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

Abstract

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.

Synthesis of Indenones via Persulfate Promoted Radical Alkylation/Cyclization of Biaryl Ynones with 1,4-Dihydropyridines

“…To date, the cyclization of alkynes toward the synthesis of cyclic compounds has been effectively investigated [ 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 ]. Recently, the radical cascade cyclization of biaryl ynones has been demonstrated to be a useful approach to construct six-membered spiro[5,5]trienones [ 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 ] with the development of different types of radical precursors ( Scheme 1 a–c). For example, Chen and Zhou reported an iron-catalyzed cascade silyl radical addition/6- exo -trig cyclization/dearomatization of biaryl ynones with silane, affording silylated spiro[5.5]trienones in good yields [ 35 ].…”

“…For example, Chen and Zhou reported an iron-catalyzed cascade silyl radical addition/6- exo -trig cyclization/dearomatization of biaryl ynones with silane, affording silylated spiro[5.5]trienones in good yields [ 35 ]. In addition, several groups have demonstrated some elegant examples based on C-, N-, P-, S-, Si-, Se-centered radicals-induced cascade cyclization of biaryl ynones in the presence of transition-metal catalysts, oxidants or photocatalysts [ 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 ]. For example, Duan and Yang reported the preparation of alkyl-functional spiro[5.5]trienone through alkylative dearomatization and the spirocyclization of biaryl ynones, respectively [ 29 , 33 ].…”