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PubMed1998

Polychlorinated naphthalenes: an environmental update.

Polychlorinated naphthalenes (PCNs; CNs) form a complex mixture of up to 75 congeners containing from one to eight chlorine atoms per naphthalene molecule. Chloronaphthalenes are widespread global environmental pollutants which accumulate in biota. All chloronaphthalenes are planar compounds and can contribute to the aryl hydrocarbon (Ah) receptor-mediated mechanism of toxicity with a combination of various 2,3,7,8-tetrachlorodibenzo-p-dioxin like toxic responses. There are three known main sources of environmental pollution with PCNs: technical PCN formulations, technical polychlorinated biphenyl formulations, and thermal and other processes in the presence of chlorine. The purpose of this paper is to briefly review the most recent data on environmental pollution, chemistry, analysis, sources, formation, persistence, toxicity and behavior of PCNs.

Journal Article↗
PubMed2000

Vibrational spectra of the adduct of 1,8-bis(dimethylamino)naphthalene with dichloromaleic acid (DMAN x DCM).

The infra-red (IR), Raman (R) and inelastic incoherent neutron scattering (IINS) spectra, particularly in low frequency region, of the title ionic adduct were studied. It is shown that all low frequency vibrations (below 200 cm(-1)) of (CH3)2N groups of protonated 1,8-bis(dimethylamino)naphthalene (DMAN)--clearly observed in IINS spectra--are sensitive to the environment, i.e. to the type of counterion forming short contacts with C-H bonds of methyl groups. The internal frequencies were also calculated by ab initio method. The results are consistent with numerous observations of the counteranion effect on the geometry of the protonated DMAN. The conclusions are compared with structural and NMR studies reported recently for the 1,8-bis(dimethylamino)naphthalene with dichloromaleic acid (DMAN x DCM) adduct. The single crystal R polarized spectra taken over the frequency range 20-3200 cm(-1) were analyzed in detail. We have shown that a substantial difference in the IR spectrum of the dichloromaleic acid (DCM) anion in the DMAN adduct and in the potassium salt results from different geometries of OHO hydrogen bonds. In the case of potassium salt the chains of longer intermolecular hydrogen bonds are formed described by means of a double minimum potential.

1-Naphthylamine↗
PubMed2007

Separation of C2-naphthalenes by gas chromatography x fourier transform infrared spectroscopy (GC x FT-IR): two-dimensional separation approach.

A two-dimensional separation approach involving gas chromatography (GC) and Fourier transform infrared spectroscopy (FT-IR) is used to separate C2-naphthalene isomers at or near baseline resolution. In addition to GC separation, the FT-IR also plays an important role in the separation, as well as its traditional role of detection and identification. This two-dimensional separation approach for the analysis of a C2-naphthalene isomeric mixture is a good example of separation design based on molecular difference and the characteristics of an analytical instrument. The details of this two-dimensional separation are discussed, along with the advantages and limitations of this approach. While two-(or multiple-)dimensional separations have demonstrated superior capabilities in the characterization of complex, largely unknown mixtures, the development of GCxFT-IR illustrates the applicability of this analytical approach in the separation of simpler but still challenging, mixtures. The GCxFT-IR results have extended this approach toward its application to the analysis of samples of more complicated composition.

Journal Article↗
PubMed1981

Fluorescent substrate analogue for adenosine deaminase: 3'-O-[5-(dimethylamino)naphthalene-1-sulfonyl]adenosine.

The synthesis of the fluorescent derivative of adenosine, by reaction with 5-(dimethylamino)naphthalene-1-sulfonyl chloride in dry pyridine at low temperature, yielding 3'-O-[5-(dimethylamino)naphthalene-1-sulfonyl]adenosine (3'-O-dansyladenosine), is here described. 3'-O-Dansyladenosine is partially soluble in water (approximately 10(-4) M) and upon excitation at 325 nm exhibits maximum fluorescence emission at 516 +/- 22 nm (corrected) in buffered aqueous solution at pH 7.6 with a quantum yield of 0.21 and a lifetime of 11.8 +/- 0.2 ns. The fluorescence of 3'-O-dansyladenosine is sensitive to the polarity of its solvent: in pyridine, a quantum yield of 0.61 at the emission maximum of 435 nm was observed. 3'-O-Dansyladenosine is a reversible competitive inhibitor of adenosine deaminase with a moderate inhibitive dissociation constant K1 = (1.54 +/- 0.13) X 10(-5) M. The enzyme-substrate analogue association constant was determined by equilibrium dialysis to be K = (0.69 +/- 0.05) X 10(5) M-1, very close to KI-1. The hydrophobic nature of its binding site in adenosine deaminase is evident from the strong blue shift of the fluorescence emission maximum to 440 nm, the 4-fold increase in fluorescence quantum yield, and the longer lifetime of 15.8 +/- 0.2 ns; the tight, rigid nature of the complex is evident from its high fluorescence polarization value, 0.23. An 85% decrease in the fluorescence emission intensity of the adenosine deaminase-3'-O-dansyladenosine complex in the presence of adenosine indicates the selective binding to the enzyme active site. Correlation between the conformation of the probe, either when free in various solvents or when bound to the enzyme, and its fluorescence quantum yield is noted. 3'-O-Dansyladenosine is suitable for fluorescent labeling of adenosine deaminase in cell systems.

