Alkali Metal Cation Selectivity of an Oligo-Ether Substituted Norbornadiene and Quadricyclane Isomer



We present investigations aimed at using the norbornadiene/quadricyclane system as a framework for a switchable cation carrier. According to molecular modelling studies a suitably substituted system should perform a pincer-like motion upon isomerisation and reisomerisation, while complexing and decomplexing a host molecule. To this end, we investigated the complexing abilities of a 1,5,6,7-tetrasubstituted quadricyclane polyether and its corresponding norbornadiene isomer. The picrate extraction method was applied to determine the extraction constants Kex and the association constants Kass of these polyethers with alkali metal cations in chloroform. As expected, the complexes of the quadricyclane polyether with Li+, Na+, K+ and Cs+ are more stable than those of the norbornadiene isomer. This difference reaches a maximum for potassium. The interconversion between the norbornadiene polyether and the corresponding quadricyclane was also examined. The norbornadiene can be converted photochemically to the quadricyclane with 50% conversion while the reverse reaction is catalysed by neutral aluminium oxide in 71% yield. No by-product was detected in either of the isomerization reactions.