Abstract:

Chiral amines have been widely used as intermediates in pharmaceutical, agro-chemicals and additive chemistry. They could be obtained by employing chiral catalysts. Thus, to design and synthesize various chiral catalysts have become an important target in this area, and many kinds of chiral catalysts were reported. The key backbone of axial chiral N,N'-dioxides has been designed and reported in early reseach. In this study, five new chiral axial 3,3'-bis[(substitutes)carbamoyl]-9,9'-dimethyl-9H,9'H-[1,1'-bipyrido[3,4-b]indole]2,2'-dioxide(4A—4E) were synthesized from natural amino acids via several steps, such as L-tryptophan by hydrolysis, acylation and N-oxidation reactions, etc. These new chiral ligands 4A—4E were employed in catalytic enantioselective reductions of ketoimines with trichlorosilane. The effects of reaction temperatures, catalyst percentages and the various substrates on the enantioselectivity were investigated as well. The results exhibited that the chiral ligand 3,3'-bis(cyclohexylcarbamoyl)-9,9'-dimethyl-9H,9'H-[1,1'-bipyrido(3,4-b)indole]2,2'-dioxide(4B) possessed a good enantioselectivity of 68% with a conversion yield of 98% when 5%(molar fraction) of catalyst 4B was used in the reactions in solvent CH₂Cl₂ at 0 ℃ for 16 h. The lower or higher temperature than 0 ℃ could result in a lower enantioselectivity. In addition, the different structures of substrates also had an influence on the enantioselectivity. The substrates with an electron donor group had a better enantioselectivity than that with an electron withdrawing group. The best catalyst 4B could be a leading catalyst for further chiral catalyst design.

Key words: Biscarboline N—O amide, Asymmetric hydrosilation, Ketoimine, Trichlorosilane