Methanol as a Hydrogen Source in the Pd-II-Bis(N-Heterocyclic Carbene)-Mediated Catalytic Semireduction of Alkynes Under Mild Reaction Conditions

摘要

A series of six new mono-organopalladium complexes of the form [LPdX2] (L=1,2-bis[(N-methylimidazol-2-ylidene)methyl]ethane (6), 1,2-bis[(N-vinylimidazol-2-ylidene)methyl]ethane (7), 1,2-bis[(N-isopropylimidazol-2-ylidene)methyl]ethane (8), 1,2-bis[(N-2,6-dimethylphenylimidazol-2-ylidene)methyl]ethane (9), 1,2-bis[(N-4-ethyl benzoate imidazol-2-ylidene)methyl]ethane (10), 4,5-bis[(N-isopropylimidazol-2-ylidene)methyl]acridine (11); X=Br for complexes 6-9 and 11, X=Cl for complex 10) were synthesized and characterized by using FTIR spectroscopy, H-1 and (CNMR)-C-13 spectroscopy, elemental analysis, and X-ray crystallography. In the solid state, complexes 6-11 were of neutral charge and each contained one bis(NHC) ligand (L) and two halide ligands in an almost-ideal square-planar geometry. These molecules were catalytically competent for the semihydrogenation of diphenylacetylene. Notably, MeOH was used as a hydrogen source in catalytic processes for the first time. Among these six new catalysts, the acridine-bridged palladium-bis(N-heterocyclic carbene) complex gave excellent selectivity for the formation of Zalkenes from internal alkynes. A range of 15 different alkynes were tested under our optimized conditions and the reactions all proceeded with very high diastereoselectivities under mild conditions.