Enamine catalysis enantioselective organocatalytic

One of the first highly enantioselective examples of multicomponent cascade reactions in orgnocatalysis was developed by Enders et al. [62] in 2006. In this report they describe an asymmetric organocatalytic triple cascade reaction for the construction of tetrasubstituted cyclohexenecarbaldehydes (93) starting from from enals (15), nitroalkenes (28), and enolizable aldehydes (94) (Scheme 10.27). In this work, they did the sequential creation of three bonds by a high enantioselective combination of enamine-iminium-enamine catalysis for a triple cascade reaction. 

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]. 

The first enantioselective organocatalytic a-allylation of cyclic ketones has been accomphshed via singly occupied molecular orbital catalysis. " Geometrically constrained radical cations (i.e., 196), forged from the one-electron oxidation of transiently generated enamines, readily undergo aUylic alkylation with a variety of commercially available allyl silanes to give 197 (Scheme 25.91). 

As for the base-promoted thioetheriiication, Shen and Wang reported in 2005 that highly electrophilic alkenes such as fluorinated vinylstannanes 172 could be converted to fluorinated vinyl sulfides 173 with ( )-selectivity via a nucleophilic addition of thiols to alkenes and defluorination 174 followed by destannylation (Scheme 46.22). In 2008, Cordova developed an organocatalytic enantioselec-tive aminosulfenylation of a,(3-unsaturated aldehydes 175 using A -(benzylthio)succinimides 176 in the presence of a TMS-protected chiral diarylprolinol. ° The reaction proceeded with low diastereoselectivities but with excellent enantioselectivities. The reason for the low diastereoselec-tivity might lie in the rapid epimerzation of Ca-H bond under the enamine catalysis 178. 

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