Electronic structure hydrazones

The mechanisms of the reaction involving oc,(3-unsaturated ketones and hydrazines were studied in several publications [30, 66, 67, 68, 69, 70, 71, 72, 73, 74]. The first stage of the reaction is the addition of hydrazine to the carbonyl group of the ketone (compound 61, Scheme 2.15). Subsequent cycliza-tion by addition of the second nucleophilic center to the ethylene bond under acidic conditions is the rate-determining stage—its rate significantly depends on the stereochemistry and electronic structure of the intermediate hydrazone 62 (Scheme 2.15). 

The correlation analysis was performed at pH 3.0 and showed that in all the series the half-wave potential did not depend on the nature of the substituent. This indicates that the hydrazone group did not conduct electronic displacements from one aromatic ring to another, although spectral data indicated some decrease in absorbance. This can be explained by the fact that rearrangement of the electronic structure takes place in the adsorbed molecule under the influence of the electrode field, leading to uncoupling of the aromatic rings. 

Iminomethyl-l-phenyl-l,2,3-triazoles (106, R = CH3) and their structural isomer 4-(iminophenyl)-l-methyl-l,2,3-triazole (108, R = CH3) are interconvertible when they are heated in DMSO at 80°C (Scheme IV.43) (84JHC627 89JHC701 90JHC2021). The equihbrium position is dependent on the electronic properties of the substituent R. When R is alkyl, a benzyl or anisyl compound 108 is favored when R is p-chlorophenyl or p-nitrophenyl, 106 is the favored isomer. The hydrazone 106 R = NH2) or oxime 106 R = OH) do not rearrange. A diazoimine 107 can be postulated as intermediate, a species whose structure is similar to the one proposed... 

Elnagdi et al. (88AP851) found that the reaction of 86 with tetracya-noethylene does not afford the expected structure 101 (X = Y = CN). Instead the hydrazones (88) (X = R = CN R = Ph) were isolated. Similarly, compounds 88 were formed upon treatment of 86 with cinnamonitrile derivatives. It was thus postulated that the reaction of these electron-poor double bonds with 86 proceeded via formation of an acyclic intermediate (e.g. 102) by reaction with 103, which then decomposes into isolable 101 by the elimination of water. In support, compounds 86 were recovered unreacted when treated with cinnamonitriles or with tetracyanoethylene in the absence of water (88AP851). 

The synthesis and characterization of V02+ complexes of hydrazones (94) and (95) derived from benzoylhydrazine and o-hydroxy aromatic aldehydes and ketones were also reported772 and molecular weight determinations, IR, electronic spectra and magnetic properties were explained assuming a dimeric structure (96) with each unit having a five-coordinate square pyramidal geometry. 

Nitroquinoline 209 enters into a direct cyclocondensation with aromatic hydra-zones in NaOH/DMF giving pyrazolo[3,4-/]quinolines 210 and (or) triazino[6,5-/ quinolines 211 in low to moderate yield (Scheme 62) (OOOL413). Their ratio mainly depends on the structure of the starting hydrazone. For example, electron-donating substituents in its aryl moiety assist triazine ring closure. Evidently, pyrazoles 210 are products of two consecutive SNH and SN ipso reactions, whereas conversion of 209 into 211 looks rather complicated and better corresponds with cascade hetero-cyclizations considered in Section III.D.l. 

The smaller coordination shift for the 15N nucleus in [73c] is consistent with its lone pair of electrons remaining free, whereas they are used for the coordination of the 15N atom in [73b]. The possibility of hydrazone structures [72] contributing to the molecule, rather than the azo-forms [73], is excluded on the basis that the hydrazine nitrogen... 

The bond distances in the hydrazone group indicated a considerable TT-electron delocalization in the system because of the contribution of resonance structure, which are energetically preferred to bicyclic system 59. It was then concluded that a disruption of the furanoid ring follows the introduction of a hydrazone residue at C-2, which induces a rr-electron delocalization in the system. This was due to the sp2 hybridization at C-3, which precludes the necessary fourth valence for ring formation (82MI9). 

Reactions of cyclic and acyclic 1,2,3-triones with phenylhydrazine give rise to mono- and bis-(phenylhydrazones). The central carbonyl group of a vicinal tricarbonyl system is electron-deficient and highly electrophilic,255 which is why treatment with an aryldiazonium ion affords 2-phenylhydrazones. The structure of such bis(phenylhydrazones) as cyclopentane-1,2,3-trione 1,3-bis(phenylhydrazone), dehydroascorbic acid 2,3-bis(phenylhydrazone), and cy-clobutanetetraone l,3-bis(phenylhydrazone) has been studied by UV, IR,1H-, 13C-, and 15N-NMR spectroscopy.265-268 Quantum-mechanical calculations to predict the most stable tautomeric forms of some 1,2- and l,3-bis(phenylhydrazones) revealed that the chelated bis(hydrazone) structure was usually more stable than the azoene-hydrazine structure.269-271 This does not mean that such structures do not exist, for example cyclobutanetetraone l,2,3-tris(phenylhydrazone) exists in a stable phenylazoene-hydrazine structure (see 49 Scheme 8).257-263... 

Oxidation of the precursor hydrazones to the corresponding diazo compounds 10 and 12 can be carried out with active Mn02 but not with yellow HgO [8]. When photolyzed in 2-methyltetrahydrofuran (MTHF) glass at 10 K, they show EPR fine structures characteristic of unoriented triplet species. The zero-field splitting (zfs) parameter D/hc of a carbene is governed by the dipolar interaction between the unpaired electrons in the a- and n-orbitals at the carbenic center. The smaller D/hc value of thep isomer 11 than that of the m isomer 13 shows the extended delocalization of the n-spin in the former. The corresponding value diminishes further in the p-substituted 1,3-butadiyne 21 [8]. 

Cyclocondensation takes place on reacting 6-nitroquinoline with substituted hydrazones 69 (Scheme 34) in the presence of NaH in DMF, thus giving rise to 3-aryl-l(3)H-pyrazolo[3,4-f]quinolines 70 and/or 3-aryl[1.2.4]triazino[6,5-f] quinolines 71 [63, 64]. Yields are varied from low to moderate, while the direction of the reaction depends mainly on the structure of hydrazones electron-donating groups in the benzene ring of hydrazones favor the triazine ring formation. 

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