Synthesis and structural chemistry of oxazolinyl-carbene copper(I) complexes

Reaction of a series of directly connected oxazoline–imidazolium salts with potassium tert-butoxide and in the presence of CuBr · SMe₂ at −78 °C cleanly gave the corresponding 2-oxazolinyl-(N-mesityl)imidazolidenecopper(I) complexes which are monomeric in solution but aggregate in the solid state. X-ray diffraction studies established a dimeric structure for [{2-(4,4-dimethyl)-oxazolinyl-(N-mesityl)imidazolidene}(bromo)copper(I)]₂ (2a) whereas the chiral derivative [{2-(4-S-isopropyl)-oxazolinyl-(N-mesityl)imidazolidene}(bromo)-copper(I)]_∝ (2b) forms infinite chains of a coordination polymer.     

Synthesis and asymmetric catalytic application of chiral imidazolium–phosphines derived from (1R,2R)-trans-diaminocyclohexane

Reaction between a chiral imidazole–amine precursor derived from (1R,2R)-trans-diaminocyclohexane and P¹Cl (where P¹ = PPh₂, P(1,3,5-Me₃C₆H₃)₂, P(2,2′-O,O′-(1,1′-biphenyl), P((R)-(2,2′-O,O′-(1,1′-binaphthyl))) and P((S)-(2,2′-O,O′-(1,1′-binaphthyl)))) followed by RX (where R = ⁿPr, ⁱPr, CHPh₂, X = Br; R = ⁱPr, X = I), respectively, gives a selection of chiral imidazolium–phosphine compounds. Deprotonation of the imidazolium salt gives the corresponding NHC–P ligands that can be used in metal-mediated asymmetric catalytic applications. Catalytic reactions show that NHC–P ligands give a significantly greater rate of reaction for a palladium catalysed allylic substitution reaction in comparison to analogous di-NHC or NHC–imine ligands and that NHC–P hybrids are also effective for iridium catalysed transfer hydrogenation.     

Enantioselective Michael reaction of β-keto esters organocatalyzed by recoverable Cinchona-derived dimeric ammonium salts

Dimeric anthracenyldimethyl-derived Cinchona ammonium salts are used as chiral organocatalysts in 1–10 mol % for the enantioselective conjugate addition of 2-alkoxycarbonyl-1-indanones to β-unsubstituted Michael acceptors. The corresponding adducts bearing a new all-carbon quaternary center were usually obtained in high yield and with up to 94% ee when using ammonium salts derived from quinidine and its pseudoenantiomer quinine as organocatalysts. These catalysts can be almost recovered quantitatively by precipitation in ether and reused.     

Molecular mechanism of DCR phenomenon observed in (RS)-1-cyclohexylethylamine–mandelic acid resolution system

In the resolution system of (RS)-1-cyclohexylethylamine (CHEA) with (S)-mandelic acid (MA), the DCR (dielectrically controlled resolution) phenomenon was clearly observed. (S)-CHEA and (R)-CHEA were obtained as less-soluble salts from 2-propanol (ε = 18) and water (ε = 78), respectively. Although (S)-MAs pack similarly in the crystals of both of the salts, the chiral spaces formed between the columns of (S)-MAs are significantly different in shape. The present study showed that these chiral spaces are crucial to molecular recognition and their shapes are greatly influenced by the dielectric property of the solvent employed.     

Viscosities and refractive indices of binary mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate and 1-butyl-2,3-dimethylimidazolium tetrafluoroborate with water at 298 K

Viscosities and refractive indices have been determined for (water + 1-butyl-3-methylimidazolium tetrafluoroborate) and (water + 1-butyl-2,3-dimethylimidazolium tetrafluoroborate) mixtures at 298.15 K, over the whole composition range. The refractive indices were compared with the predictions of the Lorentz–Lorenz, Wiener, and Gladstone–Dale equations. Viscosity deviations (Δη) and refractive index deviations (Δn_D) have been calculated and fitted to the Redlich–Kister polynomial equations. Δn_D are positive whereas Δη are negative over the entire mixture composition for the two salts. The influence of the structure of imidazolium cation on the above physicochemical properties was discussed.     

Synthesis of chiral monoaza-15-crown-5 ethers from a chiral amino alcohol and enantiomeric recognition of potassium and sodium salts of amino acids

The enantiomeric recognition of chiral monoaza-crown ethers for amino acids as their sodium and potassium salts has been investigated by UV–vis. The highest discrimination was observed for TrpK (d/l = 6.47). The reversed enantioselectivity of chiral monoaza-crown ether II was observed for TrpK.     

