N-Heterocycle (tetrazole)-stabilized pseudocyclic λ3-iodane: Synthesis and reactivity

Article abstract of DOI:10.1016/j.tetlet.2019.150995

An N-heterocycle (tetrazole)-stabilized pseudocyclic λ³-iodane was synthesized and its thermal stability and redox activity were studied. Simultaneous thermogravimetric analysis and differential scanning calorimetry revealed that the polyvalent iodine compound started to degrade at 127 °C and showed a notable exothermic decomposition peak at 147 °C, which was markedly more pronounced than the parent tetrazole with monovalent iodine. The reduction potential was determined by cyclic voltammetry to be ?0.38 V vs AgNO₃|Ag in dimethyl sulfoxide at a scan rate of 0.02 V/s. The oxidative power of the polyvalent iodine compound was demonstrated by conducting oxidation of thioanisole to sulfoxide and oxidative dimerization of thiobenzamide to 3,5-diphenyl-1,2,4-thiadiazole.

Full text of DOI:10.1016/j.tetlet.2019.150995

Journal Pre-Proof
N-heterocycle (tetrazole)-stabilized pseudocyclic λ 3-iodane: Synthesis and re-
activity
Avichal Vaish, Kapil Dev Sayala, Nicolay V. Tsarevsky
PII:
S0040-4039(19)30750-6
DOI:
https://doi.org/10.1016/j.tetlet.2019.150995
TETL 150995
Reference:
Tetrahedron Letters
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Accepted Date:
27 June 2019
25 July 2019
29 July 2019
Please cite this article as: Vaish, A., Sayala, K.D., Tsarevsky, N.V., N-heterocycle (tetrazole)-stabilized pseudocyclic
λ 3-iodane: Synthesis and reactivity, Tetrahedron Letters (2019), doi: https://doi.org/10.1016/j.tetlet.2019.150995
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JOURNAL PRE-PROOF
N-heterocycle (tetrazole)-stabilized pseudocyclic λ3-iodane: Synthesis and
reactivity
Avichal Vaish, Kapil Dev Sayala, and Nicolay V. Tsarevsky*
Department of Chemistry, Southern Methodist University, Dallas, TX 75206 USA
* Email: nvt@smu.edu, Tel: +1-214-768-3259
Abstract
An N-heterocycle-stabilized pseudocyclic λ3-iodane with tetrazole as the stabilizing group
was synthesized and its thermal stability and redox activity were studied. Simultaneous
thermogravimetry analysis and differential scanning calorimetry revealed that the hypervalent
iodine(III) compound started to degrade at 127 ºC and showed a notable exothermic decomposition
peak at 147 ºC. The reduction potential was determined by cyclic voltammetry to be -0.38 V vs
AgNO3|Ag (CV) in dimethyl sulfoxide (at a scan rate of 0.2 V/s). The oxidative power of the
polyvalent iodine compound with a tetrazole ligand was demonstrated by conducting oxidation of
thioanisole to sulfoxide and dimerization of thiobenzamide to 3,5-diphenyl-1,2,4-thiadiazole.
Keywords: polyvalent iodine; tetrazole; oxidation; energetic materials
Introduction
Organic hypervalent (HV) iodine compounds have emerged as efficient, versatile and
environmentally benign reagents for a plethora of synthetically useful oxidative transformations.1-
14
including oxidation and electrophilic group transfer reactions. Early examples of HV iodine-
based oxidants such as iodosyl benzene (PhIO) had critical drawbacks including poor solubility
and short bench lifetime. In a pioneering work, Carmalt and co-workers demonstrated that the
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extensive secondary intermolecular I···O interactions of unstabilized λ3-iodanes produces highly
insoluble and thermally labile polymeric networks.15 To overcome this problem, Protasiewicz et
al. developed stable and soluble ortho-sulfonyl- substituted iodosylbenzene (1) and ortho-
phosphoryl-substituted iodylbenzene (2), which possess improved reactivity toward organic
substrates.16-19 Subsequently, Zhdankin and co-workers reported the first example of the
pseudocylic benziodoxole triflate and tosylate (3).20,21 Furthermore, several pseudocyclic O-
donor-stabilized λ3-iodanes with ether (4),22,23 amide (5)24 and nitro (6)25 groups interacting with
the I(III) centers were described. In 2017, Muniz reported first N-heterocycle-stabilized
pseudocyclic HV iodine reagent (8) with an ortho-pyridinyl group.26 Recently, Nachtsheim et al.
reported a series of N-heterocycle stabilized pseudocyclic HV iodine reagents with different atoms
in the ring (9) and showed that their reactivity can be easily tuned by altering the heterocycle.27
Figure 1. O- and N-stabilized pseudocyclic polyvalent iodine compounds.
