Abstract:Chiral triads composed of a central aromatic diimide moiety and two outer aromatic molecules connected by one- or two-carbon linkers exist in an equilibrium of C and S conformers, as inferred from analysis of circular dichroism and nuclear magnetic resonance spectra, as well as from molecular modeling.
“…The case of trimer 3 is different in that two diastereomeric conformers can be envisaged. Their structures, designated with the letters C and S ,17 are shown in Figure 2. Their computed (AM1) heats of formation are equal to within 0.3 kcal mol −1 , and hence nearly equal equilibrium populations of the two conformers can be assumed.…”
The chiral but highly symmetrical acyclic and cyclic pyromellitic diimide dimers and trimers 2–5 have been obtained and characterized for the first time. The pyromellitdiimide chromophores in these molecules are linked by a rigid diequatorially 1,2‐disubstituted cyclohexane skeleton. The structures of the compounds have been determined in detail by molecular modeling and, in the case of cyclic dimer 4 and trimer 5, by means of X‐ray diffraction analysis. The electronically excited states of the pyromellitdiimide chromophore (1 a) have been studied in these and other model compounds by means of linear dichroism (LD), magnetic circular dichroism (MCD), and circular dichroism (CD) spectroscopy. CD spectra of the rigid cyclic trimer 5 have provided the most detailed information on the excited states of the pyromellitdiimide chromophore. The low‐energy tail (340–360 nm) of the absorption envelope can be assigned to out‐of‐plane polarized n–π* transitions (I, II). The higher energy bands are due to contributions from up to six π–π* transitions, these being polarized either along the long (IV–VI, VIII) or short axis (III, VII). The results of ab initio CIS/cc‐pVDZ and semiempirical INDO/S‐CI calculations have been compared with the experimental data. CD Cotton effects in the region 200–260 nm, which result from exciton interactions between electric dipole allowed transitions of two pyromellitdiimide chromophores in compounds 2–5, provide reliable and useful information concerning the conformation and absolute configuration of these molecules, which may be extrapolated to other oligoimide systems.
“…The case of trimer 3 is different in that two diastereomeric conformers can be envisaged. Their structures, designated with the letters C and S ,17 are shown in Figure 2. Their computed (AM1) heats of formation are equal to within 0.3 kcal mol −1 , and hence nearly equal equilibrium populations of the two conformers can be assumed.…”
The chiral but highly symmetrical acyclic and cyclic pyromellitic diimide dimers and trimers 2–5 have been obtained and characterized for the first time. The pyromellitdiimide chromophores in these molecules are linked by a rigid diequatorially 1,2‐disubstituted cyclohexane skeleton. The structures of the compounds have been determined in detail by molecular modeling and, in the case of cyclic dimer 4 and trimer 5, by means of X‐ray diffraction analysis. The electronically excited states of the pyromellitdiimide chromophore (1 a) have been studied in these and other model compounds by means of linear dichroism (LD), magnetic circular dichroism (MCD), and circular dichroism (CD) spectroscopy. CD spectra of the rigid cyclic trimer 5 have provided the most detailed information on the excited states of the pyromellitdiimide chromophore. The low‐energy tail (340–360 nm) of the absorption envelope can be assigned to out‐of‐plane polarized n–π* transitions (I, II). The higher energy bands are due to contributions from up to six π–π* transitions, these being polarized either along the long (IV–VI, VIII) or short axis (III, VII). The results of ab initio CIS/cc‐pVDZ and semiempirical INDO/S‐CI calculations have been compared with the experimental data. CD Cotton effects in the region 200–260 nm, which result from exciton interactions between electric dipole allowed transitions of two pyromellitdiimide chromophores in compounds 2–5, provide reliable and useful information concerning the conformation and absolute configuration of these molecules, which may be extrapolated to other oligoimide systems.
“…The linear alignment induced by the linker resulted in observable conformer populations, in which the alkaloid units were oriented either syn ( or anti ( (Fig. 32 ) [ 89 ]. Fig.…”
Section: Dimers Connected At the Central C-9 Positionmentioning
confidence: 99%
“…However, the ratio varied in response to carboxylic and dicarboxylic acids [ 90 ]. The prevalent conformation and their equilibria were studied by circular dichroism (CD) [ 89 ] and NMR spectroscopy. In addition, an analytical system composed of e QD-124 and bromophenol blue allowed for visual identification of -hydroxycarboxylic acids, as well as their spectrophotometric estimation.…”
Section: Dimers Connected At the Central C-9 Positionmentioning
Nature is full of dimeric alkaloids of various types from many plant families, some of them with interesting biological properties. However, dimeric Cinchona alkaloids were not isolated from any species but were products of designed partial chemical synthesis. Although the Cinchona bark is amongst the sources of oldest efficient medicines, the synthetic dimers found most use in the field of asymmetric synthesis. Prominent examples include the Sharpless dihydroxylation and aminohydroxylation ligands, and dimeric phase transfer catalysts. In this article the syntheses of Cinchona alkaloid dimers and oligomers are reviewed, and their structure and applications are outlined. Various synthetic routes exploit reactivity of the alkaloids at the central 9-hydroxyl group, quinuclidine, and quinoline rings, as well as 3-vinyl group. This availability of reactive sites, in combination with a plethora of linker molecules, contributes to the diversity of the products obtained.
“…The title compound was prepared from commercially available 3,4dimethoxybenzoic acid (Acros) according to a four-step synthesis briefly outlined in the literature (Gawró nski et al, 1999). Full experimental procedures and crystallographic data of the intermediates will be reported in a forthcoming communication.…”
Key indicatorsSingle-crystal X-ray study T = 100 K Mean (C-C) = 0.004 Å R factor = 0.052 wR factor = 0.124 Data-to-parameter ratio = 11.0For details of how these key indicators were automatically derived from the article, see
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