New organic nitrates. II. Synthesis of 2H-pyrido[2,3-e]-1,3-oxazine-2,4(3H)-diones, 2H-pyrido[4,3-e]-1,3-oxazine-2,4(3H)-diones, 2H-pyrido[4,3-e]-1,3-oxazin-4(3H)-ones, 2H-thieno[2,3-e]-1,3-oxazin-4(3H)-ones and 2H-thieno[3,4-e]-1,3-oxazin-2,4(3H)-diones

The preparation and the physico-chemical characterization of 2H-pyrido[2,3-e]-1,3-oxazine-2,4(3H)-diones, 2H-pyrido[4,3-e]-1,3-oxazine-2,4(3H)-diones, 2H-pyrido[4,3-e]-1,3-oxazin-4(3H)-ones, 2H-thieno[2,3-e]-1,3-oxazin-4(3H)-ones and 2H-thieno[3,4-e]-1,3-oxazine-2,4(3H)-diones are reported.     

Hydrido-Carbonyl Rhenium Clusters with a Square Geometry of the Metal Core. Synthesis and X-Ray Characterization of the Novel [Re4(μ-H)3(CO)16]− Anion

Three and tetranuclear ring clusters have been obtained by treatment of [Re₂(CO)₈(THF)₂] with carbonyl-rhenates containing two terminal hydrides. The reaction with [ReH₂(CO)₄]^- provided a selective route to the previously known [Re₃(-H)₂(CO)₁₂]^- triangular cluster anion 1. The reaction with [Re₂H₂(-H)(CO)₈]^- gave the novel [Re₄(-H)₃(CO)₁₆]^- anion 2, containing a rare example of a puckered-square metal cluster. Protonation of 1 is known to afford the neutral [Re₃(-H)₃(CO)₁₂] species 3. Analogously the reaction of 2 with a strong acid afforded the previously known square metal clusters [Re₄(-H)₄(CO)₁₆] 4. The reaction could not be reversed by treatment with bases. Photolysis of 4 gave the unsaturated complex [Re₂(-H)₂(CO)₈] 5: this is the reverse of the dimerization reaction, that in THF at room temperature produces 4 from 5. Thermal treatment (reflux in cyclohexane for 24 h) left 4 almost unchanged. A single crystal X-ray analysis of [NEt₄]2 showed a s/e/s/s (e=eclipsed, s=staggered) conformation of the Re(CO)₄ units, leading to a puckered geometry of the ring, at variance with the square-planar geometry of 4 (all eclipsed). Two of the three hydrides of 2 have been located as bridging the Re–Re edges from inside the metal ring, as previously observed in 4. Density functional computations indicated a puckered conformation as the most stable for both 2 and 4, with very low activation energies for ring inversion (6.6 and 2.2 kcal·mol^-1, respectively), but ruled out solid state fluxionality for 4, whose observed planar geometry must be attributed to packing stabilization.     

Methylene‐Bridged Metallocenes with 2,2′‐Methylenebis[1H‐inden‐1‐yl] Ligands: Synthesis,Characterization, and Polymerization Catalysis of a Synthetically Simple Class of C2‐ and C2v‐Symmetric ansa‐Metallocenes

