Conversion aryl hydrazones

Table 6. Conversion of Aryl Hydrazones to Geminal Difluorides...

The heat profile shows that the reaction has zero order kinetics at first, and then switches to positive order kinetics. The benzophenone hydrazone reacts first only when it is completely consumed, the reaction involving hexylamine begins. Samples were taken and analyzed by and NMR. One sample was taken when the aryl halide conversion was low, at about 5%, and the profile was overall zero order the second when the profile had switched to positive order and the conversion of the halide was greater than 50%. 

The conversion of aryl hydrazones to indoles requires elevated temperatures and the addition of Br msted or Lewis acids. For example a milder conversion when iV-trifluoroacetyl enehydrazines are used as substrates. 

Watanabe reports a new method for the direct conversion of o-choroacetaldehyde N,N-disubstituted hydrazones into 1-aminoindole derivatives 93 by palladium-catalyzed intramolecular ring closure of 92 in the presence of P Bu3 or the bisferrocenyl ligand 94 <00AG(E)2501>. When X = Cl, this cyclizative process can be coupled with other Pd-catalyzed processes with nucleophilic reagents (e.g., amines, azoles, aryl boronic acids) so as to furnish indole derivatives with substituents on the carbocyclic ring. 

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. 

Since the isolation of reaction intermediate (32 equation 30) by Hurd and Mori there has been no further work to illustrate the mechanism for its formation, nor has there been any further work to unravel the mechanism for its conversion to product under the reaction conditions. Only recently for hydrazone precursor (33), where R = Me and R1 = alkyl or aryl groups, it has been reported that cyclization predominates at the more reactive methylene site rather than the methyl site (81CB2938). Other workers have shown that when sulfur dichloride (SC12) is substituted for thionyl chloride, 1,2,3-thiadiazoles are still formed from a-methylene hydrazones. In several instances sulfur dichloride afforded products in higher yield than did thionyl chloride (81G289). 

Hydrazine too reacts with activated aryl halides as readily as ammonia or amines. The important carbonyl reagent 2,4-dinitrophenylhydrazine is obtained in 80% yield from an aqueous-alcoholic solution of l-chloro-2,4-dinitrobenzene and hydrazine.555 Moreover, hydrazones can be arylated in the same way, which can be of interest when the 2,4-dinitrophenylhydrazones cannot be prepared with ease directly from the ketone. The conversion of acetone hydrazone into the 2,4-dinitrophenylhydrazone556 will be described here as a typical example ... 

C-Labelling showed that the thermal conversion of the phenyl trimethylsilyl diazo-compound (168) into (169) involved isomerization of a phenyl carbene into a cycloheptatrienylidene, silacyclopropanes were not involved. It was shown that the P-naphthyl carbene obtained by rearrangement of 4,5-benzocycloheptatrienylidene enters the singlet-triplet manifold as a singlet. Evidence has been reported that aryl carbenes can show both electrophilic and nucleophilic properties in their intramolecular ring expansion. Low-temperature photolysis of the sodium salt of the toluene-p-sulphonyl hydrazone (170) gave, by intramolecular carbene addition, the dibenzobicyclo[4,l,0]heptatriene (171) which was trapped by buta-l,3-diene. ... 

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