Platinum(II) complex-catalyzed enantioselective aldol reaction with ketene silyl acetals in DMF at room temperature

[{(R)-binap}Pt(μ-OH)]₂2X is a weak Lewis acid, which can catalyze the enantioselective aldol reaction of aldehydes with ketene silyl acetals in DMF at room temperature. The platinum(II) complex-catalyzed the enantioselective aldol reaction of aldehydes with 1-methoxy-2-methyl-(1-trimethylsilyloxy)propene gave the corresponding aldols in high yields with enantioselectivity up to 92%. With 5 mol % loading of the complexes, the enantioselective aldol reaction of aldehydes with 1-benzyloxy-1-(trimethylsilyloxy)propene smoothly proceeded in DMF containing 10% HMPA as to predominantly give anti-propionates with enantioselectivity up to 89%, irrespective of the silyl nucleophile geometry.     

(μ-Hydroxo)-platinum complex-catalyzed enantioselective aldol reaction of aldehydes with 1-methoxy-2-methyl-1-(trimethylsilyloxy)propene in DMF

[((R)-BINAP)Pt(μ-OH)]₂·2OTf catalyzed the enantioselective aldol reaction of aldehydes with 1-methoxy-2-methyl-(1-trimethylsilyloxy)propene at room temperature in dry DMF in high yields with enantioselectivity up to 92%. This is a versatile example of the catalytic enantioselective aldol reaction using a silyl ketene acetal promoted by (μ-hydroxo)-platinum complexes under mild conditions.     

Chiral Imidazolium Prolinate Salts as Efficient Synzymatic Organocatalysts for the Asymmetric Aldol Reaction

Chiral imidazolium l-prolinate salts, providing a complex network of supramolecular interaction in a chiral environment, have been studied as synzymatic catalytic systems. They are demonstrated to be green and efficient chiral organocatalysts for direct asymmetric aldol reactions at room temperature. The corresponding aldol products were obtained with moderate to good enantioselectivities. The influence of the presence of chirality in both the imidazolium cation and the prolinate anion on the transfer of chirality from the organocatalyst to the aldol product has been studied. Moreover, interesting match/mismatch situations have been observed regarding configuration of chirality of the two components through the analysis of results for organocatalysts derived from both enantiomers of prolinate (R/S) and the trans/cis isomers for the chiral fragment of the cation. This is associated with differences in the corresponding reaction rates but also to the different tendencies for the formation of aggregates, as evidenced by nonlinear effects studies (NLE). Excellent activities, selectivities, and enantioselectivities could be achieved by an appropriate selection of the structural elements at the cation and anion.     

Versatile One‐Step One‐Pot Direct Aldol Condensation Promoted by MgI2

A true one‐step one‐pot aldol‐reaction procedure has been developed for the synthesis of β‐hydroxy ketones and esters. The reaction can be run at room temperature by simply mixing four components in CH₂Cl₂, with medium‐to‐high yields of aldol products obtained after regular workup. Mechanistically, the process probably proceeds via Mg‐enolate formation of the ketone or ester component, followed by addition to the electrophilic aldehyde.     

Application of high pressure, induced by water freezing, to the direct asymmetric aldol reaction

High pressure, induced by water freezing, has been successfully applied to the direct catalytic asymmetric aldol reaction, in which higher yield and better enantioselectivity can be realized than in the reaction at room temperature under 0.1 MPa.     

Organocatalytic kinetic resolution of a planar-chiral ferrocenecarbaldehyde

The proline-catalyzed aldol reaction of racemic 2-(2′-pyrimidyl)ferrocenecarbaldehyde with acetone in DMSO at room temperature constitutes as the first example of an organocatalytic kinetic resolution of a planar-chiral compound. The selectivity factor of the kinetic resolution is 9.2, and the stereochemical outcome of the process can be easily rationalized by the standard mechanistic model of the proline-catalyzed aldol reaction.     

