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Chemistry of Heterocyclic Compounds

, Volume 50, Issue 9, pp 1214–1243 | Cite as

Chemistry of Pyrazole-3(5)-Diazonium Salts (Review)*

  • I. V. LedenyovaEmail author
  • V. V. Didenko
  • Kh. S. Shikhaliev
Article

The methods of synthesis, structure, reactivity, and synthetic utility of pyrazole-3(5)-diazonium salts are reviewed here for the first time, with emphasis on heterocyclization reactions of these compounds.

Keywords

fused pyrazole systems pyrazole-3(5)-diazonium salts azo coupling heterocyclization 

More than one hundred years have passed since α-diazo derivatives of pyrazole were discovered. A substantial amount of knowledge has been accumulated over this time regarding the stability, structure, and transformations of these compounds. However, the chemistry of pyrazole-3(5)-diazonium salts developed rapidly only in the second half of the 20th century, with many studies performed in various countries, in particular Germany, Egypt, Russia, and Japan. Unfortunately, no systematic review has been compiled for the wide range of data about pyrazole-3(5)-diazonium salts. Some aspects regarding the properties of these compounds have been discussed in reviews and monographs on other topics [1-17].

1. Structure, Preparation, and Reactivity Evaluation of Pyrazole α-Diazo Derivatives

Pyrazole α-diazo derivatives are divided in two groups – pyrazole-3(5)-diazonium salts 1 and 3-diazo-pyrazoles 2.

Detailed discussion of 3-diazopyrazole 2 chemistry would be beyond the scope of this review. Short summaries of the structure and properties of these compounds are available from many sources [5-7, 10, 18-21].

The stability and high reactivity of pyrazole-3(5)-diazonium salts is based on the significant resonance stabilization of N-unsubstituted ion 1'.

The terminal nitrogen atom is the main reactive site during transformations of pyrazole-3(5)-diazonium salts that do not involve elimination of nitrogen. One of the principal factors affecting the reactivity of any organic compound is the nature of its substituents. Electron-withdrawing groups, similarly to the ring nitrogen atoms, generally increase the electrophilic properties of a diazo group [5, 22, 23]. The presence of electron-donating substituents stabilizes the diazonium cation, but reduces the positive charge at the terminal nitrogen atom and decreases the electrophilicity of diazo group. In line with this, 4-nitro- and 4-cyanopyrazole-3(5)-diazonium salts can be heated in aqueous solutions to near boiling without substantial decomposition [24].

No X-ray structural data are available for pyrazole-3(5)-diazonium salts. However, X-ray structural data for the isomeric 1H-3,5-dimethylpyrazole-4-diazonium chloride [25] indicate that compounds 1 should have structures similar to their aromatic carbocyclic analogs. Shortened С–N bonds (1.351 Å) in the molecule of pyrazole-4-diazonium salts point to the strong conjugation of diazonium group with the heteroaromatic ring.

The structure and tautomeric transformations of three cationic species of pyrazole-3(5)-diazonium hexafluorophosphate in acidic medium were studied by computational approach [26]. The calculated heat of formation and protonation entalphy for three forms of cation according to AM1 method showed that the equilibrium was shifted towards the formation of 3-diazo tautomer , while its protonation leading to dication was energetically unfavorable.
The structure of ethyl 5-diazo-3-methylsulfanylpyrazole-4-carboxylate (2) was studied by X-ray structural analysis, IR and 1H NMR spectroscopy [27]. The authors could not obtain the expected pyrazole-3(5)-diazonium hydrogen sulfate 1 through a diazotization reaction.
The molecules of compound 2 were linked in crystal by electrostatic π–π interactions of carbonyl oxygen atom with the diazo nitrogen atom bonded to the ring. The strong conjugation of latter with the heterocycle also affected the length of exocyclic С–N bond (1.358 Å).

The earliest reports of 5-aminopyrazole derivative reactions with nitrous acid were by German chemists Seidel [28] and Knorr [29]. They were the first to prepare α-aminopyrazoles 3 and proposed that the products formed from the reaction of compound 3 with sodium nitrite in acidic medium were nitrosoamino compounds 4,which could be used in further condensation reactions to obtain a range of other products. However, their assumptions about the identity and structure of the obtained compounds were not correct, as the reaction conditions precluded the formation of pyrazole-3(5)-diazonium salts.

Other sources [30-32] report successful diazotization of α-aminopyrazoles by various methods, including the "direct" approach (sodium nitrite, threefold excess of strong acid, 0°С) [33]. The unsubstituted position 4 of pyrazole ring is quite reactive, leading to the presence of oxime 5 as impurity upon regular diazotization procedure with nitrous acid. Increasing the concentration of mineral acid and decreasing the temperature resulted in higher yields of salts 1 [34]. Under certain conditions, С-nitrosation of pyrazoles 3 is possible while leaving the amino groups intact [35-38].

