Hyperaromatic stabilization of arenium ions.

Benzene-cis- and trans-1,2-dihydrodiols undergo acid-catalyzed dehydration at remarkably different rates: k(cis)/k(trans) = 4500. This is explained by formation of a ß-hydroxycarbocation intermediate in different initial conformations, one of which is stabilized by hyperconjugation amplified by an aromatic no-bond resonance structure (HOC(6)H(6)(+) ↔ HOC(6)H(5) H(+)). MP2 calculations and an unfavorable effect of benzoannelation on benzenium ion stability, implied by pK(R) measurements of -2.3, -8.0, and -11.9 for benzenium, 1-naphthalenium, and 9-phenanthrenium ions, respectively, support the explanation.

ß-Hydroxy carbocation intermediates in solvolyses of di- and tetra-hydronaphthalene substrates.

Solvolysis of trichloroacetate esters of 2-methoxy-1,2-dihydro-1-naphthols shows a remarkably large difference in rates between the cis and trans isomers, k(cis)/k(trans) = 1800 in aqueous acetonitrile. This mirrors the behaviour of the acid-catalysed dehydration of cis- and trans-naphthalene-1,2-dihydrodiols to form 2-naphthol, for which k(cis)/k(trans) = 440, but contrasts with that for solvolysis of tetrahydronaphthalene substrates, 1-chloro-2-hydroxy-1,2,3,4-tetrahydronaphthalenes, for which k(cis)/k(trans) = 0.5. Evidence is presented showing that the trans isomer of the dihydro substrates reacts unusually slowly rather than the cis isomer unusually rapidly. Comparison of rates of solvolysis of 1-chloro-1,2,3,4-tetrahydronaphthalene and the corresponding (cis) substrate with a 2-hydroxy group indicates that a ß-OH slows the reaction by nearly 2000-fold, which represents a typical inductive effect characteristic also of cis-dihydrodiol substrates. The slow reaction of the trans-dihydrodiol substrate is consistent with initial formation of a ß-hydroxynaphthalenium carbocation with a conformation in which a C-OH occupies an axial position ß to the carbocation centre preventing stabilisation of the carbocation by C-H hyperconjugation, which would occur in the conformation initially formed from the cis isomer. It is suggested that C-H hyperconjugation is particularly pronounced for a ß-hydroxynaphthalenium ion intermediate because the stability of its no-bond resonance structure reflects the presence of an aromatic naphthol structure.

Toluene dioxygenase-catalyzed synthesis of cis-dihydrodiol metabolites from 2-substituted naphthalene substrates: assignments of absolute configurations and conformations from circular dichroism and optical rotation measurements.

cis-Dihydrodiol metabolites have been isolated from naphthalene and six 2-substituted naphthalene substrates. Their structures and absolute configurations have been determined by a combination of calculated (TDDFT) and experimentally based circular dichroism (CD) and optical rotation (OR) methods. The "inverse" styrene helicity rule is shown to be incorrect for the interpretation of the CD spectra of cis-dihydrodiols. A striking conclusion is that CD spectra correlate directly with the helicity of the styrene chromophore: that is, the sign of the long-wavelength Cotton effect is identical with the sign of styrene torsion angle, whereas the OR sign is dependent on the absolute configuration of the allylic carbon atom. The results demonstrate that a predictive model previously used for the determination of preferred regio- and stereoselectivity associated with TDO-catalyzed cis-dihydroxylation of substituted benzene substrates can now be successfully extended to substituted naphthalene substrates.