Synthesis and hydrogenation of (E)-γ-aryl-γ-morpholino-α-trifluoromethylated allyl alcohols through the reaction of trifluoroacetaldehyde ethyl hemiacetal with enamines

Treatment of trifluoroacetaldehyde ethyl hemiacetal with enamines, derived from acetophenone derivatives, at room temperature gave (E)-1,1,1-trifluoro-4-morpholino-4-aryl-but-3-en-2-ols, which are intermediates for preparation of the β-trifluoromethylated aldol products, 4,4,4-trifluoro-3-hydroxy-1-aryl-butan-1-ones. The structure of the intermediate (E)-1,1,1-trifluoro-4-morpholino-4-(4-nitrophenyl)-but-3-en-2-ols could be assigned by ¹H, ¹³C NMR, IR, and X-ray crystallography. Furthermore, hydrogenation and reductive deamination of the intermediate (E)-1,1,1-trifluoro-4-morpholino-4-aryl-but-3-en-2-ols with hydrogen in the presence of a catalytic amount (10 mol %) of palladium on carbon in trifluoroethanol proceeded smoothly at room temperature to give 1,1,1-trifluoro-4-aryl-2-butanols in good to excellent yields.     

New indenyl phosphinooxazoline complexes of iron and their catalytic activity in the Mukaiyama aldol reaction

New phosphinooxazoline (PHOX) η⁵-indenyl complexes of iron were synthesized and applied as catalysts in the Mukaiyama aldol reaction. Reaction of three different PHOX ligands with [Fe(η⁵-Ind)I(CO)₂] afforded the iodide salts of three complexes of the general formula [Fe(η⁵-Ind)(CO)(PHOX)]⁺ in 73-81% isolated yields. The molecular structure of one of the new complexes was determined, revealing a pseudo octahedral coordination geometry about the iron center. The iron complexes are catalytically active in the Mukaiyama aldol reaction between aldehydes and 1-(tert-butyldimethylsilyloxy)-1-methoxyethene to give the corresponding aldol adducts (3 mol % catalyst, 15 min, room temperature, 48-83% isolated yields). A previously synthesized iron complex of the general formula [Fe(η⁵-Cp)(CO)(PHOX)]⁺ was found to be catalytically active in the title reaction as well, but needed three hours at room temperature to convert the starting materials to the products.     

New N-terminal prolyl-dipeptide derivatives as organocatalysts for direct asymmetric aldol reaction

A series of new N-terminal prolyl-dipeptide derivatives have been synthesized and evaluated as organocatalysts for the direct asymmetric aldol reaction of acetone with electron-deficient aromatic aldehydes. At room temperature, the presence of 10 mol % of catalysts 2 and 5 efficiently catalyzes the direct asymmetric aldol reaction to give the aldol adducts with modest to excellent enantiomeric excesses (ee) values, which are up to 96%.     

Organocatalysts of tertiary-phosphines and amines catalyzed reactions of α-keto esters with cyclopent-2-enone

The reaction of α-keto esters with cyclopent-2-enone catalyzed by tertiary-phosphines provides the corresponding Baylis-Hillman adducts. Among the catalysts, diphenylmethylphosphine was found to be the most effective promoter allowing the reaction to proceed smoothly at room temperature and to give the corresponding adducts in higher yields in the presence of p-nitrophenol. Whereas, the similar reaction of α-keto esters with cyclopent-2-enone catalyzed by tertiary-amine of DBU furnishes the corresponding aldol adducts with syn-configuration exclusively.     

Synthesis, characterization, luminescence and nonlinear optical (NLO) properties of truxene-containing platinum(II) alkynyl complexes

A series of luminescent trinuclear platinum(II) alkynyl complexes containing dihydro-5H-diindeno[1,2-a;1′,2′-c]fluorene (truxene) as the core and aryl alkynyl ligands with different electronic properties at the periphery has been successfully synthesized and characterized. The electronic absorption, emission, nanosecond transient absorption and electrochemical properties of these complexes have been reported. These complexes showed long-lived emissions in degassed benzene solution at room temperature, and their emissions have been assigned to originate from triplet states of intraligand (IL) character with some mixing of metal-to-ligand charge-transfer (MLCT) character. The luminescent platinum(II) alkynyl complexes are found to show two-photon absorption (2PA) and two-photon induced luminescence (TPIL) properties, and their two-photon absorption cross-sections have been determined to be 6-51 GM upon excitation at 720 nm.     

