Chiral recognition in surface explosion

The vast majority of chiral compounds crystallize into racemic crystals. It has been predicted and was experimentally established as a rule that chiral molecules on surfaces are more easily separated into homochiral domains due to confinement into a plane and lower entropic contributions. We investigated the formation and stability of two-dimensional tartrate crystals on a Cu(110) surface for the racemic mixture for the first time by means of temperature-programmed desorption (TPD), low-energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS). At low coverage, a bitartrate species becomes separated into homochiral domains, while at high coverage a monotartrate species forms a racemic mixture. At the same coverage and lateral arrangement, the thermally induced autocatalytic decomposition reaction occurs for the monotartrate racemate at a lower temperature than for the pure enantiomers. The stereochemistry in this so-called "surface explosion" reaction is explained by a higher stability of the enantiopure lattice due to lateral hydrogen-bond formation. The higher stability of the enantiopure two-dimensional lattice is in contrast to the higher stability of racemic three-dimensional tartaric acid crystals but is consistent with the observation that homochirality is preferred in hydrogen-bonded self-assembled biomolecular structures.     

Photochromism of O-Analogs of Thiathiophthenes

Irradiation of O-analogs of thiathiophthenes yield a photoproduct which reverts thermally to the starting material. Kinetic measurements for this thermal reaction yield E_a values in the range of 10–15 kcal/mol and Δ S* values between ?16 and ?41 eu. Based on NMR. and IR. results the structure of the photoproduct is established. The electronic spectra of the starting material and of the photoproduct are discussed. The experimental results are compared with MO-calculations.     

Oligonucleotide Synthesis on Polystyrene-Grafted Poly(tetrafluoroethylene) Support

Supports consisting of a thin layer (2–10%) of polystyrene (PS) grafted onto a poly(tetrafluoroethylene) (PTFE) core are an interesting alternative to controlled pore glass (CPG) carriers in oligonucleotide synthesis. The beads are mechanically stable, do not show significant swelling, and allow effective removal of substrates by short washing steps with organic solvents. PTFE-PS as an organic polymer has generally more hydrophobic properties than inorganic polymer supports and, therefore, is well compatible with organic solvents such as anhydrous MeCN. We found PTFE with a content of 2–3% PS graft to be a very good support for the synthesis of oligonucleotides of extended length. In comparison, PTFE with 5–10% grafted PS is especially useful for large-scale syntheses. Functionalization procedures minimized in the extent of side reactions are described as well as examples for the use of the supports in syntheses of oligonucleotides both on large scale and of extended chain length.     

Cycloaddition of single oxygen and triazolinedione (TAD) with spirocyclopentadienyl-1,3,5-cycloheptatrienes1: tropilidene versus norcaradiene reactivity

4-Methyl-1,2,4-triazoline-3,5-dione (MTAD) gives with 7-spiro-fluorenyl-1,3,5-cycloheptatriene ($\text{1d}$) initially he norcaradine-type urazole ($\text{N3d}$) which at ambient temperatures rearranges into ($\text{6d}$), while ¹0₂ gives the tropilidene-type endoperoxide ($\text{T4d}$).     

Photochemisch induzierte oxidative addition von aldehyden an dekacarbonyldirhenium(0)

Decacarbonyldirhenium(O) (I) reacts photochemically with acetaldehyde or propionaldehyde (II, III) to give predominantly tetradecacarbonyl-μ-hydridotrirhenium (VI). In addition, μ-acyloctacarbonyl-μ-hydridodirhenium complexes (IV, V) are obtained in oxidative addition reactions with simultaneous loss of two CO ligands. Complexes IV and V were characterized by their elemental analyses, IR, ¹H NMR, and ¹³C NMR spectra. Furthermore, the molecular structure of IV was determined by X-ray structure analysis. IV shows approximate (C_s-symmetry. Two Re(CO₄) groups are connected via the C and O atoms of the carbonyl function of an acetyl bridge, and from the steric arrangement of the Re(CO)₄ moieties and NMR spectroscopic evidence, a hydrido bridge has to be assumed between the Re atoms, which rules out a ReRe bond. The two coordination octahedrons are joined by the hydrido bridge. The connection of two further edges by the acetyl bridge causes an eclipsed arrangement of the CO ligands in IV.     

