Construction of vicinal 4°/3°-carbons via reductive Cope rearrangement

Herein reported is a strategy for constructing vicinal 4°/3° carbons via reductive Cope rearrangement. Substrates have been designed which exhibit Cope rearrangement kinetic barriers of ∼23 kcal mol⁻¹ with isoenergetic favorability (ΔG ∼ 0). These fluxional/shape-shifting molecules can be driven forward by chemoselective reduction to useful polyfunctionalized building blocks.

Herein reported is a strategy for constructing vicinal 4°/3° carbons via reductive Cope rearrangement.

Constructing sterically congested vicinal quaternary–tertiary carbons (4°/3° carbons) via Cope rearrangement is currently quite limited with only a handful of papers on the subject published over the past 40 years. This stands in stark contrast to the plethora of other methods for establishing sterically congested vicinal carbons.^1–5 Central to the challenge are kinetic and thermodynamic issues associated with the transformation. In the simplest sense, Cope rearrangements proceed in the direction that results in highest alkene substitution (Fig. 1).^6,7 To forge 4°/3° motifs by Cope rearrangement, additional driving forces must be introduced to reverse the [3,3] directionality and compensate for the energetic penalty associated with the steric and torsional strain of the targeted vicinal 4°/3° motif. With limited reports in all cases, oxy-Cope substrates (Scheme 1, eqn (1)),^8–14 divinylcyclopropanes (Scheme 1, eqn (2)),^15–20 and vinylidenecyclopropane-based 1,5-dienes²¹ (Scheme 1, eqn (3)) have demonstrated favourability for constructing vicinal 4°/3° carbons. Malachowski et al. put forth a series of studies on the construction of quaternary centers via Cope rearrangement driven forward by a conjugation event (Scheme 1, eqn (4)).^22–25 In their work, a single example related to the construction of vicinal 4°/3° centers was disclosed, though kinetic (180 °C) and thermodynamic (equilibrium mixtures) challenges are also observed.²³ And of particular relevance to this work, Wigfield et al. demonstrated that 3,3-dicyano-1,5-dienes with the potential to generate vicinal 4°/3° carbons instead react via an ionic mechanism yielding the less congested products (Scheme 1, eqn (5)).²⁶Open in a separate windowFig. 1Cope equilibrium of 1,1,6-trisubstituted 1,5-dienes.Open in a separate windowScheme 1(A) Cope rearrangements for constructing vicinal 4°/3°-centers (B) this report.Our group has been examining strategies to decrease kinetic barriers and increase the thermodynamic favourability of 3,3-dicyano-1,5-diene-based Cope substrates.^27–31 Beyond the simplest, unsubstituted variants, this class of 1,5-diene is not particularly reactive in both a kinetic and thermodynamic sense (e.g.Scheme 1, eqn (5)).^26,32 Reactivity issues aside, these substrates are attractive building blocks for two main reasons: (1) they have straightforward accessibility from alkylidenemalononitriles and allylic electrophiles by deconjugative allylic alkylation.³³ (2) The 1,5-diene termini are substantially different (malononitrile vs. simple alkene) thus allowing for orthogonal functional group interconversion facilitating target and analogue synthesis.³⁴ Herein we report that a combination of 1,5-diene structural engineering^28,31 and reductive conditions (the reductive Cope rearrangement^29,30) can result in the synthesis of building blocks containing vicinal gem-dimethyl 4°/3° carbons along with orthogonal malononitrile and styrene functional groups for interconversion (Scheme 1B). On this line, malononitrile can be directly converted to amides³⁴ yielding functionally dense β-gem-dimethylamides, important pharmaceutical scaffolds.³⁵This project began during the Covid-19 pandemic lockdown (ca. March–May 2020). As such, we were not permitted to use our laboratory out of an abundance of caution. We took this opportunity to first computationally investigate a Cope rearrangement that could result in vicinal 4°/3° carbons (Scheme 2). Then, when permitted to safely return to the lab, we would experimentally validate our findings (vide infra). From our previous work, it is known that by adding either a 4-aromatic group²⁸ or a 4-methyl group³¹ to a 3,3-dicyano-1,5-diene, low barrier (rt – 80 °C) diastereoselective Cope rearrangements can occur. Notably, the 4-substituent was found to destabilize the starting material (weaken the C3–C4 bond, conformationally bias the substrate for [3,3]), and stabilize the product side of the equilibrium via resonance (phenyl group) or hyperconjugation (methyl group). In this study, we modelled substrates 1, 3, and 5 that have variable 4-substitution and would result in vicinal gem-dimethyl- and phenyl-containing 4°/3° carbons upon Cope rearrangement to 2, 4, or 6, respectively. We chose to target this motif due to likely synthetic accessibility from simple starting materials but also because of the important and profound impact that gem-dimethyl groups impart on pharmaceuticals.