Modular chiral thiazolidine catalysts in asymmetric aryl transfer reactions

Modular chiral thiazolidine derivatives were synthesized in a single step from inexpensive and commercially available starting materials. These ligands catalyzed enantioselective arylation of different aldehydes using aryl boronic acids as a source of transferable aryl groups. The products were obtained in excellent yields and good enantioselectivities.     

Catalyzed asymmetric aryl transfer reactions to aldehydes with boronic acids as aryl source

Chiral diaryl methanols are important intermediates for the synthesis of biologically active compounds. Here, we describe a flexible method for their catalyzed asymmetric synthesis from readily available starting materials. Noteworthy is the fact that with a single catalyst both enantiomers of the product are accessible simply by choosing the appropriate combination of aryl boronic acid or aldehyde as aryl donor and acceptor, respectively. The catalysis with a planar-chiral ferrocene is easy to perform and yields a broad range of products with excellent enantioselectivities (up to 98% ee).     

Enantioswitchable catalysts for the asymmetric transfer hydrogenation of aryl alkyl ketones

A subtle change in the ligand structure, replacing the carbonyl oxygen with sulfur in simple alpha-amino acid amides, resulted in a dramatic activity and selectivity improvement in the rhodium- or ruthenium-catalyzed reduction of ketones under hydrogen transfer conditions. In addition, in most cases, a switch of the product's absolute configuration was observed on going from amides to the corresponding thioamides. Under optimized conditions, we obtained the secondary alcohol products in high yield and enantioselectivity (up to 97% ee) using only 0.25 mol % catalyst loading. [structure: see text]     

Ruthenium amino carboxylate complexes as asymmetric hydrogen transfer catalysts

The synthesis and characterization of optically active amino carboxylate complexes of formula [(η(6)-arene)Ru(Aa)Cl] (arene = C(6)H(6), C(6)Me(6), Aa = amino carboxylate) as well as those of the related trimers [{(η(6)-arene)Ru(Aa)}(3)][BF(4)](3) are reported. Trimerization takes place with chiral self-recognition: only diastereomers equally configured at the metal, R(Ru)R(Ru)R(Ru) or S(Ru)S(Ru)S(Ru), are detected. The crystal structures of the complexes [(η(6)-C(6)H(6))Ru(Pip)Cl] and [{(η(6)-C(6)Me(6))Ru(Pro)}(3)][BF(4)](3) have been determined by X-ray diffraction methods. Both types of complexes catalyse the hydrogen transfer reaction from 2-propanol to ketones with moderate enantioselectivity (up to 68% ee). The enantiodifferentiation achieved can be accounted for by assuming that Noyori's bifunctional mechanism is operating.     

Chiral thiophosphoramide catalyzed asymmetric aryl transfer reactions for the synthesis of functional diarylmethanols

In this investigation, chiral thiophosphoramide 3d was easily prepared from chiral (1R,2R)-1,2-diphenylethylenediamine and then applied as an efficient chiral ligand in the catalytic asymmetric arylation reactions of various aromatic aldehydes. The corresponding diarylmethanol products were produced with good to excellent yields (up to 98%) and enantioselectivities (up to 94%). The recovery of chiral ligand 3d could be as high as 96%.     

Recyclable polymer-supported Pd catalysts for aryl amination reactions

Polymer-supported palladium catalysts were prepared from three commercially available phosphine-functionalised polymers (PS-PR₂), Pd₂(dba)₃ and P(t-Bu)₃. Catalyst stability was investigated using VT ³¹P NMR spectroscopy. One of the catalysts can be reused in the amination of bromobenzene and chlorotoluene, up to three times, without loss in yield. Recyclability of the catalysts is dependent on the method of preparation and the nature of the polymer-bound phosphine.     

