On-Surface Synthesis of Cumulene-Containing Polymers via Two-Step Dehalogenative Homocoupling of Dibromomethylene-Functionalized Tribenzoazulene

Cumulene compounds are notoriously difficult to prepare and study because their reactivity increases dramatically with the increasing number of consecutive double bonds. In this respect, the emerging field of on-surface synthesis provides exceptional opportunities because it relies on reactions on clean metal substrates under well-controlled ultrahigh-vacuum conditions. Here we report the on-surface synthesis of a polymer linked by cumulene-like bonds on a Au(111) surface via sequential thermally activated dehalogenative C−C coupling of a tribenzoazulene precursor equipped with two dibromomethylene groups. The structure and electronic properties of the resulting polymer with cumulene-like pentagon–pentagon and heptagon–heptagon connections have been investigated by means of scanning probe microscopy and spectroscopy methods and X-ray photoelectron spectroscopy, complemented by density functional theory calculations. Our results provide perspectives for the on-surface synthesis of cumulene-containing compounds, as well as protocols relevant to the stepwise fabrication of carbon–carbon bonds on surfaces.     

On‐Surface Synthesis of Cumulene‐Containing Polymers via Two‐Step Dehalogenative Homocoupling of Dibromomethylene‐Functionalized Tribenzoazulene

Cumulene compounds are notoriously difficult to prepare and study because their reactivity increases dramatically with the increasing number of consecutive double bonds. In this respect, the emerging field of on‐surface synthesis provides exceptional opportunities because it relies on reactions on clean metal substrates under well‐controlled ultrahigh‐vacuum conditions. Here we report the on‐surface synthesis of a polymer linked by cumulene‐like bonds on a Au(111) surface via sequential thermally activated dehalogenative C?C coupling of a tribenzoazulene precursor equipped with two dibromomethylene groups. The structure and electronic properties of the resulting polymer with cumulene‐like pentagon–pentagon and heptagon–heptagon connections have been investigated by means of scanning probe microscopy and spectroscopy methods and X‐ray photoelectron spectroscopy, complemented by density functional theory calculations. Our results provide perspectives for the on‐surface synthesis of cumulene‐containing compounds, as well as protocols relevant to the stepwise fabrication of carbon–carbon bonds on surfaces.     

Reversible Dehalogenation in On‐Surface Aryl–Aryl Coupling

In the emerging field of on‐surface synthesis, dehalogenative aryl–aryl coupling is unarguably the most prominent tool for the fabrication of covalently bonded carbon‐based nanomaterials. Despite its importance, the reaction kinetics are still poorly understood. Here we present a comprehensive temperature‐programmed x‐ray photoelectron spectroscopy investigation of reaction kinetics and energetics in the prototypical on‐surface dehalogenative polymerization of 4,4′′‐dibromo‐p‐terphenyl into poly(para‐phenylene) on two coinage metal surfaces, Cu(111) and Au(111). We find clear evidence for reversible dehalogenation on Au(111), which is inhibited on Cu(111) owing to the formation of organometallic intermediates. The incorporation of reversible dehalogenation in the reaction rate equations leads to excellent agreement with experimental data and allows extracting the relevant energy barriers. Our findings deepen the mechanistic understanding and call for its reassessment for surface‐confined aryl–aryl coupling on the most frequently used metal substrates.     

The credit risk+ model with general sector correlations

We consider an enhancement of the credit risk⁺ model to incorporate correlations between sectors. We model the sector default rates as linear combinations of a common set of independent variables that represent macro-economic variables or risk factors. We also derive the formula for exact VaR contributions at the obligor level.     

Discovery of novel inhibitors for <Emphasis Type="Italic">Leishmania</Emphasis> nucleoside diphosphatase kinase (NDK) based on its structural and functional characterization

Nucleoside diphosphate kinases (NDKs) are ubiquitous enzymes that catalyze the transfer of the γ-phosphate moiety from an NTP donor to an NDP acceptor, crucial for maintaining the cellular level of nucleoside triphosphates (NTPs). The inability of trypanosomatids to synthesize purines de novo and their dependence on the salvage pathway makes NDK an attractive target to develop drugs for the diseases they cause. Here we report the discovery of novel inhibitors for Leishmania NDK based on the structural and functional characterization of purified recombinant NDK from Leishmania amazonensis. Recombinant LaNDK possesses auto-phosphorylation, phosphotransferase and kinase activities with Histidine 117 playing an essential role. LaNDK crystals were grown by hanging drop vapour diffusion method in a solution containing 18% PEG-MME 500, 100 mM Bis-Tris propane pH 6.0 and 50 mM MgCl₂. It belongs to the hexagonal space group P6₃22 with unit cell parameters a?=?b?=?115.18, c?=?62.18 Å and α?=?β?=?90°, γ?=?120°. The structure solved by molecular replacement methods was refined to crystallographic R-factor and R_free values of 22.54 and 26.52%, respectively. Molecular docking and dynamics simulation-based virtual screening identified putative binding compounds. Protein inhibition studies of selected hits identified five inhibitors effective at micromolar concentrations. One of the compounds showed ~45% inhibition of Leishmania promastigotes proliferation. Analysis of inhibitor-NDK complexes reveals the mode of their binding, facilitating design of new compounds for optimization of activities as drugs against leishmaniasis.     

