Pillar[4]pyridinium: a square-shaped molecular box.

Numerous applications of Stoddart's 'blue-box', a pyridinium containing macrocycle of rectangular shape, encouraged us to seek successors of this amazing molecule. Using a one-step cyclization reaction we synthesized a square-shaped cyclic tetramer consisting of 4-methylenepyridinium units - pillar[4]pyridinium (P[4]P). Pillar[4]pyridinium is a quadruply positively charged water-soluble macrocycle with a highly symmetric, strained structure and an electron-deficient cavity. These features impel the macrocycle to assemble into channel networks in the solid state and render it an effective fluoride receptor in water.

Hybrid [n]arenes through thermodynamically driven macrocyclization reactions.

Hybrid [n]arenes, the class of medium-sized macrocyclic compounds consisting of different alkoxybenzene units, were obtained by a simple, one-pot, direct condensation of two different alkoxybenzenes with formaldehyde catalyzed by a Brønsted acid (trifluoroacetic acid). We have shown that, under Brønsted acid catalysis, this reaction is reversible and therefore governed by the relative stability of the products. The main macrocyclic products are hybrid [n]arenes consisting of four alkoxybenzene units of [2 + 2] or [3 + 1] stoichiometry. However, an unusual [3 + 2] hybrid macrocycle was also obtained as a main product of the condensation between 1,4-dimethoxybenzene, 1,3,5-trimethoxybenzene, and formaldehyde. The stability of the hybrid products and the reversibility of the reaction were further confirmed by a scrambling experiment, involving pillar[5]arene and per-O-methylated resorcin[4]arene. The scrambling experiment has given hybrid macrocycles in yields comparable with those obtained in condensation reactions. NMR spectra and X-ray structures of hybrid [n]arenes indicate that 1,2- and 1,3-dialkoxybenzene units are flexible parts of macrocyclic rings. However, the 1,4-dialkoxybenzene units present considerable steric hindrance, resulting in the formation of isomers and inherently chiral macrocycles due to inhibited rotation. The recognition properties toward various organic cations were also determined. Highly selective recognition of the N-methylpyridinium cation was observed for the [3 + 2] hybrid macrocycle.