Homogeneous versus Supported ONN Pincer‐Type Gold and Palladium Complexes: Catalytic Activity

Abstract

The ONN-tridentate unsymmetrical pincer-type (S)-1-((6-(2-hydroxyphenyl)pyridin-2-yl)methyl)-N-methyl-N-(3-(triethoxysilyl)pro-pyl)pyrrolidine-2-carboxamide ligand has been synthesized by an easy method, in high purity and good yield. Its respective palladium(II) and gold(III) complexes have been prepared as air-stable solids. After deprotonation of the -OH group the coordination of the metal ion is completely stereospecific and gives rise to only one diastereoisomer. The complexes, immobilized onto ordered mesoporous silica (MCM-41), are shown to be very active catalysts in the hydrogenation of prochiral olefins (98% ee was achieved with the heterogenized chiral Au(MCM-41) complex), hydrosilylation, and Suzuki and Heck C-C coupling reactions, under mild conditions. The reactions were studied with the soluble catalysts as well as their heterogenized counterparts. The high accessibility introduced by the structure of the supports allows the preparation of highly efficient immobilized catalysts. The repeated use of the immobilized catalyst in four cycles demonstrates "homogeneous" catalysis with "heterogeneous" catalysts, thus reducing solvent waste and loss of precious metal and/or ligand.

Synthesis of Electron-Rich CNN-Pincer Complexes, with N-Heterocyclic Carbene and (S)-Proline Moieties and Application to Asymmetric Hydrogenation

“…Our research is focused on the synthesis of neutral unsymmetrical pyridine pincer-type ligands with a lateral ( S )- N - tert- butyl-methylpyrrolidine-2-carboxamide donor function and an N-heterocyclic carbene moiety. Previously, symmetrical NCN-pincer complexes with two ester prolinates or prolinols as the nitrogen donor substituents have been described. , We also have reported the synthesis of chiral tridentate ( 1a , 1b in Figure ) and pincer-type ONN ( 2a , 2b ) (Figure ), which contain O phenol , N pyridil , and chiral N carboxamide donor group derivatives of the natural amino acid ( S )-proline as N-donor groups on the pincer backbone, resembling coordination environments present in previously described Schiff base ligands ( 1a , 1b ) (Figure ) . These are commercially available and enantiomerically pure building blocks for the synthesis of chiral ligands, and their stereogenic information can be maintained by choosing the appropriate reaction conditions during the synthesis and purification of the complexes.…”

“…The new neutral N-heterocyclic carbene pincer-type ligands were designed with the objective of stabilizing metal complexes with enhanced catalytic properties. In this regard two anionic ONN-tridentate unsymmetrical pincer-type ligands, 2-(6-(pyrrolidin-1-ylmethyl)pyridin-2-yl)phenol ( 2a ) and N - tert -butyl-1-((6-(2-hydroxyphenyl)pyridin-2-yl)methyl)pyrrolidine-2-carboxamide ( 2b ), have been recently reported, and their gold and Pd complexes showed activity toward several organic reactions. In this work we have obtained the unsymmetrical CNN pincer-type ligands 2-[(3-aryl-2,3-dihydro-1 H -imidazol-1-yl)methyl]-6-(pyrrolidin-1-ylmethyl)pyridine ( 3a , 4a ) and (2 S )- N - tert -butyl-1-((6-((3-aryl-2,3-dihydro-1 H -imidazol-1-yl)methyl)pyridin-2-yl)methyl)pyrrolidine-2-carboxamide ( 3b , 4b ).…”

Gold-Catalyzed Carbon−Heteroatom Bond-Forming Reactions

“…For instance, Caporusso and co-workers have used gold nanoparticles, supported by means of the metal vapor synthesis technique on different metal oxides and carbon, as catalysts for the hydrosilylation of 1-hexyne, achieving the E -isomer with high selectivity. Similarly, Iglesias, Sánchez, and co-workers have supported an O , N , N -tridentate pincer-type gold(III) complex onto ordered mesoporous silica MCM-41, which acts as heterogeneous catalyst for the hydrosilylation reaction. The single gold(III) complex was also studied as catalyst in homogeneous phase.…”

Commercial Gold(I) and Gold(III) Compounds Supported on Carbon Materials as Greener Catalysts for the Oxidation of Alkanes and Alcohols

“…Corma et al. also used compounds with ONN‐tridentate and CNN‐tridentate pincer‐type ligands, chiral (NHC)‐dioxolane‐amino Au I and Au III complexes, and a mononuclear asymmetrical N‐heterocyclic carbene‐Au complex . A N‐heterocyclic carbene‐Au I complex was also reported by Fujita and co‐workers .…”