Selective Formylation of Azacalixpyridine Macrocycles and Their Transformation to Molecular Semicages

The aromatic electrophilic formylation
reaction of azacalix[2]­arene[2]­pyridine
and azacalix[4]­pyridine 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 azacalix[2]­arene[2]­pyridines
and azacalix[4]­pyridines. 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 azacalix[2]­arene[2]­pyridines
and azacalix[4]­pyridines 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.