DESIGN AND SYNTHESIS OF A MOLECULAR TURNSTILE

Macrobicycles 1-3 have been prepared by palladium-catalyzed double macrocyclization. The planar geometry of this system is such that the para axis of the inner ring exactly matches the inner diameter of the macrocycle. Molecular models suggest that the inner ring of 1 should be able to rotate freely about its long axis, acting like the spindle of a turnstile. For large spindle substituents, this rotation will become hindered while for intermediate sized substituents, a pair of energetically equivalent conformational states should rapidly interconvert on an experimentally observable time scale. Substituted derivatives 2 and 3 possess diastereotopic methylene protons which become operationally enantiotopic upon fast rotation. Based on variable-temperature H-1 NMR and longitudinal T-1 relaxation experiments, examples of what are believed to be freely rotating 2 and conformationally locked 3 spindles are reported. These molecules represent the first iteration in the development of phenylacetylene macrocycles possessing conformational bistability that may eventually lead to new types of solids or liquid crystals that respond rapidly to external electric fields.