Acylierung von Heterocyclen mit Kohlensäurederivaten. I. Kinetik und Mechanismus der Reaktion von 2-Aminobenzimidazolen mit Arylcyanaten

Acylation of Heterocycles with Carbonic Acid Derivatives. I. Kinetics and Mechanism of the Reaction of 2-Aminobenzimidazoles with Aryl Cyanates The second order rate constants for the reaction of 2-amino-benzimidazoles (2-ABI) with aryl cyanates forming 2-amino-benzimidazole aryl ester imide 3 have been determined in dependence on substituent effects by u. v. measurements. The results are interpreted by a six-membered cyclic transition state in which the electrophilic attack of the cyanate on the endocyclic N atom is catalyzed by an H bridge interaction of the exocyclic amino group of 2-ABI with the OCN group.     

Über die Reaktionen von Benzimidazol-2-carbonsäurehydrazid mit Elektrophilen

About the Reactions of Benzimidazole-2-carbohydrazide with Electrophilic Compounds Benzimidazole-2-carbohydrazide ( 1 ) reacts with aldehydes and ketones to form N-alkylidene-benzimidazole-2-carbohydrazides( 2a–j ). With carbon disulfide, diphenyl carbonate and cyanates, ring closure takes place to afford 1,2,4-triazines and 1,3,4-oxadiazoles. Attempts to synthezise 2-isocyanato-benzimidazole failed. ¹H-n.m.r. and ¹³C-n.m.r. spectroscopic data of the compounds are given.     

Environmental Control of Triplet Emission in Donor–Bridge–Acceptor Organometallics

Carbene‐metal‐amides (CMAs) are a promising family of donor–bridge–acceptor molecular charge‐transfer (CT) emitters for organic light‐emitting diodes. A universal approach is demonstrated to tune the energy of their CT emission. A blueshift of up to 210 meV is achievable in solid state via dilution in a polar host matrix. The origin of this shift has two components: constraint of thermally‐activated triplet diffusion, and electrostatic interactions between guest and polar host. This allows the emission of mid‐green CMA archetypes to be tuned to sky blue without chemical modifications. Monte‐Carlo simulations based on a Marcus‐type transfer integral successfully reproduce the concentration‐ and temperature‐dependent triplet diffusion process, revealing a substantial shift in the ensemble density of states in polar hosts. In gold‐bridged CMAs, this shift does not lead to a significant change in luminescence lifetime, thermal activation energy, reorganization energy, or intersystem crossing rate. These discoveries offer new insight into coupling between the singlet and triplet manifolds in CMA materials, revealing a dominant interaction between states of CT character. The same approach is employed using materials which have been chemically modified to alter the energy of their CT state directly, shifting the emission of sky‐blue chromophores into the practical blue range.     

Synthesis and reactivity of 4-chloro-3,3,4-trifluoro-methoxycarbonyl cyclobutene (CTMC)

An improved synthesis of CTMC is described. This cyclobutene is thermally stable towards electrocyclic ring opening. Its reactivity is explored and a number of radical adducts are described. A more complicated picture is observed with various nucleophiles. The strong dienophilic character of CTMC is illustrated by numerous examples. CTMC reacts well as a dipolarophile with diphenylnitrone, diphenylnitrilimine and diazoalkanes. The high reactivity of this cyclobutene is due to its inherent polarity and strain.     

The critical role of water in spider silk and its consequence for protein mechanics

Due to its remarkable mechanical and biological properties, there is considerable interest in understanding, and replicating, spider silk's stress-processing mechanisms and structure-function relationships. Here, we investigate the role of water in the nanoscale mechanics of the different regions in the spider silk fibre, and their relative contributions to stress processing. We propose that the inner core region, rich in spidroin II, retains water due to its inherent disorder, thereby providing a mechanism to dissipate energy as it breaks a sacrificial amide-water bond and gains order under strain, forming a stronger amide-amide bond. The spidroin I-rich outer core is more ordered under ambient conditions and is inherently stiffer and stronger, yet does not on its own provide high toughness. The markedly different interactions of the two proteins with water, and their distribution across the fibre, produce a stiffness differential and provide a balance between stiffness, strength and toughness under ambient conditions. Under wet conditions, this balance is destroyed as the stiff outer core material reverts to the behaviour of the inner core.     