The acid‐catalyzed reaction between formaldehyde and 1H‐indene, 3‐alkyl‐ and 3‐aryl‐1H‐indenes, and six‐membered‐ring substituted 1H‐indenes, with the 1H‐indene/CH₂O ratio of 2 : 1, at temperatures above 60° in hydrocarbon solvents, yields 2,2′‐methylenebis[1H‐indenes] 1 – 8 in 50–100% yield. These 2,2′‐methylenebis[1H‐indenes] are easily deprotonated by 2 equiv. of BuLi or MeLi to yield the corresponding dilithium salts, which are efficiently converted into ansa‐metallocenes of Zr and Hf. The unsubstituted dichloro{(1,1′,2,2′,3,3′,3a,3′a,7a,7′a‐η)‐2,2′‐methylenebis[1H‐inden‐1‐yl]}zirconium ([ZrCl₂( 1′ )]) is the least soluble in organic solvents. Substitution of the 1H‐indenyl moieties by hydrocarbyl substituents increases the hydrocarbon solubility of the complexes, and the presence of a substituent larger than a Me group at the 1,1′ positions of the ligand imparts a high diastereoselectivity to the metallation step, since only the racemic isomers are obtained. Methylene‐bridged ‘ansa‐zirconocenes’ show a noticeable open arrangement of the bis[1H‐inden‐1‐yl] moiety, as measured by the angle between the planes defined by the two π‐ligands (the ‘bite angle’). In particular, of the ‘zirconocenes’ structurally characterized so far, the dichloro{(1,1′,2,2′,3,3′,3a,3′a,7a,7′a‐η)‐2,2′‐methylenebis[4,7‐dimethyl‐1H‐inden‐1‐yl]}zirconium ([ZrCl₂( 5′ )] is the most open. The mixture [ZrCl₂( 1′ )]/methylalumoxane (MAO) is inactive in the polymerization of both ethylene and propylene, while the metallocenes with substituted indenyl ligands polymerize propylene to atactic polypropylene of a molecular mass that depends on the size of the alkyl or aryl groups at the 1,1′ positions of the ligand. Ethene is polymerized by rac‐dichloro{(1,1′,2,2′,3,3′,3a,3′a,7a,7′a‐η)‐2,2′‐methylenebis[1‐methyl‐1H‐inden‐1‐yl]}zirconium ([ZrCl₂( 2′ )])/MAO to polyethylene waxes (average degree of polymerization ca. 100), which are terminated almost exclusively by ethenyl end groups. Polyethylene with a high molecular mass could be obtained by increasing the size of the 1‐alkyl substituent.     

Design strategies for controlling the molecular weight and rate using reversible addition–fragmentation chain transfer mediated living radical polymerization

Living radical polymerization has allowed complex polymer architectures to be synthesized in bulk, solution, and water. The most versatile of these techniques is reversible addition–fragmentation chain transfer (RAFT), which allows a wide range of functional and nonfunctional polymers to be made with predictable molecular weight distributions (MWDs), ranging from very narrow to quite broad. The great complexity of the RAFT mechanism and how the kinetic parameters affect the rate of polymerization and MWD are not obvious. Therefore, the aim of this article is to provide useful insights into the important kinetic parameters that control the rate of polymerization and the evolution of the MWD with conversion. We discuss how a change in the chain‐transfer constant can affect the evolution of the MWD. It is shown how we can, in principle, use only one RAFT agent to obtain a polymer with any MWD. Retardation and inhibition are discussed in terms of (1) the leaving R group reactivity and (2) the intermediate radical termination model versus the slow fragmentation model. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3189–3204, 2005     

Designing generalized statistical ensembles for numerical simulations of biopolymers

Conformational properties of polymers, such as average dihedral angles or molecular alpha-helicity, display a rather weak dependence on the detailed arrangement of the elementary constituents (atoms). We propose a computer simulation method to explore the polymer phase space using a variant of the standard multicanonical method, in which the density of states associated to suitably chosen configurational variables is considered in place of the standard energy density of states. This configurational density of states is used in the Metropolis acceptance/rejection test when configurations are generated with the help of a hybrid Monte Carlo algorithm. The resulting configurational probability distribution is then modulated by exponential factors derived from the general principle of the maximal constrained entropy by requiring that certain average configurational quantities take preassigned (possibly temperature dependent) values. Thermal averages of other configurational quantities can be computed by using the probability distributions obtained in this way. Moments of the energy distribution require an extra canonical sampling of the system phase space at the desired temperature, in order to locally thermalize the configurational degrees of freedom. As an application of these ideas we present the study of the structural properties of two simple models: a bead-and-spring model of polyethylene with independent hindered torsions and an all-atom model of alanine and glycine oligomers with 12 amino acids in vacuum.     

