Synthesis, stereodynamics and applications of highly congested axially chiral 1,8-diheteroarylnaphthalenes.

摘要

Axially chiral 1,8-bis(2,2'-disubstituted-4,4 '-dipyridylnaphthalenes and 1,8-bis(2,2'-disubstituted-4,4 '-diquinolyl)naphthalenes have been prepared via Pd-catalyzed Stille cross-coupling of 1,8-dihalonaphthalenes and appropriately substituted 4-trimethylstannylpyridines and 4-trimethylstannylquinolines. Investigation of reversible first-order isomerization kinetics of these 1,8-diheteroarylnaphthalenes using chromatographic and spectroscopic techniques revealed activation energies ranging from 65 to 73 kJ/mol and 115 to 122 kJ/mol for the anti/syn -isomerization of 1,8-dipyridylnaphthalenes and 1,8-diquinolylnaphthalenes, respectively. 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°C for several months without any sign of racemization. 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.; Fluorescence and NMR studies showed that 1,8-bis(2,2'-diisopropyl-4,4 '-diquinolyl)naphthalene undergoes selective recognition of the cis- and trans-isomer of diaminocyclohexane through substrate-controlled interconversion between its syn- and anti-isomer exhibiting strikingly different fluorescence quantum yields. NMR spectroscopy experiments confirmed that trans -1,2-diaminocyclohexane stabilizes the C2-symmetric anti-isomers, whereas cis-1,2-diaminocyclohexane favors formation of the less fluorescent meso syn-isomer, which results in a reduced fluorescence response. Titration studies using enantiopure 1,8-bis(2,2'-diisopropyl-4,4' -diquinolyl)naphthalene N,N'-dioxide showed that this compound can discriminate between the enantiomers of trans-1,2-diaminocyclohexane with an enantioselectivity, alpha, of 1.6. Screening of a variety of chiral hydrogen bond donors revealed that the N,N' -dioxide sensor is selective for 1,2-diaminocyclohexane which may provide a tool for real-time determination of the enantiomeric purity of 1,2-diaminocyclohexane in the presence of other analytes.; Fluorescence titration experiments using the anti-isomer of 1,8-bis(2,2 '-diisopropyl-4,4'-diquinolylnaphthalene showed highly efficient quenching by Fe(III) and Fe(II) in aqueous solutions, whereas almost no quenching effects were observed with other alkali and transition metal chloride salts in aqueous solutions. The excellent selectivity of the sensor for Fe(III) was not diminished even in the presence of other metal ions.