Chirality Transfer from 4-Methylcyclohexylidenemethyl(phenyl)iodonium Tetrafluoroborate to 4-Methylcycloheptanone during Solvolysis: Evidence against a Primary Vinylic Cation as Intermediate
Chirality Transfer from 4-Methylcyclohexylidenemethyl(phenyl)iodonium Tetrafluoroborate to 4-Methylcycloheptanone during Solvolysis: Evidence against a Primary Vinylic Cation as Intermediate
复制标题
溶剂分解过程中从 4-甲基亚环己基甲基（苯基）碘鎓四氟硼酸盐到 4-甲基环庚酮的手性转移：反对初级乙烯基阳离子作为中间体的证据
DOI：
10.1021/ja001509x
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发表时间：
2000
影响因子：
15
通讯作者：
T. Okuyama
中科院分区：
文献类型：
--
作者：
Morifumi Fujita;Y. Sakanishi;T. Okuyama
在R碳上带有一个氢原子的初级乙烯基阳离子公认是不稳定的，1-3并且只能在特殊条件下产生;例如，通过氚化乙烯的核衰变，4通过光解，5在超强酸中，6或在强硫酸中。7乙烯基阳离子被β-烷基取代基稳定，3，8并且有迹象表明在正常条件下形成伯β，β-二烷基乙烯基阳离子。9-11亚环己基甲基三氟甲磺酸酯被认为是通过一个短暂的主要乙烯基阳离子在醇的水溶液中在高温下进行溶剂分解（方程1）。9在这篇文章中，我们报告了手性4-甲基亚环己基甲基碘鎓盐1的溶剂分解导致完全手性转移到重排产物4-甲基环庚酮（2），这是反对初级乙烯基阳离子中间体参与的证据（方案1）。我们最近发现，不对称取代的β，β-二烷基乙烯基碘鎓盐的溶剂分解通过β-烷基参与发生广泛的重排，并得出结论，在水溶液中不形成伯乙烯基阳离子。10尽管如此，有人提出，伯阳离子可能部分参与亲核性较低的溶剂，如2，2，2-三氟乙醇（TFE）和乙酸。β，β-二烷基乙烯基碘鎓三氟甲磺酸盐在氯仿中的热分解也是通过伯乙烯基阳离子进行的。[11]这些结论是基于从几何异构底物获得的未重排取代产物的E/Z立体化学。然而，由于存在两个不同的β-烷基取代基，导致产物分布相当复杂，这导致三对取代产物和三种通过重排的消除产物。[10] β-烷基迁移能力的差异进一步使反应分析复杂化。使用手性4-取代亚环己基甲基衍生物1作为底物，避免了大部分复杂情况。这两个β-烷基部分在电子和空间上是相同的。唯一的区别是手性，由1形成的伯乙烯基阳离子I1是非手性的，这是由于阳离子碳原子的线性结构1，2。如果生成伯阳离子I1，则手性将丧失，导致产物的对映体纯度降低。预期的唯一产物是未重排和重排形式的对映体取代产物（方案1）。七元环乙炔和联烯的张力大，消除产物难以生成。光学活性的（R）-4-甲基亚环己基甲基（苯基）-碘鎓四氟硼酸盐（1）12，13由根据文献方法15（方案2）获得的（R）-4-甲基亚环己基甲基溴制备。最后一步，乙烯基硅烷转化为相应的乙烯基碘鎓盐，建立为在完全保留构型的情况下进行，16并且1的对映体过量（ee）与前体乙烯基硅烷的对映体过量（ee）相同，除非在后处理期间发生1的外消旋化或光学拆分。发现粗1的结晶降低其ee。69%ee 17的结晶1用于溶剂分解实验。
Primary vinylic cations, bearing a hydrogen atom at the R carbon, are admittedly unstable, 1-3 and can only be generated under special conditions; for example, by nuclear decay of tritiated ethene, 4 by photolysis, 5 in a superacid, 6 or in strong sulfuric acid. 7 Vinyl cations are stabilized by β-alkyl substituents, 3, 8 and there are indications for formation of primary β, β-dialkylvinyl cations under normal conditions. 9-11 Cyclohexylidenemethyl triflate was suggested to undergo solvolysis via a transient primary vinylic cation in aqueous alcohols at high temperature (eq 1). 9 In this contribution we report that solvolysis of the chiral 4-methylcyclohexylidenemethyl iodonium salt 1 results in complete chirality transfer to the rearranged product, 4-methylcycloheptanone (2), which serves as evidence against the involvement of a primary vinylic cation intermediate (Scheme 1). We have recently shown that the solvolysis of unsymmetrically substituted β, β-dialkylvinyliodonium salts takes place with extensive rearrangements through β-alkyl participation, and concluded that primary vinylic cations are not formed in aqueous solution. 10 Nonetheless, it was suggested that primary cations may be partially involved in less nucleophilic solvents such as 2, 2, 2-trifluoroethanol (TFE) and acetic acid. The thermolysis of β, β-dialkylvinyliodonium triflates in chloroform is also proposed to occur via primary vinylic cations. 11 The conclusions are based on the E/Z stereochemistry of the unrearranged substitution products obtained from geometrically isomeric substrates. However, product profiles are quite complicated due to the presence of two different β-alkyl substituents, which leads to three pairs of substitution products and three elimination products via rearrangements. 10 The difference in migratory aptitudes of the β-alkyl groups further complicates the analysis of the reaction. Most of the complications are avoided by using the chiral 4-substituted cyclohexylidenemethyl derivative 1 as substrate. The two β-alkyl moieties are electronically as well as sterically the same. The only difference is the chirality and the primary vinylic cation I1 formed from 1 is achiral due to the linear structure1, 2 of the cationic carbon atom. If the primary cation I1 were generated, the chirality should be lost leading to reduced enantiomeric purity of the products. The only products expected are enantiomeric substitution products of unrearranged and rearranged form (Scheme 1). Elimination products are hard to produce since sevenmembered cyclic acetylene and allene are subject to serious strain. Optically active (R)-4-methylcyclohexylidenemethyl (phenyl)-iodonium tetrafluoroborate (1) 12, 13 was prepared from (R)-4-methylcyclohexylidenemethyl bromide which was obtained according to a literature procedure15 (Scheme 2). The last step, conversion of the vinylsilane to the corresponding vinyliodonium salt is established to proceed with complete retention of configuration, 16 and the enantiomeric excess (ee) of 1 is the same as that of the precursor vinylsilane unless racemization or optical resolution of 1 occurs during the workup. Crystallization of the crude 1 was found to diminish its ee. Crystallized 1 of 69% ee17 was used for the solvolysis experiments.