Update 1 of: C₂-Symmetric Chiral Bis(oxazoline) Ligands in Asymmetric Catalysis

Abstract

In 1991 two back-to-back communications appeared in The Journal of the American Chemical Society: one by Evans et al. dealt with the asymmetric cyclopropanation of alkenes 9 and a second one by Corey et al. that covered enantioselective DielsÀAlder reactions. 10 They reported the use of chiral Cu(I)À and Fe(III)Àbox complexes as catalysts, respectively.These two communications induced a small revolution in the field of asymmetric catalysis. The box ligands quickly became widely adopted as bidentate ligands for their easy and flexible synthesis and for the excellent enantioselectivity induced first in two very useful reactions, and later in a large variety of other reactions. These communications also anticipated the usual research protocol to be ultimately used in box chemistry: (a) synthesis of the ligand following a sequence that, for a long time, would become a standard (reaction of dialkylmalonyl dichloride with an optically active 1,2-amino alcohol, conversion of the bis-hydroxyamide to the corresponding bis-chloroamide, and ring closure under basic conditions); (b) preparation of the chiral catalyst by reaction of the box ligand with an inorganic salt [CuOTf and FeCl 2 /I 2 ]; (c) test of the chiral Lewis acid complex as a catalyst for asymmetric induction in the reaction; (d) proposal of a reacting intermediate in which the PR286 dx.

Sulfonimidation via ring-opening of 2-oxazolines with acidic sulfonimide nucleophiles

“…As a result, we suspected that dibenzenesulfonimide is acidic enough to protonate the nitrogen atom on 1. Indeed, 1 moved to the baseline on TLC upon contact with HN(SO 2 Ph) 2 . We initially surveyed the solvent effect on transformation 1→5.…”

“…However, this methodology did not work for electron-poor substrates. For example, no reaction was observed when 2-(4-nitrophenyl)-2-oxazoline (13) and HN(SO 2 Ph) 2 were heaterd togegther at reflux in 1,4-dioxane. Further, addition of 0.5 equiv of sodium hydride to boost the nucleophilicity and heating up to 150 °C for six hours did not provide any ring-opened product either.…”

Asymmetric copper-catalyzed Diels–Alder reaction revisited: control of the structure of bis(oxazoline) ligands

“…Conversion of bis(hydroxyamide) 3 to the corresponding bis(oxazoline) 4 was carried out in one step by reaction with diethylaminosulfur trifluoride in CH 2 Cl 2 at -78°C followed by base induced cyclisation 16,9 in 50-80% yield. For the ligand 4d, the bis(hydroxyamide) 3d was first converted into the bis(amide) dichloride with SOCl 2 and then the cyclization was achieved with aqueous NaOH in 73% yield.…”

“…[1][2][3][4][5][6][7][8][9] Among them, a novel class of oxazoline ligands with multiple elements of chirality has been synthesized from readily available chiral diacid and chiral amino alcohol (Fig. 1).…”

Asymmetric cyclopropanation in ionic liquids promoted by dicopper complexes of ditopic ligands

“…1,2 In this regard, bis(oxazolines) (Box) are versatile chiral ligands that are able to form complexes with different metals, which catalyse a variety of organic reactions. 3,4 The immobilisation of these complexes, using a large variety of strategies, allows obtaining recoverable catalysts for a wide range of organic reactions. [5][6][7] An enantioselective catalysts recycling strategy, usually used with metal complexes of chiral ligands, consists in the use ionic liquids phases.…”