Metal-free access to 4-indolyl coumarin from coumarin 3-carboxylic acid and indole <i>via</i> a Michael addition-decarboxylation and dehydrogenation protocol: a photo-physical study and utility as an invisible ink

Herein, on water tandem Michael addition-decarboxylation of coumarin-3-carboxylic acids with indoles under catalysis of Amberlite IR120 afforded 4-indolyl 3,4-dihydrocoumarin. The process was extended to the synthesis of 4-indolyl coumarin derivatives upon oxidation using an organic oxidant. The wide substrate scope, high product yields, catalytic recyclability and large-scale preparation of intermediates are the noteworthy features of this methodology. This two-step process provides better yields of 4-indole-substituted coumarins as compared to the corresponding single-step palladium-catalyzed cross-coupling reactions. We also report herein for the first time that the indole-coumarin hybrid can be used as a thermal- and moisture-resistant invisible ink.