Multifaceted applications of crown ethers exploiting nitroxide labelling- cation sensing, synthesis of paramagnetic rotaxanes and dissipative processes of radical molecular machines

Abstract

Crown ethers are one of the most studied class of macrocycles for host-guest interactions. They have been employed in a wide range of possible applications in the field of biochemistry as biomimetic models, ionophores, metal sensing and mechanically interlocked molecules (MIMs). In the field of cation sensing an alternative approach consists in introducing radical probes as an active moiety in the complexation of a crown ether structure. In these systems, EPR spectroscopy acquires a key role in the detection of binding events thanks to its high sensitivity to the chemical environment surrounding the paramagnetic units. To this aim nitroxides based on aza-crown ethers were prepared and employed as selective sensors for the detection of inorganic and organic cations by EPR analysis of the corresponding host-guest complexes, particularly with Alkali, Alkali-earth and di-benzyl ammonium cations. The possibility to extend spin probe methodology in stable nitroxide macrocycle involved in molecular machines, like rotaxanes, makes the radical behaving as a direct recognizing unit for a guest hosted inside containing more than one station. In the present investigation, a [2]rotaxane based on a synthetic radical aza-crown ether derivative was studied in an aerobic oxidation reaction of alcohol to aldehyde as a mechanically blocked catalyst. Lastly, it is discussed a novel methodology for the synthesis of a series of 24-crown-8 chiral derivatives containing ethers, alcohols and carboxylic groups as side chains. Furthermore, these macrocycles were tested as catalysts for the synthesis of MGI-1 type Rotaxanes using the Metal-free active template approach, and the parameters to induct a face-selectivity of the macrocycles were also investigated