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

Abstract

The complexes [Pd(η 3-1-PhC 3H 4)(L–L′)] + [L–L′=2-(PPh 2)C 6H 4-1-CHNR (R=Me ( 1a), i-Pr ( 1b) , t-Bu ( 1c), ( R)-bornyl ( 1d), C 6H 4OMe-4 ( 1e), C 6H 3Me 2-2,6 ( 1f), C 6H 3(i-Pr) 2-2,6 ( 1g)), 6-MeC 5H 3N-2-CHNC 6H 4OMe-4 ( 2a), C 5H 4N-2-CHN-t-Bu ( 2b) and C 5H 4N-2-CH 2S-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 CH 2 allyl terminus, yielding [Pd(η 2-fn)(L–L′)] and the allylamines ( E)-PhCHCHCH 2Y. Under pseudo-first-order conditions the amination rates ( k obs) are found to depend on the k 2[HY] term for 2a and 3a, and on the sum k 2[HY]+ k 3[HY] 2 for the other complexes. The second-order term k 2 is related to direct nucleophilic attack on the CH 2 allyl terminus of the substrate whereas the third-order term k 3 is ascribed to parallel attack by a further HY molecule on the intermediate [Pd(1-PhC 3H 4)(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, the 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 CH 2 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 CH 2 allyl terminus with the 6-Me pyridine group in close proximity in the predominant isomer.