³⁵ Substrate 1 lacking 4-substitution had an extremely unfavourable kinetic and thermodynamic profile (ΔG^‡ = 31.6; ΔG = +5.3 kcal mol⁻¹). When a 4-methyl group was added, the kinetic barrier (ΔG^‡) dropped appreciably to 28.2 kcal mol; however, the thermodynamics were still quite endergonic (ΔG = +4.4 kcal mol⁻¹). Most excitingly, it was uncovered that the 4-phenyl group dramatically impacted the kinetics and thermodynamics: the [3,3] has a barrier of 22.9 kcal mol⁻¹ (ΔG^‡) and is ∼isoenergetic (ΔG = +0.17 kcal mol⁻¹). Thus, the reaction appears to be fluxional/shape-shifting at room temperature.^36–40 For this substrate, we also modelled the dissociative pathway (Scheme 2D). It was found that bond breakage to two allylic radical intermediates is a higher energy process than the concerted transition state (Scheme 2Cvs.Scheme 2D). Specifically, the dissociative pathway was found to be kinetically less favourable (ΔG^‡ ∼ 27.6 kcal mol; ΔG = 26.2 kcal mol⁻¹) than the concerted process (ΔG^‡ = 22.9 kcal mol⁻¹). While the dissociative pathway is less favourable than the concerted transformation, we surmised that the two-step process becomes accessible at elevated temperature (vide infra). Finally, the ionic pathway was calculated to be significantly higher for this substrate (see the ESI^†).Open in a separate windowScheme 2Computational analysis of 3,3-dicyano-1,5-diene that in theory could result in vicinal 4°/3° carbons. (A) 4-Unsubstituted 3,3-dicyano-1,5-diene. (B) 4-Methyl 3,3-dicyano-1,5-diene. (C) 4-Phenyl 3,3-dicyano-1,5-diene. (D) The dissociative mechanism for substrate 5 is higher than the closed transition state. (E) visualization of the kinetic- and thermodynamic differences of transformations (A–D).The class of substrate uncovered from our computational investigation could be accessed from γ,γ-dimethyl-alkylidenemalononitrile (7a) and 1,3-diarylallyl electrophiles (such as 8a) by Pd-catalyzed deconjugative allylic alkylation (Scheme 3A).³³ As such, model 1,5-diene 5a was prepared to verify the computational results. It was found that upon synthesis of 5a, an inseparable 21 : 79 mixture of 1,5-diene 5a and the 1,5-diene 6a was observed. The predicted ratio of 5a to 6a was 57 : 43 (Scheme 2C). These two results are within the error of the calculations (predicted; slightly endergonic, observed; slightly exergonic). To determine whether the transformation was progressing through the predicted concerted pathway (Scheme 2C) over the dissociative pathway (Scheme 2D), substrate 5b was prepared by an analogous deconjugative allylic alkylation reaction. Similarly, two Cope equilibrium isomers 5b and 6b are observed at room temperature in a 12 : 88 ratio. Upon heating at 100 °C for 3 h, the 1,5-dienes “scramble” (e.g. iso-6b is observed; 0.2 : 1.0 : 1.5 ratio of 5b : 6b : iso-6b) indicating that the dissociative pathway is only accessible at elevated temperature. This is all in good agreement with the calculated kinetics and thermodynamics of this system (Scheme 2).Open in a separate windowScheme 3(A) Observation of fluxional [3,3] and confirmation of calculated predictions. (B) Optimization of a reductive Cope rearrangement protocol for constructing vicinal 4°/3° centers. (C) The Pd-catalyzed deconjugative allylic alkylation must be regioselective.With respect to the synthetic methodology, we aimed to increase the overall efficiency and applicability of the sequence (Scheme 3B). Specifically, we wanted to avoid [3,3] equilibrium mixtures and sensitive/unstable substates and intermediates. It was found that the direct coupling of 7a with diphenylallyl alcohol 9a could take place in the presence of DMAP, Ac₂O, and Pd(PPh₃)₄. When the coupling was complete, methanol and NaBH₄ were added to drive the Cope equilibrium forward, yielding the reduced Cope rearrangement product 10a in 76% isolated yield. In terms of practicality and efficiency, this method utilizes diphenylallyl alcohols, which are more stable and synthetically accessible than their respective acetates, and the [3,3] equilibrium mixture can be directly converted dynamically to a single reduced product.With an efficient protocol in hand for constructing malononitrile–styrene-tethered building blocks featuring central vicinal 4°/3° carbons, we next examined the scope of the transformation (Scheme 4). We chose diarylallyl alcohols with the propensity to react regioselectively via an electronic bias (Scheme 3C).