Chiral propargylic cations as intermediates in SN1-type reactions: substitution pattern, nuclear magnetic resonance studies, and origin of the diastereoselectivity.
Chiral propargylic cations as intermediates in SN1-type reactions: substitution pattern, nuclear magnetic resonance studies, and origin of the diastereoselectivity.
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手性炔丙阳离子作为 SN1 型反应的中间体：取代模式、核磁共振研究和非对映选择性的起源
DOI：
10.1021/ja411772n
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发表时间：
2014
影响因子：
15
通讯作者：
T. Bach
中科院分区：
文献类型：
--
作者：
D. Nitsch;S. M. Huber;A. Pöthig;A. Goeppert;G. A. Olah;G. K. S.Prakash;T. Bach
制备了9种炔丙基乙酸酯，它们在亲电碳原子附近带有立体异构中心（−C* HXR 2），并使用三氟甲磺酸铋[Bi（OTf）3]作为刘易斯酸与各种甲硅烷基亲核试剂进行SN 1型取代反应。当烷基R2为叔丁基时，反应的非对映选择性高，与取代基X无关。产物形成一致，非对映体比例大于95：5，有利于反式非对映体。如果烷基取代基R2是仲烷基，则非对映选择性降低至80：20。该反应被证明进行立体收敛，并阐明了相关的产品配置。反应的结果是通过调用一个手性炔丙基阳离子作为中间体，这是优先攻击的亲核试剂从它的两个非对映异构体面之一解释。密度泛函理论（DFT）计算表明，一个优选的构象中，基团R2是几乎垂直于由在阳离子中心的三个取代基所定义的平面，与亲核试剂接近亲电中心R2的对面。过渡态计算的烯丙基三甲基硅烷与两个代表性的阳离子的反应支持这一假设。在α位具有立体中心（−C* HXR 2）的叔炔丙基阳离子是通过在−80 °C下在SO2ClF中用FSO3H电离相应的醇前体而产生的。核磁共振（NMR）光谱获得的五个阳离子，和化学位移可以明确地分配。从核Overhauser实验中提取的阳离子的优选构象与负责二级炔丙基阳离子反应的优选构象一致。
Nine propargylic acetates, bearing a stereogenic center (−C*HXR2) adjacent to the electrophilic carbon atom, were prepared and subjected to SN1-type substitution reactions with various silyl nucleophiles employing bismuth trifluoromethanesulfonate [Bi(OTf)3] as the Lewis acid. The diastereoselectivity of the reactions was high when the alkyl group R2was tertiary (tert-butyl), irrespective of the substituent X. Products were formed consistently with a diastereomeric ratio larger than 95:5 in favor of theanti-diastereoisomer. If the alkyl substitutent R2was secondary, the diastereoselectivity decreased to 80:20. The reaction was shown to proceed stereoconvergently, and the relative product configuration was elucidated. The reaction outcome is explained by invoking a chiral propargylic cation as an intermediate, which is preferentially attacked by the nucleophile from one of its two diastereotopic faces. Density functional theory (DFT) calculations suggest a preferred conformation in which the group R2is almost perpendicular to the plane defined by the three substituents at the cationic center, with the nucleophile approaching the electrophilic center opposite to R2. Transition states calculated for the reaction of allyltrimethylsilane with two representative cations support this hypothesis. Tertiary propargylic cations with a stereogenic center (−C*HXR2) in the α position were generated by ionization of the respective alcohol precursors with FSO3H in SO2ClF at −80 °C. Nuclear magnetic resonance (NMR) spectra were obtained for five cations, and the chemical shifts could be unambiguously assigned. The preferred conformation of the cations as extracted from nuclear Overhauser experiments is in line with the preferred conformation responsible for the reaction of the secondary propargylic cations.
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