Abstract
Seven polyoxazoline ligands were synthesized in high yield in a one-pot reaction by heating polycarboxylic acids or their esters and chiral β-amino alcohols under reflux with concomitant removal of water or the alcohol produced in the reaction. The method is much simpler and more efficient in comparison to those methods reported in the literature.The compounds were used as chiral ligands in the rhodium-catalyzed asymmetric hydrosilylation of aromatic ketones, and the effects of the linkers and the substituents present on the oxazoline rings on the yield and enantioselectivity investigated. Compound 2 was identified as the best ligand of this family for the hydrosilylation of aromatic ketones.
Highlights
The design and development of effective chiral oxazoline ligands have played a significant role in the advancement of asymmetric catalysis and have attracted a great deal of attention
We report the results of heating polycarboxylic acids or their esters with chiral β-amino alcohols under reflux conditions with the simultaneous removal of water or the alcohol produced in the reaction in a one-step process for the preparation of novel chiral polyoxazoline ligands (Figure 1)
We have successfully developed facile procedures for the syntheses of polyoxazolines starting from polycarboxylic acids or their esters, such as 3,4-dihydroxyfuran-2,5-dicarboxylic acid (DFA) and its dimethyl ester (DDFA), diglycolic acid and its dimethyl ester, triglycine and its triethyl ester, 1,3,5-benzenetricarboxylic acid (BTA) and its trimethyl ester (TBTA), ethylenediaminetetraacetic acid (EDTA) and ethylene glycol-bis(2-aminoethylether)-N,N,N′,N′tetraacetic acid (EGTA)
Summary
The design and development of effective chiral oxazoline ligands have played a significant role in the advancement of asymmetric catalysis and have attracted a great deal of attention. Various chiral oxazoline ligands have been developed and applied in many catalytic asymmetric reactions to prepare enantiomerically pure compounds; in particular, a range of monoand bisoxazolines have been widely used as effective templates for metal-catalyzed asymmetric reactions over the last 30 years [1-13]. Polyoxazoline ligands were reported to have good catalytic activities and high enantioselectivities in various asymmetric reactions [14-20]. Chiral 1,2,2-tris[2(4-isopropyloxazolinyl)]propane was shown to lead to high enantioselectivities in the Cu(II)-catalyzed asymmetric Michael addition reaction between indole and alkylene malonates [19]. Excellent catalytic activities and enantioselectivities in the Wacker-type cyclization of allylphenols with up to 99% ee [20]. Despite their great application potential, until now, only a few polyoxazoline ligands have been reported in the literature due to synthetic difficulties. Simpler and more efficient synthetic strategies are required for the preparation of polyoxazoline ligands
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