14-electron T-shaped [PdRXL] complexes: Evidence or illusion? Mechanistic consequences for the Stille reaction and related processes

摘要

A recent claimed spectroscopic observation (by H-1 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(AsPh3)(2)] (R =Ph, C6Cl2F3, X = Cl, I) in different solvents (S = CDCl3, THF, DMF) suggests that: 1) there is no NMR-detectable amount of [PdRX(AsPh3)], and 2) in the presence of free arsine (AsPh3/ [PdRX(AsPh3)(2)] 2:1) the concentration of [PdRX(AsPh3)(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 condition,, the dominant pathway is the associative reaction of the stannane with the square-planar complex [PdRX(AsPh3)(2)]. [References: 46]