Stereospecific living Ziegler-Natta polymerization via rapid and reversible chloride degenerative transfer between active and dormant sites

The successful realization of a stereospecific chloride degenerative transfer living Ziegler-Natta polymerization process that provides isotactic polyolefins of narrow polydispersity (Mw/Mn 相似文献   

Nucleophilic displacements in pyridine rings

The reactions of halopyridines containing an electron withdrawing group (-CN, -CO₂R,-COM_e, -NO₂) with sulphur nucleophiles is reported.     

Macrocycles in the construction of acyclic stereochemistry

The conformations of macrocyclic intermediates provide a useful medium through which distant chiral centers may control chemical reactions. In this paper, we show that macrocycles made by cyclization of simple acyclic starting materials with an auxiliary spacer may be used to prepare stereochemically complex acyclic products.     

Multiply doped nanostructured silicate sol-gel thin films: spatial segregation of dopants, energy transfer, and distance measurements

Physical and chemical strategies that place designed molecules in spatially separated regions of surfactant-templated mesostructured silicate thin films are used to prepare films containing rhodamine 6G (R6G), lanthanide complexes, and both simultaneously. Fluorescence and photoexcitation spectra of R6G in amorphous and structured thin films show that it is located inside the surfactant micelles of structured thin films. A silylated ligand that binds lanthanides condenses to form part of the silica framework and causes the lanthanide to localize in the silica. Luminescence and photoexcitation spectra show that energy transfer from the metal complex to R6G occurs in the films. R6G quenches Tb emission in a concentration-dependent manner. Energy transfer efficiency is calculated using the Tb luminescence lifetime, and this quantity is used to calculate the distance between Tb and R6G with the aid of Forster theory.     

Analysis of Ginkgo biloba for the presence of ginkgotoxin and ginkgotoxin 5'-glucoside

Hot water extracts of Ginkgo biloba seeds were analyzed for the presence of ginkgotoxin (4'-O-methylpyridoxine) by reversed-phase liquid chromatography (LC) using methanol-0.05M KH2PO4 (1 + 9, v/v) adjusted to pH 3 as mobile phase. Detection was by fluorescence (excitation 280 nm, emission 370 nm). A straight line calibration curve was obtained for the 10-100 ng injected. After addition of beta-glucosidase (37 degrees C/h), an earlier eluting peak disappeared and the ginkgotoxin peak increased. The identity of the ginkgotoxin was confirmed by LC/MS and LC/MS/MS. LC/MS/MS also confirmed the 5'-glucoside by comparison with the 3-glucoside. This is the first identification of a glucoside of ginkgotoxin in Ginkgo biloba. An unknown compound of MW 267 also observed in the Ginkgo biloba seed extract was shown not to be 3,5'-diacetylginkgotoxin by its different LC retention time. Extraction of ground Ginkgo biloba seeds with boiling water in a Soxhlet for 2 x 2 h yielded a total of 179 microg/g of free ginkgotoxin. The concentration in powder from Ginkgo biloba capsules was several times lower than this (17-64 microg/g) in 3 samples but higher in another (457 microg/g). Canned ginkgo seeds (white nuts) contained no detectable free ginkgotoxin but the glucoside was present. Different extraction times were studied: 0.5 h gave only 52 microg/g free ginkgotoxin in the ginkgo seeds. However, boiling an extract for 4 h showed about 15% loss of ginkgotoxin and its glucoside.     

Proton spin-lattice relaxation studies. N-aryl-1-isoindolinones

Proton spin-lattice relaxation rates (R₁ values) have been measured, at 270 MHz, for a series of N-aryl isoindolinones. A normalization procedure has been used to enable comparison of R₁ values in different compounds by minimizing the effects on relaxation rates of changes in motional correlation times accompanying changes in substitution patterns. A substantial (4.3-fold) dynamic range of R₁ values has been observed, and individual values have been correlated with the molecular environments of the nuclei. There is evidence for an interring relaxation process.     

Computational strategies for evaluating barrier heights for gas-phase reactions of lithium enolates

Gas-phase activation energies were calculated for three lithium enolate reactions by using several different ab initio and density functional theory (DFT) methods to determine which levels of theory generate acceptable results. The reactions included an aldol-type addition of an enolate to an aldehyde, a proton transfer from an alcohol to a lithium enolate, and an S(N)2 reaction of an enolate with chloromethane. For each reaction, the calculations were performed for both the monomeric and dimeric forms of the lithium enolate. It was found that transition state geometry optimization with B3LYP followed by single point MP2 calculations generally provided acceptable results compared to higher level ab initio methods.     

