Hyperaromatic Stabilization of Arenium Ions: A Remarkable Cis Stereoselectivity of Nucleophilic Trapping of 尾-Hydroxyarenium Ions by Water

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• 作者：David A. Lawlor ; Jaya Satyanarayana Kudavalli ; Aoife C. MacCormac ; Dara A. Coyne ; Derek R. Boyd ; Rory A. More O鈥橣errall
• 刊名：Journal of the American Chemical Society
• 出版年：2011
• 出版时间：December 14, 2011
• 年：2011
• 卷：133
• 期：49
• 页码：19718-19728
• 全文大小：993K
• 年卷期：v.133,no.49(December 14, 2011)
• ISSN：1520-5126

文摘

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 尾-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鈥?-fold, these results imply a high cis stereoselectivity for nucleophilic trapping of a 尾-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 鈥渇orward鈥?reaction) of different conformations of carbocation from cis- and trans-dihydrodiol reactants with respectively 尾-C鈥揌 and 尾-C鈥揙H 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鈥揌 than a C鈥揙H 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⁺ 鈫?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 鈥渉yperaromatic鈥?