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.