A practical anodic oxidation of aminofurazans to azofurazans: an environmentally friendly route

Abstract

Nickel oxyhydroxide, NiOOH, anode has been shown to be an effective tool for the oxidation of aminofurazans to azofurazans in ca. 1% aqueous alkali at room temperature. The electrochemical reaction is simple and convenient, eliminating the use of expensive and toxic organic or inorganic oxidants. The green economic preparation of desired azo compounds is very clean, producing only H₂ as a result of cathodic reduction.

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Introduction

Green chemical processes play a crucial role in sustainable development, and atom-economical strategies for the enhancement of molecular complexity are the key elements for the design of new environmentally favorable synthetic processes. According to the trend, we have focused our effort on the development of a safe and efficient method to prepare furazan (1,2,5-oxadiazole) derivatives.

Furazan chemistry has extended over a period of about 140 years, and many useful derivatives have been synthesized and investigated. Applications of these compounds are highly diverse, ranging from medicinal chemistry to explosive and propellant ingredients.¹ In this family, azofurazan structural motif is present in a range of crown ether analogs² and biologically active molecules that inhibit the NO-dependent activation of the soluble guanylate cyclase.³ However, compounds incorporated azofurazan framework with a high nitrogen content and large positive heats of formation have been most extensively studied as energetic materials.⁴ 4,4′-Diaminoazofurazan 1,^4c,5 3,4-di(4-nitrofurazan-3-azo)furazan 2,^4a,6 4,4′-di(5-tetrazolyl)-azofurazan 3,⁷ 4,4-dinitrodiazenoxyazofurazan 4,⁸ and tetrakis(furazano)[3,4-c:3′,4′-g:3′′,4′′-k:3′′′,4′′′]-[1,2,5,6,9,10,13,14]-octaazacyclohexa-decine 5 ^4c,9 are examples of the energetic compounds (Fig. 1). Given their utility, the development of synthetic methodology to access azofurazan derivatives is continually warranted.¹

Fig. 1 Some high energetic azofurazans.

Azofurazans are typically prepared from aminofurazans by an oxidation reaction. Traditionally, this has been carried out using such reagents as KMnO₄/H⁺,¹⁰ CrO₃/AcOH,¹¹ (NH₄)₂S₂O₈,¹² NaOCl or NaOBr,¹³ nitronium tetrafluoroborate,¹⁴ Br₂/H₂O,¹⁵ or a variety of organic reagents (dibromoisocyanuric acid¹⁶ and trichloroisocyanuric acid).^4e,17 These procedures, however, require the use of hazardous, corrosive or expensive reagents. In a chemical oxidation, stoichiometric amounts or excess of an oxidant are needed. As a result the corresponding amounts of metal salts or acids are formed as waste. Separation of the products from the waste is often difficult. Our goal was to develop a convenient, economical and environmentally friendly synthetic protocol for a quick assembly of the azofurazan core.

The growing social pressure for new green technologies and the promise of organic electrosynthesis to deliver them has led to high academic and industrial interest in electrochemical methods.¹⁸ The electron in electrosynthesis is one of the most environmentally friendly reagents, as it produces no waste in contrast to chemical reagents; furthermore, it is the cheapest reagent in chemistry.

Organic electrooxidation has attracted much attention as one of the effective methods in carbon–carbon, carbon–nitrogen, and carbon–oxygen bond formations.¹⁹ At the same time, electrochemical N=N bond formations is nearly unexplored. Thus, an alkyl azo compound, di(^tbutyl) diazene, than have been obtained in low yield by electrooxidation of ^tbutylamine, was described.^20a The only known electrochemical synthesis of azo-(het)arenes from amino(het)areneis oxidation of anilines to azo-benzenes (3–47% yield; the largest yields were recorded for compounds bearing electron-withdrawing substituents).^20b,c However, the electro-oxidation of the aniline nitrogen atom usually resulted in the formation of a radical cation, that can exist in multiple resonance forms. The latter provides an opportunity for three possible couplings: (i) nitrogen–nitrogen (head-to-head, HH), (ii) nitrogen–arene (head-to-tail, HT) and (iii) arene–arene (tail-to-tail, TT). HT and TT coupling occurs predominately, and the polymerization of anilines is a common result.²¹

Recently, it was shown that electrooxidation of 3-methoxy-4-nitraminofurazan 6 leads to the formation of 4,4′′-dimethoxyazofurazan 7 in undivided cell in ca. 40% yield (Scheme 1).²² This procedure required preparation and the use of hazardous starting nitramine 6.

