Synthetic approaches to guanacastepenes. Enantiospecific syntheses of BC and AB ring systems of guanacastepenes and rameswaralide

[reaction: see text] A simple and efficient approach for the BC and AB ring systems of the novel diterpenes guanacastepenes and rameswaralide starting from the readily and abundantly available monoterpene (R)-carvone employing RCM reaction as the key step is described.     

A solvolytic C-C cleavage reaction of 6-acetoxycyclohexa-2,4-dienones: mechanistic implications for the intradiol catechol dioxygenases

6-Acetoxycyclohexa-2,4-dienones are found to undergo a rapid reaction in methanol/water under mildly basic conditions to give an acyclic ketoester as the major product for 6-phenyl and 6-methyl substrates. Reaction monitoring by UV spectroscopy indicates the formation of an unsaturated ketone reaction intermediate (lambda(max) 275 nm, R = Ph) and the transient appearance of a highly conjugated species. Reaction of the 6-phenyl substrate (4.95 x 10(-6) s(-1)) is 2-fold faster than the 6-methyl substrate (2.47 x 10(-6) s(-1)). The reaction rate is first order with respect to substrate concentration, and the final step in the reaction is pH-dependent. No cleavage was observed for a substrate lacking an acetyl substituent. A reaction mechanism for C-C cleavage is proposed involving a benzene oxide-oxepin interconversion. The possible relevance to the catalytic mechanism of the intradiol catechol dioxygenases is discussed.     

Tungsten allylimido complexes Cl4(RCN)W(NC3H5) as single-source CVD precursors for WNxCy thin films. Correlation of precursor fragmentation to film properties

A mixture of the tungsten allylimido complexes Cl(4)(RCN)W(NC(3)H(5)) (3a, R = CH(3) and 3b, R = Ph) was tested as a single-source precursor for growth of tungsten nitride (WN(x)) or carbonitride (WN(x)C(y)) thin films. Films deposited from 3a,b below 550 degrees C contained amorphous beta-WN(x)C(y), while those deposited at higher temperatures were polycrystalline. Film growth rates from 3a,b ranged from 5 to 10 A/min over a temperature range of 450-650 degrees C, and the apparent activation energy for film growth was 0.15 eV. A plot of the E(a) values for deposition from Cl(4)(RCN)W(NR') [R' = Ph, (i)Pr, allyl] against the N-C imido bond strengths for the analogous amines R'NH(2) is linear, implicating cleavage of the N-C bond as the rate-determining step in film growth. The correlation of mass spectral fragmentation patterns for Cl(4)(RCN)W(NR') with film properties such as nitrogen content supports the significance of facile N-C bond cleavage in film growth.     

Heteronuclear diatomic force constants clarified through perturbation theory II

A simple relationship between the heteronuclear diatomic force constant (K(AB)) and the homonuclear diatomic force constants (K(AA), K(BB)), which was proposed in a previous report, has been improved through the second-order perturbation theory as K(AB) = zeta3(K(AA) x K(BB))(1/2); zeta = (R(AA) x R(BB))(1/2)/R(AB) where zeta denotes the correction factor in which R(AB), R(AA), and R(BB) are the equilibrium internuclear distances of diatomic molecules AB, AA, and BB, respectively. To test the above expression, a large number of heteronuclear diatomic force constants have been calculated and compared with those obtained from normal coordinate analyses as well as ab initio quantum mechanical methods (Gaussian 98W). We have found that the above modified expression better reproduces the force constants of most heteronuclear diatomic molecules than the previous expression. It is therefore expected that the expression may also be applied to the prediction of stretching force constants between heteronuclear diatomics in various polyatomic molecules.     

