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Murahashi T, Nakashima H, Nagai T, Mino Y, Okuno T, Jalil MA, Kurosawa H: Stereoretentive elimination and trans-olefination of the dicationic dipalladium moiety [Pd2Ln] 2+ bound on 1,3,5-trienes. J Am Chem Soc. 2006 Apr 5;128(13):4377-88.
The reaction of [Pd (2)(CH (3) CN)(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-eta (3)-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 CD (2) Cl (2) under aerobic conditions, while a mixture of two diastereomers was formed under N (2) 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 (PPh (3) 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 [Pd (2)(PPh (3))(2)(CH (3) CN)(4)][BF (4)](2) (2) or [Pd (2)(dppm)(2)(CH (3) CN)(2)][BF (4)](2) (6). During the PPh (3)-induced dinuclear elimination, the phosphine adducts 7 that retain bi-eta (3)-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 CH (2) Cl (2) afforded unique dipalladium sandwich compounds [Pd (2)(mu-eta (3):eta (3)-DPHT)(2)][BF (4)](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 [Pd (2)(CH (3) CN)(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 pi-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. |
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