Scheme 1

It should be noted that the electrooxidations for N=N bond formation in the syntheses of azocompounds mentioned above^20,21 have been carried out with great expensive Pt anode in a non-aqueous medium (acetonitrile). An attractive perspective for synthesis of (het)aromatic azo compounds is electrooxidation of corresponding (het)aryl amines on Ni anode under galvanostatic electrolysis. The basic electrochemical reaction at Ni anode²³ involves the formation of nickel hydroxide Ni(OH)₂ and its oxidation in alkaline solution (typical electrolyte is 1 M NaOH/H₂O) to nickel oxyhydroxide NiOOH²⁴ which is an equivalent to the well-known oxidant, nickel peroxide.²⁵ Electrochemical synthesis is based on a combination of an electron transfer at the Ni anode with a chemical reaction of electrodeposited NiOOH (Fig. 3). The electron transfer (re)generates the NiOOH reagent. The anode is usually used in organic synthesis for oxidation of C–H and C–C bonds of organic compounds.²⁶ It should be noted that an example of N=N bond formation at the NiOOH anode have been described: 1,1-disubstituted hydrazines were oxidized in good yield to 1,1,4,4-tetrasubstituted tetrazenes.²³ To the best of our knowledge, a synthesis of (het)aryl azo compounds using NiOOH anode has not been reported. We hypothesized that it might be possible to use the anode for oxidation of a (het)aryl amine to the corresponding azo compound.

Fig. 2 CVs of Ni/NiOOH in 0.2 M NaOH with (black line) and without (gray line) amine 8a (20 mmol L⁻¹, 100 mV s⁻¹).

Fig. 3 Schematic illustration of the possible electrochemical mechanism for the oxidation of aminofurazans on Ni(OH)₂.

As a part of a program aimed at the development of ecological synthetic methods for the construction of energetic materials,²⁷ we now report a facile and reliable electrochemical oxidation of aminofurazans. This clean process allows the synthesis of azofurazansin one step with high atom economy, and driven by loss of environmentally benign hydrogen gas.

Results and discussion

We were gratified to find that this NiOOH electrode was successful in the oxidative preparation of the azofurazans, giving clean reaction profiles (Fig. 3). We started our study by examining the oxidation of 3-amino-4-methylfurazan 8a to 4,4′-dimethylazofurazan 9a. Cyclic voltammetry (CV) was used for an initial evaluation of the electrocatalytic process. The typical voltammetric response is exemplified in Fig. 2. Compared to the voltammogram of the Ni²⁺/Ni³⁺ pair (E^1/2 = 430 mV) in absence of a substrate (solid line), the peak current increases when amine 8a was added (dotted line). The observation is similar to that fixed at the electrooxidation urea²⁸ and alcohols²⁹ on NiOOH catalyst surface in alkaline medium.

However, preparative-scale evaluation is crucial in order to establish the actual performance and efficiency of the Ni²⁺/Ni³⁺ catalytic systems under synthetic conditions. Preparative electrolysis of compound 8a under galvanostatic conditions involved using a four-neck jacketed flask as an undivided cell, a cylindrical Ni anode, a cylindrical net Ti cathode, and an aqueous solution of an alkali as the supporting electrolyte.

Anodic oxidations of amine 8a were carried out at stirring under various conditions. The reactions were generally clean when monitored by NMR, only target azo compound 9a and unreacted starting material have been identifiable components. Details on the experimental protocol are summarized in Table 1.

