Hydrazones, isomerization

Fluorothiiranes are readily desulfurized by a variety of reagents. Triphenylpho-sphine, for example, reacted at RT with aminosulhde 102 to yield Dewar pyrrole 103. °° For the desulfurization of azo compound 104, a nonbasic thiophile was required because of the extremely facile azo-to-hydrazone isomerization. An imi-dazolethione accomplished this catalytically and cleanly to give 105 plus elemental sulfur, presumably via zwitterion 106. ° Thiirane 30 was transformed into perfluor-oisobutylene (107) by both amines and antimony pentafluoride. ... 

Tautomerism has been discussed in Section 4.04.1.5.2. It concerns prototropic tautomerism and the decreasing order of stability is (hydrazone) >A (azo)> A (enehydrazine). The isomerization A -> A occurs via a A -pyrazoline (65BSF769). Pyrazolidones and amino-A -pyrazolines exist as such. The only example of non-prototropic tautomerism deals with the isomerization (403) —> (404) (74CJC3474). This intramolecular process is another example (Section 4.04.1.5) of the thermodynamic analogy between prototropy and metallotropy. 

Clean examples of diaziridine to hydrazone rearrangements are rare. Diaziridine (119) mentioned above rearranges to the isomeric enhydrazone in boiling toluene, and 2,4-dinitrophenyldiaziridine (125) under the same conditions affords the 2,4-dinitrophenylhy-drazone (145) within 4 h. On blocking this rearrangement by iV-methyl, conversion with loss of cyclohexanone occurred to give benzotriazole iV-oxide (146) (72JOC2980). 

Katritzky and co-workers studied the mechanism of this reaction in detail. His work involved a NMR study of 16 reactions of methyl-, phenyl-, 1,2-dimethyl-, and l-methyl-2-phenylhydrazine with /3-keto esters. In many cases starting materials, intermediates, and products were detected simultaneously. Most reactions proceed by nucleophilic addition of the less hindered hydrazine nitrogen atom to the keto carbon of the keto ester. For example, the pathway given in Scheme 3 for the reaction of methyl 3-oxobutanoate 9 with methyl- or phenyUiydrazine 2 (R = Me or Ph) was found to be dominant. The initially formed addition product 10 dehydrates to hydrazone 11, which then isomerizes to hydrazone 12. Intermediate 12 then cyclizes to pyrazol-3-one 13, which tautomerizes to the kinetically more stable pyrazol-3-otie 14 [87JCS(P2)969]. 

The hydrazone structure 40 can be eliminated at once many examples of this class of compounds are known and their properties are completely different from the diaziridines. For example, 3,3-dimethyldiaziridine has a heat of combustion of about 35 kcal higher than the isomeric acetone hydrazone. Further pairs of isomers of diaziridines and hydrazones are known. The spectrum eliminates both the hydrazone structure and the betaine structure 41. The diaziridines do not absorb in the UV range. In the infrared spectrum, absorption is completely absent in the double-bond region. - The NMR spectrum of 3,3-dimethyldiaziridine is in agreement with a formulation that has two equivalent iV-protons. ... 

Hydrazine 746 could be a precursor for this ring system through the formation of its respective hydrazones and oxidation with ferric chloride to give 748 (93BCJ00) and not the isomeric [1,2,4]triazolo[4,3-6][l,2,4] triazine ring. 

Hydrogen shifts are often observed in thermal isomerizations of vinylaziridines. Heating of compounds 221 at 180 °C produced mixture of 3-pyrrolines 222 and hydrazones 223 (Scheme 2.54) [87]. The formation of 223 can be explained in terms either of a concerted hydrogen shift as depicted in 224 or of diradical intermediates 225, both of which would be followed by thermal isomerization of the (Z)-carbon-carbon double bonds to provide the ( ) isomers 223. 

Aliphatic azo compounds in which the carbon containing the azo group is attached to a hydrogen are unstable and tautomerize to the isomeric hydrazones (15), which are therefore the products of the reaction. 

Autoxidation of the hydrazone (O2/C6H6/UV) gives the explosive isomeric 1,2-dihydroperoxy-l,2-bis(benzeneazo)cyclohexane [1], and the same is true for COT derivatives [2],... 

