Collect. Czech. Chem. Commun.
2002, 67, 1719-1728
Speed-Controlled Molecular ShuttlesMartin Bělohradskýa, Arkadij M. Elizarovb and J. Fraser Stoddartb,*
a Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, CZ-166 10 Prague 6, Czech Republic
b Department of Chemistry and Biochemistry, University of California, 405 Hilgard Avenue, Los Angeles, CA 90095, U.S.A.
Five potential molecular shuttles based on degenerate rotaxanes - wherein the ring component is a bis-1,4-phenylenecrown-10 and the dumbbell components are terminated by tetraarylmethane (slippage) stoppers and contain two identical bipyridinium recognition sites and a central 1,3-phenylene unit carrying a CH2OR (R = Me, Et, Ph, i-Pr, and t-Bu) substituent on its C-5 position - have been prepared by acylating the parent hydroxymethyl derivative with the appropriate reagents. Dynamic 1H NMR spectroscopy, carried out on these five potential molecular shuttles in CD3CN solutions, has revealed average Gibbs energy barriers to shuttling of 11.5, 12.0, 14.6, and 14.8 kcal mol-1 for the rotaxanes where R = Me, Et, i-Pr, and Ph, respectively. For the rotaxane where R = t-Bu, the Gibbs energy barrier to shuttling must exceed 17.2 kcal mol-1.
References: 52 live references.