^41,42 The combination of p-nitrophenyl and phenyl (10b) or p-methoxyphenyl (10c) yielded regioselective outcomes with the electron-deficient arene at the allylic position. This is consistent with the expected regiochemical outcome where the nucleophile reacts preferentially at the α-position and the electrophile reacts at the allylic position bearing the donor-arene (Scheme 3C).^41,42 Then, reductive Cope rearrangement occurs to position the electron-deficient arene adjacent to the gem-dimethyl quaternary center. This is an exciting outcome as many pharmaceutically relevant (hetero)arenes are electron deficient. Thus, fluorinated arenes were installed at the allylic position of products 10d–10k. While the phenyl group resulted in poor regioselectivity (1 : 1–3 : 1), the p-methoxyphenyl group enhanced the regiomeric ratios in all cases (3 : 1–15 : 1). The degree of selectivity is correlated with the number and position of fluorine atoms. N-Heterocycles could be incorporated with excellent regioselectivity, generally speaking (10l–10q). For example, 3-chloro-4-pyridyl (10l/10m) groups were installed at the allylic position with >20 : 1 rr. 4-Chloro-3-pyridyl was poorly regioselective (10n), but the combination of 4-trifluomethyl-3-pyridyl/p-methoxyphenyl (10o) gave good regioselectivity of 11 : 1. 2-Pyridyl/p-methoxyphenyl (10q) was also a regioselective combination. We also examined a few other heterocycles including quinoline (10s) and thiazole (10t and 10u) with excellent and modest regioselectivity observed, respectively. As a general trend, when the arenes on the allylic electrophile become less polarized, poor regioselectivity is observed in the Pd-catalyzed allylic alkylation. For example, the combination of p-chlorophenyl and p-methoxyphenyl (10v) or phenyl (10w) yields regioisomeric mixtures of products. This can be circumvented by utilizing symmetric electrophiles (to 10x).Open in a separate windowScheme 4Scope of the 4°/3°-center-generating reductive Cope rearrangement.The phenyl or the p-methoxyphenyl group is necessary to achieve the 4°/3° carbon-generating Cope rearrangement: it functions as an “activator” by lowering the kinetic barrier and increasing thermodynamic favourability. These activating groups can be removed through alkene C C cleavage reactions (e.g. metathesis (Scheme 5) and ozonolysis (Scheme 6B)). In this regard, highly substituted cycloheptenes 11 were prepared by allylation and metathesis (Scheme 4).^28,43 The yields were modest to excellent over this two-step sequence. In many cases, where 10 exists as a mixture of regioisomers, the major allylation/RCM products 11 could be chromatographically separated from their minor constituents. As shown in Scheme 6A, the malononitrile can be transformed via oxidative amidation³⁴ to products 12 containing a dense array of pharmaceutically relevant functionalities (amides, gem-dimethyl, fluoroaromatics, and heteroaromatics). Following this transformation, ozonolysis terminated with a NaBH₄ quench installs an alcohol moiety on small molecule 13a.Open in a separate windowScheme 5Removal of the “activating group” by ring-closing metathesis.Open in a separate windowScheme 6(A) oxidative amidation of malononitrile. (B) Removal of “activating group” by ozonolysis.These first computational and experimental studies utilizing 3,3-dicyano-1,5-dienes as substrates for constructing vicinal 4°/3° centers sets the stage for much further examination and application. For example, while we focused our efforts on gem-dimethyl-based quaternary carbons, it is likely that other functionality can be installed at this position. For example, while unoptimized, it appears the protocol is reasonably effective at incorporating a piperidine moiety in addition to heteroarenes from the allylic electrophile (7b + 9f → 14a; Scheme 7A). Similar functional group interconversion chemistry as described in Schemes 5 and ​and66 can thus yield functionally dense building blocks 15 and 16 in good yields.Open in a separate windowScheme 7(A) The construction of 4/3° centres on piperidines. (B) Promoting endergonic [3,3] rearrangements is possible, assuming the [3,3] kinetic barrier is sufficiently low.While the 4,6-diaryl-3,3-dicyano-1,5-dienes offered the most attractive energetic profile (low kinetic barrier, isoenergetic [3,3] equillibrium; Scheme 2C), the 4-methyl analogue is also intriguing to consider as a viable substrate class for reductive Cope rearrangement (Scheme 2B). The challenge here is that the kinetics and thermodynamics are quite unfavourable (not observable by NMR), but potentially not prohibitively so. It is extremely exciting to find that Cope equilibria that are significantly endergonic in the desired, forward direction (e.g.3a to 4a) can be promoted by a related reductive protocol (Scheme 7B). While unoptimized, we were able to isolate product 17 in xx% yield by heating at 90 °C in the presence of Hantzsch ester in DMF.     

