Photoredox-Catalyzed Thiocyanative Cyclization of Biaryl Ynones to Thiocyanated Spiro[5.5]trienones: An External-Oxidant- and Transition-Metal-Free Approach

A photoredox-catalyzed, efficient protocol for the direct insertion of a -SCN group onto spiro[5.5]trienone via a dearomative cascade cyclization of biaryl ynone with inexpensive NH4SCN has been developed, employing 4CzIPN as a potent photocatalyst under blue-light irradiation without external oxidant. This scalable 6-exo-trig cyclization led to the cascade formation of C-C and C-S bonds and incorporate diverse thiocyanated spiro compounds with excellent yield (up to 92 %). Additionally, mechanistic investigations with fluorescence quenching, cyclic voltammetry, and radical scavenging are presented. Gram-scale synthesis and further functionalization of thiocyanated compounds highlight the potential utility of this methodology.