Hydrazones, dehydrogenation

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... 

The aliphatic diazo-compounds can also be prepared by careful dehydrogenation of the hydrazones (with HgO) (Curtius, Staudinger) and, conversely, they are converted into the latter by hydrogenation ... 

Reduction of dehydro-c-ascorbic acid phenylhydrazone (40) with LiAlH4 resulted in hydrogenation of the hydrazone residue and cyclization to bicyclic compound 41, which was dehydrogenated with boiling acetic anhydride during acetylation to give diacetate 43, then partly hydrolyzed to monoacetate 42 (Scheme 7) (72JOC3523). 

Numerous methods to prepare individual classes of aliphatic diazo compounds have been extensively developed. The major strategies for their synthesis involve the alkaline cleavage of N-alkyl-N-nitroso-ureas, -carboxamides and -sulfonamides, dehydrogenation of hydrazones, as well as diazo group transfer from sulfonyl and related azides to active methylene compounds, and electrophilic diazoalkane substitution reactions. These synthetic methods have been comprehensively reviewed (15,16). Useful information on the preparation of selected diazo compounds can be found elsewhere (6,17). 

One of the common chemical methods for determining carbonyl compounds consists of converting them into hydrazones [7], This has been used for (1) oxycellulose [28], (2) nylon-6 and nylon-6,6 [29], (3) dehydrogenated poly (vinyl chloride) [30], (4) in irradiated polyethylene films [31], and (5) grafted polyethylene glycol [32],... 

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... 

So far so good. But how can the fourth double bond be created There is a possibility of dehydrogenation of -CH-NH- group by hydrazine, similar to that of dehydrogenation of -CH-OH bond in saccharides by phenylhydrazine, which is hydrogenolyzed to ammonia and aniline. Here the molecule of cyclobutanetetronetetrakis(hydrazone), Z, is obtained, and hydrazine is split to ammonia [52]. 

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 oxidmt. Whilst this unusual reaction affords some advantages over earlier methods it is unlikely to be the method of choice in most instances. 

The formation of pyridazines from 1,4-ketoacids (10) or their esters and unsubstituted or substituted hydrazines is one of the most widely used methods of synthesis. It is possible to conduct the reaction in a single step or via the intermediate hydrazones or semicarbazones. The resulting 4,5-dihydro-3(2.fif)-pyridazinones (11) are then converted into the corresponding 3(2fl )-pyridazinones (12) upon dehydrogenation. Bromine in glacial acetic acid is the commonest and most useful... 

The same condensation-cyclization sequence with 4-methoxy-phenylhydrazine (292), however, proceeded in only 17% yield (113), so an alternative synthesis was devised for the important 8-oxygenated derivatives, such as 293 and 294, which employed a more conventional Fischer indole reaction (Scheme 47) (113). In the event, condensation of keto ketal 295 with hydrazine 280 gave hydrazone 296. Cyclization in hot diphenyl ether gave ketone 297, which, upon dehydrogenation, protection of the phenol as the benzoate, and chlorination gave 298. Deprotection and/or methylation afforded the target chloro-y-carbolines (299-301). 

Cyclopentadimylidene (2). 2 was very easily obtained by photolysis or thermolysis of the corresponding diazo-cyclopentadienes (72). These were prepared by straightforward procedures (diazo-group transfer, dehydrogenation of hydrazones, or Bamford-Stevens reaction of tosyl-hydrazones) 26-80) from either cyclopentadienes (77) or substituted cyclopentadienones (73). 

In carrying out such a dehydrogenative cyclization on alkyl hydrazones 280 derived from 6-hydrazinotetrazolo[l, 5-/ ]pyridazine (279) using lead... 

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