Abstract
A series of new C2-symmetric fluoren-9-ylidene malonate-derived bis(oxazoline) ligands were synthesized from fluoren-9-ylidene malonate and enantiomerically pure amino alcohols via a convenient route. Their asymmetric catalytic properties in the Friedel-Crafts reactions of indoles with arylidene malonates were evaluated, and the Cu(OTf)2 complex of a fluoren-9-ylidene malonate-derived bis(oxazoline) bearing a phenyl group showed moderate to good enantioselectivity (up to 88% ee).
Highlights
During the past two decades, a plethora of bis(oxazoline) ligands have been synthesized and successfully applied in a variety of asymmetric catalytic reactions [1,2,3]
One straightforward strategy to tune the bridge angle was introduced to BOX ligands, in which two oxazoline rings are attached to a sp2 hybridized carbon and provide a larger bridge angle than those with sp3 hybridized bridge carbon
The diacyl chloride condensed with chiral β-amino alcohols in the presence of Et3N to give the corresponding chiral intermediate dihydroxydiamides 3 in good yields, which were treated with methanesulfonyl chloride and excess Et3N in dichloromethane to afford the desired bis(oxazoline)s 1a~d in good yield (75-84%)
Summary
During the past two decades, a plethora of bis(oxazoline) ligands have been synthesized and successfully applied in a variety of asymmetric catalytic reactions [1,2,3]. Ligands derived from malonate and its analogues, the bridge angle φ, correlating with the bite angle θ of the BOX-metal complex, is an important structural factor influencing the enantioselectivity of the catalysis [4,5,6]. One straightforward strategy to tune the bridge angle was introduced to BOX ligands, in which two oxazoline rings are attached to a sp hybridized carbon and provide a larger bridge angle than those with sp hybridized bridge carbon. We reported that heteroarylidene malonate-derived bis(oxazoline) ligand III-copper(II) complexes demonstrated excellent enantioselectivities (up to >99% ee) in the catalytic Friedel-Crafts reactions between indoles and diethyl alkylidenemalonates [10]. As a continuation of our ongoing endeavor to explore these novel chiral ligands and their application in synthetic methodology, we wish to document the synthesis and application of fluoren-9-ylidene malonate derived bis(oxazoline) 1
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