A novel regiodivergent resolution reaction mediated by a homochiral lithium amide base

The reaction of a racemic perhydroisoquinolene derivative 9 with the homochiral lithium amide basse 3 in the presence of Me₃SiCl in an regiodivergent fashion to give the two non-racemic regioisomeric enol silanes 10 and 11. Conversion of 10 into enone 15, an intermediate useful in the synthesis of the alkaloid (+)-yohimbine, was also possible.     

Kinetics and mechanisms in carbocationic polymerization: The quest for true rate constants

This article is a critical analysis of kinetic dataavailable on carbocationic polymerizations. A survey of published propagation rate constant (k_p) data revealed several orders of magnitude differences. In this article, an explanation of this apparent discrepancy is offered with a case study involving the carbocationic polymerization of 2,4,6‐trimethylstyrene (TMS). With the polymerization mechanism originally proposed for this system, k_p = 1.35 × 10⁴ L mol^?1 s^?1 was extracted from experimental data with the Predici polyreaction package. The alternative mechanism yielded k_p = 1.01 × 10⁷ L mol^?1 s^?1, close to that predicted by Mayr's Linear Free Energy Relationship (LFER). We propose that true rate constants can only be obtained from direct competition experiments or from kinetic interpretation based on independently proven mechanisms. The second part of this review discusses critical analysis of the temperature and concentration dependence of various living IB systems. Comparison of the temperature dependence in systems initiated with 2‐ chloro‐2,4, 4‐ trimethylpentane (TMPCl)/TiCl₄ from various laboratories yielded of ΔH ～?25 and ?34.5 kJ/mol for high and low TMPCl/TiCl₄ ratios, respectively. Aromatic (cumyl‐type) initiators show ΔH ～ ?40 kJ/mol, whereas H₂O/TiCl₄ in the presence of the strong electron‐ pair donor dimethylacetamide gave ΔH = ?12 kJ/mol. The significant differences indicate different underlying mechanisms with complex elementary reactions. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5394–5413, 2005     

Rocks,Shocks and Asteroids,and Some Interesting Research Directions in Mechanics

Mechanics lies at the heart of many of the underpinnings of modern technological civilization: materials, infrastructure, transportation, health and security. The mechanics of dynamic failure processes also has a major bearing on the potential catastrophes that threaten civilization, including airbursts and major asteroid impacts. Recent events (such as the Chelyabinsk meteoroid) have demonstrated the need to understand major impact and fragmentation events. Many of the fundamental problems of current interest in national security also involve impact and fragmentation, typically studied through large-scale computational simulations. In the Murray lecture, these issues were addressed by describing fundamental high-strain-rate experiments, high-speed visualization, theoretical and computational modeling of failure processes, and simulations of asteroid damage and disruption. This paper focuses on experimental results on meteorites and a basalt.     

Modeling Equilibrium Behavior of Nylon 6, Nylon 6,6 and Nylon 6/6,6 Copolymer

Nylon 6 and nylon 6,6 reaction equilibria depend in a complex way on water concentration and temperature. For example, data sets from six research groups reveal that the apparent equilibrium constant for polycondensation increases with water at low water concentrations, reaches a maximum, and then decreases as the water concentration rises further. In this article, semi‐empirical expressions are proposed to describe the experimentally observed equilibrium behavior for the five main reactions that occur during nylon 6 and nylon 6,6 polymerization. Nine side reactions involving amidine ends, cyclopentanone ends, and hydrated carboxyl ends are used to develop expressions that account for the influence of both water and temperature on these equilibrium constants. Excellent fit to the data, over the entire range of the available nylon 6 and nylon 6,6 literature data, suggests that the proposed equations will be helpful for modeling reaction equilibria for nylon 6/6,6 copolymerization.     

Synthesis of molybdenum nitrido complexes for triple-bond metathesis of alkynes and nitriles

Complexes of the type N≡Mo(OR)(3) (R = tertiary alkyl, tertiary silyl, bulky aryl) have been synthesized in the search for molybdenum-based nitrile-alkyne cross-metathesis (NACM) catalysts. Protonolysis of known N≡Mo(NMe(2))(3) led to the formation of N≡Mo(O-2,6-(i)Pr(2)C(6)H(3))(3)(NHMe(2)) (12), N≡Mo(OSiPh(3))(3)(NHMe(2)) (5-NHMe(2)), and N≡Mo(OCPh(2)Me)(3)(NHMe(2)) (17-NHMe(2)). The X-ray structure of 12 revealed an NHMe(2) ligand bound cis to the nitrido ligand, while 5-NHMe(2) possessed an NHMe(2) bound trans to the nitride ligand. Consequently, 17-NHMe(2) readily lost its amine ligand to form N≡Mo(OCPh(2)Me)(3) (17), while 12 and 5-NHMe(2) retained their amine ligands in solution. Starting from bulkier tris-anilide complexes, N≡Mo(N[R]Ar)(3) (R = isopropyl, tert-butyl; Ar = 3,5-dimethylphenyl) allowed for the formation of base-free complexes N≡Mo(OSiPh(3))(3) (5) and N≡Mo(OSiPh(2)(t)Bu)(3) (16). Achievement of a NACM cycle requires the nitride complex to react with alkynes to form alkylidyne complexes; therefore the alkyne cross-metathesis (ACM) activity of the complexes was tested. Complex 5 was found to be an efficient catalyst for the ACM of 1-phenyl-1-butyne at room temperature. Complexes 12 and 5-NHMe(2) were also active for ACM at 75 °C, while 17-NHMe(2) and 16 did not show ACM activity. Only 5 proved to be active for the NACM of anisonitrile, which is a reactive substrate in NACM catalyzed by tungsten. NACM with 5 required a reaction temperature of 180 °C in order to initiate the requisite alkylidyne-to-nitride conversion, with slightly more than two turnovers achieved prior to catalyst deactivation. Known molybdenum nitrido complexes were screened for NACM activity under similar conditions, and only N≡Mo(OSiPh(3))(3)(py) (5-py) displayed any trace of NACM activity.     

