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

The complexes [Pd(eta (3)-1-PhC3H4)(L-L')(=) [L-L' = 2-(PPh2)C6H4-1-CH=N (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 solutio n as two geometrical isomers, the relative abundance of which depends essen tially on the steric requirements of the L-L' ligand. In the presence of fu maronitrile the cationic complexes undergo a regioselective amination by se condary amines HY at the CH, allyl terminus, yielding [Pd(eta (2)-fn)(L-L') ] and the allylamines (E)-PhCH=CHCH2Y. Under pseudo-first-order conditions the amination rates (k(obs)) are found to depend on the k(2)[HY] term for 2 a and 3a, and on the sum k(2)[HY] + k(3)[HY](2) for the other complexes. Th e second-order term k(2) is related to direct nucleophilic attack on the CH , allyl terminus of the substrate whereas the third-order term k(3) is ascr ibed to parallel attack by a further HY molecule on the intermediate [Pd(1- PhC3H4)(L-L')(HY)](+). The k(2) values depend on the steric and electronic properties of both the amine HY and the ligand L-L'. For complexes 1a-1g, t he relatively higher k(2) 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 tra ns 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 t he predominant isomer. (C) 2001 Elsevier Science B.V. All rights reserved.