Comparative Structural Study of Three Tetrahalophthalic Anhydrides: Recognition of X···O(anhydride) Halogen Bond and πh···O(anhydride) Interaction

Structures of three tetrahalophthalic anhydrides (TXPA: halogen = Cl (TCPA), Br (TBPA), I (TIPA)) were studied by X-ray diffraction, and several types of halogen bonds (HaB) and lone pair···π-hole (lp···πh) contacts were revealed in their structures. HaBs involving the central oxygen atom of anhydride group (further X···O(anhydride) were recognized in the structures of TCPA and TBPA. In contrast, for the O(anhydride) atom of TIPA, only interactions with the π system (π-hole) of the anhydride ring (further lp(O)···πh) were observed. Computational studies by a number of theoretical methods (molecular electrostatic potentials, the quantum theory of atoms in molecules, the independent gradient model, natural bond orbital analyses, the electron density difference, and symmetry-adapted perturbation theory) demonstrated that the X···O(anhydride) contacts in TCPA and TBPA and lp(O)···πh in TIPA are caused by the packing effect. The supramolecular architecture of isostructural TCPA and TBPA was mainly affected by X···O(acyl) and X···X HaBs, and, for TIPA, the main contribution provided I···I HaBs.     

A Nanoporous Supramolecular Metal–Organic Framework Based on a Nucleotide: Interplay of the π···π Interactions Directing Assembly and Geometric Matching of Aromatic Tails

