Iron-promoted sulfur sequestration for the substituent-dependent regioselective synthesis of tetrazoles and guanidines

Article abstract of DOI:10.1080/17415993.2021.1909589

We have established a facile and versatile synthetic methodology for the construction of tetrazoles and guanidines in the presence of an eco-friendly, inexpensive, easily available iron reagent. Aromatic thioureas with electron-donating substituents produced their respective target products in quantitative yield. In contrast, when electron-withdrawing substituted aromatic thioureas were used, the expected products were obtained in reduced yield. However, the desired products were obtained in good yield at moderate temperature. In addition, mechanistic studies revealed that the synthetic route involved iron-based subsequent reactions of addition and removal of sulfur.

Full text of DOI:10.1080/17415993.2021.1909589

Journal of Sulfur Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/gsrp20
Iron-promoted sulfur sequestration for the
substituent-dependent regioselective synthesis of
tetrazoles and guanidines
Venkata Bhavanarushi Pendem, Ramana Tamminana & Madhavi
Nannapaneni
To cite this article: Venkata Bhavanarushi Pendem, Ramana Tamminana & Madhavi
Nannapaneni (2021): Iron-promoted sulfur sequestration for the substituent-dependent
regioselective synthesis of tetrazoles and guanidines, Journal of Sulfur Chemistry, DOI:
10.1080/17415993.2021.1909589
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JOURNAL OF SULFUR CHEMISTRY
https://doi.org/10.1080/17415993.2021.1909589
Iron-promoted sulfur sequestration for the
substituent-dependent regioselective synthesis of
tetrazoles and guanidines
a
Venkata Bhavanarushi Pendemb, Ramana Tamminanac and Madhavi Nannapaneni
aDepartment of Chemistry, J. K. C. College, Guntur, India; bDepartment of Chemistry, Rajiv Gandhi University
of Knowledge Technologies, Srikakulam, India; cDepartment of Chemistry, GITAM Deemed University,
Bengaluru, India
ABSTRACT
ARTICLE HISTORY
Received 24 October 2020
Accepted 15 March 2021
We have established a facile and versatile synthetic methodology
for the construction of tetrazoles and guanidines in the presence
of an eco-friendly, inexpensive, easily available iron reagent. Aro-
matic thioureas with electron-donating substituents produced their
respective target products in quantitative yield. In contrast, when
electron-withdrawing substituted aromatic thioureas were used, the
expected products were obtained in reduced yield. However, the
desired products were obtained in good yield at moderate temper-
ature. In addition, mechanistic studies revealed that the synthetic
route involved iron-based subsequent reactions of addition and
removal of sulfur.
KEYWORDS
Sulfur-extracting reagent
iron; room temperature;
tetrazoles; guanidines;
desulfurization
Introduction
The conversion of a simple substrate into diverse libraries of more complex molecules
constitutes a great challenge in modern organic synthesis from both academic and indus-
trial standpoints [1,2]. Recently, multicomponent reactions have emerged as important
synthetic approaches involving at least three or more than three variable components
CONTACT Madhavi Nannapaneni
rtamminana17@gmail.com
Department of Chemistry, J. K. C. College,
Guntur, AP 522006, India
© 2021 Informa UK Limited, trading as Taylor & Francis Group