Diazotization of the respective aminopyrazoles is the current standard method for preparation of pyrazole-3(5)-diazonium salts. A threefold excess of concentrated mineral acid (HCl, Н3PO4, H2SO4, HBF4, etc.) and performing the reaction in aqueous or alcoholic solutions minimize the occurrence of side reactions.
An alternate method for the preparation of pyrazole-3-diazonium salts is a single-electron oxidation followed by diazotization with aromatization of 3-amino-1-aryl-Δ2-pyrazoline 6. The radical cation 7 is not capable of dimerization due to blocked para position in the benzene ring and is relatively stable, with lifetime of several weeks. Adding sodium nitrite to a solution of aminopyrazoline 6 in hydrochloric acid initially induces the red color of radical cation, then diazotization and dehydrogenation, giving the pyrazole diazonium chloride 8 [39].

An analogous method was applied to the synthesis of diazonium salts 1 (R = R1 = H), with mechanism involving a sequence of nitrosation at endocyclic nitrogen atom, single-electron oxidation, diazotization, denitrosation, and aromatization [40].

Interestingly, the formation of diazonium salts by interaction of partially hydrogenated aminopyrazoles with nitrous acid was dismissed in earlier publication [41].

Methods for the preparation of pyrazole diazonium salts [22, 42, 43] include Knoevenagel reaction (isoamyl or amyl nitrite, concentrated mineral acid, organic solvent), which allowed to isolate the solid salts 1 in pure form. Diazotization of 5-aminopyrazoles was also performed in anhydrous medium, using nitrosyl chloride or alkyl nitrites in chloroform [44].

A non-obvious method for the preparation of diazonium hydrogen sulfates 1 is based on the treatment of pyrazolylhydrazone 9 with concentrated sulfuric acid [45].
We should note that diazotization of heterocyclic substrates usually produces the corresponding diazonium salts, and only in rare cases, such as pyrazole series, it is possible to isolate diazo compounds 2. The latter are usually obtained by adding base (Na2CO3, AcONa, NaOH, Et3N) to a freshly prepared solution of diazonium salts in order to bind the acid [1, 2, 20, 22, 46-48].

Often no distinction is made whether diazo compounds or diazonium salts are the reactive species, and whether there are prototropic equilibria between the forms and how they are affected by solvents. It has been noted that dissolution of diazonium salts in chloroform leads to deprotonation forming 3-diazopyrazoles 2 [22].

Pyrazole-3(5)-diazonium salts are unstable and explosive as solids [24, 38], therefore used in further transformations without isolation, as solutions at temperatures from 0 to 10°С.

The reactivity of pyrazole-3(5)-diazonium salts is comparable to that of phenyldiazonium chloride, while 3-diazopyrazoles have properties similar to aliphatic diazo compounds [22, 23, 46, 47]. The stretching frequency of N2 + group in compounds 1 is close to that of aromatic diazonium salts [23]. However, 3(5)-diazo derivatives of pyrazole are considerably more stable compared to carbocyclic analogs, enabling their isolation when necessary [1, 3, 22, 24, 38].

The considerable interest in pyrazole-3(5)-diazonium salts has several reasons. First of all, these compounds are thermally and chemically more stable than aromatic and many heterocyclic analogs. For example, triazole diazonium salts easily eliminate a nitrogen molecule, thus are prepared in nitrate form [48]. For the same reason, both series of aminoimidazoles are diazotated in concentrated sulfuric and tetrafluoroboric acids [49, 50]. Tetrazole diazonium salts are even less stable [3]. Secondly, the availability of the starting amines, the simple conditions of diazotization, high reactivity of pyrazole-3(5)-diazonium salts, and the presence of a reactive nucleophilic center (endocyclic nitrogen atom of pyrazole) at ortho position relative to diazonium group offers possibilities for using these compounds as building blocks in heterocyclic synthesis.

α-Pyrazole diazonium salts have been studied in most of the typical arenediazonium reactions. The general pattern of reactivity and types of transformations of these compounds are formalized below.
According to this scheme, several types of reactions can be emphasized for pyrazole-3(5)-diazonium salts:
  • nucleophilic substitution of N2 + group (Nu = Hal, N3, NO2, OH, etc.) (a);

  • reduction of diazonium group (b);

  • azo coupling with aromatic and heterocyclic compounds, as well as primary and secondary amines, leading to azo compounds and triazenes (Z = Ar, Het, NR2) (c). The products of such reactions may undergo intra-molecular cyclization in some cases (d);

  • intramolecular azo coupling, which occurs in the presence of substituents X susceptible to electrophilic attack (NH2, NH, Ar, multiple carbon–carbon bonds, etc.) at the ortho position relative to diazo group and leading to pyrazoloazines (e);

  • the interaction with compounds containing a methylene group activated by electron-withdrawing groups, leading to pyrazolylhydrazones (f), as well as heterocyclization of the latter, resulting in linearly linked (g) and condensed systems (h) with a pyrazole fragment.

2. Reactions of Pyrazole-3(5)-Diazonium Salts Without Ring Formation

2.1. Reactions involving the loss (substitution) of diazo group

Nucleophilic substitution of diazo group is one of the most important synthetic directions in the chemistry of diazo compounds. This type of transformations is known as dediazotization and allows to obtain various functional derivatives (halides, cyanides, thiocyanates, hydroxides, thiols, nitrites, azides, etc.).

