Synthesis of Iridium Complexes with New Planar Chiral Chelating Phosphinyl-imidazolylidene Ligands and Their Application in Asymmetric Hydrogenation (original) (raw)
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Synthesis of iridium complexes with novel planar chiral chelating imidazolylidene ligands
Tetrahedron: Asymmetry, 2003
New planar chiral imidazolium salts such as (S p)-1-[4-(4,4-dimethyl-4,5-dihydrooxazolyl)[2.2]paracyclophane-12ylmethyl]-3-methyl imidazolium bromide (S p)-8a have been synthesized starting from enantiopure 4,12dibromo[2.2]paracyclophane (S p)-4. Deprotonation of these salts followed by reaction with [Ir(COD)Cl] 2 yielded chelating iridium imidazolylidene complexes 9a-c, which were applied in asymmetric hydrogenations of alkenes. The solid state structure of (S p)-9c was determined by single-crystal X-ray structure analysis.
Journal of Organometallic Chemistry - J ORGANOMET CHEM, 2006
The relatively inexpensive chiral monodentate phosphoramidite (S)-MONOPHOS may be used in combination with pyridines to prepare iridium complexes effective for catalysis of asymmetric imine hydrogenation with comparable enantioselectivity to some of those containing more costly chiral bidentate phosphines. [Ir(cod)((S)-MONOPHOS)(L)]BArF (cod=1,5-cyclooctadiene; L=3-methylisoquinoline, acridine, 2,6-lutidine, acetonitrile, or 2,3,3-trimethylindolenine; BArF=tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) are efficient catalysts for the asymmetric hydrogenation of 2,3,3-trimethylindolenine. An important observation is that the catalyst containing acridine is more enantioselective than the catalyst derived from 2,3,3-trimethylindolenine which suggests that the other N-donor ligands are not readily displaced by the substrate during the catalytic cycle.
Journal of Molecular Structure, 2019
New chiral C 2-symmetric bis(phosphinite) ligands containing imine group were synthesized from 1-({[(1R,2R)-2-{[(2-hydroxynaphthalen-1-yl)methylidene] amino}cyclohexyl]-imino}methyl)-naphthalen-2-ol and two equivalents of Ph 2 PCl, (i-Pr) 2 PCl or (Cy) 2 PCl, in high yields. Binuclear C 2-symmetric half sandwich h 6-p-cymene-Ru(II) complexes of the chiral phosphinite ligands were synthesized by treating of [Ru(h 6-p-cymene)(m-Cl)Cl] 2 with the phosphinites in 1:1 M ratio in CH 2 Cl 2. Their catalytic activities in asymmetric transfer hydrogenation (ATH) were investigated for the reaction of acetophenone derivatives with isopropyl alcohol. The corresponding optically active secondary alcohols were obtained in excellent levels of conversion (96e99%) and moderate enantioselectivity (up to 60% ee). Among three complexes investigated, complex 4 was the most efficient one.
Angewandte Chemie International Edition, 2007
Bidentate chiral ligands were the rule in metal-catalyzed asymmetric hydrogenation for more than 30 years as chelation was believed to be necessary to impart the necessary rigidity to the metal complex for an efficient transfer of chirality. Recently, however, a few groups have demonstrated that monodentate ligands can also induce high enantioselectivity [2] as long as two of these ligands are present in the active species. Herein, we describe the first example of a highly asymmetric hydrogenation that is induced by a metal catalyst containing only one monodentate ligand. Iridium is an important metal in hydrogenation. The Crabtree catalyst, its enantioselective version, developed by Pfaltz, based on chiral P,N ligands, or the celebrated Metolachlor process catalyst are prime examples of Irbased hydrogenation catalysts (Scheme 1). We were interested in investigating whether iridium complexes of chiral monodentate phosphoramidites could also act as efficient enantioselective hydrogenation catalysts. Although such Ir complexes have already been reported, [7] which has led to the discovery of new cyclometalated species that are active in allylic substitution, [8] there are no reports of their use in enantioselective hydrogenation.