Thermal decomposition of trimethylgallium Ga(CH3)3: a shock-tube study and first-principles calculations

The thermal decomposition of Ga(CH3)3 has been studied both experimentally in shock-heated gases and theoretically within an ab-initio framework. Experiments for pressures ranging from 0.3 to 4 bar were performed in a shock tube equipped with atomic resonance absorption spectroscopy (ARAS) for Ga atoms at 403.3 nm. Time-resolved measurements of Ga atom concentrations were conducted behind incident waves as well as behind reflected shock waves at temperatures between 1210 and 1630 K. The temporal variation in Ga-atom concentration was described by a reaction mechanism involving the successive abstraction of methyl radicals from Ga(CH3)3 (R1), Ga(CH3)2 (R2), and GaCH3 (R3), respectively, where the last reaction is the rate-limiting step leading to Ga-atom formation. The rate constant of this reaction (R3) was deduced from a simulation of the measured Ga-atom concentration profiles using thermochemical data from ab-initio calculations for the reactions R1 and R2 as input. The Rice-Ramsperger-Kassel-Marcus (RRKM) method including variational transition state theory was applied for reaction R3 assuming a loose transition state. Structural parameters and vibrational frequencies of the reactant and transition state required for the RRKM calculations were obtained from first-principles simulations. The energy barrier E3(0) of reaction R3, which is the most sensitive parameter in the calculation, was adjusted until the RRKM rate constant matched the experimental one and was found to be E(0) = 288 kJ/mol. This value is in a good agreement with the corresponding ab-initio value of 266 kJ/mol. The rate constant of reaction R3 was found to be k 3/(cm(3) mol(-1)s(-1)) = 2.34 x 10(11) exp[-23330(K/ T)].     

Influence of response factors on determining equilibrium association constants of non-covalent complexes by electrospray ionization mass spectrometry

A method for determining the equilibrium association constant of a complexation reaction A + B left harpoon over right harpoon AB by electrospray ionization mass spectrometry is described. The method consists in measuring the relative intensities of the peaks corresponding to A and to AB in equimolar A-B solutions at different concentrations C(0). The results are fitted by a non-linear least-squares procedure, with the two variable parameters being the equilibrium association constant K(a) and a factor R, defined by I(AB)/I(A) = R x [AB]/[A]. The factor R is the ratio between the response factors of AB and A, and corrects for the relative electrospray responses of the complex and the free substrate A, mass discrimination of instrumental origin and/or moderate in-source dissociation. The method is illustrated with the following two systems: complexes between a double-stranded 12-base pair oligonucleotide and minor groove binders, and cyclodextrin complexes with alpha,omega-dicarboxylic acids. For the oligonucleotide complexes, it is found that the response of the complex is not dramatically different to the response of the free oligonucleotide duplex, as the double helix conformation is disturbed by the drug only to a minor extent. In the case of cyclodextrin complexes, these complexes were found to have a much higher response than free cyclodextrin. This may be due to the fact that cyclodextrin is neutral in solution, whereas the complex is charged, but it can also stem from the fact that a significant proportion of the complex is in a non-inclusion geometry. The present method requires the exact determination of the concentrations of the reactants and is applicable to 1 : 1 complexes.     

Mechanistic study on gold(I)‐catalyzed crosscoupling of diazo compounds: A DFT study

The reaction mechanisms of the gold(I)‐catalyzed cross‐coupling reaction of aryldiazoacetate R1 with vinyldiazoacetate R2 leading to N‐substituted pyrazoles have been theoretically investigated using density functional theory calculations. Two possible reaction mechanisms were examined and discussed. The preferred reaction mechanism (mechanism A) can be characterized by five steps: the formation of the gold carbenoid A2 via the attack of catalyst to R1 (step I), nucleophilic addition of another reactant R2 to generate intermediate A3 (step II), intramolecular cyclization of A3 to form intermediate A4 (step III), hydrogen migration to give intermediate A5 (step IV), and catalyst elimination affording the final product P1 (step V). Step IV is found to be the rate‐determining step with an overall free energy barrier of 28.3 kcal/mol. Our calculated results are in good agreement with the experimental observations. The present study may provide a useful guide for understanding these kinds of gold(I)‐catalyzed cross‐coupling reactions of diazo compounds.     

