In-situ generation of silica

Mus Musto, P., Ragosta, G., Scarinzi, G., Mascia, L. Toughness enhancement of polyimides by in situ generation of silica particles. Polymer 45 (2004) 4265—4274. 

Several heterogeneous catalysts have been developed for the hydroxylation of alkanes under mild conditions.68,69 One of them is the bi-catalytic system, which combines the ability of palladium to convert hydrogen and oxygen to hydrogen peroxide, with the capability of the iron ions to activate the hydrogen peroxide to hydroxylate hydrocarbons.70 Iron oxide and palladium supported on silica have been used as efficient catalysts for the oxidation of cyclohexane to the alcohol and ketone, via the in situ generation of hydrogen peroxide in an acetone solvent.71... 

In situ generation of Pd-NHC complexes in an imidazolium-type ionic liquid matrix and grafting of the catalyst on the surface of silica was reported by Karimi and Enders [41]. Pal-ladium(II) anchored on an ionic Uquid matrix, derived from A-3-(3-trimethoxysilylpropyl)-3-methyl imidazolium chloride, displays high thermal stabihty up to 280 °C. Catalyst 8... 

As the immobilization requires high temperatures (T = 180 C), which would cause catalyst decomposition, the preligand system was permanently bonded onto the glass surface of the fused-silica capillary via Si-O-Si bonds before the in situ generation of the catalytically active system. Therefore, it was necessary to convert the permanently bonded diamine to the permanently bonded polymeric 2-(pentafluorophenyl)imidazolidine by using pentafluorobenzaldehyde [40]. 

Another approach for the in situ generation of aldehydes in the presence of stabilized phosphonium ylides is the oxidative cleavage of glycols using NaI04 on silica gel (Scheme 19) [99]. The simultaneous, one-pot oxidative cleavageAVittig reaction of carbohydrates and amino acid derivatives affords a number of synthetically useful alkenes with high E selectivity. 

Terminal and disubstituted internal alkenes react rather slowly with HC1 in nonpolar solvents. The rate is greatly accelerated in the presence of silica or alumina in noncoordinating solvents such as dichloromethane or chloroform. Preparatively convenient conditions have been developed in which HC1 is generated in situ from SOCl2 or (ClCO)2.2 These heterogeneous reaction systems also give a Markovnikov orientation. 

The practical applications of NaBH4 reductions on mineral surfaces for in situ generated SchifFs bases have been successfully demonstrated. The solid-state reductive amination of carbonyl compounds on various inorganic solid supports such as alumina, clay, silica etc. and especially on K 10 clay surface rapidly afford secondary and tertiary amines [126]. Clay behaves as a Lewis acid and also provides water from its interlayers thus enhancing the reducing ability of NaBH4 [22],... 

A final analogous situation occurs in the in-situ silica generation of silica particles within a polymer network. At least in some cases, the particles are constrained in size by the network strands, in that the maximum particle size observed decreases with decrease in the molecular weight between cross-links (increase in extent of cross-linking) [262, 263]. 

The in situ regeneration of Pd(II) from Pd(0) should not be counted as being an easy process, and the appropriate solvents, reaction conditions, and oxidants should be selected to carry out smooth catalytic reactions. In many cases, an efficient catalytic cycle is not easy to achieve, and stoichiometric reactions are tolerable only for the synthesis of rather expensive organic compounds in limited quantities. This is a serious limitation of synthetic applications of oxidation reactions involving Pd(II). However it should be pointed out that some Pd(II)-promoted reactions have been developed as commercial processes, in which supported Pd catalysts are used. For example, vinyl acetate, allyl acetate and 1,4-diacetoxy-2-butene are commercially produced by oxidative acetoxylation of ethylene, propylene and butadiene in gas or liquid phases using Pd supported on silica. It is likely that Pd(OAc)2 is generated on the surface of the catalyst by the oxidation of Pd with AcOH and 02, and reacts with alkenes. 

These cobaloximes (1) are generated by reaction of alkyl halides with in situ generated Co (dmgH)2py. They are orange crystals that are relatively stable to air and silica gel, but on photolysis they generate R-, which can be trapped in generally high yield or which cyclize in the case of alkenyl radicals.1... 

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