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首页> 外文期刊>Chemistry: A European journal >On the origin of the stereoselectivity in organocatalysed reactions with trimethylsilyl-protected diarylprolinol
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On the origin of the stereoselectivity in organocatalysed reactions with trimethylsilyl-protected diarylprolinol

机译：关于三甲基甲硅烷基保护的二芳基脯氨醇在有机催化反应中立体选择性的起源

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摘要

The origin of the enantioselectivity in the TMS-protected (TMS = trimethylsilyl) prolinol-catalysed alpha-heteroatom functionalisation of aldehydes has been investigated by using density functional theory calculations. Eight different reaction paths have been considered which are based on four different conformers of the TMS-protected prolinol-enamine intermediate. Optimisation of the enamine structures gave two intermediates with nearly the same energy. These intermediates both have an E configuration at the C-C bond and the double bond is positioned anti or syn, relative to the 2-substituent in the pyrrolidine ring. For the four intermediates, the chiral TMS-protected-diaryl substituent effectively shields one of the faces of the reacting C = C bond in the enamine intermediate. A number of transition states have been calculated for the enantioselective fluorination by N-fluorobenzenesulfonimide (NFSI) and based on the transition-state energies it has been found that the enantioselectivity depends on the orientation of the C = C bond, being anti or syn, relative to the 2-substituent on the pyrrolidine ring, rather than the approach of the electrophilic fluorine to the face of the reacting carbon atom in the enamine which is less shielded relative to the face with the highest shielding. The calculated enantiomeric excess of 96% ee (ee=enantiomeric excess) for the fluorination reaction corresponds well with the experimentally found enantiomeric excess-97% ee. The transition state for the alpha-amination reaction with the same type of intermediate has also been calculated by using diethyl azodicarboxylate as the amination reagent. The implication of the intermediate structures on the stereoselection of alpha-functionalisation of aldehydes is discussed.
机译：已通过使用密度泛函理论计算研究了TMS保护的（TMS =三甲基甲硅烷基）脯氨醇催化的醛的α-杂原子官能化中对映选择性的起源。已经考虑了八种不同的反应路径，这些路径基于TMS保护的脯氨醇-烯胺中间体的四个不同构象体。烯胺结构的优化给出了两种具有几乎相同能量的中间体。这些中间体在C-C键上均具有E构型，并且相对于吡咯烷环中的2-取代基，双键位于反位或同位。对于这四个中间体，手性TMS保护的二芳基取代基有效地屏蔽了烯胺中间体中正在反应的C = C键的一个面。对于N-氟苯磺酰亚胺（NFSI）的对映选择性氟化，已经计算出许多过渡态，并且基于过渡态能量，发现对映选择性取决于C = C键的方向，是反键还是顺键，相对于吡咯烷环上的2-取代基，而不是亲电氟接近烯胺中反应性碳原子的表面（相对于具有最高屏蔽性的表面而言，屏蔽性较低）。对于氟化反应，计算出的96％ee的对映体过量（ee ＝对映体过量）与实验发现的对映体过量97％ee很好地对应。还通过使用偶氮二羧酸二乙酯作为胺化试剂来计算与相同类型中间体的α胺化反应的过渡态。讨论了中间结构对醛的α-官能化的立体选择的影响。

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