Homogeneous precipitation of mullite precursors

The synthesis of mullite powders or gels from an aqueous precursor solution has been studied. The starting solution is made by hydrolyzing TEOS into an aqueous solution of aluminium nitrate. When this solution is sprayed into a solution of ammonia in isopropanol, a nearly monophasic mullite precursor is obtained, whereas if this precipitation is made into an aqueous solution of ammonia, or ammonium carbonate, a diphasic colloidal precipitate is formed. By a slow and homogeneous precipitation in aqueous solution, induced by in-situ generation of ammonia by thermal hydrolysis of urea, a monophasic gel is also obtained. The samples have been studied by DTA, DSC, TGA, XRD and dilatometry.     

Nickeladihydrofuran. Key intermediate for nickel-catalyzed reaction of alkyne and aldehyde

The formation of a nickeladihydrofuran by oxidative cyclization of an alkyne and an aldehyde with nickel(0) has been demonstrated; the transformation of the nickeladihydrofuran into an enone by decomposition, a lactone by carbonylation and an allylic alcohol by treatment with ZnMe(2) suggests that nickeladihydrofuran is an important key intermediate in a variety of catalytic reactions.     

Diffusioosmosis of electrolyte solutions along a charged plane wall

The steady diffusioosmotic flow of an electrolyte solution along a dielectric plane wall caused by an imposed tangential concentration gradient is analytically examined. The plane wall may have either a constant surface potential or a constant surface charge density of an arbitrary quantity. The electric double layer adjacent to the charged wall may have an arbitrary thickness, and its electrostatic potential distribution is determined by the Poisson-Boltzmann equation. The macroscopic electric field along the tangential direction induced by the imposed electrolyte concentration gradient is obtained as a function of the lateral position. A closed-form formula for the fluid velocity profile is derived as the solution of a modified Navier-Stokes equation. The direction of the diffusioosmotic flow relative to the concentration gradient is determined by the combination of the zeta potential of the wall and the properties of the electrolyte solution. For a given concentration gradient of an electrolyte along a plane wall, the magnitude of fluid velocity at a position in general increases with an increase in its electrokinetic distance from the wall, but there are exceptions. The effect of the lateral distribution of the induced tangential electric field in the double layer on the diffusioosmotic flow is found to be very significant and cannot be ignored.     

Mechanism of the second sulfenylation of indole

Sulfenylation of indole using a sulfenyl chloride occurs initially at the 3-position of the ring, leading to a 3-indolyl sulfide. When an excess of sulfenyl chloride is used, a second sulfide group is introduced at the 2-position, and an indolyl 2,3-bis-sulfide results. We have demonstrated that this second sulfenylation occurs not by direct introduction of the second sulfide at the 2-position but via initial formation of an indolenium 3,3-bis-sulfide intermediate, followed by migration of one of the sulfide groups to the 2-position. This was achieved by the isolation of two examples of 3H-indole 3,3-bis-sulfides and by subsequent demonstration that they rearrange to the indolyl 2,3-bis-sulfides by treatment with sulfenyl halides.     

Characterization of the switch in the mechanism of an intramolecular Diels-Alder reaction

Changing the dienophile moiety of an intramolecular Diels-Alder (IMDA) cycloaddition from an allyl ether to an allenyl ether can dramatically change the regioselectivity. We hereby show by density functional theory computations that such unprecedented divergence is produced by an underlying change in the mechanism of the reaction. The allyl ether yields a fused tetrahydrofuran through a classical Diels-Alder reaction, whereas the allenyl ether yields a (methylidene)tetrahydropyran through a stepwise process. The latter reaction involves an extreme asynchronism in the bond-forming events with a diradicaloid intermediate that is stabilized by conjugation and synergistic (captodative) effects. Comparison with intermolecular model D-A reactions, which are concerted processes with various degrees of asynchrony, helps explain the change in regioselectivity for the IMDA reaction of allyl systems and the shift in mechanism for the IMDA reaction of the allenyl derivatives studied.     

Phase inversion of the Pickering emulsions stabilized by plate-shaped clay particles

We investigated the phase inversion of Pickering emulsions stabilized by plate-shaped clay particles. Addition of water induced a phase inversion from a water-in-oil (W/O) emulsion to an oil-in-water (O/W) emulsion when the amount of the oil phase exceeded a limiting amount of oil absorption to solid particles. On the other hand, a phase inversion from a powdery state to an O/W emulsion state through an oil-separated state is observed when the amount of an oil phase is less than the limiting amount of the oil absorption. Interestingly, the oil separated is re-dispersed as emulsion droplets into the O/W emulsion phase. This type of phase inversion, which is a feature of the Pickering emulsions stabilized by the clay particles, is caused by a change in the aggregate structures of particles.     

