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Synthesis and Application of N-Ar Axially Chiral N-Heterocyclic Carbene-Oxazoline Ligands

Author: LiShengKe
Tutor: ZhaoMeiXin；ShiMin
School: East China University of Science and Technology
Course: Organic Chemistry
Keywords: Axial chirality N-Heterocyclic carbene Oxazoline Asymmetric allylation MBH adduct Kinetic resolution
CLC: O621.25
Type: Master's thesis
Year: 2012
Downloads: 41
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Abstract


This thesis mainly focused on the synthesis and asymmetric application of N-Ar axially chiral N-heterocyclic carbene (NHC)-oxazoline ligands. It can be divided into two parts:(ⅰ) the synthesis of NHC-oxazoline ligands and their coordination study with palladium(Ⅱ) and gold(Ⅰ); (ⅱ) the application of N-Ar axially chiral Pd(Ⅱ) complex in the asymmetric allylation of Morita-Baylis-Hillman(MBH) adducts with arylboronic acid.1. A novel type of N-Ar axially chiral NHC-oxazoline ligands was successfully synthesized from methyl 1-hydroxy-2-naphthoate through six steps. The two distereomeric isomers of ligand (aS,S)-I-7 and (aR,S)-I-7 could be easily separated through silica gel column chromatography. An appropriate condition under which the two isomers could transform into each other was developed. The coordination of these ligands with Pd(OAc)2 and AuCl·Me2S was studied as well. Axially chiral ligands with S configuration coordinated with metals smoothly, none bidentate palladium complexes and monodentate gold complexes were obtained. However,, presumably due to the steric repulsion of the substituents on the oxazoline ring, ligands with R configuration couldn’t furnish desired palladium complexes, and the yields of their monodentate gold complexes decreased sharply. All the ligands and complexes were characterized by 1H NMR,13C NMR, HRMS and IR spectroscopies et al. Moreover, the structures of complexes (aS,S)-I-8a and (aS,S)-I-9a were also confirmed by their single-crystal X-ray diffraction analysis.2. With palladium complex (aS,S)-I-8 in hand, the Pd-catalyzed asymmetric allylation reaction between hindered MBH adducts and arylboronic acids was examined. The trisubstituted alkenes with up to 99% ee value were obtained, and 92% ee of recovered Morita-Baylis-Hillman adducts were also obtained. Arylboronic acids and MBH adducts derived from Aromatic aldehydes were better substrates when there is a withdrawing group at the 4-position of the phenyl ring. MBH adducts derived from alkyl aldehyde were the best substrates. Besides, the absolute configurations of the asymmetric allylation adducts were confirmed by the single-crystal X-ray diffraction analysis and the comparation of the optical rotation. The reaction mechanism was also proposed, which is Pd-catalyzed 1,4-addition and the subsequent elimination of the original chiral center via Pd-OH elimination mechanism. The palladium complex was further utilized to catalyze the asymmetric asymmetric allylation of (E)-1,3-diphenylprop-2-en-I-ol with arylboronic acids.

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