Schiff Base Macrocycles Containing Pyrroles and Pyrazoles

Abstract

A double nucleophilic substitution reaction of 3,5-bis(chloromethyl)pyrazole with pyrroles generates a novel pyrrole-pyrazole hybrid building block, the pyrazole analogue to tripyrrane. Vilsmeier-Haack formylation produces the corresponding dialdehyde, which was used in the formation of a series of nonaromatic Schiff base macrocycles. NMR and UV/Vis spectroscopy and single-crystal diffractometry were used to characterize the novel macrocycles. The solid-state structures of select free bases and protonated members of this class of macrocycles display a range of intra- and intermolecular hydrogen-bonding patterns that suggest their use in molecular-recognition systems. They also contain an acid-sensitive chromophore. Their acid-base and anion-recognition properties were ascertained; alas, only modest anion-selective spectroscopic signatures could be detected by using UV/Vis and (1)H NMR spectroscopy. The macrocycles proved resistant toward oxidation to their aromatic congeners. The pyrrole-pyrazole building blocks presented are potentially useful for the synthesis of a range of pyrazole analogues of all-pyrrole macrocycles.

Siamese‐Twin Porphyrins: Variation of Two meso‐Aryl Groups

“…The choice of the substituents and the particular substituent pattern was guided by practical synthesis considerations (i.e., the symmetric 3,4‐diethylpyrrole is readily accessible and does not give rise to the formation of regioisomers) and to induce the proper conformation of the precursor Siamese‐twin porphyrinogen to allow its oxidation to the final product 1a . We also found that the meso ‐phenyl groups adjacent to the pyrazole groups were necessary for the oxidation of this position to take place; left unsubstituted, these positions resisted oxidation , …”

Siamese‐Twin Porphyrin: A Pyrazole‐Based Expanded Porphyrin of Persistent Helical Conformation

“…Unfortunately, the 3 + 3-type syntheses of the target macrocycles could not be achieved at the time, though other interesting, non-conjugated macrocycles, such as 5, were prepared and studied. [23] Three major obstacles became evident in our work, some of which mirrored the problems in Linds work: Whereas mass spectrometry (MS) indicated that a 3 + 3 macrocyclization of the pyrrolyl-pyrazole building blocks had likely taken place, their conformational flexibility and the large number of stereoisomers formed posed great technical difficulties. As a result, the macrocycles could not be isolated or spectroscopically unambiguously characterized.…”

“…Only one family of porphyrin analogues incorporating a pyrazole, carbaporphyrins 4, recently introduced by the group of Lash, is known. [22] One other multidentate cyclic pyrazole-pyrrole hybrid ligand was reported, compound 5, [23] but this macrocycle cannot be categorized as a true porphyrinoid, that is, containing a fully conjugated framework.…”

“…Based on our experience with the functionalization of pyrroles and pyrazoles, we devised an alternative strategy towards the assembly of the key building block, bis(pyrrolyl-methylene)-pyrazoles 14, which was based on an alkylation of an a-unsubstituted pyrrole with a pyrazole. [23] We thus directly delivered sp 3 -linkages between the pyrazole and pyrroles. A double Vilsmeyer-Haak formylation of 14 generated bisaldehydes 15.…”

Schiff bases derived from hydroxyaryl aldehydes: molecular and crystal structure, tautomerism, quinoid effect, coordination compounds

“…These properties are greatly influenced by the topochemistry of the Schiff base molecules which in turn is highly affected by the crystal structure. Therefore the study of crystal packing and the intermolecular interactions in the crystal structures of various Schiff bases can lead to valuable data for the design and synthesis of new materials (Figure 1) [79][80][81][82][83][84][85].…”