Synthesis of Shape-Persistent meta-Arylene-Butadiynylene Macrocycles with a Different Ring Size

Abstract

A series of meta-phenylene-butadiynylene macrocycles was obtained by homocoupling of terminal bis(ethynyl) precursors using Eglinton conditions. Cyclotetramers and cyclopentamers were isolated from the mixture. However, a hexameric analogue was synthesized using a step-by-step approach. Molecular structures of the resulting macrocycles obtained by X-ray single-crystal analysis manifest conformational diversity, indicating significant flexibility of the meta-arylene-butadiynylene macrocycle conjugated skeleton.

Better big or strained? Deformations of shape persistent arylene butadiynylene macrocycles

“…The cyclic compounds ( 1 - 2 - t -Bu) 4–5 have been previously synthesized using a statistical shotgun approach from diacetylene precursors, while the hexameric compound ( 1 - t -Bu) 6 was obtained via a step-by-step procedure (called Moore's approach). 45…”

“…Analysis of the 1 H NMR spectra of the macrocyclization products of 2 -i-Pr was not informative since signals from aromatic protons for the pivaloyl derivatives were not separated (for different products) as in the case of the methyl derivatives. 45 The experiment was repeated several times (under the same conditions) and, although we were extra careful at the separation step, we were not able to identify smaller or larger homologs. Nevertheless, based on our previous experience and literature data, we are reluctant to claim that such species do not form.…”

“…The trimers that have formed for the precursor with unprotected hydroxy groups did not form for compounds with larger substituents (methyl or pivaloyl) placed at the oxygen atoms of the phenolic fragment. 45…”

“…As published already, the hexamer ( 1 - t -Bu) 6 was obtained via an iterative, step-by-step approach and we were able to obtain its crystals suitable for rentgenostructural characterization. 45 The compound was crystallized from a CH 2 Cl 2 /hexanes mixture as a solvate with a highly disordered hexane molecule in the crystal lattice. The compound crystallizes in the monoclinic system in the C 2/ c space group with Z = 4 and its structure is presented in Fig.…”

“…45 Finally, an important factor for carrying out these studies was that, despite the fact that quite a few arylene butadiynylene macrocycles have been described, interestingly, there were no crystallographic data available for such systems before our publication. 45…”