Table 1 Electrooxidative coupling^a 3-amino-4-methylfyrazan 8a on NiOOH anode^b

Entries	Base (concentration, mol × L^−1)	Current density (mA × cm^−2)	Electricity passed, Q/Qtheor^c	Time (min)	Yield^d (%)
					9a	Recovery of 8a
a Reactions were run under galvanostatic conditions on 2 mmol scale of compound 8a in 100 ml of H2O (0.02 mol × L^−1) at room temperature.b Before the experiment, the Ni anode (ca. 48 cm^2) was activated according to the next procedure:^24 a thin NiOOH surface layer was deposited on the anode at preliminary electrolysis in a solution 0.1 M NiSO4, 0.1 M NaOAc, and 0.005 M NaOH, at ja = 1 mA × cm^−2 with periodical reverse of the electrode polarization.c Here Q is the amount of electricity (F), and Qtheor = 2 F per a mole of compound 8a.d Determined by ^1H NMR spectroscopy using 4,4′-dimethoxyazofurazan as an internal standard added after the reaction (averages of two runs); isolated yields are shown in parentheses.e Reaction conducted at 70 °C.
1	LiOH (0.1)	6	1	22	57	42.0
2	NaOH (0.1)	6	1	22	59	39.5
3	KOH (0.1)	6	1	22	46	52.5
4	NaOH (0.2)	6	1	22	76 (74)	22 (20)
5	NaOH (0.3)	6	1	22	67	31.0
6	NaOH (0.5)	6	1	22	61	38.0
7^e	NaOH (0.2)	6	1	22	73^e	25.5
8	NaOH (0.2)	2	1	66	59	39.5
9	NaOH (0.2)	3	1	45	53	45.5
10	NaOH (0.2)	4	1	34	65	33.0
11	NaOH (0.2)	8	1	17	58	41.0
12	NaOH (0.2)	16	1	8.5	64	33.5
13	NaOH (0.2)	6	1.5	33	83	17
14	NaOH (0.2)	6	2	44	97 (95)	1.5
15	NaOH (0.2)	7	1	19	66	33.0

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The results in Table 1 indicate that in the electrooxidation, all of the bases (entries 1–3) favored the formation of azo compound 9a with comparable selectivity. It appeared that NaOH is optimum with respect to overall yield and cost. We found that the use of 0.1 M NaOH in H₂O at 22 °C under an air atmosphere produced 9a after 22 min in 59% yield with a moderate conversion of 8a (Table 1, entry 2). Using 0.2 M NaOH as a basic medium, the conversion was enhanced, but the resulting yield of 9a remained below 80% (Table 1, entry 4). Increase of the temperature (entry 7) or prolongation of the reaction time (entry 8) did not lead to an improvement in the yield. However, an excellent result was obtained when double quantity of electricity was passed; a 97% yield of azo compound 9a and almost complete conversion of amine 8a were observed (Table 1, entry 14).

Next, the scope of this oxidation was examined (Table 2). It was found that the solubility of aminofurazans in water is a key parameter to the success of the electrochemical process. Thus, the solubility of alkyl and alkoxy compounds 8a–g is much better than that of aryl 8i and hetarylfurazans 8j, 8k, and 8m. As a result, a variety of groups, including Me (a), Et (b), Pr (c), Bu (e), cyclopropyl (d), MeO (f), PrO (g), and OH (h) were perfectly tolerated under these reaction conditions to produce the corresponding azofurazans 9a–h in excellent yields. On the other hand, aminofurazans bearing aryl and pyridyl substituents showed little conversion into the desired azofurazans 9i and 9j. Aminofurazan 8k with 1,2,4-oxadiazolyl group is insoluble in water. As a result, attempts to oxidize this amine to azocompound 9k were unsuccessful and only unreacted starting material was observed. On the other hand, tetrazolylfurazanylamine 8l have higher water solubility and produced the desired azo compound 9l in excellent yield. Obviously, this protocol could be feasible for large scale production of the product 9l.

Table 2 Electrooxidative synthesis of azofurazans from aminofurazans on NiOOHanode^a

a Reaction conditions: substrate 8 (2 mmol), 0.2 M NaOH in H₂O (100 ml), 25 °C, current density j_a = 6 mA cm⁻², Q_theor = 2F per a mole of compound 8, NiOOH coated anode, Ti cathode. General procedure was used. Isolated yields are an average of three runs; in all cases where the reaction was unsuccessful or the yield of azo compound 9 was low, significant amounts of starting material were recovered.b Electricity passed: Q/Q_theor = 1.c Electricity passed: Q/Q_theor = 2.5.d Electricity passed: Q/Q_theor = 3.e Reaction was performed at 70 °C.f 3-Amino-4-nitrofurazan 8r was used in place of 3-amino-4-hydroxyfurazan 8h.

