Trisoxazoline ligands

Scheme 15.1 Modular assembly of trisoxazoline ligands by reaction of the metal-lated bisoxazolines with 2-bromooxazoline derivatives. This allows the synthesis of Cj-chiral ligands (A), the inversion of one...

The enantiopure C3 symmetric trisoxazoline 173 proved to be a suitable supporting ligand for scandium-catalyzed olefin polymerization whilst invoking sufficient stereocontrol over the substrate to induce a high level of tacticity in the polymer microstructure <05AG(E)1668>. 

Sterically hindered trisoxazoline (TOX) ligands, such as 165, were exploited in Cu(II)-catalyzed enantioselective intramolecular Cannizzaro reactions of aryl and alkyl glyoxals with alcohols (13JA16849). 

Oxazolines are nowadays essential ligands in asymmetric catalysis and also important synthons for stereoselective synthesis [8]. The success of the Cj-symmetric bis(oxazolines) ( BOX ) and pyridine-bis(oxazolines) ( Pybox ) discovered in the early 1990s has established them as a privileged class of ligands [9]. In contrast, the development and application of trisoxazolines lagged behind for a long time. Katsuki and collaborators [10] reported the first example of a chiral trisoxazoline in 1995 and their use in the allylic oxidation of alkenes (Kharasch-Sosnovsky reaction), as well as the enantioselective addition of diethylzinc to aldehydes. 

Trisox as a Bidentate Ligand Chiral Trisoxazolines in Copper(ll) Lewis Acid Catalysis and Palladium-Catalyzed Asymmetric Allylic Substitutions... 

Pfaltz and coworkers [29] had previously investigated the allylic alkylation of l,3-diphenylprop-2-enyl acetate with dimethyl malonate as nucleophile (in the presence of A/ ,0-bis(trimethylsilyl)acetamide, BSA) with the well-established BOX ligands as stereodirecting ligands, and this particular system therefore provided the point of reference for the catalytic study at hand (Table 15.3, entry 1) [28]. Under the reaction conditions displayed in Scheme 15.4, the trisoxazoline-based catalysts generally induce a better enantioselectivity compared to their BOX analogs, and this behavior appears to be independent of the substituent, as shown in Table 15.3. 

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