The opening of Plutarch's Life of Themistokles

Plutarch begins the biography by selecting incidents and an epigram that anticipate the basic themes of Themistocles' life--his greatness, sometimes ambiguous, and his benefactions to all of Greece.

High resolution photoacoustic overtone spectroscopy of monofluoroacetylene, HCCF, with a titanium:sapphire ring laser

Intracavity photoacoustic overtone spectrum of monofluoroacetylene, HCCF, has been recorded in the wave number region 10 750–14 500 cm−1 with a titanium:sapphire ring laser. The spectrum contains many dense vibration–rotation band systems which can be resolved with Doppler limited resolution. Altogether 58 individual overtone bands have been analyzed rotationally. Many of the observed bands show perturbations of which some have been attributed to anharmonic resonance interactions. A Fermi resonance model based on conventional rectilinear normal coordinate theory has been used to assign vibrationally bands from this work and from earlier studies. Many of the observed vibrational term values and rotational constants can be reproduced well with this model. The results show the importance of the Fermi resonance interactions at the high overtone excitations.

Alkene-arene meta photocycloadditions with a four-carbon-atom tether: efficient approach toward the polycyclic ring systems of aphidicolin and stemodinone

At the end of its tether! The fusion of a six-membered ring onto the four-carbon-atom tether of substrate 1 provides an efficient approach toward the polycyclic ring systems of the natural products aphidicolin and stemodinone. The reaction represents a unique example of a preference for product formation from an endo exciplex in an intramolecular system (exo:endo 2:3=1.0:1.2).

Aromatic-aromatic interaction between metallacycle and phenyl ring

The X-ray crystal structure of [CuL2]ClO4 where L is the 1:1 condensate of benzil-monohydrazone and 2-pyridinecarboxalde-hyde, reveals unprecedented pi - pi interaction between the metallacycles and phenyl rings. The interaction is intramolecular. (C) 2003 Elsevier Science B.V. All rights reserved.

Intermediate ion stability and regio selectivity polymerization using neutral salicyladiminato in propene nickel(II) and palladium(II) complexes as catalysts

The mechanism of the Heck reaction has been studied with regard to transition metal catalysis of the addition of propene and the formation of unsaturated polymers. The reactivity of nickel and palladium complexes with five different bidentate ligands with O,N donor atoms has been investigated by computational methods involving density functional theory. Hence, it is possible to understand the electronic and steric factors affecting the reaction and their relative importance in determining the products formed in regard of their control of the regiochemistry of the products. Our results show that whether the initial addition of propene is trans to O or to N of the bidentate ligand is of crucial importance to the subsequent reactions. Thus when the propene is trans to 0, 1,2-insertion is favoured, but when the propene is trans to N, then 2,1-insertion is favoured. This difference in the preferred insertion pathway can be related to the charge distribution engendered in the propene moiety when the complex is formed. Indeed charge effects are important for catalytic activity but also for regioselectivity. Steric effects are shown to be of lesser importance even when t-butyl is introduced into the bidentate ligand as a substituent. (C) 2007 Elsevier B.V. All rights reserved.

A DFT study of propylene polymerization using neutral salicyladiminato nickel(II) and palladium(II) as catalysts

Propylene polymerization using salicyladiminato metal catalalysts has been studied using density functional theory at the B3LYP/LANL2DZ level. In particular, the effects on the reaction mechanisms of changing the metal from Pd(II) to Ni(II) have been investigated. While the reaction mechanisms involving the salicyladiminato Ni(II) catalyst have been found to be similar to those established previously for the salicyladiminato Pd(II) catalyst, the nickel catalyst was found to differentiate the trans-O intermediate from the trans-.N intermediate with an energy difference of 46.63 U mol(-1) significantly more than the palladium catalyst for which the energy difference was calculated as 35.82 kJ mol(-1). The energy difference between the trans-O configuration and the trans-N configuration is decreased significantly when combining a molecule of propylene with the catalyst. For the Ni catalyst, the trans-O isomer is more stable than the trans-N isomer to a greater extent than for Pd, so that the insertion of propylene from 20 is relatively less favoured for Ni than for Pd. It is predicted that the mechanism of isomerization from 20 to 2N through a rotational transition state TS2O2N is more appropriate for the Ni catalyst system. The palladium system shows a larger preference for pi-coordination than its nickel counterpart, although the latter possesses a lower reaction barrier. It was found that the occupation of the trans-O position in the asymmetric salicyladiminato catalyst is also more favored by the alkene as it is by the alkyl so that insertion of the alkene may always start from a particular configuration so that specific products are obtained. (c) 2005 Elsevier B.V. All rights reserved.

Ring-rearrangement metathesis of bicyclic amino acid derivatives

In this letter, we describe the ring-rearrangement metathesis (RRM) of bicyclic amino acid derivatives. The procedure is of use for the synthesis of constrained amino acid and peptide derivatives with potential as reverse-turn inducers. (c) 2005 Elsevier Ltd. All rights reserved.

Ring-rearrangement metathesis of substituted 2-aminonorbornenes

In this report we describe the ring-rearrangement metathesis of 2-aminonorbornene derivatives. Ail efficient ruthenium-catalysed metathesis reaction Occurs with a wide range of pendent alkenes and alkynes to generate bicyclic amines and amides.

