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

Abstract

This paper 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, non-chelation controlled transformations LPdIVCl4 reacted with alkenes and alkynes to 1,2-dichlorination adducts. Aromatic, benzylic, and aliphatic CH-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 down 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.

Chelating Bis(N‐Heterocyclic Carbene) Palladium‐Catalyzed Reactions

“…In the functionalization of methane and propane, Pd IV –bis(NHC) was assumed to be involved in the catalytic cycle . Isolated Pd IV –bis(NHC) tetrachloride complexes were reported by Kraft and co‐workers, and the chlorination of aromatic C−H bonds proceeded in a stoichiometric manner, as well as aliphatic C−H bonds …”

“…[90] Isolated Pd IV -bis(NHC) tetrachloride complexes were reported by Kraft and co-workers, and the chlorination of aromatic CÀHb onds proceeded in as toichiometricm anner,a s well as aliphatic CÀHb onds. [91] In addition to CÀHf unctionalization of methane and propane, Hou and co-workers investigated the regioselective functionalization (i.e.,o xidative trifluoroacetoxylation) of the methylene sp 3 -CÀHbond of linear trifluoroacetic acidesters. [92] Complex 10 a (Figure 1) displayed the highest activity among six candidates.…”

Chelating Bis-N-heterocyclic Carbene–Palladium(II) Complexes for Oxidative Arene C–H Functionalization

“…7 Recently, Kraft et al advanced one more step and actually synthesized (bis-NHC)Pd IV Cl 4 complexes for further direct chlorination of aromatic and aliphatic C−H bonds, albeit in a stoichiometric manner ( Figure 1). 8 In a similar study, Arnold and Sanford also implemented a chelating anionic alkoxy-tethered NHC ligand to achieve Pd II /Pd IV -based aromatic C−H halogenations. 5 In parallel to the alkane activation, oxidative functionalization of arene C−H bonds is another highly demanding transformation.…”

Oxidation and Reduction

“…296 The mechanistic pathway for the formation of (140) instead of anticipated aldehyde (141), when Pummerer rearrangement of sulfoxide (142) was attempted by treating it with trifluoroacetic anhydride and subsequent treatment with aqueous sodium bicarbonate has been discussed. The intermediate (143) was presumably generated via trifluoroacetylation of a sulfoxide precursor (142). Ionization of trifluoroacetate generated a dipositive 'sulfenium' equivalent (144), which transformed to (145).…”