19117-19-2Relevant academic research and scientific papers
Pyridine-assisted chlorinations and oxidations by palladium(IV)
McCall, A. Scott,Kraft, Stefan
scheme or table, p. 3527 - 3538 (2012/06/16)
The reactivity of the bis-NHC complex LPdIVCl4 (L = κ2-[R-NHCCH2NHC-R] with R = C14H 29) 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 PdIV complexes. LPdIVCl3(py)+ could be formed from Cl -/py exchange or from the oxidation of LPdIICl(py) + by Cl2. Taking advantage of the enhanced reactivities that pyridine coordination imparted on both PdII and PdIV complexes allowed for the catalytic chlorination of styrene with LPd IVCl4 as a sacrificial oxidant, thereby establishing the principal feasibility of PdII/PdIV catalyses that obviates PdII activations of the substrate.
Bis- N -heterocyclic carbene palladium(IV) tetrachloride complexes: Synthesis, reactivity, and mechanisms of direct chlorinations and oxidations of organic substrates
McCall, A. Scott,Wang, Hongwang,Desper, John M.,Kraft, Stefan
supporting information; experimental part, p. 1832 - 1848 (2011/04/15)
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)CH 2(NHC)] from LPdIICl2 and Cl2. In intermolecular, nonchelation-controlled transformations LPdIVCl 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 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 LPdIVCl 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 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.