Synthetic and mechanistic studies of metal-free transfer hydrogenations applying polarized olefins as hydrogen acceptors and amine borane adducts as hydrogen donors

Metal-free transfer hydrogenation of polarized olefins (RR'C=CEE': R, R' = H or organyl, E, E' = CN or CO(2)Me) using amine borane adducts RR'NH-BH(3) (R = R' = H, AB; R = Me, R' = H, MAB; R = (t)Bu, R' = H, tBAB; R = R' = Me, DMAB) as hydrogen donors, were studied by means of in situ NMR spectroscopy. Deuterium kinetic isotope effects and the traced hydroboration intermediate revealed that the double H transfer process occurred regio-specifically in two steps with hydride before proton transfer characteristics. Studies on substituent effects and Hammett correlation indicated that the rate determining step of the H(N) transfer is in agreement with a concerted transition state. The very reactive intermediate [NH(2)=BH(2)] generated from AB was trapped by addition of cyclohexene into the reaction mixture forming Cy(2)BNH(2). The final product borazine (BHNH)(3) is assumed to be formed by dehydrocoupling of [NH(2)=BH(2)] or its solvent stabilized derivative [NH(2)=BH(2)]-(solvent), rather than by dehydrogenation of cyclotriborazane (BH(2)NH(2))(3) which is the trimerization product of [NH(2)=BH(2)].     

Complex Formation of Phosphoryl- and Thiophosphorylacylacetonitriles with Different Cations

The enols R 1 R 2 P(E)(CN)C = CR 3 OH (E = O or S) gave in solutions either neutral metal complexes ML x or M(OH) y L x . The anionic ambidentate ligands are coordinated through E and O atoms in solutions, and O, E, and N atoms in in crystals.     

Solid-state NMR study of Li-assisted dehydrogenation of ammonia borane

The mechanism of thermochemical dehydrogenation of the 1:3 mixture of Li(3)AlH(6) and NH(3)BH(3) (AB) has been studied by the extensive use of solid-state NMR spectroscopy and theoretical calculations. The activation energy for the dehydrogenation is estimated to be 110 kJ mol(-1), which is lower than for pristine AB (184 kJ mol(-1)). The major hydrogen release from the mixture occurs at 60 and 72 °C, which compares favorably with pristine AB and related hydrogen storage materials, such as lithium amidoborane (LiNH(2)BH(3), LiAB). The NMR studies suggest that Li(3)AlH(6) improves the dehydrogenation kinetics of AB by forming an intermediate compound (LiAB)(x)(AB)(1-x). A part of AB in the mixture transforms into LiAB to form this intermediate, which accelerates the subsequent formation of branched polyaminoborane species and further release of hydrogen. The detailed reaction mechanism, in particular the role of lithium, revealed in the present study highlights new opportunities for using ammonia borane and its derivatives as hydrogen storage materials.     

Kinetics and mechanisms of the oxidation of phenols by a trans-dioxoruthenium(VI) complex

The kinetics of the oxidation of phenols by trans-[Ru(VI)(L)(O)(2)](2+) (L = 1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane) have been studied in aqueous acidic solutions and in acetonitrile. In H(2)O the oxidation of phenol produces the unstable 4,4'- biphenoquinone, as evidenced by a rapid increase and then a slow decrease in absorbance at 398 nm. The first step is first-order in both Ru(VI) and phenol, and rate constants are dependent on [H(+)] according to the relationship k(f) = k(x) + (k(y)K(a)/[H(+)]), where k(x) and k(y) are the rate constants for the oxidation of PhOH and PhO(-), respectively. At 298 K and I = 0.1 M, k(x) = 12.5 M(-1) s(-1) and k(y) = 8.0 x 10(8) M(-1) s(-1). At I = 0.1 M and pH = 2.98, the kinetic isotope effects are k(H(2)O)/k(D(2)O) = 4.8 and 0.74 for k(x) and k(y), respectively, and k(f)(C(6)H(5)OH)/k(f)(C(6)D(5)OH) = 1.1. It is proposed that the k(x) step occurs by a hydrogen atom abstraction mechanism, while the k(y) step occurs by an electron-transfer mechanism. In both steps the phenoxy radical is produced, which then undergoes two rapid concurrent reactions. The first is a further three-electron oxidation by Ru(VI) and Ru(V) to give p-benzoquinone and other organic products. The second is a coupling and oxidation process to give 4,4'-biphenoquinone, followed by the decay step, k(s). A similar mechanism is proposed for reactions in CH(3)CN. A plot of log k(x) vs O-H bond dissociation enthalpies (BDE) of the phenols separates those phenols with bulky tert-butyl substituents in the ortho positions from those with no 2,6-di-tert-butyl groups into two separate lines. This arises because there is steric crowding of the hydroxylic groups in 2,6-di-tert-butyl phenols, which react more slowly than phenols of similar O-H BDE but no 2,6-tert-butyl groups. This is as expected if hydrogen atom abstraction but not electron transfer is occurring.     