The problem of the 27Al neutron spin-dependent scattering length

Early measurements of the neutron spin-dependent scattering length of ²⁷Al by neutron precession have given two values, consistent within the limits of accuracy, whose mean value is b₊?b_? = + 0.52 ± 0.02 fm. The positive sign was recently put in doubt from known values of neutron resonance parameters. New experiments were undertaken on a sample of ²⁷Al, a compound sample of ²⁷Al and ⁴⁵Sc, together with a redetermination of the sign of the ²⁷Al gyromagnetic ratio. These experiments confirmed in sign and magnitude the b₊?b_? value previously found. We have so far no explanation of the discrepancy.     

Synthesis and characterisation of low density porous polymers by reversible addition-fragmentation chain transfer (RAFT)

PolyHIPE foams with densities of 0.05–0.1 g cm^?3 have been prepared by the polymerisation of the continuous phase of high internal phase emulsions (HIPEs). The internal aqueous phase in HIPE occupies more than 74 % of the total volume, which leads to highly porous and open-cell morphologies. In this paper a method of preparing polyHIPE foams by using reversible addition-fragmentation chain transfer (RAFT) polymerisation has been investigated. Polystyrene-co-polymethyl methacrylate (PS-co-PMMA) has been studied and by using a variety of characterisation methods, it was possible to compare the polyHIPEs prepared by the conventional free radical polymerisation (FRP) to those by RAFT polymerisation. Scanning electron microscopy images have confirmed the presence of a cellular polyHIPE structure. PS-co-PMMA polyHIPEs made by RAFT have significantly narrower molecular weight distribution with values for the polydispersity index (PDI) for PS-co-PMMA between 1.46 and 2.08 compared to 4.68 observed by FRP. The effects of different concentrations of the RAFT agent on structure, glass transition temperature (T _g) and PDI of PS-co-PMMA polyHIPE foams are presented.     

A comparison of the binding of metal complexes to duplex and quadruplex DNA

Electrospray ionisation mass spectrometry (ESI-MS) and circular dichroism (CD) spectroscopy were used to compare the binding of mononuclear nickel, ruthenium and platinum complexes to double stranded DNA (dsDNA) and quadruplex DNA (qDNA). CD studies provided evidence for the binding of intact complexes of all three metal ions to qDNA. ESI mass spectra of solutions containing platinum or ruthenium complexes and qDNA showed evidence for the formation of non-covalent complexes consisting of intact metal molecules bound to DNA. However, the corresponding spectra of solutions containing nickel complexes principally contained ions consisting of fragments of the initial nickel molecule bound to qDNA. In contrast ESI mass spectra of solutions containing nickel, ruthenium or platinum complexes and dsDNA only showed the presence of ions attributable to intact metal molecules bound to DNA. The fragmentation observed in mass spectral studies of solutions containing nickel complexes and qDNA is attributable to the lower thermodynamic stability of the former metal complexes relative to those containing platinum or ruthenium, as well as the slightly harsher instrumental conditions required to obtain spectra of qDNA. This conclusion is supported by the results of tandem mass spectral studies, which showed that ions consisting of intact nickel complexes bound to qDNA readily undergo fragmentation by loss of one of the ligands initially bound to the metal. The ESI-MS results also demonstrate that the binding affinity of each of the platinum and ruthenium complexes towards qDNA is significantly less than that towards dsDNA.     

Temperature and pressure dependence of the rate coefficient for the reaction between ClO and CH3O2 in the gas-phase

A temperature and pressure kinetic study for the CH(3)O(2) + ClO reaction has been performed using the turbulent flow technique with a chemical ionisation mass spectrometry detection system. An Arrhenius expression was obtained for the overall rate coefficient of CH(3)O(2) + ClO reaction: k(10)(T) = (1.96(?0.24)(+0.28)) × 10(-11) exp[(-626 ± 35)/T] cm(3) molecule(-1) s(-1) where the uncertainty associated with the rate coefficient is given at the one standard deviation level. Over a range of pressure (100-200 Torr) and temperature (298-223 K) no pressure dependence is observed. The smaller rate coefficients measured at lower temperatures compared with both previous low temperature studies are believed to arise through the reduction of secondary chemistry and greater sensitivity in terms of reactant detection (hence much lower initial concentrations were employed). These new data reduce the effectiveness of ozone loss cycles involving reaction of CH(3)O(2) + ClO in the polar stratosphere by around a factor of 1.5 and restrict the importance of the reaction to the tropical and extra-tropical clean marine environments in the troposphere.