Chiral ethylene-bridged flavinium salts: the stereoselectivity of flavin-10a-hydroperoxide formation and the effect of substitution on the photochemical properties

A series of chiral non-racemic N¹,N¹⁰-ethylene bridged flavinium salts 4 was prepared using enantiomerically pure 2-substituted 2-aminoethanols (R?=?isopropyl, phenyl, benzyl, 4-methoxybenzyl, 4-benzyloxybenzyl) derived from amino acids as the sole source of chirality. The flavinium salts were shown to form 10a-hydroperoxy- and 10a-methoxy-adducts with moderate to high diastereoselectivity depending on the ethylene bridge substituent originating from the starting amino acid. High diastereoselectivities (dr values from 80:20 to >95:5) were observed for flavinium salts bearing benzyl substituents attached to the ethylene bridge. The benzyl group preferred the face-to-face (syn) orientation relative to the flavinium unit; thereby effectively preventing nucleophilic attack from one side. This conformation was found to be the most stable according to the DFT calculations. Consequently, the presence of benzyl groups causes intermolecular fluorescence quenching resulting in a significant decrease in the fluorescence quantum yield from 11% for 4a bearing an isopropyl substituent to 0.3% for 4c containing a benzyl group and to a value lower than 0.1% for the benzyloxybenzyl derivative 4e.     

Reactions of P4S10 and pyPS2Cl with N,N′-diphenylurea and N,N′-diphenylthiourea

The reaction of P₄S₁₀ (1) with N,N′-diphenylurea (PhNH)₂CO (2) results in new heterocyclic compounds: the pyridinium salt of 1,3-diphenyl-2-sulfido-2-thioxo-1,3-diaza-2λ⁵-phosphetidine (3) (with a P–N–C–N cycle) and the pyridinium salt of 1,4-diphenyl-2,5-disulfido-2,5-dithioxo-1,4-dithiadiaza-2λ⁵,5λ⁵-diphosphinane (4), containing the (P–S–N)₂ cycle and the cyclic thiophosphates [pyH]₂[P₂S₈] (5), [pyH]₂[P₂S₇] (6) and [pyH]₃[P₃S₉] (7). A similar reaction, but carried out with N,N′-diphenylthiourea (PhNH)₂CS (8), leads to the formation of 4 and 6. pyPS₂Cl (9), used as an alternative starting material, also yields compounds 3, 4, 5, and further [pyH][PS₂Cl₂] (10) and S₈ after reaction with 2. Compound 3 reacts with Pd(CH₃COO)₂, with the formation of the complex [Pd(Ph₂N₂COPS₂)₂] (11). The crystal structures of 3 and 7 were determined by single-crystal X-ray diffraction.     

Inorganic-Organic Hybrid Materials: Synthesis and X-Ray Structure of N,N′-Dimethylimidazolium Salts [(Me2Im)2][Cd2(SCN)6] and N,N′-Dicyclohexylimidazolium [(Cy2Im)2][Cd2(SCN)6]·C3H6O

Two novel N,N′-dialkylimidazolium thiocyanate-cadmium complexes [(R₂Im)₂][Cd₂(SCN)₆] for R=Me (3), and cyclohexyl (4) have been synthesized and characterized by single-crystal X-ray diffraction. Compound 3 crystallizes in the monoclinic unit cell dimensions of 17.468(3), 7.7273(12), 10.6750(16) Å, 104.833(2)°, and space group C2 with two [(Me₂Im)₂] [Cd₂(SCN)₆] per unit cell. The two cadmium atoms in 3 are octahedrally coordinated in 4N2S and 2N4S coordination environment, and linking into one-dimensional zigzag chains. Compound 4 belongs to the monoclinic space group Cc with unit cell of dimensions 13.3049(12), 17.5550(16), 20.8012(19) Å, 101.494(2)°, and four [(Cy₂Im)₂][Cd₂(SCN)₆]·C₃H₆O per unit cell. The cadmium atoms in 4 are all 3N3S hexa-coordinated with six bridging SCN⁻ ions in an fac configuration and form infinite zigzag polymeric chains. The infinite chains in 3 form an approximate hexagonal array, making triangular channels which are occupied by N,N′-dimethylimidazolium ions, whereas the chains in 4 form layered structure, and the layers are stacked perpendicularly with respect to the orientation of the infinite anionic chains alternatively. N,N′-dicyclohexylimidazolium cations and solvent molecules fill in between layers.     

