Reactions of Electron-Rich Arylpalladium Complexes with Olefins. Origin of the Chelate Effect in Vinylation Catalysis

Reaction of (dippp)Pd(Ph)Cl (1) with norbomene or styrene yields (dippp)PdCl₂ (8) and (dippp)Pd(η²-olefin). Kinetic follow-up reveals fast formation of (dippp)Pd(phenylnorbornyl)-Cl (10), followed by its slow decomposition, with k_insertion = 0.50 × 10^-3 L mol^-1 s^-1 and k_{decomposition} = 0.90 × 10^-4 s^-1. Phenylnorbomane and (with styrene) stilbenes are also formed. Faster reaction is observed with (dppp)Pd(Ph)Br (2) and faster still with (dippe)Pd(Cl (4) to yield, in the latter case, the stable (dippe)Pd(phenylnorbomyl)Cl (18). The rates of these reactions are strongly solvent dependent (DMF ≫ dioxane), are strongly retarded by added Cl^-, and are unaffected by added phosphine, indicating that halide dissociation, followed by olefin coordination and rate-determining olefin insertion, are involved. In contrast, reaction of trans-(PⁱPr₂ⁿBu)₂Pd(Ph)X (X = Cl, 5; X = Br, 6) with norbomene (or styrene) involves phosphine dissociation and leads to formation of (PⁱPr₂ⁿBu)₂Pd(H)X. In the case of norbomene, β-carbon elimination of the unobserved intermediate phenylnorbomyl complexes followed by β-H elimination yields 1-methylene-2-phenylcyclohexenes. Complexes of the ligand dippb are unique in that both η¹ and η² coordination modes are easily accessible. While reaction products are similar to those obtained with dippp and dippe complexes, dependence of the reaction rate on reaction variables is intermediate between those observed for complexes of chelating and monodentate phosphines. The implications of these findings on catalysis are outlined.