Adenosine↗
PubMed2006

Characterization of a naphthalene dioxygenase endowed with an exceptionally broad substrate specificity toward polycyclic aromatic hydrocarbons.

In Sphingomonas CHY-1, a single ring-hydroxylating dioxygenase is responsible for the initial attack of a range of polycyclic aromatic hydrocarbons (PAHs) composed of up to five rings. The components of this enzyme were separately purified and characterized. The oxygenase component (ht-PhnI) was shown to contain one Rieske-type [2Fe-2S] cluster and one mononuclear Fe center per alpha subunit, based on EPR measurements and iron assay. Steady-state kinetic measurements revealed that the enzyme had a relatively low apparent Michaelis constant for naphthalene (K(m) = 0.92 +/- 0.15 microM) and an apparent specificity constant of 2.0 +/- 0.3 mM(-)(1) s(-)(1). Naphthalene was converted to the corresponding 1,2-dihydrodiol with stoichiometric oxidation of NADH. On the other hand, the oxidation of eight other PAHs occurred at slower rates and with coupling efficiencies that decreased with the enzyme reaction rate. Uncoupling was associated with hydrogen peroxide formation, which is potentially deleterious to cells and might inhibit PAH degradation. In single turnover reactions, ht-PhnI alone catalyzed PAH hydroxylation at a faster rate in the presence of organic solvent, suggesting that the transfer of substrate to the active site is a limiting factor. The four-ring PAHs chrysene and benz[a]anthracene were subjected to a double ring-dihydroxylation, giving rise to the formation of a significant proportion of bis-cis-dihydrodiols. In addition, the dihydroxylation of benz[a]anthracene yielded three dihydrodiols, the enzyme showing a preference for carbons in positions 1,2 and 10,11. This is the first characterization of a dioxygenase able to dihydroxylate PAHs made up of four and five rings.

Benz(a)Anthracenes↗
PubMed2005

Occurrence and solid-liquid partition of sulfonated naphthalene-formaldehyde condensates in the aquatic environment.

Sulfonated naphthalene-formaldehyde condensates (SNFC) are high production volume chemicals used in a variety of applications, for example, as concrete plasticizers, tanning agents, or dye dispersants. They enter the aquatic environment primarily by the wastewater path. The occurrence and fate of the monomers, which are different isomers of mono- and disulfonated naphthalene, was intensively investigated in previous studies. However, the environmental fate of the persistent higher molecular SNFC is so far widely unknown. This paper describes an ultrasonic extraction under alkaline conditions, followed by ion-pair HPLC with fluorescence detection for the analysis of SNFC oligomers from solid environmental matrixes such as sewage sludge, suspended solids, and river sediments. Limits of quantification of about 0.1 mg kg-1 d.m. were well below the measured concentrations in environmental samples. SNFC were adsorbed to suspended solids and river sediments in three major German rivers (Rhine, Neckar, and Danube) in concentrations typically up to several mg kg(-1) d.m. A total content of about 4 g kg(-1) d.m. was measured in a sewage sludge of a municipal wastewater treatment plant, which receives wastewater from a textile dyeing plant. Furthermore, the first quantitative field data on the partition of SNFC and their monomers between the aqueous phase and solid environmental compartments are presented. Solid-liquid partition coefficients (Kd) of oligomers with a chain-length ranging from three to six naphthalenesulfonate units were derived from the analysis of corresponding wastewater and sewage sludge samples and from suspended solids and river water samples, respectively. Determined Kd values were in the range from 10(2) to 10(4) L kg(-1).

