An efficient asymmetric aldol reaction of Chan's diene promoted by chiral Ti(IV)–BINOL complex

1,3-Bis-(trimethylsilyloxy)-1-methoxy-buta-1,3-diene (Chan's diene) can be conveniently used in asymmetric aldol reaction with aromatic, heteroaromatic, α,β-unsaturated and aliphatic aldehydes in the presence of catalytic amounts (2–8% mol) of chiral Ti(IV)/(R)-BINOL complex.     

Novel Fréchet-type dendritic BINOL ligands and their applications in Ti(IV) complex catalyzed asymmetric addition of diethylzinc to aldehydes

New Fréchet-type dendritic BINOL ligands bearing several BINOL units at the periphery have been successfully synthesized. The (dendritic BINOL) Ti(Ⅳ) complexes were proved to be efficient catalysts for the enantioselective addition of diethylzinc to aromatic aldehydes.Moreover,these ligands could be quantitatively recovered and reused at least five times without a loss of the yield and enantioselectivity.     

Novel Frechet-type dendritic BINOL ligands and their applications in Ti（IV） complex catalyzed asymmetric addition of diethylzinc to aldehydes

New Frechet-type dendritic BINOL ligands bearing several BINOL units at the periphery have been successfully synthesized. The （dendritic BINOL） Ti（IV） complexes were proved to be efficient catalysts for the enantioselective addition of diethylzinc to aromatic aldehydes. Moreover, these ligands could be quantitatively recovered and reused at least five times without a loss of the yield and enantioselectivity.     

Enantioselective addition of AlEt3 to aldehydes catalyzed by titanium(IV)–TADDOLate complex

Enantioselective addition of AlEt₃ to aldehydes catalyzed by a titanium(IV)–TADDOLate complex was carried out with ee values up to 99% for 2-chlorobenzaldehyde as a substrate. The key success of the study resides in the choice of THF as a solvent. THF is capable of coordinating to Al(III) and is speculated to lower the Lewis acidity of AlEt₃ and thus to suppress the competing reaction from AlEt₃ itself.     

Chiral diphenylperhydroindolinol silyl ether catalyzed domino oxa-Michael–aldol condensations for the asymmetric synthesis of benzopyrans

Asymmetric domino oxa-Michael–aldol reactions between trans-cinnamaldehydes and salicylic aldehyde derivatives have been developed. Using (2S,3aS,7aS)-diphenylperhydro indolinol silyl ether 4i as the catalyst, most corresponding chiral benzopyrans can be obtained with excellent chemo- and enantioselectivities.     

Insights into the stereoselective BF3-catalyzed hetero Diels–Alder reaction of Garner’s aldehyde with Danishefsky’s diene

A trans-2,6-disubstituted tetrahydropyranone was prepared in high yield via a BF₃-mediated hetero Diels–Alder reaction between Danishefsky’s diene and Garner’s aldehyde. Only two of the four possible reaction products were obtained, and excellent diastereoselectivity toward the exo–syn adduct was observed (de = 80%). The origin of this somewhat unusual selectivity can be attributed to the steric interactions between the bulky protecting groups present in the diene and dienophile.     

Identification of additives in poly(vinylacetate) artist’s paints using PY-GC-MS

Commercial poly(vinyl acetate) (PVAc) paint formulations for artists include a number of compounds in addition to the PVAc polymer and pigments to improve the physical and chemical properties of the resulting product. Among the most common additives are surfactants, coalescing agents, defoamers, freeze–thaw agents and thickeners. These products significantly influence the behaviour of the dried film. Nevertheless, they are usually difficult to detect with conventional analytical methods given their low concentration. In order to identify these additives, present in the dried film as minor components, an analytical method based on in situ thermally assisted pyrolysis–silylation gas chromatography–mass spectrometry (GC-MS) using hexamethyldisilazane as a derivatisation reagent is proposed. This method improves the conventional GC-MS analysis performed by direct pyrolysis and enables the simultaneous identification of the PVAc binding medium and the additives included by the manufacturer in the commercial paint. Five different commercial PVAc paints have been analysed, namely, armour green, burnt umber, oriental red, raw umber and white from Flashe®. Internal plasticiser VeoVa consisting of C₁₀ fatty acids with highly branched chains has been recognised from the MS spectra. On the other hand, the differences found in the additive content of the studied paints, in particular the poly(ethylene glycol)-type surfactant, are in good agreement with their mechanical properties.