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The electrooxidation of an amino group in 3,4-diaminofurazan 8o was first expected to provide the desired azo compound 9o. An amino group of this product can further react with itself or with the amino group of remaining starting amine 8o to generate oligomeric azofurazan byproducts. The easiest way to remove these impurities is via crystallization, thus giving diaminoazofurazan 9o in acceptable yields (42%, Table 2).

It is significant that 1,2-bis(3-aminofurazan-4-yl)hydrazine 10 is nearly quantitatively converted to azocompound 9o in 0.2 M NaOH/H₂O at electro oxidation on NiOOH anode with Q = Q_theor = 2F per a mole for ca. 20 min at room temperature. In contrast, there is a report that bubbling of air through methanol solution convert hydrazine 10 to azo compound 9o in good yield (92%);^5a however, the reaction takes up to 20 h to complete.

Attempted electrooxidation of 3-amino-4-nitrofurazan (8r, R = NO₂) resulted exclusively in the formation of hydroxyl product 9h instead of the desired 4,4-dinitroazofurazan (9r, R = NO₂), which was undetectable. Hydrolysis of the nitro group proceeded cleanly to provide the hydroxy derivative 9h in 94% yield; in contrast, only 63% yield of compound 9h was obtained from oxidation of 3-amino-4-hydroxyfurazan 8h.

Table 2 illustrates a generalized protocol implemented for the synthesis of azo compounds of interest in the energetic material chemistry arena such as 4,4-dihydroxyazofurazan 9h,^4d,30 4,4′-di(5-tetrazolyl)-azofurazan 9l (identical with compound 3 from Fig. 1),⁷ compound 9m,^4c 4,4′-diaminoazofurazan 9o (identical with compound 1 from Fig. 1),^4c,5 and compound 9p³¹ (the latter is an intermediate in the synthesis of nitrodiazenoxides, R′–N(O)=N–NO₂ ⁸).

Notably, these electrochemical conditions provide access to the products derived from 3-amino-4-azidofurazan 8n.³² At room temperature, highly sensitive azo compound 9n can be made in 74% yield (Table 2). On the other hand, at higher temperatures, energetic pentalene 11 ³³ (Scheme 2) was synthesized in good yield.

Scheme 2

The products were identified from their characteristic spectroscopic properties by comparison with those of similar compounds in the literature. Their ¹³C NMR data are all consistent with the presence of the azofurazan core in the proposed structures. In particular, the δ_C values for C–N=N (155–163 ppm), and C–R (148–151 ppm) are typical of azofurazans.³⁴

The structures of 4,4-dicyclopropylazofurazan 9d and 5-(4-azidofurazan-3-yl)-[1,2,3]triazolo[4,5-c][1,2,5]oxadiazol-5-ium-4-ide 11 were established by X-ray crystallography (Fig. 4).³⁵

Fig. 4 General view of the structures 9d and 11 with the atom numbering scheme. Displacement ellipsoids are drawn at the 50% probability level.

An asymmetric unit cell of azo compound 9d contains half of the molecule located at the center of symmetry which adopts planar structure and ap–ap–ap conformation that is typical for azofurazans.^10i,36

An asymmetric unit cell of fused furazan 11 contains one molecule, and this structure is characterized by high density (1.859 g cm⁻³ at 100 K). The molecule is somewhat nonplanar probably due to steric repulsion between substituents at the furazan ring. For the analysis of the crystal packing we used combination of the common visual analysis of the crystal packing based on consideration of the close contacts and the approach based on intermolecular pair energies³⁷ for which the M052X/aug-cc-pvdz level of approximation was utilized. Both methods and the basis set were successfully applied in our recent studies on isolated molecules and their aggregates.³⁸ The details of the crystal packing analysis are given as ESI.†

Conclusions

In summary, we have reported a novel, efficient, and easy to perform green method for N=N bond formation to the synthesis of azofurazans. Key features of the approach include the electrooxidation of aminofurazans in ca. 1% aqueous alkali on NiOOH anode without the use of inorganic/organic oxidants or other solvents. The process run in water produces from good to excellent yields of the azofurazans, promising candidates for energetic material formulations. Given the high yield, “greenness”, and possibility of scaling-up, the process has considerable potential for adoption by pilot plant.

Current research from this laboratory is directed towards developing new applications of the electrooxidation in azoheterocycle synthesis.

