Solution behavior and X-ray structure of cationic allylpalladium(II) complexes with iminophosphine ligands. Kinetics and mechanism of allyl amination by secondary amines

The solution behavior of the cationic complexes [Pd(eta(3)-allyl)(P-N)](+) (P-N = o-(PPh2)C6H4CH=NR (R = C6H4OMe-4, Me, CMe3, (R)-bornyl); allyl = propenyl (1a-4a) and 3-methyl-2-butenyl (1b-4b)) consists essentially of three dynamic processes: (i) a very fast conformational change of the P-N chelate ring, which moves above and below the P-Pd-N coordination plane, (ii) a relatively fast eta(3)-eta(1)-eta(3) interconversion which brings about a syn-anti exchange only for the allylic protons cis to phosphorus; (iii) a slower apparent rotation of the eta(3)-allyl ligand around its bond axis. For 1b-3b, two geometrical isomers are observed, the predominant one having the allyl CMe2 group trans to phosphorus. The complexes 4a and 4b, containing the chiral(R)-bornyl group, are present in solution with two and four diastereomeric species, respectively. The X-ray structural analysis of 4b(ClO4) shows the presence of two diastereomeric molecules in the unit cell, both having distorted-square-planar coordination geometries, characterized by rather elongated Pd-CMe2 bonds trans to phosphorus and by a marked distortion of the allyl ligand, which is rotated away from the PPh2 group. The complexes [Pd(eta(3)-allyl)(P-N)](+) react with secondary amines HY in the presence of fumaronitrile, yielding [Pd(eta(2)-fn)(P-N)] and allylamines. Under pseudo-first-order conditions the amination rates obey the laws k(obs) = k(2)[HY] + k(3)[HY](2) for 1a-4a and K-obs = k(2)[HY] for 1b, 3b, and 4b. The k(2) term is related to direct bimolecular attack on a terminal allyl carbon of the substrate, whereas the k(3) term is ascribed to parallel attack by a further amine molecule on the intermediate [Pd(allyl)(P-N)(HY)](+). The k(2) values increase with increasing basicity and decreasing steric hindrance of the amine, and with increasing electron-withdrawing ability and increasing bulkiness of the P-N nitrogen substituent. The higher amination rates for [Pd(eta(3)-allyl)(P-N)](+), compared to [Pd(eta(3)-allyl)(alpha-diimine)](+), are essentially due to lack of displacement equilibria of the P-N ligand by amines.