Reactions of diazonium group substitution in pyrazole series are currently well understood. The first attempts of performing such reactions were made by Mohr in 1914 [38, p. 509], but Lund [24, 51] was the first to synthesize pyrazole 10 and 3-iodopyrazole 11 by deamination and diazo group substitution with iodine. The interaction of diazonium salts 1 (R =3-Py, R1 = H, NO2) with ammonia solution led to formation of the starting aminopyrazole and evolution of nitrogen [52].

Sandmeyer and Balz–Schiemann reactions were used to convert compounds 1 into a series of functionalized 3(5)-derivatives of pyrazole: fluorides 12 [53-55], chlorides 13 and bromides 14 [44, 52, 53, 56-61], iodides 15 [22, 44, 51, 52], nitropyrazoles 16 [56, 62-64], azides 17 [57, 61, 64-66], methyl sulfides 18 [44], and sulfonyl chlorides 19 [67-69].

The mechanism of these reactions remains subject to discussion, because experimental data exist both in favor of ionic, as well as radical mechanisms. Achieving a smooth reaction and high yields requires the presence of electron-withdrawing groups in the ring, which increases the stability of diazo compounds in aqueous solutions. However, in some cases the situation is complicated by substituents at the nitrogen atom of pyrazole ring. For example, diazotization of 5-amino-1,3-dimethyl-4-nitropyrazole in hydrochloric acid results in rapid substitution of diazo group with chlorine, while 5-bromo-1,3-dimethyl-4-nitropyrazole is obtained in hydrobromic acid [60]. The isomeric 3-amino-1,5-dimethyl-4-nitropyrazole is smoothly diazotated under analogous conditions. The presence of two electron-withdrawing substituents (NO2) in pyrazole nucleus decreases the basicity of amino group but has no effect on the diazotization process and subsequent nucleophilic substitution of N2 + group [61].

The Balz–Schiemann reaction may occur both during the thermolysis of dry tetrafluoroborates 1, as well as photochemically during irradiation of these starting materials in solution [54, 55].

The attempts at converting 5-amino-3-methyl-4-nitropyrazole into nitrile by diazotization and subsequent treatment with potassium tricyanocuprate results in the formation of halogen derivative [52]. Nitriles of pyrazole series have not yet been obtained by the dediazotization method.

Practically no data are available about syntheses of 3(5)-hydroxypyrazoles from the respective diazonium salts possibly due to the high stability of compounds 1 in solution. Nevertheless, amino derivatives of other azoles may undergo hydroxydeamination [3, p. 142]. Adding base to pyrazole-3(5)-diazonium salts, as mentioned before, leads to bipolar 3-diazopyrazoles. Hydroxydeamination of α-aminopyrazole 3 is known to proceed photochemically upon irradiation [70].
Reduction of α-pyrazole diazonium salts by substituting diazo group with hydrogen should be specifically mentioned. The appropriate reducing agents are lower alcohols [51, 64], hypophosphorous acid, [56, 58], or sulfur dioxide in the presence of catalyst (CuCl2) in carbon tetrachloride [71]. The deamination products in the latter case were formed in low yields (3-20%) along with the major chloro derivatives 13. A one-pot synthesis of pyrazoles 21 can be performed by direct action of alkyl nitrite in THF [44, 72] or DMF [73] on the starting amines containing electron-withdrawing groups at position 4.

These processes are believed to involve either intermediate formation of pyrazolyl cation or free radicals, as in the case of arenediazonium salts [74].

A brief review of literature published prior to 1991 about dediazotization reactions, including reactions in pyrazole series, is available [75, p. 643].

2.2. Reactions with conservation of diazo group

The reactions of pyrazole-3(5)-diazonium salts without elimination of nitrogen open broad possibilities for the synthesis of pyrazole derivatives: hydrazines, triazenes, formazans and, finally, the highly important hydrazones and/or azo compounds. These transformations are widely described in scientific periodicals and patents. The value of pyrazole-3(5)-diazonium salt reactions with conservation of diazo group (reduction of N2 + group to hydrazines, azo coupling with activated NH and CH reagents) is mostly in the possibilities of obtaining intermediates for heterocyclic synthesis. Occasionally such compounds are used for various in situ transformations, and in some cases hydrazones or azo compounds undergo spontaneous intramolecular condensation but in other cases are unusually stable and inert against further reactions.

Several studies have been reported about the synthesis of 3(5)-hydrazinopyrazoles 22 through reduction. Fischer reaction (Na2SO3, OH - ) [76-78] and Meyer reaction (SnCl2, HCl) [57, 77-79] provided satisfactory yields of products (30-40%), which were not air-stable, and thus not isolated as bases.

It has been pointed out [80] that the reduction of diazonium salt is not successful when R1 = NO2. Such hydrazines are obtained indirectly [57, 77]. Hydrazinopyrazoles are quite attractive precursors for building various polyheterocycles, for example, physiologically active pyrazolo[5,1-c]-s-triazoles [79, 81].

The interaction of pyrazole-3(5)-diazonium salts with NH acids represents the simplest method for the preparation of heterocyclic triazenes (amino azo compounds) and other related derivatives. The pioneering works of late 19th and early 20th centuries [28, 31, 37] report the formation of bipyrazolyltriazenes 23 upon diazotization of aminopyrazoles 3 by one-pot procedure with insufficient amount of strong acid, as well as when coupling the obtained diazonium salt with the starting amine [22].
It has been reported [82-84] that pyrazole-3(5)-diazonium salts interact with primary and secondary amines forming 3-substituted 1-pyrazolyltriazenes 24.

