Formazan oxidation

Fischer indole synthesis, 91-95 Formamidine disulfide, salts of, 281, 282 Formazans, oxidation of, 37 Furans, alkyl-, ring-opening of, 63... 

Although the formation of tetrazolium salts of structure 92 by the oxidation of formazans under acid conditions has been known for many years, it is only recently that the quatemization of 1,5-disubstituted... 

Tetrazolium salts are an important class of leuco dyes that have found a wide range of applications. They are unique among the synthetic leuco dyes in that the colored form of the dye is the reduced form rather than the oxidized form. There are at least three extensive reviews on tetrazolium salts and the corresponding formazan dyes1-3 and one textbook.96 This chapter will describe some recent developments as well as some of the material covered in these reviews. 

In general, tetrazolium salts are prepared by (a) oxidation of formazans and (b) alkylation of tetrazoles. Therefore, synthetic methods for formazans and tetrazoles are discussed first, followed by some direct and miscellaneous methods. 

Ethyl formate34,52 or orthoformate5 3,54 reacts with two equivalents of phenylhydrazine to yield 1,5-diphenylformazan (11) the reaction takes place under acidic conditions and involves an oxidation. Under basic conditions, ethyl nitrate reacts at the methylene position to yield 3-methyl-1,5-diphenylformazan (37) which can also be obtained from the reaction of phenyl-azoethane (38) with isoamyl nitrite (Scheme 4).8,68 Aryl hydrazines react with a variety of s-triazines (39) to yield 1,5-diaryl formazans with hydrogen, methyl, or phenyl groups in the 3-position as in 40 (Eq. 6).56 Hydrazines have also been reported to react with benzotrichloride55,658 and sym-diamino tetrazine659 to yield formazans. 

Diaryl carbazides and thiocarbazides, e.g., 43, can be oxidized at alkaline pH to yield C-hydroxy or C-mercapto formazans (44) (Eq. 8).68,129 The replacement of C-halo formazans is considered to be a better method60 for the preparation of 44. A class of cyclic formazans (45) can be obtained by air oxidation of amidrazones (46) (Eq. 9) 9,73,101,102 jhough this is formally a formazan, there are no reports of its oxidation reactions. In a... 

A variety of oxidizing agents have been used to effect the transformation of formazans to tetrazolium salts. The choice of the reagent is still empirical and depends on the solubility properties of the formazan as well... 

The earliest oxidations were effected with nitrous fumes and later with mercuric oxide and isoamyl nitrite.74 Lead tetraacetate in acetic acid is in many cases the reagent of choice, but the removal of by-products can present some difficulties.75 IV-Haloimides and amides in alcoholic solutions have been reported to yield essentially pure tetrazolium salts76 and have been found specially useful in the preparation of heteroaryl-substituted tetrazolium salt.77,78 The novel formazans 49 have been successfully oxidized to 50 using 7V-chloro succinimide (Eq. II).79 tert-Butyl hypo-... 

Electrochemical oxidation has also been reported to result in good yields of tetrazolium salts.97,98 Treatment of formazans with boron trifluor-... 

Table 3. Synthesis of Tetrazoliums from Oxidation of Formazans... 

Wedekind and Stauwe" studied the oxidation of 3-substituted formazans and concluded that ease of oxidation depended on the steric effects of the 3-substituent. More recently, Hegoraty et al. 100 studied the reaction of formazans with bromine. It proceeds via an odd-electron species such as 52 favoring an electronic substituent effect (Scheme 5). The rate of reaction increases with electron-donating substituents. Similar conclusions have been reached using thalium(III) as the oxidant.101,102... 

Ferricyanide oxidation of 1,5-disubstituted thiocarbazones (116) give the mesoionic tetrazolium salts (117) under mild conditions (Eq. 21).190 This is in contrast to the strongly alkaline oxidation of carbazides leading to mercapto formazans as shown in Eq. 8 (Section 7.3.1.4). The heterocyclic triazine (118), obtained by the action of a diazonium salt on 2-... 

Formazans are stable in alkaline solution. Alkaline hydrolysis of functionalities on formazans such as nitriles, esters, and amides leads to the acids (Section 7.3.1.1). The case of 3-nitroformazans (198) is unique. Reaction with hydroxide ion gives 3-hydroxy formazan (199) which can be readily oxidized to the tetrazolium betaine. In the presence of hydrosulfide, a reduction of the nitro group takes place giving 3-aminoformazan (200) with traces of the 3-mercaptoformazan (201), which by contrast is the main product when ammonium polysulfide is used (Scheme 30).45,346... 

Verdrazyls, e.g., 210, are a class of cyclic free radicals, with characteristic ESR spectra and intense colors. They represent a special case of oxidation of formazans and can be obtained from the methylation of and simultaneous or subsequent dehydrogenation of formazans.359-361 Ver-... 

A few electrochemical reductions of formazans to hydrazidine have been reported.370,372 Ho wever, as discussed in Section 7.4.2.6, electrochemical techniques have been widely used to study the redox chemistry of tetrazolium salts and formazans. Opinions about the reversibility of the electron transfer step, the number of electrons involved, and the identity of the rate-determining step differ widely.369- 371,656 The electrochemical oxidation of some novel formazans, e.g., 215 produces the dicationic species 216 (Eq. 28). The mechanism is not clearly understood.372,373... 

Triphenylformazan behaves as a bidentate ligand forming 2 1 complexes (217) with divalent copper, nickel, and cobalt.377 Formazan metal complexes can be compared to complexes of azo dyes or beta diketones due to structural similarity.301,302 In general, formazan metal complexes have low stability toward acids. However, when electron-donating substituents are added to the aromatic ring, a considerable enhancement in stability is observed. Cationic complexes of type 218 are also known. The complexation of formazan with metal cation can be accompanied by oxidation to the tetrazolium salt and the formation of a complex... 

