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.