A Novel Borosilicate Cage Compound with an Incomplete B(4)Si(4) Cube Structure:

Silane-Bearing Borasilsesquioxanes: Synthetic Protocol and Unsuspected Redistribution Reactions

The presented work is a continuation of research on the reactivity of unsaturated borasilsesquioxanes under the conditions of common organometallic transformations. The catalytic hydrosilylation reaction with silanes, siloxanes and silsesquioxanes in presence of platinum catalyst was explored. The majority of the products were obtained in high yields (>90 %) and their structures were confirmed and characterized by spectroscopy and spectrometry, i. e. NMR and MALDI-TOF-MS. The most significant segment of the work is research on the spontaneous redistribution reaction of the alkoxy group from silane to borane moiety occurring in the obtained products, not being limited to the heterosilsesquioxane chemistry, however. The products were confirmed using GC-MS, ESI-MS methods and B3LYP exchange-correlation, in order to ascertain formation of the silicon-boron hybrid molecule.

Second-order nonlinear optical properties of eight-membered centrosymmetric cyclic borasiloxanes

SHG efficiencies of eight-membered centrosymmetric cyclic borasiloxanes arise form distorted silicon atom, strong electron withdrawing group and non-covalent interactions in crystal packing.

Synthesis of borasiloxanes by oxidative hydrolysis of silanes and pinacolborane using Cu3(BTC)2 as a solid catalyst

This work reports the synthesis of borasiloxanes from silanes and pinacolboranes using Cu₃(BTC)₂ as a heterogeneous catalyst in acetonitrile under mild conditions.

Ruthenium-catalysed multicomponent synthesis of borasiloxanes

Ruthenium-catalysed multicomponent synthesis of borasiloxanes directly from silanes, boranes and water is reported.

A convenient and clean synthetic method for borasiloxanes by Pd-catalysed reaction of silanols with diborons

Pd-Catalysed O-borylation of silanols with diborons selectively proceeded to give the corresponding boryl silyl ethers in high yields.

Dioxygen-promoted regioselective oxidative heck arylations of electron-rich olefins with arylboronic acids

Arylations of electron-rich heteroatom-substituted olefins were performed with arylboronic acids. This appears to constitute the first example of palladium(II)-catalyzed internal Heck arylations. The novel protocol exploits oxygen gas for environmentally benign reoxidation and a stable 1,10-phenanthroline bidentate ligand to promote the palladium(II) regeneration and to control the regioselectivity. Internal arylation is strongly favored with electron-rich arylboronic acids. DFT calculations support a charge-driven selectivity rationale, where phenyls substituted with electron-donating groups prefer the electron-poor alpha-carbon of the olefin. Experiments, verified by calculations, confirm the cationic nature of the catalytic route. This Heck methodology provides a facile and mild access to functionalized enamides. Controlled microwave heating and increased oxygen pressure were used to further reduce the reaction time to 1 h.

Microwave-Promoted and Chelation-Controlled Double Arylations of Terminal Olefinic Carbon of Vinyl Ethers

Herein we report a rapid, palladium-catalyzed terminal diarylation of the chelating olefin N,N-dimethyl(2-ethenyloxy)ethanamine under noninert conditions utilizing controlled microwave heating as a convenient energy source. Among the aryl bromides examined, both electron-rich and electron-poor substrates were demonstrated to furnish useful yields after only 10-120 min of directed microwave heating at 160-200 degrees C. The good terminal regioselectivity suggests that the precatalyst (Herrmann's palladacycle) serves as a source of weakly coordinated palladium(0) in the investigated high-temperature Heck process.

Molybdenum-catalyzed asymmetric allylic alkylations

The highly regio- and enantioselective molybdenum-catalyzed allylic alkylation reaction has become a powerful synthetic tool during the past few years. This Account describes the achievements gained so far in the area, with special attention directed to the different chiral ligands that have been used for inducing chirality in the products, the range of allylic substrates and nucleophiles employed, mechanistic studies, and applications of the reaction in asymmetric syntheses.

Microwave-Assisted Ring-Closing Metathesis Revisited. On the Question of the Nonthermal Microwave Effect

The ring-closing metathesis reactions (RCM) of six standard diene substrates leading to five-, six-, or seven-membered carbo- or heterocycles were investigated under controlled microwave irradiation. RCM protocols were performed with standard Grubbs type II and a cationic ruthenium allenylidene catalyst in neat and ionic liquid-doped methylene chloride under sealed vessel conditions. Very rapid conversions (15 s) were achieved utilizing 0.5 mol % Grubbs II catalyst under microwave conditions. Careful comparison studies indicate that the observed rate enhancements are not the result of a nonthermal microwave effect.

