Cooperative Reactivity in an Extended-Viologen-Based Cyclophane

A tetracationic
pyridinium-based cyclophane with a box-like geometry,
incorporating two juxtaposed alkyne functions, has been synthesized.
The triple bonds are reactive through cycloadditions toward dienes
and azides, promoted by the electron-withdrawing nature of the pyridinium
rings, as well as by the strain inherent in the cyclophane. The cycloadditions
proceeded in high yields, with the cyclophane reacting faster than
its acyclic analogue. While the cyclophane contains two reactive triple
bonds, there is no evidence for a stable monofunctional intermediateonly
starting material and the difunctional product have been detected
by 1H NMR spectroscopy. Molecular modeling of the energy
landscape reveals a lower barrier for the kinetically favored second
cycloaddition compared with the first one. This situation results
in tandem cascading reactions within rigid cyclophanes, where reactions
at a first triple bond induce increased reactivity at a distal second
alkyne.