Enantioselective amination of silylketene acetals with (N-arylsulfonylimino)phenyliodinanes catalyzed by chiral dirhodium(II) carboxylates: asymmetric synthesis of phenylglycine derivatives

The first catalytic enantioselective amination of silylketene acetals with (N-arylsulfonylimino)phenyliodinanes is described. The reaction of silylketene acetals derived from methyl phenylacetates with [N-(2-nitrophenylsulfonyl)imino]phenyliodinane (NsN = IPh) under the catalysis of dirhodium(II) tetrakis[N-tetrachlorophthaloyl-(S)-tert-leucinate], Rh₂(S-TCPTTL)₄, proceeds in benzene at room temperature to give N-(2-nitrophenylsulfonyl)phenylglycine derivatives in high yields and with enantioselectivities of up to 99% ee.     

Enantio- and diastereoselective cyclopropanation with tert-butyl α-diazopropionate catalyzed by dirhodium(II) tetrakis[N-tetrabromophthaloyl-(S)-tert-leucinate]

The first successful example of a catalytic asymmetric cyclopropanation with α-diazopropionates is described. The cyclopropanation reaction of 1-aryl-substituted and related conjugated alkenes with tert-butyl α-diazopropionate has been achieved by catalysis with dirhodium(II) tetrakis[N-tetrabromophthaloyl-(S)-tert-leucinate], Rh₂(S-TBPTTL)₄, providing the corresponding cyclopropane products containing a quarternary stereogenic center in good to high yields and with high diastereo- and enantioselectivities (trans:cis = 90:10 to >99:1, 81-93% ee).     

N-tert-Butoxycarbonylation of amines using H3PW12O40 as an efficient heterogeneous and recyclable catalyst

The commercially available heteropoly acid H₃PW₁₂O₄₀ (0.5 mol %) is a very efficient and environmentally benign catalyst for N-tert-butoxycarbonylation of amines (primary, secondary) with di-tert-butyl dicarbonate at room temperature in short reaction times (<10 min). No competitive side products such as isocyanates, ureas, N,N-di-tert-butoxycarbonyls, O-Boc and oxazolidinones were observed. Chiral α-amino alcohols and esters afforded the corresponding N-Boc derivatives chemoselectively in excellent yields.     

Acid catalyzed monodehydro-2,5-diketopiperazine formation from N-α-ketoacyl amino acid amides

We have developed a new synthetic method for monodehydro-2,5-diketopiperazines (monodehydroDKPs), which is based on an acid catalyzed cyclization of N-α-ketoacyl amino acid amides. Using this cyclization reaction, monodehydroDKP was formed with no or slight racemization in case that N-α-ketoacyl amino acid amides with β-aliphatic-α-ketoacyl groups and sterically unhindered N-substituting groups at the C-terminal amide nitrogen were used in the presence of catalytic amount of p-TsOH (3-5 mol %) or 10% TFA. In the case of β-aryl-α-ketoacyl amino acid derivatives, in which an enol form predominantly exists by conjugation with the aromatic ring, racemization could be minimized by optimizing the reaction conditions (5 mol % p-TsOH, reflux for 6 h), although the chemical yield could not be dramatically improved. However, this reaction condition was successfully applied to the synthesis of a tubulin depolymerization agent, (−)-tert-butyl-oxa-phenylahistin, with no racemization.     

EPR study of mono-o-iminobenzosemiquinonato nickel(II) complexes with Ni-C σ-bond

New square-planar (Ph₃P)Ni^II(o-Tol)(ISQ-Prⁱ) (1), (Ph₃P)Ni^II(o-Tol)(ISQ-Me) (2), (Ph₃P)Ni^II(o-Tol)(ISQ-Bu^t) (3) nickel complexes (where ISQ-Prⁱ = 4,6-di-tert-butyl-N-(2,6-di-iso-propylphenyl)-o-iminobenzosemiquinonate, ISQ-Me = 4,6-di-tert-butyl-N-(2,6-di-methylphenyl)-o-iminobenzosemiquinonate, ISQ-Bu^t = 4,6-di-tert-butyl-N-(2,5-di-tert-butylphenyl)-o-iminobenzosemiquinonate, o-Tol = o-tolyl ligand) have been synthesized. Complexes contain σ-bound o-tolyl and neutral donor ligand Ph₃P. The sterical hindrances of N-aryl in o-iminobenzosemiquinonate ligands lead to the tetrahedral distortion of square-planar configurations of complexes as it was established using EPR spectroscopy.     

