Kinetics and mechanism of regioselective amination of the 1-phenylallyl group in cationic palladium(II) complexes bearing bidentate ligands

The complexes [Pd(η3-1-PhC3H4)(L–L′)]+ [L–L′=2-(PPh2)C6H4-1-CHNR (R=Me (1a), i-Pr (1b), t-Bu (1c), (R)-bornyl (1d), C6H4OMe-4 (1e), C6H3Me2-2,6 (1f), C6H3(i-Pr)2-2,6 (1g)), 6-MeC5H3N-2-CHNC6H4OMe-4 (2a), C5H4N-2-CHN-t-Bu (2b) and C5H4N-2-CH2S-t-Bu (3a)] are generally present in solution as two geometrical isomers, the relative abundance of which depends essentially on the steric requirements of the L–L′ ligand. In the presence of fumaronitrile the cationic complexes undergo a regioselective amination by secondary amines HY at the CH2 allyl terminus, yielding [Pd(η2-fn)(L–L′)] and the allylamines (E)-PhCHCHCH2Y. Under pseudo-first-order conditions the amination rates (kobs) are found to depend on the k2[HY] term for 2a and 3a, and on the sum k2[HY]+k3[HY]2 for the other complexes. The second-order term k2 is related to direct nucleophilic attack on the CH2 allyl terminus of the substrate whereas the third-order term k3 is ascribed to parallel attack by a further HY molecule on the intermediate [Pd(1-PhC3H4)(L–L′)(HY)]+. The k2 values depend on the steric and electronic properties of both the amine HY and the ligand L–L′. For complexes 1a–1g, the relatively higher k2 values and their increase with increasing steric crowding at the nitrogen-bonded carbon of substituent R are interpreted in terms of a greater reactivity of the isomer with the CH2 allyl terminus trans to phosphorus and cis to the NR group. The high amination rate of 2a, as compared with that of 2b, is related to substantial steric interaction of the CH2 allyl terminus with the 6-Me pyridine group in close proximity in the predominant isomer.