Ionic Liquid Effect on the Reversal of Configuration for the Magnesium(II) and Copper(II) Bis(oxazoline)‐Catalysed Enantioselective Diels–Alder Reaction

Abstract

Ionic liquids have been used to support a range of magnesium-and copper-based bis(oxazoline) complexes for the enantioselective DielsAlder reaction between N-acryloyloxazolidinone and cyclopentadiene. Compared with reaction performed in dichloromethane or diethyl ether, an enhancement in ee is observed with a large increase in reaction rate. In addition, for non-sterically hindered bis(oxazoline) ligands, that is, phenyl functionalised ligands, a reversal in configuration is found in the ionic liquid, 1-ethyl-3-methylimidazolium bis[(trifluoromethanesulfonyl)imide], compared with molecular solvents. Supported ionic liquid phase catalysts have also been developed using surface-modified silica which show good reactivity and enantioselectivity for the case of the magnesiumbased bis(oxazoline) complexes. Poor ees and conversion were observed for the analogous copperbased systems. Some drop in ee was found on supporting the catalyst due a drop in the rate of reaction and, therefore, an increase in the contribution from the uncatalysed achiral reaction.

Asymmetric Carbon‐Carbon Bond Forming Reactions Catalysed by Metal(II) Bis(oxazoline) Complexes Immobilized using Supported Ionic Liquids

“…Analysis of the supernatant and product following recycle showed that, for all the supports studied, the metal leaching was below 0.01 mol% by ICP per reaction and HPLC analysis showed that ligand leaching was below the detection limit. The decrease in enantioselectivity on recycle was, therefore, attributed to the build up of cyclopentadiene dimer20 on the support surface as has been previously observed in other SILP catalysed Diels–Alder reactions 16. This proposal was supported by the fact that the addition of cyclopentadiene dimer (1.5 molar equivalents with respect to substrate) to the ethereal layer of fresh MCM‐41 and CNT SILP catalyst 1 led to little change in the conversion but a marked decrease in ee from 95% to 8% for MCM‐41 and from 85% to 3% ee for graphite supported SILP catalyst 1 .…”

“…In all cases, immobilisation of the chiral complexes onto silica using an ionic liquid resulted in similar ee values and conversions compared with the same reactions conducted under homogeneous conditions. In contrast, moderate to poor ee values were observed for the bis(oxazoline)‐Cu(II)‐catalysed Diels–Alder reaction between N ‐acryloyloxazolidinone ( 1 ) and cyclopentadiene (Scheme ) when immobilised onto silica using SILP technology despite high ee values being obtained under homogeneous conditions in the ionic liquid 16. This paper reports on the general applicability of SILP metal‐bis(oxazoline) catalysts for asymmetric Diels–Alder and Mukaiyama‐aldol reactions.…”

An efficient Cu(ii)-bis(oxazoline)-based polymer immobilised ionic liquid phase catalyst for asymmetric carbon–carbon bond formation

“…This by-product results from a Mukaiyama-aldol reaction between 1-phenyl-1-trimethylsiloxyethene and acetophenone, the latter of which is generated via hydrolysis of the 1-phenyl-1-trimethylsiloxyethene substrate. 2 In addition, both SILP catalysts gave complete conversion in short reactions times with only a slight reduction in ee compared with the homogenous reaction conducted in ionic liquid (entries 5-6). In contrast to the SILP-based catalysts, in general, the ee's and conversions obtained with the PIILP and PIILP/IL system were lower than found under homogeneous conditions.…”

“…1 In this respect, a range of chiral metal complexes of Pt, Zn, Cu and Mg have been immobilised in ILs under homogeneous or heterogeneous (Supported Ionic Liquid Phase-SILP) conditions and shown to be excellent catalysts for a host of important asymmetric C-C bond forming reactions such as Diels-Alder and Mukaiyama-aldol reactions, cyclopropanations and oxidations, often with significant and marked enhancements in rate and selectivity compared with the corresponding reaction in conventional solvents. 2 In the case of SILP catalysed Diels-Alder reactions (Scheme 1), the presence of an IL film on the silica oxide or carbon support proved crucial in maintaining high conversions, good endo-selectivity and high levels of enantioselectivity (ee). 3 However, despite the success of SILP catalysts some issues still limit the potential applications of this concept including their low/poor mechanical stability, leaching of IL into the solvent phase and the high cost of the IL.…”

Multiphase enantioselective Kharasch–Sosnovsky allylic oxidation based on neoteric solvents and copper complexes of ditopic ligands

“…Studies on oxidation catalysis in ionic liquids have been recently reviewed 30 and, to the best of our knowledge, the only precedent in allylic oxidation is the use of a non-chiral Cu (salen), 31 with an ionic pyridinium tag in the backbone of the ligand, in the allylic oxidation of cyclohexene with oxygen. There are also several examples describing the use of catalysts bearing ligands of the bis(oxazoline) family in ionic liquids, [32][33][34][35][36] but only one of them 36 describe an oxidation reaction, namely the epoxidation of alkenes using dioxomolybdenum(VI) complexes containing chiral oxazolines.…”