A simple and efficient biphasic method for the preparation of 4-nitrophenyl N-methyl- and N-alkylcarbamates

Treatment of 4-nitrophenyl chloroformate with alkylammonium hydrochloride salts and solid anhydrous Na₂CO₃ in either CH₂Cl₂ or CH₃CN gave 4-nitrophenyl N-methylcarbamate and other N-alkylcarbamate analogues in excellent yields. Of particular interest is the observation that 4-nitrophenyl N-methylcarbamate, a safer alternative to the highly toxic methyl isocyanate, is obtained in quantitative yield (?95% pure as determined by ¹H NMR) after simple filtration and solvent evaporation.     

Reaction of 1,2-dibromoethane with primary amines: formation of N,N′-disubstituted ethylenediamines RNH-CH2CH2-NHR and homologous polyamines RNH-[CH2CH2NR]n-H

The reaction of primary amines RNH₂ (R: Me, Et, iPr, tBu and Ph) with 1,2-dibromoethane gave N,N′-disubstituted ethylenediamines R-NH-CH₂CH₂-NH-R (1) in yields ranging from 10% (1a; R=Me) to 70% (1d, R=tBu; 1e, R=Ph). Piperazines and N-substituted polyethyleneimines were identified (¹H NMR, ¹³C NMR and EI-MS) as side products of the reaction and isolated by fractional distillation. The piperazines 2 are formed in yields of 3-10% and can be separated from the diamines 1 in all cases, except for R=Me and Ph. The polyamine homologues RNH-[CH₂CH₂NR]_n-H (3-5) were isolated in yields ranging from 0.1% (n=4, R=iPr) to 14% (n=2, R=iPr). The yields of 1 increase with the size of the substituent R, no obvious trend exists for the yields of the side products.     

Imino- and bis-imino-pyridines with N-ter-butyl-N-aminoxyl group: synthesis, oxidation and use as ligand towards M ( Mn, Ni, Zn) and Gd

Imino- and bis-iminopyridine ligands bearing N-ter-butylhydroxy groups were synthesized by the condensation reaction between a N-ter-butylhydroxy substituted aniline and 2-formylpyridine, 2-acetylpyridine and 2,6-bis-acetylpyridine. The N-ter-butylaminoxy radicals obtained after oxidation using PbO₂ or Ag₂O were studied by EPR spectroscopy in solution. Indeed, complete decomposition was observed during isolation of these radical derivatives. The N-ter-butylhydroxy substituted ligands obtained were complexed with Mn²⁺, Ni²⁺, Zn²⁺ and Gd³⁺ salts; the obtained complexes were characterized and oxidized to give the aminoxy analogs, which were studied using IR, UV and EPR spectroscopies.     

Structure and properties of protonated N-alkyl-2-aza[3]ferrocenophanes

Protonation of N-alkyl-2-aza[3]ferrocenophanes by HCl and NH₄PF₆ affords hexafluorophosphate salts having a trialkylammonium group. Structures of the protonated and unprotonated N-(p-methylbezyl)-2-aza[3]ferrocenophanes were determined by X-ray crystallography. Variable temperature ¹³C{¹H} NMR spectra of the N-protonated N-hexyl-2-aza[3]ferrocenophane revealed inversion of the nitrogen of the 2-aza[3]ferrocenophane on the NMR time scale probably via partial deprotonation of the nitrogen atom. Cyclic voltammograms of the N-protonated compounds exhibited reversible redox peaks at higher potentials than those of the corresponding neutral ferrocenophanes.     

Radical polymerization behavior of 3-(N-2-methacryloyloxyethyl-N,N-dimethyl)ammonatopropanesulfonate in water

The homogeneous polymerization of 3-(N-2-methacryloyloxyethyl-N,N-dimethyl)ammonatopropanesulfonate (MDAPS) with potassium peroxydisulfate (KPS) was kinetically in situ investigated in water by means of FT-near IR spectroscopy. The overall activation energy of the polymerization was calculated to be 16.0 kcal/mol. The initial polymerization rate (R_p) at 40 °C was expressed by R_p=k[KPS]^0.65[MDAPS]^1.0. The presence of alkaline metal salts was observed to accelerate the polymerization. The order of acceleration at 40 °C was CsCl > KCl > NaCl > LiCl when the chloride salts were used. NaCl showed higher acceleration effect than NaF. NaBr and NaI exhibited retardation and inhibition effect, respectively, because of reduction of KPS and its primary radical with bromide and iodide ions. The polymerization of MDAPS with KPS in water in the presence of NaCl at 2.0 mol/l gave R_p=k[KPS]^0.70[MDAPS]^1.4 at 40 °C. The overall activation energy of the polymerization in the presence of NaCl was estimated to be 11.6 kcal/mol being considerably lower value compared with that in its absence. The syndiotacticity of poly(MDAPS) tended to increase with rising temperature and decrease in the presence of NaCl.     

