Recycling without supports

Reports on homogeneous catalysis did not underline the recyclability of the catalysts. The supported catalysts were recycled without any significant loss of activity and selectivity Table 4 presents the recycling results for the first 3 runs using the substrate la in the presence of both catalysts. The supported catalytic system can easily be separated by centrifugation under ambient conditions without any additional treatment. 

Ishii and co-workers [109] reported the aerobic oxidation of various organic compounds catalyzed by (NH4)5H6[PV8Mo4O40] supported on active carbon. The catalyst showed high activity for oxidative dehydrogenation of various benzylic and allylic alcohols to give the corresponding carbonyl compounds in moderate to high yields. The catalyst can be recycled without loss of activity for the... 

A partially soluble polyallylscandium triflamide ditriflate 45 was prepared and used to catalyze a three-component coupling reaction.67 An aldehyde, an aromatic amine, and an alkene were mixed in the presence of the catalyst to afford tetrahydroquinolines (equation 17). The catalyst was recovered from the reaction mixtures by precipitation with hexane and could be recycled without loss of activity. Another polymer-supported scandium catalyst was prepared by treating Nafion with scandium chloride to afford the Nafion-scandium catalyst 46.68 This catalyst was used in allylation reactions of carbonyl compounds by tetraallyltin (equation 18). It could be easily recovered by filtration and reused without appreciable loss of activity. 

The development of supported catalysts has permitted the elimination of the expensive catalyst removal stage. Therefore, in slurry processes taking advantage of highly active catalysts, the diluent is recovered after centrifugation and recycled without purification (Figure 3.53) [51]. 

Scandium triflate-catalyzed aldol reactions of silyl enol ethers with aldehyde were successfully carried out in micellar systems and encapsulating systems. While the reactions proceeded sluggishly in water alone, strong enhancement of the reactivity was observed in the presence of a small amount of a surfactant. The effect of surfactant was attributed to the stabiMzation of enol silyl ether by it. Versatile carbon-carbon bondforming reactions proceeded in water without using any organic solvents. Cross-linked Sc-containing dendrimers were also found to be effective and the catalyst can be readily recycled without any appreciable loss of catalytic activity.Aldol reaction of 1-phenyl-l-(trimethylsilyloxy) ethylene and benzaldehyde was also conducted in a gel medium of fluoroalkyl end-capped 2-acrylamido-2-methylpropanesulfonic acid polymer. A nanostmctured, polymer-supported Sc(III) catalyst (NP-Sc) functions in water at ambient temperature and can be efficiently recycled. It also affords stereoselectivities different from isotropic solution and solid-state scandium catalysts in Mukaiyama aldol and Mannich-type reactions. 

Aral et al. investigated Heck coupling of iodobenzene with methyl acrylate by heterogeneous palladium catalysts based on silica or charcoal as a support. Under reaction conditions a substantial portion of the palladium was already present in the liquid phase in the early stages of the reaction, and activities correlated with the palladium concentration in solution [51, 52]. The presence of only base or olefin did not result in detectable palladium concentrations. By comparison to silica, charcoal had a stronger propensity for readsorbing palladium from solution. Under appropriate conditions, a catalyst could be recycled without loss in activity [53]. 

Moreover, an MGM-41 mesoporous silicas-supported water-sol TPPTS-Rh complex in the IL can also be used for the catalyst recycling without reducing the activity and selectivity in the hydroformylation of 1-hexene. ... 

Recyclable polymer-supported Ag(I)-NHC complexes XLin [104] and XLIV [105] have also been reported allowing to perform the reaction at room temperature (with or without solvent), albeit with long reaction times. 

Novel heterogeneous catalysts containing a palladium complex anchored on meso-porous supports for hydrocarboxylation of aryl olefins and alcohols were found to give high regioselectivity, activity, and recyclability without leaching of palladium complex from the supports. In styrene hydrocarboxylation at 115 °C and 31 bar CO pressure, 2-phenyl-propionic acid is formed with 99% selectivity at 2600 mol styrene mol palladium h turnover frequency [137]. 

Handy and Okello prepared ionic liquid-supported stable and recyclable hypervalent iodine(III) reagents 19 and 20 (Figure 6) and used in the preparation of a-fxmctionalized ketones [8]. The by-product supported monovalent iodine could be easily separated from the product and recycled without compromising the product yield. In 2006, Zhang et al. reported an environmentally benign ionic liquid-supported iodine(III) reagent 21 (Figure 6) which was foxmd to be quite effective in the selective oxidation of primary alcohols [9]. 

MCM-41-supported metal (Yb, Zn) bis[(perfluoroalkyl)sulfonyl]imides were reported as effective catalysts for nitration of aromatic compounds with 1 eq. of 65 wt% nitric acid in the liquid phase Equation (8.68). The enhanced electron-drawing and steric effects from longer perfluori-nated alkyl chains were found to promote this action. Water exhibits also a positive effect. The catalysts were recycled without substantial loss of catalytic activity [103]. 

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