Synthesis of E-3-Alkenyl 2H-Indazoles via Pd/Cu-Catalyzed Cross-coupling/Cyclization of 2-Iodoazoarenes with Terminal Allylenes and Visible-Light-Promoted Isomerization

We report a straightforward one-pot synthesis of E-3-alkenylindazole scaffolds under mild reaction conditions, involving successive processes of catalyzed coupling, cyclization, 1,2-hydrogen migration, and E/Z isomerization. Complemented by theoretical calculations, the results rationalized the reaction mechanism and elucidated that the low-energy light-cyclized intermediate C-3a (T1) is the main driving factor for the rare Z -> E photoisomerization. This synthetic approach would be an attractive and practical strategy for preparing diverse E-3-alkenyl 2H-Indazoles, owing to its good substituent tolerance and high E/Z selectivity. Notably, molecular engineering allows for the tuning of absorption and emission properties across the visible spectrum by introducing various electron-donating groups into specific styrene subunits, making them valuable dyes for photonic or optoelectronic applications.