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Transformations and Tautomeric Equilibrium among Different Intermediates in Proline-Catalyzed Reactions of Aldehydes or Ketones
Li-hua Gan*,Qing Chang,Jin Zhou
Author NameAffiliationE-mail
Li-hua Gan* School of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China ganlh@swu.edu.cn 
Qing Chang School of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China  
Jin Zhou School of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China  
Abstract:
The enamines, iminium ions, and oxazolidinones are thought to be the key intermediates in the proline-catalyzed reactions of aldehydes or ketones, but there is an extensive contro-versy about their roles. Here, the corresponding transition states connecting any two of the three kinds of species are located at the ωB97XD/6-311++G** level of theory. The calcula-tions demonstrate that the oxazolidinones are the predominant species in both the gas phase and solvents; there exists tautomeric equilibrium among these species and the equilibriums are controlled by the employed solvents and temperature in the reaction. These results demonstrate that the concentration and role of the mentioned species are controlled by the employed solvent and temperature. A new reaction pathway is presented herein for the trans-formation between iminium ions and oxazolidinones through iminium ion-water complex and oxazolidinone-water complex. The calculations demonstrate that the rate-limiting step in proline-catalyzed Mannich reaction between acetaldehyde/keteones and N-Boc imines is the formation of the C-C bond rather than the intermediates tautomerization. These calcula- tions rationalize the available experimental observations and can be valuable in optimizing the experimental conditions of asymmetric organic-catalyzed reactions of aldehydes or ke-tones.
Key words:  Asymmetric organocatalysis, Tautomerism, Transition state, Intermediate, Density functional theory
FundProject:
脯氨酸催化的醛或酮反应中不同中间体的相互转换及异构平衡
甘利华*,苌青,周瑾
摘要:
烯胺、亚胺离子和恶唑烷酮是脯氨酸催化醛、酮反应中的关键中间体,在ωB97XD/6-311++G**理论水平上找到了连接其中任意两个中间体的过渡态. 计算结果表明无论在气相还是溶液中恶唑烷酮都是优势物种,这些物种是可以相互转化的,且异构平衡受控于所用溶剂和实验温度,这些物种存在与否以及发挥的作用取决于实验条件.提出了一条亚胺离子和恶唑烷酮通过亚胺水合物和恶唑烷酮水合物进行相互转换的新路径. 计算结果还表明在醛/酮与N-Boc亚胺的脯氨酸催化反应中,决速步骤是C-C键的形成而不是中间体的相互转换. 这些计算结果合理地解释了已有实验现象,并有利于优化醛、酮不对称有机催化反应的条件.
关键词:  不对称有机催化,互变异构,过渡态,密度泛函理论
DOI:10.1063/1674-0068/26/01/54-60
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