Easy Access to Enantiomerically Pure Heterocyclic Silicon-Chiral Phosphonium Cations and the Matched/Mismatched Case of Dihydrogen Release

Phosphonium ions are widely used in preparative organic synthesis and catalysis. The provision of new types of cations that contain both functional and chiral information is a major synthetic challenge and can open up new horizons in asymmetric cation-directed and Lewis acid catalysis. We discovered an efficient methodology towards new Si-chiral four-membered CPSSi* heterocyclic cations. Three synthetic approaches are presented. The stereochemical sequence of anchimerically assisted cation formation with B(C₆F₅)₃ and subsequent hydride addition was fully elucidated and proceeds with excellent preservation of the chiral information at the stereogenic silicon atom. Also the mechanism of dihydrogen release from a protonated hydrosilane was studied in detail by the help of Si-centered chirality as stereochemical probe. Chemoselectivity switch (dihydrogen release vs. protodesilylation) can easily be achieved through slight modifications of the solvent. A matched/mismatched case was identified and the intermolecularity of this reaction supported by spectroscopic, kinetic, deuterium-labeling experiments, and quantum chemical calculations.     

Local layer structure of the steep field line defect in surface-stabilized ferroelectric liquid crystal cells

The local layer structure of one type of DC electric field induced line defect observed in CS-1014 surface stabilized ferroelectric liquid crystal electro-optic cells has been determined using X-ray scattering and optical microscopy. The characterized defect is a high contrast defect line distinct from other field lines in that a defect mirror image is not produced when the DC field direction is reversed.     

Robotic preparation of metal samples for analysis by plasma emission spectrometry

Summary A method utilizing a laboratory robotic system to automate sample preparation for the chemical analysis of metals was developed. Anticipated elemental concentration values for samples are entered into the robotic system, and the system determines the needed sample weights and calibration solution concentrations. The robot then weighs, dissolves, and dilutes the samples and prepares calibration solutions prior to multi-elemental analyses by inductively-coupled plasma — atomic-emission spectrometry. Zinc-base alloy standard reference materials were used to evaluate this method. For a batch of ten samples, operator times compared with a similar manual method were reduced by about 5-fold. Precision and accuracy data for samples prepared by robotic and manual methods were equivalent.

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Automatische Probenvorbereitung für die Analyse durch Plasma-Emissions-Spektrometrie mit Hilfe eines Laborroboters

     

High-brightness terawatt KrF* (248 nm) system

19 W/cm² range with improved shot-to-shot energy stability. This system gives a resulting brightness of ∼3.3×10²¹ W cm^-2 sr^-1. Received: 5 November 1996/Revised version: 29 November 1996     

FT-IR and raman spectra of a series of metallo-β-cyclodextrin complexes

A series of metallo--CD complexes were prepared and formulated as [M₂(OH)₂ -CD·2 H₂O] ^n– . Changes in the FT-IR and Raman Spectra of-CD on coordination may be taken as evidence for complexation and support for a hydroxy bridged binuclear structure. Further support was obtained from uv/visible and magnetic moment measurements.     

Mean squared prediction error in the spatial linear model with estimated covariance parameters

The problem considered is that of predicting the value of a linear functional of a random field when the parameter vector of the covariance function (or generalized covariance function) is unknown. The customary predictor when is unknown, which we call the EBLUP, is obtained by substituting an estimator j for in the expression for the best linear unbiased predictor (BLUP). Similarly, the customary estimator of the mean squared prediction error (MSPE) of the EBLUP is obtained by substituting j for in the expression f for the BLUP's MSPE; we call this the EMSPE. In this article, the appropriateness of the EMSPE as an estimator of the EBLUP's MSPE is examined, and alternative estimators of the EBLUP's MSPE for use when the EMSPE is inappropriate are suggested. Several illustrative examples show that the performance of the EMSPE depends on the strength of spatial correlation; the EMSPE is at its best when the spatial correlation is strong.This research was partially supported by a University of Iowa Old Gold Fellowship (Zimmerman) and by the NSF under grant DMS-8703083 (Cressie).     

Manganese Catalyzed Hydrogenation of Organic Carbonates to Methanol and Alcohols

The first example of a homogeneous catalyst based on an earth‐abundant metal for the hydrogenation of organic carbonates to methanol and alcohols is reported. Based on the mechanistic investigation, which indicates metal‐ligand cooperation between the manganese center and the N?H group of the pincer ligand, we propose that the hydrogenation of organic carbonates to methanol occurs via formate and aldehyde intermediates. The reaction offers an indirect route for the conversion of CO₂ to methanol, which coupled with the use of an earth abundant catalyst, makes the overall process environmentally benign and sustainable.     

Room‐Temperature Functionalization of N2 to Borylamine at a Molybdenum Complex

Intermolecular, stepwise functionalization by BH bonds of a (triphosphine)Mo^IV–nitrido complex generated by N₂ splitting is reported. The imido–hydride and di‐hydride–amido Mo^IV complexes have been isolated and characterized. Addition of PinBH to the [Mo(H)₂(N(BPin)₂)]⁺ complex at room temperature results in the liberation of borylamines from the metal center.