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V. B. PENDEM ET AL.
Figure 1. Examples of biological important tetrazole compounds.
participating in such a way that to provide a single product [3–9]. Multicomponent reac-
tions provide a valuable and convenient methodology to create several natural, bioactive,
and non-bioactive organic molecules. Over the past few decades, these MCRs have had a
significant impact due to their huge advantages such as speed, efficiency, green reaction
conditions, and atom economy. Based on the better productivity and considerable process
time reduction of MCRs compared to the classical techniques, MCRs have become popular
in organic chemistry [10–15].
Tetrazoles are an interesting class of heterocyclic compound, and they are resistant to
metabolic degradation as well as towards chemical oxidants [16] and as structural com-
ponents of many biological important molecules [17]. In this regard, a few examples of
tetrazoles, such as bearing anti-bacterial drug [18], receptor modulator [19], and anti-HIV
[20,21] activity, are shown in Figure 1.
Initially, the synthesis of substituted tetrazole has been developed using traditional
methods, such as the addition of NaNO2 to aminoguanidine [22], the addition of NaN3
to carbodiimides or cyanamides [23–26], the reaction of amines with−a leaving group
in tetrazoles 5-position [27,28], and nucleophilic substitution by N3 of chlorine in
α-chloroformamidines [29]. Later the traditional methods of tetrazole syntheses were
replaced by modern methods that have used various transition metals like copper [30,31],
mercury [32,33], lead salts [34], zinc salts [35–41], and cobalt sources [42]. Furthermore,
tetrazoles have also been constructed in the presence of TBAF [43] and iodine [44].
However, these methods require either toxic reagents or harsh reaction conditions such
as high temperature. They also lack regioselectivity [45], suffer from unavailable start-
ing materials, lower yields, and are limited to aryl substituents. Furthermore, many of
the reports have focused on the formation of tetrazoles from cyanides and cyanamides.
On the other hand, very few reports [44] are available for the synthesis of tetrazole from
thiourea. Therefore, we initiated research to develop the formation of tetrazoles from
thiourea using ‘sulfur-extracting reagent iron’ as a reagent. The proposed reaction involves
desulfurization, nucleophilic substitution, and electrocyclization. In addition, we would
like to confirm that no report is available for the synthesis of tetrazole using an iron
reagent.
In extension, we wish to report the synthesis of substituted guanidines, which have bio-
logical and medicinal importance (Figure 2) [46,47]. Recently, Izdebski and co-workers
have synthesized protected guanidines from amine using DMAP at room temperature
[48,49]. Later, Giacomelli [50] have used cyanuric chloride (TCT) to prepare di-Boc-
guanidines instead of classical HgCl2 to avoid heavy-metal waste. In this century, Mack
[51] has developed a methodology for the preparation of protected guanidines from the

JOURNAL OF SULFUR CHEMISTRY
3
Figure 2. Examples for biological important guanidine molecules.
treatment of protected imines and amines in the presence of acetonitrile under mild reac-
tion conditions. In addition, recently Yanina Lamberti group has reported novel guanidines
from acyl thiourea, and they found the good biological activity of those compounds [52].
Furthermore, the existed methods have some other drawbacks like (i) the use of toxic
reagents; (ii) extremely alkaline conditions; (iii) the use of expensive reagents; (iv) the use
of high reaction temperature; (v) low yields involving tedious purification procedures; and
(vi) the need for longer reaction time. To the best of our knowledge, no report is available
for the synthesis of guanidines from thiourea in the presence of iron reagents. Therefore,
we would like to report the synthesis of substituted guanidines from thiourea using cheap,
readily available, and air-stable iron reagents under mild reaction conditions.
Experimental materials and methods
General information
Aniline, CS2, FeCl3, Fe(NO3)3.H2O, Fe2(SO4)3.H2O, FeCl2, Et3N, pyridine, sodium bicar-
bonate, sodium acetate, and sodium hypo phosphate are purchased from Aldrich and used
without further purification. The solvents are purchased and dried according to the stan-
dard procedure prior to use.1H NMR (400 MHz) spectra are recorded with a Varian 400
spectrometer. Infrared (IR) spectra are recorded as a Perkin Elmer Spectrum on FT-IR
1
spectrometer. H NMR (400 MHz) and 13C NMR (100 MHz) spectra were recorded on
either Varian or Bruker Advance spectrometer. We have used Perkin Elmer Spectrum One
FT-IR spectrometer for recording the IR spectra. CHNS analyzer (Perkin Elmer) has been
used for analyzing the elemental analysis.
Representative experimental procedure for the synthesis of phenylamino tetrazole
Substituted thiourea (2 mmol, 304 mg) was completely dissolved in the DMSO (3 mL) at
room temperature. To this, Et3N (2 mmol, 202 mg) and catalyst were added slowly at room
temperature. The resulting reaction mixture was stirred at room temperature for 2 h. After
that, NaN3 (2 mmol, 130 mg) was added, and stirring was further continued for 3 h. The
development of the reaction was examined by TLC (30% ethyl acetate in hexane). The
black color solid, which was observed during the reaction, separated using the centrifuga-
tion process. The clear reaction mixture extracted with ethyl acetate (2 × 10 mL), and the
organic layer was washed with brine (1 × 5 mL) and water (2 × 5 mL). Drying and evap-
oration of the solvent gave a residue that was purified by column chromatography using
30% ethyl acetate in hexane as eluent to obtain phenyl aminotetrazole as a solid.

