Efficient dual catalytic enantioselective diethylzinc addition to the exocyclic C [dbnd]N double bond of some 1,2,4- N-triazinylarylimines using polymer-supported chiral β-amino alcohols derived from norephedrine

Abstract

Chiral N, N-dialkylnorephedrines and their corresponding copolymers were evaluated as chiral ligands for the enantioselective diethylzinc addition to the exocyclic C N double bond of some 4-arylideneamino-3-mercapto-6-methyl-4 H-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-4 H-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.