Palladium(II)-Catalyzed Direct Carboxylation of Alkenyl C–H Bonds with CO₂

Pd-catalyzed direct carboxylation of alkenyl C–H bonds with carbon dioxide was realized for the first time. Treatment of 2-hydroxystyrenes and a catalytic amount of Pd­(OAc)₂ with Cs₂CO₃ under atmospheric pressure of CO₂ afforded corresponding coumarins in good yield. Furthermore, isolation of the key alkenylpalladium intermediate via C–H bond cleavage was achieved. The reaction was proposed to undergo reversible nucleophilic addition of the alkenylpalladium intermediate to CO₂.