Hydrazones, dehydrogenation oxidation

Diazoalkanes are fairly stable toward oxidation reagents. This is evident from one of the major reactions for their synthesis, the hydrazone dehydrogenation (see Subsect. 2.5.1). The diazoalkanes formed are stable against attack by the transition-metal oxides used as dehydrogenation reagents. 

Abolishing the need for heavy metal oxidants, Shechter and coworkers introduced a novel method for hydrazone dehydrogenation by reacting lithium hydrazraiides with phosphonium azide 55 (Scheme 21) [53]. The inherently basic reaction... 

Hydrazones of thiochroman-4-ones are converted into thiochromans under Wolff-Kishner-Huang conditions65 and into the azo dimers by silver oxide.66 Fisher indolization of the phenylhydrazones (33) gives 6,ll-dihydrobenz[6]indolo[2,3-d]thiopyrans (34), which by hydride loss form the thiopyrylium salts (35), or on dehydrogenation produce the pseudoazulenes (36) (heterocyclic analogs of the carcinogen, benz[a]-carbazole), as shown in Scheme 6.87-73... 

Little woric has been carried out on sulfmylation reactions on those systems having thiocaibonyl and imino moieties. However, hydrazones are converted to a-sulfinyl derivatives on reaction of their anions (prepared from LDA in THF) at -78 C with sulrinate esters, although the full utility of this reaction remains to be explored. Furthermore, in an unusual reaction, p-toluenesulfmyl chloride has been shown to effect a facile one-step dehydrogenation of the thiolactam (17 equation 7) in good yield. These reactions contrast with the oxidative removal of thiocarbonyl, hydrazonyl and similar functionalities with Se species (see Section 2.2.4.2). 

An interesting dehydrogenation of hydrazones (51) has been reported by Barton which relies on the available oxygen of aromatic nitro groups (equation 22). In a detailed study, quantitative yields were obtained using 4-nitrobenzoic acid as the oxidant.Whilst this unusual reaction affords some advantages over earlier methods it is unlikely to be the method of choice in most instances. 

The domain of oxidations with silver oxide includes the conversion of aldehydes into acids [63, 206, 362, 365, 366, 367 and of hydroxy aromatic compounds into quinones [171, 368, 369]. Less frequently, silver oxide is used for the oxidation of aldehyde and ketone hydrazones to diazo compounds [370, 371], of hydrazo compounds to azo compounds [372], and of hydroxylamines to nitroso compounds [373] or nitroxyls [374] and for the dehydrogenation of CH-NH bonds to -C=N- [375]. Similar results with silver carbonate are obtained in oxidations of alcohols to ketones [376] or acids [377] and of hydroxylamines to nitroso compounds [378]. 

Mercuric oxide, HgO (yellow modification or the less reactive red modification), resembles silver oxide in its oxidizing properties. This reagent transforms phenols and hydroquinones into quinones [383, 384] and is used especially for the conversion of hydrazones into diazo compounds [355, 386, 387, 388, 389, 390, 391, 392]. Dihydrazones of a-diketones furnish acetylenes [393, 394, 395, 396], A -Aminopiperidines are dehydrogenated to tetrazenes [397] or converted into hydrocarbons [395]. 

Nickel peroxide, an undefined black oxide of nickel, is prepared from nickel sulfate hexahydrate by oxidation in alkaline medium with an ozone-oxygen mixture [929] or with sodium hypochlorite [930, 931, 932, 933]. Its main applications are the oxidation of aromatic side chains to carboxyls [933], of allylic and benzylic alcohols to aldehydes in organic solvents [929, 932] or to acids in aqueous alkaline solutions [929, 930, 932], and of aldehydes to acids [934, the conversion of aldehyde or ketone hydrazones into diazo compounds [935] the dehydrogenative coupling of ketones in the a positions with respect to carbonyl groups [931] and the dehydrogenation of primary amines to nitriles or azo compounds [936]. 

Curtius discovered both the first synthetic route to aliphatic diazo compounds by nitrosation of amines (1883, see Sect. 2.3) and, in 1889, also their preparation by dehydrogenation of hydrazones, i.e., reaction (5) in Table 2-1. He treated the mono-and the bis-hydrazone of benzil (1,2-diphenylethanedione, 2.60) with yellow mercury (ii) oxide (2-23). With the monohydrazone 2.61, he obtained 2-diazo-l,2-diphenyl-ethan-l-one (azibenzil, 2.62). The corresponding bis-diazo compound (2.64) of... 

Stoichiometrically, one equivalent of HgO is necessary for the dehydrogenation of a hydrazone. Examples have been published in Organic Syntheses Smith and Howard (1955) described the procedure for diphenyldiazomethane, obtained from benzophenone hydrazone in petroleum ether in 89-96% yield. The necessity for the absence of moisture is emphasized, but no activation of the mercury(n) oxide seems to be required. Andrews et al. (1988) have reported on the dehydrogenation of acetone hydrazone to 2-diazopropane (70-90% yield) in ether in the presence of catalytic amounts of KOH in ethanol. There are also cases where two equivalents are used, e.g., the procedure for (benzoyl)(phenyl) diazomethane (2.62, yield 87-94%) published in Organic Syntheses by Nenitzescu and Solomonica (1943). Neither these nor other authors have explained, however, why two equivalents would be necessary. 

Recently, an efficient CuBr-catalyzed aerobic intramolecular dehydrogenative cyclization reaction of AfAf-disubstituted hydrazones to pyrazoles by a double C(sp )-H bond functionalization was developed in Ge s group (Scheme 8.67). This transformation includes C(sp )-H oxidation, cyclization, and aromatization for the formation of pyrazole products. This is the first example of an intramolecular copper-catalyzed dehydrogenative coupling reaction via an iminium ion intermediate by a C(sp )-H bond functionalization process [113]. 

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