Stereoretentive Elimination and Trans-olefination of the Dicationic Dipalladium Moiety [Pd2L2]2+ Bound on 1,3,5-Trienes.

The reaction of [Pd2(CH3CN)6][BF4]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-aIlyIdipaIladium 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,F)-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(PPh3)2(CH3CN)4][BF4]2 (2) or [Pd2(dppm)2(CH3CN)2][BF4]2 (6). During the PPh3-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(CH3CN)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 pairwise 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 Iigands for metal-metal moieties. [ABSTRACT FROM AUTHOR]