Palladacycles bearing COOH-/ester-functionalized N-heterocyclic carbenes: Divergent syntheses and catalytic applications

摘要

Two new [C^N]-type palladacyclic dinuclear complexes bearing carboxylate-containing N-heterocyclic carbenes (NHCs) were synthesized, and in both cases the carboxylato-NHC ligand adopts a bridging mode. Both complexes proved to be suitable precursors, which can be used to divergently access palladacycles bearing ester- or COOH-functionalized NHCs upon esterification or acidolysis. In the esterification reactions, alkyl halides are found to selectively react with the carboxylato moieties, and the palladacycle scaffold is retained even when excess haloalkane is employed. In the acidolysis reactions, the desired COOH-tethered complexes can only be obtained when stoichiometric acid (with respect to Pd) is used, while excess acid destroys the metallacycle scaffold. Finally, a preliminary catalytic study reveals the good performances of all newly synthesized complexes in direct aromatic CH functionalization reactions with alkynes. Poisoning experiments indicate that these hydroarylation reactions are likely to be homogeneously catalyzed.