Syntheses and Crystal Structure of the Ternary Silicides RE2Si2Mg (RE = Y, La–Nd, Sm, Gd–Lu) and Structure Refinement of Dy5Si3

The rare earth metal–magnesium–silicides RE₂Si₂Mg (RE = Y, La–Nd, Sm, Gd–Lu) were prepared by induction melting of the elements in sealed tantalum tubes in a water-cooled sample chamber of a high-frequency furnace. The silicides were investigated via X-ray powder diffraction. The structures of Sm₂Si₂Mg and Dy₂Si₂Mg were refined from X-ray single crystal diffractometer data: Mo₂FeB₂ type, P4/mbm, a = 727.86(7), c = 428.16(6) pm, wR2 = 0.0194, 206 F² values, 13 variable parameters for Sm₂Si₂Mg and a = 713.85(7), c = 419.07(6) pm, wR2 = 0.0331, 286 F² values, 12 variable parameters for Dy₂Si₂Mg. The samarium compound shows a small homogeneity range Sm_2+xSi₂Mg_1−x. The investigated single crystal had the refined composition Sm_2.022(3)Si₂Mg_0.978(3). The RE₂Si₂Mg silicides are 1:1 intergrowth structures of CsCl and AlB₂ related slabs of compositions REMg and RESi₂. Crystals of the binary silicide Dy₅Si₃ were obtained as side product. The structure was refined from X-ray single crystal data: Mn₅Si₃ type, P6₃/mcm, a = 841.0(2), c = 631.3(1) pm, wR2 = 0.0661, 269 F² values, 12 variable parameters.     

Antifolate chemistry: synthesis of 4-[N-[(6RS)-2-methyl-4-oxo-3,4,7,8-tetrahydro-6H-cyclopenta[g]quinazolin-6-yl]-N-(prop-2-ynyl)amino]benzoic acid via a (propargyl)Co2(CO)6+ complex

A new route to compound 3 (4-[N-(6RS)-2-methyl-4-oxo-3,4,7,8-tetrahydro-6H-cyclopenta[g]quinazolin-6-yl]-N-(prop-2-ynyl)amino]benzoic acid), a crucial intermediate for the synthesis of potent inhibitors of thymidylate synthase (TS), is described. In this sequence the C6-N10 bond was constructed first, by the reductive amination of 5-acetamido-6-bromoindan-1-one 6 with tert-butyl 4-aminobenzoate, then the cyclopenta[g]quinazolinone ring was formed and the propargyl group was introduced on the N10-position using the (propargyl)Co2(CO)6+ complex as the electrophilic propargyl reagent.     

Optimization and application of lithium parameters for PM3

Lithium parameters have been optimized for Stewart's standard PM3 method. The average deviation of the heats of formation calculated for 18 reference compounds is 6.2 kcal/mol from the experimental or high-level ab initio data; the average deviation with Li/MNDO is 18.9 kcal/mol. The average error in bond lengths is also reduced by a factor of two to three. Ionization potentials and dipole moments are reproduced with comparable accuracy than Li/MNDO. However, the mean deviation for the heats of formation of both methods increases when being applied to other systems, especially to small inorgnic molecules. The applicability of the new parameter set is demonstrated further for various compounds not included in the reference set, for the calculation of the activation barriers of several lithiation reactions, as well as for the estimation of oligomerization energies of methyl lithium (including the tetramer). Li/PM3 gives reliable results even for large dimeric complexes, like [{4-(CH₃CR)C₅H₄N}Li]₂, containing TMEDA or THF as coligands and reproduces the haptotropic interaction between Li⁺ and π-systems (e.g., in benzyl lithium) as well as the relative energies and structural features of compounds with “hypervalent” atoms (e.g., in lithiated sulfones). © John Wiley & Sons, Inc.     

Mass spectrometric fragmentation of S-(1-aryltetrazol-5-yl)-monothiocarbonic acid esters

The mass spectra of S-(1-aryltetrazol-5-yl)-monothiocarbonic acid esters have been studied. The stability of the ester molecular ions is lower than the stability of the corresponding 1-aryl-5-mercaptotetrazoles. Substituents at the phenyl group increase the stability, whereas the influence of the ester alkyl groups is very small. The esters undergo fragmentation via four different fragmentation pathways. The elimination of carbon dioxide is influenced by an ‘ortho effect’ of the substituents.     

Non-destructive depth profile analysis using synchrotron radiation excited XPS

We have used synchrotron radiation as excitation source in an X-ray photoelectron spectroscopy (XPS) experiment to analyse surface-near element depth profiles non-dectructively. By tuning the photon energy one can vary the kinetic energy of the photoelectrons and in turn the information depth of the measurement. To quantify the sample geometry (e.g. layer thicknesses) model calculations similar as for angle-resolved XPS (ARXPS) measurements are necessary. We have successfully applied this technique to several samples. We will show how to calculate the relative intensities of the peaks, using photoionization cross sections and an experimentally determined analyzer transmission function and the procedure to quantify the geometry for a model sample: natively oxidized Ta covered by carbon contamination. At Sn-doped indium oxide samples we found a sub-monolayer of segregated Sn at the surface which was expected from previous investigations.     

Synthesis and reactions of the cyclic silyl peroxide 1,1,4,4-tetramethyl-2,3-dioxa-1,4-disilacyclohexane

The first simple cyclic silyl peroxide, namely 1,1,4,4,-tetramethyl-1,4-disila-2,3-dioxane (1), was prepared by classical synthetic methodology from its corresponding cyclic disilazane and the urea complex of hydrogen peroxide.     

Synthesis of R-(-)-imperanene from the natural lignan hydroxymatairesinol

A convenient and high yielding method for the synthesis of R-(-)-imperanene, starting from the readily available natural lignan hydroxymatairesinol from Norway spruce, was developed. Hydroxymatairesinol was degraded in strongly basic aqueous conditions to (E)-4-(4-hydroxy-3-methoxyphenyl)-2-(4-hydroxy-3-methoxyphenylmethyl)but-3-enoic acid, which was esterified and then reduced by LiAlH(4) to afford R-(-)-imperanene. The configuration at the crucial stereocenter was preserved in the synthesis, and the obtained product was identified by optical rotation measurements and chiral HPLC analyses as the R-(-)-enantiomer (ee 86-92%).     

Iminopropadienones R–NCCCO and carbon suboxide, C3O2. Theoretical and experimental C NMR spectra

The ¹³C NMR data of five iminopropadienones R–NCCCO as well as carbon suboxide, C₃O₂, have been examined theoretically and experimentally. The best theoretical results were obtained using the GIAO/B3LYP/6-31+G**//MP2/6-31G* level of theory, which reproduces the chemical shifts of the iminopropadienone substituents extremely well while underestimating those of the cumulenic carbons by 5–10 ppm. The computationally faster GIAO/HF/6-31+G**//B3LYP/6-31G* level is also adequate.