Synthesis and Reactivity of Monohydrocarbyl Palladium(IV) Complexes Using Hydrogen Peroxide as Oxidant in Protic Solvents

Abstract

Mild, and selective transition metal catalyzed processes for the functionalization of C-H bonds utilizing environmentally benign and inexpensive dioxygen and/ or HOOH oxidants are extremely attractive, as they render these transformations more atom economical and practical for large-scale syntheses. Our approach towards this end involves optimizing the oxidation and C-X reductive elimination steps of the proposed catalytic cycle using tridentate facially chelating ligands, which include 1-hydroxy-1,1-di(2-pyridyl)methoxide, a derivative of di(2-pyridyl)ketone (dpk) and 6-(2-pyridinoyl)pyridine-2-carboxylic acid (ppc).

Oxidation of the dpk- and the ppc-ligated palladacycles with HOOH in water and acetic acid solvents produces the corresponding monohydrocarbyl Pd(IV) complexes quantitatively. The mechanism of oxidation of these complexes was investigated, and was proposed to involve addition of HOOH across the C=O bond of the ligand, followed by heterolytic cleavage of the O-O bond via nucleophilic attack of Pd(II) onto the hydroperoxo adduct.

The dpk- and ppc-ligated monohydrocarbyl Pd(IV) complexes undergo C-O reductive elimination at room temperature in acetic acid and/ or water to produce the corresponding phenols and/ or aryl acetates quantitatively. Mechanistic studies led us to propose a C-O reductive elimination reaction that proceeds either from a 5-coordinate intermediate, produced upon dissociation of the pyridine group of the dpk chelate or from a 6-coordinate Pd(IV) species.

Addition of HX (X=Cl, Br, and I) to aqueous solutions of the dpk-supported hydroxo-ligated monohydrocarbyl Pd(IV) complexes leads to quantitative formation of C-X bond-coupling products. Some of the corresponding X-ligated monohydrocarbyl Pd(IV) complexes were isolated from these solutions (X=Cl and Br), and could be independently prepared by oxidation of the hydrocarbyl Pd(II) precursors with the corresponding N-halogenosuccinimides (NXS).

Palladium catalyzed C-H functionalization reactions were performed in the presence of tridentate, facially chelating bis(6-methyl-2-pyridyl)methanesulfonate ligand. Substituted 2-phenylpyridine substrates underwent predominantly C-C coupling reactions with minor C-O coupling products produced, while 2-benzyl- and 2-phenoxypyridine substrates that form 6-membered palladacycles produced the corresponding C-O coupling products selectively in high yields. These reactions were significantly slower in the absence of the ligand, and no reactions took place in the absence of palladium acetate.