Speed-Controlled Molecular Shuttles

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Abstract
Five potential molecular shuttles based on degenerate [2]rotaxanes - wherein the ring component is a bis-1,4-phenylene[34]crown-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 [2]rotaxanes where R = Me, Et, i-Pr, and Ph, respectively. For the [2]rotaxane where R = t-Bu, the Gibbs energy barrier to shuttling must exceed 17.2 kcal mol-1.

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