Stereoretentive elimination and trans-olefination of the dicationic dipalladium moiety [Pd2Ln]2+ bound on 1,3,5-trienes

Article abstract of DOI:10.1021/ja0576740

The reaction of [Pd₂(CH₃CN)₆][BF ₄]₂ (1) with 1,3,5-hexatriene, 1,6-diphenyl-1,3,5- hexatriene (DPHT), or 2,2,9,9-tetramethyl-3,5,7-decatriene (DBHT) afforded bi-η³-allyldipalladium complexes 3, 4, or 5. The reaction of 1 and DBHT proceeded in a stereospecific (syn) manner when the reaction was carried out in CD₂Cl₂ under aerobic conditions, while a mixture of two diastereomers was formed under N₂ atmosphere. The two diastereomers (5-E,Z,E-antifacial and 5-E,E,E-antifacial) formed from DBHT were isolated, and the structure of 5-E,Z,E-antifacial, which was kinetically formed from the reaction of 1 and (E,E,E)-DBHT, was determined by X-ray diffraction analysis. Addition of phosphine ligands (PPh₃ or dppm) to the dinuclear adduct 5-E,Z,E-antifacial or 5-E,E,E-antifacial in acetonitrile resulted in the stereospecific (syn) elimination of [Pd₂(PPh ₃)₂(CH₃CN)₄][BF₄] ₂ (2) or [Pd₂(dppm)₂(CH₃CN) ₂][BF₄]₂ (6). During the PPh ₃-induced dinuclear elimination, the phosphine adducts 7 that retain bi-η³-allyldipalladium structure were observed initially. The phosphine adduct generated from 5-E,E,E-antifacial was isolated and structurally characterized by X-ray diffraction analysis. The reaction of 1 and DPHT in CH₂Cl₂ afforded unique dipalladium sandwich compounds [Pd₂(μ-η³:η³-DPHT) ₂][BF₄]₂ (8). Interconversion between the sandwich complexes and half-sandwich complexes occurred in a stereoretentive manner. The structure of the sandwich complex 8-E,Z,E formed from 4-E,E,E-antifacial and (E,Z,E)-DPHT was determined by X-ray diffraction analysis. Transfer of the dipalladium moiety [Pd₂(CH ₃CN)₄]²⁺ from DPHT ligand of 4-E,E,E-antifacial onto DBHT ligand proceeded in a Stereoretentive manner. The observed stereoretentive dinuclear process is featured by the painwise behavior of two palladium atoms sitting on the triene π-plane. In the dinuclear elimination, the two Pd atoms that are initially in the divalent state and bound on the opposite faces (antifacial) come to the synfacial positions to form a Pd-Pd bond prior to dissociation. These results represent the unique property of conjugated olefin as the multidentate ligands for metal-metal moieties.