Thermal decomposition behaviors of β-cyclodextrin,its inclusion complexes of alkyl amines,and complexed β-cyclodextrin at different heating rates

The present work revealed there was a conceptual difference in the thermal decomposition behaviors between the complexed β-cyclodextrin (CD) in an inclusion system and the β-CD complex of guest. The thermal decomposition behaviors of the solid inclusion complexes of β-CD with ethylenediamine (Eda), diethylenetriamine (Dta) and triethylamine (Tea) were investigated using nonisothermal thermogravimetry (TG) analysis based on weight loss as a function of temperature. In view of TG profiles, a consecutive mechanism describing the formation and thermal decomposition of the three solid supermolecules of β-CD was presented. Heating rate has very different effects on the thermal decomposition behaviors of these complexes. The faster the heating rate is, the higher the melting-decomposition point of the complexed β-CD in an inclusion system is, and on the whole the bigger the rate constant (k) of the thermal decomposition reaction of the complexed β-CD is. The thermal decomposition process of the complexed β-CD for each inclusion system is determined to be simple first-order reaction using Ozawa method. The apparent activation energies (E _a) and frequency factors (A) of the thermal decomposition reactions of the complexed β-CD molecules have been also calculated. It is found that when the decomposition reaction of the complexed β-CD encountered a large value of E _a, such as that in Dta–β-CD system, an apparent compensation effect of A on E _a can provide enough energy to conquer the reaction barrier in prompting the k value of thermal decomposition reaction of the complexed β-CD according to Arrhenius equation.     

High-pressure rate rules for alkyl + O2 reactions. 2. The isomerization, cyclic ether formation, and β-scission reactions of hydroperoxy alkyl radicals

The unimolecular reactions of hydroperoxy alkyl radicals (QOOH) play a central role in the low-temperature oxidation of hydrocarbons as they compete with the addition of a second O(2) molecule, which is known to provide chain-branching. In this work we present high-pressure rate estimation rules for the most important unimolecular reactions of the β-, γ-, and δ-QOOH radicals: isomerization to RO(2), cyclic ether formation, and selected β-scission reactions. These rate rules are derived from high-pressure rate constants for a series of reactions of a given reaction class. The individual rate expressions are determined from CBS-QB3 electronic structure calculations combined with canonical transition state theory calculations. Next we use the rate rules, along with previously published rate estimation rules for the reactions of alkyl peroxy radicals (RO(2)), to investigate the potential impact of falloff effects in combustion/ignition kinetic modeling. Pressure effects are examined for the reaction of n-butyl radical with O(2) by comparison of concentration versus time profiles that were obtained using two mechanisms at 10 atm: one that contains pressure-dependent rate constants that are obtained from a QRRK/MSC analysis and another that only contains high-pressure rate expressions. These simulations reveal that under most conditions relevant to combustion/ignition problems, the high-pressure rate rules can be used directly to describe the reactions of RO(2) and QOOH. For the same conditions, we also address whether the various isomers equilibrate during reaction. These results indicate that equilibrium is established between the alkyl, RO(2), and γ- and δ-QOOH radicals.     

Metadynamics simulations of R–NHC reductive elimination in intermediate palladium complexes of cross-coupling and Mizoroki–Heck reactions

Exploring the free energy surface of the R–NHC coupling reaction in the key intermediates of the Mizoroki–Heck and cross-coupling catalytic cycles has been conducted by the methods of biased and unbiased molecular dynamics. Molecular dynamics simulations were carried out both in vacuum and in a polar solvent, with the following main observations on the influence of the media: (1) the solvent prevents the dissociation of the solvate ligand, so the R–NHC coupling proceeds in a four-coordination complex (rather than in a three-coordination one, as in the case of a gas-phase reaction); (2) in the condensed phase, the potential barrier of the reaction is significantly higher compared to the same process in vacuum (17.7 vs. 21.8 kcal mol^-1); (3) polar solvent stabilizes the R–NHC coupling product. The reaction in a polar medium is exergonic (ΔG = −3.9 kcal mol^-1), in contrast to the in vacuum modeling, where the process is endergonic (ΔG = 0.4 kcal mol^-1).     