4
V. B. PENDEM ET AL.
Representative experimental procedure for the synthesis of phenylguanidines
Et3N (2 mmol, 202 mg), catalyst (50 mol %), and thiourea (2 mmol) were stirred in DMSO
at room temperature, and the reaction mixture was stirred for 2 h. After that, Aq NH3
(2 mL) was added, and further stirring was continued for 3 h. The progress of the reaction
was monitored by TLC using 30% ethyl acetate in hexane as eluent. After completion of
the reaction, the target reaction mixture was centrifuged for 10 min by using a centrifuga-
tion machine to separate the black color solid. The clear solution mixture extracted with
ethyl acetate (2 × 10 mL), and the organic layer was washed with brine (1 × 5 mL) and
water (2 × 5 mL). Drying and evaporation of the solvent gave a residue that was purified
by column chromatography using 30% ethyl acetate in hexane as eluent to obtain phenyl
aminoguanidine as a solid.
Results and discussion
First, reaction conditions were optimized using readily available N-phenyl thiourea and
sodium azide as model substrates, together with different iron reagents, bases, and solvents.
The reactions were also carried out at room temperature. Initially, solvent optimization
was carried out using Et3N as a base and ferric sulfate as a catalyst. In this connection, the
formation of tetrazole was more efficient when using DMSO as solvent compared to the use
of other solvents (Acetone, CH3CN, DMF, EtOH, MeOH, H2O, n-hexane, and n-heptane).
The control experiment confirmed that the reaction did not give the target product in the
absence of solvent. Since thiourea does not dissolve in nonpolar solvents like n-hexane
and n-heptane, the reaction could not give the target product. No tetrazole formation was
observed in the presence of other solvents, which would not make complex with the iron
reagent.
However, a high yield of tetrazole was observed when using Et3N and pyridine instead
of inorganic bases (NaOAc, NaOH, and Na2HPO4). Organic bases like Et3N and Pyri-
dine complex with iron salts that increase their solubility, which leads to an increase in the
yield of tetrazole product. Among the inorganic bases, sodium acetate is better than oth-
ers. It might be reasoned that NaOAc may produce Fe(OAc)2, which is more soluble than
other iron sources. To confirm, we have performed the control experiment with an organo-
soluble iron source like Fe(acac)2, and it gave tetrazole product in good yield (Scheme 1). In
the case of disodium phosphate and sodium hydroxide, we could observe tetrazole product
along with some other undetermined spots, which are in TLC (bond might be broken in
the presence of strong bases).
Both Iron (II) and iron (III) sources (FeCl2, Fe2(SO4)3.H2O, Fe(NO3)3.9H2O, FeCl3)
exhibited similar levels of activity and selectivity. Reaction with lower amounts of iron
Scheme 1. Synthetic route for the synthesis of 1-alkyl-5-amino tetrazoles.

JOURNAL OF SULFUR CHEMISTRY
5
Scheme 2. Substrate scope for the synthesis of tetrazoles[a]. [a] Reaction conditions: substituted
thiourea (2 mmol), Fe2(SO4)3.H2O (50 mol %), Et3N (1 eq), DMSO, room temperature, 2 h, then NaN3 (1
eq), rt, 3 h. [b] Isolated yield. [c] K2CO3 (1 eq) was used.
source (25 mol %) gave lower yields of tetrazole. In a control reaction in which an iron
source was absent, the tetrazole was not formed, and the starting material was recovered
intact.
Having the optimal conditions in our hand, the scope of the protocol was next explored
(Scheme 2). The process was found to be generally applicable; various aryl- and alkyl-
substituted thioureas have reacted with sodium azide to provide the corresponding tetra-
zoles 1a–m in moderate to good yield. Aryl thiourea having electron-donating substituents
on the phenyl ring (4-Me, 4-OMe, and 2,4-diMe) were more reactive than those hold
electron-withdrawing substituents on the aryl ring (4-F, 4-CN, 2-NO2, and 4-COOMe
groups). In addition, aryl thioureas were partially more reactive than alkyl thioureas. The
thiourea having strong electron-withdrawing groups like 4-CN, 2–NO2, and 4-COOMe
on phenyl ring gave the corresponding tetrazoles in less yield under optimized reaction
conditions. Therefore, we have carried out further optimization to improve the yield of
the products. In this connection, they were tested using a strong base (anhydrous potas-
sium carbonate (K2CO3)), and they gave their respective target products 1h–j in moderate
yield. On the other hand, the thioureas contain strong EWG like 4-CN, 2–NO2, and 4-
COOMe on phenyl ring provided other isomeric products 3a–c in 46%, 48%, and 52%
yields, respectively.

