Efficient Solvent‐Free Syntheses of [2]‐ and [4]Rotaxanes

Abstract

Rotaxanes have potential applicability as molecular actuators and switches within mesoscale molecular electronic devices. [1] Among the protocols devised for preparing rotaxanes, threading followed by stoppering [2] has attracted the most attention. Nevertheless, synthesizing rotaxanes in high yields by this approach can be challenging because several factors affect the formation of the precursor pseudorotaxanes in solution-for example, low association constants for the interactions between the thread-and beadlike components, the use of competing solvents and/or elevated temperatures, and the formation of interfering by-products during the stoppering process.[3] Although solvent-free conditions [4] would, in theory, minimize the degree of dissociation of the pseudorotaxane complexes during the stoppering reaction and allow high-order rotaxanes to be generated more efficiently, a new challenge arises in choosing a suitable stoppering reaction that can be performed by grinding a wellmixed solid phase. To the best of our knowledge, only two types of rotaxanes have been synthesized through solid-tosolid grinding: one through the reaction of a mixture of bis-pphenylene[34]crown-10, a benzyl bromide derivative incorporating a bipyridinium recognition site, and a pyridinecontaining stopper, [5] and the other through ball-milling of polypseudorotaxane complexes-comprising a-cyclodextrin and poly(tetrahydrofuran) components-with isocyanate stoppers.[6] Both of these cases gave low-to-moderate yields of their desired products (< 45 %), thus suggesting that these reactions are not suitable for the efficient syntheses of higherorder rotaxanes. Herein, we report a new solid-state ballmilling reaction that produces both [2]-and [4]rotaxanes efficiently and in high yield.The formation of imines through the dehydration of aldehydes and primary amines can be achieved in high yield by solid-state ball-milling. [7] As imines are in general easily hydrolyzed, we were not inclined to use this condensation reaction to construct higher-order rotaxanes. Instead, we turned our attention toward the formation of hexahydropyrimidines by condensing carbonyl compounds with 1,3-diamines.[8] We chose 1,8-diaminonaphthalene (3) as a suitable diamine for the reaction with a threadlike moiety terminated with a formyl group because of its steric bulk and the stability of the resulting dihydropyrimidine stopper units.[9]Previously, we reported that the oxygen-deficient macrocycle 1 forms a complex with a dibenzylammonium (DBA) ion in CD 3 CN (K a = 200 m À1 ).[10] Thus, as the first step toward preparing a [2]rotaxane under solvent-free conditions from such components, we concentrated an equimolar mixture of the macrocycle 1 and the dialdehyde 2-H·PF 6 in CH 3 CN under reduced pressure to afford a white solid, which we assumed to contain predominantly the (Figure 1). After 1 h, these signals were predominant (Figure 1 e), so we subjected the mixture to column chromatography and isolated the [2]rotaxane 4-H·PF 6 in 80 % yield.[11] The yield in...

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