N-Heterocyclic Carbene/Lewis Acid Catalyzed Enantioselective Aerobic Annulation of α,β-Unsaturated Aldehydes with 1,3-Dicarbonyl Compounds

Abstract

A novel and efficient aerobic asymmetric cyclization reaction of cinnamaldehydes and 1,3-dicarbonyl compounds through oxidative NHC-catalysis has been developed, and it allows the synthesis of a wide range of enantiomeric enriched dihydropyranone derivatives in good yields with good to excellent enantioselectivities. Various α,β-unsaturated aldehydes with aliphatic and aromatic substitution groups and 1,3-dicarbonyl compounds were well tolerated. The air was directly used as the oxidant, which made this asymmetric cyclization reaction in a highly efficient, cheap, and green manner.

Carboranes as Aryl Mimetics in Catalysis: A Highly Active Zwitterionic NHC‐Precatalyst

“…Finally, we turned our attention to reactions typically mediated by more electron‐rich NHCs, in which N ‐substituents such as o,o′ ‐dimethoxyphenyl as electron‐donating N ‐aryl residues, have been shown to improve intramolecular hydroacylation reactions of unactivated double bonds (Scheme , top right) . However, here our zwitterionic precatalyst 5 (10 mol %) proved to be ineffective for this transformation and thus denoting the scope limitation of its reactivity …”

“…To further illustrate the stability of triazolium precatalyst 5 towards air and moisture, we performed a lactonization of aldehyde 12 and acetylacetone by in situ oxidation in a vessel open to air (Scheme , top center) . Lactone 14 was obtained in 62 % yield, further underlining the extraordinary versatility of 5…”

“…To further illustrate the stabilityo ft riazolium precatalyst 5 towardsa ir and moisture, we performed al actonization of aldehyde 12 and acetylacetone by in situ oxidation in av essel open to air (Scheme2,t op center). [46] Lactone 14 was obtained in 62 %y ield, further underlining the extraordinary versatility of 5. [30] Finally,w et urned our attention to reactions typically mediated by more electron-rich NHCs, in which N-substituents such as o,o'-dimethoxyphenyla se lectron-donating N-aryl residues, have been shown to improve intramolecular hydroacylation reactions of unactivated double bonds (Scheme 2, top right).…”

“…[47] However,h ere our zwitterionic precatalyst 5 (10 mol %) proved to be ineffective for this transformation and thus denoting the scope limitation of its reactivity. [30,46] The reactivity profile highlightsthe broad applicability of the novel aryl mimetic nido-carboranyl NHC, which already largely exceeds the versatility of the analogous N-phenyl-derivative. In addition, this reactivity map also allows for ap reliminary classification of the aryl mimetic nido-carborate's combined electronic and stericp roperties as compared to typical arene residues.…”

Polyhalides as Efficient and Mild Oxidants for Oxidative Carbene Organocatalysis by Radical Processes

“…For example, while O 2 (and air) is in principle an excellent choice as oxidant, its application in NHC catalysis is mostly limited to the conversion of aldehydes into the corresponding carboxylic acids and esters . When O 2 was used as the oxidant for functionalization of the α, β, or γ‐carbon of aldehydes, they are unproductively consumed to form simple carboxylic acids or esters as the side products …”