Synthesis of crystalline skutterudite superlattices using the modulated elemental reactant method

A series of samples ((AB)(x)(CD)(y))(z) were prepared containing both short repeat units (AB and CD) and long repeat units ((AB)(x)(CD)(y)), where the short repeat units were designed to have the composition appropriate to form square M(4)Sb(12) skutterudites (M = Fe, Co, or Ir; square = vacancy, La, or Y). X-ray diffraction and reflectivity were used to follow the evolution of the films from amorphous, layered materials to crystalline skutterudite superlattices as a function of annealing temperature and time. In all cases, the short repeat units interdiffused and crystallized the expected skutterudite, while the long repeat period persisted after annealing. The skutterudites crystallize with random crystallographic orientation with respect to the substrate. The observed splitting of the peaks in the high-angle diffraction data from the IrSb(3)/CoSb(3) sample indicates the formation of a novel superlattice structure with each grain having a random crystallographic orientation of the skutterudite lattice with respect to the superlattice direction.     

Advances in the chemistry of nanosized zirconium phosphates: a new mild and quick route to the synthesis of nanocrystals

Simple addition of zirconyl propionate to phosphoric acid in alcoholic media surprisingly led to the formation, in few minutes, of transparent gels containing solvent intercalated zirconium phosphate (ZrP) nanoparticles with hexagonal shape and a planar size of about 40 nm. With the help of elemental analysis, inductively coupled plasma-optical emission spectrometry (ICP-OES), and (31)P magic angle spinning (MAS) NMR, the nanoparticle composition was formulated as Zr(R)(w)(HPO(4))(x)(H(2)PO(4))(y), in which R can be an hydroxyl or a propionate group. The stoichiometric coefficients for propanol intercalated ZrP are x = 1.43, y = 0.83, and w = 0.32. Solvent elimination at 60 °C gave rise to an increase in the x value and a decrease in the y and w values. X-ray powder diffraction analysis and transmission electron microscopy (TEM) observations showed a concomitant increase in the particle size: planar size and thickness ranged from 90 to 200 nm and from 20 to 85 nm, respectively, depending on the nature of the solvent. A possible mechanism explaining the change in the x, y, and w values, the growth of nanoparticles, and the role of the solvent is proposed. Finally, the possibility of using these gels to disperse the ZrP nanoparticles within the polymer matrix of Nafion117 is shown.     

Effect of anisotropic diffusion and external electric field on the rate of diffusion-controlled reactions

In this paper we investigate theoretically the effect of an external electric field on the rate constant of steady-state bulk diffusion-controlled reactions. We generalize previously derived results for isotropic diffusion in the absence of interparticle interaction [J. Chem. Phys. 87, 4622 (1987)] to the case where translational diffusion is anisotropic. A frequently occurring situation of transverse isotropy where D(x)=D(y) not equal to D(z) is considered in detail. We derive the first-order expansion for the reaction rate constant in terms of the electric field strength E, k(E)=k(0) (1+1/2epsilongamma), where gamma=k(0)/4piRD( perpendicular ), epsilon=qER/k(B)T, q is the charge, R is the contact distance, and D( perpendicular ) is the transverse diffusion coefficient. Numerical calculations show that this first-order expansion works well in the whole range of applicability of the Nernst-Einstein relation, i.e., for epsilon<1.     