Ion Dynamics in a Mixed‐Cation Alkoxy‐Ammonium Ionic Liquid Electrolyte for Sodium Device Applications

The ion dynamics in a novel sodium‐containing room‐temperature ionic liquid (IL) consisting of an ether‐functionalised quaternary ammonium cation and bis(trifluoromethylsulfonyl)amide [NTf₂] anion with various concentrations of Na[NTf₂] have been characterised using differential scanning calorimetry, impedance spectroscopy, diffusometry and NMR relaxation measurements. The IL studied has been specifically designed to dissolve a relatively large concentration of Na[NTf₂] salt (over 2 mol kg^?1) as this has been shown to improve ion transport and conductivity. Consistent with other studies, the measured ionic conductivity and diffusion coefficients show that the overall ionic mobility decreases with decreasing temperature and increasing salt content. NMR relaxation measurements provide evidence for correlated dynamics between the ether‐functionalised ammonium and Na cations, possibly with the latter species acting as cross‐links between multiple ammonium cations. Finally, preliminary cyclic voltammetry experiments show that this IL can undergo stable electrochemical cycling and could therefore be potentially useful as an electrolyte in a Na‐based device.     

Acceleration of the imine base/isocyanate (IBI)‐mediated polymerization of MMA caused by ionic liquid traces

The brutto rate of the imine base/isocyanate (IBI)‐mediated radical polymerization of methyl methacrylate (MMA) can be significantly increased by use of ionic liquid (IL) traces. At least, catalytic amounts of IL influence both the value of the brutto polymerization rate ν_Br,0 and the necessary reaction temperature of the used IBI mixture. Combinations of 2‐phenyl‐2‐oxazoline (POX) or 1‐methyl pyrazole (1MP) with isocyanates are IBI systems that usually do not initiate MMA at room temperature. By adding traces of 1‐ethyl‐3‐methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([Emim]FAP), polymethyl methacrylate (PMMA) with high average molecular weight can be obtained whereas the initiator mixture (imine base/isocyanate) concentration can be decreased by a factor of 10. The polymerization kinetics of several IBI combinations in the presence of ILs has been determined and a comparison to non‐IL containing initiator mixtures is given. Additionally, the temperature dependence of the IL‐containing polymerizations was measured. The interaction of the IL with MMA and the individual IBI initiator components is studied by means of attenuated total reflection Fourier transformation middle infrared spectroscopy (ATR FT MIR). Furthermore, the IBI brutto polymerization rate constants k_Br,0 were brought into relation to the IL hydrogen bond donating ability α. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

New simple and recyclable O-acylation serine derivatives as highly enantioselective catalysts for the large-scale asymmetric direct aldol reactions in the presence of water

New classes of O-acylation serine derived organocatalysts have been synthesized one-step by rational combination of serine with acyl chlorides at room temperature in trifluoroacetic acid. No protecting groups or chromatographic techniques are involved in any of the procedures, and certain combined serine-surfactant organocatalysts mediate the direct aldol reactions of ketones with a series of aromatic aldehydes to provide the aldol products in high yields (up to 99%) and enantioselectivities (up to 99% ee). The catalyst 1b can be easily recovered and reused, and without significant decrease of enantioselectivity was observed for five cycles. This novel catalyst can be efficiently used in large-scale reactions with the enantioselectivities being maintained at the same level, which offers a great possibility for application in industry.     