Formation and structures of Pd(II) N,S-heterocyclic carbene-pyridyl mixed-ligand complexes

Mononuclear mixed-ligand complexes of Pd(II) containing a N,S-heterocyclic carbene (NSHC) with a secondary alkyl N-substituent and pyridyl ligand, with the general formula [PdI₂(C₁₀H₁₁NS)L] (C₁₀H₁₁NS = 3-isopropylbenzothiazolin-2-ylidene; L = pyridine, 2-aminopyridine, 3-iodopyridine and 4-tert-butyl-pyridine) have been synthesized and characterized by X-ray single-crystal crystallography. Both solution and solid-state structures, as evident from their ¹H NMR spectra and X-ray structures, show anagostic γ-hydrogen interactions of metal with methine of the substituent on the carbene or pyridyl ligand giving 5-membered-chelate-like structures.     

Asymmetric addition of trimethylsilylcyanide to N-benzylimines catalyzed by recyclable chiral dimeric V(V) salen complex

Chiral dimeric vanadium (V) salen complex (10 mol%) derived from 5,5-Methylene di-[(S,S)-{N-(3-tert-butyl salicylidine)-N′-(3′,5′-di-tert-butyl salicylidene)]-1,2-cyclohexanediamine] with vanadyl suphate followed by auto oxidation was used as efficient catalyst for enantioselective Strecker reaction of N-benzylimines with TMSCN at −30 °C. Excellent yield (92%) of α-aminonitrile and high chiral induction was achieved (ee up to 94%) in case of 2-methoxy substituted N-benzylimines in 10 h. The catalytic system worked well up to four cycles with retention of enantioselectivity.     

Synthesis and nonlinear optical properties of new symmetrically substituted stilbenes

Three new chromophores and trans-4-(N-(ethyl 4″-nitrobenzoate)-N-ethyl amino)-4′-(dimethyl amino) stilbene (DMANHAS) have been synthesized and whose chemical structures have been characterized by ¹H NMR, IR, and elemental analyses. Linear absorption, single-photon-induced fluorescence and two-photon-induced fluorescence are experimentally studied. Trans-4-(N-2-hydroxyethyl-N-ethyl amino)-4′-(dimethyl amino)stilbene (DMAHAS) and trans-4-(N-2-hydroxyethyl-N-ethyl amino)-4′-(diethyl amino)stilbene (DEAHAS) have effective two-photon absorption cross-sections of σ₂=0.91×10⁻⁴⁶ cm⁴ s/photon and σ₂=1.19×10⁻⁴⁶ cm⁴ s/photon at 532 nm by using an open aperture Z-scan technique, respectively. When pumped with 800 nm laser irradiation, DMAHAS and DEAHAS indicate strong two-photon-induced blue fluorescence of 436 and 440 nm, respectively, while trans-4-(N-(ethyl 4″-nitrobenzoate)-N-ethyl amino)-4′-(diethyl amino) stilbene (DEANHAS) and DMANHAS exhibit no fluorescence.     

Catalytic efficacy of Schiff-base copper(II) complexes: Synthesis,X-ray structure and olefin oxidation

Three copper(II) Schiff-base complexes, [Cu(L¹)(H₂O)](ClO₄) (1), [Cu(L²)] (2) and [Cu(L³)] (3) have been synthesized and characterized [where HL¹ = 1-(N-ortho-hydroxy-acetophenimine)-2-methyl-pyridine], H₂L² = N,N′-(2-hydroxy-propane-1,3-diyl)-bis-salicylideneimine and H₂L³ = N,N′-(2,2-dimethyl-propane-1,3-diyl)-bis-salicylideneimine]. The structure of complex 1 has been determined by single crystal X-ray diffraction analysis. In complex 1, the copper(II) ion is coordinated to one oxygen atom and two nitrogen atoms of the tridentate Schiff-base ligand, HL¹. The fourth coordination site of the central metal ion is occupied by the oxygen atom from a water molecule. All the complexes exhibit high catalytic activity in the oxidation reactions of a variety of olefins with tert-butyl-hydroperoxide in acetonitrile. The catalytic efficacy of the copper(II) complexes towards olefin oxidation reactions has been studied in different solvent media.     

