text: In the presence of palladium acetate under 1-30 bar of CO, simple aromatic compounds convert to aromatic carboxylic acids. A PSiP-pincer ligand (5) promotes carboxylation of allene without using pre-functionalized substrates. Catalyst regeneration, EtAl was added to do transmetallation with palladium. Catalyst is regenerated by the following β-H elimination. Apart from terminal allenes, some of internal allenes are also tolerated in this reaction, generating allyl carboxylic acid with the yield between 54% and 95%. This system was also applied to 1,3-diene, generating carboxylic acid in 1,2 addition fashion. In 2015, Iwasawa et al. reported the germanium analogue (6) and combined CO source together with hydride source to formate salts.
   Palladium has shown huge power to catalyze C-H functionalization. If the Pd-C intermediate in carboxylation reaction comes from C-H activation, such methodology must promote metal catalyzed carboxylation to a much higher level in utility. Iwasawa and co-workers reported direct carboxylation by styrenyl C-H activation generating coumarin derivatives. Benzene rings with different electronic properties and some heteroaromatic rings are tolerated in this reaction with yield from 50% to 90%. C-H activation was demonstrated by crystallography study.