Experimental section

Caution! Some substances prepared herein are highly energetic compounds and sensitive to various stimuli. Safety precautions, such as face shields, a leather apron, gloves, and hearing protection should be employed. These compounds should be handled with great care.

General

All the reagents and solvents were of analytical grade, purchased from commercial sources, and used as received. Alkylfurazan 8a–e,³⁹ alkoxy- and hydroxyfurazans 8f–h,⁴⁰ aryl derivatives 8i and 8j,⁴¹ 1,2,4-oxadiazole 8k,⁴² tetrazole 8l,⁴³ compound 8m,⁴⁴ and ^tbutylazoxy derivative 8p⁴⁵ were synthesized by using previously reported methods. Infrared spectra were determined in KBr pellets on a Perkin-Elmer Model 577 spectrometer. Mass-spectra were recorded on a Varian MAT-311A instrument. High resolution mass-spectra (HRMS) were measured on the Bruker micrOTOF II instrument using electrospray ionization (ESI). The ¹H, ¹³C and ¹⁵N (external standard: CH₃NO₂) spectra were recorded at 300.13, 75.47 and 50.7 MHz, respectively. The chemical shift values (δ) are expressed relative to the chemical shift of the solvent-d or to external standard without correction nitromethane (¹⁵N). Analytical TLC was performed using commercially precoated silica gel plates (Silufol UV₂₅₄), and visualization was effected with short wavelength UV light. Melting points were determined on Gallenkamp melting point apparatus and they are uncorrected.

Electrolysis

Electrochemical experiments were performed on a galvanostatic mode using the direct current source B5-8 in an 200 mL 4-neck jacketed flask as an undivided cell equipped with a cylindrical Ni anode (S = 48 cm²) and a cylindrical Ti cathode (S = 20 cm²). The distance between the electrodes was 1.1 cm. During the electrolysis, the reaction mixture was stirred with a magnetic stir bar at constant temperature (thermostat U-1). Before the experiment, the Ni anode was activated according to the procedure described earlier:²⁴ preliminary electrolysis was carried out in the solution containing 0.1 M NiSO₄, 0.1 M NaOAc, and 0.005 M NaOH, at j_a = 1 mA cm⁻² with periodical reverse of the electrode polarization. This procedure is necessary for the formation of multi-layer coating containing NiOOH on the Ni anode surface.

Electrooxidation of 3-amino-4-methylfurazan 8a (general procedure). A 0.2 M solution of NaOH (100 mL) and amine 8a (0.2 g, 0.002 mol) were placed in the cell, and electrolysis was carried out at a current of 290 mA and 25 °C. After 2 F per mole of starting amine of electricity were passed (Q = 386 C), the solution usually changed from colorless to red-orange and the electrolysis was stopped. Then the reaction mixture was stirred for 15 min. Product was extracted with CH₂Cl₂ (3 × 30 mL). The combined extract was washed with water, dried over MgSO₄ and the solvent was removed under vacuum. The residue was purified by recrystallization.

4,4′-Dimethylazofurazan (9a). Yellow solid, R_f = 0.62 (1 : 1 hexane/CH₂Cl₂); mp 108–109 °C (lit.⁴⁶ mp 107 °C); ¹H and ¹³C NMR spectra are identical with those reported in the literature.^34a

Azofurazans 9b–9g, 9i–9j, 9l–9n and 9p were obtained by analogous methodology.

4,4′-Diethylazofurazan (9b). Yellow solid, R_f = 0.62 (1 : 1 hexane/CH₂Cl₂); mp 59–60 °C (hexane); IR (KBr) ν/cm⁻¹: 2987, 2930, 1562, 1460, 1425, 1387, 1324, 1205, 1035, 979, 916, 800, 738, 719, 618; ¹H NMR (300 MHz, DMSO-d₆): δ 1.98 (t, J = 7.4 Hz, 6H, CH₃), 2.98 (k, J = 7.4 Hz, 4H, CH₂); ¹³C NMR (75.5 MHz, DMSO-d₆): δ 10.9 (s, CH₃), 18.1 4 (s, CH₂), 150.7 (s, C–R), 162.5 (s, C–N=N); anal. calcd for C₈H₁₁N₆O₂ (222.40): C 43.24, H 4.54, N 37.82; found: C 43.28, H 4.50, N 37.76. HRMS (ESI-TOF) cacld for C₈H₁₀N₆NaO₂⁺ ([M + Na⁺]) = 245.0763, found 245.0779.