Self-assembly is the most powerful force for creating ordered supramolecular architectures from simple components under mild conditions. π···π stacking interactions have been widely explored in modern supramolecular chemistry as an attractive reversible noncovalent tool for the nondestructive fabrication of materials for different applications. Here, we report on the self-assembly of cytidine 5’-monophosphate (CMP) nucleotide and copper metal ions for the preparation of a rare nanoporous supramolecular metal-organic framework in water. π···π stacking interactions involving the aromatic groups of the ancillary 2,2’-bipyridine (bipy) ligands drive the self-assemblies of hexameric pseudo-amphiphilic [Cu6(bipy)6(CMP)2(µ-O)Br4]²⁺ units. Owing to the supramolecular geometric matching between the aromatic tails, a nanoporous crystalline phase with hydrophobic and hydrophilic chiral pores of 1.2 and 0.8 nanometers, respectively, was successfully synthesized. The encoded chiral information, contained on the enantiopure building blocks, is transferred to the final supramolecular structure, assembled in the very unusual topology 8T6. These kinds of materials, owing to chiral channels with chiral active sites from ribose moieties, where the enantioselective recognition can occur, are, in principle, good candidates to carry out efficient separation of enantiomers, better than traditional inorganic and organic porous materials.     

The Bifurcated σ-Hole···σ-Hole Stacking Interactions

The bifurcated σ-hole···σ-hole stacking interactions between organosulfur molecules, which are key components of organic optical and electronic materials, were investigated by using a combined method of the Cambridge Structural Database search and quantum chemical calculation. Due to the geometric constraints, the binding energy of one bifurcated σ-hole···σ-hole stacking interaction is in general smaller than the sum of the binding energies of two free monofurcated σ-hole···σ-hole stacking interactions. The bifurcated σ-hole···σ-hole stacking interactions are still of the dispersion-dominated noncovalent interactions. However, in contrast to the linear monofurcated σ-hole···σ-hole stacking interaction, the contribution of the electrostatic energy to the total attractive interaction energy increases significantly and the dispersion component of the total attractive interaction energy decreases significantly for the bifurcated σ-hole···σ-hole stacking interaction. Another important finding of this study is that the low-cost spin-component scaled zeroth-order symmetry-adapted perturbation theory performs perfectly in the study of the bifurcated σ-hole···σ-hole stacking interactions. This work will provide valuable information for the design and synthesis of novel organic optical and electronic materials.     

Strength of the [Z–I···Hal]− and [Z–Hal···I]− Halogen Bonds: Electron Density Properties and Halogen Bond Length as Estimators of Interaction Energy

Bond energy is the main characteristic of chemical bonds in general and of non-covalent interactions in particular. Simple methods of express estimates of the interaction energy, E_int, using relationships between E_int and a property which is easily accessible from experiment is of great importance for the characterization of non-covalent interactions. In this work, practically important relationships between E_int and electron density, its Laplacian, curvature, potential, kinetic, and total energy densities at the bond critical point as well as bond length were derived for the structures of the [Z–I···Hal]⁻ and [Z–Hal···I]⁻ types bearing halogen bonds and involving iodine as interacting atom(s) (totally 412 structures). The mean absolute deviations for the correlations found were 2.06–4.76 kcal/mol.     

Schiff Base in Ketoamine Form and Rh(η4-cod)-Schiff Base Complex with Z′ = 2 Structure from Pairwise C-H···Metallochelate-π Contacts