Reversal of enantioselection in an aldol reaction catalyzed by sterically congested bis(oxazoline)–Cu(II) complexes

The use of sterically congested C₂-symmetrical bis(oxazoline) ligands with methylene and ethylene spacers between the oxazoline rings results in the reversal of the enantioselection for aldol reactions catalyzed by bis(oxazoline)–Cu(II) complexes.     

Proline–calixarene thiourea host–guest complex catalyzed enantioselective aldol reactions: from nonpolar solvents to the presence of water

A proline–calix[4]arene thiourea host–guest complex catalyzed intermolecular aldol reaction of aromatic aldehydes with cyclohexanone has been developed. The anti-configured products were obtained in good yields and with high enantioselectivities. The reaction is proposed to work via a modified Houk–List model, where the carboxylate part of the proline constitutes as a supramolecular system with the thiourea. The outcome of the study indicates the influence of the calix[4]arene thiourea on both the reactivity and selectivity in a non-polar reaction medium, even in the presence of water at moderate temperatures.     

Enantio- and diastereoselective hetero-Diels–Alder reactions between 4-methyl-substituted Rawal’s diene and aldehydes catalyzed by chiral dirhodium(II) carboxamidates: catalytic asymmetric synthesis of (−)-cis-aerangis lactone

The first catalytic asymmetric hetero-Diels–Alder (HDA) reaction between 3-tert-butyldimethylsilyloxy-1-dimethylamino-1,3-pentadiene (4-methyl-substituted Rawal’s diene) and aldehydes is described. With 3 mol % of dirhodium(II) tetrakis[N-benzene-fused-phthaloyl-(S)-piperidinonate], Rh₂((S)-BPTPI)₄, the cycloaddition reaction proceeded exclusively in an endo fashion and gave, after a novel sequential treatment with dimethyl acetylenedicarboxylate and acetyl chloride, the corresponding 2,3-cis-disubstituted dihydropyranones with up to 98% ee and perfect diastereoselectivity. The utility of this catalytic protocol was demonstrated by an asymmetric synthesis of the (−)-cis-aerangis lactone.     

Role of anions in the kinetics of the acid catalyzed dissociation of tris-(2,2′bipyridine) Fe(II) complex

Excess of Cl⁻ and NO₃⁻ ions bring about a significant increase in the rate of dissociation of the tris-(2,2′bipyridine) Fe(II) complex in HCl, HClO₄ and H₂SO₄ in a very narrow range of H⁺ concentration (∼0·1−0·3 M). Above and below this concentration range, these anions do not lead to significant variation in the rate of dissociation. This points to the involvement of these ions in a kinetic step which is only important in the rate expression in this narrow range. It is suggested that the anions occupy the coordination site left vacant by the partial dissociation of the complex leading to the half bonded intermediate. This would significantly lower the free energy of activation for the subsequent protonation step leading to complete dissociation and thus lead to acceleration. The lowering of the free energy of activation will be mainly due to an increase in entropy of this intermediate on account of charge reduction and lower rigidity due to lack of hydrogen bonding. The retarding effect of SO₄²⁻ concentrations and the unambiguously interpreted as its addition leads to simultaneous changes in H⁺ and HSO₄⁻ concentrations and the possibility of ion pair formation with the complex cannot be ruled out.     

Highly stereoselective direct aldol reaction of 4-formylcoumarins with acetone catalyzed by L-proline in water–acetone mixtures

Organocatalyzed direct intermolecular aldol reactions have been developed for substituted 4-formylcoumarins with acetone in water using L-proline and phthalimido-prolinamide catalysts without use of additives. Stereoselective products obtained were in excellent yields (up to 97%) with high purity (up to 99%) and enatioselectivities (up to 95%). The isolated compounds were confirmed by infrared, NMR, high-performance liquid chromatography, and mass spectrometry and some of them by single-crystal x-ray crystallography.     