Chromatography, High Pressure Liquid↗
PubMed2003

Intramolecular excimer formation in a naphthalene-appended dinuclear iron-oxo complex.

The synthesis, structure, and physical properties of a Heisenberg exchange-coupled cluster containing naphthalene groups are described. [Fe2(O)(O2CCH2C10H7)2(TACN-Me3)2]2+ (3) crystallizes in space group P1 with unit cell parameters a = 12.94(2) A, b = 14.84(2) A, c = 15.23(2) A, alpha = 101.12(7) degrees, beta = 90.8(1) degrees, gamma = 114.14(7) degrees, V = 2605(6) A3, and Z = 2 with R = 0.0425 and wR2 = 0.1182. Variable-temperature magnetic susceptibility data indicate that the two high-spin FeIII centers are antiferromagnetically coupled with J = -105 cm-1 (H = -2 JS1.S2), which is typical for this class of compounds. The room-temperature static emission spectrum of the compound in deoxygenated CH3CN solution is centered near 335 nm and has features reminiscent of both methyl-2-naphthylacetate (1) and [Zn2(OH)(O2CCH2C10H7)2(TACN-Me3)2]+ (2) with the following two caveats: (1) the overall emission intensity is roughly a factor of 10 less than that of the free ester (1, phi = 0.13) or the ZnII analogue (2, phi = 0.14), and (2) there is significant broadening of the low-energy shoulder of the emission envelope. Time-correlated single photon counting data revealed biphasic emission for 3 with tau 1 = 4.6 +/- 1 ns and tau 2 = 47 +/- 1 ns. The latter compares favorably with that found for 2 (tau = 47 +/- 1 ns) and is assigned as the S0-S1 fluorescence of naphthalene. Emission anisotropy, time-gated emission spectra, and nanosecond time-resolved absorption measurements all support the assignment of the 4.6 ns component as being due to a singlet excimer that forms between the two naphthylacetate groups of 3, a process that is likely mediated by the structural constraints of the oxo-bis-carboxylato diiron core. No direct evidence for intramolecular electron and/or energy transfer from the photoexcited naphthyl group to the iron-oxo core was obtained, suggesting that the short-lived excimer may contribute to circumventing such pathways in this type of system.

Journal Article↗
PubMed2006

Structural and electrochemical investigations of the high fluoride affinity of sterically hindered 1,8-bis(boryl)naphthalenes.

The reaction of 10-bromo-9-oxa-10-boraanthracene with the tetrakis(tetrahydrofuran)lithium salt of dimesityl-1,8-naphthalenediylborate in diethyl ether affords 1-(dimesitylboryl)-8-(10'-bora-9'-oxaanthryl)naphthalene (2). This diborane reacts with [Me3SiF2][S(NMe2)3)] to afford the anionic complex [2-mu2-F]-, which has been isolated as a [S(NMe2)3]+ salt. The cyclic voltammograms of diborane 2 as well as 1-(dimesitylboryl)-8-(10'-bora-9'-thiaanthryl)naphthalene (1) exhibit two reversible reductions at E(1/2) = -2.200 and -2.566 V (vs FcH/FcH+) for 1 and E(1/2) = -2.248 and -2.620 V (vs FcH/FcH+) for 2 corresponding to the sequential reduction of the two boron centers. These two waves simultaneously disappear upon fluoride addition, thus indicating the formation of fluoride chelate complexes [1-mu2-F]- and [2-mu2-F]-. To identify the origin of the high fluoride affinity displayed by these diboranes, the structures of 2 and [2-mu2-F]- have been studied experimentally and computationally. The crystallographic studies show that the structure of 2 is distorted, thus indicating the presence of important steric repulsions between the neighboring boryl moieties. By contrast, the structure of the anionic complex [2-mu2-F]- is much more sterically relaxed than that of 2, as indicated by a reduction of the B-B distance from 3.279(4) A in 2 to 2.922(7) A in [2-mu2-F]-. The structural results suggest that the high fluoride affinity displayed by 2 results, at least in part, from the relief of steric repulsions induced by fluoride binding. Finally, the nature of the bonding as well as the strength of the interactions involved in the B-F-B bridge of [2-mu2-F]- has been studied using density functional theory calculations and Atoms-In-Molecules analyses. These calculations indicate that the enthalpic gain associated with the formation of two B-F bonds in [2-mu2-F]- only amounts to a fraction of the energy of a terminal B-F bond. These calculations also suggest that the relief of steric repulsions induced by fluoride binding in 2 may contribute to the high fluoride affinity of these types of molecules.

