Recycling with supports

A large membrane pack of this type will act like an artificial gill, permitting a swimmer to breathe like a fish and remain submerged for much longer periods of time than are possible with scuba equipment. Speculative fiction has man returning to live in the seas, and this type of application may make it possible. Their application in spacecraft is obvious as a part of a continuously recycled air support system. The oxygen permeability of silicone materials is just one example of the selective permeability of plastics. 

Harmer et al.196 used 1,1,2,2-tetrafluoroethanesulfonic acid in the alkylation of para-xylene with 1-dodecene. The silica-embedded catalyst prepared by the sol-gel method showed much higher activity than the neat acid (almost complete conversion in 15 min at 100°C over the sol-gel-derived material versus 10% conversion, using the same molar amounts of acid). Practically no leaching was detected and the catalyst could be recycled with a slight decrease in conversion. It is in sharp contrast with silica-supported triflic acid, which showed much lower activity due to the loss of volatile triflic acid. 

Fig. 6. Polymer-supported IBX (resin 5) can be activated and recycled with monoperoxy sulfonic acid (Caro s acid). The polymer reagent is capable of alcohol oxidations, dehydrogenations, and radical cyclization reactions.

In addition, minor variation of the catalyst in combination with immobilization on a resin support gave an analogous recyclable solid-supported organocatalyst. Varying the derivatization method by trapping the a-amino nitrile intermediate with formic acid and acetic anhydride gives the crystalline formamides 19 in excellent yield and with high enantioselectivity. These features of this catalytic process have been demonstrated by results from the synthesis of r-tert-leucine (Scheme 14.8) [49]. 

Catalyst recycling is an important subject, especially when dealing with supported catalyst. Heterogeneous catalysts tend to lose activity when used in consecutive reactions. In case of the immobilised Ils, the reason can be either leaching of the active phase, deposits on the surface (adsorption on the acid sites) or the deterioration of the catalyst due to water adsorption. Leaching of the IL from the surface of the support was studied for the catalysts mentioned and it was found to be negligible (Table 3). 

Very recently, Pericas reported a new strategy to immobihze trans-4-hydroxypro-line onto an insoluble Merrifield-type polymer by exploiting Cu(I)-catalyzed 1,3-dipolar cycloaddition ( click chemistry ) [42]. The supported catalyst 25 was successfully employed in the a-aminoxylation of ketones and aldehydes (Scheme 8.13). Under the optimized reaction conditions (20mol%/cat, 2 equiv. ketone, DMF, 23 °C, 3 h), the reaction of cyclohexanone with nitrosobenzene catalyzed by 25 gave the product in 60% yield and 98% ee (Scheme 8.13 Equation a). It should be noted that the reaction rates of cyclic ketones with supported catalyst are faster than those reported with (S)-proline. The use of a supported catalyst allowed for a simplification of the work-up procedure, as the product could often be obtained after simple filtration of the catalyst and evaporation of the solvents. Furthermore, 25 was recycled up to three times without any decrease in either the chemical and/or stereochemical efficiency. 

During the past 20 years, solid-supported organic catalysts have become powerful synthetic tools readily available to the chemical community. The reasons for developing an immobihzed version of a chiral catalyst go weU beyond the simple-yet still fundamental-aspect of the recovery and recycling of the precious catalytic species. Catalyst stabihty, structural characterization, catalytic behavior, new or different solubihty properties, simphfication of the reaction work-up, catalyst discovery and optimization, use in environmentally friendly or green solvents are all issues that may be converhently addressed working with supported systems. 

Nevertheless, with the exception of uses in ROMP processes, only a limited number of industrial processes use olefin metathesis. This is mainly due to difficulties associated with removing ruthenium from the final products and recycling the catalyst. To tackle these problems, there is tremendous activity in this area, dealing with supported or tagged versions of homogeneous catalysts. 

TEMPO, polystyrene-supported TEMPO, sol-gel TEMPO, PEG-TEMPO, supported oxammoium salts, etc.). These catalysts have shown good activity and selectivity, and were also readily recyclable. The aerobic oxidation of alcohols with supported TEMPO employing oxygen as co-oxidant has been explored [19k, 191, 20], either in the presence of Co(N03)2 and Mn(N03)2 (Minisci s conditions) or in the presence of Cu(II), giving interesting results. 

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