Triazenes of pyrazole series have been synthesized from amino acids (glycine, proline) [85] and from (hetero)aromatic amines (4-chloroaniline, α-aminopyrazoles, 4-aminoantipyrine) [82, 86]. The preparation of pyrazolyltriazenes from amidines has been patented [87].

There have been practically no studies of formazans (azohydrazones) containing a pyrazole ring. Azo coupling of diazonium salts 1 with various aldehyde hydrazones 25 provided a long series of 1(5)-pyrazolyl-3,5(1,3)-diarylformazans 26 in 40-80% yields [88].
Other methods for the synthesis of pyrazolylformazans 27a,b functionalized at position 3 by employing the respective diazonium salts as starting materials were tested by authors from Russia [89]. The activated CH components in bisazo coupling were acetoacetic ester and nitromethane. In one case the formazan 27с (Y = CO2Et) was obtained in 81% yield using the potassium salt of malonic ester [90].

One of the most important transformations that characterize the chemical properties of pyrazole-3(5)-diazonium salts is С-azo coupling with aromatic and heterocyclic azo components. This reaction occurs as electrophilic aromatic substitution similar to azo coupling reactions of arenediazonium salts with phenols and anilines.

The synthetic study of salts 1 by Mohr [37, 38] and Meyer [32] by using β-naphthol (28) provided the first colored pyrazolylazo compounds 29.

In later studies α- and β-naphthols were quite frequently used for illustration of azo coupling reactions involving diazonium salts 1 (R = H, Alk, SAlk, Ar; R1 = H, Alk, NO2, CO2Et, CONH2, CN; X = Cl, HSO4) [22, 39-41, 53, 64, 91-93]. Syntheses of pyrazolylazo compounds have been described in publications and

patents, based on various aromatic compounds: polyhydroxy-, polymethoxy-, ethoxycarbonyl-, arylamino-, sulfo-, and other derivatives of naphthalene [53, 93-95], phenols, cresols, anisoles [23, 53, 75, 96], anilines, and N,N-dialkylanilines [23, 39, 40, 51, 53, 94, 97, 98] (compounds 30). A remarkable feature of these reactions is the possibility of subsequent intramolecular cyclocondensation of products (provided the azo component has a nucleofuge group at the ortho position relative to its azo group), leading to annelated polycyclic structures. The orientation in azo coupling between pyrazole-3(5)-diazonium salts and aromatic compounds follows the general principles and has been thoroughly studied by Reimlinger [53].

1-(5-Pyrazolylazo)-2-naphthol and 1-(4-carboxy-5-pyrazolylazo)-2-naphthol are used in the field of inorganic analysis for quantitative photometric determination of d-metals. Other pyrazolylazo compounds with aromatic/heterocyclic fragments find applications as dyes for synthetic and natural fabrics and coatings. These compounds are also characterized by pronounced fungicidal activity and lightfastness.

The information available about reactions between diazonium salts 1 and heterocyclic azo components (derivatives of pyrazole, isoxazole, thiazole, pyridine, quinoline, etc.) is rather incomplete. The interaction of salts 1 with 5-aminopyrazoles 31 has been studied [91, 93, 94] and resulted in the formation of azo derivatives 32. The reaction with 5-pyrazolones 33 proceeded analogously [31]; [53; 75 p. 598; 91; 96; 99; 100-103], giving products that can exist as two tautomers: the azo form (structure 34) and hydrazone (structure 34А).

The synthesis of azo compounds 34 was performed in alcohol, acetone, dioxane, or pyridine, while maintaining a weakly alkaline medium. Occasionally the azo coupling with aminopyrazoles was achieved in the presence of acetic acid [104].

Thiazol-4-one and 5-aminoisoxazole, containing reactive sites susceptible to electrophilic attack, are successfully used as azo components in reactions with pyrazole-3(5)-diazonium salts [105, 106]. Examples were found for azo coupling of diazonium salts 1 with 4-alkyl-3-cyano-6-hydroxypyridin-2-one [102, 103, 107], 8-hydroxyquinoline [92], 1,2,3,4-tetrahydroquinolines [43, 94, 97], 4-hydroxy-2-pyrone [102], and [1, 3]thiazo-lo[3,2-a]benzimidazol-3-one [108].

Azo–hydrazone equilibrium 35 ⇄ 35A was identified in some cases. The IR and UV spectra of some of the products have been recorded in acidic and basic solvents [102, 103]. No successful intramolecular heterocyclization of pyrazolylazo compounds 34 and 35 has been reported.

Involving the hetaroylacetonitrile 36 in reaction with the salts 1 led to the hydrazone 37, the heterocyclization of which by heating in pyridine was difficult [109].
The methyl group in quinoxaline derivative 38а was characterized by increased acidity and therefore underwent coupling with pyrazole-3(5)-diazonium salts. An analogous result was achieved when using the methylene methoxycarbonyl derivative 38b [110].

