Palladium-catalyzed aromatic coupling and allylic substitution

Abstract: Different types of palladium(II)-catalyzed aromatic couplingreactions have been investigated.The cyclization of diphenyl ether was found to be moresensitive to reaction conditions than was the cyclization ofthe diphenylamine derivatives. A more electrophilic catalyst,palladium(II) trifluoroacetate, together with a cocatalyst,tin(II) acetate, was required for diphenyl ether cyclization,while diphenylamine could be cyclized using catalytic amountsof palladium(II) acetate. The primary limitation to catalyticreactivity appeared to be palladium metal formation, whichinhibits the subsequent reoxidation of palladium(0) back to thecatalytically active palladium(II) species. Addition of highsurface area materials or species that can retain palladium insolution without reducing the catalytic activity ensured theformation of reproducible yields of dibenzofuran.The dimerization of 4-methylpyridine and its derivatives wasshown to be catalyzed by palladium(II) on a carbon support. Thecatalyst deactivation was partly caused by poisoning of theactive catalytic surface. The identification of the poison wasdifficult with the experimental techniques employed, but it ismost likely either the product,4,4’-dimethyl-2,2’-bipyridine, or the by-product,4,4’,4"-trimethyl-2,2’,6’,2"-terpyridine. Undersome conditions, the catalytic palladium was removed from thesurface by dissolution into the reaction mixture.Oxygen could be efficiently used as single oxidant in all ofthe above reactions.A theoretical study of the palladium-assisted allylicsubstitution reaction was also conducted.Using density functional theory, the transtition-statestructure was determined for a few model complexes indichloromethane and water. The importance of solvation wasevident since the reaction of an anionic nucleophile and acationic palladium allyl complex was found not to have a gasphase barrier. Molecular mechanicsparameters were developedfor calculations on both palladium allyl complexes containingphosphorus and/or nitrogen ligands and palladium olefincomplexes carrying phosphorus ligands. The developed forcefield parameters were used in a study of catalyst systems thathave been shown to give excellent enantioselectivities in thepalladium-assisted allylic substitution reaction. Thecalculations on the complexes were used succesfully torationalize the experimentally observed results.Keywords: Palladium, Catalysis, Aromatic Coupling,Allylic Substitution, Transition State, Solid State NMR, DFTCalculations, Force Field Parameterization, Molecular Mechanicscalculations.