DFT-PCM studies of solvent effects on the cross-interaction constants in benzhydryl cation and anion formation

The Hammett rho+ and rho- values have been determined by varying substituent Y' for a given Y in the benzhydryl cation and anion formation (YH4C6-CH-C6H4Y' where C is a cationic or an anionic center) at the RHF/3-21G, RHF/6-31G, RHF/6-31+G, and B3LYP/6-31+G levels. The failure of RHF theory in accounting for the stabilization by delocalization leads to the smaller magnitudes of rho+ and rho- with electron-donating and -withdrawing substituents, Y, respectively, than the corresponding DFT values. The effects of solvent (benzene, dichloroethane, and acetonitrile) on the rho values were calculated by applying the conductor polarizable continuum model method to the DFT results. Finally, the cross-interaction constants (rho(YY)') and their variation with solvent were determined. As the polarity (dielectric constant, epsilon) of the solvent is increased, the magnitude of rho+ and rho- decreased, whereas that of rho(YY)' increased. Satisfactory correlations were obtained between rho values (rho+, rho- and rho(YY)') and the Kirkwood function f(k) (= epsilon - 1/2epsilon + 1). The rho(YY)' values are negative with a magnitude greater for the anionic (rho(YY)'-) than the cationic (rho(YY)'+) system.     

Selective reductive cleavage of 2-(phenylthio)pyrimidines for efficient synthesis of 2-(H)pyrimidines

A reaction method is described for selective reductive cleavage of 2-(phenylthio)pyrimidines using Pd(OAc)₂ and Et₃SiH to produce 2-(H)pyrimidines. The reaction proceeds efficiently with a wide range of 2-(phenylthio)pyrimidines. Considering the ready availability of 2-(arylthio)pyrimidines derived from oxidative CS cross coupling of 3,4-dihydropyrimidin-1H-2-thiones (DHPMs), this method unambiguously provides a shortcut to the preparation of 2-(H)pyrimidines with unprecedented diversity.     

Shape evolution of single-crystalline iron oxide nanocrystals

Shape- and size-controlled synthesis of single-crystalline maghemite (gamma-Fe2O3) nanocrystals are performed by utilizing a solution-based one-step thermolysis method. Modulating the growth parameters, such as the type and amount of capping ligands as well as the growth time, is shown to have a significant effect on the overall shape and size of the obtained nanocrystals and on the ripening process itself. The resulting shapes of the novel structures are diverse, including slightly faceted spheres, diamonds, prisms, and hexagons, all of which are in fact truncated dodecahedron structures with different degrees of truncation along the {111}, {110}, or {100} faces. Spherical nanocrystals are easily assembled into the three-dimensional superlattices, demonstrating the uniformity of these nanocrystals. The size-dependent magnetic properties are examined, and large hexagon-shaped gamma-Fe2O3 nanocrystals are shown to be ferrimagnetic at room temperature.     

Novel examples of high-dimensional mixed-valence copper cyanide complexes

The first examples of high-dimensional mixed-valence homometallic cyano-bridged copper complexes were synthesized and characterized: net-structured [Cu(CN)(4){Cu(cyclam)}(1.5)](2)(n)()(H(2)O)(5)(n) (1), ladder-type double-chain-structured [Cu(CN)(2){Cu(CN)(2)Cu(cyclam)}](n)()(H(2)O)(n) (2), layer-structured [{Cu(CN)(2)}(2)Cu(cycalm)](n) (3), and hydrogen-bond-based 2-D [Cu(CN)(3)Cu(cyclam)](n)()(CH(3)OH)(n) (4) (cyclam = 1,4,8,11-tetraazacyclotetradecane). (1) Crystallizes in triclinic space group P with a = 8.3589(11) A, b = 13.478(2) A, c = 14.828(2) A, alpha = 66.895(2) degrees , beta = 77.916(3) degrees , gamma = 85.939(3) degrees , and Z = 1; (2) crystallizes in triclinic space group P with a = 8.2305(12) A, b = 9.8861(15) A, c = 13.219(2) A, alpha = 84.863(3) degrees , beta = 75.744(3) degrees , gamma = 89.818(3) degrees , and Z = 2; 3 crystallizes in monoclinic space group P2(1)/c with a = 6.830(2) A, b = 8.482(2) A, c = 17.306(4) A, beta = 98.144(4) degrees , and Z = 2; 4 crystallizes in triclinic space group P with a = 9.470(1) A, b = 10.034(1) A, c = 12.064(1) A, alpha = 67.325(2), beta = 75.593(2), gamma = 70.672(2), and Z = 2. The coordination sphere of Cu(I) sites in the complexes shows diverse structures: tetrahedral [CuC(4)] for (1), tetrahedral [CuC(3)N] and triangular [CuC(2)N] for (2), triangular [CuC(2)N] for (3), and triangular [CuC(3)] for 4. In particular, (1) constitutes the first example of a structurally characterized system containing a bridging tetrahedral [Cu(CN)(4)](3)(-) unit. The diverse structural nature of these complexes is governed by the capping amines and the content of water in the reaction media. The magnetic interactions are negligible in these mixed-valence complexes.     

