1402556-62-0Relevant articles and documents
η1-Allylpalladium complexes with a tridentate PNP ligand with different phosphino groups
Crociani, Bruno,Antonaroli, Simonetta,Paoli, Paola,Rossi, Patrizia
, p. 12490 - 12500,11 (2012/12/13)
The iminodiphosphine 2-(PPh2)C6H4-1- CHNC6H4-2-(PPh2) (P-N-P′) is used for the preparation of the complexes [Pd(η1-CHR1-CHCR 2R3)(P-N-P′)]BF4 [R1 = R 2 = 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-CHCH 2)(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 SN2 mechanism with nucleophilic attack at the Pd-CHR1 carbon atom. The high regioselectivity in the reaction of 4 with piperidine implies an SN2′ 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.
