Qingbin Liu

Enhancing catalytic activity towards Heck-reaction by using 4,5,9,10-tetra(arylimino)pyrenylidenyldipalladium tetrachlorides

A series of binuclear 4,5,9,10-tetra(arylimino)pyrenylidenyldipalladium(II) tetrachloride complexes (Pd1–Pd4) was synthesized and characterized by FT-IR and NMR spectroscopy as well as elemental analysis. The molecular structures of representative complexes Pd1 and Pd2 were determined by single crystal X-ray diffraction to reveal a square planar geometry around the palladium centre. The complex Pd1 dissolved in the presence of ethanol to form mono-palladium complex C1, whose structure was also confirmed by single crystal X-ray diffraction. All binuclear palladium complexes promote the Heck cross-coupling reactions with high activities; the reaction can be carried out for various substrates and activators (basic compounds) and also in various solvents. The binuclear palladium complexes exhibit significantly higher efficiency than do their mono-palladium complexes bearing 4,5-bis(arylimino)pyrenylidenes.

Efficient acceptorless dehydrogenation of secondary alcohols to ketones mediated by a PNN-Ru(II) catalyst

Four types of ruthenium(II) complexes, [fac-PNN]RuH(PPh3)(CO) (A), [fac-PNHN]RuH(η1-BH4)(CO) (B), [fac-PNHN]RuCl2(PPh3) (C) and [fac-PNHN]RuH(η1-BH4)(PPh3) (D) (where PNHN and PNN are N-(2-(diphenylphosphino)ethyl)-5,6,7,8-tetrahydroquinoline-8-amine and its deprotonated derivative), have been synthesized and assessed as catalysts for the acceptorless dehydrogenation of secondary alcohols to afford ketones. It was found that C, in combination with t-BuOK, proved the most effective and versatile catalyst allowing aromatic-, aliphatic- and cycloalkyl-containing alcohols to be efficiently converted to their corresponding ketones with particularly high values of TON achievable. Furthermore, the mechanism for this PNN-Ru mediated process been proposed on the basis of a number of intermediates that have been characterized by EI-MS and NMR spectroscopy. These catalysts show great potential for applications in atom-economic synthesis as well as in the development of organic hydride-based hydrogen storage systems.

Cooperative interplay between a flexible PNN-Ru(II) complex and a NaBH4 additive in the efficient catalytic hydrogenation of esters

A catalyst loading of between 0.001–0.05 mol% of the PNN-bearing ruthenium(II) complex [fac-PNN]RuH(PPh3)(CO) (PNN = 8-(2-diphenylphosphinoethyl)amidotrihydroquinoline), in combination with 5 mol% NaBH4, efficiently catalyzes the hydrogenation of esters to their corresponding alcohols under mild pressures of hydrogen. Both aromatic and aliphatic esters can be converted with high values of TON or TOF achievable. Mechanistic investigations using both DFT calculations and labeling experiments highlight the cooperative role of NaBH4 in the catalysis while the catalytically active species has been established as trans-dihydride [mer-PNHN]RuH2(CO) (PNHN = 8-(2-diphenylphosphinoethyl)aminotrihydroquinoline). The stereo-structure of the PNHN-ruthenium species greatly affects the activity of the catalyst, and indeed the cis-dihydride isomer [fac-PNHN]RuH2(CO) is unable to catalyze the hydrogenation of esters until ligand reorganization occurs to give the trans isomer.

Direct Hydrogenation of a Broad Range of Amides under Base‐free Conditions using an Efficient and Selective Ruthenium (II) Pincer Catalyst

The ruthenium(II) complex, [fac‐PNHN]RuH(η1‐BH4)(CO) (B; PNHN=8‐(2‐diphenylphosphinoethyl)aminotrihydroquinoline), is a highly versatile and effective catalyst (loadings of 0.1–1 mol %) for the hydrogenation of a multitude of amides, which include primary, secondary, and tertiary amides, to give their corresponding alcohols and amines in high yields under base‐free conditions. All products were confirmed by using GC and GC–MS.

An air and moisture tolerant iminotrihydroquinoline-ruthenium(II) catalyst for the transfer hydrogenation of ketones

Reaction of 8-amino-5,6,7,8-tetrahydroquinoline with RuCl2(PPh3)3 at room temperature affords the ruthenium(ii) chelate (8-NH2-C9H10N)RuCl2(PPh3)2 (E), in which the two triphenylphosphine ligands are disposed mutually cis. By contrast, when the reaction is performed at reflux ligand oxidation/dehydrogenation occurs along with cis-trans reorganization of the triphenylphosphines to form the 8-imino-5,6,7-trihydroquinoline-ruthenium(ii) complex, (8-NH-C9H9N)RuCl2(PPh3)2 (F). Complex F can also be obtained in higher yield by heating a solution of E alone to reflux. Comparison of their molecular structures highlights the superior binding properties of the bidentate imine ligand in F over its amine-containing counterpart in E. Both complexes are highly effective in the transfer hydrogenation of a wide range of alkyl-, aryl- and cycloalkyl-containing ketones affording their corresponding secondary alcohols with loadings of as low as 0.1 mol%. Significantly, F can deliver excellent conversions even in bench quality 2-propanol in reaction vessels open to the air, whereas the catalytic efficiency of E is diminished by the presence of air but only operates efficiently under inert conditions.