Chemistry of C‐heteroarylnitrilimines. Synthesis and cycloaddition reactions of N‐phenyl‐C‐(2‐thienyl)nitrilimine

The nitrilimine 7, generated in situ from N‐phenyl‐C‐(2‐thienyl)formohydrazidoyl chloride 5 in chloroform in the presence of triethylamine, reacts with acrylonitrile, acrylamide and arylidene acetophenones to give exclusively the 5‐substituted 2‐pyrazoline derivatives 9, 10 and 16, respectively. On the other hand, the cycloaddition of 7 to coumarin, benzalmalononitrile and the enolate anions of active methylene compounds yield the pyrazole derivatives 12, 14 and 23, respectively. The structures of the cycloadducts prepared were attributed on the basis of spectroscopic and chemical evidence. The regioselectivity in the studied reactions is discussed in terms of the frontier orbital theory.