Binuclear complexes of chromium,manganese and rhodium withN-hydroxyethyl-N,N′,N′-tribenzimidazolylmethylethylenediamine

Summary Three novel binuclear complexes, Cr₂Cl₆L·EtOH, Mn₂OAc₄L·EtOH, Rh₂Cl₆L.2H₂O, where L =N-hydroxyethyl-N,N,N-tribenzimidazolylmethylethylenediamine, have been prepared and characterized by elemental analyses, i.r., u.v.-vis. and e.s.r. spectra, and by magnetic susceptibility measurements. A weak antiferromagnetic spin exchange interaction exists between the metal ions in a binuclear unit for the dimanganese complex. Preliminary bioassays show that the manganese complex possesses antitumour activityin vitro.     

New conjugates of β-cyclodextrin with manganese(III) salophen and porphyrin complexes as antioxidant systems

Oxidative stress is the hallmark of several pathologies like arthritis, hypertension and many neurodegenerative disorders such as Parkinson's and Alzheimer's diseases. In this scenario, antioxidant compounds can play a pivotal role in treating these severe pathologies. The synthesis of molecules able to mimic physiologically-relevant proteins is nowadays of particular interest. Several transition metal complexes, especially manganese(III) complexes with porphyrin and salen-type ligands, have been reported to be superoxide scavengers. Here we report the synthesis and spectroscopic characterization of manganese(III) complexes of 5[4-(6-O-β-cyclodextrin)-phenyl],10,15,20-tri(4-hydroxyphenyl)-porphyrin and of 6(A)-deoxy-6(A)[(S-cysteamidobenzoyl(3,4-diamino)-N,N'-bis(salicylidene))]-β-cyclodextrin. The superoxide dismutase activity of the metal complexes was investigated by indirect methods. The catalase and peroxidase activities were tested using ABTS assays.     

Abstraction of β-hydrogen vs. alkyl groups in reactions of dialkylzinc compounds and bis(oxazolinyl)borane

An ambiphilic bis(oxazolinyl)borane proligand and zinc dialkyls react via alkyl group transfer or β-hydrogen abstraction. The latter process is favored by formation of a bis(oxazolinyl)borane-zinc adduct that positions a β-hydrogen in the proximity of the Lewis acid center.     

Significant electrostatic effects in π-facial stereoselection of nucleophilic addition reactions to β,γ-unsaturated carbonyl compounds

The Significance of electrostatic effects on the origin of π-facial stereoselectivity in nucleophilic additions to β,γ-unsaturated carbonyl systems was shown through theoretical studies (ab initio MO calculations of transition structures for NaH + HCOCH₂CCH) and experimental results (M(BH₄)_n reduction of PhCOCH(CH₃)R (M  Na, Zn, Cd; R  C₂H₅, CHCH₂, CCH)).     

Arene complexes of transition metals in reactions with nucleophilic reagents: XXX. Reaction of π-halomesitylene [Tetramethyl(ethyl)cyclopentadienyl]rhodium(II) complexes with anions derived from CH acids

Reactions of fluoro-and chloromesitylene π-complexes [(η⁶-1-Hlg-2,4,6-Me₃C₆H₂)(η⁵-C₅EtMe₄)Rh]-(BF₄)₂ (Hlg = F, Cl) with diethyl malonate anion in THF or acetone-d ₆ at 20°C initially (within the first 5–30 min) involve nucleophile addition at unsubstituted carbon atom in the arene ligand with formation of π-cyclohexadienyl complexes {[η⁵-1-(EtOCO)₂CH-1-H-3-Hlg-2,4,6-Me₃C₆H₂](η⁵-C₅EtMe₄)Rh}(BF₄). The subsequent replacement of the halogen atom yields {[η⁶-1-(EtOCO)₂CH-2,4,6-Me₃C₆H₂](η⁵-C₅EtMe₄)Rh}(BF₄)₂, where the arene ligand is readily withdrawn from π-coordination by the action of chloride ion or the solvent. Dimethyl mesitylmalonate was isolated in 76% yield. Likewise, the reactions with anions derived from malononitrile and ethyl cyanoacetate gave 25–38% of the corresponding derivatives 1-R-2,4,6-Me₃C₆H₂ where R = (NC)₂CH or EtOCO(NC)CH.     

