The optical activity of carvone: a theoretical and experimental investigation

The optical rotatory dispersion (ORD) and circular dichroism of the conformationally flexible carvone molecule has been investigated in 17 solvents and compared with results from calculations for the "free" (gas phase) molecule. The G3 method was used to determine the relative energies of the six conformers. The optical rotation of (R)-(-)-carvone at 589 nm was calculated using coupled cluster and density functional methods, including temperature-dependent vibrational corrections. Vibrational corrections are significant and are primarily associated with normal modes involving the stereogenic carbon atom and the carbonyl group, whose n → π? excitation plays a significant role in the chiroptical response of carvone. Without the inclusion of vibrational corrections the optical rotation calculated with CCSD and DFT has the opposite sign of experimental data. Calculations of optical rotation performed in solution using the polarizable continuum model were also opposite in sign when compared to that of the experiment.     

Charge-transfer interactions in a multichromophoric hexaarylbenzene containing pyrene and triarylamines

Two different hexaarylbenzenes with three pyrene and three triarylamine substituents in different positions (trigonal symmetric and asymmetric arrangement) were synthesized, and their charge-transfer states were investigated by optical spectroscopy. In these multichromophoric systems triarylamine acts as the electron donor and pyrene as the electron acceptor. A reference chromophore with only one donor-acceptor pair was also investigated. All these chromophores form charge-transfer states upon photoexcitation which relax with a moderate fluorescence quantum yield to the ground state. The compounds do not differ significantly concerning most of their fluorescence properties, which shows that the fluorescent charge-transfer state is very similar in all chromophores. This observation indicates symmetry breaking for the symmetric chromophore within fluorescence lifetime of several tens of ns. This interpretation was substantiated by fluorescence excitation anisotropy measurements in a sucrose octaacetate matrix.     

A New Two-Dimensional Pulse Sequence for T2* Measurements of Protons in C Isotopomers

A new two-dimensional pulse sequence for T₂* measurement of protons directly coupled to ¹³C spins is proposed. The sequence measures the tranverse relaxation time of heteronuclear proton single-quantum coherence under conditions of free precession and is therefore well suited to evaluate relaxation losses of proton magnetization during preparation delays of heteronuclear pulse experiments in analytical NMR. The relevant part of the pulse sequence can be inserted as a “building block” into any direct or inverse detecting H,C correlation pulse sequence if proton spin–spin relaxation is to be investigated. In this contribution, the building block is inserted into a HETCOR as well as into a HMQC pulse sequence. Experimental results for the HETCOR-based sequence are given.     

Enhanced conductivity in ionic conductor-insulator composites: numerical models in two and three dimensions

We describe a two-dimensional (2D) and a three-dimensional (3D) percolation model for ionic conductor-insulator composites such as copper(I) bromide-titanium dioxide (CuBr-TiO₂) or lithium iodide-alumina (LiI-Al₂O₃). These composites present an enhanced conductivity closely related to the insulator concentration. This effect is explained by the formation of highly conducting space charge regions near the phase boundaries which are represented by good conductor bonds. Our numerical model takes into account grain size and correlation effects. The dimension has a leading role for the conduction properties. In the 2D case, the good conductor bonds do not percolate, whatever the insulator concentration, and the maximum conductivity of the composite samples is of the same order as that of the ionic conductor grains. The behavior of the system is very different in the 3D case where, for a large domain of composition, the good conductors percolate through the regions between the conductor grains. For the CuBr-TiO₂ composites the conductivity versus composition curve is bell-shaped. Conversely, in the LiI-Al₂O₃ system, a linear relation between the conductivity and the insulator volume fraction is obtained in the experiments. Our model gives a plausible interpretation of the conductivity in both systems. Received 10 April 2001     

SPSA vs Simplex in statistical machine translation optimization

Most statistical machine translation systems are combinations of various models and tuning scaling factors is an important step. However, this optimisation problem is hard because the objective function has many local minima and the available algorithms cannot achieve a global optimum. Consequently, optimisations starting from different initial settings can converge to fairly different solutions. We present tuning experiments with the Simultaneous Perturbation Stochastic Approximation (SPSA) algorithm, and compare them with the widely used downhill simplex method. With IWSLT 2005 Chinese-English data, both methods showed similar performance, but SPSA was more robust to the choice of initial settings. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Polycyclic heteroaromatics via hydrazine-catalyzed ring-closing carbonyl–olefin metathesis

The use of hydrazine-catalyzed ring-closing carbonyl–olefin metathesis (RCCOM) to synthesize polycyclic heteroaromatic (PHA) compounds is described. In particular, substrates bearing Lewis basic functionalities such as pyridine rings and amines, which strongly inhibit acid catalyzed RCCOM reactions, are shown to be compatible with this reaction. Using 5 mol% catalyst loadings, a variety of PHA structures can be synthesized from biaryl alkenyl aldehydes, which themselves are readily prepared by cross-coupling.

Hydrazine catalysis enables the ring-closing carbonyl–olefin metathesis (RCCOM) to form polycyclic heteroaromatics, especially those with basic functionality.

