- A combined experimental and theoretical study on the conformation of multiarmed chiral aryl ethers
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(Graph Presented) Four series of multiarmed chiral aryl ethers carrying two, three, five, or eight side-chains on a variety of aromatic core molecules (2-5) were prepared. The structure and conformation of 2 and 3 (in the solid state) were determined by the X-ray crystallographic analyses. While a pair of alternated (anti) conformers (i.e, up-down and down-up) were found in the crystal of 2, three side-arms in 3 were aligned in the same direction to give a C3-symmetric syn-conformation. Examinations by dispersion-corrected density functional (DFT-D) calculations revealed that two out of six anti- and two out of four syn-conformers of 2 are energetically most important. Two calculated structures of anti-conformers are in good agreement with those found in the solid state by X-ray analysis. Similarly, relevant conformations of syn-3, fully alternated 4, and C5-symmetric 5 were optimized at the DFT-D-B-LYP/TZVP level. The structure and conformation of the side-arms in 2-5 in solution were further studied by temperature dependent 1H NMR and UV-vis spectroscopy. In addition, comparative experimental and theoretical CD spectral studies were carried out in order to elucidate the contribution of the thermodynamically less-stable minor isomers in solution. The CD spectral changes observed for 2 and 3 at varying temperatures were quite different, while the parent chiral arene 1, as well as 4 and 5, only showed an increased intensity of the negative Cotton effect for the 1Lb band. The latter behavior is readily accounted for in terms of the conformational freezing of the chiral groups at low temperatures. The unusual CD spectral behavior observed for 2 and 3 was rationalized by the conformational alteration of the side-arms. Because of attractive van der Waals interactions between the aromatic units of the arms in nonpolar solvents, the syn-conformations become gradually more important for 2 at low temperatures, which eventually results in a weak positive Cotton effect for the 1Lb band. This was also supported by the SCS-MP2/TZVPP single-point energy calculations for the relevant conformers of 2. For 3, the contribution of the C3-symmetrical conformer becomes more important than the less-symmetrical isomers at low temperatures. The conformations of 2 and 3 in their excited states as well as in the oxidized states were also examined.
- Mori, Tadashi,Grimme, Stefan,Inoue, Yoshihisa
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p. 6998 - 7010
(2008/02/11)
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- Intervalence (Charge-Resonance) Transitions in Organic Mixed-Valence Systems. Through-Space versus Through-Bond Electron Transfer between Bridged Aromatic (Redox) Centers
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Intervalence absorption bands appearing in the diagnostic near-IR region are consistently observed in the electronic spectra of mixed-valence systems containing a pair of aromatic redox centers (Ar+/Ar) that are connected by two basically different types of molecular bridges. The through-space pathway for intramolecular electron transfer is dictated by an o-xylylene bridge in the mixed-valence cation radical 3.+ with Ar = 2,5-dimethoxy-p-tolyl (T), in which conformational mobility allows the proximal syn disposition of planar T.+/T redox centers. Four independent experimental probes indicate the large through-space electronic interaction between such cofacial Ar.+/Ar redox centers from the measurements of (a) sizable potential splitting in the cyclic voltammogram, (b) quinonoidal distortion of T.+/T centers by X-ray crystallography, (c) "doubling" of the ESR hyperfine splittings, and (d) a pronounced intervalence charge-resonance band. The through (br)-bond pathway for intramolecular electron transfer is enforced in the mixed-valence cation radical 2a.+ by the p-phenylene bridge which provides the structurally inflexible and linear connection between Ar.+/Ar redox centers. The direct comparison of intramolecular rates of electron transfer (kET) between identical T.+/T centers in 3.+ and 2a.+ indicates that through-space and through-bond mechanisms are equally effective, despite widely different separations between their redox centers. The same picture obtains for 3.+ and 2a.+ from theoretical computations of the first-order rate constants for intramolecular electron transfer from Marcus-Hush theory using the electronic coupling elements evaluated from the diagnostic intervalence (charge-transfer) transitions. Such a strong coherence between theory and experiment also applies to the mixed-valence cation radical 7.+, in which the aromatic redox S center is sterically encumbered by annulation.
- Sun,Rosokha,Lindeman,Kochi
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p. 15950 - 15963
(2007/10/03)
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- Structure and Reactivity of Isoannulated Heterocyclic Systems with 4n-? and (4n + 2)-? Electrons, XVIII. - Benzothiophenes with Symmetric Structure: Modified and Optimized Preparations by the S-Oxide Route
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Benzothiophenes (15) with symmetric structure have been prepared efficiently from 1,3-dihydrobenzothiophene 2-oxides (9) by reaction with aluminium oxide, by O-acylation with trifluoroacetic anhydride, or O-alkylation with methyl trifluoromethanesulfonate.The aromatization of the S-oxides 9 is achieved by O-functionalization, subsequent elimination, and consecutive deprotonation.
- Kreher, Richard P.,Kalischko, Juergen
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p. 645 - 654
(2007/10/02)
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