We should note that reactions of pyrazole-3(5)-diazonium salts that do not create a new ring are synthetically highly valuable. The mild conditions of these transformations and the high reactivity of substrates allow to synthesize a significant number of linearly linked pyrazole derivatives. Azides, sulfonyl chlorides, hydrazones, formazans, hydrazines, and triazenes of these series have been characterized as biologically active compounds, dyes, complexing agents, and synthetic intermediates. The interest towards these reactions continues to this day.

3. Heterocyclization Reactions of Pyrazole Diazonium Salts and Related Intermediates

Reactions of pyrazole diazonium salts involving the closure of a new ring have been extensively studied and described in the literature. Intramolecular heterocyclization may occur in two main directions: by diazonium group attack at the adjacent nucleophilic fragment and by ring closure of azo coupling products (hydrazones, azo compounds, triazenes, etc.) at the endocyclic nitrogen atom of pyrazole ring or the hydrazone fragment nitrogen atom. All these reactions lead to condensed or linearly linked polyheterocycles.

Examples of fused bicyclic pyrazole systems, obtained from pyrazole-3(5)-diazonium salts and their intermediates, are presented in the scheme below as molecular frameworks.

3.1. Intramolecular azo coupling

Arenediazonium salts containing substituents susceptible to electrophilic attack (NH2, NH, OH, SH, Alk, Ar, multiple carbon-carbon bonds, etc.) at the ortho position relative to diazo group are unstable and undergo spontaneous intramolecular cyclization forming the corresponding heterocycles [10, p. 1068; [33, p. 241; 39, p. 131; 111]. This is true also for some pyrazole-3(5)-diazonium salts, the intramolecular azo coupling of which produces pyrazolo[3,4-c]pyridazines, pyrazolo[3,4-d][1-3]triazines, pyrazolo[3,4-e]-[1-4]tetrazines, and other polycyclic systems.

The first example of such reactions was described by Mohr [38]. For example, attempts to substitute diazonium group in compound 40 with hydroxyl group led to derivatives of pyrazolo[5,1-c][1, 2, 4]benzotriazine 41 but not those of pyrazolone 42. Analogous results were obtained also for other pyrazole-5-diazonium salts in the case of thermal cyclization [112] or treatment with sodium acetate solution [113].

Intramolecular azo coupling reactions of compounds 1 containing 3,4-dimethoxyphenyl and indolyl substituents at position 4 of pyrazole ring have been described [114]. These examples may be viewed as original methods for obtaining derivatives of pyrazolo[3,4-c]cinnoline 43 and pyrazolo[3',4':5,6]pyridazino[3,4-b]-indole 44 according to the click chemistry approach ([115] and references therein, [116]).

The classical Richter reaction [111, p. 357] has been used successfully for the synthesis of cinnolines with acetylene derivatives of N-alkylpyrazole diazonium salts [117, 118]. Compounds 45 were subject to cyclo-condensation with the formation of halo-substituted pyrazolo[3,4-с]pyridazines 46; heating of 4-(arylethynyl)-pyrazole-3-diazonium halides 47 resulted in the formation of mainly 6-hydroxy-2Н-pyrazolo[3,4-с]pyridazines 48a,b and also the halogenated analogs 49 as minor components.
One of the methods for preparation of pyrazolo[3,4-d]-v-triazines is based on intramolecular azo coupling of α-pyrazole diazonium salts with the carboxamide group at position 4. This effective approach was apparently used for the first time by Justoni for the case of 1,3-diphenylpyrazole-5-diazonium salts, which were transformed into pyrazolo[3,4-d][1-3]triazines 51 in the stage of formation from the corresponding amines 50 [119].
Analogous reactions were reported for the diazonium salts 1 (R = H, NHPh; R1 = CONH2, CONHHet), which cyclized either in the presence of acid or alkali, or spontaneously over 2 h [120, 121].
The formation of 1,2,3-triazine ring in reactions with 1,4-substituted salts of pyrazole-3(5)-diazonium has been illustrated with many examples in a series of publications from recent decades. The range of substituents at the N-1 atom includes fragments of heterocycles, β-D-ribofuranose, 2-deoxy-β-D-ribofuranose, and their derivatives; at the С-4 atom – amidine, nitrile, hydrazide, heterocyclic, and other fragments [122-124].

Derivatives of another representative of bicyclic nitrogen heterocycles with 10 π-electrons, 2H-pyrazolo-[3,4-e][1-4]tetrazine 54, were formed in good yields (64-82%) from 4-arylazopyrazole-3(5)-diazonium salts by treatment with AcONa solution [125]. The mechanism of this reaction was proposed to involve the delocalized zwitterion 53 as intermediate [126].

Thus, intramolecular azo coupling reactions of pyrazole-3(5)-diazonium salts are successfully used in the synthesis of azines annelated with azoles. The steady interest towards these transformations is motivated by the applicability of click chemistry, as well as the structural similarity of products to indoles and natural purine bases and the associated diverse biological activity.

3.2. Intermolecular azo coupling

Reactions of salts 1 with various СH and NH acids opens broad possibilities for heterocyclic synthesis. The main reason for this is the great variety of components suitable for use in these reactions (aliphatic, alicyclic, aromatic, heterocyclic), and another reason is the possible cyclocondensation of the obtained acyclic products either at the endocyclic nitrogen atom of pyrazole, or at other substituents. The range of products available by this approach is immense and not limited by the number, nature, and type of bonds between the rings in the molecule [11; 78, p. 628, 656].

