A general organocatalyst for direct alpha-functionalization of aldehydes: stereoselective C-C, C-N, C-F, C-Br, and C-S bond-forming reactions. Scope and mechanistic insights

The development of a general organocatalyst for the alpha-functionalization of aldehydes, via an enamine intermediate, is presented. Based on optically active alpha,alpha-diarylprolinol silyl ethers, the scope and applications of this catalyst for the stereogenic formation of C-C, C-N, C-F, C-Br, and C-S bonds are outlined. The reactions all proceed in good to high yields and with excellent enantioselectivities. Furthermore, we will present mechanistic insight into the reaction course applying nonlinear effect studies, kinetic resolution, and computational investigations leading to an understanding of the properties of the alpha,alpha-diarylprolinol silyl ether catalysts.     

Thermochemische Untersuchungen zu den Systemen Ti/Ni und Ti/Co

Thermochemical Investigations of the Systems Ti/Ni and Ti/Co By treatment of solid Ni or Co with a H₂/TiCl₄-gas mixture at sufficient high temperature (T ≥ 900°C) the intermetallic phases TiNi₃ and TiCo₃, resp., are formed. The conversion grade depends on the H₂/TiCl₄-ratio. From the experimentally determined conversion grades and the known thermodynamic data of all other species existing in equilibrium the free enthalpies and the heats of formation of TiN₃ and TiCo₃ have been calculated (TiNi₃: ΔH(298) = ?133.3 ± 6 kJ/mol; TiCo₃: ΔH(298) = ?104.7 ± 6 kJ/mol).     

Adsorption of CF4 on the internal and external surfaces of opened single-walled carbon nanotubes: a vibrational spectroscopy study

Infrared spectroscopy has been used to make the first experimental discrimination between molecules bound by physisorption on the exterior surface of carbon single-walled nanotubes (SWNTs) and molecules bound in the interior. In addition, the selective displacement of the internally bound molecules has been observed as a second adsorbate is added. SWNTs were opened by oxidative treatment with O(3) at room temperature, followed by heating in a vacuum to 873 K. It was found that, at 133 K and 0.033 Torr, CF(4) adsorbs on closed SWNTs, exhibiting its nu(3) asymmetric stretching mode at 1267 cm(-1) (red shift relative to the gas phase, 15 cm(-1)). Adsorption on the nanotube exterior is accompanied by adsorption in the interior in the case of opened SWNTs. Internally bound CF(4) exhibits its nu(3) mode at 1247 cm(-1) (red shift relative to the gas phase, 35 cm(-1)). It was shown that, at 133 K, Xe preferentially displaces internally bound CF(4) species, and this counterintuitive observation was confirmed by molecular simulations. The confinement of CF(4) inside (10,10) single-walled carbon nanotubes does not result in the production of lattice modes that are observed in large 3D ensembles of CF(4).     

High-pressure dissociation of crystalline para-diiodobenzene: optical experiments and Car-Parrinello calculations

We have investigated the high-pressure properties of the molecular crystal para-diiodobenzene, by combining optical absorption, reflectance, and Raman experiments with Car-Parrinello simulations. The optical absorption edge exhibits a large red shift from 4 eV at ambient conditions to about 2 eV near 30 GPa. Reflectance measurements up to 80 GPa indicate a redistribution of oscillator strength toward the near-infrared. The calculations, which describe correctly the two known molecular crystal phases at ambient pressure, predict a nonmolecular metallic phase, stable at high pressure. This high-density phase is characterized by an extended three-dimensional network, in which chemically bound iodine atoms form layers connected by hydrocarbon bridges. Experimentally, Raman spectra of samples recovered after compression show vibrational modes of elemental solid iodine. This result points to a pressure-induced molecular dissociation process which leads to the formation of domains of iodine and disordered carbon.     

Reaction of tellurium(iv) with dithizone

With an excess of dithizone over tellurium, the extraction of Te(IV) from 1 M perchloric acid solutions into a carbon tetrachloride solution (o) of dithixone follows the relation

When the acidity is varied, again with a sufficiently large excess of dithissone, the following relation seems to be approached;

(μ=1.0).     

Miscibility of zinc sulfide and zinc phosphide

Solid solutions in the system zinc sulfide/zinc phosphide (Zn(2+)(x)S(2-2xP(2x)) were investigated using the cyclic cluster model within the semiempirical MSINDO method. Results of cyclic cluster calculations for binding energies of the perfect ZnS and Zn(3)P(2) are presented and compared with the experimental data. The miscibility of ZnS and Zn(3)P(2) over the whole composition range of 0 < x < 1 was investigated by calculating the Gibbs free energy of mixing Delta(M)G for different values of x. A miscibility gap was found at both ends of the composition range and compared with experimental data.     

Zur Bildung von Xylol-tricarbonylchrom (Dimethylbenchrotrenen) aus Di-n-butylether und Hexacarbonylchrom

The reaction of fluorenone ethyleneketal (9), diphenic anhydride as well as of biphenyls such as4 with Cr(CO)₆ in refluxing di-n-butylether (BE) gives dimethylbenchrotrenes [xylene-Cr(CO)₃,7] which are also formed by refluxing pureBE with Cr(CO)₆—although with much lower yields. Similarily, from di-n-pentylether and Cr(CO)₆ isobutyl- and 1-methyl-3-propyl-benchrotrene (13 and14) were obtained, whilst from di-n-propyl- and-hexylether, resp., and Cr(CO)₆ no benchrotrenes could be isolated.Tentative assumptions on the catalytic action of certain functional groups were confirmed by the reaction ofBE and Cr(CO)₆ in the presence of carbonamides, such as acetamide, urea and pyrrolidone, where 1,4-dimethylbenchrotrene (7 c) was formed with appreciable yields.Studies using dideuteratedBE showed that—at least for the reaction of9 with Cr(CO)₆ (giving a much lower yield of7 than with undeuteratedBE)—a simple bimolecular mechanism can be excluded.

     

EFFECT OF PHYTOCHROME ON THE BIOSYNTHESIS OF ACYCLIC AND CYCLIC CAROTENOIDS IN HIGHER PLANTS

Radish plants were grown in the presence of three different herbicides that interfere with the formation of the normal range of cyclic carotenoids, leading to an accumulation of acyclic biosynthetic intermediates, mainly phytoene (SAN 6706 and amitrole) and zeta-carotene (3852). Plants were then irradiated by four different light programs in order to gain more insight into the first steps of carotenoid biosynthesis and their control by light and phytochrome. In all cases, herbicide-treated and control, carotenoid biosynthesis was greatly enhanced by red light consistent with an effect of phytochrome on the early steps of the pathway. However, similar enhancement was also obtained after treatment with far-red light. Indeed with SAN 6706-treated plants synthesis of phytoene was stimulated to a much greater extent by far-red light given alone, than by red light. The involvement of phytochrome in the regulation of carotenoid biosynthesis appears not to be as simple as previously supposed.     

Ab initio calculations of the pi electron hamiltonian: singlet-triplet splittings

Some results of approximate ab initio calculations of the “correlation” contribution to the true parameters of the pi electron hamiltonian are presented for the ethylene molecule. In particular, by using sum-of-the-pairs type generalized perturbation theory, it is shown that there is a large core “correlation” contribution to singlet-triplet splittings within pi electron theories that results from the difference in the degree of ionicity of the isoconfigurational states.