NHC-catalysed retro-aldol/aldol cascade reaction enabling solvent-controlled stereodivergent synthesis of spirooxindoles

An N-heterocyclic carbene (NHC)-catalysed retro-aldol/aldol cascade reaction of spirooxindole-based β-hydroxyaldehyde has been developed. The ring opening-closure process enables the diastereodivergent synthesis of spirocyclopentaneoxindole products with four consecutive stereocenters by simply changing the reaction solvents (THF or DCE). The Michael/aldol/retro-aldol/aldol sequential protocol allows the diastereodivergent synthesis of spirocyclopentaneoxindoles from 3-substituted oxindole and α,β-unsaturated aldehyde under the relay catalysis of a chiral secondary amine and an NHC catalyst. Moreover, four stereoisomers of the product can be selectively provided by using different combinations of a chiral secondary amine and a solvent.     

固载脯氨酸类手性催化剂的制备及在不对称Aldol反应中的催化作用

制备了两种二氧化硅-甲基纤维素-脯氨酸类手性小分子复合催化剂,证明了它们可以在室温下催化不对称Aldol反应.所得产物产率较高,ee值最高能达到86.8%.这些催化剂比较稳定,并具有一定的重复使用的性能.     

syn-Diastereoselective glycolate aldol addition reactions of an N(3)-(p-methoxyphenoxy)acetyloxazolidine-2-thione

An N₃-(p-methoxyphenoxy)acetyloxazolidine-2-thione has been synthesized and employed in glycolate asymmetric aldol addition reactions with aromatic and aliphatic aldehydes. It was determined that the titanium tetrachloride medicated aldol reaction afforded diastereoselectivities that ranged from 75:25 to 94:6 when the reaction was conducted at ?78 °C. The absolute stereochemistry of the aldol adducts was determined by ¹H NMR spectroscopy and X-ray crystallography. The ¹H NMR spectra of the aldol adducts contained a signal (the α-proton of the glycolate position of the aldol side chain) that was highly deshielded due to conformational restriction about the N(3)-(p-methoxyphenoxy)acetyl side chain and the oxazolidine-2-thione auxiliary.     

Recoverable silica-gel supported binam-prolinamides as organocatalysts for the enantioselective solvent-free intra- and intermolecular aldol reaction

Silica-gel supported binam-derived prolinamides are efficient organocatalysts for the direct intramolecular and intermolecular aldol reaction under solvent-free conditions using conventional magnetic stirring. These organocatalysts in combination with benzoic acid showed similar results to those obtained under similar homogeneous reaction conditions using an organocatalyst of related structure. For the intermolecular process, the aldol products were obtained at room temperature and using only 2 equiv of the ketone with high yields, regio-, diastereo- and enantioselectivities. Under these reaction conditions, also the cross aldol reaction between aldehydes is possible. The recovered catalyst can be reused up to nine times providing similar results. More interestingly, these heterogeneous organocatalysts can be used in the intramolecular aldol reaction allowing the synthesis of the Wieland–Miescher and ketone analogues with up to 92% ee, with its reused being possible up to five times without detrimental on the obtained results.     

Enantioselective Aldol Reaction with Difluoroenoxysilanes Catalyzed by Cationic Palladium Aqua Complexes

The enantioselective addition of difluoroenoxysilanes to aromatic aldehydes, catalyzed by cationic palladium aqua complexes, has been described. The reaction is operationally simple, does not require anhydrous conditions and performs at room temperature. The palladium catalysts, previously discovered and reported by Sodeoka, are easily prepared in two steps from PdCl₂(MeCN)₂. The aldol compounds were obtained in good yields and with enantiomeric ratios up to 93 : 7, the reaction being more efficient using electron-rich aldehydes.     

A novel and efficient direct aldol condensation from ketones and aromatic aldehydes catalyzed by proline-TEA through a new pathway

A novel and efficient direct aldol condensation from various ketones and a wide range of aldehydes was catalyzed by l-proline-TEA (triethylamine) in MeOH at room temperature, affording the corresponding (E)-α,β-unsaturated ketones in excellent yields. By using the method, some complicated (E)-α,β-unsaturated ketone C-glycosides were obtained from unmodified ketone C-glycosides and aldehydes. This reaction proceeds through a new pathway, in which the specific intermediate was captured and identified.