The oxidation of 2-alkoxy-3,6-di-tert-butylphenols. The reversible dimerization of 2-alkoxy-3,6-di-tert-butylphenoxy radicals

The oxidation of 2-alkoxy-3,6-di-tert-butylphenols has been studied. It was found that 2-RO-3,6-di-tert-butylphenoxy radicals (R = Et, Pr, Ph) undergo dimerization by C-O coupling. The X-ray structure of 2-ethoxy-3,6-di-tert-butylphenoxy dimer has been determined. Radical dimers dissociate reversibly in solution to give two phenoxy radicals at 200-350 K. By using EPR spectroscopy the equilibrium concentration of the phenoxy radicals, equilibrium constants and ΔH and ΔS of dissociation have been determined.     

Synthesis and characterisation of N-1,10-phenanthrolin-5-ylalkylamides and their photosensitising heteroleptic Ru(II) complexes

In the context of our studies on ruthenium(II) complexes containing polyazaheterocyclic ligands as functionalised photosensitisers for singlet molecular oxygen generation in heterogeneous phase, we describe the synthesis and spectroscopic characterisation of different amide-functionalised N-1,10-phenanthrolin-5-ylalkylamides. These chelators are used to obtain heteroleptic [Ru(phen)₂L]²⁺ complexes, where L stands for 2-iodo-N-1,10-phenanthrolin-5-ylacetamide (5-iap), 4-oxo-4-(1,10-phenanthrolin-5-ylamino)butanoic acid (5-suap), 5-oxo-5-(1,10-phenanthrolin-5-ylamino)pentanoic acid (5-glap) and tert-butyl 4-oxo-4-(1,10-phenanthrolin-5-ylamino)butylcarbamate (BOC-5-ngap). The spectroscopic data, excited state lifetimes and quenching rate constants with O₂ (ca. 3.7×10⁹ L mol⁻¹ s⁻¹) of these novel complexes are also reported.     

Tertiary alkoxyl radicals from 3-alkoxythiazole-2(3H)-thiones

This study deals with the synthesis of tert-O-alkyl thiohydroxamates and their use as tert-alkoxyl radical precursors. tert-Alkoxyl radicals were applied in mechanistic studies to determine rate constants of (i) p-chlorocumyloxyl radical addition to bicyclo[2.2.1]heptene (k=1×10⁷ M⁻¹ s⁻¹), (ii) 2-phenylhex-5-en-2-oxyl radical 5-exo-trig-cyclization (k^cis=3×10⁹ s⁻¹, k^trans=1×10⁹ s⁻¹), and (iii) 2-methyl-5-phenylpent-2-oxyl to 2-hydroxy-2-methyl-5-phenylpent-5-yl radical isomerization (1,5-H-atom shift; k=0.4-1.5×10⁸ s⁻¹). The reactions pose key steps in synthesis of 2,2,5-substituted tetrahydrofurans and 2-bromo-3-alkoxybicyclo[2.2.1]heptanes. Stereoselectivity in 5-exo-trig cyclization (2,5-cis) and intermolecular addition (exo/endo>99:1), originates from torsional strain in transition structures of alkoxyl radical reactions.     

Synthesis of a new bidentate ferrocenyl N-heterocyclic carbene ligand precursor and the palladium (II) complex trans-[PdCl2(CfcC)], where (CfcC) = 1,1′-di-tert-butyl-3,3′-(1,1′-dimethyleneferrocenyl)-diimidazol-2-ylidene

A new ferrocenyl-N-heterocyclic carbene ligand precursor 1,1′-bis[(1-tert-butylimidazolium)-3-methyl]ferrocene dichloride has been synthesised and structurally characterised. The imidazolium salt was readily deprotonated in situ with KN(SiMe₃)₂ and reacted with [PdCl₂ (cod)] to afford the structurally characterised palladium (II) complex trans-[PdCl₂(C^∧fc^∧C)], where (cod) = 1,5-cyclooctadiene and (C^∧fc^∧C) = 1,1′-di-tert-butyl-3,3′-(1,1′-dimethyleneferrocenyl)-diimidazol-2-ylidene.     