Preparation of various xanthene derivatives over sulfonic acid functionalized imidazolium salts (SAFIS) as novel,highly efficient and reusable catalysts

In this work, some novel sulfonic acid functionalized imidazolium salts (SAFIS), as a new category of ionic liquids, are synthesized by eco-friendly and simple procedures, and used as highly efficient and reusable catalysts to promote the following one-pot multicomponent organic transformations under solvent-free conditions: (i) the synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes from β-naphthol (2 eq.) and arylaldehydes (1 eq.), (ii) the preparation of tetrahydrobenzo[a]xanthene-11-ones from β-naphthol, arylaldehydes and dimedone, and (iii) the synthesis of 1,8-dioxo-octahydroxanthenes from dimedone (2 eq.) and aromatic aldehydes (1 eq.). Environmentally benign, simple methodologies, easy workup procedure, clean reaction, short reaction time, high yield and easy preparation of the catalysts are some advantages of this work.     

(Liquid + liquid) equilibria for (acetate-based ionic liquids + inorganic salts) aqueous two-phase systems

In the present work, (liquid + liquid) equilibrium data have been determined experimentally for aqueous two-phase systems formed by the imidazolium ionic liquids of [C_nmim][CH₃COO] (n = 4, 6, 8) and inorganic salts of K₃PO₄, K₂HPO₄, and K₂CO₃ at T = 298.15 K. Combined with available data in the literature, the effect of alkyl chain length of cations, type of anions of the ionic liquids, and nature of the inorganic salts were examined on the binodal curves of the systems. Then the binodal curves were fitted to a four-parameter empirical equation, and the tie-lines were described by the Othmer–Tobias and Bancroft equations. In addition, the extraction capacity of the {[C_nmim][CH₃COO] (n = 4, 6, 8) + K₃PO₄} aqueous two-phase systems was evaluated through their application to the extraction of l-tryptophan. The high extraction efficiency suggests that these aqueous two-phase systems are feasible to be used in the extraction and separation process.     

Ionic liquids as entrainers for water + ethanol,water + 2-propanol,and water + THF systems: A quantum chemical approach

Ionic liquids (ILs) are used as entrainers in azeotropic systems such as water + ethanol, water + 2-propanol, and water + tetrahydrofuran (THF). Ionic liquids consisting of a cation and an anion has limitless combinations, thereby making experimentation expensive and time taking. For the prediction of the liquid phase nonidealities resulting from molecular interactions, “COnductor-like Screening MOdel for Real Solvents” (COSMO-RS) approach is used in this work for the screening of potential ionic liquids. Initially benchmarking has been done on 12 reported isobaric IL based ternary systems with an absolute average deviation of 4.63% in vapor phase mole fraction and 1.07% in temperature. After successful benchmarking, ternary vapor + liquid equilibria for the azeotropic mixture of (a) ethanol + water, (b) 2-propanol + water, and (c) THF + water with combinations involving 10 cations (imidazolium, pyridinium, quinolium) and 24 anions were predicted. The VLE prediction, which gave the relative volatility, showed that the imidazolium based ionic liquid were the best entrainer for the separation of the three systems at their azeotropic point. ILs with [MMIM] cation in combination with acetate [OAc], chloride [Cl], and bromide [Br] anion gave the highest relative volatility.     

Phase behavior of ionic liquids 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imides with halogenated benzenes

(Liquid + liquid) miscibility temperatures as a function of composition have been determined experimentally for the binary systems formed by imidazolium based ionic liquids with bis(trifluoromethylsulfonyl)imide ([C_nMIM][NTf₂]: n = 3 to 10) with fluorobenzene, chlorobenzene, bromobenzene, iodobenzene and 1,2-dichlorobenzene. In addition, the phase diagrams for deuterated chlorobenzene, bromobenzene and 1,2-dichlorobenzene have been obtained. All the measured systems show the limited miscibility with the upper critical solution temperature behavior. Similarly to the other systems with the imidazolium cation the increase of the alkyl chain length in this cation improves the miscibility. The impact of the halogenobenzene is also very visible. The miscibility is improving in the order: iodobenzene < bromobenzene < chlorobenzene < fluorobenzene. This arrangement corresponds to the decreasing molar volume of the substituted benzenes. The disubstituted chlorobenzene is a better solvent for ionic liquids than chlorobenzene. The replacement of hydrogen for deuterium in halogenobenzenes in all cases improves the miscibility and the isotope shifts of the UCSTs are very large.     

