新轴手性N,N-Dioxide Biscarbolin衍生物催化酮亚胺的不对称硅氢化反应

New Chiral Biscarboline N,N-Dioxide Derivatives as Catalysts over Enantioselective Hydrosilylation of Keteoimines

以L-色氨酸为原料合成了5个伯酰胺结构的轴手性双咔啉N—O催化剂N2,N2'-二氧-9,9'-二甲基-3,3'-取代甲酰胺-β-双咔啉(4A^4E),并用于不对称催化酮亚胺的还原反应.结果表明,催化剂的催化转化率较高(80%~98%),立体选择性(e.e.值)较好,其中催化剂N2,N2'-二氧-9,9'-二甲基-3,3'-环己基甲酰胺-β-双咔啉(4B)的催化转化率达到了98%,e.e.值达68%.

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 vari- ous chiral catalysts have become an important target in this area, and many kinds of chiral catalysts were re- ported. 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- I 1,1 ＇-bipyrido（3 ,g-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 CH2C12 at 0 ~C for 16 h. The lower or higher temperature than 0 ~C 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.