Single and Multiple Insertion of Alkynes into Pd−Acyl and Pd−Aryl Bonds in Cationic Palladium Complexes with Phosphine−Imine (P∼N) Ligands

Neutral and cationic Pd(II)−alkyl and Pd(II)−aryl complexes with phosphine−imine (P∼N)
ligands show unusual reactivity toward alkynes. No insertion of alkyne has been observed
into the neutral palladium complexes or the cationic Pd−alkyl complex. On the other hand,
smooth insertion of alkynes into the cationic complexes [(P∼N)Pd(COMe)(MeCN)]+ (4) and
[(P∼N)Pd(Ph)(MeCN)]+ (5) was observed, resulting in the formation of single- and double-insertion products, respectively. All the inserted products were isolated and characterized
by spectroscopic methods. Single-crystal X-ray analyses for some alkyne insertion complexes
indicate that the products are stabilized by intramolecular coordination via either a carbonyl
oxygen or a π-phenyl coordination with η2 mode. Higher order insertions of ethyl propiolate
in the complexes [(P∼N)Pd(Ph)(MeCN)]+ (5) and [(P∼N)Pd(C(Ph)C(Ph)C(Ph)CPh2)(MeCN)]+ (13) leading to the oligomeric species are found to proceed smoothly, but
disubstituted alkynes such as diphenylacetylene do not undergo such insertion.