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-
-
We recently reported that tetrazolates RCN4 , similar to carboxylates RCO2 , can serve as
ligands that bind to HV iodine(III) centers, and described the synthesis, structural characterization,
thermal degradation, electrochemical behavior, and reactivity of a number of tetrazole-containing
HV iodine(III) compounds with the general formula RCN4-[I(Ar)-O)]n-I(Ar)-N4CR (R = CH3,
C6H5, or 4-CH3C6H4, and n = 0, 1, ≥2; the compounds with n = 0 being the terazole analogues
ArI(N4CR)2 of (diacyloxyiodo)arenes Ar(O2CR)2).28 As a continuation of our studies, we now
report the synthesis and properties of an N-tetrazole-stabilized pseudocyclic HV iodine(III)
reagent, namely (2-(1H-tetrazol-5-yl)phenyl)(hydroxy)iodonium tosylate (10).
Results and Discussion
The precursor of the title compound, 5-(2-iodophenyl)-1H-tetrazole 11, was synthesized in
good yield (75%) by the reaction of 2-iodobenzonitrile with sodium azide in the presence of a
protic acid (ammonium chloride) in N,N-dimethylformamide at 130 ºC for 20 h using a simplified
literature29 procedure. The monovalent iodine compound was then oxidized with meta-
chloroperoxybenzoic acid (mCPBA) in the presence of p-toluenesulfonic acid monohydrate
(TsOH.H2O) in CH2Cl2 at room temperature for 15 h to afford 10 in good yield (72%), as shown
in Scheme 1. The pseudocyclic HV iodine(III) compound with a single tetrazole ligand 10 was
obtained in an analytically pure form as off-white powder after washing with diethyl ether (all
starting materials are well soluble in excess of diethyl ether) and was found to be stable at room
temperature for several weeks when kept in a vacuum desiccator.
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Scheme 1. Synthesis of (2-(1H-tetrazol-5-yl)phenyl)(hydroxy)iodonium tosylate 10.
The thermal stability of the λ3-iodane 10 was examined by simultaneous thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC). Samples were heated from 25 ºC to
550 ºC at a rate of 5 ºC/min under nitrogen. Both 10 and 11 were analyzed twice, and the averaged
thermal decomposition curves are shown in Figure 2. The TGA curve of 10 exhibits a two-step
degradation profile with a Td (onset degradation temperature, at which 5% weight loss has taken
place) of 142 ºC, and steep weight loss between 127 ºC and 152 ºC, with an exothermic signal in
the DSC curve, due to the dissociation of the weak HV bond. Compound 11, which also contains
the thermally labile tetrazole group but no HV bonds, was more stable than 10 and started to
degrade at 183 ºC without showing any prominent exothermic peak in the DSC curve save for a
small peak at 210 ºC. The enthalpies of degradation (ΔHd) were calculated by integrating the
mentioned exothermic peaks and the average ΔHd for compound 10 was 548±86 J/g, which was
about 20-fold higher than that of compound 11 (27.5±7.5 J/g). This shows that compound 10 and
potentially other tetrazole-containing hypervalent iodine(III) compounds can be used as energetic
materials. Similarly, linear tetrazole-containing HV iodine(III) compounds were shown to degrade
explosively at elevated temperatures,28 although their enthalpies of thermolysis were not measured.
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150
145
140
135
130
125
120
115
1.0
0.8
0.6
0.4
0.2
0.0
10
11
10
11
100
200
300
400
500
o
Temperature [ C]
Figure 2. Simultaneous TGA and DSC analysis of compounds 10 (solid lines) and 11 (dashed
lines).
HV iodine compounds are often used as oxidizing agents. Therefore, newly synthesized
compound 10 was further analyzed by cyclic voltammetry (CV) to examine its oxidizing power.
The starting materials used to prepare 10, namely the tetrazole 11 and TsOH.H2O were also
analyzed to understand the origin of all the peaks in the cyclic voltammogram of 10. The CV
measurements (0.5 V to -1.5 V) were performed in dry and deoxygenated dimethyl sulfoxide
(DMSO) (a good solvent for all compounds), using a glassy carbon electrode at a scan rate of 0.02
V/s using 0.1 M (n-Bu)4NPF6 as the electrolyte. The CV of DMSO containing only the electrolyte
was studied also in the same voltage range to check its redox stability. DMSO did not show any
peak in the above-mentioned region and hence it was concluded to be appropriate to use as solvent
for the electrochemical studies. The cyclic voltammogram of 10 showed two main reduction peaks
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and several oxidation peaks (Figure 3). The first reduction peak at -0.38 V was due to the two-
electron reduction of iodine(III) in 10 to iodine(I), i.e. formation of 11 as shown in Scheme 2.
