- Cooperative Catalysis with Coupled Chiral Induction in 1,3-Dipolar Cycloadditions of Azomethine Ylides
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1,3-Dipolar cycloadditions (1,3-DC) between imino esters (as precursors of N-metallated azomethine ylides) and π-deficient alkenes are promoted by cooperative asymmetric Lewis acid/Br?nsted base catalysis. The components of these catalytic pairs are silver salts derived from enantiopure commercially available BINOL-based phosphoric acids and Cinchona alkaloids. Chiral phosphoric silver(I) salts promote HOMO raising of in situ formed 1,3-dipoles, whereas protonated cinchona alkaloids generate a LUMO lowering for the dipolarophiles resulting in a global acceleration of the 1,3-DC. The best results were obtained with BINOL-derived silver phosphate and hydrocinchonine. Matching between both cooperative metallo- and organocatalyst results in an enhanced enantiomeric excess, superior to that reached by both separate components. NOESY experiments and DFT calculations are compatible with a non-covalent interaction (hydrogen bond) between both catalysts, which results in close contacts and mutually coupled chiral environments.
- Cayuelas, Alberto,Larra?aga, Olatz,Selva, Verónica,Nájera, Carmen,Akiyama, Takahiko,Sansano, José M.,de Cózar, Abel,Miranda, José I.,Cossío, Fernando P.
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supporting information
p. 8092 - 8097
(2018/05/30)
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- Organocatalytic enantioselective Michael addition of β-diketones to β-nitrostyrene: The first Michael addition of dipivaloylmethane to an activated olefin
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The addition of a family of β-diketones to β-nitrostyrene was explored using a library of cinchona organocatalysts. A thiourea organocatalyst, under improved reaction conditions, is shown to be much more efficient at catalyzing this reaction than previously reported giving excellent yields and enantioselectivites (up to 95% yield and 97% ee). The same thiourea organocatalyst was employed in the first successful Michael addition of the sterically challenging dipivaloylmethane to β-nitrostyrene (99% ee). ARKAT-USA, Inc.
- Gavin, Declan P.,Stephens, John C.
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experimental part
p. 407 - 421
(2011/08/07)
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