1.1 Porphyrins - Friedrich-Alexander-Universität Erlangen-Nürnberg

1 Introduction
8
[H]
4 + 4
O H
O H
±[H] +
N
H
±[H] +
N
H
[H]
HO
O
H
HN
16
1. condensation by catalyst [H] +
2. aromatization by oxidation
±[H] +
HN HN
NH 17
OH
±[H] +
N
NH HN
N
H
H
N
H
NH HN
H
N
H
H
3 HO
3
O
X X
Y
X
Y
X
Y Y
Scheme 7. Iterative reaction steps in the porphyrin synthesis and subsequent oxidation of
porphyrinogen 18 by a benzoquinone reagent. 29
Thus it appears that homogeneously substituted systems – porphyrins of A4 substitution
pattern – are easy to synthesize and obtainable in good yields. Systems arising from different
aldehydes can give complex product mixtures since the combination of several fragments
occurs statistically. Hence, the yield of desired products can drop drastically and their
purification can be considerably hampered. To overcome these disadvantages, specific
fragments, so-called dipyrromethanes or tripyrranes prepared in advance, can be used for
synthesis. 28,29 As aryldipyrromethane fragments are of particular importance for this work,
they should be focused on next.
Although such compounds were already known for a long time 35 , it took until the early
1990s when LEE & LINDSEY 36 and VIGMOND et al. 37 made them efficiently accessible. They form
when arylaldehydes are reacted with excessive pyrrole under the same conditions as above
(Scheme 8) and represent moderately stable and isolable compounds. As they are to be used
as building blocks, they should be stable enough to withstand the conditions of the
formation of porphyrinogens. Else, decomposition would lead back to the parental aldehyde
15
18
OH
O