Enantioselective addition of terminal alkynes to N-(diphenylphosphinoyl)imines catalyzed by Zn–BINOL complexes

Chiral nonracemic N-(diphenylphosphinoyl)-protected propargylic amines have been prepared by addition of terminal alkynes to N-(diphenylphosphinoyl)aldimines in the presence of dimethylzinc and 3,3′-dibromo-BINOL as catalyst. The reaction works with a variety of aromatic and heteroaromatic aldimines and with different alkynes, providing the expected products in generally good yields and enantiomeric excesses (up to 96%).     

(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.     

One-step,efficient synthesis of combined threonine–surfactant organocatalysts for the highly enantioselective direct aldol reactions of cyclic ketones with aromatic aldehydes in the presence of water

In this work, a new class of organocatalysts have been designed and synthesized in one step by a rational combination of threonine with acyl chlorides at room temperature in trifluoroacetic acid to make a series of novel combined threonine-surfactant organocatalysts readily available on a large-scale. No protecting groups or chromatographic techniques are involved in the procedures, while certain combined threonine-surfactant organocatalysts mediate the direct aldol reactions of cyclic ketones with aromatic aldehydes to provide the aldol products in good yields with enantioselectivities of up to 99%.     

Structure and properties of the ion pair charge-transfer complex of octacyanotungstate(IV) with the 2,2′-bipyridinium dication

The X-ray crystal structure of (bpyH2)2[W(CN)8]·4H2 O (bpyH2=2,2-bipyridinium) is described. The [W(CN)8]4– anion has an approximately square antiprismatic (D4d) conformation, seemingly imposed by strong anion–water–cation hydrogen-bonding interactions. Bond distances in the anion are: WC 2.150(5), 2.163(5); CN 1.128(6), 1.145(6)Å and the angles WCN are 177.6(5), 178.3(5)°. The dihydrate and the anhydrous salt are both intensely black solids, exhibiting ion-pair charge-transfer interaction between cation and anion. E.s.r. spectra indicate that 30% of the tungsten is present as WV in the solid state, but that in solution only the WIV complex is present. The electron withdrawing effect of the cation is discussed and compared with that in a series of salts with different bipyridinium cations.     

Friedel-Crafts alkylation of indoles with nitroalkenes catalyzed by Cu(Ⅱ)-imine complex

A series of new ligands L₁-L₇ were readily prepared in one step.Friedel-Crafts alkylation of indoles with nitroalkenes catalyzed by a novel Cu（Ⅱ）-L complex has been developed.The remarkable advantages of this reaction are mild reaction conditions,simple workup procedure,high yields of products and the use of ethanol as a green solvent.     

1-Naphthyl-TADDOL/Emim BF<Subscript>4</Subscript>: A new catalytic system for the asymmetric addition of Chan’s diene to aromatic aldehydes

A new organocatalytic system was tested as a promoter for the asymmetric addition of Chan’s diene to aldehydes under solvent-free conditions. This new organocatalyst generated in situ by mixing 1-naphthyl-TADDOL derivative and Emim BF₄ was able to give enantioenriched vinylogous aldols and hetero-Diels-Alder cycloadducts. A mechanistic investigation through the detection of nonlinear effects confirmed the involvement of the ionic liquid in the formation of a new catalytic supramolecular species.     

Synthesis and crystal structure of a trinuclear titanium(IV) β -diketonate complex with a six-membered [Ti–(μ-O)]3 ring

The trinuclear titanium(IV) β-diketonate complex, [Ti₃(μ-O)₃(dppd)₆]?·?(DMF)(0.5EtOH) (1) (H-dppd?=?1,3-diphenylpropane-1,3-dione) has been synthesized and its crystal structure and spectroscopic properties have been studied. Complex 1 crystallizes in monoclinic system with space group C2/c and a?=?20.954(5)?Å, b?=?18.705(4)?Å, c?=?24.617(6)?Å, β?=?111.957(4)°, Z?=?4. The complex consists of a six-membered [Ti–(μ-O)]₃ ring, of which three titanium(IV) ions are linked together by bridging oxygen atoms. Each titanium(IV) possesses an octahedral geometry, with four oxygen atoms from the two β-diketonate ligands and two bridging oxygen atoms.