Temperature dependence of the surface tension and 0.1 MPa density for 1-Cn-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate with n = 2, 4, and 6

Experimental air–liquid interfacial tension data and density data are presented for three 1-C_n-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphates (FAP), [C_nMIM][(C₂F₅)₃PF₃], with n = 2, 4, and 6, measured at atmospheric pressure in the temperature range from 267 K to 360 K using the Krűss K100MK2 tensiometer. The accuracy of the surface tension measurements was checked by employing the Wilhelmy plate and the du Noüy ring methods in parallel. The combined standard uncertainty associated with the Wilhelmy plate method is estimated to be ±0.1 mN · m⁻¹. The density data were obtained using buoyancy method with an estimated standard uncertainty less then ±0.4 kg · m⁻³ (3 · 10⁻⁴ϱ). The chloride anions decrease the density of the tris(pentafluoroethyl)trifluorophosphates of interest up to six times more effectively than they decrease the density of the imidazolium based tetrafluoroborates. A QSPR analysis of the surface tension of imidazolium based ionic liquids with BF₄, TFA, DCA, FAP, NTf₂, and PF₆ anions indicates, that the FAP ionic liquids fit well into the analyzed group of imidazolium based ionic liquids while those having hexafluorophosphate anion show anomalously high deviations of the experimental surface tension from the values predicted by the QSPR model.     

Effect of ether group on the electrochemical stability of zwitterionic imidazolium compounds

The cathodic stability of the zwitterionic imidazolium compounds was significantly enhanced by the introduction of an ether group at 1 or 2-position on the imidazolium ring. The cycle performance tests showed that the initial cell capacity was maintained almost unchanged up to 100 cycles at 0.5 and 1 C when 2.5 wt.% of 2-butoxymethyl-1-methylimidazolium-3-propylsulfonate or 2-butoxymethyl-1-butylimidazolium-3-propylsulfonate was added to the model electrolyte (1 M LiPF₆ in ethylene carbonate, dimethyl carbonate and ethylmethyl carbonate (1/1/1 v/v/v)).Structures of zwitterionic compounds and their interactions with lithium ions were theoretically investigated.     

Sodium carbonate as phase promoter in aqueous solutions of imidazolium and pyridinium ionic liquids

Several methylsulfate and chloride anion-based ionic liquids, such as 1-alkyl-3-methyl imidazolium methyl sulfate, C_nMIM CH₃SO₄ (n = 1, 2, and 4), 1-benzyl-3-methyl imidazolium methyl sulfate, BzMIM CH₃SO₄, 1-benzyl, or hexyl-3-methyl imidazolium chloride, XMIM Cl (X = Bz and Hx), and methylpyridinium methylsulfate, Mpy CH₃SO₄, with sodium carbonate, Na₂CO₃, as phase forming salt, have been investigated and discussed for their potential use in separations based on aqueous two-phase systems (ATPS). Phase diagrams have been experimentally ascertained at T = 298.15 K, and Merchuck equation and a variation of this model have been used for correlating the binodal data. The alkyl chain length in the cation and the type of anion and cation have been explored and discussed due to their decisive influence in the ATPS behavior. The consistence of tie-line data was ascertained by applying the Othmer–Tobias and Bancroft equations.     

Enantiomeric recognition of amino acid derivatives by chiral schiff bases of calix[4]arene

Novel calix[4]arene Schiff bases bearing chiral substituents both on the upper and the lower rims have been developed. These chiral receptors exhibit good chiral recognition ability towards α-amino acid ester hydrochlorides (up to K_D/K_L = 4.36, ΔΔG₀ = − 3.65 kJ mol⁻¹) in CHCl₃. The molecular recognition abilities and enantioselectivities for guests are also discussed from a thermodynamic point of view.     

Silver alkoxide and amino N-heterocyclic carbenes; syntheses and crystal structures

Silver(I) complexes of heterobidentate ligands that incorporate one or two N-heterocyclic carbene moieties coupled with an alcohol or amine group have been made by direct deprotonation of ligands of the form [HOCR¹R²CH₂(1-HC{NCHCHNR})][X], H₂L¹X (X = Br, I), [H₂NR¹CHR²CHR²(1-HC{NCHCHNR})][Br]₂ H₃L²X₂ (X = Cl, Br), and [H₂N{CH₂CH₂(1-HC[NCHCHNMes])}₂][X]₃ H₄L³X₃ (X = Cl, Br). Silver(I) oxide is sufficiently basic to deprotonate both the imidazolium and the alcohol functional groups of all but one of the L¹ ligand precursors, to afford rare examples of silver alkoxide complexes [Ag(L¹)], stabilised by the soft donor carbene. Another complex of L¹ is characterised as the carbene alcohol adduct [Ag(HL¹)₂I]. The analogous reactions of silver(I) oxide with the amino imidazolium precursors afford silver amino-carbenes [Ag(HL²)Br] with the potentially bidentate L² ligand, and [Ag(HL³)X] (X = Cl, Br) with the potentially tridentate L³ ligand. A single crystal X-ray diffraction study of the latter complex confirms that the neutral amine of the potentially tridentate L³ ligand is unco-ordinated; instead the structure contains discrete chains of T-shaped silver bis(carbene) halide moieties that bridge to form a zig-zag 2-connected polymer. Protonolysis of two of the silver alkoxide and amino adducts, [Ag(L^1a)] and [Ag(HL^2a)Br], affords imidazolium complexes salts [H₂L^1a][AgCl₂] and [Ag(H₂L^2a)Br][AgBr₂] that retain the Ag(I) centre as complex counterions. The single crystal X-ray structures of these salts have been determined and show the silver(I) cations are now incorporated into ladders or chains as silver(I) halo-anions, and a silver amine dative bond is present in the latter complex.     

