Kinetics and mechanisms of methyl vinyl ketone hydroalkoxylation catalyzed by palladium(II) complexes

Palladium(II) coordination complexes such as (CH3CN)(2)PdCl2, 1, catalyze the addition of alcohols to vinyl ketones to produce ethers. During the catalytic cycle, the alcohol adds selectively to the beta -carbon (anti-Markovnikov). The kinetics for the reaction of benzyl alcohol (BA) with methyl vinyl ketone (MVK) as catalyzed by I has been investigated in detail. The experimental rate law is first-order in catalyst and BA and features saturation kinetics in MVK. Acetonitrile is a competitive inhibitor for MVK. The most consistent mechanism with the experimental findings involves substitution of an acetonitrile ligand by MVK in a preequilibrium step (K-1 = 0.020 +/- 0.004 in CDCl3 at 25 degreesC) followed by nucleophilic attack of benzyl alcohol (k(2) = (7.6 +/- 0.8) x 10(-3) M-1 s(-1) in CDCl3 at 25 degreesC). A kinetic isotope effect has been noted for the reaction in the limit of saturated MVK (k(2)(H)/k(2)(D) = 2.0). MVK coordinates to palladium affording an eta (2)-alkene adduct. The rate constants for several alcohols are reported; the catalytic reaction is sensitive to steric hindrance of the alcohol nucleophile: 1 degrees > 2 degrees much greater than 3 degrees. Appreciable kinetic effects are observed by variation of the substituents on BA. Two new palladium(II) coordination complexes containing bidentate and tridentate pyridyl imine ligands have been synthesized, fully characterized, and explored as catalysts for the hydroalkoxylation reaction. The synthesis of AgBAr4F and its use in metathesis reactions with Pd(H) complexes are described. A mechanism has been put forth where the carbonyl group of the olefin interacts with palladium and directs the alcohol addition to the beta -carbon, resulting in the anti-Markovnikov addition ether product. Finally, the charge of the palladium complex augments catalytic activity.