Journal Article↗
PubMed1999

Effect of DNA Scaffolding on Intramolecular Electron Transfer Quenching of a Photoexcited Ruthenium(II) Polypyridine Naphthalene Diimide.

Intramolecular emission quenching of a photoexcited ruthenium(II) polypyridine by a covalently linked naphthalene diimide (NDI) has been measured in aqueous buffer both without and with calf thymus DNA. The complex consists of a Ru(2,2'-bipyridine)(2)(2,2'-bipyridine-5-carboxamide)(2+) electron donor covalently attached by way of a -CH(2)CH(2)CH(2)- linker to a 1,4,5,8-naphthalene diimide acceptor (Ru-NDI, 1). The NDI portion of the complex intercalates in calf thymus DNA, as indicated by the hypochromism of its optical absorbance bands and observation of an induced circular dichroism spectrum in the same region. Emission quenching in Ru-NDI has been measured relative to a Ru tris-bpy model lacking the NDI moiety by both lifetime and emission quantum yield techniques. Using lifetime averages, the relative emission quenching is, respectively, 99.1% and 97.9% in aqueous buffer solutions without and with DNA. The emission quenching is ascribed to intramolecular electron transfer within the Ru-NDI complex with an estimated driving force (-DeltaG degrees ) of 0.33 eV. In buffer, the emission decays of Ru-NDI alone are fit well with a triexponential model with lifetimes of 0.34 (0.88), 1.99 (0.11), and 12.6 (0.008) ns (relative amplitude). The emission decays of the DNA-intercalated Ru-NDI complex are also fit well with a triexponential model with lifetimes of 0.31 (0.79), 2.00 (0.13), and 11.8 (0.08) ns. Thus, the fractional amplitudes of the lifetimes change upon DNA intercalation of the complex, while the lifetimes themselves remain essentially the same. The average rates of electron transfer in aqueous buffer without and with DNA are, respectively, 1.6 x 10(9) and 6.8 x 10(8) s(-)(1). The striking result of this study is that the overall character of electron transfer quenching in Ru-NDI is very similar whether or not it is bound to DNA. Intercalation of the NDI in DNA apparently has negligible consequences for electron transfer, implying either that the activation energy and electronic coupling in Ru-NDI are largely unaffected by this, at first glance, seemingly significant environmental change or that changes in these parameters on DNA binding cancel fortuitously.

Journal Article↗
PubMed2002

The role of hydrogen bonding on the h-atom-donating abilities of catechols and naphthalene diols and on a previously overlooked aspect of their infrared spectra.

Catechols and 1,8-naphthalene diols contain one "free" hydroxyl and one intramolecularly H-bonded hydroxyl group. The "free" hydroxyls are strong hydrogen-bond donors (HBDs) with alpha2H values (Abraham et al. J. Chem. Soc., Perkin Trans. 2 1989, 699) ranging from 0.685 to 0.775, indicating that these compounds have similar HBD properties to those of strongly acidic phenols such as 4-chlorophenol (alpha2H = 0.670) and 3, 5-dichlorophenol (alpha2H = 0.774). Kinetic effects on H-atom abstractions from the diols in HB acceptor (HBA) solvents can be quantitatively accounted for over at least 50% of the available range of solvent HBA activities (as measured by their beta2H values; see Abraham et al. J. Chem. Soc. Perkin Trans. 2 1990, 521) on the basis of a single reactive OH group, the "free" OH. This free OH group is an outstanding H-atom donor in poor HBA solvents; e.g., in hexane rate constants for reaction with the DPPH* radical are 2.1 x 104 M-1 s-1 for 3,5-di-tert-butyl catechol and 2 x 106 M-1 s-1 for 4-methoxy-1,8-naphthalene diol, but only 7.4 x 103 M-1 s-1 for alpha-tocopherol (vitamin E). The diols are much more reactive than simple phenols because the O-H bond dissociation enthalpy of the "free" OH group is weakened by 5-9 kcal/mol by the intramolecular H-bond. The IR spectra of all the diols in CCl4 show two fairly sharp O-H stretching bands of roughly equal intensity separated by 42-138 cm-1. Addition of a low concentration of DMSO, a strong HBA, causes the band due to the intramolecularly H-bonded OH group to decrease in intensity to roughly half the extent that the "free" OH band loses intensity. The latter forms an intermolecular H-bond with the DMSO, the former does not. What has been overlooked in earlier work is that as the DMSO concentration is increased the band due to the intramolecularly H-bonded OH group first broadens and then evolves into a new, lower frequency (by 19-92 cm-1) band. The magnitude of the shift in the frequency of the intramolecular OH band caused by H-bonding of HBAs to the "free" OH group, Deltanu, increases linearly as the HBA activity of the additive increases, e.g., for 3,5-di-tert-butylcatechol, Deltanu/cm-1 = 33.8 beta2H (R 2 = 0.986). This may provide a new and simple method for determining beta2H values.