P NMR study of the valence stability of tin in its 1-hydroxyethylene-diphosphonate (HEDP) and N,N′,N′-trimethylenephosphonate-polyethyleneimine (PEI-MP) complexes

The valence stability of tin in its complexes with 1-hydroxyethylene-diphosphonate (HEDP) and with N,N′,N′-trimethylenephosphonate-polyethyleneimine (PEI-MP) was investigated. With particular interest in the possible interconversion between Sn²⁺ and Sn⁴⁺, the complexes were monitored with the aid of ³¹P NMR spectroscopy. The extent of complex formation with both ligands was evaluated for systems with tin in their respective oxidation states. The Sn²⁺-complexes underwent initial, but limited oxidation upon preparation, and beyond which were rather stable, irrespective of pH or time. Both Sn²⁺- and Sn⁴⁺-complexes were found to exist in solution without change. Oxidation of Sn²⁺ was achieved by addition of hydrogen-peroxide and was partially reversed by the addition of glutathione (GSH). The amount of H₂O₂ needed for complete oxidation of the Sn²⁺- into Sn⁴⁺-complexes was determined for both ligands, as well as the time taken for that oxidation.     

Spectroscopic and structural characterization of Na2[(VO)2(ttha)]·8H2O (ttha = triethylenetetraamine-N,N,N′,N″,N′″,N′″-hexaacetate): Interpreting the results with density functional theory

Na₂[(V^IVO)₂(ttha)]·8 H₂O (ttha = triethylenetetraamine–N,N,N′,N″,N′″,N′″–hexaacetate ion), prepared by treating [VO(H₂O)₅][(VO)₂(ttha)]·4 H₂O with Na₆(ttha), has been characterized by single crystal X-ray diffraction, infrared spectroscopy, UV–Vis absorption spectroscopy, electron spin resonance spectroscopy, and modeled by density functional theory (DFT). The X-ray structure revealed a distorted octahedral geometry around each vanadium center. The electronic absorption spectrum of [(VO)₂(ttha)]²⁻ (aq) features absorptions at ca. 200 nm (ε > 13900 L mol⁻¹ cm⁻¹), 255 nm (ε = 3480 L mol⁻¹ cm⁻¹), 586 nm (ε = 33 L mol⁻¹ cm⁻¹), and 770 nm (ε = 38 L mol⁻¹ cm⁻¹). The time-dependent density functional theory (TDDFT) calculated electronic absorption spectrum was remarkably similar to the actual spectrum, and TDDFT predicts absorption peaks at 297, 330, 458, 656, and 798 nm. TDDFT assigned the peak at 798 nm to be the α spin HOMO → LUMO transition. Hence, the peak at 770 nm in the actual spectrum is most likely the α spin HOMO → LUMO transition. Moreover, the TDDFT calculations revealed that the α spin HOMO and LUMO are partly comprised of d orbitals on both vanadium centers, and the first derivative electron spin resonance spectrum also suggests that the two unpaired electrons in [(VO)₂(ttha)]²⁻ are localized near the vanadium centers.     

Ru(II) complexes imparting N2O2 donor bis chelating ligand N,N-bis(salicylidine)-hydrazine in unusual coordination mode

The synthesis and characterization of binuclear ruthenium complexes [{(η⁶-C₆H₆)Ru}₂(μ-bsh)₂] (1), [{(η⁶-C₁₀H₁₄)Ru}₂(μ-bsh)₂] (2), [{(η⁶-C₆Me₆)Ru}₂(μ-bsh)₂] (3), and rhodium complex [{(η⁵-C₅Me₅)RhCl}₂(μ-bsh)] (4) (bsh=N,N^′-bis(salicylidine)-hydrazine dianion) are reported. The complexes have been fully characterized by analytical and spectral techniques and unusual coordination mode of the ligand H₂bsh has been confirmed by single crystal X-ray analysis of the complex 2. Structural data revealed extensive inter- and intra-molecular C-H?O and C-H?π interactions and involvement of methyl and isopropyl hydrogen from the p-cymene in hydrogen bonding.     

