Prolinol Silyl Ethers as Asymmetric Organocatalysts

Department of Organic Chemistiy, Faculty of Natural Sciences, Comenius University in Bratislava, Mlynska dolina CH-2, SK-84215 Bratislava, Slovakia 

Michael additions are among the most important organocatalytic transformations.The ability of prolinol silyl ethers to form enamines, diena-mines, trienamines or iminium ions makes them appealing in a number of chemical transformations based on addition of a nucleophile to an a,p-unsaturated carbonyl compound or other Michael acceptor. The first example of Michael addition catalysed by a prolinol silyl ether was published by Hayashi and coworkers. Aliphatic aldehydes added to a range of aryl-substituted nitroalkenes. Prolinol silyl ether Cla presumably formed chiral enamines with aldehydes. The F-anh-enamine was formed and it reacted with nitroalkene via an acyclic synclinal transition state, originally proposed 

Sustainable Catalysis Without Metals or Other Endangered Elements, Part 1 

R = Me, CsHs, 3-MeCeH4,4-MeOC6H4, 4-CIC6H4, 2-thienyl,1,3-benzodioxolane 

Detailed mechanistic studies of this reaction have been undertaken hy several teams. Michael addition of the simplest enolisahle aldehyde, acetaldehyde, poses a significant challenge due to high auto-aldol reactivity of acetaldehyde. List and coworkers used prolinol silyl ether eatalysts to promote enantioselective Michael addition of acetaldehyde to nitroalkenes.The key to successful accomplishment of this transformation was slow addition of acetaldehyde via a syringe pump. 

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