Journal Article↗
PubMed2000

Easy access to 3,8-Diaryldifurano[2,3-a:2',3'-f]naphthalenes. A new extended aromatic system.

3,8-Diaryldifurano[2,3-a:2',3'-f]naphthalenes were prepared in two simple steps. First, 1,5-dihydroxynaphthalene and 1-aryl-2-bromodecan-1-ones were condensed to the corresponding naphthalene 1,5-diethers. Second, these intermediates were cyclized using methanesulfonic acid in methylene chloride. Seven examples are given, three of which are doubly substituted with octyl chains to enhance the solubility. This increased solubility allowed further modification of the 3,8-aryl groups to attach electron-withdrawing groups (formyl, nitrile, dicyanovinyl, and benzoyl).

Journal Article↗
PubMed2002

Novel [2 + 2] photocycloaddition-induced rearrangement of bichromophoric naphthalene-tethered resorcinol ethers.

The first examples of sequential photocycloaddition-rearrangement reactions of naphthalene-tethered resorcinol ethers are described. Bichromophoric aromatic compounds with naphthalene and resorcinol ether moieties were irradiated in the presence/absence of a small amount of acid to give the corresponding cycloaddition-rearrangement products. From the determination of quantum yields, steady-state fluorescence spectral studies, and fluorescence lifetime measurements, the mechanism of this novel photoinduced multistep reaction was elucidated to involve the initial intramolecular exciplex formation, followed by the intramolecular [2 + 2] photocycloaddition between the two aromatic rings and the subsequent acid-catalyzed skeletal rearrangement of the resulting cyclobutane derivative leading to the final products.

Journal Article↗
PubMed2002

Kinetic and thermodynamic stability of naphthalene oxide and related compounds. A comparative microcalorimetric and computational (DFT) study.

The kinetics of the acid-catalyzed ring opening of naphthalene 1,2-oxide (5) in highly aqueous media to give naphthols has been measured by heat-flow microcalorimetry. The reaction enthalpy of this aromatization reaction was measured as DeltaH = -51.3 +/- 1.7 kcal mol(-)(1). The unexpectedly low reactivity of naphthalene oxide is suggested to be due to an unusually large thermodynamic stability. A crude estimate of the stabilization effect, approximately 1 kcal mol(-)(1)(not a significant stabilization), is obtained by using the measured reaction enthalpies of structurally related substrates as references. A larger value (2.7 kcal mol(-)(1)) was obtained by calculation using the B3LYP hybrid functional corrected with solvation energies derived from semiempirical AM1/SM2 calculations. The origin of this effect is discussed in terms of homoconjugative stabilization and homoaromaticity. There is a good linear correlation (with slope = 0.63) between the experimentally measured free energy of activation and the calculated enthalpy of carbocation formation in water.

Journal Article↗
PubMed2003

A new titanium tetrachloride mediated annulation of alpha-aryl-substituted carbonyl compounds with alkynes: A simple and highly efficient method for the regioselective synthesis of polysubstituted naphthalene derivatives.

A new straightforward procedure has been developed for the synthesis of polysubstituted naphthalene derivatives. The reaction of alpha-aryl-substituted carbonyl compounds with terminal or internal alkynes in the presence of TiCl4 regioselectively generates substituted naphthalene derivatives in good to excellent yields.

Journal Article↗
PubMed2003

Synthesis of substituted naphthalenes and carbazoles by the palladium-catalyzed annulation of internal alkynes.

An efficient synthesis of highly substituted naphthalenes has been developed by the palladium-catalyzed annulation of a variety of internal alkynes, in which two new carbon-carbon bonds are formed in a single step under relatively mild reaction conditions. This method has also been used to synthesize carbazoles, although a higher reaction temperature is necessary. The process involves arylpalladation of the alkyne, followed by intramolecular Heck olefination and double-bond isomerization. This method accommodates a variety of functional groups and affords the anticipated highly substituted naphthalenes and carbazoles in good to excellent yields.

