Ring transformations in reactions of heterocyclic halogeno compounds with nucleophiles (XXX) Reaction of chlorophenyl- and chlorodiphenyl-pyrazines with potassium amide in liquid ammonia

While treatment of 2-chloro-3-phenylpyrazine with potassium amide in liquid ammonia yields only 2-amino-3-phenylpyrazine, from 2-chloro-6-phenylpyrazine on the contrary two ring contraction products, 4(5)-phenylimidazole and 2-cyano-4(5)-phenylimidazole are obtained. From 2-chloro-5-phenylpyrazine a mixture of 2-amino-5-phenylpyrazine, 4(5)-phenylimidazole as well as 2-cyano-4(5)-phenylimidazole is formed. In the diphenyl series it has been established that, whereas 2-chloro-3,5-diphenylpyrazine gives only the corresponding 2-aminopyrazine, 2-chloro- 5,6-diphenylpyrazine gives a mixture of the amino product and two ring contraction products: 4,5-diphenylimidazole and 2-cyano-4,5-diphenylimidazole.2-Chloro-3,6-diphenylpyrazine was found to be relatively inactive towards potassium amide in liquid ammonia. This divergent behaviour is explained by the high susceptibility of the positions 3 and 6 of the 2-chloropyrazine for a nucleophilic attack as established by 14C- and 15N-labelling studies which were made previously.