Bis- N -heterocyclic carbene palladium(IV) tetrachloride complexes: Synthesis, reactivity, and mechanisms of direct chlorinations and oxidations of organic substrates

Article abstract of DOI:10.1021/ja107342b

This Article describes the preparation and isolation of novel octahedral CH₂-bridged bis-(N-heterocyclic carbene)palladium(IV) tetrachlorides of the general formula LPd^IVCl₄ [L = (NHC)CH ₂(NHC)] from LPd^IICl₂ and Cl₂. In intermolecular, nonchelation-controlled transformations LPd^IVCl ₄ reacted with alkenes and alkynes to 1,2-dichlorination adducts. Aromatic, benzylic, and aliphatic C-H bonds were converted into C-Cl bonds. Detailed mechanistic investigations in the dichlorinations of alkenes were conducted on the 18VE Pd^IV complex. Positive solvent effects as well as kinetic measurements probing the impact of cyclohexene and chloride concentrations on the rate of alkene chlorination support a Pd^IV-Cl ionization in the first step. Product stereochemistry and product distributions from various alkenes also support Cl⁺-transfer from the pentacoordinated Pd^IV-intermediate LPd^IVCl ₃⁺ to olefins. 1-Hexene/3-hexene competition experiments rule out both the formation of π-complexes along the reaction coordinate as well as in situ generated Cl₂ from a reductive elimination process. Instead, a ligand-mediated direct Cl⁺-transfer from LPd ^IVCl₃⁺ to the π-system is likely to occur. Similarly, C-H bond chlorinations proceed via an electrophilic process with in situ formed LPd^IVCl₃⁺. The presence of a large excess of added Cl^- slows cyclohexene chlorination while the presence of stoichiometric amounts of chloride accelerates both Pd^IV-Cl ionization and Cl⁺-transfer from LPd^IVCl₃ ⁺. ¹H NMR titrations, T1 relaxation time measurements, binding isotherms, and Job plot analysis point to the formation of a trifurcated Cl^-...H-C bond in the NHC-ligand periphery as a supramolecular cause for the accelerated chemical events involving the metal center.