Condensation and structure of silicic acid in tetrahydrofuran

Silicic acid (SA) was extracted with THF from aqueous sodium metasilicate (SMS) solutions neutralized with hydrochloric acid, followed by silylation to give silylated SA from which the condensation and structure of SA in tetrahydrofuran (THF) were investigated. The degree of extraction markedly depends on SA and HCl concentrations. The condensation of 0.86 and 3.5 mol/L SA-THF solutions was followed by measuring the viscosity of the solutions at 0 and 20°C. The silylated SA was isolated as a distillate (LS) and a residue (HS) with low and high M?_ns (ca. 1300 and 3800) by vacuum distillation. The ratio of LS against HS decreased as a SA concentration in THF increased. In an aqueous solution, SA exists as lower molecular weight SAs compared with those in THF. SAs such as monomer, dimer, cyclic tri- and tetramer were the main components in a 0.1 mol/L aqueous solution. On the extraction with THF from an aqueous solution, SA was found to undergo condensations to form more polymerized SAs. From the THF solutions of SA concentration above 0.5 mol/L, the HS was obtained as a main component (73%) which was expected to have ladder-like structures. © 1992 John Wiley & Sons, Inc.     

Preparation of polysiloxanes from silicic acid. X. Preparation and properties of allyldimethylsilylated silicic acids

In order to obtain polysiloxanes capable of forming a film, the preparation and properties of allyldimethylsilylated silicic acid was investigated. The reaction in variable molar ratios of allyldimethylchlorosilane to silicic acid gave partially silylated silicic acids with different degrees of silylation. They provided not only fibrous polysiloxanes but also films from concentrated polymer solutions. Spinnability and film formation depended on the solvent, degree of silylation, molecular weight, and allyl group.     

Synthesis, Conformations and Inclusion Properties of Homocalix[3]arenes

A new type of host is introduced: homocalix[3]arenes containing three aromatic units. Phane synthesis leading to molecules which may be termed, in the most general sense, homocalix[3]arenes, are outlined in a brief overview. The design, synthesis, conformations and host properties of homocalix[3]arenes are described in detail.     

Cyclic aromatic oligoamides as highly selective receptors for the guanidinium ion

A class of six-residue, shape-persistent aromatic oligoamide macrocycles bind the guanidinium ion with very high selectivity.     

Through-space electronic interactions of [2.2]metacyclophane benzyl cations

The hydrolysis of 8-bromomethyl[2.2]metacyclophanes 3 to the corresponding 8-hydroxymethyl derivatives 4 was carried out in 83% aqueous dioxane solution at 25°C. Substituent effect through space on the rate of the hydrolysis of bromomethyl groups attached on the opposite aromatic ring was first found in this investigation. Interestingly, the introduction of the substituents at the internal position 16 tends to enhance the hydrolysis reaction rate 10–100 times. It was found also that the stabilization by both the direct through-space cation-π-interaction and the interaction through the intra-annular 8,16-position are possible in the [2.2]metacyclophane 8-benzyl cations. The good correlation with log(K/K_H) and σ_p ⁺ was observed for the hydrolysis of internally unsubstituted 5-bromomethyl[2.2]MCPs 7, in which the direct through-space cation-π-interactions are not possible. TiCl₄ and Nafion-H, a perfluorinated resinsulfonic acid, catalysed Friedel-Crafts benzylation of benzene and substituted benzenes with 8-bromomethyl- and 8-hydroxymethyl[2.2]metacyclophanes to afford 8-benzyl[2.2]metacyclophanes is described. A high substrate and positional selectivity were observed in the present benzylation reaction quite different from those obtained from the benzyl bromide and benzyl alcohol. The benzyl cation intermediate stabilized by the through-space electronic interaction among the opposite benzene ring was first demonstrated in the benzylation of [2.2]metacyclophane systems. The mild and selective transannular reaction attributable to the highly strained character of [2.2]metacyclophane skeleton and the increased stabilization of the 5-benzyl cation intermediate arising from the electronic interactions among the opposite benzene ring through the intra-annular 8,16-positions was also observed.     

"N-acyliminium ion pool" as a heterodiene in [4 + 2] cycloaddition reaction

[reaction: see text] An N-acyliminium ion pool was found to undergo cycloaddition reaction with a variety of dienophiles such as alkenes and alkynes. A concerted mechanism seems to be most likely for alkyl-substituted alkenes as suggested by the DFT calculations, whereas a stepwise mechanism plays the major role for aryl-substituted alkenes. It is also noteworthy that the present study demonstrates the potential of the combination of the cation pool method and the micromixing in both mechanistic and synthetic aspects.     

2,3-di-O-methoxymethyl-6-O-tert-butyldimethylsilyl-gamma-cyclodextrin: a new class of cyclodextrin derivatives for gas chromatographic separation of enantiomers

Octakis(2,3-di-O-methoxymethyl-6-O-tert-butyldimethylsilyl)-gamma-cyclodextrin (2,3-MOM-6-TBDMS-gamma-CD) was employed as stationary phase for capillary gas chromatographic separation of enantiomers. Selective introduction of the acetal function at positions 2 and 3 of the glucose units was achieved by reaction of 6-O-TBDMS-gamma-cyclodextrin with methoxymethyl chloride. 2,3-MOM-6-TBDMS-gamma-CD was shown to be a chiral stationary phase suitable for enantiodifferentiation of a broad spectrum of chiral volatiles from various chemical classes. A total of 125 pairs of enantiomers could be separated. Structural influences of the analytes on the enantioseparation were demonstrated. High alpha values up to 1.8 were observed for the hydroxyketone acetoin and some methyl branched ketones. Pronounced enantioseparations were also determined for cyclic pentenolone and furanone derivatives.