Chiral Squaramide-Functionalized Imidazolium-Based Organic–Inorganic Hybrid Silica Promotes Asymmetric Michael Addition of 1,3-Dicarbonyls to Nitroalkenes in Brine

Abstract

Chiral cinchona-based squaramide-functionalized organic−inorganic hybrid silica is developed through postgrafting 3-mercaptopropyltrimethoxylsilane onto imidazolium-based organic−inorganic hybrid silica, followed by the anchor of a squaramide organocatalyst via a thiolene click method. Structural characterizations and spectroscopic analyses demonstrate that a well-defined single-site chiral cinchona-based squaramide active center is incorporated onto the organic−inorganic hybrid silica. As a bifunctional heterogeneous catalyst, it displays excellent catalytic activity and high enantioselectivity in asymmetric Michael addition of 1,3-dicarbonyl compounds to nitroalkenes in brine. As presented in this study, the synergistic effect of confined site-isolated squaramide species and a salient imidazolium phase-transfer function significantly accelerates the catalytic performance. Furthermore, the organic−inorganic hybrid silica is conveniently recovered and reused at least eight times without loss of catalytic activity, showing an attractive feature in practice of organic transformation in an environmentally friendly manner.

Proximity Effect using a Nanocage Structure: Polyhedral Oligomeric Silsesquioxane‐Imidazolium Tetrachloro‐ palladate Salt as a Precatalyst for the Suzuki–Miyaura Reaction in Water

“…Indeed, ILs that contain imidazolium, phosphonium, ammonium and pyridinium cations have been shown to act as phase‐transfers catalysts for fluorination, nucleophilic substitutions, etherification and benzoin condensation reactions . Recently, it was described that an imidazolium‐based organic–inorganic hybrid silica enhanced the asymmetric Michael addition of 1,3‐dicarbonyl compounds to nitroalkenes in brine at 0.5 mol % loading by acting as a phase‐transfer catalyst . These effects cannot be operative if the bmim 2 PdCl 4 catalyst is used because of the absence of the nanocage (Figure b).…”

Ionic Liquids in Asymmetric Synthesis: An Overall View from Reaction Media to Supported Ionic Liquid Catalysis

“…Liu and co-workers describe the immobilization of ac hiral cinchona-based squaramide onto imidazolium-functionalized organic-inorganic hybrid silica to give catalyst 138 andi ts use as ab ifunctionalc atalyst in the asymmetricM ichael addition of 1,3-dicarbonyl compounds to nitroalkenes (Scheme 74). [142] The synergistic effect of the imidazolium phase-transfer function and the confined site-isolated cinchona-based squaramide organocatalyst leads to excellent catalytic activity and high enantioselectivity in the reaction (< 90 %y ield and > 94 % ee). Furthermore, this catalyst can be reused at least eight times without am arked reduction in its catalytic performance.…”

Recent Developments in Enantioselective Organocatalytic Michael Reactions in Aqueous Media

“…In 2014, Liu et al incorporated a chiral cinchona alkaloidderived squaramide organocatalyst onto imidazolium-based organic-inorganic hybrid silica to construct novel bifunctional heterogeneous catalyst 18 [38]. The latter was further investigated as catalyst in enantioselective Michael additions of 1,3-dicarbonyl compounds to nitroalkenes in brine, as shown in Scheme 15.…”