Synthesis of water-soluble palladium(ii) complexes with N-heterocyclic carbene chelate ligands and their use in the aerobic oxidation of 1-phenylethanol

Abstract

The synthesis of palladium(ii) complexes containing N-heterocyclic carbene chelate ligands is reported. These chelate ligands have methylene or ethylene ring bridges and sulfonate, carbonate or ammonium functionalities to render the complexes soluble in water. Bis- [(bis-NHC)PdX] (X = Cl, Br or I) and tetra-carbene [(bis-NHC)Pd] complexes have been prepared and the crystal structures of two of them have been determined. The NHC ligand is coordinated to the metal center in normal (C2-bound) or abnormal (C4-bound) mode. Processes involving the formation of NHC-Pd bonds or the transfer of NHC ligands have been explored in water. The behavior of the bis-carbene complexes as catalysts for the aerobic oxidation of 1-phenylethanol is also discussed.

Mesoionic and Related Less Heteroatom-Stabilized N-Heterocyclic Carbene Complexes: Synthesis, Catalysis, and Other Applications

“…This direct palladation protocol has been extended to the palladation of the related diimidazolium salt 317 containing sulfonyl groups, which yields the water-soluble bis(aNHC) palladium(II) complex 318 (Scheme 125). 287 Palladation of the 2-phenyl-protected imidazolium salt 245 with PdCl 2 in the presence of K 2 CO 3 as a base and with pyridine as solvent affords the palladium complex 319 featuring a 4-imidazolylidene ligand (Scheme 126a). 245 Application of the same method with the imidazo[1,2a]pyridinium iodide salt 320 yields the palladium complex 321 containing a mesoionic imidazolylidene-type ligand (Scheme 126b).…”

“…Complex 318 catalyzes the oxidation of 1-phenylethanol using molecular oxygen as the oxidant (Scheme 157). 287 With 0.25 mol % palladium complex and 2 bar O 2 at 100 °C (water solvent), a mere 14% conversion is achieved after 15 h. In contrast, the analogous palladium complex with 2-imidazolylidene ligands accomplishes full conversion under otherwise identical conditions.…”

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

“…Oxidation of 1‐phenylethanol with molecular O 2 (2 bar) at 100 °C was performed with a series of water‐soluble Pd–bis(NHC) complexes, such as 71 b (Scheme b and Figure ) . Among these complexes, complex 71 b , with sulfonated wingtip groups, provided the best performance.…”

“…[95] Oxidation of 1-phenylethanol with molecular O 2 (2 bar) at 100 8Cw as performed with as eries of water-soluble Pd-bis(NHC) complexes, such as 71 b (Scheme 12 ba nd Figure 13). [96] Among these complexes, complex 71 b,w ith sulfonatedw ingtip groups, provided the best performance. Palladium black was generated during the reaction and O 2 wasa ssumed to suppress the accumulation of intermediate palladium(0) species (which was supposed to be "naked" Pd 0 complex) in the formation of palladiumb lack during the catalytic cycle, and fast formation (recovery) of the palladium(II) complex was favored in this case.…”

Aqueous-Phase Chemistry of η3-Allylpalladium(II) Complexes with SulfonatedN-Heterocyclic Carbene Ligands: Solvent Effects in the Protolysis of Pd–C Bonds and Suzuki–Miyaura Reactions

“…The usefulness of this route should be noted for the preparation of cis-bis-carbenes, in contrast with the trans complexes, which are preferentially obtained by other procedures. 25 The transformation of 9 into 12 26 was very fast under the same conditions.…”