[R-C7H16N2][V2Te2O10] and [S-C7H16N2][V2Te2O10]; new polar templated vanadium tellurite enantiomers

New polar vanadium tellurite enantiomers have been synthesized under mild hydrothermal conditions through the use of sodium metavanadate, sodium tellurite and enantiomerically pure sources of either R-3-aminioquinuclidine or S-3-aminioquinuclidine. [R-C₇H₁₆N₂][V₂Te₂O₁₀] and [S-C₇H₁₆N₂][V₂Te₂O₁₀] contain [V₂Te₂O₁₀]_n²ⁿ⁻ layers constructed from [(VO₂)₂O(TeO₄)₂] monomers. Steric effects associated with the hydrogen-bonding network between the [V₂Te₂O₁₀]_n²ⁿ⁻ layers and [C₇H₁₆N₂]²⁺ result in polar structures and crystallization in the space group P2₁ (no. 4). Electron localization functions were calculated to visualize the tellurite stereoactive lone pairs. Both iterative and non-iterative Hirshfeld techniques were evaluated as means to determine atomic partial charges, with iterative Hirshfeld charges more accurately representing charge distributions in the reported enantiomers. These charges were used to calculate both component and net dipole moments. [R-C₇H₁₆N₂][V₂Te₂O₁₀] and [S-C₇H₁₆N₂][V₂Te₂O₁₀] exhibit dipole moments of 17.37 and 16.62D, respectively. [R-C₇H₁₆N₂][V₂Te₂O₁₀] and [S-C₇H₁₆N₂][V₂Te₂O₁₀] both display type 1 phase-matching capabilities and exhibit second harmonic generation activities of ∼50×α-SiO₂.     

New 1,2,4,5-tetrakis-(N-imidazoliniummethyl)benzene and 1,2,4,5-tetrakis-(N-benzimidazoliummethyl)benzene salts as N-heterocyclic tetracarbene precursors: synthesis and involvement in ruthenium-catalyzed allylation reactions

New tetraimidazolinium and tetrabenzimidazolium salts have been prepared. Upon reaction with ^tBuOK, they generate carbene ligands, which were associated in situ to [RuCp*(MeCN)₃]PF₆ to produce new ruthenium catalysts that are active for the substitution of allylic substrates by amines, phenols, and carbonucleophiles. The influence of the N-heterocyclic core as well as that of the N-substitutents at the periphery of the salts, on reactivity and regioselectivity have been examined.     

Synthesis,electrochemical and in situ spectroelectrochemical studies of new transition metal complexes with two new Schiff-bases containing N2O2/N2O4 donor groups

New Schiff-base copper and cobalt complexes, [Cu(L¹)], [Cu(L²)] and [Co(L¹)], [Co(L²)] (where L¹ = N-N′-bis(3,5-di-tert-butylsalicylaldimine)-1,4-cyclohexane bis(methylamine) and L² = N-N′-bis(3,5-di-tert-butylsalicylaldimine)-1,8-diamino-3,6-dioxaoctane), were synthesized and characterized using elemental analysis, IR spectra, UV–Vis spectra, magnetic susceptibility measurements, ¹H and ¹³C NMR spectroscopy, thermal analysis and molar conductance (Λ_M). Their electro-spectrochemical properties were investigated using cyclic voltammetric (CV) and thin-layer spectroelectrochemical techniques in a dichloromethane solution (CH₂Cl₂). The CV of [Cu(L²)] showed a lower oxidation potential than that of [Cu(L¹)] under the same experimental conditions. The oxidation wave (II) of [Cu(L²)] was accompanied by an EC process (II′), which was not observed for [Cu(L¹)]. Also, [Cu(L²)] exhibited a reduction process, but [Cu(L¹)] did not. These results indicate that the Cu(II) ion in [Cu(L²)] is coordinated by N₂O₄ donor sites while [Cu(L¹)] presents a square-planar structure with N₂O₂ donor sites. Both oxidation processes for [Co(L¹)] and [Co(L²)] are based on the cobalt center, and they are assigned to Co(II)/Co(III) couples. The spectroelectrochemical results indicate that the oxidized species of [Cu(L²)] is similar to that of [Cu(L¹)], the only difference being that the absorption bands of the oxidized species for [Cu(L²)] shift to lower energy compared with those of [Cu(L¹)] because of their different coordination environment. The geometry of [Cu(L²)] changed into square-planar after the complex was totally oxidized and the neutral complex was only recovered following the EC process, as observed from the CV of [Cu(L²)]. For the two cobalt complexes, the bands corresponding to the π → π^∗transitions disappeared and new bands with small red shifts and of lower intensity were observed during the oxidation process. These new bands are attributed to the LMCT transition as observed in the case of the oxidation processes of the cobalt complexes.     