Highly active new α-diimine nickel catalyst for the polymerization of α-olefins

A new silylated α-diimine ligand, bis[N,N′-(4-tert-butyl-diphenylsilyl-2,6-diisopropylphenyl)imino]acenaphthene 3, and its corresponding Ni(II) complex, {bis[N,N′-(4-tert-butyl-diphenylsilyl-2,6-diisopropylphenyl)imino]acenaphthene}dibromonickel 4, have been synthesized and characterized. The crystal structures of 3 and 4 were determined by X-ray crystallography. In the solid state, complex 4 is a dimer with two bridging Br ligands linking the two nickel centers, which have square pyramidal geometries. Complex 4, activated either by diethylaluminum chloride (DEAC) or methylaluminoxane (MAO) produces very active catalyst systems for the polymerization of ethylene and moderately active for the polymerization of propylene. The activity values are in the order of magnitude of 10⁷ g PE (mol Ni [E] h)⁻¹ for the polymerization of ethylene and of 10⁵ g PP (mol Ni [P] h)⁻¹ for the polymerization of propylene. NMR analysis shows that branched polyethylenes (PE) are obtained at room or higher temperatures and almost linear PE is obtained at 0 °C with 4/DEAC.     

p-tert-Butyl thiacalix[4]arenes functionalized at the lower rim by o-, m-, p-amido and o-, m-, p-(amidomethyl)pyridine fragments as receptors for α-hydroxy- and dicarboxylic acids

A series of new p-tert-butyl thiacalix[4]arenes with o-, m-, p-amido and o-, m-, p-(amidomethyl)pyridine substituents at the lower rim in cone, partial cone, and 1,3-alternate conformations were synthesized. The ability of the obtained compounds to recognize the α-hydroxy (glycolic, tartaric) and dicarboxylic (oxalic, malonic, succinic, fumaric, and maleic) acids was investigated by UV-vis spectroscopy. Also, the efficiency and selectivity of binding, the association constants log K_a (10² to 10⁷ M⁻¹) and the stoichiometry were determined for the complexes of p-tert-butyl thiacalix[4]arenes with the acids. The receptors based on p-tert-butyl thiacalix[4]arenes with (amidomethyl)pyridine substitutes are most efficient in complexation in many cases.     

Synthesis, characterization and luminescence study of dimethyl[2-(arylmethyleneimino)phenolato]gallium complexes: Crystal structure of dimethyl[N-(4-N,N′-dimethylamino) phenylmethyleneiminophenolato]gallium

Three dimethylgallium complexes of type Me₂GaL [L = 2-methoxylphenylmethyleneiminophenolato (1), N-(4-N,N′-dimethylamino)phenylmethyleneiminophenolato (2), N-(2-naphthyl)methyleneiminophenolato (3)] have been synthesized by the reaction of trimethylgallium with appropriate N-arylmethyleneiminophenol. The complexes obtained have been characterized by elemental analysis, ¹H, ¹³C{¹H} NMR, IR and mass spectroscopy, respectively. The solid structure of 2 has been determined by X-ray single crystal analysis. The gallium atom was bonded by an oxygen atom and coordinated by an imine nitrogen atom forming one five-membered ring. The stable dimmer was formed by the coordination of bridging oxygen atom of phenolate to another gallium atom. The photoluminescence of complexes 1-3 were studied. The maximum emission wavelengths of 1-3 are between 305 and 320 nm upon radiation by UV light. The electroluminescent properties of diodes using 1-3 as emitting material were measured. The blue/green electroluminescence has been observed.     

Solid phase microextraction procedure for the determination of alkylphenols in water by on-fiber derivatization with N-tert-butyl-dimethylsilyl-N-methyltrifluoroacetamide