基于高剪切分散乳化技术的黄芪中黄酮类化合物提取方法及动力学研究

利用高剪切分散乳化(High shear dispersing emulsifier,HSDE)技术为样品前处理方法,以甲醇为提取剂.通过单因素实验、L9(34)正交试验,优选出HSDE提取黄芪中4种黄酮类化合物(毛蕊异黄酮-7-O β-D葡萄糖苷、毛蕊异黄酮、芒柄花素、芒柄花苷)的最佳条件为:料液比1∶20,转速16000 r/min,时间180 s.在此提取条件下进行黄芪提取的动力学研究,结果表明,黄芪中4种黄酮类化合物的HSDE提取过程符合准二级动力学方程特征.以时间(t)为横坐标,时间与峰面积的比值(t/A)为纵坐标,进行线性拟合,方程分别为:毛蕊异黄酮-7-O-β-D-葡萄糖苷:y=0.00974x+ 0.00869,R2=0.99736;毛蕊异黄酮:y=0.00153x+0.01654,R2=0.9862;芒柄化素:y=0.00196x+0.02322,R2=0.98492;芒柄花苷:y=0.00527x+ 0.046,R2=0.99228.在实验的基础上推测其提取机理为气蚀效应、机械效应和剪切效应的协同作用.     

Photo-Induced Self-assembly of Copolymer-Capped Nanoparticles into Colloidal Molecules

Colloidal molecules (CMs) are precisely defined assemblies of nanoparticles (NPs) that mimic the structure of real molecules, but externally programming the precise self-assembly of CMs is still challenging. In this work, we show that the photo-induced self-assembly of complementary copolymer-capped binary NPs can be precisely controlled to form clustered AB _x or linear ( AB ) _y CMs at high yield (x is the coordination number of NP- B s, and y is the repeating unit number of AB clusters). Under UV light irradiation, photolabile p-methoxyphenacyl groups of copolymers on NP- A *s are converted to carboxyl groups (NP- A ), which react with tertiary amines of copolymers on NP- B to trigger the directional NP bonding. The x value of AB _x can be precisely controlled between 1 and 3 by varying the irradiation duration and hence the amount of carboxyl groups generated on NP- A s. Moreover, when NP- A * and NP- B are irradiated after mixing, the assembly process generates AB clusters or linear ( AB ) _y structures with alternating sequence of the binary NPs. This assembly approach offers a simple yet non-invasive way to externally regulate the formation of various CMs on demand without the need of redesigning the surface chemistry of NPs for use in drug delivery, diagnostics, optoelectronics, and plasmonic devices.     

A theoretical study on the mechanism and diastereoselectivity of the Kulinkovich hydroxycyclopropanation reaction

A detailed mechanism for the Kulinkovich hydroxycyclopropanation reaction has been explored with density functional theory calculations on the reactions between R(1)COOMe and Ti(OMe)(2)(CH(2)CHR(2)) (R(1) and R(2) are hydrogen and alkyl groups). Addition of ester to titanacyclopropane is found to be fast, exothermic, and irreversible. It has a preference for the alpha-addition manifold over the beta-addition manifold in which its cycloinsertion transition states suffer from the steric repulsion between the R(2) and ester. The following intramolecular methoxy migration step is also exothermic with reasonable activation energy. The cyclopropane-forming step is the rate-determining step, which affords the experimentally observed cis-R(1)/R(2) diastereoselectivity in the alpha-addition manifold by generating cis-R(1)/R(2) 1,2-disubstituted cyclopropanol when R(1) is primary alkyl groups. On the contrary, the unfavored beta-addition manifold offers the diastereoselectivity contradicting the experimental observations. The effects of R(1) and R(2) on the regio- and stereoselectivity are also discussed.     

Kinetics and Mechanism of the Reaction between Serum Albumin and Auranofin (and Its Isopropyl Analogue) in Vitro

