Selective Formylation of Azacalixpyridine Macrocycles and Their Transformation to Molecular Semicages
The aromatic electrophilic formylation reaction of azacalixarenepyridine and azacalixpyridine were systematically studied. By simply controlling the ratio of reactants and the reaction temperature, the Vilsmeier–Haack reaction selectively afforded mono-, di-, and tetra-formylated azacalixarenepyridines and azacalixpyridines. The preferential and selective functionalization reactions of macrocycles were discussed in terms of their conformational structure and conjugation effect between aromatic subunits and bridging nitrogen atoms. All resulting functionalized azacalixarenepyridines and azacalixpyridines adopted a 1,3-alternate conformation both in the crystalline state and in solution. Taking advantage of the close proximity of aldehyde groups in 1,3-alternate di- and tetra-formylated azacalixpyridine macrocycles, the McMurry reductive coupling reaction of carbonyls was accomplished to yield unique semicage molecules.