Hyperaromatic Stabilization of Arenium Ions: A Remarkable Cis Stereoselectivity of Nucleophilic Trapping of β-Hydroxyarenium Ions by Water
Hyperaromatic Stabilization of Arenium Ions: A Remarkable Cis Stereoselectivity of Nucleophilic Trapping of β-Hydroxyarenium Ions by Water
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DOI：
10.1021/ja207160z
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发表时间：
2011-12-14
影响因子：
15
通讯作者：
O'Ferrall, Rory A. More
中科院分区：
文献类型：
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作者：
Lawlor, David A.;Kudavalli, Jaya Satyanarayana;O'Ferrall, Rory A. More
苯的顺式和反式-1，2-二氢二醇异构体经过酸催化脱水形成苯酚。原则上，异构体底物通过共同的β-羟基苯阳离子（环己二烯基）碳正离子反应。尽管如此，异构体显示出反应性的大差异，k（顺式）/k（反式）= 4500。对于萘的1，2-二氢二醇和菲的9，10-二氢二醇，这种差异分别减少到k（顺式）/k（反式）= 440和50，对于苊烯的非芳族7，8-双键的二氢二醇，减少到6.9。由于顺式和反式二氢二醇的稳定性差异不应超过2-3倍，因此这些结果意味着在碳阳离子形成反应的逆过程中，水对β-羟基芳烃阳离子的亲核捕获具有高顺式立体选择性。这通过研究菲的反式-9，10-溴代醇衍生物和菲顺式-9，10-二氢二醇的一三氯乙酸酯的水溶剂分解产生的10-羟基-9-菲鎓离子得到证实。顺式和逆反应的顺式立体选择性是由顺式和反式二氢二醇反应物中碳阳离子的不同构象的形成（在“正”反应中）来解释的，所述反应物分别具有相对于超共轭最佳的碳阳离子的电荷中心的假轴位置中的β-C-H和β-C-OH键。不同构象的形成受到（质子化）OH离去基团从假轴位置离开的约束。碳正离子稳定性的差异被认为是源于（a）C-H键比C-OH键具有更强的超共轭能力和（B）由代表超共轭的无键共振结构（C6 H6 OH + C6 H5 OH H+）中的芳香环的稳定影响引起的增强的共轭。这与Mulliken的早期建议和Schleyer的证明是一致的，即苯阳离子受到超共轭芳香稳定化的影响。有人建议，在类似的术语同共轭和同芳香性，芳烃离子应被视为“超芳族”。
Cis- and trans-1,2-dihydrodiol isomers of benzene undergo acid-catalyzed dehydration to form phenol. In principle the isomeric substrates react through a common beta-hydroxybenzenium (cyclohexadienyl) carbocation. Notwithstanding, the isomers show a large difference in reactivity, k(cis)/k(trans) = 4500. This difference is reduced to k(cis)/k(trans) = 440 and 50 for the 1,2-dihydrodiols of naphthalene and 9,10-dihydrodiols of phenanthrene, respectively, and to 6.9 for the dihydrodiols of the nonaromatic 7,8-double bond of acenaphthylene. Because the difference in stabilities of cis- and trans-dihydrodiols should be no more than 2-3-fold, these results imply a high cis stereoselectivity for nucleophilic trapping of a beta-hydroxyarenium cation by water in the reverse of the carbocation-forming reaction. This is confirmed by studies of the 10-hydroxy-9-phenanthrenium ion generated from aqueous solvolyses of the trans-9,10-bromohydrin derivative of phenanthrene and the monotrichloroacetate ester of the phenanthrene cis-9,10-dihydrodiol. The cis stereoselectivity of forward and reverse reactions is explained by the formation (in the "forward" reaction) of different conformations of carbocation from cis- and trans-dihydrodiol reactants with respectively beta-C-H and beta-C-OH bonds in pseudoaxial positions with respect to the charge center of the carbocation optimal for hyperconjugation. Formation of different conformations is constrained by departure of the (protonated) OH leaving group from a pseudoaxial position. The difference in stability of the carbocations is suggested to stem (a) from the greater hyperconjugative ability of a C-H than a C-OH bond and (b) from enhanced conjugation arising from the stabilizing influence of an aromatic ring in the no-bond resonance structures representing the hyperconjugation (C6H6OH+ C6H5OH H+). This is consistent with an earlier suggestion by Mulliken and a demonstration by Schleyer that the benzenium ion is subject to hyperconjugative aromatic stabilization. It is proposed that, in analogy with the terms homoconjugation and homoaromaticity, arenium ions should be considered as "hyperaromatic".