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(η³-allyl)(P−N)]⁺ (P−N = o-(PPh<INF>2</INF>)C<INF>6</INF>H<INF>4</INF>CH&dbd;NR (R = C<INF>6</INF>H<INF>4</INF>OMe-4, Me, CMe<INF>3</INF>, (R)-bornyl); allyl = propenyl (<BO>1a</BO>−<BO>4a</BO>) and 3-methyl-2-butenyl (<BO>1b</BO>−<BO>4b</BO>)) 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 <BO>1b</BO>−<BO>3b</BO>, two geometrical isomers are observed, the predominant one having the allyl CMe<INF>2</INF> group trans to phosphorus. The complexes <BO>4a</BO> and <BO>4b</BO>, containing the chiral (R)-bornyl group, are present in solution with two and four diastereomeric species, respectively. The X-ray structural analysis of <BO>4b</BO>(ClO<INF>4</INF>) shows the presence of two diastereomeric molecules in the unit cell, both having distorted-square-planar coordination geometries, characterized by rather elongated Pd−CMe<INF>2</INF> bonds trans to phosphorus and by a marked distortion of the allyl ligand, which is rotated away from the PPh<INF>2</INF> 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 k<INF>obs</INF> = k<INF>2</INF>[HY] + k<INF>3</INF>[HY]² for <BO>1a</BO>−<BO>4a</BO> and k<INF>obs</INF> = k<INF>2</INF>[HY] for <BO>1b</BO>, <BO>3b, </BO>and<BO> 4b</BO>. The k<INF>2</INF> term is related to direct bimolecular attack on a terminal allyl carbon of the substrate, whereas the k<INF>3</INF> term is ascribed to parallel attack by a further amine molecule on the intermediate [Pd(allyl)(P−N)(HY)]⁺. The k<INF>2</INF> 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.