Conformational instability of a proximally-difunctionalized calix[4]-diquinone

We have recently described the electrochemical oxidation of 5,11,17,23-tetra-tert-butyl-25, 26-bis(diphenylphosphinoylmethoxy)-27,28-dihydroxycalix[4]arene into the corresponding diquinone 1. The solid state structure of 1 has now been determined by a single crystal X-ray diffraction study. Diquinone 1 crystallizes in the monoclinic space group with a=10.3507(12), b=25.219(4), c=20.2315(14) Å, β=101.166(8)°, V=5181.1(10) Å3, Z=4, and Dx=1.273 g cm−3. The calixarene skeleton adopts a partial cone conformation in which one quinone ring is anti-oriented with respect to the other three rings of the calixarene core. A variable temperature NMR study shows that 1 is dynamic in solution, each quinone unit undergoing fast oscillation about the axis that passes through the two meta carbon atoms bonded to the adjacent methylene groups. The motion of the quinone rings was confirmed by 2D NMR experiments.