Imidazolidinones Mukaiyama-Michael-reactions

The chiral imidazolidinone 45 also catalyzes the Mukaiyama-Michael reaction between 2-silyloxy furans and a,/ -unsaturated aldehydes, affording enantiomeri-cally highly enriched y-butenolides (Scheme 4.18) [33]. For optimum catalytic performance, hydroxyl additives are necessary, and addition of 2 equiv. water proved best. 

Alternatively, the iminium-activation strategy has also been apphed to the Mukaiyama-Michael reaction, which involves the use of silyl enol ethers as nucleophiles. In this context, imidazolidinone 50a was identified as an excellent chiral catalyst for the enantioselective conjugate addition of silyloxyfuran to a,p-unsaturated aldehydes, providing a direct and efficient route to the y-butenolide architecture (Scheme 3.15). This is a clear example of the chemical complementarity between organocatalysis and transition-metal catalysis, with the latter usually furnishing the 1,2-addition product (Mukaiyama aldol) while the former proceeds via 1,4-addition when ambident electrophiles such as a,p-unsaturated aldehydes are employed. This reaction needed the incorporation of 2,4-dinitrobenzoic acid (DNBA) as a Bronsted acid co-catalyst assisting the formation of the intermediate iminium ion, and also two equivalents of water had to be included as additive for the reaction to proceed to completion, which... 

Mukaiyama-Michael Reaction Promoted by Imidazolidinone Organocatafysts... 

In some cases, using the silyl enol ethers form of nucleophiles in the asymmetric Michael reactions is necessary for ensuring high reactivity and selectivity. MacMillan and co-workers [113] developed the first enantioselective organocata-lytic Mukaiyama-Michael reaction for the synthesis of enantioenriched 7-butenolide architecture in 2003. In the presence of chiral imidazolidinone catalyst 120 with acid additive, the reactions of silyloxy furan 118 with simple a,(3-unsaturated aldehydes... 

Chiral imidazolidinones like 20 serve as chiral organocatalysts for the enantioselective reactions of 2-siloxyfuran with a y9-unsaturated aldehydes through oc,fi-unsaturated iminium species (Scheme 3-83)J The Mukaiyama-Michael reaction with nitroolefines provides an access to 1,3-dicarbonyl compounds... 

The elfectiveness of imidazolidinone of type 11 was confirmed by successful application to a broad range of chemical transformations, including cycloadditions, conjugate additions, Friedel-Crafts alkylations, Mukaiyama-Michael additions, hydrogenations, cyclopropanations, and epoxidations. A summary of these enantio-selective iminium catalyzed processes is provided by reaction subclass. 

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