Surrogate-stoppered [2]rotaxanes: A new route to larger interlocked architectures

A novel synthetic strategy for exchanging stoppers on rotaxanes, without them losing their integrity as interlocked molecules, is presented. The surrogate-stoppered [2] rotaxane contains one inert stopper and a triphenylphosphonium group attached to a benzylic position as a second reactive stopper in the dumbbell-shaped component which contains an ammonium (NH₂⁺) ion recognition site, encircled by a crown ether (24C8 or 25C8) component. The strategy for exchanging stoppers relies upon the ability of a benzylic triphenyl-phosphonium function to undergo a Wittig reaction with a bulky aromatic aldehyde to form a 'stilbenoid' [2] rotaxane as a mixture of cis and trans isomers, without the occurrence of any dethreading of the crown ether ring component. The C=C double bonds can then be hydrogenated, using Adams' catalyst, to afford a new covalently modified [2]rotaxane with two inert stoppers. Utilizing this strategy, larger interlocked molecular structures including a two-stationed [2]rotaxane and a branched [4]rotaxane have been prepared. Furthermore, initial studies, aimed at using this methodology to gain access to poly[n]rotaxane architectures, are presented.