In-Mediated Conversion of 2-Alkynylphenyl Isothiocyanates to Quinoline-2-thiones

Significance

Reported is the indium(III)-promoted formation of quinoline-2-thiones via Friedel-Crafts reactions of 2-alkynylphenyl isothiocyanates with various aromatic systems. When aryl sulfides are used as the Friedel-Crafts reaction partners, nucleophilic addition to the triple bond occurs, forming a new C-S bond. A mechanism for this reaction is proposed involving activation of the triple bond by coordination with indium and subsequent nucleophilic attack by either the aromatic system or sulfur to give an alkenyl-indium intermediate (structures A and B above). These then undergo elimination of the t-Bu protecting group as isobutylene and a 6π-electrocyclization with the isothiocyanate unit to give the desired heterocycle after workup.