The most important products formed by intermolecular azo coupling of pyrazole-3(5)-diazonium salts are pyrazolo[5,1-с][1, 2, 4]triazines or their annelated derivatives [11, 16].

Reactions using acyclic compounds with activated СН/NH groups. The compounds used as activated aliphatic CH components in azo coupling reactions with salts 1 belong to various classes, mainly β-keto acids, β-diketones, nitriles, and enaminones. These reactions lead to the formation of pyrazol-3(5)-yl-hydrazones or the corresponding azo compounds, which may cyclize under the conditions of azo coupling reaction or after additional treatment.

Directed cyclization with the participation of endocyclic nitrogen atom of pyrazole was first accomplished by Partridge [1, 127] by reacting pyrazole-3(5)-diazonium salt 1 with acetoacetic acid and its ester, as well as with benzoylacetic ester in alcoholic sodium acetate solution.

The acyclic products 56c,d spontaneously condensed into pyrazolo[5,1-с][1, 2, 4]triazines 57c,d, which gave the carboxylic acids 57а,b after alkaline hydrolysis and were subsequently decarboxylated. At the same time, malonic esters reacted with difficulty in azo coupling with pyrazole diazonium chlorides 1 [128, 129].

A library of pyrazolo[5,1-с]-as-triazines was obtained by using 1,3-diketones, α-hetaryl ketones, and other analogous synthons in reactions with the salts 1. There are references [127, 49, 130-132] for the
preparation of derivatives 59а-d based on acetylacetone, heptane-3,5-dione, nonane-4,6-dione, and dibenzoyl-methane under the conditions of azo coupling reaction.

There are quite a few sources reporting the synthesis, structure, and properties of 2-pyrazolyl-hydrazono-1,3-dicarbonyl compounds (see the review [133]), thus there is no doubt that hydrazones serve as intermediates in the aforementioned reactions.

The issue of regioselectivity arises when pyrazole-3(5)-diazonium salts react with asymmetric β-di-ketones. The preparation of 3-acetyl-4-phenylpyrazolo[5,1-с]-as-triazine 60 was achieved by reaction of pyrazole diazonium salts 1 with benzoylacetone [134].

This interpretation has been questioned in work [135], the authors of which used counter synthesis to unequivocally prove the formation of 3-benzoyl-4-methylpyrazolo[5,1-с][1, 2, 4]triazine 61 in a similar reaction.

The interaction of perfluoroalkyl 1,3-diketones 62 with pyrazole-3(5)-diazonium salts 1 under the standard azo coupling conditions led to the formation of 4-fluoroalkyl-4-hydroxy-1,4-dihydropyrazolo[5,1-с]-as-triazines 63. The latter were formed from intermediates A by the endocyclic nitrogen atom of pyrazole attacking the carbonyl group at fluorinated substituent [136].
The interaction of pyrazole-3(5)-diazonium salt 1 with 4,4,4-trifluoro-1-(thiophen-2-yl)butan-1,3-dione (64) occurred under microwave irradiation in pyridine and gave the pyrazolotriazines 65 [137].
The hetaryl ketones 66а,b also were smoothly coupled to pyrazole diazonium salts 1. These reactions allowed to introduce various heterocyclic fragments in the side chain of 1,2,4-triazine [138, 139].
The reactions of pyrazole diazonium salts 1 with methyl and ethyl esters of acetonedicarboxylic acid 67а,b have been reported [140]. Heating acyclic intermediates of azo coupling in AcOH leads to pyrazolo[5,1-c]-[1, 2, 4]triazine derivatives 68a-e.

The condensation of 2-chloro-1,3-dicarbonyl compounds with diazonium salts 1 [127, 131, 141-143] produced the intermediate azo compounds 69а,b, from which acetyl fragment was eliminated, leading to stable hydrazonyl chlorides 70а,b (by Japp–Klingemann type reaction).

Depending on the conditions used for the further reaction, the latter were transformed either into pyrazolo[5,1-с][1, 2, 4]triazoles 71а,b (refluxing in 2-aminopyridine or alcohol in the presence of triethylamine, treatment with sodium alkoxide), or in pyrazolo[5,1-с][1, 2, 4]triazines 72а,b (refluxing in acetic acid or ethanol). An original method for the construction of pyrazolo[5,1-с][1, 2, 4]triazepine derivatives is Wittig reaction with the participation of hydrazones 70а,b [144].
The preparation of pyrazolo[5,1-с][1,2,4]triazines from salts 1 (R = H, Me, NHPh, SMe, Ar, Het; R1 = H, Ar, Het, CN; X = Cl) and various β-ketocarboxylic acid derivatives 73 under analogous conditions is described in several publications [49, 130-132, 145-147] and a patent [148].
There is an interesting method for obtaining 3-unsubstituted pyrazolo[5,1-c]-as-triazines by reacting the salts 1 with γ-bromo-substituted β-keto ester 74 [149]. The reaction involves a СН2 group activated with bromine from one side and a keto group from the other side.
Nitriles containing an activated СH group are the leading choice as synthons for the preparation of pyrazolo[5,1-с][1,2,4]triazine derivatives in reactions with diazonium salts [127, 128, 150]. For example, the interaction of pyrazole-3(5)-diazonium salts 1 with cyanoacetic esters allowed to isolate and characterize the hydrazones 76. Depending on the nature of solvent, these compounds may either cyclize through an exo-dig process forming 4-aminopyrazolotriazines 77 or convert by intramolecular heterocyclization to the 4-oxo-1,4-dihydropyrazolo[5,1-c][1,2,4]triazines 78 [150].