Synthesis of cationic dialkylgold(III) complexes: nature of the facile reductive elimination of alkane

A series of gold(III) cations of the type cis-[CH₃)₂AuL₂]⁺ X^? where L  Ph₃, PMePh₂, PMe₂Ph, PMe₃, AsPh₃, AsPh₃, SbPh₃, $\text{1}\text{2}$H₂NCH₂CH₂NH₂, $\text{1}\text{2}$ Ph₂PCH₂CH₂-PPh₂, $\text{1}\text{2}$ Ph₂AsCH₂CH₂AsPh₂, and $\text{1}\text{2}$o-C₆H₄(AsMe₂)₂ and X  BF₄^?, PF₆^?, ClO₄^?, and F₃CSO₃^? has been prepared. In addition, the cis complexes [(CH₃)(CD₃)-Au(PPh₃)₂]F₃CSO₃, [(C₂H₅)₂Au(PPh₃)₂]F₃CSO and [(n-C₄H₉)₂Au(PPh₃)₂]F₃-CSO₃ have been synthesized. All have been characterized by PMR, Raman and infrared spectroscopy. These [R₂AuL₂]X compounds yield only ethane, butane, or octane via reductive elimination, and no disproportionation is observed. The alkane eliminations have been studied in CHCl₃, CH₃Cl₂, and CH₃COCH₃ solution as a function of temperature, concentration of the complex, and concentration of added ligand L. Elimination is fastest when L is bulky (PPh₃ > PMePh₂ > PMe₂Ph > PMe₃), decreases in the sequence SbPh₃ > AsPh₃ > PPh₃, is slow with chelating ligands, is inhibited by excess ligand, and there is small anion effect as X is varied. As R is varied, the rate of elimination decreases Bu ? Et > Me. An intramolecular dissociative mechanism is proposed which involves rapid elimination of alkane from an electron deficient dialkylgold(III) complex with nonequivalent gold—carbon bonds and produces the corresponding [AuL₂]X complex.     

ESR STUDY OF ORIENTED TRIPLET MOLECULES GENERATED BY EXCITATION WITH POLARIZED LIGHT

Abstract— The lowest-lying triplet states of a variety of aromatic molecules and complexes have been generated by the irradiation of these compounds in solvent glasses with plane-polarized light. Measurements of the allowed (Δ M=± 1) ESR transitions clearly demonstrate that the triplets so formed are oriented with respect to the external magnetic field. By this method the triplet zero-field splitting parameters, D and E , can be evaluated simply and reliably. Intramolecular energy transfer is postulated to explain the triplet spectra of Zn( o -phen)₂(NO₃)₂ and Zn( o -phen)₃(NO₃)₂. It was observed that in triplet-triplet energy transfer from benzophenone to naphthalene there is no apparent orientation requirement between the donor and acceptor molecules. Further areas of significance and application of this technique are suggested.     

Efficiency in integral facility design problems

An example of design might be a warehouse floor (represented by a setS) of areaA, with unspecified shape. Givenm warehouse users, we suppose that useri has a known disutility functionf _isuch thatH _i(S), the integral off _iover the setS (for example, total travel distance), defines the disutility of the designS to useri. For the vectorH(S) with entriesH _i(S), we study the vector minimization problem over the set {H(S) :S a design} and call a design efficient if and only if it solves this problem. Assuming a mild regularity condition, we give necessary and sufficient conditions for a design to be efficient, as well as verifiable conditions for the regularity condition to hold. For the case wheref _iis thel _p-distance from warehouse docki, with 1<p<, a design is efficient if and only if it is essentially the same as a contour set of some Steiner-Weber functionf =₁ f ₁++ _m f _m ,when the _i are nonnegative constants, not all zero.This research was supported in part by the Interuniversity College for PhD Studies in Management Sciences (CIM), Brussels, Belgium; by the Army Research Office, Triangle Park, North Carolina; by a National Academy of Sciences-National Research Council Postdoctorate Associateship; and by the Operations Research Division, National Bureau of Standards, Washington, D.C. The authors would like to thank R. E. Wendell for calling Ref. 16 to their attention.