Synthesis,characterization and DNA-binding properties of mixed porphyrin–polypyridyl ruthenium(II) complexes

Two mixed porphyrin–polypyridyl Ru^II complexes [Ru(bpy)₂(MPyTPP)Cl]Cl (1) and [Ru(phen)₂(MPyTPP)Cl]Cl ( 2 ) (bpy=2,2-bipyridine; phen=1,10-phenanthroline; MPyTPP=5-monopyridyl-10,15,20–triphenylporphyrin) have been synthesized and characterized by elementary analysis, e.s.–m.s., cyclic voltammetry and u.v.–vis. spectroscopy. The DNA-binding properties of these complexes were investigated by electronic spectra, c.d. spectra and viscosity experiments. The results suggested that both complexes (1) and (2) bind to DNA in an outside binding mode. At the same time, theoretical calculations applying the ab initio and the density functional theory (DFT) methods were also performed, and the results showed that there is no good planarity on the main ligand MPyTPP of these complexes, and there are rather great distortion angles (dihedral angles ca. 72°) between the porphrin ring and each of the 10-, 15-, 20-phenyl groups. This may be the reason why the complexes bind to DNA in an outside mode, instead of an intercalative mode.     

Switching the photoinduced processes in host-guest complexes of β-cyclodextrin-substituted silicon(IV) phthalocyanines and a tetrasulfonated porphyrin

Porphyrins and phthalocyanines are two attractive classes of functional dyes for the construction of artificial light harvesting and charge separation molecular systems. The assembly of these components by supramolecular approach is of particular interest as this provides a facile route to build multi-chromophoric arrays with various architectures and tuneable photophysical properties. We report herein a series of host-guest complexes formed between a tetrasulfonated porphyrin and several silicon(IV) phthalocyanines substituted axially with two permethylated β-cyclodextrin units via different spacers. As shown by electronic absorption and fluorescence spectroscopic methods, the two components bind spontaneously in a 1:1 manner in water with large binding constants in the range of 1.1 × 10(7) to 3.5 × 10(8) M(-1). The photophysical properties of the resulting supramolecular complexes have also been studied in detail using steady-state and time-resolved optical spectroscopic methods. It has been found that two major photoinduced processes, namely fluorescence resonance energy transfer and charge transfer are involved which are controlled by the spacer between the β-cyclodextrin units and the silicon centre of phthalocyanine. Despite the fact that charge transfer is a thermodynamically favourable process for all the complexes, only the ones with a tetraethylene glycol or oxo linker exhibit an efficient charge transfer from the excited phthalocyanine to the porphyrin entity. The lifetimes of the corresponding charge-separated states have been determined to be 200 and 70 ps by picosecond pump-probe experiments.     

Syntheses, structure and ethylene polymerization behavior of β-diiminato titanium complexes

A series of new titanium complexes bearing β-diiminato ligands [(Ph)NC(R₁)CHC(R₂)N(Ph)]₂TiCl₂ (4a: R₁ = R₂ = CH₃; 4b: R₁ = R₂ = CF₃; 4c: R₁ = Ph, R₂ = CH₃; 4d: R₁ = Ph, R₂ = CF₃) has been synthesized and characterized. X-ray crystal structures reveal that complexes 4a and 4c adopt distorted octahedral geometry around the titanium center. With modified methylaluminoxane (MMAO) as a cocatalyst, complexes 4a-d are active catalysts for ethylene polymerization, and produce high molecular weight polyethylenes. Catalyst activities and the molecular weights of polymers are considerably influenced by the steric and electronic effects of substituents on the catalyst backbone under the same polymerization condition. With the strong electron-withdrawing groups (CF₃) at R₁ or/and R₂ position, complexes 4b and 4d show higher activities than complexes 4a and 4c, respectively.     

