Synthesis of stilbene crown ether Calix[4]arenes as switchable receptors

Abstract

Starting from bisbenzaldehyde and bisnitrobenzene-p-tert-butylcalix[4]arenes (1 and 2), photoswitchable stilbene-bridged and azobenzene-bridged p-tert-butylcalix[4]arenes (3 and 4) were synthesised through reductive coupling reactions. Both cis- and trans-3 were produced from the reductive coupling of the ortho-1 and meta-1 with cis-isomer being predominant for both regioisomers while the coupling of para-1 gave only cis-product. The only isolable product obtained from the reductive coupling of ortho-2 and meta-2 was the corresponding trans-4, while the coupling product from para-2 was not stable. The geometrical assignment from [superscript 1]H-NMR is in agreement with X-ray crystal structures. All the synthesised compounds showed photostationary state of their cis-trans isomerisation. The complexation with alkali metal ion was observed for only the ortho-4 derivative, suggesting that the lone pair of N-atom in the azobenzene bridge participated in the complexation. The complexation with acetonitrile and nitromethane was observed for cis- and trans-ortho-3 and cis-meta-3. The complexation constants were determined by [superscript 1]H-NMR titration. The meta-3 displayed photoswitchable complexation with acetonitrile and nitromethane as cis-meta-3 could complex with both compounds but trans-meta-3 could complex with neither one. The reaction of meta-1 with KCN (20% mole) in alcoholic solvent gave a monoester product resulting from a selective oxidation of one of the aldehyde groups followed by an intramolecular acid catalysis esterification with the alcoholic solvent.