6
V. B. PENDEM ET AL.
Scheme 3. Substrate scope for the synthesis of N-aryl/alkyl guanidines[a]. [a] Reaction conditions: sub-
stituted thiourea (2 mmol), Fe2(SO4)3.H2O (50 mol %), Et3N (1 eq), DMSO, room temperature, 2 h, then
Aq NH3 (1 eq), rt, 3 h. [b] Isolated yield. [c] K2CO3 (1 eq) was used.
Later, we have started the optimization for the synthesis of guanidine using phenyl
thiourea and Aq NH3 as model substrates. Since Aq NH3 is cheaper and more readily
available than NaN3, we have chosen NH3 as nucleophile for the synthesis of guanidine
derivatives. We gratifyingly confirmed that the reaction was best using Fe2(SO4)3.H2O
(50 mol %), Et3N (2 mmol), and Aq NH3 in the presence of DMSO at room temperature.
All the electron-donating and electron with-drawing substituents provided their respec-
tive target products 2a–k in 43–93% yields (Scheme 3). Similar to tetrazole formations,
isomeric guanidines were also obtained in the case of strong electron-withdrawing sub-
stituents of phenyl ring. In this regard, thioureas consisting of 4-CN, 4-COOMe, and
2-NO2 on phenyl gave guanidine products 4a–c in 45%, 48%, and 55% yields, respectively.
To gain insight into the nature of the iron reactivity and rationalize why both Iron (II)
and Iron (III) salts reacted equally well, we performed EPR experiments (see Supp.). The
reaction was monitored to show that a similar iron species is generated from both FeCl2
and Fe2(SO4)3.H2O prepromoters. Thus, from these experiments, we strongly believe that
Fe (III) may convert into Fe (II) species in the presence of substituted thiourea.
Based on experimental results and literature reports [53,54], we propose a possible
catalytic cycle in Scheme 4. Iron (II) species (could be formed from Fe (III) salt) [55–60] co-
ordinates to sulfur in thiourea followed by desulfurization (FeS was formed as byproduct
and an extra sulfur might have converted into polysulfide) [61,62] and nucleophilic substi-
tution with NaN3 to afford the intermediate C via intermediates A and B. Electrocycliza-
tion then occurs depending on the electron density of the phenyl ring of the intermediate.

JOURNAL OF SULFUR CHEMISTRY
7
Scheme 4. Plausible mechanism.
While the phenyl ring contains strong electron-withdrawing groups (4-CN, 4-COOMe, or
2-NO2), it is possible to obtain the target products, substituted 5-phenylaminotetrazoles
3a–c, via an intermediate D, otherwise phenyltetrazolamines 1a–m are formed via inter-
mediate C. On the other hand, desulfurization [63–69,44] of intermediate B and followed
by nucleophilic substitution with Aq NH3 to afford the target products 2a–k. Whereas
phenyl ring bearing strong electron-withdrawing substituents like CN, COOMe, and NO2
to provide possible tautomeric products 4a–c.
To confirm the formation of FeS, after completion of the reaction the solid FeS was
separated via centrifugation, and it was subjected to Powder XRD analyses (Figure 3).
The spectrum shows the peaks such as (101), (110), (102), (201), and (213) match with
JCPDS XRD data for ferrous sulfide [70–73]. No other diffraction peaks appeared in the
XRD pattern. Therefore, we strongly believe that ferrous sulfide has formed in the reaction
medium.
Conclusion
In conclusion, a simple method for the construction of aryl/alkyl tetrazoles and guanidines
has been established from thiourea under mild reaction conditions using sulfur-extracting
reagent iron, which is cheap, readily available, and air-stable. In contrast to previously
described methods, which frequently require unstable moisture-sensitive starting mate-
rials, such as isocyanide dichlorides, chloroformamidines, aminomethane sulfonic acids,

8
V. B. PENDEM ET AL.
Figure 3. Powder XRD pattern for solid FeS.
and benzotriazolyl carboximidamides, our method uses stable thiourea as a starting pre-
cursor and occurs in high yields. The ease of product purification is also enhanced.
Furthermore, functional group tolerance has been described in this paper. In addition,
we have also conducted mechanistic studies to provide a plausible mechanism for this
methodology.
Disclosure statement
No potential conflict of interest was reported by the author(s).
ORCID
Madhavi Nannapaneni http://orcid.org/0000-0002-3702-9929
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