η1-Allylpalladium complexes with a tridentate PNP ligand with different phosphino groups

Abstract

The iminodiphosphine 2-(PPh₂)C₆H₄-1-CHNC₆H₄-2-(PPh₂) (P–N–P′) is used for the preparation of the complexes [Pd(η¹-CHR¹–CHCR²R³)(P–N–P′)]BF₄ [R¹ = R² = R³ = H: (1); R¹ = R² = Ph, R³ = H: (2); R¹ = R³ = H, R² = Ph: (3); R¹ = H, R² = R³ = Me: (4)]. The P–N–P′ tridentate coordination and the η¹-allyl bonding mode in the solid are confirmed by the X-ray structural analysis of 1. In solution, the complexes 1 and 2 undergo an η¹–η³–η¹ rearrangement at 298 K interconverting the bonding site of the allyl group. A five-coordinate structure with the phosphine ligands in the axial position is proposed for the η³-allyl intermediate. For the dynamic process, a ΔG^≠ value of 53.8 kJ mol⁻¹ is obtained from ¹H NMR data of 2. In 3 and 4, the allyl ligand is rigidly bound to the metal through the less substituted terminus, in line with the higher free energy content of the corresponding isomers: [Pd(η¹-CHPh–CHCH₂)(P–N–P′)]⁺ +48.78 kJ mol⁻¹; [Pd(η¹-CMe₂–CHCH₂)(P–N–P′)]⁺ +69.35 kJ mol⁻¹. The complexes react with secondary amines in the presence of fumaronitrile at different rates yielding allylamines and the palladium(0) derivative [Pd(η²-fn)(P–N–P′)] (5). On the basis of charge distribution on the allylic carbon atoms and of steric factors, the difference in rate and the regioselectivity in the amination of 1–3 are better rationalized by a mechanism with nucleophilic attack at the η³-intermediate rather than by an S_N2 mechanism with nucleophilic attack at the Pd–CHR¹ carbon atom. The high regioselectivity in the reaction of 4 with piperidine implies an S_N2′ mechanism with nucleophilic attack at the CMe₂ allyl carbon. A dynamic process occurs also for the 18-electron complex 5 consisting in a dissociation–association equilibrium of the olefin.