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Nucleophilic Ring Opening of meso-Substituted 5‑Oxaporphyrin by Oxygen, Nitrogen, Sulfur, and Carbon Nucleophiles
journal contribution
posted on 2014-03-21, 00:00 authored by Kazuhisa Kakeya, Masakatsu Aozasa, Tadashi Mizutani, Yutaka Hitomi, Masahito KoderaNucleophilic
ring opening of 23H-[21,23-didehydro-10,15,20-tris(4-methoxycarbonylphenyl)-5-oxaporphyrinato](trifluoroacetato)zinc(II)
with various nucleophiles such as alkoxide, amine, thiolate, and enolate
gave 19-substituted bilinone zinc complexes, and they were isolated
as free base bilinones. An X-ray crystallographic study demonstrated
that the product of 5-oxaporphyrin with sodium methoxide was 21H,23H-(4Z,9Z,15Z)-1,21-dihydro-19-methoxy-5,10,15-tris(4-methoxycarbonylphenyl)bilin-1-one
with a helicoidal conformation. The structure of the product of 5-oxaporphyrin
with an enolate of ethyl acetoacetate was 21H,22H,24H-(4Z,9Z,15Z,19E)-19-(1-ethoxycarbonyl-2-oxopropylidene)-5,10,15-tris(4-methoxycarbonylphenyl)-1,19,21,24-tetrahydrobilin-1-one,
with three inner NH groups. The product with SH– was also the same tautomer, 21H,22H,24H-19-thioxo-bilin-1-one, with three NH groups,
while the products with RO–, RNH2, and
RS– nucleophiles were 21H,23H-bilin-1-ones with two inner NH groups. The first-order
rate constants of the ring opening reaction of 5-oxaporphyrin with
1 M BnOH and BnSH in toluene at 303 K were 3.0 × 10–4 and 6.1 × 10–4 s–1, respectively.
The ratio of the rate of alcohol to thiol was much higher than that
with methyl iodide, suggesting that 5-oxaporphyrin reacted as a hard
electrophile in comparison to methyl iodide. UV–visible spectra
of 19-substituted bilinones in CHCl3 at 298 K showed that
the absorption maximum of the lower energy band was red-shifted in
increasing order of O-substituted (645 nm), S-substituted (668 nm),
N-substituted (699 nm), and C-substituted bilinones (706 nm).