Insertion of olefins into palladium(Ii)-acyl bonds. mechanistic and structural studies

The reaction of norbornylene with [Pd(PPh₃)₂(CH₃CN)(COR)]BF4 (2) produced [Pd(PPh₃)₂-(C₇H₁₀COR)](BF4) (3). The crystal structure of 3a (R = Me) revealed a square-planar coordination around the palladium atom. The two triphenylphosphine ligands occupied eis positions, and the 2-acetylnorborn-l-yl residue acted as a chelating ligand by coordinating through the norbornyl carbon and the carbonyl oxygen. The two palladium-phosphorus distances, 2.238 (2) and 2.434 (2) Â, were dramatically different. Crys-tallographic data: space group Ρ2_ϊ/η; α = 11.430 (10), b = 23.042 (3), c = 16.338 (3) Â; β = 99.90 (2)°; V = 4238.7 A³; Ζ = 4; R = 0.0673, R_w = 0.0894. The reaction mechanism, determined by studying the analogous reaction of 2 with norbornadiene, involved insertion from a four-coordinate intermediate formed by olefin displacement of the acetonitrile ligand. The reaction of Pd(PPh₃)₂(Cl)(COR) (1) with norbornylene produced Pd(PPh₃)(Cl)(C₇H₁₀COR) (6). For this reaction a mechanism involving insertion from a four-coordinate intermediate formed by olefin displacement of a triphenylphosphine ligand was consistent with the experimental results. The reaction of 1 with olefin was significantly accelerated in the presence of a “phosphine sponge” (e.g., Pd(PhCN)₂Cl₂, CH₃I, or S).