Praxisnahe Bestimmung von Wärmeübergangskoeffizienten an Körpern unterschiedlicher Geometrie

A simple method is presented by which the mean heat transfer coefficient at arbitrary geometries in a steady flow field can be determined. It is based on the measurement of the time dependent temperature and an evaluation that takes into account the conservation of thermal energy. For spheres of various sizes the method is exemplified demonstrating that the results agree well with those from the heat transfer literature.     

Switchable Plasmonic Metasurface Utilizing the Electro-Optic Kerr Effect of a Blue Phase Liquid Crystal

A switchable metasurface composed of plasmonic split ring resonators and a polymer-stabilized liquid crystal blue phase is developed. Owing to field-induced birefringence (electro-optic Kerr effect), the state of polarization of the incident near infrared radiation changes, when a voltage is applied to the liquid crystal. Thus, different resonant modes of the split ring resonators can be addressed and the transmission spectrum changes accordingly. In comparison with other liquid crystal phases, blue phases have several advantages. For example, they are optically isotropic in the field-off state, so that no alignment layer is required. The results of the present study indicate that the advantages of these mesophases can be utilized for switchable metasurfaces.     

Mechanisms of Enzyme-Catalyzed Reduction of Two Carcinogenic Nitro-Aromatics, 3-Nitrobenzanthrone and Aristolochic Acid I: Experimental and Theoretical Approaches

This review summarizes the results found in studies investigating the enzymatic activation of two genotoxic nitro-aromatics, an environmental pollutant and carcinogen 3-nitrobenzanthrone (3-NBA) and a natural plant nephrotoxin and carcinogen aristolochic acid I (AAI), to reactive species forming covalent DNA adducts. Experimental and theoretical approaches determined the reasons why human NAD(P)H:quinone oxidoreductase (NQO1) and cytochromes P450 (CYP) 1A1 and 1A2 have the potential to reductively activate both nitro-aromatics. The results also contributed to the elucidation of the molecular mechanisms of these reactions. The contribution of conjugation enzymes such as N,O-acetyltransferases (NATs) and sulfotransferases (SULTs) to the activation of 3-NBA and AAI was also examined. The results indicated differences in the abilities of 3-NBA and AAI metabolites to be further activated by these conjugation enzymes. The formation of DNA adducts generated by both carcinogens during their reductive activation by the NOQ1 and CYP1A1/2 enzymes was investigated with pure enzymes, enzymes present in subcellular cytosolic and microsomal fractions, selective inhibitors, and animal models (including knock-out and humanized animals). For the theoretical approaches, flexible in silico docking methods as well as ab initio calculations were employed. The results summarized in this review demonstrate that a combination of experimental and theoretical approaches is a useful tool to study the enzyme-mediated reaction mechanisms of 3-NBA and AAI reduction.     

Polymerization of Triazolinediones with trans-3,3-Dichloro-1-phenyl-1-propene

The reaction of 4-phenyl-1,2,4-triazoline-3,5-dione (PhTD) ( 2 ) with trans-3,3-dichloro-1-phenyl-1-propene ( 4 ) was investigated at room and reflux temperatures in methylene chloride solution. Although the reaction is slow, it gives quantitative yield. This reaction leads to the formation of two 2:1 adducts via double Diels–Alder and Diels–Alder-Ene reactions in a ratio of about 1: 7. The major product was isolated by means of fractional crystallization as a pure compound and was characterized by infra-red (IR), ¹H nuclear magnetic resonance (NMR), ¹³C NMR, mass spectra and elemental analysis. The structure of the minor product was determined by IR and ¹H NMR. These compounds were used as models for the polymerization reactions. The reaction of bistriazolinediones (1,6-bis-(3,5-dioxo-1,2,4-triazoline-4-yl)hexane and bis-(p-3,5-dioxo-1,2,4-triazoline-4-ylphenyl)methane) with ( 4 ) was carried out in dimethylformamide (DMF). The reactions gave novel polymers via repetitive double Diels–Alder and Diels–Alder-Ene polyaddition reactions, with the major component being a Diels–Alder-Ene structure. These polymers have intrinsic viscosities in a range from 0·08 to 0·18dlg^-1 in DMF. The physical properties and structural characterization of these polymers have been studied and are reported. © of SCI.