Synthesis of Benzimidazole‐Fused Medium‐Sized N,S‐Heterocycles via Palladium‐Catalyzed Cyclizations.

The synthesis of unprecedented benzimidazole‐fused thiazocines, thiazonines, and thiazecines having an exocyclic double bond is reported. The process proceeds through an 8‐, 9‐, or 10‐exo‐dig cyclocarbopalladation followed by reduction. The scope and limitations were established and showed the importance of the precursor structure for the success of the reaction. A competition between the exo‐dig and the endo‐dig cyclization was observed for two substrates bearing an N‐homopropargyl chain, the endo‐cyclization leading to rarely encountered 10‐ and 11‐membered N,S‐heterocycles with a trans‐endocyclic double bond. X‐ray structures of three products were obtained by co‐crystallization with fumaric acid and show the twisted structures of these molecules. Benzimidazole‐fused medium‐sized N,S‐heterocycles (8 to 10 ring atoms) having an exocyclic double bond were synthesized by using exo‐dig Pd‐catalyzed reductive cyclizations. In the case of two substrates bearing an N‐homopropargyl chain, a competing endo‐dig cyclocarbopalladation occurred, leading to rarely encountered 10‐ and 11‐membered N,S‐heterocycles with an endocyclic double bond. [ABSTRACT FROM AUTHOR]