Pyridine-Assisted Chlorinations and Oxidations by Palladium(IV)

The reactivity of the bis-NHC complex LPd^IVCl₄ (L = κ²-[R-NHCCH₂NHC-R] with R = C₁₄H₂₉) in chlorinations and oxidations of organic substrates was considerably increased in the presence of pyridine. For alkene chlorinations, this effect was due to the in situ formation of highly reactive LPd^IVCl₃(py)⁺, which was able to transfer Cl⁺ to the CC bond in a ligand-mediated process (devoid of π complexation), which did not require py dissociation. The enhanced reactivity in the presence of pyridine also extended to the oxidation of secondary and benzylic alcohols under mild conditions in a reaction where py served as a base, broadening the known scope of reactivity for Pd^IV complexes. LPd^IVCl₃(py)⁺ could be formed from Cl^–/py exchange or from the oxidation of LPd^IICl­(py)⁺ by Cl₂. Taking advantage of the enhanced reactivities that pyridine coordination imparted on both Pd^II and Pd^IV complexes allowed for the catalytic chlorination of styrene with LPd^IVCl₄ as a sacrificial oxidant, thereby establishing the principal feasibility of Pd^II/Pd^IV catalyses that obviates Pd^II activations of the substrate

Bis-<i>N</i>-heterocyclic Carbene Palladium(IV) Tetrachloride Complexes: Synthesis, Reactivity, and Mechanisms of Direct Chlorinations and Oxidations of Organic Substrates

This Article describes the preparation and isolation of novel octahedral CH2-bridged bis-(N-heterocyclic carbene)palladium(IV) tetrachlorides of the general formula LPdIVCl4 [L = (NHC)CH2(NHC)] from LPdIICl2 and Cl2. In intermolecular, nonchelation-controlled transformations LPdIVCl4 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 PdIV 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 PdIV−Cl ionization in the first step. Product stereochemistry and product distributions from various alkenes also support Cl+-transfer from the pentacoordinated PdIV-intermediate LPdIVCl3+ 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 LPdIVCl3+ to the π-system is likely to occur. Similarly, C−H bond chlorinations proceed via an electrophilic process with in situ formed LPdIVCl3+. The presence of a large excess of added Cl− slows cyclohexene chlorination while the presence of stoichiometric amounts of chloride accelerates both PdIV−Cl ionization and Cl+-transfer from LPdIVCl3+. 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

Bis-<i>N</i>-heterocyclic Carbene Palladium(IV) Tetrachloride Complexes: Synthesis, Reactivity, and Mechanisms of Direct Chlorinations and Oxidations of Organic Substrates

This Article describes the preparation and isolation of novel octahedral CH2-bridged bis-(N-heterocyclic carbene)palladium(IV) tetrachlorides of the general formula LPdIVCl4 [L = (NHC)CH2(NHC)] from LPdIICl2 and Cl2. In intermolecular, nonchelation-controlled transformations LPdIVCl4 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 PdIV 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 PdIV−Cl ionization in the first step. Product stereochemistry and product distributions from various alkenes also support Cl+-transfer from the pentacoordinated PdIV-intermediate LPdIVCl3+ 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 LPdIVCl3+ to the π-system is likely to occur. Similarly, C−H bond chlorinations proceed via an electrophilic process with in situ formed LPdIVCl3+. The presence of a large excess of added Cl− slows cyclohexene chlorination while the presence of stoichiometric amounts of chloride accelerates both PdIV−Cl ionization and Cl+-transfer from LPdIVCl3+. 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