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14-Electron T-Shaped [PdRXL] Complexes: Evidence or Illusion? Mechanistic Consequences for the Stille Reaction and Related Processes

2002; Wiley; Volume: 8; Issue: 21 Linguagem: Inglês

10.1002/1521-3765(20021104)8

1521-3765

Juan Á. Casares, Pablo Espinet, Gorka Salas,

Cyclopropane Reaction Mechanisms

A variant of associative ligand substitution in Pd, which operates in 1, is observed in the transmetalation step under the conditions of the Pd-catalyzed coupling of organic halides or triflates and stannanes (Stille reaction). Dissociative transmetalation pathways can be discounted. Genuine 14-electron intermediates (C) are still to be detected spectroscopically, and their contribution to transmetalation in the Stille reaction, as well as that of solvent complexes E under the usual conditions, is negligible. A recent claimed spectroscopic observation (by 1H 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 conditions the dominant pathway is the associative reaction of the stannane with the square-planar complex [PdRX(AsPh3)2]. La observación espectroscópica (por 1H RMN) de complejos de paladio tricoordinados de 14 electrones anunciada en un reciente artículo resulta ser un error de interpretación. Un estudio completo de las especies formadas por [PdRX(AsPh3)2] R=Ph, C6Cl2F3; X=Cl, I) en distintos disolventes (S=CDCl3, THF, DMF) lleva a las siguientes conclusiones: 1) No hay trazas detectables por RMN de [PdRX(AsPh3)]; 2) en presencia de arsina libre (AsPh3:[PdRX(AsPh3)2]=2:1) la concentración de [PdRX(AsPh3)(S)] es despreciable. Esto define claramente la cuestión en la controversia entre vías disociativas o asociativas para la etapa de transmetalación implicada en el acoplamiento de Stille, en favor de las últimas: En las condiciones catalíticas el camino dominante es la reacción asociativa del estannano con el complejo plano-cuadrado [PdRX(AsPh3)2].