Reusable organocatalysts

The removal of the poly(ethylene oxide) fiber material can be achieved by extraction with water. Eventually, poly(amido amine) dendrimers that are entrapped in the poly(p-xylylene) tubes are obtained. These entrapped dendrimers exhibit a high catalytic activity as reusable organocatalysts [108]. 

D.L. da Silva, S.A. Fernandes, A.A. Sabino, A. de Fatima, p-Sulfonic acid calixarenes as efficient and reusable organocatalysts for the synthesis of 3,4-dihydropyrimidin-2(llT)-ones/-thiones, Tetrahedron Lett. 52 (2011) 6328-6330. 

By using of a modified proline, L-prolinamide 47 (which is known to be a more reactive catalyst than L-proline in cross-aldol reactions [80]), the enantioselectivity of the direct aldol reactions in ionic liquid [bmim][BF4] was remarkably increased as compared with the reaction carried out in acetone (69% ee) (Scheme 7.26) [81]. However, the reusability of the recovered 47 when immobilized in the ionic liquid layer was somewhat inferior to that of the L-proline catalyst this effect could be ascribed to the increased solubility of the organocatalyst 47 in the extracting organic solvents (not provided in the literature), leading to an increased leaching of the catalyst. 

Paixao and coworkers showed that a sulfur-modified catalyst C5 was able to efficiently catalyse the Michael addition of malonates to enals in EtOH/ brine mixture (Scheme 8.7). The sulfur-containing catalyst was efficient for the Michael addition of aldehydes to nitroalkenes in polyethylene glycol (PEG). The most important feature of this system was its reusability. After extraction of the reaction products, the PEG solution containing the organocatalyst was reused at least four times with no adverse effects on yield or enantioselectivity. ... 

Wang and coworkers synthesised recyclable and reusable pyrrolidine-fluorosulfonamide organocatalysts 3b,c. The fluorous sulfonamide organocatalyst 3b was used for the Michael addition of ketones and aldehydes to nitro-olefins in water, and the reaction gave products with high enantio- and diastereomeric purity. Robust organocatalyst 3b was easily separated from... 

Scheme 9.22 Recyclable and reusable pyrrolidine fluorous sulfonamide organocatalysts.

SCHEME 5.2. Reusable diarylprolinol silyl ether organocatalysts. 

For the six-membered-ring cyclic ketones, many effective organocatalysts [36] have been developed in the past decade (as shown in Scheme 5.17 for selected examples). These catalysts proved to be effective for a broad range of aryl nitroalkenes, and some of the reactions could be conducted using alcohol or even water as solvent. Interestingly, alkyl nitroalkene was also suitable electrophile when 29 [36b] was used as the catalyst. On the other hand, the lluorous (S)-pyrrolidine sulfonamide 30 [36c] and functionalized chiral ionic liguids 31 [36d] were reusable. 

A simple trifunctional L-prolinamide (48) based on 8-aminoquinoline catalyses enantioselective aldols of aromatic and aliphatic aldehydes with acetone. L-Proline-anilide (49) is a simple and cheap organocatalyst of direct aldols, giving y e Aldelee up to 99/98/99% in large-scale reactions, and is readily recoverable and reusable. Proline anthranilamide-based pseudopeptides act as bifunctional catalysts for direct aldols, giving good yields and detee up to 99/96%. ... 

Organocatalyst (53) - formed by a one-step 0-acylation of threonine - gives direct aldol y leldldelee up to 99/98/99% in water, can be used on a large scale, and is recoverable and reusable 0-acyl serines with a range of acyl substituents (alkyl, Ph, Bn, CH2CH2Ph) have similar range and reusability in water (up to 99% yield and 0... 

Fluorous thiourea organocatalyst (103) promotes Michael reaction of aldehydes with maleimides in high yield, and ee sometimes >99%. Catalyst (103), easily prepared 0 in one step from readily available materials, can be recovered by filtration and is reusable. 

M. Kargar, R. Hekmatshoar, A. Mostashari, Z. Hashemi, Efficient and green s)mthesis of 3,4-dihydropyrimidin-2(lfl)-ones/thiones using imidazol-l-yl-acetic acid as a novel, reusable and water-soluble organocatalyst, Catal. Commun. 15 (2011) 123-126. 

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