Effect of N1-substituted pyrazolic hybrid ligands on palladium catalysts for the Heck reaction

摘要

In this paper we have explored the influence of several linkers present on the [PdCl_2(L)] complexes, where L is 3,5-dimethylpyrazolic hybrid ligand N1-substituted by polyether chains and/or phenyl groups. These complexes have been used as pre-catalysts in the Heck reaction between phenyl halides and tert-butyl acrylate. The corresponding complexes efficiently catalyze the Heck olefination and provide good yields under phosphine-free conditions, even for aryl chlorides. Different reaction conditions were investigated and it was found that the nature of the ligand has an important influence on the effectiveness of the catalytic system. Ligand 1,8-bis(3,5-dimethyl-1H-pyrazol-1-yl)-3,6- dioxaoctane (L1), which has previously shown to be the most flexible and versatile, achieved high turnover numbers within very short reaction times and low catalyst loadings. Several Pd~(II) complexes with dimethylpyrazolic hybrid ligands efficiently catalyze the Heck olefination and provide good yields under phosphine-free conditions even for aryl chlorides.