Synthesis, structure, and conformation of aza[1n]metacyclophanes

[GRAPHICS] N-Benzyl substituted aza[1(n)]metacyclophanes (n = 4, 6, 8, and 10) were prepared in overall 40% isolated yields via Pd-catalyzed aminations. Analyses of the reaction mixtures showed that aza[1(4)]metacyclophane and the related polymer were the primary products (similar to 60% overall yield); aza[1(n)]metacyclophanes up to n = 14 and linear oligomers with up to 20 nitrogen atoms (with at least three types of end groups) were detected. Macrocyclic structures for n = 4, 6, and 10 were confirmed by X-ray crystallography. 1,3-Alternate (D-2d) and 1,3,5-alternate (S-6) conformations in solution on NMR time scale at low temperatures were found for macrocycles with n = 4 and n = 6, respectively; the barrier for ring inversion was considerably lower for the larger macrocycle.