Solution Behavior and X-ray Structure of Cationic Allylpalladium(II) Complexes with Iminophosphine Ligands. Kinetics and Mechanism of Allyl Amination by Secondary Amines

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Abstract

The solution behavior of the cationic complexes [Pd(η3-allyl)(P−N)]+ (P−N = o-(PPh2)C6H4CHNR (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 η3−η1−η3 interconversion which brings about a syn−anti exchange only for the allylic protons cis to phosphorus; (iii) a slower apparent rotation of the η3-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(η3-allyl)(P−N)]+ react with secondary amines HY in the presence of fumaronitrile, yielding [Pd(η2-fn)(P−N)] and allylamines. Under pseudo-first-order conditions the amination rates obey the laws kobs = k2[HY] + k3[HY]2 for 1a−4a and kobs = k2[HY] for 1b, 3b, and 4b. The k2 term is related to direct bimolecular attack on a terminal allyl carbon of the substrate, whereas the k3 term is ascribed to parallel attack by a further amine molecule on the intermediate [Pd(allyl)(P−N)(HY)]+. The k2 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(η3-allyl)(P−N)]+, compared to [Pd(η3-allyl)(α-diimine)]+, are essentially due to lack of displacement equilibria of the P−N ligand by amines.

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