A revised statistical method for the description of inhomogeneous broadening of resonance lines in solids

The shapes of inhomogeneously broadened resonance lines of defect centers in solids are generally calculated using the so-called statistical method. In this paper it is shown that this method is physically inconsistent. A revised statistical method is introduced relating the resonance lineshape to the spatial distribution of an individual defect center, the number of imperfections causing the broadening, the distribution of an individual imperfection with respect to the defect center, and the perturbation induced by an individual imperfection. Depending on the convergency properties of the first and second absolute moment of the perturbation induced by an individual imperfection the evaluation of the lineshape can be simplified considerably. The resonance lineshape in the limiting case of a large number of imperfections is investigated; under specific conditions, the lineshape is represented by a gaussian.     

Computational study of the mechanism of cyclometalation by palladium acetate

Various mechanisms for the cyclometalation of dimethylbenzylamine by palladium acetate have been studied by DFT calculations. Contrary to previous suggestions, the rate-limiting step is the electrophilic attack of the palladium on an ortho arene C-H bond to form an agostic complex rather than a Wheland intermediate. The cyclometalated product is then formed by intramolecular deprotonation by acetate via a six-membered transition state; this step has almost no activation barrier.     

Biosynthesis of the antitumor agent chartreusin involves the oxidative rearrangement of an anthracyclic polyketide

Chartreusin is a potent antitumor agent with a mixed polyketide-carbohydrate structure produced by Streptomyces chartreusis. Three type II polyketide synthase (PKS) gene clusters were identified from an S. chartreusis HKI-249 genomic cosmid library, one of which encodes chartreusin (cha) biosynthesis, as confirmed by heterologous expression of the entire cha gene cluster in Streptomyces albus. Molecular analysis of the approximately 37 kb locus and structure elucidation of a linear pathway intermediate from an engineered mutant reveal that the unusual bis-lactone aglycone chartarin is derived from an anthracycline-type polyketide. A revised biosynthetic model involving an oxidative rearrangement is presented.     

Synthesis of a chemically bonded weak cation exchange material by conversion of an allyl phase

Summary The allyl bonded phase is used as an intermediate in the synthesis of a weak cation exchange material. The double bond is first converted to an aldehyde by an ozonation reaction and then subsequent oxidation to the carboxylate group is accomplished with dichromate. Diffuse reflectance Fourier transform infrared spectroscopy (DRIFT) is used to monitor the conversion process. The presence of the aldehyde functional group is confirmed by reaction with 2,4-dinitrophenylhydrazine. ESCA spectra reveal no residual chromium on the silica surface. The product is tested chromatographically by the retention of alkylarylketones, the ion-exchange behavior of both monovalent and divalent cations, the retention of nucleosides and bases, and the separation of a mixture of hemoglobins.     

Synthesis of tetrahydrofurans by cyclization of homoallylic alcohols with iodine/iodine(III)

Tetrahydrofuran derivatives can be obtained by cyclofunctionalization of homoallylic alcohols bearing a terminal double bound by using [hydroxy(tosyloxy)iodo]benzene (HTIB, Koser's reagent) in the presence of a catalytic amount of I(2) (20 mol %) in MeOH under mild conditions. This transformation is an overall 5-endo-trig cyclization, which occurs by two different pathways. The first is a 4-exo-trig cyclization followed by ring expansion, whereas the second is an electrophilic addition followed by a 5-endo-tet cyclization.     

Thermoreversible gelation of solutions of vinyl polymers

The thermoreversible gelation of solutions of isotactic poly(methyl methacrylate) is investigated. Amorphous gels can be prepared in l-butanol by a combination of a liquid-liquid demixing with an upper critical demixing temperature and a glass transition. Annealing of the demixed solutions above their glass transition temperature T_G, results in the formation of a crystalline gel. In oxylene, crystalline gels are formed by a liquid-liquid demixing with an lower critical demixing temperature and an annealing at room temperature. Very fast gelation is observed to occur far below room temperature in solvents like 2-butanone.     

A theoretical study of alcohol oxidation by ferrate

The conversion of methanol to formaldehyde mediated by ferrate (FeO(4)2-), monoprotonated ferrate (HFeO4-), and diprotonated ferrate (H2FeO4) is discussed with the hybrid B3LYP density functional theory (DFT) method. Diprotonated ferrate is the best mediator for the activation of the O-H and C-H bonds of methanol via two entrance reaction channels: (1) an addition-elimination mechanism that involves coordination of methanol to diprotonated ferrate; (2) a direct abstraction mechanism that involves H atom abstraction from the O-H or C-H bond of methanol. Within the framework of the polarizable continuum model (PCM), the energetic profiles of these reaction mechanisms in aqueous solution are calculated and investigated. In the addition-elimination mechanism, the O-H and C-H bonds of ligating methanol are cleaved by an oxo or hydroxo ligand, and therefore the way to the formation of formaldehyde is branched into four reaction pathways. The most favorable reaction pathway in the addition-elimination mechanism is initiated by an O-H cleavage via a four-centered transition state that leads to intermediate containing an Fe-O bond, followed by a C-H cleavage via a five-centered transition state to lead to formaldehyde complex. In the direct abstraction mechanism, the oxidation reaction can be initiated by a direct H atom abstraction from either the O-H or C-H bond, and it is branched into three pathways for the formation of formaldehyde. The most favorable reaction pathway in the direct abstraction mechanism is initiated by C-H activation that leads to organometallic intermediate containing an Fe-C bond, followed by a concerted H atom transfer from the OH group of methanol to an oxo ligand of ferrate. The first steps in both mechanisms are all competitive in energy, but due to the significant energetical stability of the organometallic intermediate, the most likely initial reaction in methanol oxidation by ferrate is the direct C-H bond cleavage.     