Polycyclic heteroaromatic (PHA) structures comprise the core framework of many valuable compounds with a diverse range of applications (Fig. 1A).¹ For example, polycyclic azines (e.g. quinolines) are embedded in many alkaloid natural products, including diplamine² and eupolauramine³ to name just a few. These types of structures are also of interest for their biological activity, such as with the inhibitor of the Src-SH3 protein–protein interaction shown in Fig. 1A.⁴ Many nitrogenous PHAs are also useful as ligands for transition metal catalysis, as exemplified by the widely used ligand 1,10-phenanthroline.⁵ Meanwhile, chalcogenoarenes⁶ such as dinaphthofuran⁷ and benzodithiophene⁸ have attracted high interest for both their medicinal properties⁹ and especially for their potential use as organic light-emitting diodes (OLEDs), organic photovoltaics (OPVs), and organic field-effect transistors (OFETs).¹⁰ These and numerous other examples have inspired the development of a wide variety of strategies to construct PHAs.^1,11–14 Although these approaches are as varied as the structures they target, the wide range of molecular configurations within PHA chemical space and the challenges inherent in exerting control over heteroatom position and global structure make novel syntheses of these structures a topic of continuing interest.Open in a separate windowFig. 1(A) Examples of PHAs. (B) RCCOM strategy for PHA synthesis. (C) Lewis base inhibition for Lewis acid vs. hydrazine catalyzed RCCOM. (D) Hydrazine-catalyzed RCCOM for PHA synthesis.One potentially advantageous strategy for PHA synthesis is the use of ring-closing carbonyl–olefin metathesis¹⁵ (RCCOM) to forge one of the PHA rings, starting from a suitably disposed alkenyl aldehyde precursor 2 that can be easily assembled by cross-coupling (Fig. 1B). In related work, the application of RCCOM to form polycyclic aromatic hydrocarbons (PAHs) was reported by Schindler in 2017.¹⁶ In this case, 5 mol% FeCl₃ catalyzed the metathesis of substrates to form phenanthrenes and related compounds in high yields at room temperature. This method was highly attractive for its efficiency, its use of an earth-abundant metal catalyst, and the production of benign acetone as the only by-product. Nevertheless, one obvious drawback to the use of Lewis acid activation is that the presence of any functionality that is significantly more Lewis basic than the carbonyl group can be expected to strongly inhibit these reactions (Fig. 1C). Such a limitation thus renders this method incompatible with a wide swath of complex molecules, especially PHAs comprised of azine rings. This logic argues for a mechanistically orthogonal RCCOM approach that allows for the synthesis of PHA products with a broader range of ring systems and functional groups.We have developed an alternative approach to catalytic carbonyl–olefin metathesis that makes use of the condensation of 1,2-dialkylhydrazines 5 with aldehydes to form hydrazonium ions 6 as the key catalyst–substrate association step.^17–19 This interaction has a much broader chemoorthogonality profile than Lewis acid–base interactions and should thus be much less prone to substrate inhibition than acid-catalyzed approaches. In this Communication, we demonstrate that hydrazine-catalyzed RCCOM enables the rapid assembly of PHAs bearing basic functionality (Fig. 1D).For our optimization studies, we chose biaryl pyridine aldehyde 7 as the substrate (20 salt 11 was also productive (entry 2), albeit somewhat less so. Notably, iron(iii) chloride generated no conversion at either ambient or elevated temperatures (entries 3 and 4). Trifluoroacetic acid (TFA) was similarly ineffective (entry 5). Meanwhile, a screen of various solvents revealed that, while the transformation could occur in a range of media (entries 6–9), THF was optimal. Finally, by raising the temperature to 90 °C (entry 10) or 100 °C (entry 11), up to 96% NMR yield (85% isolated yield) of adduct 8 could be obtained in the same time period.Optimization studies^a

Stereoselectivity in nucleophilic addition to unsaturated ligands bound to molybdenum : Allylic alkylation of cyclohexanone

The stereochemistry and regiochemistry of nucleophilic addition to olefinic, allylic, or diene moieties can be controlled in reactions of molybdenum complexes. The synthesis of a wide range of -allylic cyclohexanones is feasible using (η⁵-cyclopentadienyl)Mo(CO)(NO)(allyl) cations. The stereoselective preparation of (RS,SR)-2-(1-methyl-2-butenyl)cyclohexanone from the reaction of 1-pyrrolidino-1-cyclohexene with [CpMo(CO)(NO)(η³-1,3-dimethylallyl)]BF₄ illustrates the methodology.     

Inclusion of the Full Electromagnetic Interaction in Correlations and Physical Quantities

Density-density correlations in a homogeneous, isotropic, classical plasma are calculated with inclusion of the full electromagnetic interaction. This leads to isotropic homogeneous field limits, contrary to the electrostatic approximation, that is consequently not valid for small wavenumbers. Application of the theory of generalized functions, combined with Kramer Kronig relations makes it possible to deal with the integrals that occur in the transverse part, provided a nonrelativistic approximation has been applied. As a side result Kubo's formula, applied to the electric conductivity, and generally derived under restriction to the electrostatic approximation, reappears for the case that the full electromagnetic interaction is included.     

Zum Nachweis der Phenole

Ohne Zusammenfassung