Class I fructose-1,6-bisphosphate aldolases as catalysts for asymmetric aldol reactions

The activity of two commercially available bacterial class I fructose-1,6-bisphosphate aldolases (FruA) towards a number of aldehydes has been compared with rabbit muscle aldolase (RAMA), which is the most widely used enzyme for aldol reactions with dihydroxyacetone phosphate. The kinetic properties of the three aldolases are very similar, but the bacterial aldolases were much more stable than RAMA. Reaction of butanal and dihydroxyacetone phosphate catalyzed by FruA from Staphylococcus carnosus was performed on a 5 mmol scale in 53% isolated yield. Enantiomeric and diastereomeric purity of the major reaction product [90% (3S,4R)] was determined by chiral gas chromatography.     

l-Proline catalysed asymmetric aldol reactions in PEG-400 as recyclable medium and transfer aldol reactions

l-Proline-catalysed direct asymmetric aldol reaction of acetone with various aldehydes in PEG-400 is described. Recycling of the catalyst and solvent (PEG) was possible up to ten runs without loss of catalyst activity. l-Proline was also found to be an efficient catalyst for the asymmetric transfer aldol reaction between various aldehydes and diacetone alcohol for the first time. Good yields and enantioselectivities were observed with both methods.     

Multiple dendritic catalysts for asymmetric transfer hydrogenation

The first and second generation multiple dendritic ligands based on chiral diamine were synthesized in a convergent approach and were well-characterized by NMR and MS techniques. Their ruthenium complexes prepared in situ had good solubility in the reaction medium (azeotrope of formic acid and triethylamine) and demonstrated high catalytic activity and enantioselectivity comparable to monomeric catalysts in the asymmetric transfer hydrogenation of ketones and imines. Quantitative yields and for some cases a slightly higher enantioselectivity (up to 98.7% ee) were obtained in the dendritic catalysis. Considering the high local catalyst concentrations at the periphery, diones were tested for the possible synergic reactivity between catalytic units at the surface, while no apparent differences were noted.     

Novel recoverable catalysts for asymmetric transfer hydrogenation

9-Amino(9-deoxy)epiquinine and 9-amino(9-deoxy)epicinchonine were applied in asymmetric transfer hydrogenation of aromatic ketones in both iridium and rhodium catalytic systems using i-propanol as the hydrogen source. Good to excellent conversions and enantioselectivities were observed with a variety of aromatic ketones. Moreover, the Ir complex and Rh complex of 9-amino(9-deoxy)cinchonine were recovered in high yields with dilute hydrochloric acid. The enantioselectivity of 1-phenylethanol was nearly maintained after six cycles.     

Development of new asymmetric two-center catalysts in phase-transfer reactions

A new asymmetric two-center phase-transfer catalyst was designed and a catalyst library containing more than 40 new two-center catalysts was constructed. The catalysts were applied in phase-transfer alkylations and Michael additions to afford the corresponding products in up to 93% ee and 82% ee, respectively.     

Palladium-diphosphite catalysts for the asymmetric allylic substitution reactions

[structure: see text] We have designed a series of diphosphite ligands to study the effect of the backbone, the size of the chelate ring, and the substituents of the biphenyl moieties and to determine the scope of this type of ligand in the Pd-catalyzed asymmetric substitution reactions of different types of substrates. Good-to-excellent activities and enantioselectivities have been obtained for disubstituted linear substrate 11 (TOF's up to >2000 mol x (mol x h)(-1), ee values up to 99%) and cyclic substrate 14 (TOF up to 285 mol x (mol x h)(-1), ee values up to 92%). However, these ligands are inadequate for the Pd-catalyzed allylic alkylation of monosubstituted linear substrates because they provide low enantioselectivities.     

