70
J. Setsune
45% yield was further converted to the push–pull porphyrin 75 with two benzenedicarboximide units as electron-withdrawing groups and two 3,4-di-p-anisylbenzene
units as electron-donating groups (Kumar et al. 2018).
The push–pull tetrabenzoporphyrin Zn complex 75 showed the Soret absorption
band at 507 nm that is red-shifted by 51 nm in comparison with the dibenzoporphyrin
Zn complex of 73 with two 3,4-di-p-anisylbenzo units. It is also red-shifted by 30 nm
in comparison with the dibenzoporphyrin Zn complex with two benzenedicarboximide unit derived from 72. The Q band of 75 at 721 nm appeared at by ca. 80 nm
longer wavelength than those push-only or pull-only dibenzoporphyrins. Theoretical
study on the electronic structure indicated that the HOMO–LUMO gaps of the pushonly and pull-only dibenzoporphyrins are not so different although both HOMO and
LUMO energy levels of the push-only dibenzoporphyrin are higher than those of the
pull-only dibenzoporphyrin. It is of interest that the HOMO energy level of 75 is very
similar to that of the push-only dibenzoporphyrin and the LUMO energy level of 75
is very similar to that of the pull-only dibenzoporphyrin, which significantly reduced
the HOMO–LUMO gap. This result was consistent with the cyclic voltammetric
analysis; 1.75 V for 75 is by 0.25 V smaller than those dibenzoporphyrins.
3.3.3 Reactions of Porphyrins at Pyrrole β-Positions
Since meso-tetraarylporphyrins are readily available, robust, and soluble in organic
solvents, great effort has so far been directed to structural modification at pyrrole
β-positions. Bringmann and coworkers investigated Pd-catalyzed borylation of 2bromo-meso-tetraphenylporphyrin 76 and obtained β-dioxaborolanylporphyrin 77
in 70% yield using toluene–water two phase system (Fig. 3.25) (Bringmann et al.
2008). If this reaction was run in DMF, carbopalladation on the adjacent meso-phenyl
group occurred to give almost exclusive formation of indene-fused porphyrin 78
without participation of bis(pinacolato)diboron (B 2 (pin) 2 ) in the reaction. A closely
related cyclopentadiene-fused porphyrins 80 was reported by Osuka’s group through
Pd-catalyzed reaction between meso-bromoporphyrin 79 and alkynes (Sahoo et al.
2006). The Ni and Cu complexes 80 showed split Soret bands; one at 380–390 nm
and the other at 480–490 nm. The additional π-conjugation in 80 (M = Ni) decreased
electrochemical HOMO–LUMO gap (E = 1.66 V) relative to that (E = 2.29 V)
of trans-A 2 type diarylporphyrin Ni complex. Once β-borylated porphyrin 77 was
available, it was subjected to the Pd-catalyzed Suzuki–Miyaura cross coupling with
76 leading to β,β-linked diporphyrin 81 in 73% yield. Some bimetallic complexes
of 81 could be resolved into enantiomers by HPLC on a chiral phase. The rotational
stability at the β,β-axis depends on the metal but racemization of the optically resolved
bisZn complex was not seen at room temperature.
Osuka, Shinokubo, and coworkers reported that the pyrrole β-position adjacent
to the unsubstituted meso-position could be directly borylated (Hata et al. 2005).
When the [Ir(COD)OMe] 2 -catalyzed direct borylation of aromatic compounds by
B 2 (pin) 2 was applied to 5,15-diarylporphyrin and 5,10,15-triarylporphyrin, the
J. Setsune
45% yield was further converted to the push–pull porphyrin 75 with two benzenedicarboximide units as electron-withdrawing groups and two 3,4-di-p-anisylbenzene
units as electron-donating groups (Kumar et al. 2018).
The push–pull tetrabenzoporphyrin Zn complex 75 showed the Soret absorption
band at 507 nm that is red-shifted by 51 nm in comparison with the dibenzoporphyrin
Zn complex of 73 with two 3,4-di-p-anisylbenzo units. It is also red-shifted by 30 nm
in comparison with the dibenzoporphyrin Zn complex with two benzenedicarboximide unit derived from 72. The Q band of 75 at 721 nm appeared at by ca. 80 nm
longer wavelength than those push-only or pull-only dibenzoporphyrins. Theoretical
study on the electronic structure indicated that the HOMO–LUMO gaps of the pushonly and pull-only dibenzoporphyrins are not so different although both HOMO and
LUMO energy levels of the push-only dibenzoporphyrin are higher than those of the
pull-only dibenzoporphyrin. It is of interest that the HOMO energy level of 75 is very
similar to that of the push-only dibenzoporphyrin and the LUMO energy level of 75
is very similar to that of the pull-only dibenzoporphyrin, which significantly reduced
the HOMO–LUMO gap. This result was consistent with the cyclic voltammetric
analysis; 1.75 V for 75 is by 0.25 V smaller than those dibenzoporphyrins.
3.3.3 Reactions of Porphyrins at Pyrrole β-Positions
Since meso-tetraarylporphyrins are readily available, robust, and soluble in organic
solvents, great effort has so far been directed to structural modification at pyrrole
β-positions. Bringmann and coworkers investigated Pd-catalyzed borylation of 2bromo-meso-tetraphenylporphyrin 76 and obtained β-dioxaborolanylporphyrin 77
in 70% yield using toluene–water two phase system (Fig. 3.25) (Bringmann et al.
2008). If this reaction was run in DMF, carbopalladation on the adjacent meso-phenyl
group occurred to give almost exclusive formation of indene-fused porphyrin 78
without participation of bis(pinacolato)diboron (B 2 (pin) 2 ) in the reaction. A closely
related cyclopentadiene-fused porphyrins 80 was reported by Osuka’s group through
Pd-catalyzed reaction between meso-bromoporphyrin 79 and alkynes (Sahoo et al.
2006). The Ni and Cu complexes 80 showed split Soret bands; one at 380–390 nm
and the other at 480–490 nm. The additional π-conjugation in 80 (M = Ni) decreased
electrochemical HOMO–LUMO gap (E = 1.66 V) relative to that (E = 2.29 V)
of trans-A 2 type diarylporphyrin Ni complex. Once β-borylated porphyrin 77 was
available, it was subjected to the Pd-catalyzed Suzuki–Miyaura cross coupling with
76 leading to β,β-linked diporphyrin 81 in 73% yield. Some bimetallic complexes
of 81 could be resolved into enantiomers by HPLC on a chiral phase. The rotational
stability at the β,β-axis depends on the metal but racemization of the optically resolved
bisZn complex was not seen at room temperature.
Osuka, Shinokubo, and coworkers reported that the pyrrole β-position adjacent
to the unsubstituted meso-position could be directly borylated (Hata et al. 2005).
When the [Ir(COD)OMe] 2 -catalyzed direct borylation of aromatic compounds by
B 2 (pin) 2 was applied to 5,15-diarylporphyrin and 5,10,15-triarylporphyrin, the