A combined theoretical and experimental study of efficient and fast titanocene-catalyzed 3-exo cyclizations

The mechanism of titanocene mediated 3-exo cyclizations was investigated by a combined theoretical and experimental study. A gradient corrected density functional theory (DFT) method has been scaled against titanocene dichloride, the parent butenyl radical, and in bond dissociation energy (BDE) calculations. The BP86 method using density fitting, and a basis set of triple-zeta quality emerged as a highly reliable tool for studying titanocene mediated radical reactions. The computational results revealed important kinetic and thermodynamic features of cyclopropane formation. Surprisingly, the beta-titanoxy radicals, the first intermediates of our investigations, were demonstrated to possess essentially the same thermodynamic stabilization as the corresponding alkyl radicals by comparison of the calculated BDEs. In contrast to suggestions for samarium mediated reactions, the cyclization was shown to be thermodynamically favorable in agreement with earlier kinetic studies. It was established that stereoselectivity of the cyclization is governed by the stability of the intermediates and thus the trans disubstituted products are formed preferentially. The observed ratios of products are in good to excellent agreement with the DFT results. By a combination of computational and experimental results, it was also shown that for the completion of the overall cyclopropane formation the efficiency of the trapping of the cyclopropylcarbinyl radicals is decisive.     

Phosphaniminate von Alkalimetallen: Die Kristallstrukturen von [KNPCy3]4, [KNPCy3]4·2OPCy3, [CsNPCy3]4·4OPCy3 und [Li4(NPPh3)(OSiMe2NPPh3)3(DME)]

The alkalimetal phosphoraneiminates [KNPCy₃]₄, ( 1 ) [KNPCy₃]₄·2OPCy₃ ( 2 ) and [CsNPCy₃]₄·4OPCy₃ ( 3 ) (Cy = cyclohexyl) which are obtainable by the reaction of pottassium amide or cesium amide with Cy₃PI₂ or Cy₃PBr₂ in liquid ammonia, as well as the lithium derivative [Li₄(NPPh₃)(OSiMe₂NPPh₃)₃(DME)] ( 4 ) have been characterized by crystal structure determinations. 4 has been formed by the insertion reaction of silicon greaze (‐OSiMe₂)n into the LiN bonds of [LiNPPh₃]₆ in DME solution (DME = 1, 2‐dimethoxyethane). 1 : Space group P&1macr;, Z = 2, lattice dimensions at 193 K: a = 1389.8(1); b = 1408.1(1); c = 2205.2(2) pm; α = 78.952(10)?; β = 81.215(10)?; γ = 66.232(8)?; R1 = 0.0418. 2 : Space group Pbcn, Z = 4, lattice constants at 193 K: a = 2943.6(2); b = 2048.2(1); c = 1893.8(1) pm; R1 = 0.0428. 3 : Space group Cmc2₁, Z = 4, lattice dimensions at 193 K: a = 2881.6(2); b = 2990.2(2); c = 1883.7(2) pm; R1 = 0.0586. 4 ·1/2DME: Space group R&3macr;c, Z = 12, lattice dimensions at 193 K: a = b = 1583.5(1); c = 11755.3(5) pm; R1 = 0.0495. All complexes have heterocubane structures. In 1‐3 they are formed by four alkali metal atoms and by the nitrogen atoms of the (μ₃‐NPCy₃^‐) groups, whereas 4 forms a "heteroleptic" Li₄NO₃ heterocubane.     

The Ideal Ionic Liquid Salt Bridge for the Direct Determination of Gibbs Energies of Transfer of Single Ions,Part I: The Concept

Described is a procedure for the thermodynamically rigorous, experimental determination of the Gibbs energy of transfer of single ions between solvents. The method is based on potential difference measurements between two electrochemical half cells with different solvents connected by an ideal ionic liquid salt bridge (ILSB). Discussed are the specific requirements for the IL with regard to the procedure, thus ensuring that the liquid junction potentials (LJP) at both ends of the ILSB are mostly canceled. The remaining parts of the LJPs can be determined by separate electromotive force measurements. No extra‐thermodynamic assumptions are necessary for this procedure. The accuracy of the measurements depends, amongst others, on the ideality of the IL used, as shown in our companion paper Part II.     

