Imidazolidinone organocatalysts

The asymmetric a-chlorination of aldehydes has also be achieved using SOMO catalysis (Scheme 13.21) [49]. In these reactions, saturated aldehydes condensed with MacMillan s imidazolidinone organocatalyst to form enamines. The oxidant combination of Cu(TFA)2 and Na2S20g oxidized the enamine to the radical cation (inset in Scheme 13.21), the reactive intermediate in this transformation containing... 

Figure 23.8 Precursors for supported MacMillan imidazolidinone organocatalysts.

Michael additions using proline (2) as the organocatalyst have proven disappointing in terms of enantiocontrol, ° ° ° stimulating the search for a more selective enamine catalyst system. In this context, imidazolidinones (initially... 

The preparation of immobilized catalysts related to the imidazolidinone-type organocatalyst 13 and their application in the asymmetric Diels-Alder reaction was reported by Pihko and co-workers [9]. The reactivity of the immobilized catalysts depended on the type of solid support. The silica-supported imidazolidinone 24, which was prepared starting from N-Fmoc-protected L-phenylalanine, was found to be a highly active organocatalyst. Several dienes and a,/i-un saturated aldehydes have been successfully used in the presence of only 3.3 to 20 mol% 24, usually... 

In addition, immobilized catalysts related to the MacMillan imidazolidinone-type organocatalyst 5 have been used for the asymmetric Diels-Alder reaction (Section... 

Enantioselective Michael additions of aldehydes to enones using imidazolidinones as organocatalysts show evidence of enamine intermediates.217 Several co-catalysts -mainly phenols - raise the yield and/or ee. 

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. 

The imidazolidinone salt (5.110) has also been used as a catalyst in the asymmetric chlorination of aldehydes. For example, octanal (5.112) is converted into the a-chloroaldehyde with high ee using the antipode of (5.110) and quinone (5.113) as the chlorine source. Jorgensen and coworkers have achieved similar levels of enantioselectivity in this transformation using C2-symmetric diphenylpyrrolidine (5.114) as the organocatalyst in combination with NCS. ... 

Notably, for linear-chained aldehydes, imidazolidinone-derived catalysts gave the highest enantioselection (88% ee). The use of NFSI provided access to quaternary a-fluoroaldehydes in high yields and modest enantio-selectivies using proline, albeit several other organocatalysts were more promising in accessing enantiopure fluorinated-substrates. 

The iminium strategy, elegantly illustrated by MacMillan, using chiral imidazolidinones as organocatalysts in a Diels-Alder reaction was then... 

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