Molybdenum(VI) dioxo and oxo-imido complexes of fluorinated β-ketiminato ligands and their use in OAT reactions

Substitution of a methyl by a trifluoromethyl moiety in well-known β-ketimines afforded the ligands (Ar)NC(Me)CH(2)CO(CF(3)) (HL(H), Ar = C(6)H(5); HL(Me), A r= 2,6-Me(2)C(6)H(3); HL(iPr), Ar = 2,6-(i)Pr(2)C(6)H(3)). Subsequent complexation to the [MoO(2)](2+) core leads to the formation of novel complexes of general formula [MoO(2)(L(R))(2)] (R = H, 1; R = Me, 2; R = iPr, 3). For reasons of comparison the oxo-imido complex [MoO(N(t)Bu)(L(Me))(2)] (4) has also been synthesized. Complexes 1-4 were investigated in oxygen atom transfer (OAT) reactions using the substrate trimethylphosphine. The respective products after OAT, the reduced Mo(IV) complexes [MoO(PMe(3))(L(R))(2)] (R = H, 5; R = Me, 6; R = iPr, 7) and [Mo(N(t)Bu)(PMe(3))(L(Me))(2)] (8), were isolated. All complexes have been characterized by NMR spectroscopy, and 1-4 also by cyclic voltammetry. A positive shift of the Mo(VI)-Mo(V) reduction wave upon fluorination was observed. Furthermore, molecular structures of complexes 2, 4, 5, and 8 have been determined via single crystal X-ray diffraction analysis. Complex 8 represents a rare example of a Mo(IV) phosphino-imido complex. Kinetic measurements by UV-vis spectroscopy of the OAT reactions from complexes 1-4 to PMe(3) showed them to be more efficient than previously reported nonfluorinated ones, with ligand L' = (Ar)NC(Me)CH(2)CO(CH(3)) [MoO(2)(L')(2)] (9) and [MoO(N(t)Bu)(L')(2)] (10), respectively. Thermodynamic activation parameters ΔH(?) and ΔS(?) of the OAT reactions for complexes 2 and 4 have been determined. The activation enthalpy for the reaction employing 2 is significantly smaller (12.3 kJ/mol) compared to the reaction with the nonfluorinated complex 9 (60.8 kJ/mol). The change of the entropic term ΔS(?) is small. The reaction of the oxo-imido complex 4 to 8 revealed a significant electron-donating contribution of the imido substituent.     

Unexpected results of the reactions of manganese and vanadium β-diketiminate halide complexes with Na[HBEt3]

The reaction of [(^Menacnac^Dipp)Mn(μ-Cl)]₂(2) (^Menacnac^Dipp = HC(C(Me)NDipp)₂; Dipp = 2,6-Prⁱ₂C₆H₃) with sodium triethylborohydride in a toluene—THF mixture afforded the complex [(^Menacnac^Dipp)Mn(μ-H)₂BEt₂(THF)] (3). The reaction of 2 with Na[HBEt₃] in toluene under THF-free conditions gave a mixture of products. The set and the ratio of these products in the resulting crystalline mixture were established by quantitative powder X-ray diffraction analysis: [(^Menacnac^Dipp)Mn(μ-H)]₂(1), [(^Menacnac^Dipp)?Mn(μ-H)₂BEt₂] (4), and unreacted compound 2 in the ratio of 15:4:1 and traces of an unknown crystalline phase. The reaction of [(^Menacnac^Dipp)VCl₂] (5) with Na[HBEt₃] yielded the compound [(^Menacnac^Dipp)V(μ-H)(μ,κ^1:1?C:C′?C₂H₄)BEt₂] (6) containing the unusual ligand [HBEt₂(CH₂CH₂)]^2?. The vanadium analog of compound 3, [(^Menacnac^Dipp)V(μ-H)₂BEt₂(THF)] (7), was isolated in one experiment. Besides. a small amount of the complex [(^Menacnac^Dipp)V(μ-H)BEt₃(THF)] (8) was detected in the mixture of crystalline products. The structures of compounds 3, 4, 6, 7, and 8 were determined by single-crystal X-ray diffraction.