Condensation of 2-hydroxybenzaldehyde (salicylaldehyde) or 2-hydroxy-1-naphthaldehyde with 2-ethylaniline yields the Schiff base compound of (E)-2-(((2-ethylphenyl)imino)methyl)phenol (HL¹) or (E)-1-(((2-ethylphenyl)imino)methyl)naphthalen-2-ol (HL²), which in turn react with the dinuclear complex of [Rh(η⁴-cod)(µ-O₂CCH₃)]₂ (cod = cycloocta-1,5-diene) to afford the mononuclear (η⁴-cod){(E)-2-(((2-ethylphenyl)imino)methyl)phenolato-κ²N,O}rhodium(I), [Rh(η⁴-cod)(L¹)] (1) or (η⁴-cod){(E)-1-(((2-ethylphenyl)imino)methyl)naphthalen-2-olato-κ²N,O}rhodium(I), [Rh(η⁴-cod)(L²)] (2) (L¹ or L² = deprotonated Schiff base ligand). The X-ray structure determination revealed that the HL² exists in the solid state not as the usual (imine)N···H-O(phenol) form (enolamine form) but as the zwitterionic (imine)N-H⁺···^–O(phenol) form (ketoamine form). ¹H NMR spectra for HL² in different solvents demonstrated the existence of keto-enol tautomerism (i.e., keto ⇆ enol equilibrium) in solution. The structure for 1 and 2 showed that the deprotonated Schiff base ligand coordinates to the Rh(η⁴-cod)-fragment as a six-membered N^O-chelate around the rhodium atom with a close-to-square-planar geometry. Two symmetry-independent molecules (with Rh1 and Rh2) were found in the asymmetric unit in 1 in a structure with Z’ = 2. The supramolecular packing in HL² was organized by π-π and C-H···π contacts, while only two recognized C-H···π contacts were revealed in 1 and 2. Remarkably, there were reciprocal or pairwise C-H···π contacts between a pair of each of the symmetry-independent molecules in 1. This pairwise C-H contact to the Rh-N^O chelate (metalloaromatic) ring may be a reason for the two symmetry-independent molecules in 1. Differential scanning calorimetry (DSC) analyses revealed an irreversible phase transformation from the crystalline-solid to the isotropic-liquid phase and subsequently confirmed the thermal stability of the compounds. Absorption spectra in solution were explained by excited state properties from DFT/TD-DFT calculations.     

Stimuli Responsive Materials Supported by Orthogonal Hydrogen and Halogen Bonding or I···Alkene Interaction

Smart materials represent an elegant class of (macro)-molecules endowed with the ability to react to chemical/physical changes in the environment. Herein, we prepared new photo responsive azobenzenes possessing halogen bond donor groups. The X-ray structures of two molecules highlight supramolecular organizations governed by unusual noncovalent bonds. In azo dye I-azo-NO₂, the nitro group is engaged in orthogonal H···O···I halogen and hydrogen bonding, linking the units in parallel undulating chains. As far as compound I–azo–NH–MMA is concerned, a non-centrosymmetric pattern is formed due to a very rare I···π interaction involving the alkene group supplemented by hydrogen bonds. The Cambridge Structural Database contains only four structures showing the same I···CH₂=C contact. For all compounds, an ¹⁹F-NMR spectroscopic analysis confirms the formation of halogen bonds in solution through a recognition process with chloride anion, and the reversible photo-responsiveness is demonstrated upon exposing a solution to UV light irradiation. Finally, the intermediate I–azo–NH₂ also shows a pronounced color change due to pH variation. These azobenzenes are thereby attractive building blocks to design future multi-stimuli responsive materials for highly functional devices.     

Essential Oil Microemulsions Inactivate Antibiotic-Resistant Bacteria on Iceberg Lettuce during 28-Day Storage at 4 °C

The objective of this study was to investigate the antimicrobial activities of essential oil-based microemulsions in the wash water against Escherichia coli O157:H7 and Pseudomonas fluorescens on Iceberg lettuce. Evaluated wash microemulsions included oregano oil, lemongrass oil, and cinnamon oil, along with a plant-based emulsifier for improved solubility. Iceberg lettuce was inoculated for 2 min with E. coli O157:H7 (6.0 log CFU/g) or P. fluorescens (6.0 log CFU/g) and then dip-treated in a phosphate buffered saline (PBS) control, 50 ppm chlorine, 3% hydrogen peroxide treatment or a 0.1%, 0.3%, or 0.5% microemulsion solution. Treated leaves were stored at 4 °C, and analyzed for surviving bacteria on days 0, 3, 7, 10, 14, 21, and 28. Efficacies of the antimicrobials were concentration and storage-time dependent. There was a 1.26–4.86 log CFU/g reduction in E. coli O157:H7 and significant reductions (0.32–2.35 log CFU/g) in P. fluorescens during storage at days 0–28 (p < 0.05). The 0.1% oregano oil microemulsion resulted in the best visual appeal in Iceberg leaves inoculated with E. coli O157:H7 and showed better improvement in the quality of the Iceberg leaves inoculated with spoilage bacteria P. fluorescens. The results suggest that 0.5% cinnamon and 0.3% oregano oil treatments have the potential to provide natural, eco-friendly, and effective alternatives to chemicals for the decontamination of leafy greens, eliminating E. coli O157:H7 and P. fluorescens.     