Journal Article↗
PubMed2004

Synthesis and stereodynamics of highly constrained 1,8-bis(2,2'-dialkyl-4,4'-diquinolyl)naphthalenes.

The syn and anti isomers of axially chiral 1,8-diquinolylnaphthalenes have been synthesized via Pd-catalyzed Stille coupling of 1,8-dibromonaphthalene and 2-alkyl-4-trimethylstannylquinolines. Optimization of the cross-coupling reaction allowed the preparation of highly constrained 1,8-bis(2,2'-dimethyl-4,4'-diquinolyl)naphthalene, 2, and 1,8-bis(2,2'-diisopropyl-4,4'-diquinolyl)naphthalene, 3, in 42% and 41% yield, respectively. Employing Pd(PPh(3))(4) and CuO as the cocatalysts in the coupling reaction of 1,8-dibromonaphthalene and 2-alkyl-4-trimethylstannylquinolines proved to be superior over other catalysts such as PdCl(2)(dppf), Pd(2)(dba)(3)/P(t-Bu)(3), and POPd. The C(2)-symmetric anti isomers of 2 and 3 were found to be more stable than the corresponding meso syn isomer. The ratio of the two enantiomeric anti conformers to the syn conformer was determined as 7.9:1 for 2 and 8.6:1 for 3 by NMR and HPLC analysis. The atropisomers of 2 and 3 were found to be stable to rotation about the chiral axis at room temperature and all three stereoisomers of 2 were isolated by semipreparative HPLC on a Chiralpak AD column. The diastereoisomers of 3 were separated via preferential crystallization of the anti isomers from diethyl ether. Slow syn/anti interconversion was observed for both atropisomers at enhanced temperature, and the diastereomerization and enantiomerization processes were monitored by NMR and HPLC. The Gibbs activation energy, DeltaG++, for the isomerization of 2 was determined as 116.0 (112.1) kJ/mol for the conversion of the anti (syn) to the syn (anti) isomer at 71.0 degrees C. The rotational energy barrier of 3 was determined as 115.2 (111.1) kJ/mol for the conversion of the anti (syn) to the syn (anti) isomer at 66.2 degrees C.

Journal Article↗
PubMed2005

Synthesis and stereodynamics of highly constrained 1,8-bis(2,2'-dialkyl-4,4'-diquinolyl)naphthalenes (2).

Axially chiral 1,8-bis(2,2'-diphenyl-4,4'-diquinolyl)naphthalene, 8, and 1,8-bis(2,2'-diisopropyl-4,4'diquinolyl)naphthalene N,N'-dioxide, 9, have been prepared to study the stereodynamics of these and other 1,8-diheteroarylnaphthalenes based on reversible first-order isomerization kinetics and crystallographic data. The ratio of the two enantiomeric anti-conformers to the meso syn-isomer of 8 and 9 was determined as 1.2:1 and 9.6:1. Investigation of the conformational stability of the atropisomers at enhanced temperatures using HPLC and NMR spectroscopy revealed a Gibbs activation energy of 122.4 (121.8) kJ/mol and 115.2 (109.0) kJ/mol for the anti/syn- (syn/anti)-isomerization of 8 and 9, respectively. Comparison of the conformational stability of a series of 1,8-dipyridylnaphthalenes and 1,8-diquinolylnaphthalenes shows that the latter exhibit a significantly higher rotational energy barrier. While the syn- and anti-isomers of 1,8-dipyridylnaphthalenes interconvert rapidly at room temperature the stereoisomers of 1,8-diquinolylnaphthalenes can be isolated by chromatography or crystallization and stored at 25 degrees C for several months without any sign of racemization. The conformational stability of 1,8-diquinolylnaphthalenes is a consequence of significantly increased steric hindrance to isomerization in a highly congested T-shaped transition state. Conversion of 1,8-diheteroarylnaphthalenes to their corresponding N,N'-dioxides was found to result in an increased anti/syn-ratio and decreased rotational energy barrier, which was attributed to synergistic repulsive dipole/dipole interactions destabilizing the diastereomeric ground states and facilitated out-of-plane bending reducing the steric hindrance in the T-shaped transition state.

Journal Article↗