Surrogate‐stoppered [2]rotaxanes: a new route to larger interlocked architectures

Abstract

AbstractA 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 triphenylphosphonium 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. Two ammonium ion/crown ether‐based [2]rotaxane monomers–each incorporating (i) a dumbbell‐shaped component, possessing the surrogate benzylic triphenylphosphonium stopper, and (ii) a crown ether ring component, bearing an aldehyde function on a fused benzenoid ring–undergo a sequence of Wittig reactions in which the surrogate benzylic triphenylphosphonium stopper is exchanged for a crown ether ring component either, (i) in the same rotaxane molecule, to give cyclic daisy‐chains by an intramolecular, chain‐terminating reaction or, (ii) in another rotaxane molecule, to give acyclic daisy chains by an intermolecular chain‐propagating reaction. Copyright © 2003 John Wiley & Sons, Ltd.