4,4′-Dipropylazofurazan (9c). Orange oil, R_f = 0.62 (1 : 1 hexane/CH₂Cl₂); IR (KBr) ν/cm⁻¹: 2967, 2937, 2877, 1559, 1463, 1245, 1201, 1092, 1027, 918, 806, 720, 616; ¹H NMR (300 MHz, CDCl₃): δ 1.01 (t, J = 7.4 Hz, 6H, CH₃), 1.76 (k, J = 7.4 Hz, 4H, CH₂), 2.95 (t, J = 7.4 Hz, 4H, CH₂); ¹³C NMR (75.5 MHz, CDCl₃): δ 13.5 (s, CH₃), 20.2 (s, CH₂), 26.3 (s, CH₂), 148.3 (s, C–R), 162.6 (s, C–N=N); anal. calcd for C₁₀H₁₄N₆O₂ (250.26): C 47.99, H 5.64, N 33.58; found: C 48.01, H 5.60, N 33.46.

4,4′-Dicyclopropylazofurazan (9d). Yellow solid, R_f = 0.62 (1 : 1 hexane/CH₂Cl₂); mp 108–110 °C (MeOH); IR (KBr) ν/cm⁻¹:1557, 1461, 1413, 1341, 1213, 1177, 1094, 1064, 1025, 925, 885, 819, 724, 617, 607; ¹H NMR (300 MHz, CDCl₃): δ 1.16 (m, 4H, CH₂), 2.37 (m, 1H, CH); ¹³C NMR (75.5 MHz, CDCl₃): δ 5.5 (s, CH), 10.0 4 (s, CH₂), 151.5 (s, C–R), 162.9 (s, C–N=N); anal. calcd for C₁₀H₁₀N₆O₂ (246.23): C 48.78, H 4.09, N 34.13; found: C 48.82, H 4.02, N 34.02. HRMS (ESI-TOF) cacld for C₁₀H₁₀N₆NaO₂⁺ ([M + Na⁺]) = 269.0763, found 269.0765.

4,4′-Dibutylazofurazan (9e). Orange oil, R_f = 0.62 (1 : 1 hexane/CH₂Cl₂); IR (KBr) ν/cm⁻¹: 2962, 2935, 2875, 1558, 1466, 1382, 1238, 1197, 1102, 1031, 917, 766, 730, 656, 616; ¹H NMR (300 MHz, CDCl₃): δ 0.96 (t, J = 7.3 Hz, 6H, CH₃), 1.44 (m, J = 7.4 Hz, 4H, CH₂), 1.72 (m, J = 7.3 Hz, 4H, CH₂), 2.99 (t, J = 7.4 Hz, 4H, CH₂); ¹³C NMR (75.5 MHz, CDCl₃): δ 13.7 (s, CH₃), 22.2 (s, CH₂), 24.2 (s, CH₂), 29.0 (s, CH₂), 148.6 (s, C–R), 162.7 (s, C–N=N); anal. calcd for C₁₂H₁₈N₆O₂ (278.31): C 51.79, H 6.52, N 30.20; found: C 51.82, H 6.47, N 30.08. HRMS (ESI-TOF) cacld for C₁₂H₁₈N₆NaO₂⁺ ([M + Na⁺]) = 301.1383, found 301.1421.

4,4′-Dimethoxyazofurazan (9f). Yellow solid, R_f = 0.62 (1 : 1 hexane/CH₂Cl₂); mp 180–182 °C (EtOH) (lit.⁴⁰ mp 177–178 °C); ¹H NMR (300 MHz, CDCl₃): δ 4.22 (s, 3H, OMe); ¹³C NMR (75.5 MHz, CDCl₃): δ 60.0 (s, OCH₃), 155.0 (s, C–R), 158.8 (s, C–N=N); anal. calcd for C₆H₆N₆O₄ (226.15): C 31.87, H 2.67, N 37.16; found: C 31.91, H 2.62, N 37.02. IR spectra are identical with those reported in the literature.⁴⁰

4,4′-Dipropoxyazofurazan (9g). Yellow solid, R_f = 0.62 (1 : 1 hexane/CH₂Cl₂); mp 52–53 °C (hexane); ¹H NMR (300 MHz, DMSO-d₆): δ 1.02 (t, J = 7.4 Hz, 6H, CH₃), 1.78 (k, J = 7.4 Hz, 4H, CH₂), 4.45 (t, J = 7.4 Hz, 4H, OCH₂); anal. calcd for C₁₀H₁₄N₆O₄ (282.26): C 42.55, H 5.00, N 29.77; found: C 42.59, H 4.55, N 29.52. HRMS (ESI-TOF) cacld for C₁₀H₁₄N₆NaO₄⁺ ([M + Na⁺]) = 305.0974, found 205.0977.