With few exceptions, by far most applications of tetrazolium salts (e.g., analytical, photographic, and biochemical) utilize their reducibility to the corresponding formazan dyes. Tetrazolium salts, due to their resistance to acid and oxidation and the presence of a positive charge, find use in other applications such as antistatic agents and phase transfer catalysts. Over the past two decades, there have been thousands of citations regarding the applications of tetrazolium salts. Most of these citations are patents with similar or overlapping and even identical claims. Any attempt at completeness would be futile. The applications are sorted where discernible, and the earliest or the broadest claims are cited. 

A colorimetric version of the method uses a tetrazolium salt. The oxidation of the NADH is coupled to the reduction of 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium bromide by a diaphorase (EC 1.6.4.3) and a deep blue formazan develops. The absorbance is read at a wavelength between 540 and 600 nm. This is more sensitive than the ultraviolet method but is more prone to interference, especially from any reducing agents present in the sample, which will result in a positive error. 

In order to illustrate the application of LSV in mechanistic analysis we can look at the redox behavior of the formazan-tetrazolium salt system which we studied some years ago [17], 1,3,5-Triphenyl formazane was oxidized at controlled potential in CH3CN-Et4NC104 solution to 2,3,5-triphenyl tetrazolium perchlorate which was then isolated in quantitative yield. Coulometry showed that the overall electrode reaction was a two-electron oxidation. It has been shown that the rate of variation of Ep with log v was 30 mV per decade of sweep rate and that there was no variation of the peak potential with the concentration of 1,3,5-triphenylformazan. According to Saveant s diagnostic criteria (Table 1), four mechanistic schemes were possible e-C-e-p-p, e-C-d-p-p, e-c-P-e-p and e-c-P-d-p. If cyclization is the rate-determining step, then the resulting e-C-e-p-p and e-C-d-p-p mechanisms would not imply variation of Ep with the concentration of base. However, we have observed the 35 mV shift of Ep cathodically in the presence of 4-cyanopyridine as a b e. These observations ruled out the first two mechanisms. The remaining possibilities were then e-c-P-e and e-c-P-d, as shown in Scheme 3. 

Anodic oxidation of formazane 18 [17], 1-arylmethylenesemicarbazide 19 [55], p-nitrobenzaldehyde phenylhydrazone 20 [56], and 2-benzoylpyridine phenylhydrazone 21 [57] afforded the corresponding heterocycles in a very good yield (Scheme 14). The homogeneous oxidation of compounds 18-20 was carried out by indirect electrolysis by the mediators generated in situ [58]. 

Formation of the Nitrogen-Nitrogen Bond 3.4.1 Oxidation of Hydrazones and Formazans... 

The chemical oxidation of 1,3,5-triarylformazans to tetrazolium salts was first accomplished in 1894 [127], Almost no attention was given to these compounds for about 50 years after their discovery. This situation began to alter markedly because of the application of tetrazolium salts in histochemical, pharmacological, and other biomedical research areas [128]. Specifically, the tetrazolium salt is reduced to a colored formazan derivative by reducing enzymes found only in metabolically active cells. Anodic transformation of for-mazans to tetrazolium salts was performed in acetonitrile solution using cotrolled potential electrolysis [17,129], In our view this reaction could be considered as a method of choice for the preparation of tetrazolium salts. The products were obtained in high yield and the electrolysis can be performed in a divided cell under constant current and decoloration of the solution indicates the end point of the reaction. Recently the anodic oxidation of formazans to tetrazolium salts was performed successfully in aqueous ethanol solution [130]. 

Oxidation of kerosene (fuel oil no. 1) and diesel fuel (fuel oil no. 2) by soil microbes, as determined by dehydrogenase activity, increased with increasing loading rates for both fuel oils (up to 60% w/w oil/dry soil) for up to 7 days of incubation but decreased thereafter. Dehydrogenase activity in soil treated with diesel fuel was almost twice that of soils treated with kerosene (56 and 32 g formazan/g soil/24 hours, respectively) (Frankenberger and Johanson 1982). 

Trisubstituted tetrazolium salts (57) are the oxidation products of formazans. They have been used to produce tetrazolinyl radicals by controlled reduction <73AG(E)455>. 7V-Alkyltetrazolium... 

The bishydrazones of the 1,2-diketones from inositols have also been converted into triazoles.222,223 The conversion of arylosazones into the corresponding osotriazoles requires the presence of an oxidant, and it is obvious that simple removal of aniline from the osazone, as suggested by the equation, is not involved. In addition to copper(II) sulfate, the reagent most commonly used, other oxidizing heavy-metal salts, such as ferric sulfate and chloride,224 and mercuric acetate,223 have been used, as well as halogens225 and nitro-sulfonates.226 The osazone acetates are converted into osotriazoles by nitrous acid,227 which decomposes the unacetylated osazones to the aldosuloses228 and the osazone formazans are cyclized with warm... 

Acyclic sugar hydrazones react with benzenediazonium chloride to give red formazans. When oxidized with N-bromosuccinimide (or, after acetylation, with lead tetraacetate), these yield tetrazolium salts. 

Oxidative ring closure by formation of a nitrogen-nitrogen bond may be obtained, using suitable heterocyclic hydrazones, formazans, and certain other azomethine functions as substrates. 

Anodic oxidation of formazans (102) in CH3CN-Et4NX (X=C104,p-TsO, or BF4) at a platinum anode produces tetrazolium salts (103) in quantitative yield162-164 [Eq. (79)]. 

---