The solid phase microextraction (SPME) technique with on-fiber derivatization was evaluated for the analysis of alkylphenols (APs), including 4-tert-octylphenol (4-t-OP), technical nonylphenol isomers (t-NPs) and 4-nonylphenol (4-NP), in water. The 85 μm polyacrylate (PA) fiber was used and a two-step sample preparation procedure was established. In the first step, water sample of 2 mL was placed in a 4 mL PTFE-capped glass vial. Headspace extraction of APs in water was then performed under 65 °C for 30 min with 800 rpm magnetic stirring and the addition of 5% of sodium chloride. In the second step, the SPME fiber was placed in another 4 mL vial, which contained 100 μL of N-tert-butyl-dimethylsilyl-N-methyltrifluoroacetamide (MTBSTFA) with 1% tert-butyl-dimethylchlorosilane (TBDMCS). Headspace extraction of MTBSTFA and on-fiber derivatization with APs were performed at 45 °C for 10 min. Gas chromatography/mass spectrometry (GC/MS) was used for the analysis of derivatives formed on-fiber. The adsorption-time profiles were also examined. The precision, accuracy and method detection limits (MDLs) for the analysis of all the APs were evaluated with spiked water samples, including detergent water, chlorinated tap water, and lake water. The relative standard deviations were all less than 10% and the accuracies were 100 ± 15%. With 2 mL of water sample, MDLs were in the range of 1.58-3.85 ng L⁻¹. Compared with other techniques, the study described here provided a simple, fast and reliable method for the analysis of APs in water.     

Shape- and stereo-selective esterase activities of cross-linked polymers imprinted with a transition-state analogue for the hydrolysis of amino acid esters

Various cross-linked (with N,N′-ethylene (C₂), butylene (C₄), hexamethylene (C₆), or decamethylene (C₁₀)-bisacrylamide) polymer catalysts containing l-histidine and quaternary trimethylammonium groups were imprinted with a racemic transition-state analogue of phenyl 1-benzyloxycarbonyl-3-methylpentylphosphonate for the hydrolysis of p-nitrophenyl N-(benzyloxycarbonyl)-l (or d)-leucinate (Z-l (or d)-Leu-PNP). Among these polymer catalysts, N,N′-C₄-bisacrylamide-cross-linked polymer catalyst, which was copolymerized with styrene monomer, exhibited the notable substrate-stereospecificity for the Z-l-Leu-PNP hydrolysis among the hydrolyses of enantiomeric l (or d)-N-protected (such as tert-butyloxycarbonyl (Boc-), acetyl (C₂-), decanoyl (C₁₀-) or benzyloxycarbonyl (Z-)) amino acid (Leu, Ala, or Phe) p-nitrophenyl esters in 10 vol.% MeCN-Tris buffer (pH 7.15) at 30°C.     

Synthesis of isotopomers of N-(tert-butoxycarbonyl)-4-cyano-l-phenylalanine methyl ester: choice of cyanation solvent

N-(tert-butoxycarbonyl)-4-cyano-l-phenylalanine methyl ester and three isotopomers (C¹⁵N, ¹³CN, and ¹³C¹⁵N) were successfully synthesized in two steps to expand the utility of the nitrile symmetric stretch vibration of this modified amino acid as a vibrational reporter of local environments. The choice of cyanation solvent directly impacted the level of isotopic enrichment of the isotopomers. The commonly used solvent acetonitrile resulted in an isotopic enrichment of only ∼80% with a cyanation reaction time of 4.5 h, however, the cyanation solvent N,N-dimethylformamide afforded the isotopomers with >98% isotopic enrichment.     

Synthesis, characterization and luminescence study of dimethyl(β-ketoiminato)gallium (-indium) complexes: crystal structure of dimethyl[1-phenyl-3-N-(4-methoxyphenylimino)-1-butanonato]gallium

Six dimethylgallium (indium) complexes of type Me₂ML [M = Ga, L = 1-phenyl-3-N-(phenylimino)-1-butanonato (1), 1-phenyl-3-N-(p-methoxyphenylimino)-1-butanonato (2), 1-phenyl-3-N-(o-chloro phenylimino)-1-butanonato (3); M = In, L = 1-phenyl-3-N-(phenyl imino)-1-butanonato (4), 1-phenyl-3-N-(p-methoxyphenylimino)-1-butanonato (5), 1-phenyl-3-N-(o-chlorophenylimino)-1-butanonato (6)] have been synthesized by reaction of trimethylgallium (indium) with appropriate 1-phenyl-3-N-(arylimino)-1-butanones. The complexes obtained have been characterized by elemental analysis, ¹H NMR, IR and mass spectroscopy. Structure of 2 has been determined by X-ray single-crystal analysis, in which Ga atom is four coordinated. Complexes 1-6 emit colors from blue to green (463-491 nm) when irradiated by UV light. The electroluminescent (EL) properties of 1-6 were examined by fabricating EL devices using 1-6 as emitter, respectively. The EL bands are located in the green region (509-522 nm).