An iodine-catalyzed sustainable, cost-effective, and atom-economic synthetic methodology is developed to synthesize a wide variety of valuable sulfenyl-phenanthrenes and polycyclic heteroaromatics, i.e., naphthothiophenes in moderate to high yield through electrophilic thiolative annulation of 2-alkynyl biaryls (6-<i>endo</i>-dig cyclization) using inexpensive, easy-to-handle, and commercially available methyl sulfoxides such as dimethyl sulfoxide (DMSO) as the sulfur-source, under transition-metal-free conditions. The transformation requires only a couple of inexpensive reagents, iodine in catalytic amount and trifluoroacetic anhydride (TFAA). A wide variety of functional groups were also tolerated. Mechanistic studies revealed the <i>in situ</i> decomposition of DMSO by TFAA to formaldehyde and methanethiol, which participates in the iodine-catalyzed methylthiolative annulation reaction with 2-alkynyl biaryls under aerobic atmosphere. Notably, DMSO played multiple roles such as the methylthiolating reagent, oxidant, and solvent in this reaction. The synthetic utility of the synthesized product, i.e., 10-phenyl-9-sulfenylphenanthrene was demonstrated through its selective and sustainable oxidation to other new but potential molecules, 9-(methylsulfinyl)-10-phenylphenanthrene and 9-(methylsulfonyl)-10-phenylphenanthrene.

Iodine catalyzed methylthiolative annulation of 2-alkynylbiaryls with DMSO: A sustainable approach to 9-sulfenylphenanthrenes and polycyclic heteroaromatics

ACS Fall 2021

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