Olefin Metathesis in Confinement: Towards Covalent Organic Framework Scaffolds for Increased Macrocyclization Selectivity

Covalent organic frameworks (COFs) offer vast structural and chemical diversity enabling a wide and growing range of applications. While COFs are well-established as homogeneous catalysts, so far, their high and ordered porosity has scarcely been utilized to its full potential when it comes to spatially confined reactions in COF pores to alter the outcome of reactions. Here, we present a highly porous and crystalline, large-pore COF as catalytic support in diene ring-closing metathesis reactions, leading to increased macrocylization selectivity. COF pore-wall modification by immobilization of a Grubbs-Hoveyda-type catalyst via a mild silylation reaction provides a molecularly precise heterogeneous metathesis catalyst. An increased macro(mono)cyclization (MMC) selectivity over oligomerization (O) for the heterogeneous COF-catalyst (MMC:O = 1.30) of up to 44% compared to the homogeneous catalyst (MMC:O = 0.90) was observed along with a substrate-size dependency in selectivity, pointing to diffusion limitations induced by the pore confinement.

Emerging Contaminants in Aqueous Matrices Determined by Gas Chromatography-Mass Spectrometry

Reports on the determination of emerging contaminants (EC) in aqueous samples have been increasingly common. Due to the low levels of concentration of the analytes as well as the complexity of this matrix, the analysis is done preferably by liquid chromatography (LC). Owing to the polar character of most of the EC determination by gas chromatography is deprecated. One way to overcome this barrier is through derivatization, which, in some cases, can be a lengthy step, presents risks to the analyst as well as to the environment due to the toxicity of the derivatizing agent, and, thus, ends up favoring the use of LC. An analytical protocol was developed in this work to increase the efficiency of derivatization in a shorter reaction time for the determination of ibuprofen, 4-octylphenol, 4-nonylphenol, triclosan, bisphenol A, diclofenac, estrone, 17-β-estradiol, estriol, coprostanol, and cholesterol. The proposal then was to carry out the silylation reaction of the analytes with the aid of a domestic microwave oven. The results indicated that the use of the device provided an increase in the efficiency of the reaction, due to the homogeneous heating of the solution. Besides, there was a significant decrease in the derivatization time of the analytes from 30 min to 5 min. Additionally, through a design of experiments (DOE), it was possible to perceive the influence of some instrumental parameters of GC-MS, such as temperature, pressure intensity, and pressure pulse time in the injector on the detectability of the investigated analytes. This study allowed a satisfactory separation of the analytes and an average increase in their areas of up to 35%. These aspects made it possible to obtain an analytical method with limits for the detection and quantification of EC between 0.03-11.00 ng mL-1 and 0.10-33.35 ng mL-1, respectively, and uncertainties below 9%. The developed method was applied in the determination of the analytes in coastal seawater and the determined concentrations varied from 0.24 ng L-1 for estriol and 43.60 ng L-1 for cholesterol. Thus, the improvement of the silylation reaction, combined with the strategy of instrumental optimization, proved to be simple, efficient, and fast, as well as being a comparable alternative to liquid chromatography.

Nickel-catalyzed silylation reaction of alkyl aryl sulfoxides with silylzinc reagents

Nickel-catalyzed silylation of alkyl aryl sulfoxides with silylzinc reagents was carried out. This protocol allows the alkyl aryl sulfoxides to convert to arylsilicon compounds under mild reaction conditions and tolerates...

Intermolecular C−H silylation through cascade carbopalladation and vinylic to aryl 1,4-palladium migration

A palladium-catalyzed remote C–H silylation reaction has been developed through vinylic to aryl 1,4-palladium migration. By using alkyne-tethered aryl iodides as the starting materials and hexamethyldisilane as the silylating reagent,...

Influence of Reaction Parameters on the Gelation of Silanised Linseed Oil

The subject of this work was to characterize the catalytic course of the linseed oil silylation reaction with vinyltrimethoxysilane (VTMOS), carried out under elevated pressure and temperature conditions, and an explanation of the reasons for rapid gelation of the reaction product. To explain and describe the process, analytical methods were used, i.e., 1H and 13C NMR (nuclear magnetic resonance), GC-FID (gas chromatography coupled with flame ionisation detection), and GPC (gel permeation chromatography). Reaction products were monitored after 3, 6 and 12 h. The molar mass of the VTMOS-modified oil in only 3 h was comparable with the molar mass of the product obtained by conventional polymerisation. An increase in the reaction time resulted in further transformations resulting from the hydrolysis and condensation reactions taking place. In contrast to reactivity of soybean oil, the silanisation of linseed oil occurred much faster and without the need for cross-linking catalysts. The reason for the high reactivity of linseed oil to VTMOS and rapid gelation of the resulting product was primarily the amount of double bonds present in linseed oil and their high availability, in particular the double bond in the acid linolenic acid located at the C16 carbon.

PAPER STRAWS: AN INVESTIGATION INTO SURFACE MODIFICATION AND HYDROPHOBIZATION OF CELLULOSE

In order to improve the quality of paper straws, experiments involving the hydrophobization of paper, in a silylation reaction with chloro(dimethyl)octadecylsilane using various solvents, were conducted. The ImageJ program was used to quantify hydrophobicity by calculating the contact angle between a water droplet and a small piece of paper, which were compared between treatment groups as well as with untreated paper and plastic straws. Samples were exposed to a variety of liquids in one-hour periods for a total of six hours. After each hour, contact angle measurements were taken. Results suggested that hydrophobicity declines with time due to leaching of silanol from the treated paper. Contact angles between water droplets and the treated paper remained larger than that of untreated paper straws throughout testing, indicating higher hydrophobicity. Furthermore, samples that were silylated using dioxane as a solvent were better able to maintain hydrophobicity than samples silylated using toluene as a solvent.

Palladium-Catalyzed Silylation of Aryl Chlorides with Bulky Dialkoxydisilanes

Arylsilanes bearing a bulky alkoxy group on the silicon were synthesized from aryl chlorides and dialkoxydisilanes under reaction conditions utilizing SingaCycle-A3 as a palladium precatalyst and lithium benzoate in wet DMA. This report proposes the first direct and catalytic method for introducing tert-butoxy- or 1-adamantyloxysilyl groups onto various aryl moieties through the silylation reaction.

Pd-Catalyzed C–H Silylation Reactions with Disilanes

Pd-catalyzed C–H silylation reactions remain underdeveloped. General strategies usually rely on the use of complex bidentate directing groups. C,C-Palladacycles exhibit extremely high reactivity towards hexamethyldisilane and can be disilylated very efficiently. The C,C-palladacycles are prepared through halide-directed C–H activation. This account introduces Pd-catalyzed C–H silylation reactions with di­silanes as the silyl source, and is focused on studies on the silylation of C,C-palladacycles.1 Introduction and Background2 Allylic C–H Silylation Reaction3 Coordinating-Ligand-Directed C–H Silylation Reaction4 Disilylation of C(sp2),C(sp2)-Palladacycles That are Generated by C(sp2)–H activation5 Disilylation of C(sp2),C(sp3)-Palladacycles That are Generated by C(sp3)–H Activation6 Disilylation of C,C-Palladacycles That are Generated through Domino Processes7 Summary and Outlook