2.5 Aza-m-Benziporphyrins
Aza-m-benziporphyrins – pyriporphyrins [68] – are structural analogues of mbenziporphyrin, in which the properly oriented pyridine motif replaces the benzene
unit affording three isomeric structures: 22-aza-m-benziporphyrin 6 [22], 3-aza-mbenziporphyrin 7 [21], and 2-aza-m-benziporphyrin 8 [69]. 7 and 8 preserve the
carbaporphyrinoid features conserving the characteristic (CNNN) set of inner core
donor atoms, whereas 6 contains the (NNNN) porphyrin-like donor set.
2.5.1 3-Aza-m-Benziporphyrin
N
R
R
R
R
The condensation of 3,5-bis[2-pyrrolyl(aryl)methyl] pyridine (pyritripyrrane) 30,
pyrrole, and aryl aldehyde in dichloromethane, catalyzed by TFA and followed by
oxidation with DDQ, resulted in the formation of meso-tetraryl-3-aza-mbenziporphyrin 7 [21]. The strict [3 + 1] methodology was applied to afford mesotetraaryl-24-thia-3-aza-m-benziporphyrin (N-confused 21-thiapyriporphyrin) 7-S
(Scheme 13). The condensation of pyritripyrrane 30 with 2,5-bis
[arylhydroxymethyl]thiophene 24 in dichloromethane catalyzed by TFA and
followed by oxidation with DDQ afforded 7-S. The yields varied in the 2.5–18%
range depending on the meso substitution.
The protonation of 7 led directly to the dicationic species 7-H 2
2+ (Scheme 14).
The feasible monocations 7-H A
+ or 7-H B
+ were not detected in the course of the
1 H
NMR titration. The tricationic structure 7-H 3
3+ was observed at a very high concentration of acid.
N
N
H
N
Ph
Ph
Tol
Tol
R
OH
Ph
HO
Ph R
O
O
R
N
N
R
a
b
c
26-R
27-R
28-R
29-R
Scheme 12 Synthesis of 22-alkyl-m-benziporphyrins 29-R: (a) DIBAH, 4 h, 183 K, (b) PhMgBr,
1 h, 293 K, (c) pyrrole, ArCHO, BF 3 Et 2 O, DDQ,1 h. R ¼ Me, Et
A Pincer Motif Etched into a meta-Benziporphyrin Frame
191