On the growth and shape of sodium taurodeoxycholate micellar aggregates: a spin-label and quasielastic light scattering investigation

Electron spin resonance (ESR) and quasielastic laser scattering (QELS) measurements have been carried out on sodium taurodeoxycholate (NaTDC) micellar aqueous solutions. Computer simulation of the ESR line shape has been used to quantitatively analyze the rotational dynamics of the cholestan-spin label (CSL) dissolved by the NaTDC micellar aggregates as a function of temperature and NaCl concentration. The local reorientation of CSL has been accounted for motionally-averaged g- and A-tensors assuming fast oscillation around the spin-probe long molecular axis. The overall Brownian tumbling of CSL-micelle complexes has been modeled by an axially symmetric rotational tensor. Good agreement with experimental spectra is obtained. Best-fit rotational parameters and QELS data suggest that, in the circumstance of large aggregation, NaTDC micelles have cylindrical shape and micellar growth occurs along the cylinder axis.     

Coordinating properties of C2-symmetric chiral bis(oxazolinylpyridinyl)dioxolane ligands

Metal complexes of general formula M(L)X₂ and M(L)X₃ [L = (4S,5S)-2,2-dimethyl-4,5-bis{6-[(4,5-dihydro-4-(S)-(1-methylethyl)oxazol-2-yl)pyridin-2-yl]}-1,3-dioxolane] were obtained by reacting, respectively, Co^II, Cu^II, Ni^II, and Zn^II nitrate salts and the Rh^III chloride salt, with a chiral C₂-symmetric bis(oxazolinylpyridinyl)dioxolane (L) ligand, in MeOH/CHCl₃ solution. A single crystal X-ray analysis was carried out on [Ni(L)(OH₂)₂](NO₃)₂ · 2H₂O and the molecular structure of L was also determined. In the free ligand the two symmetric arms are essentially planar and oriented nearly perpendicular to the dioxolane average plane. In the Ni complex one seven-membered and two five-membered chelation rings are formed. The metal atom also lies on the C₂ axis, and two symmetry-related water molecules complete the octahedral coordination environment. Both compounds crystallize in chiral space groups; the ligand crystallizes in orthorhombic system, space group C 2 2 2₁, Z = 4; the nickel complex crystallizes in tetragonal system, space group P 4₃ 2₁ 2, Z = 4.     

Pd nanoparticles as efficient catalysts for Suzuki and Stille coupling reactions of aryl halides in ionic liquids

Pd-catalyzed Suzuki and Stille cross-couplings of aryl bromides and chlorides were carried out in quaternary ammonium salts as solvents under mild conditions and with the recycling of the catalyst.     

Molecular-level control of the photoluminescence from PPV nanostructured films

The fabrication of varied molecular architectures in layer-by-layer (LbL) films is exploited to control the photoluminescence (PL) of poly(p-phenylene vinylene) (PPV) in an unprecedented way. This was achieved by controlling the F?rster energy transfer between PPV layers (donors) and layers of a commercial azodye, Brilliant Yellow (BY) (acceptors). Energy transfer was controlled by inserting spacer layers of inert polymers between PPV and BY layers and by photoaligning the BY molecules via trans-cis-trans isomerization. The PPV/BY LbL films displayed polarized PL whose intensity could be varied almost continuously by changing the time of photoalignment, which was carried out by impinging a linearly polarized laser light simultaneously to the PL experiments. For PPV/BY films with no spacer layers, PL was completely quenched, but its intensity increased with the number of spacing layers. Further increase in PL was obtained by photoaligning the BY molecules perpendicularly to the PPV molecules. This minimizes energy transfer, since F?rster processes are directional, dipole-dependent resonant transfers. Energy transfer is also controlled by imparting a preferential orientation of the PPV chains on PPV/BY LbL films deposited onto flexible Teflon substrates that may be stretched.     

Reaction of singlet oxygen with some benzylic sulfides

Product distribution, total quenching rate (k_T), and rate of chemical reaction (k_r) with singlet oxygen have been determined for some alkyl, benzyl, α-methylbenzyl, and cumyl sulfides. Their contributions depend on the steric hindering around the sulfur atom. In protic solvents, the sulfoxide is the main product via a hydrogen-bonded persulfoxide. In apolar solvents, intramolecular α-H abstraction leads to oxidative C-S bond cleavage, with varying efficiency. The behavior of sulfides is compared to that of alkenes and amines.