What a turnover! An efficient chiral iridium catalyst that bears a tridentate spiro aminophosphine ligand catalyzes the asymmetric hydrogenation of ketones with excellent enantioselectivities (up to 99.9 % ee) and extremely high turnover numbers (TONs; as high as 4 550 000).
We constructed a series of novel optical sensors for determination of broad-range pH based on a single fluorophore and multi-ionophores with different pK(a) values. These optical sensors use photoinduced electron transfer (PET) as the signal transduction and follow the design concept of "fluorophore-spacer-receptor (ionophore)" which employs 4-amino-1,8-naphthalimide as the single fluorophore, ethyl moiety as the spacer, and a series of phenols and anilines as the receptors. Key to the successful development of this sensor system is that coupling the receptors with six different pK(a) values with a single fluorophore produces the correct optical properties. This rational design affords a series of optical pH sensors with unique fluorescence property and accurately tunable pH measurement ranging from 1 to 14 pH units. Because of covalent immobilization of the indicators, these sensors demonstrate excellent stability, adequate reversibility, and satisfactory dynamic range up to full pH ranges (pH 1-14).
Dedicated to Professor Christian Bruneau on the occasion of his 60th birthdayOptically active b-hydroxy acids and their derivatives are versatile chiral building blocks for many useful molecules, including pharmaceuticals and natural products.[1] Catalytic asymmetric hydrogenation of b-ketoesters is an efficient and economically feasible method for preparing these important chiral compounds. Pioneered by Noyori and co-workers, [2] the chiral ruthenium diphosphine complexes [RuX 2 -(diphosphine)] (X = Cl or Br) and their analogues have become by far the most popular catalysts for this transformation.[3] Many of them show excellent enantioselectivity [> 99 % enantiomeric excess (ee)] and extraordinarily high activity (turnover number (TON) of up to 100 000) for the hydrogenation of b-alkyl b-ketoesters.[4] However, only a few of these complexes exhibit high enantioselectivity for the hydrogenation of b-aryl b-ketoesters. Zhang et al. reported that the ruthenium catalysts bearing the ligands xylyl-obinapo [5] (3,3'-bis(3,5-dimethylphenyl)-2,2'-bis(diphenylphosphinoxy)-1,1'-binaphthyl) and C 3 *-TunePhos [6] give up to 99 % ee for the hydrogenation of b-aryl b-ketoesters. Using ruthenium complexes of 4,4'-substituted binap ligands (binap = 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl), Lin et al. [7] obtained up to 99.8 % ee for the hydrogenation of a range of b-aryl b-ketoesters. The highest TON (10 000) was achieved by Saito and co-workers [8] in the asymmetric hydrogenation of methyl 3-oxo-3-phenylpropanoate. Note that chiral rhodium or iridium complexes, which efficiently catalyze olefin and imine hydrogenation, are seldom used for the asymmetric hydrogenation of b-ketoesters.[9] Furthermore, chiral [RuCl 2 (diphosphine)(diamine)] complexes, which catalyze the hydrogenation of simple ketones extremely efficiently, are also inert for the hydrogenation of b-ketoesters.[10] The major reason for the inertness may be that the strong base, such as KOtBu, that is required for activation of the [RuCl 2 (diphosphine)(diamine)] catalysts enolizes the b-ketoester substrates instead of activating the catalysts.Recently, we developed chiral iridium catalysts containing a chiral SpiroPAP ligand, and these catalysts show excellent enantioselectivity (up to 99.9 % ee) and an extremely high TON (as high as 4 550 000) for the hydrogenation of simple ketones.[11] These Ir/SpiroPAP catalysts are likely to have a "metal-ligand bifunctional catalysis" mechanism, similar to the [RuCl 2 (diphosphine)(diamine)] catalysts.[12] The aromatic N À H of the Ir/SpiroPAP catalysts is more acidic than the aliphatic NÀH of [RuCl 2 (diphosphine)(diamine)] catalysts (the proton resonances of the NH or NH 2 group of the catalysts are as follows:.3 and 3.5 ppm (C 6 D 6 ) [13] ), thus indicating that the Ir/SpiroPAP catalysts may be more easily activated with a relatively weak base such as the enolate salt of a b-ketoester. To confirm this possibility, we tested Ir/SpiroPAP catalysts for the hydrogenation of bketoesters and found that the catalysts were...
The iridium complexes of chiral spiro aminophophine ligands, especially the ligand with 3,5-di-tert-butylphenyl groups on the P atom (1c) were demonstrated to be highly efficient catalysts for the asymmetric hydrogenation of alkyl aryl ketones. In the presence of KOtBu as a base and under mild reaction conditions, a series of chiral alcohols were synthesized in up to 97% ee with high turnover number (TON up to 10,000) and high turnover frequency (TOF up to 3.7×10(4) h(-1)). Investigation on the structures of the iridium complexes of ligands (R)-1a and 1c by X-ray analyses disclosed that the 3,5-di-tert-butyl groups on the P-phenyl rings of the ligand are the key factor for achieving high activity and enantioselectivity of the catalyst. Study of the catalysts generated from the Ir-(R)-1c complex and H(2) by means of ESI-MS and NMR spectroscopy indicated that the early formed iridium dihydride complex with one (R)-1c ligand was the active species, which was slowly transformed into an inactive iridium dihydride complex with two (R)-1c ligands. A plausible mechanism for the reaction was also suggested to explain the observations of the hydrogenation reactions.
The highly efficient asymmetric hydrogenation of alpha-arylmethylene cycloalkanones catalyzed by Ir-complexes of chiral spiro aminophosphine ligands was developed, providing chiral exo-cyclic allylic alcohols at high yields with excellent enantioselectivities (up to 97% ee) and high turnover numbers (S/C up to 10,000). This new reaction provided an efficient method for the synthesis of the key intermediate of the active form of the anti-inflammatory loxoprofen.
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