REACTIONS OF ELECTRON-RICH ARYLPALLADIUM COMPLEXES WITH OLEFINS - ORIGIN OF THE CHELATE EFFECT IN VINYLATION CATALYSIS

Reaction of (dippp)Pd(Ph)Cl(1) with norbornene or styrene yields (dippp)PdCl2 (8) and (dippp)Pd(eta(2)-olefin). Kinetic follow-up reveals fast formation of (dippp)Pd(phenylnorbornyl)Cl (10), followed by its slow decomposition, with k(insertion) = 0.50 x 10(-3) L mol(-1) s(-1) and k(decomposition) = 0.90 X 10(-4) S-1. Phenylnorbornane 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(phenylnorbornyl)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-(P(i)Pr(2)(n)Bu)(2)Pd(Ph)X (X = Cl, 5; X = Br, 6) with norbornene (or styrene) involves phosphine dissociation and leads to formation of (P(i)Pr(2)(n)Bu)(2)Pd(H)X. In the case of norbornene, beta-carbon elimination of the unobserved intermediate phenylnorbornyl complexes followed by beta-H elimination yields 1-methylene-2-phenylcyclohexenes. Complexes of the ligand dippb are unique in that both eta(1) and eta(2) 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.