Relaxing Toward Rotaxanes

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Science  18 Aug 2006:
Vol. 313, Issue 5789, pp. 892
DOI: 10.1126/science.313.5789.892c

Traditional approaches to the chemical synthesis of complex molecular topologies, such as knots and interlocked rings, have focused on reducing kinetic barriers. More recently, an alternative approach has relied on the reversible assembly of comparatively simple building blocks that relax eventually into the desired topological conformation because it is the most favorable thermodynamic arrangement. Northrop et al. apply this second strategy to the preparation of [4]pseudorotaxanes, in which a Y-shaped core bears a ring on each of its three axes, and the rings in turn are linked to one another through either one or two central capping groups parallel to the planar core.

The rings in this case are crown ether derivatives attracted to the core axes through hydrogen bonding to cationic ammonium groups. Capping is achieved by reversible imine bond formation between formyl groups appended to the ends of the rings and amine groups on the phenyl cap. The singly capped complex assembled within 2 hours of mixing the components in solution, whereas the doubly capped analog (in which the caps straddled the core) required 8 days to wend through assorted kinetic intermediates. Both complexes were characterized by nuclear magnetic resonance and mass spectrometry. — JSY

Org. Lett. 8, 10.1021/ol061262u (2006).

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