Iridium complexes chirality transfer

The type 6 carbenes have been used as ligands in the rhodium(I) and iridium(I)-catalyzed transfer hydrogenation of ketones displaying low to moderate stereoselectivities in the conversion of most substrates. Somewhat higher enantioselectivities were obtained with complex 8b containing the chiral C2-symmetric carbene derived from 7 (Scheme 9). 

Owing to strategic reasons, Lonza exited the dextrometorphan business before the process could be transferred. Subsequently a further study of the hydrogenation of 10, using iridium complexes with chiral amidophosphine-phosphonite ligands, was published by another group [14]. An enantioselectivity of 86% ee was reached, but the chemoselectivity was low. 

Asymmetric reductions of aiylalkylketones by hydride transfer from i-PrOH are catalyzed by iridium complexes, and the most efficient chiral ligands are bis-oxazolines 3.57 (R = i-Pr) and pyridine derivatives 3.63 and 3.64 [339, 873,942],... 

These complexes also formed stable Langmuir films, which could be transferred onto a range of various substrates to yield Langmuir-Blodgett (LB) films.A pyroelectric effect was observed in alternate layers incorporating an iridium complex, and the electronic hyperpolarizability of some chiral derivatives was measured. ... 

Asymmetric transfer hydrogenation of ketones in the presence of soluble transition metal catalysts has been developed [8-10], enantioselectivities up to 99% ee being obtained using a ruthenium catalyst bearing mono-N-tosylated diphenyl-ethylenediamine as a ligand. Iridium complexes associated with fluorous chiral diimines 3a-3c or diamines 4a—4b have also been shown to be effective catalysts in hydrogen-transfer reduction of ketones [11,12]. 

Scheme 8.24 Photoinduced electron transfer from ketone 56 to the iridium complex occurs coti-certedly with protonation by the chiral phosphate. In the process, the ketyl radical becomes significantly more basic than phosphoric acid, and rapid radical cyciization from the neutral ketyl as an H-bonded adduct to the chiral phosphate occurs. Asymmetric induction results, due to the rapid radical cyciization. Hantsch dihydropyridine esters provide the required proton and hydrogen atoms...

Murata, K. Ikariya, T. Noyori, R. New chiral rhodium and iridium complexes with chiral diamine ligands for asymmetric transfer hydrogenation of aromatic ketones. /. Org. Chem. 1999,64,2186-2187. 

The treatment of [Cp MCl2]2 (M = Rh and Ir) with (S,S)-TsDPEN gave chiral Cp Rh and Cp Ir complexes (12a and 12b Scheme 5.9). An asymmetric transfer hydrogenation of aromatic ketones using complex 12 was carried out in 2-propanol in the presence of aqueous KOH (1 equiv.) the results obtained are summarized in Table 5.4. In all of the reactions, the (S)-alcohols were obtained with more than 80% enantiomeric excess (ee) and in moderate to excellent yields. The rhodium catalyst 12a was shown to be considerably more active than the iridium catalyst... 

Arylation, olefins, 187, 190 Arylketimines, iridium hydrogenation, 83 Arylpropanoic acid, Grignard coupling, 190 Aspartame, 8, 27 Asymmetric catalysis characteristics, 11 chiral metal complexes, 122 covalently bound intermediates, 323 electrochemistry, 342 hydrogen-bonded associates, 328 industrial applications, 8, 357 optically active compounds, 2 phase-transfer reactions, 333 photochemistry, 341 polymerization, 174, 332 purely organic compounds, 323 see also specific complexes Asymmetric induction, 71, 155 Attractive interaction, 196, 216 Autoinduction, 330 Axial chirality, 18 Aza-Diels-Alder reaction, 220 Azetidinone, 44, 80 Aziridination, olefins, 207... 

Promise is held in MPV reactions carried out under catalytic conditions. Instead of, for example, stoichiometric amounts of aluminum as the metal ion activator, catalytic quantities of complexes of rhodium and iridium can sometimes be used to bring about the same reactions. Although the catalytic mechanisms have not been established, postulation of the usual six-membered transition state in the critical step of hydride transfer appears reasonable. The strongly basic conditions of the MPV reaction are avoided. Reductions of aryl ketones (69 equation 30) using (excess) isopropyl alcohol as hydrogen donor and at partial conversions have led to the formation of alcohol (70) in modest enantiomeric excesses with various chiral ligands. " ... 

Three classes of catalysts have been studied for the asymmetric hydrogenation of imines. One class of catalyst is generated from late transition metal precursors and bisphosphines. These catalysts have typically been generated from rhodium and iridium precursors. A second class of catalyst is based on the chiral titanocene and zirconocene systems presented in the previous section on the asymmetric hydrogenation of unfunctionalized olefins. The third class of catalyst is used for the transfer hydrogenation of imines and consists of ruthenium or rhodium complexes containing diamine, amino tosylamide, or amino alcohol ligands. " ... 

Asymmetric Transfer Hydrogenation of Ketones. The first reports on asymmetric transfer hydrogenation (ATH) reactions catalyzed by chiral metallic compounds were published at the end of the seventies. Prochiral ketones were reduced using alcohols as the hydrogen source, and Ru (274,275) or Ir (276) complexes were used as catalysts. Since then, many chiral catalytic systems for homogeneous ATH of ketones, imines, and olefins have been developed (37,38,256,257,277-289). The catalytic systems are usually based on ruthenium, rhodium, or iridium, and the ATH of aryl ketones is by far the most studied. Because of the reversibility of this reaction, at high conversions, a gradual erosion of the ee of the product has been frequently reported. An azeotropic 5 2 mixture of formic acid/triethylamine can be used to overcome this limitation. 

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