(Liquid + liquid) equilibrium of aqueous biphasic systems composed of 1-benzyl or 1-hexyl-3-methylimidazolium chloride ionic liquids and inorganic salts

(Liquid + liquid) equilibria for {1-benzyl-3-methylimidazolium chloride ([BzMIM]Cl) or 1-hexyl-3-methylimidazolium chloride ([HMIM]Cl) + inorganic salts (potassium phosphate K₃PO₄, potassium carbonate K₂CO₃, or dipotassium hydrogen phosphate K₂HPO₄) + H₂O} aqueous biphasic systems (ABSs) are presented at T = 298.15 K. An empirical equation was used to correlate the binodal data. The experimental tie lines were appropriately correlated by the Othmer–Tobias and Brancroft empirical equations. The influence of the selected inorganic salts in the phase segregation was investigated by means the calculated effective excluded volume (EEV) and Setschenow-type equation. The salting-out ability of salts was also evaluated in terms of the Gibbs energy of hydration of salt (ΔG_hyd) and assessed with EEV values.     

Pyridine-containing chiral macrocycles for the enantioselective recognition of amino acid derivatives and their molecular dynamics simulations

Two novel C₂-symmetric optically active pyridine-15-crown-5 type ligands containing lipophilic chains at the stereogenic centres, macrocycles 5 and 6, were prepared from (S)-1,2-propanediol and (S)-3-aryloxy-1,2-propanediol for the enantiomeric recognition of amino acid ester derivatives. These novel macrocycles have been shown to be strong complexing agents for primary organic ammonium salts (with K values of up to 1363.5 M^?1, ΔG^o of up to 17.86 kJ mol^?1 and a selectivity ratio of 80:20) by ¹H NMR titration method. These macrocyclic host exhibited enantioselective binding towards the l-enantiomer of phenylalanine methyl ester hydrochloride with K_L/K_D up to 8.57 in CDCl₃ containing 0.25% CD₃OD. Experimental results have been detailed with molecular dynamic calculations at atomic level concerning the molecular recognition and discrimination properties of a chiral pyridino-15-crown-5. The binding free energies were calculated as ～?25 kJ mol^?1. The results indicated that the host binds and discriminates valine salts better than phenylalanine salts. The molecular dynamics, MM/PBSA calculations are consistent with the ¹H NMR results.     

Chiral 1,4-morpholin-2,5-dione derivatives as α-glucosidase inhibitors: Part 2

A series of chiral 1,4-morpholin-2,5-dione derivatives were synthesized starting from chiral synthons 1 and 2, diastereomeric monolactim ethers derived from l-valine. The compounds investigated, were inactive toward β-glucosidase, α-mannosidase and α-galactosidase but behave as noncompetitive inhibitors against the α-glucosidase (from Saccharomices cervisiae) with some showing a good inhibition ability (0.05 < K_i < 0.18 mM).     

Solubility of inorganic salts in pure ionic liquids

The solubility of different conventional salts in several room-temperature ionic liquids – containing ammonium, phosphonium or imidazolium cations combined with acetate, sulfate, sulfonate, thiocyanate, chloride, tetracyano-borate, tris(pentafluoroethyl)trifluoro-phosphate, l-lactate, bis(trifluoromethylsulfonyl)imide or trifluoromethylsulfonate anions – were screened using a visual detection method. The most promising mixtures were then re-measured using an ATR–FTIR (Attenuated Total Reflection Fourier Transform Infra Red) spectroscopy technique in order to accurately and quantitatively determine the corresponding solubility at 298.15 K.     

trans-(2R,3R)-2,3-Diphenylcyclopropane-1,1-dimethanol: a pivotal diol for the synthesis of novel C2-symmetric ligands for asymmetric transition metal catalysis

Various chiral ligands bearing phosphorus or nitrogen donor atoms were obtained in a straightforward manner starting from trans-(2R,3R)-diphenylcyclopropane-1,1-dimethanol as a key structure. These chiral ligands were tested and compared in palladium(0)-catalyzed asymmetric allylic alkylation reactions (up to 71% ee) and rhodium(I)-catalyzed asymmetric hydrogenations (up to 88% ee). Moreover, in the asymmetric allylic alkylation, we observed excellent activity with a diphosphinite ligand (TOF = 600 mol 17 × [mol Pd × h)^?1].