Dynamic Stereochemistry of N-Acyl-9-triptycylamine derivatives.

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Dynamic stereochemistry of N-9-triptycylacetamide (1) and its N-methyl-, N-ethyl-, N-benzyl-, and N-phenyl derivatives was studied. These compounds were synthesized by alkylation of 9-triptycylamine followed by acetylation. X-Ray crystallographic analysis of all the compounds were made, and revealed that the nitrogen atom is significantly pyramidalized in any compound except for the parent one 1. For the N-ethyl compound, both Z- and E-isomers exist in a single crystal, while the other compounds adopt Z-configuration in crystal. In solution, 1 and the N-phenyl compound exist almost exclusively as the Z-isomer as for the amide bond, while the other compounds exist as an equilibrium mixture of Z- and E-isomers. The equilibrium constant (Z/E) tends to increase with the solvent polarity in any compound. Rotation of the amide bond was studied by dynamic NMR spectroscopy. The barrier to rotation also tends to increase with the solvent polarity. At low temperatures, the methylene protons of the N-ethyl and N-benzyl compounds are nonequivalent to each other, indicating the pyramidality of the nitrogen atom in solution. Stereomutation of the Tp-N bond was measured and discussed.