Tertiary Amide Rotation in a Nanoscale Host

A self-assembled cylindrical capsule provides a nanoscale environment that affects the rotational barriers of tertiary amides. Measurements of the activation energies for the rotations and behaviors of the amides inside the capsule were determined by using 1H NMR spectroscopic methods in deuterated mesitylene solution. For amides 3-8, rotation rates can decrease or increase in the capsule by up to an order of magnitude from those of the free amides in solution depending on the structure of the amides. The acceleration/deceleration of the rotation results from selective destabilization/stabilization of the ground state or the transition state. In the case of compound 10, the rotation generates two isomers that are equimolar in solution but inside the capsule only one of them is observed. Accordingly, the rotation rate is slowed by several orders of magnitude inside the capsule. In the case of amide 8, a competition experiment indicates that the acceleration of the rotation inside the capsule is due to destabilization of the ground state. © Wiley-VCH Verlag GmbH & Co. KGaA, 2007.