Metal triflates immobilization

Ionic liquids in combination with supercritical fluids are a versatile tool for the immobilization and recycling of homogeneous catalysts, allowing continuous Friedel-Crafts acylation reactions to be realized. The acylation of anisole with acetic anhydride is carried out in a flow system using a metal triflate immobilized in the ionic liquid 1 -butyl-4-methylpyridinium bis(trifluoro-methylsulfonyl)imide as catalyst and scCO as continuous extraction phase [22]. Different metal triflates are utilized under continuous flow conditions using high pressure yttrium triflate possesses the best balance between sufficient acidity for catalytic activity and softness to release the product and so permits a good catalyst reuse (TONs up to 190). 

Rare earth metal triflates have been employed as mild Lewis acid in many organic transformations including Mukaiyama-aldol, Michael addition, Diels-Alder, aziridination, oxidation, reduction, rearrangement, and protection/deprotection reactions [102]. Several methods for the immobilization of these Lewis acids have been developed [102, 103]. Janda and coworkers developed a new method for the immobilization of Yb(OTf)3 on the cross-linked pyridine/styrene copolymers [104]. Since rare earth metal triflates are known to be catalytically active in the presence of bases, the cross-linked pyridine/styrene polymers could serve as suitable... 

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. 

Alkylations are Friedel-Crafts-type electrophilic reactions requiring the participation of a strong Lewis catalyst. Obviously they were carried out in the presence of hazardous Lewis acid catalysts as metal halides. Switching from these molecular compounds to solid embedded triflates led to catalysts on which such reactions occurred with turnover frequency (TOFs) having at least one order of magnitude higher than on classic acid solid catalysts as zeolites [100,101]. Examples of such reactions are alkylations of phenols or naphthols occurring on immobilized La, Ag, or r rr-butyldimethylsilyl-trifluoromethane-sulfonate catalysts. 

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