Efficient dual catalytic enantioselective diethylzinc addition to the exocyclic CN double bond of some 1,2,4-N-triazinylarylimines using polymer-supported chiral β-amino alcohols derived from norephedrine
- 作者：Ashraf A. El-Shehawy
- 关键词：Polymeric reagents and catalysts ; Chiral polymers ; 1 ; 2 ; 4-Triazines ; Arylimines ; Dual catalytic ; Enantioselective ethylation ; Lewis acids
- 刊名：Tetrahedron
- 出版年：2007
- 期刊代码：107_00404020
- 类别：ch
- 出版时间：18 June 2007
- 卷：63
- 期：25
- 页码：5490-5500
- 文件大小：346 K
摘要
Chiral N,N-dialkylnorephedrines and their corresponding copolymers were evaluated as chiral ligands for the enantioselective diethylzinc addition to the exocyclic CN double bond of some 4-arylideneamino-3-mercapto-6-methyl-4H-1,2,4-triazin-5-ones 2a–f. The use of a dual catalytic system (amino alcohol/halosilane) in the titled asymmetric reaction was examined. The enantioselective ethylation reaction has been successfully carried out in the heterogeneous system even at low temperature. The corresponding 4-(1-arylpropyl)amino-3-mercapto-6-methyl-4H-1,2,4-triazin-5-ones 4a–f were obtained in high yields with high enantioselectivities using chiral polymers (up to 91%ee), which are almost the same as those obtained from homogeneous analogues (up to 92%ee). The diethylzinc reagent neither opened the 1,2,4-triazinyl heterocyclic ring nor attacked the carbonyl or the thione groups of the 1,2,4-triazinyl heterocyclic ring and the addition reaction took place exclusively at the exocyclic electrophilic carbon atom yielding the C-ethylated products 4a–f. Reductive cleavage of the 1,2,4-triazinyl heterocyclic ring led smoothly to the corresponding primary aromatic amines 11a–f without significant loss of enantiomeric purity. A suggestion about the possible transition state for the addition reaction is also presented.