Five-Membered Ring Imidoyl Halides

In 1907 Tafel and Wassmuth treated pyrrolidone with phosphorus pentachloride and isolated a compound, m.p. 50-51°C, for which the cyclic imidoyl chloride structure XX has been postulated. 

The compound was crystallized from aqueous acetone, indicating either a rather low reactivity, or a different structure. 

The perchlorinated derivatives XXI and XXII are obtained upon reaction of dichloromaleic imide with phosphorus pentachloride ( ). 

While XXI can be hydrolyzed with warm water to afford the starting dichloromaleic imide, XXII is considerably more stable. 

Similarly, perfluorosuccinimide can be converted to 2,5,5-trichloro-3,3,4,4-tetrafluoro-l-pyrroline (XXIII), using phosphorus pentachloride, or, preferably, phenylphosphorus tetrachloride ( ). 

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The fully aromatic heterocyclic five-membered ring imidoyl halides have only a limited degree of imidoyl reactivity. However, their benzo derivatives are often considerably more reactive. For example, 2-chloro-benzthiazole (XXXI) reacts considerably faster than 2-chlorothiazole (XXXII) in nucleophilic substitution reactions ( ). 

In the cyclic series imidoyl halides are reported only for five-, six-, and seven-membered ring systems. However, it was demonstrated recently that 2,2-dichloroaziridines, which are obtained by addition of dichloro-carbene to C=N double bond compounds, react as iminium chlorides. The synthetic methods for the cyclic imidoyl halides are similar to those used for the preparation of the linear species. Therefore, emphasis is focused on the reactions of cyclic imidoyl halides rather than on their synthesis. The organization of this chapter also differs from that of the previous chapters because it is based on ring size rather than on the difference attributable to substituents attached to the C=N double bond system. 

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