The thermolysis of N-t-alkyl-, and N-aryl-(2,2-dichlorocyclopropyl)methyleneamines yielded directly 1-t-alkyl, and 1-aryl-3-chloropyrroles as the major products along with a slight amount of the 2-chloro isomers. The transformation to the 3-chloropyrroles was enhanced in polar solvents. But basic additives directed the thermal rearrangement into the 1,2-bond cleavage of the cyclopropane ring, leading to the 2-chloropyrroles. On the other hand, (difluorocyclopropyl)methyleneamines were pyrolyzed exclusively to 3-fluoropyrroles under 1,3-bond cleavage. The ionic mechanism involving a heterolytic dissociation of chlorine atom under 1,3-bond scission of the cyclopropane ring is proposed for the rearrangement to 3-chloropyrroles, while a homolytic cleavage pathway is proposed for 3-fluoropyrroles. The present thermolysis supplies a unique preparative tool for 2-, and 3-halo-4-phenylpyrrole derivatives.
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Thermolysis of A/-t-butyl-and N-cumyl-2,2-dichlorocyclopropylimines gives selectively 3-chloropyrroles in polar solvents and 2-chloro derivatives with added base.
Manipulated Thermal Rearrangement of N-t-Alkyl-2,2-dichlorocyclopropylimines to N-Alkylchloropyrroles.-Thermolysis of the dichloro(iminomethyl)cyclopropanes (I) in benzene results in elimination of hydrogen chloride and rearrangement, yielding the 3-chloropyrroles (II). Only in the case of (Ia), the regioisomer (IIIa) of (IIa) is obtained as a by-product. However, ( IIIa) is the main product, when the thermolysis is performed in refluxing phenetole in the presense of a base. A mechanism involving cationic acyclic intermediates is proposed. -(KAGABU, S.; KAWAI, I.; J. Chem.
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