Methane hydrate dissociation above 0 °C and below 0 °C

The kinetic data of methane hydrate dissociation at various temperatures and pressures were measured in a sapphire cell apparatus by depressurizing method. When the temperature was higher than 0 °C, the experimental results showed that the hydrate dissociation rate was controlled by intrinsic dissociation reaction. When the temperature was lower than 0 °C, water generated from the hydrate dissociation would transform into ice rapidly at the surface of hydrate crystal. The released gas diffused from the hydrate and ice mixture to the bulk of gas phase. With the hydrate continuous dissociation, the boundary of ice–hydrate moved toward water/ice phase. The hydrate dissociation was controlled by gas diffusion, and the hydrate dissociation process was treated as a moving boundary problem. Corresponding kinetic models for hydrate dissociation were established and good agreements with experimental data were achieved.     

DSC calibration below 0°C

Summary This contribution deals with the calibration of a DSC apparatus between −100 and 0°C using compounds with well-known temperature and heat of transition. Only a few suitable substances are mentioned in literature. For that reason another compound, adamantane, is proposed with a solid-solid transition at −64.56°C and a heat of reaction being 24.78 J/g.

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Zusammenfassung Unter Verwendung von Substanzen mit genau bekannten Umwandlungs-temperaturen und-w?rmen befa?te man sich mit der Kalibrierung von DSC-Ger?ten im Temperaturbereich von −100 bis 0°C. In der Literatur werden nur einige wenige geeignete solcher Substanzen erw?hnt. Aus diesem Grunde wird als weitere Substanz Adamantan mit einem Feststoff-Feststoff-Umwandlungspunkt bei −64.56°C und einer Reaktionsw?rme von 24.78 J/g vorgeschlagen.

     

Selective 17‐β‐estradiol molecular imprinting

An efficient novel method for the synthesis of a covalent molecularly imprinted polymer (MIP) highly specific to β‐estradiol have been developed. MIP prepared by both covalent and non covalent techniques, demonstrated high selectivity toward β‐estradiol. MIPs were synthesized by radical polymerization of 17‐β‐estradiol 4‐vinyl‐benzene carboxyl or sulfonyl esters used as covalent functional monomers, methacrylic acid as noncovalent functional monomer, ethylene glycol dimethacrylate as crosslinking agent, and acetonitrile as swelling and porogenic component. Almost 35% (w/w) of 17‐β‐estradiol was successfully removed from the polymer network by basic hydrolysis. The binding ability of MIP was 10.73 μg/mg MIP following removal of 17‐β‐estradiol in the 2 mg/mL β‐estradiol solution. Selective rebinding of β‐estradiol toward MIP was tested in the presence of competitive binders including estrone, 19‐nortestosterone, epiandrosterone, and cholesterol. Estrone having closest similar chemical structure to β‐estradiol exhibited only 0.6 μg/mg MIP competitive binding, being exposed to equivalent concentrations. Moreover, other competitive steroids demonstrated negligible affinity toward MIP indicating high selectivity of novel MIP system toward β‐estradiol. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5534–5542, 2009     

A Mononuclear η2‐Bonded Phosphaalkyne Nickel(0) Complex

The synthesis and characterization of the new complex [Ni(Im^iPr)₂(η²‐P≡C‐tBu)] ( 1 ) is reported. Compound 1 represents the first structurally characterized example of a mononuclear nickel(0) complex with a side on coordinated phophaalkyne ligand.     