     

Asymmetric synthesis of β,β-difluoroamino acids via cross-coupling and Strecker reactions

β,β-Difluoroamino acids were synthesized from commercially available ethyl bromodifluoroacetate using cross-coupling and Strecker reactions as key steps. The coupling reaction of aryl iodides with ethyl bromodifluoroacetate gave the corresponding coupling products, which were transformed to 2-difluoromethyl-1,3-oxazolidines in two steps. Boron trifluoride etherate promoted Strecker reaction of 2-difluoromethyl-1,3-oxazolidines gave α-amino nitriles in good yields and diastereoselectivities. After removal of chiral auxiliary and hydrolysis of the nitrile group, β,β-difluorophenylalanine was obtained with 73% ee. Partial racemization occurred during the hydrolysis of nitrile group.     

Synthesis and biological evaluation of some new class of benzothiazole–pyrazole ligands containing arene ruthenium,rhodium and iridium complexes

Transition Metal Chemistry - Metal complexes 1–9 have been synthesized by reacting the benzothiazole–pyrazole derivative ligands (L1, L2 and L3) with the metal precursors of ruthenium...     

Syntheses of chiral β- and γ-amino ethers, morpholines, and their homologues via nucleophilic ring-opening of chiral activated aziridines and azetidines

Lewis acid catalyzed quaternary ammonium salt mediated highly regioselective ring-opening of chiral activated aziridines and azetidines with alcohols to nonracemic β- and γ-amino ethers has been developed. The reaction mainly proceeds via an S(N)2 pathway, and the partial racemization of the starting substrate was effectively controlled by using quaternary ammonium salts. β- and γ-amino ethers are obtained with high enantio- and diastereospecificity (ee up to >99%, de up to 99%). The methodology was further extended to synthesize morpholines and their homologues with high enantiospecificity (ee up to 90%) when halo alcohols were employed as the nucleophiles.     

Rate constants for the gas-phase reactions of OH and O3 with β-ocimene, β-myrcene, and α- and β-farnesene as a function of temperature

The rate constants for the gas-phase reactions of hydroxyl radicals and ozone with the biogenic hydrocarbons β-ocimene, β-myrcene, and α- and β-farnesene were measured using the relative rate technique over the temperature ranges 313-423 (for OH) and 298-318 K (for O?) at about 1 atm total pressure. The OH radicals were generated by photolysis of H?O?, and O? was produced from the electrolysis of O?. Helium was used as the diluent gas. The reactants were detected by online mass spectrometry, which resulted in high time resolution, allowing large amounts of data to be collected and used in the determination of the Arrhenius parameters. The following Arrhenius expressions have been determined for these reactions (in units of cm3 molecules?1 s?1): for β-ocimene + OH, k = (4.35(-0.66)(+0.78)) × 10?11 exp[(579 ± 59)/T]; for β-ocimene + O?, k = (3.15(-0.95)(+1.36)) × 10?1? exp[-(626 ± 110)/T]; for β-myrcene + O?, k = (2.21(-0.66)(+0.94)) × 10?1? exp[-(520 ± 109)/T]; for α-farnesene + OH, k(OH) = (2.19 ± 0.11) × 10?1? for 23-413 K; for α-farnesene + O?, k = (3.52(-2.54)(+9.09)) × 10?12 exp[-(2589 ± 393)/T]; for β-farnesene + OH, k(OH) = (2.88 ± 0.15) × 10?1? for 323-423 K; for β-farnesene + O?, k = (1.81(-1.19)(+3.46)) × 10?12 exp[-(2347 ± 329)/T]. The Arrhenius parameters here are the first to be reported. The reactions of α- and β-farnesene with OH showed no significant temperature dependence. Atmospheric residence times due to reactions with OH and O? were also presented.     

The reactions of perfluorinated α- and β-alkoxyradicals with hydrogen chloride, chlorine and fluorine

The kinetics of the reactions of the macroradicals R_f′OCF₂ (I) and R_f′OCF₂CF₂ (II) with HCl, Cl₂ and F₂ have been studied in the liquid phase, being R_f′ a poly(oxydifluoromethylene-oxytetrafluoroethylene) chain with average molecular weight of about 10⁴ Da. Radical (I) showed a higher reactivity compared to radical (II) with all the three radical transfer agents.In case of HCl the activation energy for the reaction: