A new, stepwise synthesis of calix[4]phyrins is described. It relies on the condensation of a ketone with pyrrole to form a dipyrromethane containing a quaternary carbon center that is subsequently condensed with an aromatic aldehyde. This methodology, in contrast to the previous rational approach described by this group (involving formation of a trisubstituted dipyrromethane via the condensation of an aldehyde with pyrrole, followed by condensation of this dipyrromethane with acetone), allows for a variety of bulky, ketone-derived substituents to be incorporated into the meso-like positions. The resulting systems, while all containing the same central macrocyclic core, display conformational properties that reflect the nature of these meso-like substituents; these conformational features were independently assessed by X-ray diffraction analysis, NMR spectroscopy, and quantum chemical calculations.
Reaction of 5,5-dimethyldipyrromethane (1) with electron-deficient aryl aldehydes in the presence of BF(3)-Et(2)O and NH(4)Cl in propionitrile constitutes efficient, easy access to unprecedented, functional porphomonomethenes together with the expected porphodimethenes (calix[4]phyrins). Alternatively, when the reaction was carried out in CH(2)Cl(2) in the presence of an acid and Florisil, the expected bis-arylcalix[4]phyrin was isolated in 41% yield, while no scrambled macrocycle was detected. After reduction of the nitro function, porphomonomethene 9 was efficiently condensed with the binaphthyl diacyl chloride (10) leading to the first chiral calix[4]phyrin dimer (11) that exhibits a moderate enantiorecognition toward the enantiomers of malic acid.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.