4
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zene, originates a far smaller amount of chlorin stressing the
importance of the nitroaromatic structure. The role of nitroaromat-
ics in the origin of higher amounts of chlorin is certainly related to
these particular reaction conditions with large amounts of por-
phyrinogen to be oxidized relatively to the existing oxidant spe-
cies. In one-pot pyrrole/aldehyde condensation carried out in the
presence of the nitrobenzene17 the amount of porphyrinogen at
Nuclear Magnetic Resonance Laboratory of the Coimbra Chemical
Centre-University of Coimbra (Proj/REEQ/481/QUI/2006) for obtain-
ing some of the NMR data.
Supplementary data
each moment is small and no chlorin is detected. This fact finds
support in Dolphin’s hypothesis23 stating that one reason to favor
chlorin formation can be oxidative disproportionation of the mac-
rocycle in different oxidative states.
References and notes
Temperature also seems to be an important factor, the oxida-
tion with propionic acid/nitrobenzene of the porphyrinogen 1a at
the lower temperature of 70–80 °C yields 9% of a mixture having
only 76% of chlorin.
In a further development for the oxidative step a more concen-
trated solution of propionic acid/acetic anhydride/nitrobenzene at
1
2
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05–110 °C was used. In this case we obtained 190 mg of a precip-
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itate of 2a (17% yield) with 92% of chlorin. From the chromato-
graphic isolation of the mother liquid more porphyrin/chlorin
mixture was obtained in a total yield of 28%. The visible spectrum
showed a small band at 736 nm which is probably due to the pres-
ence of a small amount of bacteriochlorin macrocycle. Following
this improvement several porphyrinogens were oxidized to mix-
tures of chlorins and porphyrins (Table 3).
The propionic acid/acetic anhydride/nitrobenzene mixture effi-
ciently oxidizes meso-tetraarylporphyrinogens originating sub-
stantial amounts of chlorins. The effect of phenyl substitution on
the chlorin selectivity is not easy to rationalize. In the absence of
substituents, porphyrin is the major oxidative product (1h). The
presence of electron-withdrawing groups tends to favor the chlorin
oxidation state with the exception of (1b). Interestingly, the same
chlorine atom at para position favors the chlorin (1i). In general
however, steric effects by substituents at the ortho position of
the phenyl group seem to favor the chlorin as in the case of 1c–
e, and also in the case of the 1-naphthyl derivative (1g). The 2,6-di-
chloro derivative 1a which has two kinds of effects gives the great-
er amount of chlorin.
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In conclusion we developed a method to efficiently oxidize por-
phyrinogens to a mixture of porphyrin and chlorin. In some cases
the chlorin is isolated as the main product. For meso-tetra(2,6-
dichlorophenyl) porphyrinogen 1a the meso-tetra(2,6-dichloro-
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The authors would like to thank Chymiotechnon and FCT-POC-
TI/QUI/55931/2004 for financial support. We acknowledge the