Kristallstruktur des Isothiocyanato‐Komplexes [Ph3PNH2(OEt2)][Sm(NCS)4(DME)2]

Crystal Structure of the Isothiocyanato Complex [Ph₃PNH₂(OEt₂)][Sm(NCS)₄(DME)₂] Colourless single crystals of [Ph₃PNH₂(OEt₂)][Sm(NCS)₄(DME)₂] ( 1 ) have been obtained besides of Ph₃PS from the reaction of the homoleptic phosphorane iminato complex [Sm(NPPh₃)₃]₂ with carbon disulfide in THF solution, followed by recrystallisation from DME/Et₂O. According to the crystal structure analysis 1 consists of [Ph₃PNH₂]⁺ cations with the diethylether molecule forming a N–H…O hydrogen bridge, and anions [Sm(NCS)₄(DME)₂]^–. Sm³⁺ realizes coordination number eight by four nitrogen atoms of the isothiocyanato ions and by four oxygen atoms of the DME chelates. 1 : Space group P 1, Z = 4, lattice dimensions at 193 K: a = 919.0(1), b = 1965.2(2), c = 2401.3(2) pm, α = 96.748(11)°, β = 94.827(10)°, γ = 91.720(11)°, R = 0.029.     

Analysis of sulphonated polyphenols, synthetic tanning agents in heavily polluted tannery wastewaters

A method is presented for the analysis of sulphonated polyphenols, acting as synthetic tanning agents, in heavily polluted tannery wastewater. It consists of ion-pair solid-phase extraction (RP-18) with tetrabutylammonium bromide followed by reversed-phase ion-pair liquid chromatography on a C₁₈ stationary phase with UV detection. The detection limit is 15 μg/1. The influence of the wastewater matrix on extraction and the effects of pH and temperature on the HPLC separation are discussed. The procedure is not affected by the high contents of dissolved organic carbon and inorganic salts of tannery wastewater. The tannery effluents investigated contained around 40 mg/1 of sulphonated polyphenols. These compounds appear to be refractory against both anaerobic and aerobic biological wastewater treatment. They might, thus, contribute to the dissolved organic sulphur content of surface waters.     

Theory and simulation of organic solar cell model compounds: from atoms to excitons

We approach the electronic properties of a simple model of organic solar cells, a binary mixture of αα'-oligothiophenes and buckminsterfullerene, from a theoretical and numerical perspective. Close-packed model geometries are generated using a Monte Carlo method, the electronic structure is described by a reparametrized semiempirical Pariser-Parr-Pople Hamiltonian. All electronic properties, such as optical absorption spectra, tightly-bound charge transfer states and exciton bands, arise from the same atomistic Hamiltonian using a configuration interaction method involving single excitations. The absorption spectra are dominated by intramolecular contributions, whereas in the optical gap low-lying charge transfer states are predicted. The efficiency of the solar cell crucially depends on the structure of the charge-transfer exciton bands and on the relaxation mechanism. We discuss how these findings may help improve the design of organic solar cells from an excitonic view.     

Enantioselective radical cyclisation reactions of 4-substituted quinolones mediated by a chiral template

Six 4-substituted quinolones 6-8, which bear an ω-iodoalkyl chain, were prepared and subjected to reductive radical cyclisation conditions employing BEt(3)/O(2) as the initiator and either Bu(3)SnH or TMS(3)SiH as hydride source. 4-(4-Iodobutyl)-quinolone (6a) and 4-(3-iodopropylthio)-quinolone (8a) gave the respective 6-endo-cyclisation products in good yields. 4-(3,3-Dimethyl-4-iodobutyl)-quinolone (6b) cyclised in a 5-exo-fashion, while the other substrates delivered only reduction products. The cyclisation reactions could be conducted in the presence of a chiral template (1) with high enantiomeric excess (94-99% ee). The association behaviour of substrate 6a to 1 was studied by NMR titration experiments. In the enantioselective cyclisation of 6b a significant nonlinearity was observed when comparing the product ee with the ee of the template.