In contrast to the Johnson s D —> A-ring construction approach, Brown devised an A —> D-ring construction approach [22]. Starting from Wieland-Miescher ketone (30), a common source of the A, B-rings in the de novo synthesis of steroids, the C-ring was introduced via hydrazone allylation, ozonolysis, aldol condensation, and olefin isomerization (31 > 32). The D-ring was assembled by a reductive alkylation... 

The thermal hydrazone-azomethine imine isomerization can be easily performed under microwave irradiation in the absence of solvent. The subsequent 1,3-dipolar cydoadditions with electron-defident dipolarophiles occur in only a few minutes to afford the corresponding cycloadducts. The use of pyrazolyl hydrazones 205 leads to valuable compounds, such as bipyrazoles 213, in good yields and this provides a new approach to the preparation of these heterocyclic derivatives [116] (Scheme 9.67). Reactions undertaken with dassical heating under comparable reaction conditions (time and temperature) lead to cydoadduct yields that are considerably lower and, indeed, several dipolarophiles do not react at all. 

Depending on the nature of the substrates, selectivity could be completely reversed between the two isomeric products. For example, switching R1 group between Buc and Ph gave high yields of the first and second product structures, respectively. The authors noted that the reaction did not proceed if the imine contained an ortho-MeO group at R2 or if the imine was replaced with an aldehyde, oxime, or hydrazone. The catalytic cycle is initiated by C-H activation of the imine, that is, the formation of a five-membered metallocycle alkyne insertion affords the intermediate drawn in Scheme 69. It is noteworthy that this is the first report of catalytic synthesis of indene derivatives via a C-H insertion mechanism (C-H activation, insertion, intramolecular addition). 

As a result of the stabilization of the coplanar structure, azo groups exhibit cis-trans isomerism. Azo compounds are generally written in the trans configuration, which is the more stable one. However, X-ray structure analyses show azo pigments exclusively in their hydrazone form (see pp. 213,272). 

Apart from cydopentadiene 309a, furan [151, 165, 321-323, 327, 329] and pyrrole derivatives [158, 325] 309b and 309c, respectively, are also often used as dienes. Recently, the synthetic applications of furan Diels-Alder chemistry were reviewed comprehensively [330]. In the case of a,/3-unsaturated hydrazones 312, isomerization and elimination of dimethylamine convert the Diels-Alder products 314, which... 

The mechanism of the coupling reaction has been very exhaustively studied. Summarising first what has already been mentioned, it must be noted that the reaction is not confined to the aromatic series, for diazo-compounds condense also with enols and with the very closely related aliphatic aci-nitro-compounds. The final products of these reactions are not azo-compounds, but the isomeric hydrazones formed from them by rearrangement. 

Cyclododecene may be prepared from 1,5,9-cyclododecatriene by the catalytic reduction with Raney nickel and hydrogen diluted with nitrogen, with nickel sulfide on alumina, with cobalt, iron, or nickel in the presence of thiophene, with palladium on charcoal, with palladimn chloride in the presence of water, with palladium on barium sulfate, with cobalt acetate in the presence of cobalt carbonyl, and with cobalt carbonyl and tri- -butyl phosphine. It may also be obtained from the triene by reduction with lithium and ethylamine, by disproportionation, - by epoxidation followed by isomerization to a ketone and WoliT-Kishner reduction, and from cyclododecanone by the reaction of its hydrazone with sodium hydride. ... 

Aminotriazoles and their derivatives can be prepared from bis hydrazones and from substituted bis hydrazones of a-diketones. The bis hydrazones may in some cases be prepared more conveniently from -bromoketones than from a-diketones. Oxidation with mercuric oxide or with manganese dioxide gives the 1-aminotriazoles directly (Scheme m-ns Although two isomeric triazoles would be ex-... 

Acyl-l,2,4-trioxolanes (61) are generally thermally labile, although their 2,4-dinitrophenyl-hydrazone (DNP) derivatives appear to be much more stable <9lCB94v>. In addition to the anhydrides (62), the previously unknown isomeric nonperoxide compounds (63) were found as products (Equation (5)). 

A cyclic silyl hydrazone derivative <83ZAAC(50i)57> as well as azo compounds isomeric to hydrazones <74PS197> have also been used for the synthesis of 2//-1,2,3-diazaphospholes by the PCI3 condensation. 2,4,5-Triphenyl-l,2,3-diazaphosphole is best prepared from the respective azoalkene and a fused benzothiadiphosphole as phosphorus donating reagent <95TL447>. 

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