Solvent‐Regulated Asymmetric Hydrogenation of Quinoline Derivatives in Oligo(Ethylene Glycol)s through Host–Guest Interactions

Abstract

The asymmetric hydrogenation of quinolines in oligo(ethylene glycol)s (OEGs) and poly(ethylene glycol)s (PEGs) with chiral cationic ruthenium diamine complexes has been investigated. Interestingly, in liquid PEGs or long-chain OEGs, the Ru catalysts lost their reactivity. Upon the addition of a little MeOH, the hydrogenation of quinoline was switched "ON". Evidence from mass spectrometry and control experiments revealed that encapsulation of the quinolinium salt by PEG or long-chain OEG molecules through supramolecular interactions is possibly the main reason for such a switchable hydrogenation reaction. Moreover, the asymmetric hydrogenation of 2-substituted quinoline derivatives was achieved in triethylene glycol (3-OEG), thereby affording 1,2,3,4-tetrahydroquinolines with excellent reactivities and enantioselectivities (up to 99 % ee). Furthermore, the Ru catalyst could be readily recycled for both pure 3-OEG and biphasic 3-OEG/n-hexane systems without a clear loss of reactivity and enantioselectivity.

Progress in the Chemistry of Tetrahydroquinolines

“…The asymmetric hydrogenation of quinolines was also achieved in oligo­(ethylene glycol)­s (OEGs) and poly­(ethylene glycol)­s (PEGs) in the presence of Ru­(TsDPEN) catalyst in excellent yields and enantioselectivities . The recycling of the catalyst was demonstrated both in pure 3-OEG and in a biphasic 3-OEG/ n -hexane system.…”

Synthesis of polymer/CNTs composites for the heterogeneous asymmetric hydrogenation of quinolines

“…Recycling of expensive chiral catalysts is crucial to practical applications. Fan and co-authors [13,15] attempted to recycle the homogeneous catalyst, Ru/Ts-DPEN, via a liquid-liquid phase separation method, in which the products were extracted by n-hexane and the catalyst remained in the solvent after the reaction; examples of such solvents include ionic liquids (ILs) or short-chain triethylene glycol. The complicated separation process led to the wastage of solvent and consequent increased costs.…”

Recent Advances in the Enantioselective Synthesis of Chiral Amines via Transition Metal-Catalyzed Asymmetric Hydrogenation

“…The same authors reported that similar Ru-diamine complexes were capable of efficiently hydrogenating S127a in solvent-free conditions, 445 in neat water, 446 in ionic liquids, 447,448 and in oligo(ethylene glycol)s through host−guest interactions. 449 On the other hand, the AH of 2-aryl-substituted quinolines was more challenging, and only a few examples have been reported. The same group extended the use of these Ru(II)−diamine complexes and found that C14h was a highly active catalyst for the AH of 2-aryl quinolines, affording the corresponding P127a (R = aryl) in excellent yields and enantioselectivities (Scheme 85).…”

“…In contrast, the AH of 2,3-dialkyl quinolines S127b provided a very low ratio of diastereoselectivity. The same authors reported that similar Ru-diamine complexes were capable of efficiently hydrogenating S127a in solvent-free conditions, in neat water, in ionic liquids, , and in oligo­(ethylene glycol)­s through host–guest interactions . On the other hand, the AH of 2-aryl-substituted quinolines was more challenging, and only a few examples have been reported.…”