It's a small 'porphyrin' world!
Porphyrins are tetrapyrrolic macrocycles that are commonly seen in heme and chlorophyll. A long-standing enigma in both laboratorial and natural porphyrin syntheses is that “pyrrole monomers selectively form tetrapyrrolic macrocycles, but those with three pyrrole monomers are never observed.” Calixpyrrole, a porphyrinogen-like macrocycle bearing three pyrrole units, might hold a key to this enigma, but has been elusive in porphyrin-related chemistry.
We have synthesized calixpyrrole for the first time using a carbonyl rope as a precursor. Calixpyrrole has been proven stable under neutral conditions. However, it underwent strain-induced ring expansion reaction under usual porphyrin cyclization conditions (in the presence of acid) within 10 seconds to give calixpyrrole. On the other hand, boron-complex of calixpyrrole tolerates acidic conditions, which explained the fact that contracted porphyrins with three pyrrole units, i.e. subporphyrin, have been known only as boron-complexed forms.
Our findings have opened the door to a new ‘small porphyrin’ world!
Chemistry of 'Carbonyl Ropes'
Our research on aliphatic polycarbonyl compounds, called 'carbonyl ropes', started with a dream of making magic ropes in chemistry.
In the biosynthesis of polyketides, various metabolites called polyketides are synthesized from polyketone chains as if they are magic ropes. Inspired by such beautiful biosynthesis, we designed an original ketone sequence of polyketone which has alternating 1,3-diketone and 1,4-diketone subunits. We are working on ‘magical synthesis’ of various functional organic compounds using the carbonyl ropes.
◆ Elongation of carbonyl ropes
Our carbonyl ropes are synthesized by stepwise oligomerization of an acetylacetone derivatives. Namely, the chain length can be doubled by terminal ketone-selective silylation and subsequent oxidative homo-coupling reaction using silver(I) oxide. With this strategy, we have synthesized up to octameric acetylacetone chain with 6 nm length.
◆ Traversable synthesis of π–conjugation
As natural polyketone chains are converted to fused six-membered π-conjugated system, our carbonyl rope can also be transformed to polycyclic compounds. Intramolecular condensation reaction of a short carbonyl rope gave pentalene-based, cross-conjugated system. After chemical modification at the remaining carbonyl groups, we have synthesized organic chromophores that emit visible light in the solid state.
◆ From carbonyl ropes to imine ropes: Weaving and coiling
The carbonyl ropes were converted to imine ropes by treatment with hydrazine. The imine ropes can bind various metal ions to form assemblies. For example, complexation with Zn(II) ions resulted in the formation of textile-like coordination polymer as if imine chains were woven. When combined with Ni(II) ions, imine chains were coiled around a nickel oxide cluster. Self-assembly of imine chains occurs on complexation with Pd(II) ions.
For the details, please visit our publication page.