Electrophilic silica-supported

In the case of methyloxirane, however, on Pt and Pd catalysts the extent of the rupture of the sterically hindered bond is indicative of the electrophilic character of the catalyst. Unsupported or silica-supported ion-exchanged catalysts cleave the sterically less hindered bond, whereas on the impregnated catalysts, the rupture of the more hindered C-O bond is dominant.290 It is likely that Pt or Pd surface metal ions are responsible for the rupture of the sterically more hindered bond and residual chlorine from the catalyst preparation can stabilize these ions in the hydrogen atmosphere. 

Both heterogeneous and homogeneous CO reduction catalyst recipes often contain electrophilic components such as silica supports, metal oxides, and A1Cl3 [1,5,33,34,35,36]. 

Alkane metathesis was first reported in 1997 [84]. Acyclic alkanes, with the exception of methane, in contact with a silica supported tantalum hydride ](=SiO)2TaH] were transformed into their lower and higher homologues (for instance, ethane was transformed into methane and propane). Later, the reverse reaction was also reported [85]. Taking into accountthe high electrophilic character ofa tantalum(III) species, two mechanistic hypotheses were then envisaged (i) successive oxidative addition/reductive elimination steps and (ii) o-bond metathesis. Further work has shown that aLkyhdene hydrides are critical intermediates, and that carbon-carbon... 

Non-metallic chemically modified solids have also been developed for liquid-phase catalytic applications. Silica-supported guanidinium chloride, for example, has been shown to have high efficiency in the decomposition of methyl chloroformate (into CH3CI and C02) and electrophilic reactions of carboxylic acids and epoxides.169... 

Analogously, in the presence of silica-supported palladium catalysts, benzene is oxidized under ambient conditions to give phenol, benzoquinone, hydroquinone and catechol [37b]. Palladium chloride, used for the catalyst preparation, is believed to be converted into metallic palladium. The synthesis of phenol from benzene and molecular oxygen via direct activation of a C-H bond by the catalytic system Pd(OAc)2-phenanthroline in the presence of carbon monoxide has been described [38]. The proposed mechanism includes the electrophilic attack of benzene by an active palladium-containing species to to produce a a-phenyl complex of palladium(ll). Subsequent activation of dioxygen by the Pd-phen-CO complex to form a Pd-OPh complex and its reaction with acetic acid yields phenol. The oxidation of propenoidic phenols by molecular oxygen is catalyzed by [A,A"-bis(salicylidene)ethane-l,2-diaminato]cobalt(ll)[Co(salen)] [39]. 

The electrophilic reactivity of silicon in (bromomethyl)chloro-dimethylsilane has also served in a large number of applications concerning the surface modification of various silica gel or silicates derivatives. Indeed, haloaUcyl-activated silica supports are promising for the preparation of stationary phases (sorhents) with high loadings of immohilized ligands. ... 

Due to the ability of imidazolium compounds to form metallic Af-heterocyclic carbene complexes, imidazolium-based ionosilicas have widely been studied for the formation of silica-supported NHC species and found wide applications in organometallic catalysis. However, ionic species recently found to promote a large variety of reactions due to their ionic nature. Cooperative nucleophilic-electrophilic activation is a widely accepted concept in catalysis, and due to their ionic nature, ionic liquid should be considered as bifunctional catalysts. 

Earlier transition metals, as zirconium and hafnium, are still more active in hydrogenolysis, which allows zirconium hydrides to be used in depolymerization reactions (hydrogenolysis of polyethylene and polypropylene) [89], In this case, the zirconium hydride was supported on silica-alumina. Aluminum hydrides close to [(=SiO)3ZrH] sites would increase their electrophilicity and, thus, their catalytic activity. A catalyst prepared in this way was able to convert low-density polyethylene (MW 125000) into saturated oligomers (after 5h) or lower alkanes at 150°C (100% conversion). It was also able to cleave commercial isotactic polypropylene (MW 250000) under hydrogen at about 190 °C (40% of the starting polypropylene was converted into lower alkanes after 15 h of reaction). 

Silica gel is also the support of choice for the activation of V-halosuccinimides. The silica functions both as a proton donor which increases the electrophilic nature of the reagent, and as a support with geometrical constraints which contributes to the stereoselectivity. Alkyl and aryl sulfoxides are readily halogenated at the a position with yields of48-80%91. The reactions are carried in the solid state on the surface of TLC plates. The conversion of the optically active alkyl 4-methylphenyl sulfoxide into 1-haloalkyl 4-methylphenyl sulfoxide is accompanied by inversion of configuration at the S-atom. The stereoselectivity in these reactions is much higher than that observed in liquid-phase halogenation. 

CuCl2 supported on silica (in presence of KC1, LaCl3 or A1C13 as cocatalysts) is also an active catalyst in oxychlorination of ethylene and other hydrocarbons104. Silica is also the support of choice for a Rh(III) complex which has been discovered to activate methane for chlorination via an electrophilic mechanism105. 

As for many other nucleophiles, the nitrite anion undergoes addition to the iodonium ion generated by the reaction of alkenes and 1,3-alkadienes with electrophilic iodine reagents. Two procedures have been described bis(pyridine)iodine(I) tetrafluoroborate136,137 [prepared from mcrcury(II) oxide and tctrafluoroboric acid supported on silica gel and pyridine on dichloromethane] and copper(II) tetrafluoroborate [prepared from copper(II) oxide and te-trafluoroboric acid] and iodine138 139. trans Addition would be expected for all products from mechanistic considerations, however, only the cyclohexene adduct 1 has been shown to have trans configuration ( H-NMR spectroscopy)139. 

The difficulty of monobrominating benzimidazole and its 1-substituted derivatives mirrors the state of affairs with the uncondensed imidazoles. Electrophilic bromination occurs at first in the 5-position, then at C-7, but excess brominating agent often substitutes all available positions on the fused benzene ring [23]. It has been found, though, that NBS supported on silica gel forms the 2-bromobenzimidazole (67%) in the first instance [32]. The same compound can also be made from 2-benzimidazolone, and it should be readily available via the 2-anion formed by reaction of an Al-protected benzimidazole with LDA, n-butyllithium or t-butyllithium. Hydroxymethyl and A -(dialkylamino)methyl protecting groups would appear to be the best choices [24, 25]. 

In benzimidazoles, the least susceptible position to electrophilic halogenation is C-2. An exception is the use of NBS supported on silica gel which 2-brominates benzimidazole, perhaps because the support holds the 2-position close to its surface in proximity to the activated NBS <86TL1051>. Anionic benzimidazoles can be iodinated in the 2-position <90JHC673>. 

Gallium(lll) oxide supported on MCM-41 mesoporous silica shows high catalytic activity with little or no moisture sensitivity in the acylation of aromatics wifh acyl chlorides. The cafalysf is utilized in 1,2-dichloro-ethane af 80°C for 3 h wifh differenf aromatic compounds, and aromatic as well as aliphatic acyl chlorides, giving ketones in 54%-82% yield. The activity order of fhe aromatic subsfrafes is benzene (43% yield) < toluene (50% yield) < mesifylene (71% yield) < anisole (79% yield), in agreement with the electrophilic substitution trend previously observed. This acylation reaction follows a probable redox mechanism similar to thaf described in Scheme 4.26. ... 

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