Conformational Transitions of Calixphyrin Derivatives Monitored by Temperature-Dependent NMR Spectroscopy. Ab Initio Interpretation of the Spectra

Abstract

Eight meso-aryl calixphyrin derivatives were synthesized and their conformational equilibria and transitions studied with temperature-dependent NMR spectroscopy. On the basis of density functional computations, several conformer species could be identified and observed changes in chemical shifts explained. In some compounds, the aryl group rotation and porphyrin ring flipping could be monitored independently, as their NMR coalescence temperatures were well-separated. Calculated relative conformer energies, transition barriers, and isotropic shieldings agree well with the experimental data. In the meso-substituted porhyrins (calixphyrins) the sp3 carbon atoms perturb their pi-electron system and significantly modify the molecular shape and the flexibility. Even when the conjugation of the pi-electron system was destroyed by the nonplanarity, far-range electronic induction effects still exist and influence chemical shielding and molecular geometry. The aryl functional groups moderately modify the structure of the calixphyrin ring and thus can be used for fine-tuning of the mechanical and chemical properties of these compounds.

Molecular Engineering of Free‐Base Porphyrins as Ligands—The N−H⋅⋅⋅X Binding Motif in Tetrapyrroles

“…Formal two-, four-, or six-electron oxidation of these nonaromatic macrocycles leads to calix[4]phyrins,which bear analogy to both porphyrins and calix[4]pyrroles by containing amixture of sp 2 -a nd sp 3 -hybridized meso-like positions.T he partial interruptions in the conjugation introduce anumber of unique structural features since the sp 3 -carbon atoms perturb the p-system and significantly modify the molecular shape and flexibility.Likewise,far-range electronic induction effects still exist so that functional groups may be used for fine-tuning of conformational and chemical properties. [80] Early examples of rational calixphyrin syntheses include acid-catalyzed cyclization reactions between ketones and pyrrolic precursors followed by partial oxidation (Figure 9B) [81] and Buchlersr eductive methylation to yield metalloporphodimethenes [82] or substitution reactions. [83] Thep roducts are usually distorted and feature structural cavities and accessible inner nitrogen atoms ( Figure 9C), which may aid the binding of substrates.…”

Synthesis of Functionalized, Sterically Congested Calix[4]phyrin Macrocycles Using Donor–Acceptor‐Substituted Cyclopropanes – First Example of a Mono‐meso‐spirolactone Incorporated into a Calix[4]phyrin

“…[29] With a multigram amount of dipyrromethane 4 in hand, we reacted it with different aldehydes in an acid-catalyzed condensation to form calixpyrrole systems, which were then oxidized to investigate the formation of functionalized calix[4]phyrin systems (Scheme 4). [2,32] As anticipated, oxidation occurred only at those meso-positions that carried the sterically less demanding aryl substituent. Therefore, as a test reaction, dipyrromethane 4 was condensed with benzaldehyde at room temperature in the presence of a catalytic amount of TFA (20 mol-%) in dichloromethane (DCM) as described for other dipyrromethane condensations [30] to furnish the corresponding calixpyrrole 5.…”

“…also Scheme 7). [2,32] As anticipated, oxidation occurred only at those meso-positions that carried the sterically less demanding aryl substituent. The steric hindrance exerted by the bulky cyclopropane-derived residues presumably interrupts the full six-electron oxidation of the calixpyrrole [10,33] and results in the preferential formation of the stable calix[4]phyrin(1.1.1.1) 6a (cf.…”

“…27 % yield; to date, this had only been isolated as a byproduct with a yield Ͻ 1 % (Scheme 6). [21,32] Information on the detailed structure of the calix[4]phyrin(1.1.1.1)s was obtained by single-crystal X-ray diffraction analyses of 6a and 6c. In calix[4]phyrin(1.1.1.1)s with unsymmetrically substituted sp 3 -meso-carbon centers there are two possible diastereomeric forms, syn and anti, which refer to the position of the residues at the saturated meso-carbon centers.…”

Ferrocene-appended porphyrins: Syntheses and properties

“…The potential applications of these new architectures are similar to those mentioned in the previous paragraphs and rely on the exciting binding, electronic, structural properties of original macrocycles with various sizes, conjugation and heteroatoms. This field is rapidly growing and numerous examples have been recently reported with analogues as oxasmaragdyrin [101,102], corroles [39], oxacorroles [101], porphodimethenes [34,117], tetraazaporphyrins and phthalocyanines [51,53,[103][104][105][106][107][108][109][110] and others [111][112][113]. Some of these structures are depicted in Fig.…”