Aromatic PCN pincer palladium complexes: forming and breaking C[sbnd]C bonds

Abstract

Through a salt metathesis reaction, (t−BuPCN)Pd-ONO2 (2) was prepared and used as a precursor for producing (t−BuPCN)Pd-OH (3) and (t−BuPCN)Pd-aryl acetylide complexes 4 (phenyl acetylide) and 5 (p-tolyl acetylide). The aryl acetylide complexes could also be prepared through another synthetic route: by condensation of 3 with the corresponding aryl acetylene. The reactivity of complexes 3 and 4 toward carbon dioxide was studied and it was found that both reactions give the hydrogen carbonate complex (6). The low reactivity of the Pd-acetylide bond was further confirmed by the fact that the propiolate complex undergoes decarboxylation to give 4. PCN palladium complexes are good catalysts for the decarboxylative cross coupling reactions between acetylene carboxylic acids and aryl halides. The yield of the cross coupling product was improved by adding a catalytic amount of CuI.