The binding of CN group to endocyclic nitrogen atom under the conditions of acidic catalysis may occur also by heating without solvent or refluxing in ethanol, tetralin, and dilute inorganic acids. In a series of cases an impurity of the alternate products 78 was formed, which became the major product when using pyridine as solvent [150]. Contradictory data on this reaction have been reported in several publications [101, 141, 151, 152], where the formation of derivatives 77 or 78 was observed under identical conditions.

The use of malonic dinitrile as the azo component in reactions with pyrazole-3(5)-diazonium salts 1 has been described in a range of publications [92, 101, 128, 129, 131-133, 141, 142, 145-147, 151-155]. These transformations also involved the formation of hydrazones 79, which either after specific treatment stage (or sometimes spontaneously) transformed into 4-amino derivatives of pyrazolo[5,1-c][1,2,4]triazines 80.
Hetarylacetonitriles have been used for the construction of pyrazolotriazine 3-azolyl and 3-azinyl derivatives 81 from pyrazole diazonium salts [156-162].
There are examples of pyrazole diazonium salts 1 reacting with amides or hydrazides of cyanoacetic acid [92, 128, 163-165] as well as its thioamide [166]. Similarly to the previous cases, as-triazine ring closure required the presence of nitrile group.
Significant synthetic potential in reactions with salts 1 was achieved by using β-keto nitriles containing aromatic and heterocyclic fragments. The simultaneous presence of two reactive electrophilic centers, ketone and nitrile, raised the issue of selectivity in this reaction, since the heterocyclization of intermediates 83 may occur either as exo-dig process (forming 3-cyano derivatives 84), or intramolecular condensation (with 3-keto derivatives 85 as products).

There is some controversy in the literature regarding the interpretation of this type of reactions: in some cases the isolation of compounds 84 is reported [49, 100, 131, 145, 154, 167], while in other cases compounds 85 were isolated [92, 108, 168-171], despite the identical starting materials and reaction conditions.

The unsaturated nitrile derivatives 86а,b were also used in reactions with pyrazole-3(5)-diazonium salts 1, yielding the pyrazolo[5,1-c][1,2,4]triazines 88а,b and 89b [129, 172].

Nitro group was found to increase the lability of adjacent CH protons, facilitating both the azo coupling as well as further cyclization. A method for the synthesis of nitroazolo[1,2,4]triazines, involving the key stage of interaction between heterocyclic diazo salts with nitrocarbonyl compounds, such as nitroacetonitrile and nitroacetic ester, was developed by Chupakhin and co-workers [173, 174]. The reaction was performed in aqueous or aqueous-alcoholic alkali solutions at low or room temperature through the stage of pyrazolylhydrazones 90 or their sodium salts 91. Treatment of the former with alkali solution gave the respective azinium salts 92, which were transformed into nitro-substituted dihydropyrazolo[5,1-с]-[1, 2, 4]triazines 93 after neutralization. The nitro derivatives 94 were obtained directly by the interaction of salts 1 with nitroacetonitrile.
Nitropyrazolo[5,1-c]-as-triazines 95 were formed in Japp–Klingemann reaction from nitromalonic aldehyde [173, 175].
Nitro compounds with activated methylene groups having the general formula 96 without groups capable of intramolecular cyclization can be used to obtain derivatives of another azoloannelated bisheterocycle, pyrazolo[5,1-с][1,2,4]triazole 98 [81].
A method for the synthesis of 4H-pyrazolo[5,1-с][1,2,4]triazines 100, using the enaminones 99 as activated methylene compounds in reactions with pyrazole-3(5)-diazonium salts, was developed by a research group from Egypt [176-180]. The proposed mechanism involves initial formation of azo compounds А, which condense under the reaction conditions, giving pyrazolotriazines 100 either directly through elimination of a dimethylamine molecule or through hydrolysis and dehydration stages. However, none of these proposed reaction intermediates were isolated.
The use of enolate 101 also enabled the preparation of 4Н-pyrazolo[5,1-с][1,2,4]triazine 100 [181].
An unusual cyclization was discovered using compound 1 and pyridinium salts 102 [182].

Stable hydrazonyl pyridinium salts 103 were converted to the derivatives 104 upon treatment with sodium carbonate solution, probably through ylides А.

Reactions of pyrazole-3(5)-diazonium salts 1 with phenacylchalcogenocyanates 105 were successfully used to synthesize substituted thiadiazoles and selenadiazoles 106 [183]. This reaction was accompanied by spontaneous heterocyclization of hydrazones A at the exo-NH and YCN sites.

Analogous products were obtained from condensation of pyrazole-3(5)-diazonium salts with phenacyldimethylsulfonium bromide with subsequent treatment of hydrazonyl bromides with potassium thio(seleno)cyanate [182, 183].

