meso-Tetraaryl-22-aza-m-benziporphyrin 6 could be considered the straightforward
structural homologue of meso-tetraarylporphyrin 1, in which the pyridine motif
replaces one of the pyrrole rings [22]. The synthesis strategy was based on
[3 + 1]-type condensation and gave a 5.5% yield (Scheme 19). In contrast to the
synthesis of 3-aza-m-benziporphyrin 7 [21], the benzoylation of the pyritripyrrane
45 to form diol 46 was necessary. The introduction of bulky meso-aryl groups
adjacently to the incorporated pyridine ring was of importance making it possible
to increase the stability of 45 during the condensation.
Two stages of the protonation of 6 were demonstrated in the
1 H NMR experiment. The addition of the first proton resulted in the formation of two symmetrical
tautomers, in which one of them 6-H
+ shows macrocyclic diatropicity (Scheme 20).
N
OHC
CHO
NH HN
HN
ROOC
ROOC
41
N
NH
HN
+
39
N
40
TFA
N
NH
HN
N
N
HN
NH
N
44
p-chloranil
(excess)
p-chloranil
(eqimolar)
N
NH
HN
N
O
43
H
N
N
N
N
42
Scheme 18 Synthesis of β-alkylated pyriporphyrin
HN
N
NH
Ar 1
Ar 1
H
N
N
N
N
Ar 1
Ar 1
Ar 2
Ar 2
O
O
N
N
N
Ar 1
Ar 1
Ar 2
Ar 2
45
6
46
1. EtMgBr
2. PhCOCl
1. NaBH 4
2. TFA
3. TEA/DDQ
Scheme 19 Synthesis of pyriporphyrin 6
A Pincer Motif Etched into a meta-Benziporphyrin Frame
195