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

Abstract

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 β-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 β-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 β-C-H and β-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 (C(6)H(6)OH(+) ↔ C(6)H(5)OH 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".

Hyperaromatic Stabilization of Arenium Ions: Acid-Catalyzed Dehydration of 2-Substituted 1,2-Dihydro-1-naphthols

“…However, these substituents also gave partial rearrangement, with formation of 1-as well as 2-substituted naphthalene products. 4 For the cis isomers it seems likely that the rearrangement must follow formation of the β-hydroxynaphthalenium ion intermediate and thus does not affect its rate of formation. In the case of the trans isomers rearrangement probably accompanies formation of the carbocation.…”

“…This is supported by computational results and calculations of ring currents for arenium ions described in a following paper. 4 An obvious question arises that will be addressed in the present paper. What is the consequence of extending the range of 2-substituents from hydroxyl to other possibilities that can be envisaged?…”

Hyperconjugative aromaticity and protodeauration reactivity of polyaurated indoliums

“…More importantly, HA helps people understand and explains some typical reactivity. For example, O’Ferrall and co-workers once correlated the HA in arenium intermediates with the enhancement of dehydration reaction rates of diols 12,13 . Recently, Houk and co-workers revealed that the reactivity and stereoselectivity of 5-substituted cyclopentadienes in Diels–Alder reactions are also related to HA 14–16 .…”

Dearomative dihydroxylation with arenophiles

“…Therefore, we decided to examine a one-pot urazole hydrolysis/bicyclic hydrazine oxidation sequence that would generate dihydrodiols via the extrusion of molecular dinitrogen. Although the hydrolysis and oxidation sequence was expected to be reasonably straightforward, the lability of such dihydrodiols was potentially troublesome 21,32 . Indeed, this turned out to be the case, as we observed significant amounts of phenol formation during the oxidation step.…”