Enantioselective Addition of Diethylzinc to Aromatic Aldehydes Using Chiral Oxazoline-Based Ligands
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nantioselective Addition of Diethylzinc to Aromatic Aldehydes Using Chiral Oxazoline-Based Ligands A. E. Aydina,* a
Department of Chemistry, Arts and Science Faculty, Hatay Mustafa Kemal University, Hatay, 31040 Turkey *e-mail: [email protected] Received April 4, 2020; revised July 21, 2020; accepted July 22, 2020
Abstract—Chiral oxazoline ligands containing an aromatic ring were prepared from norephedrine and pyrrole2-carbonitrile or 2-hydroxybenzoyl chloride. The synthesized ligands were used in the copper-catalyzed asymmetric addition of diethylzinc to aromatic aldehydes to provide optically active 1-arylpropan-1-ols with high conversion (92%) and enantioselectivity (up to 99% ee). Keywords: asymmetric catalysis, enantioselective synthesis, chiral oxazoline-based ligands, 1,2-addition
DOI: 10.1134/S1070428020070271 INTRODUCTION Enantioselective addition of dialkyzinc to carbonyl compounds is one of the most important reactions forming a new carbon–carbon bond in asymmetric synthesis [1–3]. The asymmetric addition of organometallic reagents to carbonyl substrates usually occurs with low enantiomeric excesses due to high chemical yields [4, 5]. On the other hand, enantioselective catalytic addition of dialkyzinc to aldehydes is by far one of the most studied enantioselective addition reactions. This reaction provides an important method for the synthesis of optically active secondary alcohols [6–9] that are widely found in nature and are also important building blocks in organic synthesis [9]. Enantioselective addition of diethylzinc to aldehydes was applied to obtain optically active lactones [10], cyclopropyl alcohols [11], as well as the natural product (+)-(R)-gossonorol which shows antifungal, anticancer, and antioxidant activities [12]. The enantioselectivity of a reaction depends mainly on the chiral ligand structure; therefore, search for new ligands for this important asymmetric transformation is a field of continuous interest. Among the chiral ligand families, primary amino alcohols [13–16] aminophenols [17, 18], diamines [19–21], disulfonamides [22–25], diols [26–28], and their derivatives have been widely employed in such reactions. Although a number of chiral ligands have been prepared, research focused on the development of new active, enantioselective, easily obtainable, and economically viable catalysts continues.
It is well known that oxazolines are synthetically and biologically significant molecular structures [29, 30]. These compounds are used as protecting groups for carboxylic acids and hydroxylamines [31]. Chiral oxazolines have also been extensively used in asymmetric syntheses as chiral catalysts [32, 33]. Chiral ligands containing one or more oxazoline rings have been synthesized and used to prepare enantiomerically pure compounds in many metal-catalyzed asymmetric reactions. Examples of widely used ligands of this type are pyridyloxazolines [34–37], bisoxazoline [38], bisoxazolinopyridine [39–43], BINOL-oxazoline [44–46], and phosphine-oxazoline [47–50] derivatives. These chiral lig
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