Synthesis of <i>E</i>‑3-Alkenyl 2<i>H</i>‑Indazoles\nvia Pd/Cu-Catalyzed Cross-coupling/Cyclization\nof 2‑Iodoazoarenes with Terminal Allylenes and Visible-Light-Promoted\nIsomerization

We\nreport a straightforward one-pot synthesis of <i>E</i>-3-alkenylindazole\nscaffolds under mild reaction conditions,\ninvolving\nsuccessive processes of catalyzed coupling, cyclization, 1,2-hydrogen\nmigration, and <i>E</i>/<i>Z</i> isomerization.\nComplemented by theoretical calculations, the results rationalized\nthe reaction mechanism and elucidated that the low-energy light-cyclized\nintermediate <b>C-3a</b> (T<sup>1</sup>) is the main driving\nfactor for the rare <i>Z</i> → <i>E</i> photoisomerization. This synthetic approach would be an attractive\nand practical strategy for preparing diverse <i>E</i>-3-alkenyl\n2<i>H</i>-Indazoles, owing to its good substituent tolerance\nand high <i>E</i>/<i>Z</i> selectivity. Notably,\nmolecular engineering allows for the tuning of absorption and emission\nproperties across the visible spectrum by introducing various electron-donating\ngroups into specific styrene subunits, making them valuable dyes for\nphotonic or optoelectronic applications.