Polyyne Rotaxanes: Stabilization by Encapsulation

Active metal template Glaser coupling has been used to synthesize a series of rotaxanes consisting of a polyyne, with up to 24 contiguous sp-hybridized carbon atoms, threaded through a variety of macrocycles. Cadiot-Chodkiewicz cross-coupling affords higher yields of rotaxanes than homocoupling. This methodology has been used to prepare [3]rotaxanes with two polyyne chains locked through the same macrocyde. The crystal structure of one of these [3]rotaxanes shows that there is extremely close contact between the central carbon atoms of the threaded hexayne chains (C center dot center dot center dot C distance 3.29 A vs 3.4 A for the sum of van der Waals radii) and that the bond-length alternation is perturbed in the vicinity of this contact. However, despite the close interaction between the hexayne chains, the [3]rotaxane is remarkably stable under ambient conditions, probably because the two polyynes adopt a crossed geometry. In the solid state, the angle between the two polyyne chains is 74 degrees, and this crossed geometry appears to be dictated by the bulk of the supertrityl end groups. Several rotaxanes have been synthesized to explore gem-dibromoethene moieties as masked polyynes. However, the reductive Fritsch-Buttenberg-Wiechell rearrangement to form the desired polyyne rotaxanes has not yet been achieved. X-ray crystallographic analysis on six [2]rotaxanes and two [3]rotaxanes provides insight into the noncovalent interactions in these systems. Differential scanning calorimetry (DSC) reveals that the longer polyyne rotaxanes (C16, C18, and C24) decompose at higher temperatures than the corresponding unthreaded polyyne axles. The stability enhancement increases as the polyyne becomes longer, reaching 60 degrees C in the C24 rotaxane.