Efficient and regioselective synthesis of 2-alkyl-2H-indazoles

An efficient and regioselective synthesis of 2-methyl-2H-indazoles and 2-ethyl-2H-indazoles using trimethyloxonium tetrafluoroborate or triethyloxonium hexafluorophosphate is reported.     

A Simple Novel Approach to Valuing Risky Zero Coupon Bond in a Markov Regime Switching Economy

We have addressed the problem of pricing risky zero coupon bond in the framework of Longstaff and Schwartz structural type model by pricing it as a Down-and-Out European Barrier Call option on the company’s asset-debt ratio assuming Markov regime switching economy. The growth rate and the volatility of the stochastic asset debt ratio is driven by a continuous time Markov chain which signifies state of the economy. Regime Switching renders market incomplete and selection of a Equivalent martingale measure (EMM) becomes a subtle issue. We price the zero coupon risky bond utilizing the powerful technique of Risk Minimizing hedging of the underlying Barrier option under the so called “Risk Minimal” martingale measure via computing the bond default probability.     

Comparing the Value at Risk Performance of the CreditRisk + and its Enhancement: A Large Deviations Approach

The standard CreditRisk^?+? (CR^?+?) is a well-known default-mode credit risk model. An extension to the CR^?+? that introduces correlation through a two-stage hierarchy of randomness has been discussed by Deshpande and Iyer (Central Eur J Oper Res 17(2):219–228, 2009) and more recently by Sowers (2010). It is termed the 2-stage CreditRisk^?+? (2-CR^?+?) in the former. Unlike the standard CR^?+?, the 2-CR^?+? model is formulated to allow correlation between sectoral default rates through dependence on a common set of macroeconomic variables. Furthermore the default rates for a 2-CR^?+? are distributed according to a general univariate distribution which is in stark contrast to the uniformly Gamma distributed sectoral default rates in the CR^?+?. We would then like to understand the behaviour of these two models with regards to their computed Value at Risk (VaR) as the number of sectors and macroeconomic variables approaches infinity. In particular we would like to ask whether the 2-CR^?+? produces higher VaR than the CR^?+? and if so then for which type of credit portfolio. Utilizing the theory of Large deviations, we provide a methodology for comparing the Value at risk performance of these two competing models by computing certain associated rare event probabilities. In particular we show that the 2-Stage CR^?+? definitely produces higher VaR than the CR^?+? for a particular class of a credit portfolio which we term as a “balanced” credit portfolio. We support this statistical risk analysis through numerical examples.     

Metal–organic framework-based mixed-matrix membranes for gas separation: An overview

Mixed-matrix membranes (MMMs) have been studied widely in the field of gas separation due to their potential to overcome performance barriers found in traditional polymeric membranes. Most polymeric membranes exhibit a trade-off between permeation and selectivity, which has limited their development in many challenging separation applications. One solution to this issue utilizes the introduction of fillers into the polymer matrix to produce MMMs. Out of the many different fillers, metal–organic frameworks stand out as a promising candidate due to their highly tunable structure, molecular sieving effect, and superior compatibility with the polymer matrix. This review will provide an in-depth look into the basic mechanisms of MMMs for gas separation and different approaches to model the permeation of gases through the membrane. In addition, challenges facing the field and recent research trends for MMMs will be discussed as well as their many applications for different gas separations. Finally, some insight on the future direction for MMMs will be covered, focusing on many intriguing opportunities and challenges that must be further explored to advance this technology.     

Reversible Dehalogenation in On-Surface Aryl–Aryl Coupling

In the emerging field of on-surface synthesis, dehalogenative aryl–aryl coupling is unarguably the most prominent tool for the fabrication of covalently bonded carbon-based nanomaterials. Despite its importance, the reaction kinetics are still poorly understood. Here we present a comprehensive temperature-programmed x-ray photoelectron spectroscopy investigation of reaction kinetics and energetics in the prototypical on-surface dehalogenative polymerization of 4,4′′-dibromo-p-terphenyl into poly(para-phenylene) on two coinage metal surfaces, Cu(111) and Au(111). We find clear evidence for reversible dehalogenation on Au(111), which is inhibited on Cu(111) owing to the formation of organometallic intermediates. The incorporation of reversible dehalogenation in the reaction rate equations leads to excellent agreement with experimental data and allows extracting the relevant energy barriers. Our findings deepen the mechanistic understanding and call for its reassessment for surface-confined aryl–aryl coupling on the most frequently used metal substrates.