Allylpalladium(II) Histidylidene Complexes and Their Application in Z-Selective Transfer Semihydrogenation of Alkynes

We have studied the use of amino acid histidine as a precursor for N-heterocyclic carbene (NHC) ligands. This natural amino acid possesses an imidazole substituent, which makes it an interesting NHC precursor that contains both an acid and an amino functionality. These functionalities may be used for further tuning of NHC complexes. We have developed routes for the synthesis of symmetric and dissymmetric alkyl, benzyl, and aryl-substituted histidinium salts. Subsequently, the corresponding Ag and Pd histidylidenes were synthesized and the palladium complexes were tested in the Z-selective transfer semihydrogenation of alkynes. Histidylidene palladium complexes that contain additional donor functionalities were found to display good selectivities. The best catalytic results were obtained with a Pd-histidylidene complex that contains two picolyl functional groups.