Bifunctional catalysts recyclable

Lewis acid and the oxygen atom of the phosphane oxide, respectively. With this catalyst system, N-allyl- and N-benzhydrylimines generally gave lower enantioselectivities. The addition of phenol was found to have a beneficial effect on the reaction rates. The JandaJEL -supported bifunctional catalyst of 14 has also been shown by Shibasaki and co-workers to promote the Strecker-type reaction of aromatic and a,/ -unsaturated imines in excellent yields with 83-87% ee in the presence of tert-butanol (110%) [11]. The reactivity of the Janda/EL catalyst was comparable to the homogeneous analogue 14, and the catalyst could be recycled at least four times. 

Recently, a nice example has been studied in which biphasic catalysis was not only used for catalyst recycling but also to increase the selectivity of the catalytic reaction. In the telomerization of butadiene with the bifunctional nucleophile ethylene glycol both monotelomers and ditelomers can be formed [113, 114]. When the reaction is carried out in a single liquid phase, for instance in tetrahydrofurane with a catalyst formed from palladiumbis-acetylacetonate and triphenylphosphine, the monotelomers are formed in a maximum yield of 60% with more than 20% of ditelomers as byproducts. When palladiumcarbene-complexes were used, the mono/ di-ratio switched to 1 3, thus increasing the formation of the ditelomer. However, if the monotelomers are the favored products, for instance for the production of detergents, no sufficient reaction control by choice of the ligand was possible so far. 

Likewise, efficient recyclable bifunctional catalysts 112 and 113 (Eigure 4.3) bearing ionic liquid-supported TEMPO and iodoarene moieties have been developed and used for the environmentally benign catalytic oxidation of alcohols [92]. Compounds 112 and 113 have been tested as catalysts for the oxidation of alcohols to the corresponding carbonyl compounds using peracetic acid as a green and practical oxidant. Ion-supported catalysts 112 and 113 can be conveniently recovered from the reaction mixture and reused without any loss of catalytic activity (Section 5.5). 

Efficient and recyclable bifunctional catalysts bearing silica-supported RuCls and iodoarene moieties have been developed and used for the environmentally benign oxidation of alcohols or alkylarenes at the benzylic position. In the presence of these catalysts, the oxidation of alcohols or alkylbenzenes by Oxone as the stoichiometric oxidant affords the corresponding carbonyl compounds in high yields under mild conditions and convenient work-up. Furthermore, these Si02-supported bifunctional catalysts can be recovered by simple filtration and directly reused (Section 5.5) [100]. 

Highly thermally stable, three-dimensional, spongelike mesoporous Ce Zri 02 nanocrystallines acted as acid-base bifunctional solid solutions for the Knoevenagel condensation of aldehydes with active methylene compounds [44]. This catalyst can be recycled at least twice for the reaction of benzaldehyde with malononitrile. 

Enantioselective organocatalytic a-chlorination of aldehydes, via enamine catalysis, was independently reported by the groups of MacMillan and Jprgensen in 2004 (Scheme 13.20) [46, 47]. MacMillan utilized his imidazolidinone catalyst and a perchlorinated quinone as the chlorine source, to obtain the S-enantiomer of the a-chloroaldehyde products. Jprgensen employed NCS as the chlorine source, and either a prolinamide catalyst to access the / -enantiomer of the a-chloroaldehyde products, or a Ci-symmetric amine catalyst to access the 5-enantiomer. A recyclable fluorous pyrrolidine-thiourea bifunctional organocatalyst was later employed as an enamine catalyst in this transformation [48]. 

Yao and coworkers [ 142] were the first to report a fluorous-functionalized olefin metathesis catalyst. A random, bifunctional, fluorous polyacrylate material, containing both a perfluoroalkyl section and a styrenic Hoveyda ligand section in a 10 1 ratio, was prepared. This was then treated with catalyst 4 to generate the heavy fluorous catalyst 156. This catalyst was then effectively used in a variety of RCM reactions at 50 C, employing a solvent mixture of PhCFj/CHjClj (1 19 v/v). This catalyst system was shown to be efficiently recycled a total of 20 times using a fluorous extraction technique. Unfortunately, no results were reported pertaining to any residual Ru contamination within the organic fractions. 

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