Structures and stereodynamics of N-9-triptycylacetamide and its N-alkyl derivatives

X-Ray crystallographic and dynamic NMR spectroscopic studies were performed on N-9-triptycylacetamide (1), N-methyl-N-9-triptycylacetamide (2), N-ethyl-N-9-triptycylacetamide (3), and N-benzyl-N-9-triptycylacetamide (4). In the crystalline state, compounds 1, 2, and 4 exist solely as the Z-isomer for the amide bond, while equal amounts of the Z- and E-isomers exist in a single crystal of compound 3. The amide nitrogen atom in 2, 3, or 4 is significantly pyramidalized. In solution, 1 exists solely as the Z-isomer, while 2, 3, and 4 exist as an equilibrium mixture of the Z- and E-isomers. The isomer ratio is not correlated with the bulkiness of the N-alkyl group; the N-ethyl compound 3 seems to behave anomalously. The isomer ratio is significantly dependent on the solvent; the population of the Z-isomer in 2 or 3 or 4 increases with the solvent polarity. The Z reversible arrow E interconversion takes place on the Nh IR time scale around ambient temperature, and the barrier is dependent on the solvent; hydroxylic solvents, methanol and ethanol, increase the barrier. The methylene protons in the ethyl or benzyl group in any isomer of 3 or 3 become diastereotopic at low temperatures suggesting that the nitrogen atom in either compound is pyramidal in solution. Stereomutation around the Tp-N bond is discussed in some detail.