8-Amino-5,6,7,8-tetrahydroquinolines as ligands in iridium(III) catalysts for the reduction of aryl ketones by asymmetric transfer hydrogenation (ATH)

Aqua iridium(III) complexes with 8-amino-5,6,7,8-tetrahydroquinolines CAMPY L1 and its derivatives as chiral ligands proved to be very efficient catalysts for the reduction of a wide range of prochiral aryl ketones, revealing a variety of behaviours in terms of reaction rate and stereoselectivity. As standard substrates, differently substituted acetophenones were studied and good enantioselectivity (86% ee) was achieved in the reduction of 1-(o-tolyl)ethan-1-one 6. Particularly interesting was the ATH reaction in the case of β-amino keto esters, precursors of β-lactams and azetidinones. The best results were obtained with [Cp¹Ir(H₂O)(L1)]SO₄ affording the corresponding diastereomeric alcohols in an (R,S)-configuration with an excellent 99% ee in the reduction of 2-(benzamido methyl)-3-oxo-3-(4-(trifluoromethyl)phenyl)propanoate 12.     

手性金属催化剂在不对称环丙烷化反应中的应用进展

手性金属催化剂在不对称环丙烷化反应中的应用一直是不对称合成领域的一个热点。对三十多年来的这一领域的手性金属催化剂的发展作了详细的归纳和评述。     

Tertiary amine oxides as phase transfer catalysts for substitution and dichlorocyclopropanation reactions

Tertiary amine oxides catalyze a variety of phase transfer reactions, primarily by $\text{in}$$\text{situ}$ deoxygenation to form the tertiary amine or derived catalyst.     

Hydrogen transfer reactions of supported metallocene catalysts

The evolution of methane from methylaluminoxane (MAO) solutions is enhanced in the presence of homogeneous metallocenes. This reaction serves as a model for the deactivation of metallocene catalysts. By supporting different metallocenes on a silica/MAO carrier the deactivation reaction by alpha-hydrogen transfer among metallocene active sites and aluminum alkyls can be suppressed. The suppression of alpha-hydrogen transfer is proven for different Al/Zr ratios and by near independence of the polymerization activity on the catalyst aging time, after reaching maximum activity. Aluminum alkyls and MAO leach Cp₂ZrCl₂ from the carrier, the leached metallocene is only active in polymerization if MAO is present.     

Polymer catalysts for important photoelectron transfer reactions

Photozymes are novel water-soluble polymers made by the copolymerization of mixtures of hydrophobic and hydrophilic monomers, some of which contain chromophores capable of absorbing light and transmitting the excitation energy to selected traps by means of the antenna effect. The interactions between these groups and water force the polymer to adopt a hypercoiled conformation with hydrophobic pockets similar to those in the catalytic sites of natural enzymes. Hydrophobic organic compounds in the water solution will seek out and localize themselves in these regions, where they are subjected to electronic energy transfer from the light-excited antenna chromophores. The chemical reactions which occur are often different and more specific than in the case of photoreactions in common organic solvents. In a number of cases the reactions appear to proceed by an electron transfer mechanism. This paper summarizes recent results on the dechlorination of chlorinated aromatic and aliphatic compounds, and laser studies of multiphoton processes in aromatic compounds such as 9-(acetoxymethyl)phenanthrene (AMP).     

Synthesis of dendritic catalysts and application in asymmetric transfer hydrogenation

Frechet-type core-functionalized chiral diamine-based dendritic ligands and hybrid dendritic ligands condensed from polyether wedge and Newkome-type poly(ether-amide) supported multiple ligands were designed and synthesized. The solubility of hybrid dendrimers was found to be finely controlled by the polyether dendron. The catalytic efficiency and recovery use of dendritic ruthenium complexes were compared in the transfer hydrogenation of acetophenone. The core-functionalized dendritic catalysts demonstrated much better recyclability, which verified the stabilizing effects of the bulky polyether wedge on the catalytically active complex. Moreover, the dendritic catalysts were applied in the asymmetric transfer hydrogenation of ketones, enones, imine, and activated olefin, and moderate to excellent enantioselectivitiy was achieved comparable to that of monomeric catalysts.