The first detailed kinetic analysis and mechanistic interpretation of the reactions between serum albumin and the second-generation gold drug Auranofin [Et(3)PAuSATg = (triethylphosphine)(2,3,4,6-tetra-O-acetyl-1-beta-D-glucopyranosato-S-) gold(I)] and its triisopropylphosphine analogue, iPr(3)PAuSATg, in vitro are reported. The reactions were investigated using Penefsky spun columns and NMR saturation transfer methods. Under the Penefsky chromatography conditions with 0.4-0.6 mM albumin and a wide range of Et(3)PAuSATg concentrations, the reaction is biphasic. The fast phase is apparently first order in albumin with a rate constant [k(1) = 3.4 +/- 0.3 x 10(-)(2) s(-)(1)] that decreases slightly in magnitude and becomes intermediate in order at low gold concentrations, [Et(3)PAuSATg] < [AlbSH]; it accounts for approximately 95% of the Au(I) that binds. A minor, slower step [k(2) = 2.3 +/- 0.3 x 10(-)(3) s(-)(1)), which accounts for only 5% of the reaction, is also first order with respect to albumin, and zero order with respect to auranofin. For iPr(3)PAuSATg, only the first step was observed, k(1) = (1.4 +/- 0.1) x 10(-)(2) s(-)(1), and is first order in albumin and independent of the iPr(3)PAuSATg concentration. (31)P-NMR saturation transfer experiments utilizing iPr(3)PAuSATg, under equilibrium conditions, yielded second-order rate constants for both the forward (1.2 x 10(2) M(-)(1) s(-)(1)) and the reverse (3.9 x 10(1) M(-)(1) s(-)(1)) directions. A multistep mechanism involving a conformationally altered albumin species was developed. Albumin domain IA opens with concomitant Cys-34 rearrangement, allowing facile gold binding and exchange, and then closes. In conjunction with the steady-state approximation, this mechanism accounts for the different reaction orders observed under the two set of conditions. The rate-determining conformational change of albumin governs the reaction as monitored by the Penefsky columns. Rapid second order exchange of R(3)PAuSATg at the exposed Cys-34 residue is observed under the NMR conditions. The mechanism predicts that under physiological conditions where [Et(3)PAuSATg] is 10-25 &mgr;M, the reaction will be second order and rapid with a rate constant of 8 +/- 2 x 10(2) M(-)(1) s(-)(1). The Penefsky spun columns revealed a previously unreported and novel binding mechanism, association of auranofin in the pocket of albumin-disulfide species, which was confirmed by Hummel-Dreyer gel chromatographic techniques under equilibrium conditions. This albumin-auranofin complex (AlbSSR-Et(3)PAuSATg) is weakly bound and readily dissociates during conventional gel exclusion chromatography.     

Adsorption of multiblock copolymers onto a chemically heterogeneous surface: a model of pattern recognition

We present a statistical mechanical model, which is used to investigate the adsorption behavior of two-letter (AB) copolymers on chemically heterogeneous surfaces. The surfaces with regularly distributed stripes of two types (A and B) and periodic multiblock copolymers (Al)B(l))(x) are studied. It is assumed that A(B)-type segments selectively adsorb onto A(B)-type stripes. It is shown that the adsorption strongly depends on the copolymer sequence distribution and the arrangement of selectively adsorbing regions on the surface. The polymer-surface binding proceeds as a two-step process. At the first step, the copolymer having short blocks adsorbs onto the surface as an effective homopolymer, which does not feel chemical pattern. At the second step, when the polymer-surface attraction is sufficiently strong, the adsorbed chain adjusts its equilibrium conformation to reach the perfect bound state, thereby demonstrating ability for pattern recognition. The key element of this mechanism is the redistribution of strongly adsorbed copolymer diblocks A(l)B(l), which behave as surfactants, between multiple AB interfaces separating A and B stripes on the adsorbing surface. Such redistribution is accompanied by a well-pronounced decrease in the system entropy. We have found that marked pattern recognition is possible for copolymers with relatively short blocks at high polymer/surface affinities, beyond the adsorption threshold.     

Construction and Properties of Sierpiński Triangular Fractals on Surfaces

Fractal structures are of fundamental importance in science, engineering, mathematics, and aesthetics. Construction of molecular fractals on surfaces can help to understand the formation mechanism of fractals and a series of achievements have been acquired in the preparation of molecular fractals. This review focuses on Sierpiński triangles (STs), representatives of various prototypical fractals, on surfaces. They are investigated by Monte Carlo simulations and ultra-high vacuum scanning tunneling microscopy. STs are bonded through halogen bonds, hydrogen bonds, metal-organic coordination bonds and covalent bonds. The coexistence of and competition between fractals and crystals are realized for a hydrogen-bonded system. Electronic properties of two types of STs are summarized.