An efficient route to disymmetrically substituted calix[6]arenes. Synthesis of novel ligands presenting a N2S or N3CO2 binding core

The C_3v tris-methoxy calix[6]arene was selectively mono-alkylated by dibromoethane yielding a key intermediate for the design of disymmetrically O-substituted calix[6]arenes. Indeed, subsequent reactions with various functional groups afforded novel calix[6]arene-based biomimetic ligands that present a mixed donor N₂S or N₃CO₂⁻ environment in an efficient way.     

Tetraphenylboroxinate(1-) salts of monoborate cations: Synthesis and single-crystal X-ray structures of [Ph2B{OCH2CH2N(Me)(CH2)n}2][Ph4B3O3] (n = 4, 5)

The salts, [Ph₂B{OCH₂CH₂N(Me)(CH₂)_n}₂][Ph₄B₃O₃] (n = 4, 5), were prepared in moderate yields in MeOH solution from reaction of Ph₂BOBPh₂ with [N(CH₂)_n(Me)(CH₂CH₂OH)][OH] and PhB(OH)₂ in a 1:2:4 ratio. The reactions also lead to Ph₃B₃O₃. Both salts were characterized by NMR (¹H, ¹³C, ¹¹B) IR, and single-crystal XRD studies. The salts are comprised of cationic monoborates (zwitterionic, 2N⁺ and 1B⁻) and tetraphenylboroxinate anions.     

N-Polyfluoroethyl and N-2-chlorodifluorovinyl derivatives of azoles

Reactions of tetrafluoroethylene, chlorotrifluoroethylene and 1,2-dichlorodifluoroethylene with N-potassium salts of imidazole, 2-methylbenzimidazole, 3,5-dimethylpyrazole, 1,2,4-triazole, and benzotriazole lead to the formation of the corresponding N-(1,1,2,2-tetrafluoroethyl), N-(2-chloro-1,1,2-trifluoroethyl), and N-(2-chloro-1,2-difluorovinyl) azoles. Treatment of N-(2-chloro-1,2-difluorovinyl) and N-(2-chloro-1,1,2-trifluoroethyl) derivatives of azoles with tetramethylammonium fluoride is a useful synthetic method for the preparation of heterocycles with 1,2,2,2-tetrafluoroethyl group attached to nitrogen.     

Pd2dba3/P(i-BuNCH2CH2)3N: a highly efficient catalyst for the one-pot synthesis of trans-4-N,N-diarylaminostilbenes and N,N-diarylaminostyrenes

A Pd₂dba₃/P(i-BuNCH₂CH₂)₃N catalyzed one-pot synthesis of unsymmetrically substituted trans-4-N,N-diarylaminostilbenes and both symmetrically and unsymmetrically substituted N,N-diarylaminostyrene derivatives is reported. The procedure involves two or more palladium catalyzed sequential coupling reactions (an amination and an inter-molecular Heck reaction) in one-pot using the same catalyst system with two different aryl halides, including aryl chlorides and hetero aryl halides as the coupling partners.     

Alkynylation of N-tosylimines with aryl acetylenes promoted by ZnBr2 and N,N-diisopropylethylamine in acetonitrile

We found a suitable condition for the effective alkynylation of N-tosylimines with aryl acetylenes. The reaction of N-tosylimines and aryl acetylenes in the presence of ZnBr₂ and DIEA (N,N-diisopropylethylamine) in CH₃CN afforded the desired N-tosyl propargylamines in moderate to good yields.     

AcOH-catalyzed aza-Michael addition/N-nitrosation: An efficient approach to CF2HCH2-containing N-nitrosoamines

A simple and highly efficient protocol for the AcOH-catalyzed three-component reaction among nitroalkenes, difluoroethylamine and tert-butyl nitrite through cascade aza-Michael addition/N-nitrosation has been developed. A range of CF₂HCH₂-containing N-nitrosoamines were obtained in good to excellent yields. This approach features easily available cheap materials, without using additional organic solvents, simple operation under room temperature, gram scalable preparation and functionally diverse products.