Benzalazine, reduction

Whether reaction route A or B (or both) operates is not yet known because both 1,2-dihydrophthalazine (155) and the aldimine (156) are reduced at a less negative or approximately the same potential as 153. The reduction of 153 thus resembles the reduction of benzalazine. 

Benzalazine is in slightly acid solution reduced to benzylamine in a six-electron reduction, whereas it forms benzaldehyde benzylhydrazone in a two-electron reaction in alkaline solution [1]. Benzaldehyde benzylhydrazone is reduced in acid solution to benzylamine in a four-electron reaction at potentials slightly less negative than those of benzalazine, so it is not quite clear whether benzaldehyde benzylhydrazone is an inteimediate in acidic solution or whether there is initially a cleavage of the central nitrogen-nitrogen bond. 

In DMF, benzalazine (VII) shows two, one-electron steps in CV. Preparative reduction of VII in the presence of an alkylating agent yields alkylated bis-hydrazines (VIII) or the dialkylated hydrazone (IX) [51], as in Eq. (14). An assignment of the stereochemistry of the bis-hydrazones, obtained by reduction of VII followed by protonation, could be achieved by cyclization with aldehydes to imidazolidines. 

In some cases the radical anions of azines in DMF are stable enough to allow their electron spin resonance (esr) spectra to be obtained. The radical anion of fluorenone azine is thus stable for months in DMF [52,53]. In the absence of a proton donor, the radical anion is reduced to the dianion, which is stable on the time scale of CV. In the presence of a relatively strong proton donor, such as hexafluoro-2-propanol, fluorenone fluorenylhy-drazine is obtained [53] this is analogous to the reduction of benzalazine in aqueous alkaline solution [1]. 

The reduction of carbonyl derivatives as oximes and hydrazones is similar to the electrolytic reaction. Oximes are reduced to amines [59], whereas hydrazones in alkaline solution may form hydrazines benzalhydrazine can thus be reduced to benzylhydrazine [92], benzalazine to benzaldehyde benzylhydrazone, or further to dibenzylhydrazine [93,94], and benzaldehyde semicarbazone to benzylsemicarbazide [95]. 

Until recently only benzalazine [28, 106, 107] and naphthalazine [107] had been investigated. Lund found that in acidic medium benzalazine gave a six-electron wave for reduction to benzylamine, and this wave decreased at pH > 7.0 owing to kinetic control of the limiting current resulting in protonation ... 

Other investigators have also observed similar waves in buffered solutions of benzalazine [106]. With quaternary ammonium salts as supporting electrolyte in unbuffered solutions benzalazine and naphthalazine each form two waves of approximately equal height [106, 107], and according to the Ilkovic equation each wave corresponds to the addition of two electrons, i. e., reduction probably gives 1, 2-dibenzylhydrazine, like reduction with sodium amalgam in alkaline solution. 

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