Formation of 11 is also observed in chemical reductions of 10 e.g. by thioanisole (Figure S7). This
reaction is described in more detail below. The wide reduction peak observed at more negative
voltages (in the range from 0.95 V to 1.05 V) is likely the result of overlap of the reduction peaks
seen in the individual CV traces of 11 and TsOH (also shown in Figure 3). The nature of the
products of reduction of TsOH and/or TsO- is difficult to ascertain. The mechanism of reduction
of aromatic sulfonic acids and/or sulfonate anions is complex and it depends upon the composition
of the solvent, and often involves the cleavage of C-S bonds.30 CV has been employed previously
to study the reduction potential of various λ3-iodanes.28,31-33 HV iodine compounds show
irreversible reduction peak, like compound 10, due to the formation of iodine(I) compounds,
typically iodoarenes. The reduction potentials of various HV iodine(III) compounds with tetrazole
ligands were between -0.25 V to -0.43 V, i.e., comparable to the reduction potential of compound
10.28
Scheme 2. Proposed pathway of electrochemical reduction of 11.
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10
TsOH.H2O
11
0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4
Voltage [V vs AgNO |Ag]
3
Figure 3. Cyclic voltammograms of 1 mM solutions of compounds 10 and 11, and TsOH in
anhydrous DMSO containing 0.1 M (n-Bu)4NPF6 at scan rate of 0.02 V/s using a glassy carbon
electrode.
Compound 10 was then employed to oxidize thioanisole to the corresponding sulfoxide 12.
First, the preliminary reaction was carried out in DMSO-d6 (Figure S7) and after confirming the
formation of 12, the reaction was then conducted in CH3CN at room temperature for 45 min to
simplify the workup and to increase the conversion by taking advantage of poor solubility of 11 in
CH3CN. Indeed, under these conditions the product was isolated in excellent yield (94%).
Compound 10 was proved very useful in the synthesis of N,S-heterocycles as well. For instance,
the oxidative dimerization of thiobenzamide in the presence of reagent 10, conducted in CH3CN
at room temperature for 30 min, afforded 3,5-diphenyl-1,2,4-thiadiazole (13) in high isolated yield
(92%). Both reactions are presented in Scheme 3.
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Scheme 3. (i) Oxidation of thioanisole, (ii) oxidative rearrangement of thiobenzamide in the
presence of λ3-iodane 10.
Conclusions
Tetrazole is demonstrated to serv as a suitable N-donor to stabilize pseudocyclic λ3-iodane,
(2-(1H-tetrazol-5-yl)phenyl)(hydroxy)iodonium tosylate, which was stable at room temperature
for weeks. The identity of the tetrazole-containing HV iodine(III) compound was proved by NMR
spectroscopy and high-resolution mass spectrometry. Although stable at ambient temperatures, the
compound decomposed with the release of significant amount of heat at temperatures exceeding
140 ºC. The oxidative power of the iodonium salt was quantified by cyclic voltammetry and it was
taken advantage of in the high-yield oxidation of thioanisole to sulfoxide and the oxidative
dimerization of thiobenzamide to 3,5-diphenyl-1,2,4-thiadiazole.
Acknowledgements
The authors gratefully acknowledge financial support by the National Science Foundation
through a CAREER grant (CHE-1455200) to N.V.T. Professor Isaac Garcia-Bosch and Khashayar
Rajabimoghadam are acknowledged for helping with the electrochemical measurements.
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N-heterocycle (tetrazole)-stabilized pseudocyclic λ3-iodane: Synthesis and
reactivity
Avichal Vaish, Kapil Dev Sayala, and Nicolay V. Tsarevsky*
Department of Chemistry, Southern Methodist University, Dallas, TX 75206 USA
* Email: nvt@smu.edu, Tel: +1-214-768-3259
Highlights
-
Synthesis of tetrazole-containing hypervalent iodine compound
-
-
-
Studies of electrochemical behavior
Studies of thermal stability and demonstration of highly exothermic decomposition
Utility in oxidation and oxidative cyclization reactions
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JOURNAL PRE-PROOF
N-heterocycle (tetrazole)-stabilized pseudocyclic λ3-iodane: Synthesis and
reactivity
Avichal Vaish, Kapil Dev Sayala, and Nicolay V. Tsarevsky*
Department of Chemistry, Southern Methodist University, Dallas, TX 75206 USA
* Email: nvt@smu.edu, Tel: +1-214-768-3259
ToC graphic
12