Palladium-Catalyzed Intramolecular Arylative Carboxylation of Allenes with CO2 for the Construction of 3-Substituted Indole-2-carboxylic Acids.

Arylative carboxylation of allenes proceeded in an intramolecular manner to afford the corresponding β,γ-unsaturated carboxylic acids in high yields using PdCl2/PAr3 (Ar = C6H4-p-CF3) and ZnEt2 under 1 atm of CO2. The intermediate of the cyclization/carboxylation sequence is thought to be a nucleophilic η1-allylethylpalladium, which reacts with CO2 at the γ-position of palladium. The products obtained could be efficiently converted into 3-substituted indole-2-carboxylate derivatives. One-pot synthesis of strychnocarpine, a β-carboline alkaloid, from the carboxylated product was also demonstrated.