Chiral Tectonics: VCD and ECD Application for Epimerization of a Star-Burst Tetranuclear Complex with a Labile Central Core

The present article reports the application of vibrational circular dichroism (VCD) and temperature-dependent electronic circular dichroism (ECD) methods to reveal the dynamical aspects of a star-burst tetranuclear metal complex with a labile central core in a solution. One-handed chiral inert tecton, ∆- or Λ-[Ru(III)(acac)2(taetH)] (acacH = acetylacetone, taetH2 = tetraacetylethane), was prepared by reacting [Ru(acac)3] with taetH2 in solid at 120 °C. The ∆Λ-pair of pure enantiomers was obtained chromatographically. On adding Al(ClO4)3 to its enantiopure solution, three units of one-handed tecton were assembled spontaneously around an aluminum(III) ion to form a star-burst tetranuclear complex, [{∆- or Λ-Ru(acac)2(taet)}3Al(III)]. The VCD spectrum recorded on the CDCl3 solution of the complex showed that the central chirality around an Al(III) ion took dominantly the absolute configuration antipodal to those of peripheral Ru(III) units at the temperature lower than −10 °C. The complex underwent interconversion between the ∆- and Λ-configurations around a central Al(III) core (or epimerization) in solution. The activation energy barrier was determined from the time courses of ECD spectra in CHCl3 and CH3OH.

Determination of the enol form of asymmetric 1,3-dicarbonyl compounds: 2D HMBC NMR data and DFT calculations

In this study, a series of asymmetric aryl 1,3-dicarbonyl compounds were synthesized and their enol forms were observed via experimental data and theoretical calculations. According to the 1H- and 13C-NMR results, all the investigated compounds were found as a single enol form in CDCl3 solution. Moreover, their HMBC spectra were applied to identify the observed enol forms and correlations between certain protons and carbon atoms were considered. The dihedral angles of the asymmetric compounds that have aryl units on both sides were calculated by DFT to understand the reason for the observed enol forms. Small dihedral angles caused longer conjugation, resulting in more stable compounds and it was found that the observed enol forms were based on small dihedral angles, namely, resonance is the driving force. Furthermore, the compounds possessing both aryl and alkyl moieties prefer the enol form towards the aromatic ring side due to longer conjugation.

An Efficient Synthesis of Rubin’s Aldehyde and its Precursor 1,3,5-Tribromo-2,4,6-tris(dichloromethyl)benzene

2,4,6-Tribromobenzene-1,3,5-tricarboxaldehyde (4) can be efficiently prepared in two reaction steps from 1,3,5-tribromobenzene. The intermediate 1,3,5-tribromo-2,4,6-tris(dichloromethyl)benzene (3) crystallizes from petroleum ether in its C3h structure. However, in CDCl3 solution it exists at room temperature in two isomeric forms: 3a (C3h) and 3b (Cs) (1:1.15).The intramolecular Br・・・Cldistances are much smaller than the sum of the van der Waals radii. Therefore, the exocyclic C-C bonds show a hindered rotation.