Conformational properties of cyanomethoxy calix[4]arenes

Abstract

O-Alkylation of the dinitro calix[4]arene 2, easily available by selective ipso-nitration of the di-cyanomethyl ether 1, with allylbromide (DMF/Cs₂CO₃) gave tetraethers 3 and 4 with anti- and syn-orientations of the two allyl ether residues. The two possible stereoisomers of 3 in the partial cone and 1,2-alternate conformation exist as an equilibrium mixture which could be quantitatively analysed by ¹H NMR spectroscopy. The temperature dependence of this equilibrium leads to ΔH₀ = − 7.6 to −9.7 kJ mol⁻¹ in different solvents (tetrachloroethane, benzene, dimethylsulfoxide). Since 3(1,2-alt) could be obtained in pure form, its isomerisation to the equilibrium mixture with 3(paco) could be followed also kinetically. An activation energy of E_a = 110.5 kJ mol⁻¹ was found for this reaction in DMSO-d₆. The results were confirmed by similar studies with tetraethers 5 and 6 obtained by O-allylation of 1, although exact thermodynamic and kinetic studies were not possible in this case, since the NMR signals of the respective isomers were strongly overlapping. Single crystal X-ray structures were obtained for 2, 3(1,2-alt), 4(paco) and 5(1,2-alt).