Synthesen und Eigenschaften von Bis(perfluoralkyl)zink‐Verbindungen

Syntheses and Properties of Bis(perfluoroalkyl)zinc Compounds The conditions for the syntheses of bis(perfluoroalkyl)zinc compounds Zn(R_f)₂ · 2 D (R_f = C₂F₅, n‐C₃F₇, i‐C₃F₇, n‐C₄F₉, n‐C₆F₁₃, n‐C₇F₁₅, and n‐C₈F₁₇; D = CH₃CN, tetrahydrofurane, dimethylsulfoxide) are described. Mass spectra, thermal decompositions, ¹⁹F‐ and ¹³C‐NMR spectra are discussed.     

Synthesis and Structure of Dimeric Chloro(cyclopentadienyl)bismuthenium Cations

A new modification of the cyclopentadienylbismuth dihalide [(C₅HR₄)BiCl(μ-Cl)]₂ ( 1 ) (R = CHMe₂) has been characterized by X-ray diffraction. The coordination of the five-membered ring as well as the arrangement of the chloro bridges differ from those of the known modification. On reaction of 1 or its pentaalkylated derivative [(C₅Me₂R₃-1,2,4)BiCl(μ-Cl)]₂ ( 2 ) with AlCl₃ the bismuthenium salts [(C₅Me₂R₃)Bi(μ-Cl)]₂²⁺ [AlCl₄^–]₂ ( 3 ) and [(C₅HR₄)Bi(μ-Cl)]₂²⁺ [AlCl₄^–]₂ ( 4 ) have been obtained. Both are centrosymmetric dimers without solvent coordination. Only for 4 a very weak interaction with the counterion could be detected.     

Study of the Interactions between Poly(vinyl pyrrolidone) and Sodium Dodecyl Sulfate by Fluorescence Correlation Spectroscopy

Polymer–surfactant interactions are operative in a variety of industrial processes and important consumer products. Fluorescence correlation spectroscopy is a powerful method for investigating the complex formation and was used to study the well-known reference system involving the aggregation of sodium dodecyl sulfate (█) and poly(vinyl pyrrolidone) (█) in the presence of a fluorescence-marked sodium decylsulfate probe (█, see picture). C_T=surfactant concentration, d_fl=hydrodynamic size of the probe.     

[Me3Si(CF3)F]− and [Me3Si(CF3)2]−: Reactive Intermediates in Fluoride-Initiated Trifluoromethylation with Me3SiCF3— An NMR Study

The long-postulated reactive intermediates of polar fluoride-inititated trifluoromethylations with Me₃SiCF₃ (see scheme) were identified by NMR spectroscopy as [Me₃Si(CF₃)F]⁻ and [Me₃Si(CF₃)₂]⁻_.     

How To Stabilize or Break β-Peptidic Helices by Disulfide Bridges: Synthesis and CD Investigation of β-Peptides with Cysteine and Homocysteine Side Chains

all-L -β³-Penta-, hexa-, and heptapeptides with the proteinogenic side chains of valine, leucine, serine, cysteine, and methionine have been prepared by previously described procedures ( 12 , 13 , 14 , 15 ; Schemes 2 – 5). Thioether cleavage with Na/NH₃ in β-HMet residues has also provided a β³-hexapeptide with homocysteine (CH₂CH₂S) side chains ( 13e ). The HS−(CH₂)_n groups were positioned on the β-peptidic backbone in such a way that, upon disulfide-bridge formation, the corresponding β-peptide was expected to maintain either a 3₁-helical secondary structure ( 1 , 2 ) (Fig. 1) or to be forced to adopt another conformation ( 3 , 4 ). The 13-, 17-, 19-, and 21-membered-ring macrocyclic disulfide derivatives and their open-chain precursors, as well as all synthetic intermediates, were purified (crystallization, flash or preparative HPL chromatography; Fig. 5) and fully characterized (m.p., [α]_D, CD, IR, NMR, FAB or ESI mass spectroscopy, and elemental analysis, whenever possible; Fig. 2 and Exper. Part). The structures in MeOH and H₂O of the new β-peptides were studied by CD spectroscopy (Figs. 3 and 4), where the characteristic 215-nm-trough/200-nm-peak pattern was used as an indicator for the presence or absence of (M)-3₁-helical conformations. A CH₂−S₂−CH₂ and, somewhat less so, a (CH₂)₂−S₂−(CH₂)₂ bracket between residues i and i+3 ( 1 vs. 12d , and 2 vs. 13e in Fig. 3) give rise to CD spectra which are compatible with the presence of 3₁-helical structures, while CH₂−S₂−CH₂ brackets between residues i and i+2 ( 3 vs. 14c ) or i and i+4 ( 4 vs. 15c in Fig. 4) do not.     

(−)-Sparteine-Mediated Asymmetric Intramolecular Carbolithiation of Alkenes: Synthesis of Enantiopure Cyclopentanes with Three Consecutive Stereogenic Centers

An asymmetric intramolecular carbolithiation reaction was developed by combining the (−)-sparteine-mediated enantiotopos-differentiating deprotonation and the anionic 5-exo-trig cyclization. Achiral 6-phenylhex-5-enyl carbamates were efficiently cyclized furnishing regio-, diastereo- (dr >99 : 1), and enantioselectively (er >98 : 2) 1,2-trans-substituted cyclopentanes. The intermediate primary benzylic lithium-carbanion pairs were – in spite of their configurative lability – diastereoselectively substituted by versatile electrophiles creating a third consecutive stereogenic center. Additionally, some 4-functionalized 6-phenylhex-5-enyl carbamates were also cyclized in high yield to provide enantiomerically pure cyclopentanes incorporating three adjacent stereogenic centers.