Super Aryl‐Extended Calix[4]pyrroles: Synthesis, Binding Studies, and Attempts To Gain Water Solubility

Abstract

We describe the synthesis of unprecedented calix[4]pyrrole receptors featuring "super aryl extended" (SAE) cavities. We elaborated the aromatic cavity provided by the αααα-isomer of para-tetraiodo-meso-phenyl calix[4]pyrrole by installing ethynyl-aryl substituents at its upper rim. We report the binding properties of the prepared SAE-calix[4]pyrrole tetraester towards pyridyl-N-oxides. The binding data revealed the formation of thermodynamically and kinetically highly stable 1:1 complexes. The complexation-induced chemical shifts indicated the formation of hydrogen bonds and aromatic interactions with the calix-core adopting the cone conformation. We quantified the additional interactions established between the four terminal aryl groups and the para-phenyl substituent of 4-phenyl pyridine N-oxide to be in the order of 1 kcal mol(-1) . The complex formation rate was found to be close to the diffusion control suggesting that the free host adopted a 1,3-alternate conformation. Finally, we attempted to gain water solubility of SAE-calix[4]pyrroles using derivatives that display four ionizable or charged groups at their upper rims.

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

“…[56a] Nevertheless,alarge number of neutral receptorsubstrate H-bonded complexes of calix[4]pyrroles were described;f or example,w ith alcohols,a mides,abroad range of oxygen-containing species,a nd pyridyl-N-oxides. [56,60] Based on relatively simple calix[4]pyrroles,n umerous more Top: while the discrete ligand 8 (CCDC:VUSFIY01) is 1,3alternate with regard to the pyrrole rings, the tetrapyrrole switches to a"cone" in 8·Cl À (CDDC:TEQKIJ) to accommodate anions. [51a] Bottom:t he complexes 8·2 MeOH (RECPEU)a nd 8·2 DMF (RECPIY; noncritical hydrogen atoms omitted)w ith neutral ligands are 1,3-and 1,2-alternate, respectively.…”

“…[58, 56a] [47] sophisticated receptors have been prepared through an evergrowing toolbox of functionalization techniques (e.g., 16-19, Figure 6). [53b,c,56b, [60][61][62][63][64] Thev arious calixpyrrole-dependent recognition techniques were reviewed extensively by Gale and Sessler. [65] Optical sensors are often based on covalently linked colorimetric or fluorescent reporter groups where perturbation of the electronic properties upon complexation results in avisual or fluorescence-based signal (fluorescence quenching/"turn off" or "turn on").…”

Synthesis and Binding Studies of a Tetra‐α Aryl‐Extended Photoresponsive Calix[4]pyrrole Receptor Bearing meso‐Alkyl Substituents

“…We had estimated above that the binding constants for the inclusion complexes of two studied guests, 6 and 7· NMe 4 , with the all ‐trans and the cis ‐enriched isomers of 5 were indeed larger than 10 4 m –1 . In previous investigations, we determined that “four‐wall” aryl‐extended calix[4]pyrroles bind pyridine N ‐oxide derivatives with association constant values larger than 10 4 m –1 both in organic solvent and in water , . However, to the best of our knowledge, the binding affinities of aromatic carboxylates for “four‐wall” aryl‐extended calix[4]pyrroles have not been reported.…”

“…The ratio of the binding constant values for the complexes of 4‐methyl pyridine N ‐oxide and 4‐phenyl pyridine N ‐oxide with the super‐aryl extended receptor was ca. 3 . This corresponds to an energy difference of ca.…”

“…We also studied the binding of tttt ‐ 5 with both guests, 6 and 7· NMe 4 , in CD 2 Cl 2 solutions. In non‐polar solvents, such as dichloromethane or chloroform, calix[4]pyrrole tttt ‐ 5 is expected to adopt an alternate conformation, and the binding of a polar guest i.e. N ‐oxide 6 or carboxylate 7 induces the receptor to adopt the cone conformation.…”