Sequential ring-closing metathesis and nitrone cycloaddition on glucose-derived substrates: a divergent approach to analogues of spiroannulated carbanucleosides and conformationally locked nucleosides

The carbohydrate-derived substrate 3-C-allyl-1,2: 5,6-di-O-isopropylidene-alpha-D-allofuranose was judiciously manipulated for preparing suitable synthons, which could be converted to a variety of isoxazolidino-spirocycles and -tricycles through the application of ring-closing metathesis (RCM) and intramolecular nitrone cycloaddition (INC) reactions. Cleavage of the isoxazolidine rings of some of these derivatives by tranfer hydrogenolysis followed by coupling of the generated amino functionalities with 5-amino-4,6-dichloropyrimidine furnished the corresponding chloropyrimidine nucleosides, which were elaborated to spiroannulated carbanucleosides and conformationally locked bicyclo[2.2.1] heptane/ oxa-bicyclo[3.2.1]octane nucleosides. However, use of higher temperature for the cyclization of one of the chloropyrimidines led to the dimethylaminopurine analogue as a sole product, formed via nucleophilic displacement of the chloro group by dimethylamine generated from DMF.

Anion-templated assembly of pseudorotaxanes: importance of anion template, strength of ion-pair thread association, and macrocycle ring size

A wide range of pseuclorotaxane assemblies containing positively charged pyridinium, pyridinium nicotinamide, imidazolium, benzimidazolium and guanidinium threading components, and macrocyclic isophthalamide polyether ligands have been prepared using a general anion templation procedure. In noncompetitive solvent media, coupling halide anion recognition by a macrocyclic ligand with ion-pairing between the halide anion and a strongly associated cation provides the driving force for interpenetration. Extensive solution H-1 NMR binding studies, thermodynamic investigations, and single-crystal X-ray structure determinations reveal that the nature of the halide anion template, strength of the ion-pairing between the anion template and the cationic threading component, and to a lesser extent favorable second sphere pi-pi aromatic stacking interactions between the positively charged threading component and macrocyclic ligand, together with macrocyclic ring size, affect the efficacy of pseudorotaxane formation.

Thermo-responsive poly(methyl methacrylate)-block-poly(N-isopropylacrylamide) block copolymers synthesized by RAFT polymerization: micellization and gelation

RAFT polymerization was used to prepare PMMA-b-PNIPAM copolymers. Two different chain transfer agents, tBDB and MCPDB, were used to mediate the sequential polymerizations. Micellar solutions and gels were prepared from the resulting copolymers in aqueous solution. When heated above T-c of PNIPAM (about 31 degrees C), DLS revealed that PNIPAM coronas collapsed, resulting in aggregation of the original micelles. The micellar gels underwent syneresis above T-c as water was expelled from the ordered gel structure, the lattice periodicity of which was determined by SANS. A large decrease in lattice spacing was observed above T-c. The gel became more viscoelastic at high temperature, as revealed by shear rheometry which showed a large increase in G".

Synthesis of oxepane ring containing monocyclic, conformationally restricted bicyclic and spirocyclic nucleosides from d-glucose: a cycloaddition approach

Carbohydrate-derived substrates having (i) C-5 nitrone and C-3-O-allyl, (ii) C-4 vinyl and a C-3-O-tethered nitrone, and (iii) C-5 nitrone and C-4-allyloxymethyl generated tetracyclic isoxazolidinooxepane/-pyrart ring systems upon intramolecular nitrone cycloaddition reactions. Deprotection of the 1,2acetonides of these derivatives followed by introduction of uracil base via Vorbruggen reaction condition and cleavage of the isooxazolidine rings as well as of benzyl groups by transfer hydrogenolysis yielded an oxepane ring containing blicyclic and spirocyclic nucleosides. The corresponding oxepane based nucleoside analogues were prepared by cleavage of isoxazolidine and furanose rings, coupling of the generated amino functiontalities with 5-amino-4,6-dichloropyrimidine, cyclization to purine rings, and finally aminolysis.

Thermal Fractionation and isothermal crystallization of polyethylene nanocomposites prepared by in situ polymerization

Nanocomposites of high-density polyethylene (HDPE) and carbon nanotubes (CNT) of different geometries (single wall, double wall, and multiwall; SWNT, DWNT, and MWNT) were prepared by in situ polymerization of ethylene on CNT whose surface had been previously treated with a metallocene catalytic system. In this work, we have studied the effects of applying the successive self-nucleation and annealing thermal fractionation technique (SSA) to the nanocomposites and have also determined the influence of composition and type of CNT on the isothermal crystallization behavior of the HDPE. SSA results indicate that all types of CNT induce the formation of a population of thicker lamellar crystals that melt at higher temperatures as compared to the crystals formed in neat HDPE prepared under the same catalytic and polymerization conditions and subjected to the same SSA treatment. Furthermore, the peculiar morphology induced by the CNT on the HDPE matrix allows the resolution of thermal fractionation to be much better. The isothermal crystallization results indicated that the strong nucleation effect caused by CNT reduced the supercooling needed for crystallization. The interaction between the HDPE chains and the surface of the CNT is probably very strong as judged by the results obtained, even though it is only physical in nature. When the total crystallinity achieved during isothermal crystallization is considered as a function of CNT content, it was found that a competition between nucleation and topological confinement could account for the results. At low CNT content the crystallinity increases (because of the nucleating effect of CNT on HDPE), however, at higher CNT content there is a dramatic reduction in crystallinity reflecting the increased confinement experienced by the HDPE chains at the interfaces which are extremely large in these nanocomposites. Another consequence of these strong interactions is the remarkable decrease in Avrami index as CNT content increases. When the Avrami index reduces to I or lower, nucleation dominates the overall kinetics as a consequence of confinement effects. Wide-angle X-ray experiments were performed at a high-energy synchrotron source and demonstrated that no change in the orthorhombic unit cell of HDPE occurred during crystallization with or without CNT.