4,4′-Diphenylazofurazan (9i). Yellow solid, R_f = 0.62 (1 : 1 hexane/CH₂Cl₂); mp 134–135 °C (hexane), (lit.⁴⁷ mp 134–135 °C); ¹H and ¹³C NMR spectra are identical with those reported in the literature.⁴⁷

4,4′-Di-4-(pyridin-3-yl)-azofurazan (9j). Yellow solid, R_f = 0.62 (1 : 1 hexane/CH₂Cl₂); mp 145–146 °C (hexane); IR (KBr) ν/cm⁻¹: 1593, 1576, 1531, 1476, 1465, 1416, 1383, 1337, 1283, 1133, 1074, 1028, 993, 917, 872, 827, 732, 707, 688, 621, 587; ¹H NMR (300 MHz, DMSO-d₆): 7.48 (dd, J = 4.8, 1H, CH), 8.10 (d, J = 7.9, 1H, CH), 8.74 (d, J = 4.6, 1H, CH), 8.91 (s, 1H, CH); ¹³C NMR (75.5 MHz, DMSO-d₆): δ 120.4, 123.7, 136.8, 148.7 (s, C–R), 149.2, 152.0, 161.8 (s, C–N=N); anal. calcd for C₁₄H₈N₈O₂ (320.27): C 52.50, H 2.52, N 34.99; found: C 52.53, H 2.49, N 34.85.

4,4′-Bis(4-(1H-tetrazol-5-yl)-azofurazan(9l). Yellow solid, mp 249–250 °C (lit.⁷ mp 238 °C); ¹H and ¹³C NMR spectra are identical with those reported in the literature.⁷

4,4′-[1,2,3]Triazolo[4,5-c][1,2,5]oxadiazol-azofurazan (9m). Orange solid, R_f = 0.80 (CH₂Cl₂); mp 128–130 °C (dec), (lit.³³ mp 100–102 °C (dec)); ¹³C NMR spectra are identical with those reported in the literature.³²

3,3′-Diazidoazofurazan (9n). Yellow solid, R_f = 0.5 (1 : 1 hexane/CH₂Cl₂); dec ca. 70 °C, (lit.⁴⁴ dec 68 °C); ¹³C NMR (75.5 MHz, CDCl₃): δ 147.5(s, C–N₃), 156.6 (s, C–N=N); ¹⁵N NMR (CDCl₃, 30.4 MHz): −143.8, −136.8, 19.5, 44.3, 49.4, 141.9; ¹³C NMR spectra are identical with those reported in the literature.⁴⁴

4,4′-Bis(tert-butyl-NNO-azoxy)-3,3′-azofurazan(9p). Orange solid, R_f = 0.60 (1 : 1 hexane/CH₂Cl₂); mp 128–129 °C, (lit.³⁰ mp 128–129 °C); ¹H and ¹³C NMR spectra are identical with those reported in the literature.³⁰

Electrooxidation of 3,4-diaminofurazan (8o). A 0.2 M solution of NaOH (100 mL) and amine 8o (0.2 g, 0.002 mol) were placed in the cell, and electrolysis was carried out at a current of 290 mA and 25 °C. After 6F per mole of starting amine were passed (Q = 1158 C), the electrolysis was stopped, the reaction mixture was stirred for 15 min. Product was extracted with ethyl acetate (3 × 80 mL). The combined extract was dried over MgSO₄ and the solvent was removed under vacuum. The residue was purified by recrystallization from DMSO/H₂O. 4,4′-Diaminoazofurazan (9o), yellow-orange solid, mp > 300 °C (lit.^5a mp 298 °C); ¹H and ¹³C NMR spectra are identical with those reported in the literature.^5a