Dating Sediments by EPR Using Al-h Centre: A Comparison between the Properties of Fine (4–11 µm) and Coarse (>63 µm) Quartz Grains

The possibility of EPR dating for sediments using Al-h signals of fine (4–11 μm) grains of quartz has not been previously discussed. Here, the Al-h and peroxy EPR spectra of fine (4–11 μm) and coarse (63–90, 125–180 μm) sedimentary quartz from thoroughly investigated loess sites in Eastern Europe were examined. By comparing experimental spectra with a simulated signal, we evaluated the overestimation observed when using the standard approach established by Toyoda and Falguères to measure Al-h intensity for different doses of radiation, up to 40,000 Gy. This overestimation, caused by the presence of peroxy signals, was much more pronounced for fine grains. Fine grains exhibited some additional dose-dependent signals, which, for some samples, caused a complete distortion of the Al-h spectra at high doses, making it impossible to measure the standard amplitude. We propose a new approach to measuring Al-h signal intensity, focusing on the peak-to-baseline amplitude of the part of the signal at g ≈ 2.0603, which is not affected by the peroxy signals and therefore has the potential of providing more accurate results. The shapes of dose response curves constructed for coarse and fine grains using the new approach show considerable similarity, suggesting that Al-h centre formation in fine and coarse grains upon artificial radiation at room temperature follows the same pattern.     

Effect of tin tetrachloride on thermal decompositions of α,α′-azobisisobutyronitrile and dimethyl α,α′-azobisisobutyrate

Thermal decomposition of α,α′-azobisisobutyronitrile (AIBN) and dimethyl α,α′-azobisisobutyrate (MAIB) in the presence of a large amount of tin tetrachloride was investigated to determine the effect of complex formation on the decomposition rates and yields of the recombination products. The addition of tin tetrachloride significantly increased the decomposition rates; the observed first-order rate constant increased by factors of 4.5 and 17 at molar ratios of [SnCl₄]/[AIBN] = 21.65 and [SnCl₄]/[MAIB] = 19.53, respectively. It was found that the decomposition of these azo compounds was also accelerated by the addition of a comparable amount of donor solvent such as ethyl acetate or propionitrile to tin tetrachloride and that the enhancement in rate was accounted for by a larger frequency factor in the Arrhenius equation. Furthermore, the addition of tin tetrachloride seemed to suppress the formation of recombination products, tetramethyl succinonitrile and dimethyl tetramethylsuccinate, of the radicals produced by decomposition.     

A molecular mechanics study on novel Ln(0) π-arene compounds

To examine the steric effects on the stability of Ln(0) π-arene compounds, molecular mechanics (MMP2) calculations are performed on Gd(η-C₆H₆)₂ and Ln(η-Bu^t₃C₆H₃)₂ (where Ln is Gd, Yb and Y ). The small potential-well depth ( ≈ 2 kcal mol⁻¹) and the large Gd-C equilibrium distance ( > 3.3 Å) explains the instability of Gd(η-C₆H₆)₂, while the difference in the stability between Gd(η-Bu^t₃C₆H₃)₂ and Yb(η-Bu^t₃C₆H₃)₂ can be attributed to the difference in the van der Waalsradii of the two metals and the more contracted 5d orbitals on the Yb atom.     

Specific volume studies of γ-irradiated polytetrafluoroethylene from 40 to 150°C

The effect of γ-irradiation and post-irradiation heat treatment on the specific volume versus temperature relationships of polytetrafluoroethylene (PTFE) samples (1/2-in. diameter rods) have been studied over the 40–150°C. temperature range for radiation doses up to 8.9 X 10⁸ rad. At low doses the specific volume at any temperature decreased with dose, but above about 10⁸ rad it increased with dose. Similarly, the rate of volumetric expansion initially decreased with dose, while, at very high doses (8.9 X 10⁸ rad) the rate of expansion at temperatures above 100°C. exceeded that of the unirradiated PTFE. Heating at 150°C. for 100 hr. produced a substantial decrease in the specific volume and a decrease in the rate of expansion for the irradiated samples. Irradiation effects in PTFE are considered to be a result of such factors as radiation-induced chain scission, increased crystallinity, and increased void content. Changes resulting from post-irradiation heat treatment can be attributed to increased crystallinity, decreased void content, and weight loss.