only (the perfect nontoxic reagents) or hypervalent iodine
reagent DIB, which generates iodobenzene and acetic acid as
the side compounds (less toxic than osmium and lead complexes).
Current investigations are now devoted to the chemical trans-
formations of the cyano groups to obtain more elaborated
chromophores.
We are grateful to the CNRS and the Agence Nationale de
la Recherche for financial support of the research project
ANR-09-JCJC-0089-01. Research in Mons is supported by
FNRS/FRFC, BELSPO (PAI 6/27) and Region Wallonne
´
(Programme d’excellence OPTI2MAT). DB is FNRS Research
Director.
Notes and references
Scheme 5 Synthesis of secochlorins 9-H2 and 11-H2.
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confirmed the presence of two CQO groups at d = 168.1 and
161.9 ppm. UV-visible spectrum of 9-H2 is very similar to that
of secochlorin 7-H2 with a Soret band at l = 435 and Q-bands
at l = 559, 601 and 684 nm. The same photoxidation reaction
carried out with porphyrin 10-H2 (R = Me) afforded the
corresponding secochlorin 11-H2 (R = Me, Scheme 5). As
expected for 11-H2, the signal due to the CHO group is a
broad singlet at d = 9.75 ppm due to the absence of the
neighboring NH group. Absorption bands of 11-H2 are
slightly hypsochromically shifted compared to secochlorin
9-H2: the Soret band is observed at l = 429 nm, and three
Q-bands are observed at l = 547, 597 and 673 nm.
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We presume that singlet oxygen (1O2) generated by free base
macrocycles under ambient light is the dominant oxidant.
Preliminary experiments showed that irradiation of diluted
solution of 8-H2 and 10-H2 in CH2Cl2 during 15 hours with a
slide projector (250 W) dramatically increased the yields of
secochlorins 9-H2 (24%) and 11-H2 (72%). This can also
explain why nickel(II) complexes 8-Ni and 10-Ni which are
1
not able to generate O2 are not converted into secochlorins
(Scheme 5). A possible mechanism to explain the formation of
secochlorins 9-H2 and 11-H2 is the formation of unstable
endoperoxide intermediates, which were previously described
by Foote and Ryang for imidazole derivatives (Scheme 6).13
Interestingly, free base porphyrin 12-H2 fused to one imidazolium
ring is stable and did not undergo cleavage reaction of the
b,b0-double bond of the fused pyrrole (Scheme 5): this is consistent
with the fact that imidazolium rings are electron-deficient and
disfavor the [4+2] cycloaddition of the singlet oxygen.
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To summarize, we describe here for the first time some new
synthetic pathways to obtain secochlorins, including stable
free base derivatives. The interesting aspect of this work is the
porphyrin - secochlorin transformation with light and oxygen
13 H.-S. Ryang and C. S. Foote, J. Am. Chem. Soc., 1979, 101,
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c
3462 Chem. Commun., 2012, 48, 3460–3462
This journal is The Royal Society of Chemistry 2012