Electrooxidation of 4,4-diaminohydrazofurazan (10). A 0.2 M solution of NaOH (100 mL) and amine 10 (0.2 g, 0.001 mol) were placed in the cell, and electrolysis was carried out at a current of 290 mA and 25 °C. After 2F per mole of starting amine were passed (Q = 193 C), the electrolysis was stopped, the reaction mixture was stirred for 15 min. Product was extracted with ethyl acetate (3 × 80 mL). The combined extract was evaporated in vacuo. The residue was purified by recrystallization from DMSO/H₂O. The yield of 4,4′-diaminoazofurazan 9o was 61%: mp > 300 °C (lit.^5a mp 298 °C); ¹H and ¹³C NMR spectra are identical with those reported in the literature.^5a

Electrooxidation of 3-amino-4-(tetrazol-5-yl)furazan 8l. A 0.2 M solution of NaOH (100 mL) and amine 8l (0.3 g, 0.002 mol) were placed in the cell, and electrolysis was carried out at a current of 290 mA and 25 °C. After 6F per mole of starting amine were passed (Q = 1158 C), the electrolysis was stopped, the reaction mixture was stirred for 15 min. After completion of the reaction, concentrated HCl was added (to pH 1) and the mixture was extracted with ether (3 × 30 mL). The organic phase was dried over Na₂SO₄ and then evaporated. The residue was purified by recrystallization to gave 4,4′-di(5-tetrazolyl)-azofurazan, (9i). Yellow solid, R_f = 0.62 (1 : 1 hexane/CH₂Cl₂); mp 247–248 °C (EtOH/H₂O), (lit. ⁷ mp 249–250 °C); ¹H and¹³C NMR spectra are identical with those reported in the literature.⁷

4,4-Dihydroxyazofurazan (9h). 4,4-Dihydroxyazofurazan (9h) was obtained by similarly to compound 9l: a yellow solid, mp 262–265 °C (lit. ^4d,40 mp 263–265 °C); ¹H and ¹³C NMR spectra are identical with those reported in the literature.^4d,40

Electrooxidation of 3-amino-4-nitrofurazan 8r. A 0.2 M solution of NaOH (100 mL) and amine 8r (0.26 g, 0.002 mol) were placed in the cell, and electrolysis was carried out at a current of 290 mA and 25 °C. After 6F per mole of starting amine were passed (Q = 1158 C), the electrolysis was stopped, the reaction mixture was stirred for 15 min. After completion of the reaction, aqueous HCl solution was added to pH 1 and the mixture was extracted with ether (3 × 30 mL). The organic phase was dried over Na₂SO₄ and then evaporated. The residue was purified by recrystallization to give 4,4-dihydroxyazofurazan (9h), mp 262–265 °C, (lit.⁴⁰ mp 263–265 °C); ¹H and ¹³C NMR spectra are identical with those reported in the literature.⁴⁰

5-(4-Azidofurazan-3-yl)-[1,2,3]triazolo[4,5-c][1,2,5]-oxadiazol-5-ium-4-ide(11). A 0.2 M solution of NaOH (100 mL) and amine 8n (0.26 g, 0.002 mol) were placed in the cell, and electrolysis was carried out at a current of 290 mA and 25 °C. After 6F per mole of starting amine were passed (Q = 1158 C), the electrolysis was stopped, the reaction mixture was diluted with benzene (100 mL) and stirred for 15 min. The benzene extract was dried over MgSO₄ and refluxed for 2 h. The solvent was removed under vacuum. The residue was purified by recrystallization to give the product 11; mp 103–104 °C (lit. ³³ mp 103–104 °C); ¹³C NMR (75.5 MHz, DMSO-d₆): δ 147.7(s, C–R), 149.3, 165.6 (s, C–N=N); ¹⁵N NMR (DMSO-d₆, 30.4 MHz): −139.7, −135.2, −88.2, −67.9, 23.5, 36.2, 38.8, 43.9; ¹H and ¹³C NMR spectra are identical with those reported in the literature.³³

Acknowledgements

Financial support for this work was provided by The Russian Science Foundation (grant 14-13-01153 to A.B.S., A.M.K. and N.V.P., and 14-13-00884 to K.Y.S.) and the Russian Foundation for Basic Research (grant no. 12-03-00517a to B.V.L.). We are especially grateful to Dr M. A. Syroeshkin for CV data.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1007707 and 1007708. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra05726d

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