Nucleophilic Cyclization of o-Alkynylbenzamides Promoted by Iron(III) Chloride and Diorganyl Dichalcogenides: Synthesis of 4-Organochalcogenyl-1H-isochromen-1-imines

We have developed a method for the preparation 4-organochalcogenyl-1H-isochromen-1-imines from o-alkynylbenzamides. This electrophile-promoted nucleophilic cyclization achieved by treatment with a combination of iron salts and diorganyl dichalcogenides. FeCl3 and diorganyl dichalcogenides reacted with o-alkynylbenzamides under aerobic conditions at room temperature and in the absence of additives to give 4-organochalcogenyl-1H-isochromen-1-imines in good yields and with good selectivities. The reaction took place with 0.5 equiv. of the diorganyl dichalcogenides, which demonstrates that both halves of the diorganyl dichalcogenides (RYYR -> 2RY) were incorporated into the final product. Mechanistic studies suggested that the regioselectivity of the cyclization is governed by a seleniranium ion, a key intermediate formed by the electrophilic addition of the organochalcogen moiety to the carbon-carbon triple bond. The utility of the 4-organochalcogenyl-1H-isochromen-1-imines was also demonstrated by their application as starting materials in Suzuki and Sonogashira cross-coupling reactions.