Chiral Ammonium‐Capped Rhodium(0) Nanocatalysts: Synthesis, Characterization, and Advances in Asymmetric Hydrogenation in Neat Water

Abstract

Optically active amphiphilic compounds derived from N-methylephedrine, N-methylprolinol, or cinchona derivatives possessing bromide or chiral lactate counterions were efficiently used as protective agents for rhodium(0) nanoparticles. The full characterization of these surfactants and the obtained nanocatalysts was performed by means of different techniques. These spherical nanoparticles, with sizes between 0.8-2.5 nm depending on the stabilizer, were evaluated in the hydrogenation of model substrates in neat water as a green solvent. The rhodium catalysts showed relevant kinetic properties, but modest enantiomeric excess values of up to 13 % in the hydrogenation of ethyl pyruvate. They were also investigated in the hydrogenation of disubstituted arenes, such as m-methylanisole, providing interesting catalytic activities and a preferential cis selectivity of around 80 %; however, no asymmetric induction was observed.

Chiral Surfactant-Type Catalyst for Asymmetric Reduction of Aliphatic Ketones in Water

“…Generally, amphiphilic surfactants can form micelles in water as microreactors to increase not only the solubility of nonpolar substrates and catalysts but also the reactivity and stereoselectivity of reactions due to the hydrophobic interaction and the preorganizing function of micelles . Thus, the use of micellar surfactants as additives or catalysts has attracted considerable attention in aqueous reactions in recent years. − However, there are few reports of successful asymmetric synthesis with high enantioselectivity in chiral micellar systems, ,− especially metallomicellar catalysts with chiral surfactants as the ligands. , Actually, lower enantioselectivity was usually observed with chiral metallomicellar catalysts compared to the corresponding chiral molecular catalysts in micelles. , …”

Odyssey in Polyphasic Catalysis by Metal Nanoparticles

“…value of 55% 93. More recently, inspired by our approach relying on the electrosteric stabilization of nanospecies by ammonium surfactants, we designed novel water‐soluble optically active ammonium salts (Figure ), based on various polar heads ( N ‐methylephedrine, N ‐methylprolinol or cinchona derivatives) in combination with different counterions 94–95. These quaternary hydroxylated ammonium salts proved to be efficient capping agents for spherical rhodium(0) nanospecies with mean sizes around 0.8 to 2.5 nm.…”