A long-standing question in porphyrin
chemistry is why pyrrole
monomers selectively form tetrapyrrolic macrocycles, whereas the corresponding
tripyrrolic macrocycles are never observed. Calix[3]pyrrole, a tripyrrolic
porphyrinogen-like macrocycle bearing three sp3-carbon
linkages, is a missing link molecule that might hold the key to this
enigma; however, it has remained elusive. Here we report the synthesis
and strain-induced transformations of calix[3]pyrrole and its furan
analogue, calix[3]furan. These macrocycles are readily accessed from
cyclic oligoketones. Crystallographic and theoretical analyses reveal
that these three-subunit systems possess the largest strain energy
among known calix[n]-type macrocycles. The ring-strain
triggers transformation of calix[3]pyrrole into first calix[6]pyrrole
and then calix[4]pyrrole under porphyrin cyclization conditions. The
present results help explain the absence of naturally occurring three-pyrrole
macrocycles and the fact that they are not observed as products or
intermediate during classic porphyrin syntheses.
In contrast to the general tendency that six coordinate iron(iii) porphyrin complexes with neutral oxygen ligands adopt a high-spin state in a wide range of temperature, some complexes with substituted pyridine N-oxides have exhibited spin-crossover from high-spin to low-spin states with decreasing temperature both in solution and in the solid state.
Fused benzene rings to antiaromatic compounds generally improve their stability but attenuate their antiaromaticity. The opposite case is now reported. NiII benzonorcorroles were synthesized and the effect of benzo‐fusion on the antiaromaticity was elucidated. The benzo‐fusion resulted in significant decrease of the HOMO–LUMO gaps and enhancement of the paratropic ring current effect. Furthermore, the introduction of the benzo groups induced singlet diradical character in the antiaromatic porphyrinoid.
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