Shrinkable/stretchable bis(calix[4]arenes) comprising photoreactive azobenzene or stilbene linkers

Organic Chemistry Frontiers. 2023. Vol. 10. No. 6. P. 1470–1484.

Lentin I., Gorbunov A., Bezzubov S., Nosova V., Cheshkov D., Kovalev V., Vatsouro I.

Semi-tubular bis(calixarenes) of photo-variable length were shown to be readily available by linking together two calix[4]arene macrorings by means of two bifunctional azobenzene or stilbene units. These biscalixarene cores were prepared by a direct distal alkylation of parent calix[4]arene tetrols with (E)-4,4′-bis(bromomethyl)azobenzene or (E)-4,4′-bis(bromomethyl)stilbene using potassium carbonate as a base. Alkylation of four remaining phenolic OH-groups in two calix[4]arene macrorings allowed further derivatization of the biscalixarene cores. When exposed to soft UV light, the (E)-azobenzene-derived bis(calixarenes) underwent stepwise isomerization of both linkers from the (E)- to the (Z)-form, resulting in shrunken (Z,Z)-bis(calixarenes) as the major products. As expected, these (Z,Z)-bis(calixarenes) are thermally unstable and can be stretched back into their (E,E)-shape upon heating. Successful photo-shrinking of an (E)-stilbene-containing bis(calixarene) was achieved only in the presence of 1,2-benzanthracene as a photosensitizer. In this case, due to sufficient thermal stability, the (Z,Z)-isomer of the bis(calixarene) was separated (along with the intermediate (E,Z)-isomer), and its molecular structure was obtained by X-ray crystallography and compared with the molecular structure of the initial (E,E)-isomer. The results unambiguously prove the substantial shrinkage of bis(calixarene) upon irradiation, with retention of the ‘co-axiality’ of the calix[4]arene macrorings. For the stilbene-containing bis(calixarene) with no additional substituents in its structure, another photochemical transformation was observed. Upon irradiation, irrespective of the presence of a photosensitizer in the system, this compound rapidly formed stable cyclobutane-linked bis(calixarenes) with rtct- and rctt-structures due to the intramolecular [2 + 2]-cycloaddition between two stilbene units, which were preorganized by two calix[4]arene macrocycles.

Keywords: органический синтез, organic synthesis, Calixarenes, каликсарены, cis-trans isomers, цис-транс изомерия