Asymmetric Diels-Alder reactions in ionic liquids

Recent interest in ionic liquids has developed various uses for them, including some applications by synthetic chemists. Ionic liquids have joined the potential list of non-traditional solvents for Diels-Alder reactions. We report here our own efforts to examine the rates and selectivities of carbon Diels-Alder reactions. Our investigations show that excellent diastereoselective and enantioselective carbon Diels-Alder reactions can be achieved in imidazolium ionic solvents at room temperature.     

Use of the intramolecular Heck reaction for forming congested quaternary carbon stereocenters. Stereocontrolled total synthesis of (+/-)-gelsemine

Intramolecular Heck reactions of alpha,beta-unsaturated 2-haloanilides derived from azatricyclo[4.4.0.0(2,8)]decanone 5 efficiently install the congested spirooxindole functionality of gelsemine. Depending upon the Heck reaction conditions and the nature of the beta-substituent, either products having the natural or unnatural configuration of the spirooxindole group are formed predominantly. Efforts to elaborate the hydropyran ring of gelsemine from the endo-oriented nitrile substituent of pentacyclic Heck product 18 were unsuccessful. Important steps in the ultimately successful route to (+/-)-gelsemine (1) are as follows: (a) intramolecular Heck reaction of tricyclic beta-methoxy alpha,beta-unsaturated 2-iodoanilide 68 in the presence of silver phosphate to form pentacyclic product 69 having the unnatural configuration of the spirooxindole fragment, (b) formation of hexacyclic aziridine 80 from the reaction of cyanide with intermediate 79 containing an N-methoxycarbonyl-beta-bromoethylamine fragment, (c) introduction of C17 by ring-opening of the aziridinium ion derived from aziridine 80, and (d) base-promoted skeletal rearrangement of pentacyclic equatorial alcohol 82 to form the oxacyclic ring and invert the spirooxindole functional group to provide hexacyclic gelsemine precursor 83.     

Nucleophilic vinylic substitution of halocoumarins and halo-l,4-naphthoquinones with morpholine

Reactions of morpholine with 4-halocoumarins 1 , 3-halocoumarins 3 , and 2-halo-l,4-naphthoquinones 8 yield two different products, one where halogen is replaced by a nucleophile at the same carbon and the other where the nucleophile is attached to the vicinal carbon away from that bearing the halogen. It is considered that reactions proceeds through nucleophilic vinylic substitution by comparision of the reaction products and deuterium exchange experiments. Plausible mechanisms for these routes are suggested.     

Variation of mechanical and thermal properties of the thermoplastics reinforced with natural fibers by electron beam processing

With restrictions for environmental protection being strengthened, the thermoplastics reinforced with natural fibers (NFs) such as jute, kenaf, flax, etc., appeared as an automobile interior material instead of the chemical plastics. Regardless of many advantages, one shortcoming is the deformation after being formed in high temperature of about 200 °C, caused by the poor adhesion between the natural fibers and thermoplastics. Also, the energy saving in connection with car air-conditioning becomes very important. In this study, the thermal conductivity, tensile strength, and deformation of several kinds of thermoplastic composites composing of 50% polypropylene (PP) and 50% natural fiber irradiated by the electron beam (energy: 0.5 MeV, dose: 0–20 kGy) were measured. The length and thickness of PP and NF are 80±10 mm and 40–120 μm, respectively. The results show that the thermal conductivity and the tensile strength changed and became minimum when the dose of electron beam is 10 kGy, and the deformation after the thermal cycle were reduced by the electron beam.