32.16. Found (%) C, 62.09; H, 5.84;
N, 32.18. 1H – NMR: 7.27 – 7.35 (m, 3H), 2.78 (s, 3H), 2.22 (s, 3H). 13C – NMR:
163.8, 161.7, 132.3, 127.4, 127.3, 115.5, 115.3, 23.4, 22.0.
Entry 5h: 5ꢀ(4ꢀtertꢀbutylꢀphenyl)ꢀ1Hꢀtetrazole9i: Elemental analysis (%) calculated
for C11H14N4 C, 65.32; H, 6.98; N, 27.70. Found (%) C, 65.36; H, 6.99; N, 27.73.
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1H – NMR: 7.35 (d, J = 7.6 Hz, 2H), 7.28 (d, J = 6.8 Hz, 2H), 2.00 (s, 9H). C –
NMR: 161.1, 136.3, 127.8, 127.5, 125.1, 24.9, 19.1
Entry 5i: 4ꢀ(1Hꢀtetrazolꢀ5ꢀyl)ꢀphenol9i: Elemental analysis (%) calculated for
C7H6N4O C, 51.85; H, 3.73; N, 34.55. Found (%) C, 51.89; H, 3.75; N, 34.59. 1H –
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NMR: 10.63 (s, 1H), 7.33 (d, J = 9.0 Hz, 2H), 7.26 (s, 2H). C – NMR: 163.2,
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161.7, 127.3, 127.2, 115.5, 115.2.
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Entry 5j: dimethylꢀ[4ꢀ(1Hꢀtetrazolꢀ5ꢀyl)ꢀphenyl]ꢀamine9k: Elemental analysis (%)
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calculated for C9H11N5 C, 57.13; H, 5.86; N, 37.01. Found (%) C, 57.20; H, 5.89;
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N, 37.09. H – NMR: 7.25 – 7.32 (m, 2H), 7.14 (d, J = 8.4 Hz, 2H), 3.74 (s, 6H).
13C – NMR: 160.0, 132.9, 132.8, 131.6, 130.8, 45.2.
Entry 5k: 5ꢀ(4ꢀchloroꢀphenyl)ꢀ1Hꢀtetrazole9j: Elemental analysis (%) calculated for
1
C7H5ClN4 C, 46.55; H, 2.79; N, 31.02. Found (%) C, 46.54; H, 2.81; N, 31.08. H
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– NMR: 7.28 (d, J = 8.0 Hz, 2H), 7.15 (d, J = 8.0 Hz, 2H). C – NMR: 160.3,
132.9, 131.6, 130.8, 128.6.
Entry 5l: 5ꢀ(4ꢀbromoꢀphenyl)ꢀ1Hꢀtetrazole9j: Elemental analysis (%) calculated for
C7H5BrN4 C, 37.36; H, 2.24; N, 24.90. Found (%) C, 37.38; H, 2.28; N, 24.94. 1H
– NMR: 7.18 (t, J = 6.8 Hz, 2H), 7.01 (t, J = 8.6 Hz, 2H). 13C – NMR: 163.1,
131.0, 130.9, 128.8, 115.5, 115.3.
Entry 5m: 4ꢀ(1Hꢀtetrazolꢀ5ꢀyl)ꢀbenzaldehyde9j: Elemental analysis (%) calculated
for C8H6N4O C, 55.17; H, 3.47; N, 32.17. Found (%) C, 55.18; H, 3.49; N, 32.20.
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1H – NMR: 9.83 (s, 1H), 7.36 (d, J = 7.6 Hz, 2H), 7.24 (d, J = 7.6 Hz, 2H). C –
NMR: 202.1, 159.7, 129.0, 120.3, 112.5, 110.1.
Entry 5n: 1ꢀ[3ꢀ(1Hꢀtetrazolꢀ5ꢀyl)ꢀphenyl]ꢀethanone9i: Elemental analysis (%)
calculated for C9H8N4O C, 57.44; H, 4.28; N, 29.77. Found (%) C, 57.47; H, 4.32;
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N, 29.78. H – NMR: 7.38 (d, J = 8.4 Hz, 1H), 7.32 (t, J = 7.2 Hz, 1H), 7.22 – 7.27
(m, 1H), 6.86 (s, 1H), 3.78 (s, 3H). 13C – NMR: 205.1, 159.7, 133.1, 133.0, 129.3,
128.4, 114.1, 113.7, 25.5.