Zur reaktion von metall-koordiniertem kohlenmonoxid mit yliden: XVI. Reversibler einbau von kohlenmonoxid in eine trimethylphosphonio (trimethylsiloxy) vinyl-einheit; Darstellung des zwitteriqnischen vinyl-chromkomplexes Cp(CO)(NO) CrC(OSiMe3)CHPMe3 und solvolytischer abbau sowie spaltung mit Me3P=CH2 oder MeX (X = I,SO3F)

The β-trimethylphosphonio(α-trimethylsiloxy)vinylchromium complex Cp(CO)(NO)$\text{C}$rC(OSiMe₃)=CHPMe₃ (2) can be isolated from a concentrated solution of Cp(CO)₂(NO)Cr (1) and Me₃P=CHSiMe₃ in benzene. 2 is obtained in better yield via O-silylation of the tetramethylphosphonium chromium acylate Me₄P[Cp(CO)(NO)CrC(O)CH=PMe₃] (3) with Me₃SiOSO₂CF₃. 2 decomposes readily by treatment with benzene to 1 and Me₃P=CHSiMe₃, which forms the ylide complex Cp(CO)(NO)CrCH(SiMe₃)PMe₃ (4) on photolysis. Degradation of 2 can be accelerated extraordinarily by traces of Me₃P=CH₂. With Me₃P= CH₂ (2 mol) controlled conversion of 2 to 3 and Me3P=CHSiMe₃ occurs. MeX (X = I, SO₃F) cleaves 2 to 1 and the phosphonium salt [Me₃PCH(SiMe₃)]X (5a,5b).     

Synthese und Molekülstruktur von Li[(Me2NCH2CH2CH2)4Yb]

Preparation and Structure of Li[(Me₂NCH₂CH₂CH₂)₄Yb] YbCl₃ reacts with dimethylaminopropyl lithium in tetrahydrofuran with formation of Li[(Me₂NCH₂CH₂CH₂)₄Yb]. The X-ray structure determination proves the formation of chelat rings by two of the ligands. The nitrogens of the other two ligands are coordinated to the lithium. The compound crystallizes in the monoclinic space group C2/c with the unit cell parameters a = 27.854(8) Å, b = 9.183(3) Å, c = 20.125(8) Å, β = 96.40° and Z = 8.     

Vinylierung und 91Zr-NMR-Spektren von Substituierten Zirconocendichloriden

Vinylation and ⁹¹Zr N.M.R. Spectra of substituted Zirconocene Dichlorides Substituted zirconocene dichlorides react with vinyl lithium with formation of zirconacyclopent-2-enes, Cp₂ZrCH = CHCH₂CH₂, or zirconocene butadiene complexes, Cp₂Zr(C₄H₆). The compounds obtained were characterized by their ¹H and ¹³C n.m.r. spectra. The ⁹¹Zr n.m.r. chemical shifts of substituted zirconocene dichlorides correlate with the bond angles Cp′? Zr? Cp′ and Cl? Zr? Cl respectively. They can be used to estimate the reaction behaviour of zirconocene dichlorides.     

13C-NMR-spektren von spiro[2.4]hepta-(1,4,6)-trienen und -(4.6)-dienen

The ¹³C NMR spectra of [1.2]-spirenes 2 and spiro[2.4]hepta-(4.6)-dienes 5 are described and the effects of substituents are discussed. A bonding model of the [1.2]-spirenes 2 is derived on the basis of C-H coupling constants and calculated charge densities. The ¹³C NMR shifts of 1-aryl-spiro[2.4]hepta-(4.6)-dienes 6 show a good correlation with the Hammett σp constants of the substituents in p-position of the phenyl ring. Conclusions on spiroconjugation in [1.2]-spirenes 2 are drawn from comparing the ¹³C shifts of similar substituted [1.2]-spirenes and spiro[2.4]hepta-(4.6)-dienes. The spiroconjugation in [1.2]-, [2.2]- and [2.3]-spirenes 2, 3 and 4 is discussed comparing the calculated charge densities with the δ (¹³C) values, as well as the UV and p.e. spectra.