Cooperative Catalysis with Coupled Chiral Induction in 1,3-Dipolar Cycloadditions of Azomethine Ylides.
Alberto CayuelasOlatz LarrañagaVerónica SelvaCarmen NájeraTakahiko AkiyamaJosé Miguel SansanoAbel de CózarJosé I MirandaFernando P CossíoPublished in: Chemistry (Weinheim an der Bergstrasse, Germany) (2018)
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/Brønsted 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.