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Entry 5o: 1ꢀ[4ꢀ(1Hꢀtetrazolꢀ5ꢀyl)ꢀphenyl]ꢀethanone9i: Elemental analysis (%)
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calculated for C9H8N4O C, 57.44; H, 4.28; N, 29.77. Found (%) C, 57.45; H, 4.30;
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3H). 13C – NMR: 205.1, 152.8, 132.8, 131.5, 130.7, 128.5, 21.9.
Entry 5p: 5ꢀ(4ꢀnitroꢀphenyl)ꢀ1Hꢀtetrazole9h: Elemental analysis (%) calculated for
C7H5N5O2 C, 43.98; H, 2.64; N, 36.64. Found (%) C, 44.01; H, 2.65; N, 36.68. 1H
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– NMR: 6.99 (dd, J = 1.2, 5.2 Hz, 2H), 6.90 (d, J = 1.2 Hz, 2H). C – NMR:
149.5, 148.4, 125.0, 114.5, 114.3.
Entry 5q: 5ꢀ(4ꢀtrifluoromethylꢀphenyl)ꢀ1Hꢀtetrazole9i: Elemental analysis (%)
calculated for C8H5F3N4 C, 44.87; H, 2.35; N, 26.16. Found (%) C, 44.89; H, 2.38;
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N, 26.19. H – NMR: 7.31 (d, J = 6.8 Hz, 2H), 7.25 (d, J = 6.0 Hz, 2H). C –
NMR: 159.8, 133.6, 129.5, 129.0, 128.5, 126.4.
Entry 5r: 5ꢀbiphenylꢀ4ꢀylꢀ1Hꢀtetrazole9i: Elemental analysis (%) calculated for
C13H10N4 C, 70.26; H, 4.54; N, 25.21. Found (%) C, 70.29; H, 4.55; N, 25.24. 1H –
NMR: 7.46 (d, J = 4.0 Hz, 2H), 7.44 (d, J = 12.4 Hz, 2H), 7.31 (s, 2H), 7.23 (t, J =
6.4 Hz, 1H), 6.68 (d, J = 8.0 Hz, 2H). 13C – NMR: 159.6, 136.7, 132.7, 131.7,
128.5, 128.2, 127.6, 126.5, 125.8.
Entry 5s: 5ꢀnaphthalenꢀ2ꢀylꢀ1Hꢀtetrazole9i: Elemental analysis (%) calculated for
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C11H8N4 C, 67.34; H, 4.11; N, 28.55. Found (%) C, 67.35; H, 4.14; N, 28.57. H –
NMR: 6.93 (d, J = 8.0 Hz, 2H), 6.80 (t, J = 4.6 Hz, 1H), 6.75 (d, J = 8.0 Hz, 1H),
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6.56 (s, 1H), 6.11 (t, J = 7.6 Hz, 1H), 5.90 (s, 1H). C – NMR: 163.8, 147.8,
147.2, 132.9, 130.8, 123.0, 121.1.
Entry 5t: 4ꢀ(1Hꢀtetrazolꢀ5ꢀyl)ꢀpyridine9j: Elemental analysis (%) calculated for
C6H5N5 C, 48.98; H, 3.43; N, 47.60. Found (%) C, 48.95; H, 3.47; N, 47.62. 1H –
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NMR: 6.94 (d, J = 8.4 Hz, 2H), 6.86 (d, J = 7.6 Hz, 2H). 13C – NMR: 158.0, 149.5
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Notes
The authors declare no competing financial interest.
ASSOCIATED CONTENT
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Supporting Information Available: H and C NMR spectra for all
compounds prepared. Crystallographic information file for complex 1 (CIF). This
material is available free of charge via the Internet at http://pubs.acs.org.
ACKNOWLEDGEMENTS
Mr. Arumugam Vignesh thanks the University Grants Commission (UGC),
New Delhi, India. for the award of Senior Research Fellowship (SRF) under UGCꢀ
BSR RFSMS.
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