PCN‐ and PCS‐Pincer Palladium Complexes as Tandem Catalysts in Homoallylation Reactions

Abstract

AbstractNovel PCN‐ and PCS‐pincer palladium complexes 2‐(dimethylamino)methyl‐5‐methoxy‐6‐(diphenylphosphinoxy)phenylpalladium(II) bromide (1), 2‐(phenylimino)‐5‐methoxy‐6‐(diphenylphosphinoxy)phenylpalladium(II) bromide (2), 2‐(phenylthiomethyl)‐5‐methoxy‐6‐(diphenylphosphinoxy)phenylpalladium(II) bromide (3), 2‐(phenylthio)methyl‐5‐methoxy‐6‐(diphenylphosphinoxy)phenylpalladium(II) chloride (4) have been synthesized (55–95% yield) by using a flexible and straightforward synthetic route starting from isovanillin as the common precursor. The structures of complexes 1, 2 and, 4 in the solid state were determined using X‐ray diffraction and showed a typical pincer‐type geometry. The catalytic activities of 1–3 in the homoallylation reactions of aldehydes and allyl(tributyl)stannane as well as of their corresponding cationic complexes 1a–3a in the tandem reaction of aldehydes or sulfonimines with allyl chlorides and hexamethyldistannane were investigated. It was found that the catalytic activities are very dependent on the combination of the E‐donor moieties in the pincer ligand. Generally, PCS‐pincer complex 3 and its cationic complex 3a outperform PCN‐pincer complexes 1 and 2 as well as their cationic complexes 1a and 2a in both the homoallylation and tandem reaction. PCS‐pincer palladium complexes 3 and 3a seem to benefit in a positive sense from the combination and cooperativity of a π‐accepting phosphorus donor and a σ‐donating sulfur donor.