Hydrazones, cyclization

Several functional groups in addition to carbon-carbon double and triple bonds can participate in radical cyclizations. Among these are oxime ethers, imines, and hydrazones. Cyclization at these functional groups leads to amino-substituted products. 

Semicarbazones of /J-nitrocarbonyl compounds have been coupled with aryl diazonium salts and the resulting hydrazones cyclized by acid to the 3-nitropyrazoles 43 (Scheme 9).36... 

Hydrazone cyclization and hydroalkylation [138-140] are rare examples of reactions conducted on a preparative scale, since the products were isolated in milligram amounts and not just identified in solution. As already mentioned in Section 6.2.5, photocorrosion of the semiconductor photocatalyst often prevents its use in preparative chemistry. This is very true also for colloidal semiconductors although the pseudo-homogeneous nature of their solutions allows one to conduct classical mechanistic investigations, until now they were too labile to be used in preparative chemistry [107, 141, 142]. In contrast to the above-mentioned reactions, in recent years we have isolated novel compounds on a gram-scale employing photostable zinc and cadmium sulfide powders as photocatalysts [97, 107, 143-145]. During this work we found also a new reaction type which was classified as semiconductor photocatalysis type B [45]. In contrast to type A reactions, where at least one oxidized and one reduced product is formed, type B reactions afford only one unique product, i.e., the semiconductor catalyzes a photoaddition reaction (see below). 

Numerous a, -unsaturated carbonyl compounds give anomalous reduction with hydrazine, because their hydrazones cyclize to 4,5-dihydro-l//-pyrazoles. The latter are pyrolyzed, usually in the presence of a basic catalyst, sometimes additionally using a platinum catalyst. Cyclopropanes are formed with extrusion of nitrogen. 

By the cyclization of hydrazones. A common characteristic of hydrazone cyclizations is the requirement of high temperatures when no solvents are used. Under these conditions, heating 3-[(Z)-2-(f-butyl-substituted silyl)hydrazono]butanoates 71a-c at 200 °C initially formed pyrazol-3-ones 72a-c which were not stable at that temperature but underwent a l,3-N,0-silyl group migration to the O-silylpyrazoles 73a-c (99JOM341) (Scheme 18). 

Recently, l-aryl-l//-indazoles have been prepared via palladium-catalyzed intramolecular amination of aryl halides. For example, the bromo-aryl-hydrazone cyclized to give the corresponding indazole under the influence of a palladium catalyst in the presence of a base. 

A novel cyclization of trifluoroacetylated hydrazones, prepared by TFAA treatment of -dialkyl aldehyde hydrazones, was reported by Hojo for the synthesis of 5-trifluoromethylimidazoles (Scheme 36) [50]. Formation of the 4-(trifluoro-methyl) isomers was observed in some cases along with the major 5-(trifluoro-methyl)imidazole products, presumably due to isomerization of trifluoroacetylated hydrazones (Table 2). This isomerization is suggested to be enhanced in the presence of silica gel and the isomeric trifluoroacetylated hydrazones cyclized to give the 4-CF3-imidazoles in refluxing toluene (c.f. Scheme 35). 

---