Organocatalysts fluorous catalyst

A fluorous hydrazine-carbothioate organocatalyst was recently prepared from 2-perfluorohexyl ethanol by sequentially treating it with 1,1 -thiocarbonyl diimidazole and hydrazine. The fluorous catalyst together with N-chlorosuccinimide (NCS) efficiently catalyzed the acetalization of aldehydes [48]. The fluorous catalyst was recovered by fluorous solid-phase extraction with high purity for direct reuse for four times. According to the author, the formation of Cl" from NCS and catalyst might be catalyzing the acetalization reaction (Scheme 7.32). 

Enantioselectively introducing fluorine atom into small organic compounds is of great importance for life sciences and high-performance materials. To this end, a fluorous cinchona alkaloid ester 29 was developed as an organocatalyst for the asymmetric fluorination of p-ketoesters (Scheme 7.34) [50]. The fluorous catalyst... 

More recently, a fluorous organocatalyst has been used to perform selective Diels-Alder reactions of dienes with oc,(3-unsaturated aldehydes in acetonitrile-water. The chiral fluorous imidazolidinone catalyst can be recovered using fluorous silica (80-90% recovery efficiency) and reused. Figure 7.10. Further organocatalytic reactions are presented later in this chapter. 

Figure 7.10 Stereo specific Diels-Alder reaction using an organocatalyst and fluorous silica for catalyst recovery.

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]. 

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. 

Synthetic strategy Recyclable fluorous organocatalyst-promoted multi-component reaction Catalyst Fluorous imine-l,2-b/s(carbothioate) (I), an organocatalyst... 

A fluorous chiral organocatalyst (18) promotes the formation of the anti-Mol product (with up to 96% ee) on reaction between aromatic aldehydes with ketones in brine. The enantioselectivity achieved on promotion of aldol and Mannich reactions by another di-diamine-based catalyst (19) can be reversed by the addition of an achiral acid and is to be the subject of further mechanistic investigation. ... 

Later on, a fluorous organocatalyst 27 with a perfluorooctyl propanoxyl group on the aromatic ring was prepared from i-tyrosine by the same group (Scheme 7.26) [41]. It efficiently catalyzed the direct aldol reaction of aromatic aldehydes with cyclohexanone. In accordance with previous results, catalyst 27 showed higher efficiency than its non-fluorinated analog which is probably because the fluorous tag created a hydrophobic reaction field in brine. The catalyst 27 was easily recovered with FSG extraction and reused without further purification. 

A fluorous thiourea organocatalyst 28 was conveniently prepared from a chiral diamine (Scheme 7.29) [45]. The fluorous thiourea 28 catalyzed the Michael reaction of maleimides and aldehydes with high yield and enantioselectivity. The catalyst 28 could be recycled by simple filtration and reused without significant loss of catalytic activity and enantioselectivity, while the catalyst recovery yield dropped dramatically after three cycles. 

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