Metal triflates stability

Unfortunately ZnCl2/Si02 cannot be recycled after reaction, due to irreversible catalyst deactivation which is believed to occur via hydrolysis of the Zn-Cl bond. Indeed chlorinated campholenic aldehyde was observed to form during the reaction, which would occur through reaction with HC1 from hydrolysed ZnCl2. It was anticipated that the enhanced water stability of the metal triflate would thus be advantageous for this reaction. 

The thermal stability of some thermosetting nanocomposites obtained by thermal cationic cure of diglycidilether of bisphenol A and y-valerolactone using rare earth metal triflates (trifluoromethanesulfonate) as initiators and containing different types of Cloisite was studied. The addition of clay into the polymeric matrix was found to increase the thermal stability, acting as a superior insulator and mass transport barrier to the volatile products evolved during thermal decomposition [67]. 

Lanthanum is the first element of the sixth-period transition metals. Its properties are close to those of the other rare earth elements and, as a consequence, the catalytic behavior of the lanthanum triflate is also very close to that of the entire series. An important example concerning the stability of these compounds in water is that indicating the capability of metal triflates, such as Yb(OTf)3, Eu(OTf)3, Sc(OTf)3 or La(OTf)3, to catalyze the hydration of alkynes to the corresponding ketones [56]. However, the interest in water-compatible lanthanide triflate-based catalysts is much larger and mainly includes the carbon-carbon bond-forming reactions. To increase the utility of these catalysts understanding of their aqueous mechanism is very important. For such a purpose, dynamic measurements of the water... 

Examples presented in the previous chapters demonstrate the stability of metal triflates and the possibility to preserve their activity when working in air or in water solvent. However, working with dispersed materials in solvents, in batch reactors at a larger scale, the goal of separation and recycling of the catalyst is not simple. Therefore, to achieve an industrial and economical catalyst development, the immobilization of the highly dispersed materials on easily separable solid supports may afford improved recycling and facile use in synthetic procedures. 

The evolution from homogeneous to heterogeneous catalysts implies in most cases the incorporation of the active species on or inside different solids acting as supports. There are many families of solid supports such as metals, oxides, carbons, and polymers that can be functionalized with the active species. These materials may possess specific chemical and physical properties such as porosity, high surface area, mechanic resistance, and thermal stability. The largest family corresponds to oxides and also to zeolites, clays, and mesoporous materials. Dealing with the metal triflates, polymers and silica-based materials have been the most extensively used supports. 

Metal triflates elicited a large interest in the past three decades especially due to the Lewis acid catalyst properties. The very high stability of these compounds against water and air, in a close relation with their capability to activate various functional groups, made them useful in very diverse transformations. 

The rates, selectivities, and stabilities of transition-metal-substituted poly-oxometalates in olefin epoxidation are compared with those of metallopor-phyrins, Schiff base complexes, and triflate salts, as follows (320b) ... 

Exploration of the reactivity of cyclohexene silacyclopropane led Woerpel and coworkers to discover that the inclusion of metal salts enabled silylene transfer to monosubstituted olefins at reduced temperatures (Table 7.1).11,74 A dramatic reduction in the temperature of transfer was observed when cyclohexene silacyclopropane was exposed to copper, silver, or gold salts. Silver salts were particularly effective at decomposing 58 (entries 6-11). The use of substoichiometric quantities of silver triflate enabled ra-hexene silacyclopropane 61 to be formed quantitatively at —27°C (entry 6). The identity of the counterion did affect the reactivity of the silver salt. In general, better conversions were observed when noncoordinating anions were employed. While the reactivity differences could be attributed to the solubility of the silver salt in toluene, spectroscopic experiments suggested that the anion played a larger role in stabilizing the silylenoid intermediate. 

Especially for alkyl halides 6 the transfer of a single electron from the metal center is facile and occurs at the halide via transition state 6C, which stabilizes either by direct abstraction of the halide to a carbon-metal complex radical pair 6D or via a distinct radical anion-metal complex pair 6E. This process was noted early but not exploited until recently (review [45]). Alkyl tosylates or triflates are not easily reduced by SET, and thus Sn2 and/or oxidative addition pathways are common. The generation of cr-radicals from aryl and vinyl halides has been observed, but is rarer due to the energy requirement for their generation. Normally, two-electron oxidative addition prevails. 

The use of aryl tosylates as electrophiles is attractive, since they can be synthesized from readily available phenols with less expensive reagents than those required for the preparation of the corresponding triflates. More importantly, tosylates are more stable towards hydrolysis than are triflates. However, this greater stability renders tosylates less reactive in transition metal-catalyzed coupling reactions. As a result, protocols for traditional cross-coupling reactions of these electrophiles were only recently developed [1], In contrast, catalytic direct arylations with aryl tosylates were not reported previously. However, a ruthenium complex derived from heteroatom substituted secondary phosphine oxide (HASPO) preligand 72 [81] allowed for direct arylations with both electron-deficient, as well... 

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