Quantum mechanical models of the resting state of the vanadium-dependent haloperoxidase

Density functional theory has been used to investigate structural and electronic properties of complexes related to the resting form of the active site of vanadium haloperoxidase as a function of environment and protonation state. Results obtained by studying models of varying size and complexity highlight the influence of environment and protonation state on the structure and stability of the metal cofactor. The study shows that, in the trigonal bipyramidal active site, where one axial position is occupied by a key histidine, the trans position cannot contain a terminal oxo group. Further, a highly negatively charged vanadate unit is not stable. Protonation of at least one equatorial oxo ligand appears necessary to stabilize the metal cofactor. The study also indicates that, while at rest within the protein, the vanadate unit is most likely an anion with an axial hydroxide and an equatorial plane containing two oxos and a hydroxide. For the neutral, protonated state of the vanadate unit, there were two minima found. The first structure is characterized by an axial water with two oxo and one hydroxo group in the equatorial plane. The second structure contains an axial hydroxo group and an equatorial plane composed of one oxo and two hydroxo oxygen atoms. These two species are not significantly different in energy, indicating that either form may be important during the catalytic cycle. These data support the initial crystallographic assignment of an axially bound hydroxide, but an axial water is also a possibility. This study also shows that the protonation state of the vanadate ion is most likely greater than previously proposed.     

Chiral Amine Catalyzed Reductive Aldol/Reductive Michael Addition Cascade Towards Enantioselective Synthesis of Benzannulated Diquinanes

Disclosed here an amine-catalyzed reductive aldol-condensation followed by an intramolecular reductive Michael-addition cascade employing Hantzsch ester as hydride source to a keto-bis-enone to provide enantio- and diastereoselective benzannulated diquinanes having three consecutive stereocenters, one of which is an all-carbon quaternary formyl stereocenter. Interestingly, on changing a tether connecting the ketone and an enone moiety from an aliphatic to an aromatic, a change in reactivity is observed. In this case, instead of the above-mentioned reductive aldol condensation, an asymmetric aldol reaction occurs, followed by an iminium/enamine isomerization and, finally diastereoselective Michael addition reaction occurs. As a result, a bis-benzannulated diquinane is obtained with vicinal congested quaternary chiral centers.     

Plasma Induced Degradation of Azobenzene in Water

The degradation of azobenzene (AO) by a gaseous plasma is reported. The plasma was generated in a localized zone between an electrolytic solution and a tip of an anode in contact with the surface of solution by means of contact glow discharge electrolysis. There is an optimum pH for the degradation of AO. Iron (II) had a remarkable catalytic action on it. Furthermore, the degradation followed the first‐order kinetic law. Some of the intermediate products of the degradation process were detected by HPLC.     

A Complete Switch of the Directional Selectivity in the Annulation of 2‐Hydroxybenzaldehydes with Alkynes

Controlling reaction selectivity is an eternal pursuit for chemists working in chemical synthesis. As part of this endeavor, our group has been exploring the possibility of constructing different natural product skeletons from the same simple starting materials by using different catalytic systems. In our previous work, an isoflavanone skeleton was obtained from the annulation of a salicylaldehyde and an alkyne when a gold catalyst was employed. In this paper, it is shown that a coumarin skeleton can be efficiently obtained through an annulation reaction with the same starting materials, that is, terminal alkynes and salicylaldehydes, by simply switching to a rhodium catalyst. A plausible reaction mechanism is proposed for this new annulation based on isotopic substitution experiments.     

Direct measurement of the rate of interconversion of zirconaaziridine enantiomers

The insertion of enantiopure C2-symmetric diphenylethylene carbonate into the Zr-C bonds of zirconaaziridines leads to the asymmetric synthesis of amino acid methyl esters. Because the zirconaaziridine enantiomers interconvert, the reaction is a dynamic kinetic resolution (DKR). The efficiency of the DKR (the ratio of the two diastereomeric products) is determined by the balance between the rate of enantiomer interconversion and the rate of insertion; slow addition of the inserting enantiopure carbonate is often required to maximize the stereoselectivity. For a case when enantiomer interconversion is fast, its rate constant kinv has been determined by NMR line broadening; for a case when interconversion is slow, k(inv) has been determined by computer simulation of the formation of the diastereomeric products as a function of time; for several intermediate cases, k(inv) has been determined by making the zirconaaziridine enantioenriched and monitoring its racemization by CD spectroscopy. The observed k(inv) is independent of [THF], implying that interconversion occurs with THF coordinated. Interconversion presumably occurs via an achiral intermediate, either a rapidly inverting (via an eta1-N structure) eta3-azaallyl hydride or an eta1-imine. As addition of THF slows insertion without affecting enantiomer interconversion, it produces a more efficient DKR without slow addition of the enantiopure carbonate.