We should note a study by German authors [184] regarding the interaction of nonaqueous diazonium salt 1 solutions (R = R1 = H, X = Cl) with diazomethane. The reaction leads to two products: 3(5)-(1-tetr-azolyl)pyrazole 107 (structure established by X-ray structural analysis) and pyrazolo[5,1-c]triazole 108.

A threefold excess of diazomethane gave an isomeric mixture of N-methylation products of compound 107.

There are only a few examples where annelated pyrazoles have been prepared by intramolecular cyclization of azo coupling products obtained from pyrazole diazonium salts and NH acids. Thus, pyrazolyl- triazenes 24 (see 2.2) may be dehydrogenated, forming 3H-pyrazolo[1,5-d]tetrazole derivatives [83]. For example, this method is used to introduce heterocyclic systems into nucleosides.

Interaction with cyclic components. There are relatively few published examples of azo coupling reactions between pyrazole-3(5)-diazonium salts and cyclic azo components, resulting in the formation of a new ring. This method may be used for the preparation of pyrazoloazines containing three, four, or more rings in the molecule.

The first example of using alicyclic components (cyclohexane-1,3-dione, dimedone, 2-ethoxycarbonyl-cyclopentanone) in this reaction was reported by authors from former Yugoslavia [134]. Thus, interaction of salts 1 with cyclohexanediones led to 6,7,8,9-tetrahydropyrazolo[5,1-c][1,2,4]benzotriazinones 109 in 69 and 72% yields.

The azo coupling with a substituted cyclopentanone took a different course: it was possible to isolate the hydrazone 110 as a result of ring opening and to achieve its thermal cyclization to the pyrazolotriazine 111.

The interaction of salt 1 with dimethylaminomethylene derivative of dimedone allowed to isolate only the bisazo coupling product 112 in 80% yield. The pentacyclic system 112 was also formed by azo coupling of compound 1 (R = R1 = H) with the enaminone 113 under standard conditions [185].
Analogous Yapp–Klingemann reaction was possible by using benzosuberone derivative 114 [186].
The interaction of pyrazole-3(5)-diazonium salts 1 with 1,3-indanedione has been described, resulting in the formation of a tetracyclic system, indeno[1,2-e]pyrazolo[5,1-c]-1,2,4-triazin-6-one 117 [129, 187, 188].
Among the heterocyclic agents used in azo coupling reactions with diazonium salts 1 to obtain condensed pyrazoles, special mention should be given to 3-hydroxy-1-methylindole 118 and α-pyridones 119 [93, 189]. In the first case, polycyclic compound 120 formed under the standard conditions without isolation of the intermediate, while in the second case the azo compounds 121 were additionally treated with acetic anhydride, and pyrazolo[5,1-c]pyrido[2,3-e]-as-triazines 122 were obtained.
Meldrum's acid was found to be a sufficiently strong CH acid in similar reactions. The interaction of the latter with diazonium salt 1 (R = R1 = H; X = HSO4) gave the hydrazone 123, which was recyclized by refluxing for 1 day in AcOH and gave pyrazolo[5,1-c][1, 2, 4]triazine-4(1H)-one (124) [190].
We should note the methods of constructing pyrazolo[5,1-c]-as-triazines 125 and 1-(1H-pyrazol-5-yl)-1H-1,2,4-triazoles 126 by using the salts 1 and 2-phenyloxazol-5(4H)-one [159]. The syntheses are based on intramolecular recyclization of oxazole nucleus with the participation of exo- and endo-NH groups.

Recent publications [191, 192] report the successful preparation of tricyclic systems with pyrazolo[5,1-с]-[1, 2, 4]triazine fragment based on the reactions of pyrazole-3(5)-diazonium salts 1 (R = Me, R1 = Ph) with derivatives of pyridine-2,4-dione 127, pyrimidine-4,6-dione 128, barbituric and N,N-dimethylbarbituric acids 129а,b. Heterocyclic hydrazones 130а,b and 132а,b were isolated as intermediates in all of these cases. Further cyclocondensation was performed either by heating in polyphosphoric acid (in the case of pyrimidine derivatives) or by refluxing in acetic acid (in the case of pyridine derivative). The selection of forcing conditions for the cyclocondensation of hydrazones 132а,b was explained by hindered nucleophilic attack of endocyclic nitrogen atom of pyrazole on the lactam group of adjacent heterocyclic fragment.

Thus, analysis of literature data show that pyrazole-3(5)-diazonium salts are valuable synthons for the preparation of various linearly linked and polycondensed nitrogen-, oxygen-, and sulfur-containing heterocyclic systems. A special place among the latter belongs to hetarylazo compounds and pyrazolo[5,1-c][1,2,4]triazines, which possess a range of practically useful properties.

This work received financial support from the Ministry of Education and Science of the Russian Federation (contract No. 02.G25.31.0007).

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Copyright information

© Springer Science+Business Media New York 2014

Authors and Affiliations

  • I. V. Ledenyova
    • 1
    Email author
  • V. V. Didenko
    • 1
  • Kh. S. Shikhaliev
    • 1
  1. 1.Voronezh State UniversityVoronezhRussia

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