Improved synthesis of a C3-symmetrical pyridinophane
作者:Claire Nolan、Thorfinnur Gunnlaugsson
DOI:10.1016/j.tetlet.2008.01.077
日期:2008.3
The formation of the C3-symmetrical 2,11,20-triaza[3.3.3](2,6)pyridinophane 1 was undertaken with the aim of improving the synthesis of this highly desirable macrocycle, with the future aim of functionalizing 1 with amide pendent arms for the recognition of lanthanide ions. The synthesis of 1 involves the stepwise transformation of pyridine-2,6-dicarboxylic acid into two key intermediates; N,N-bis
occurs across the lanthanide series at Sm3+; the complexes of the large Ln3+ ions (La–Nd) are chiral due to the nonplanar conformation of the macrocycle, and present effective C3v symmetries in solution as a consequence of a fast interconversion of two enantiomeric forms with C3 symmetry. The activation free energy for this enantiomerization process, as estimated by using DFT calculations, amounts to 33
structure is very similar to the structures observed in the solid state. The complexes of the light Ln3+ ions are fluxional due to a fast Δ(λλλλλλ) ↔ Λ(δδδδδδ) interconversion that involves the inversion of the macrocyclic ligand and the rotation of the acetamide pendant arms. The complexes of the small Ln3+ ions are considerably more rigid, the activation free energy determined from VT 1H NMR for the