The +IV Oxidation State in Organopalladium Chemistry. Recent Advances and Potential Intermediates in Organic Synthesis and Catalysis

Abstract

The organometallic chemistry of palladium is dominated by the +II oxidation state, and the chemistry of complexes containing simple organic groups bonded to palladium in the +IV oxidation state has developed only recently. Organic synthesis and catalytic reactions that may involve undetected palladium(IV) intermediates have been suggested frequently and the new oxidation state +IV chemistry provides some support for these proposals, and gives encouragement for the development of new systems involving palladium(IV). The chemistry of organopalladium(IV) is reviewed here, and possible catalytic roles for palladium(IV) are discussed. The synthesis and decomposition reactions of palladium(IV) complexes provide "models" for catalytic proposals. The palladium(IV) complexes are formed by oxidative addition of organohalides to palladium(II) complexes, and most complexes decompose under mild conditions by carbon-carbon bond formation in reductive elimination reactions, for example, for methyl(pheny1)(2,2'-bipyridyl)palladium(II) as a substrate, oxidative addition of benzyl bromide gives Pd(IV)BrMePh(CH2Ph)(bpy), which reductively eliminates toluene to form the complex Pd(II)Br(CH2Ph)(bpy). Organopalladium(IV) chemistry is providing new comparisons of structure, solution dynamics, and reactivity among the nickel triad elements, and its development is commencing some 80 years after the synthesis of platinum(IV) complexes by Pope and Peachey. The new chemistry is providing a firmer basis for proposals involving palladium(IV) in catalysis,in particular the occurrence of oxidative addition-reductive elimination sequences for carbon-carbon bond formation.