Diarylprolinol Silyl Ether Salts as New, Efficient, Water-Soluble, and Recyclable Organocatalysts for the Asymmetric Michael Addition on Water

摘要

The organocatalytic asymmetric Michael reaction of aldehydesnto nitroolefins is a key transformation in organic synthesis.1nMany chiral organocatalysts have been developed to exhibit highnactivities and enantioselectivities for this cornerstone transformationnin recent years.2 However, a major problem associated withnthese organocatalytic systems is that high catalyst loadingn(normally 10-30 mol %) and large excess amounts of Michaelndonors (up to 10 equiv of aldehydes) are generally required fornthe reactions to be completed in reasonable time scales withngood enantioselectivity. Furthermore, the high polarity andnvolatility of organic solvents, such as DMSO, DMF, or CH2Cl2,nwhich are typically used for these reactions, are also a majornproblem from a green chemistry perspective. Therefore, thendesign and synthesis of highly active organocatalysts aimed atnlowering catalyst loading, reducing the quantity of donor sources,nand being active in an aqueous system3 has proved to be ansignificant challenge. Most importantly would be the challengento facilitate these expensive organocatalysts’ recovery and reuse.nAlthough initial strategies toward organocatalyst recycling usingnsolid-phase, ionic liquid support, and fluorous technologies havenbeen developed,4 these methods result in low catalytic activitynand enantioselectivity for aldehydes due to heterogeneousnconditions,4a use several different polar organic solvents forncatalyst recovery,4b or require expensive fluorous solvents fornphase separation.4c In this paper, we provide the first examplenof organocatalysis using diarylprolinol silyl ether salts (Figuren1)5 as efficient, water-soluble, and recyclable organocatalystsnfor the asymmetric Michael addition on pure water.6,7 This newnapproach successfully combines the high efficiency and selectivitynof homogeneous catalysis with the ease of separation andnrecyclability of heterogeneous catalysis..