14-Electron T-shaped [PdRXL] complexes: evidence or illusion? Mechanistic consequences for the stille reaction and related processes.

A recent claimed spectroscopic observation (by (1)H NMR) of 14-electron T-shaped 3-coordinated palladium complexes turns out to be a misinterpretation. A thorough study of the species formed by [PdRX(AsPh(3))(2)] (R=Ph, C(6)Cl(2)F(3); X=Cl, I) in different solvents (S=CDCl(3), THF, DMF) suggests that: 1) there is no NMR-detectable amount of [PdRX(AsPh(3))], and 2) in the presence of free arsine (AsPh(3)/[PdRX(AsPh(3))(2)] 2:1) the concentration of [PdRX(AsPh(3))(S)] is negligible. This clearly settles matters in the controversy of dissociative or associative pathways for the transmetalation step involved in the Stille coupling in favor of the latter: under catalytic conditions the dominant pathway is the associative reaction of the stannane with the square-planar complex [PdRX(AsPh(3))(2)].