High-speed microwave-promoted hetero-Diels-Alder reactions of 2(1H)-pyrazinones in ionic liquid doped solvents

Inter- and intramolecular hetero-Diels-Alder reactions in a series of functionalized 2(1H)-pyrazinones were investigated under controlled microwave irradiation. The cycloaddition reactions were efficiently performed in sealed tubes, utilizing either a combination of 1,2-dichloroethane and a thermally stable ionic liquid, or 1,2-dichlorobenzene as reaction medium. In all cases, a significant rate-enhancement using microwave flash heating as compared to thermal heating was observed.

Microwave-accelerated homogeneous catalysis in organic chemistry

The efficiency of microwave flash heating in accelerating organic transformations (reaction times reduced from days and hours to minutes and seconds) has recently been proven in several different fields of organic chemistry. This specific account mainly summarizes our own experiences in developing rapid, robust, and selective microwave-assisted transition metal-catalyzed homogeneous reactions. Applications include selective Heck couplings, cross-couplings, and asymmetric substitutions. The science of green chemistry was developed to meet the increasing demand for environmentally benign chemical processes. We believe the combination of metal catalysis and microwave heating will be of importance in the search for green laboratory-scale synthesis.

Microwave-accelerated O-alkylation of carboxylic acids with O-alkylisoureas

[reaction: see text] Microwave-assisted O-alkylations of several carboxylic acids have been performed with three different O-alkylisoureas. All reactions are significantly faster compared to conventionally heated reactions, while retaining high chemoselectivity. The combination of microwave technology with the use of the solid-supported isourea 3 enables the synthetic chemist to obtain the pure methyl esters starting from the corresponding acids in less than an hour.

Solvent-dependent dynamic kinetic asymmetric transformation/kinetic resolution in molybdenum-catalyzed asymmetric allylic alkylations

Catalytic asymmetric alkylation reactions of branched racemic carbonates 1a and 1b with sodium dimethyl malonate, promoted by molybdenum and ligand 5, proceed by a kinetic resolution in toluene, THF, tetrahydropyran, i-PrOAc, 1,2-dichloroethane, and MeCN with k(rel) of 7-16. In THF, MeCN, tetrahydropyran, and i-PrOAc using the (S,S)-5 ligand, the fast reacting (S)-carbonate enantiomer provides the branched product with high ee (97-99.5%) and branched/linear selectivity, but the ee erodes as the reaction of the slow-reacting (R)-enantiomer takes place. This implies that the rate of equilibration of the oxidative addition complexes in these solvents is competitive with the subsequent malonate displacement step. In toluene and dichloroethane, the ee and branched/linear ratios diminish during the reaction of the slow-reacting (R)-isomer, but not nearly as much as in the other solvents. This is most likely due to either an increase in the rate of equilibration of the oxidative addition complexes relative to the malonate displacement step, or vice versa. Because of the minimal stereochemical memory effect in toluene and 1,2-dichloroethane, the reactions in these solvents can be carried to completion (dynamic kinetic asymmetric transformation) and still provide product with excellent ee (>95%). The anion of dimethyl methylmalonate also reacts via a kinetic resolution, although the ee's, rates, and k(rel) values differ from those of the reactions with dimethyl malonate.

4-Bromophenylboronate Derivatives of Ring and Cage Borosilicates

The reactions between equimolecular amounts of 4-bromophenylboronic acid and di-tert-butylsilanediol or tert-butylsilanetriol in toluene solutions at reflux temperatures afforded the borosilicate ring compound [(4-BrC6H4B)O2Si(t-Bu)2]2 (1), and the cube-based species [(t-BuSi)4(4-BrC6H4B)4O10] (2), in medium (62%) and low (13%) yields, respectively. These compounds were characterised by microanalysis, IR and NMR (1H, 11B, 13C and 29Si) spectroscopy and single crystal X-ray crystallography. During the work-up of 2, a sample of the boroxine, [(4-BrC6H4BO)3] (3), was isolated and was characterised by single crystal diffraction.

Aqueous DMF-potassium carbonate as a substitute for thallium and silver additives in the palladium-catalyzed conversion of aryl bromides to acetyl arenes

Highly selective palladium-catalyzed internal alpha-arylations of alkyl vinyl ethers with aryl and heteroaryl bromides were conveniently conducted in aqueous DMF with potassium carbonate as base and with DPPP as bidentate ligand. The corresponding acetyl arene products were, after hydrolysis, isolated in good to excellent yields. This Heck reaction procedure does not require toxic thallium or expensive silver salt additives, is promoted by water, and is suggested to proceed via charged organopalladium intermediates. Single-mode microwave irradiation was utilized in one example to shorten the reaction time.