Abstract
The preparation and characterization of products of the photochemical and thermochemical rearrangements of 19-membered azoxybenzocrowns with two, bulky, tert-butyl substituents in benzene rings in the para positions to oligooxyethylene fragments (meta positions to azoxy group, i.e., t-Bu-19-Azo-O have been presented. In photochemical rearrangement, two colored typical products were expected, i.e., 19-membered o-hydroxy-m,m′-di-tert-butyl-azobenzocrown (t-Bu-19-o-OH) and 19-membered p-hydroxy-m,m′-di-tert-butyl-azobenzocrown (t-Bu-19-p-OH). In experiments, two colored atypical macrocyclic derivatives, one 6-membered and one 5-membered ring, bearing an aldehyde group (t-Bu-19-al) or intramolecular ester group (t-Bu-20-ester), were obtained. Photochemical rearrangement led to one more macrocyclic product being isolated and identified: a 17-membered colorless compound, without an azo moiety, t-Bu-17-p-OH. The yield of the individual compounds was significantly influenced by the reaction conditions. Thermochemical rearrangement led to t-Bu-20-ester as the main product. The structures of the four crystalline products of the rearrangement—t-Bu-19-o-OH, t-Bu-19-p-OH, t-Bu-20-ester and t-Bu-17-p-OH—were determined by the X-ray method. Structures in solution of atypical derivatives (t-Bu-19-al and t-Bu-20-ester) and t-Bu-19-p-OH were defined using NMR spectroscopy. For the newly obtained hydroxyazobenzocrowns, the azo–phenol⇄quinone–hydrazone tautomeric equilibrium was investigated using spectroscopic methods. Complexation studies of alkali and alkaline earth metal cations were studied using UV-Vis absorption spectroscopy. 1H NMR spectroscopy was additionally used to study the cation recognition of metal cations. Cation binding studies in acetonitrile have shown high selectivity towards calcium over magnesium for t-Bu-19-o-OH.
Highlights
The photochemical rearrangement of azoxybenzocrowns is an interesting and multidirectional process and seems to be a relatively efficient method for obtaining hydroxy–azo compounds, and other macrocyclic compounds with a hydrazone group in macroring
A novelty of this paper is the study of the photochemical reaction with 19-membered azoxybenzocrowns containing substituents on benzene rings, in this case, tert-butyl substituents, which block the para position with respect to the oligoether chain
Another important novelty is in the isolation, besides that of the expected tert-butyl-19-ortho- and tert-butyl-19-para hydroxyazobenzocrowns, that of the macrocyclic compounds of new types—the macrocycles with the characteristic element of a 5-membered ring, resulting from the contraction of the benzene ring
Summary
Macrocycles, bearing different functionalities, are often used in many areas, e.g., as drug carriers [1] or catalysts [2,3]. Macrocyclic compounds are of interest as a synthetic targets, providing functional systems for sensors, e.g., [4,5], molecular devices and switches c.f. Such macrocycles bear additional functionalities to make them sensitive to external stimuli, e.g., pH, potential, light, etc. A azobenzene moiety is attached as a peripheral or inherent part due to its photo or redox activity [8–12]. Macrocyclization is challenging; different approaches are used for an effective synthesis of particular types of macrocyclic cores using the well-known high dilution technique and/or using various templates to more approaches sophisticated elegant
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