Deprotection of a benzyl unit induces a 22π aromatic macrocycle of 3-oxypyripentaphyrin(0.1.1.1.0) with strong NIR absorption

Article abstract of DOI:10.1039/d0ob01213k

We report aromaticity switching from a 6π pyridine ring to a 22π macrocyclic ring of 3-oxypyripentaphyrin(0.1.1.1.0). This system has potential applications in photodynamic therapy owing to macrocyclic aromaticity being selectively induced by protecting group removal and strong absorption bands produced in the NIR region especially in methanol. This journal is

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Deprotection of benzyl unit induces 22π aromatic macrocycle of
3-oxypyripentaphyrin(0.1.1.1.0) with strong NIR absorption.
Daiki Moria, Tomoki Yonedaa,b*, Masaaki Suzukic, Tyuji Hoshinoa, and Saburo Neyaa*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
We report aromaticity switching from a 6π pyridine ring to 22π
macrocyclic ring of 3-oxypyripentaphyrin(0.1.1.1.0). This system
has potential applications in photodynamic therapy owing to
macrocyclic aromaticity being selectively induced by protecting
group removal and strong absorption bands produced in the NIR
region especially in methanol.
Fig. 1. Representation of “Deprotection Induced Aromaticity” strategy.
Porphyrins and their analogs are employed for photodynamic
therapy (PDT), which is a treatment used to eliminate cancer or
abnormal
tissue
lesions.1
Conventional
porphyrin
O
O
photosensitisers have their longest absorption maxima in the
visible-light range. However, regarding penetration depth,
photosensitiser with absorbance of NIR region (700–1100 nm)
is desired.2 Expanded porphyrins with large π-conjugation
circuits3 which absorb light reaching NIR region are expected for
application of photodynamic therapy.4 However, a known
undesirable side effect of existing photosensitisers is
phototoxicity toward healthy tissue. This problem can be more
serious when expanded porphyrins with strong absorption in
the visible and NIR regions are adopted as photosensitisers. If
the strong NIR absorption of a molecule is induced by
deprotection, this system can be a candidate of photosensitizer.
This system prevents photosensitivity toward ambient light
until metabolization occurs in tumors or tissue lesions. The
removal of protecting group is theoretically applicable to new
reagents for NIR photodynamic therapy by employing
appropriate protecting groups metabolised in tumour. If the
aromaticity of expanded porphyrins could also be triggered by
metabolization, the molecule would show strong NIR
absorption and be a theoretically crucial candidates for
photodynamic therapy would be obtained (Fig. 1).
N
N
NH
HN
NH
N
HN
HN
C6F5
C6F5
C6F5
C6F5
NH HN
C6F5
C6F5
1
2
Scheme 1. Aromaticity induced by deprotection of 1.
As the example of such expanded porphyrin systems, we
herein report an expanded porphyrin with macrocyclic
aromaticity which is triggered by simple benzyl group
deprotection.
We
designed
deprotection
of
3-
benzyloxypyripentaphyrin-(0.1.1.1.0)
1
to produce 3-
oxypyripentaphyrin(0.1.1.1.0) 2 as an expanded porphyrin
system with 3-oxypyridine ring (Scheme 1). Reported
pentaphyrin(1.1.1.0.0)s with five pyrrole rings are known to
have aromatic character derived from their 22π conjugation
circuits.5 To improve the stability of such electron- rich
pentaphyrins, we suggested installing a pyridine ring instead of
pyrrole. Compared with pyrrole, in which 6π-electrons are
delocalised on five atoms, the pyridine ring is more electron-
deficient because of delocalisation of same number of electrons
on six atoms. As pyridine-containing porphyrins, macrocycles
bearing pyridine or 3-oxypyridine rings have been reported.
Among them, pyridine rings without substituents at β- or γ-
positions favour aromaticity of 6π pyridine rings, with their
macrocyclic aromaticity connected in
a. Department of Pharmaceutical Sciences, Chiba University
Inohana, Chuo-ku, Chiba, 260-8675, Japan.
b. Division of Applied Chemistry, Faculty of Engineering, Hokkaido University, Kita 13
Nishi 8 Kita-ku, Sapporo, Hokkaido, 060-8628, Japan,
E-mail: t_yoneda@eng.hokudai.ac.jp.
c. Graduate School of Natural Science and Technology, Shimane University
1060, Nishikawatsu-cho, Matsue, Shimane, 690-8504, Japan
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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Scheme 2. Synthetic procedure of 3-oxypyripentaphyrins 1 and 2.
a cross-conjugated manner.6 In contrast, in molecules with 3- condensation using p-toluenesulfonic acid (1.3 equiv.) followed
oxopyridine (pyridone) rings, macrocyclic 18π aromaticity takes by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
priority over aromaticity of 6π pyridine ring.7 The similar system (DDQ;
3.0
equiv)
afforded
benzyl-protected
of 3-hydroxybenziporphyrins and the induction of aromaticity pyripentaphyrin(0.1.1.1.0) 1 in 19% yield. The benzyl group of 1
by deprotection of methyl group was also reported.8 From these was intact under the oxidative conditions.
previous researches, the usage of 3-oxopyridine ring can be
High-resolution electrospray ionization mass spectroscopy
effectively induce aromaticity in expanded porphyrin (HR-ESI-TOF-MS) provided the parent ion peak of 1 at m/z =
macrocycles. Previously, Setsune et al. reported expanded 1036.2133 (calcd for 1036.2128). The 1H NMR chemical shifts of
porphyrins containing pyridine moieties in their structure, 1 indicated its nonaromatic conjugation character, including the
although these molecules did not show macrocyclic peaks of  protons at 6.41, 6.51, 6.75, and 6.80 ppm. In contrast,
aromaticity.9 Sessler et al. also reported the synthesis of peaks for pyridyl protons were observed at downfield-shifted
cyclo[m]pyridine[n]pyrroles, which showed aromaticity only range (7.75 and 8.31 ppm), reflecting its electron-deficient
under acidic conditions.10 In addition, pyrrole and pyridine aromatic 6π conjugation circuit of the pyridine ring. Inner NH
moieties of 1 have been designed to prevent meso-carbon protons were observed at 5.06 ppm, which seems to be derived
oxygenation.11
from fast tautomeric exchange with protons of water in CDCl3.13
The synthetic procedure for 1 is shown in Scheme 2. To
synthesise the desired 3-oxypyripentaphyrin(0.1.1.1.0), we
designed the synthesis of directly connected pyrrole-pyridine-
pyrrole (dipyrropyridine) moiety known in the synthesis of
Bn
1.394(6)
1.387(6)
b)
1.386(6)
O
expanded
pyriporphyrins.9,12
Using
Miyaura–Ishiyama
1.359(5)
a)
1.417(6)
borylation, 2-ethoxycarbonyl-5-iodo-3,4-dimethylpyrrole 3 was
transformed into a borylated analog 4 in 72% yield. Compound
4 (2 equiv.) was coupled with 2,6-diiodopyridin-3-ol to afford a
new directly bonded pyrrole-pyridine-pyrrole trimer 5 in 61%
yield. The hydroxyl group of 5 was protected with benzyl
bromide to give benzyl ether 6 quantitatively. The terminal
ethoxycarbonyl groups of 6 were completely hydrolysed by
1.462(6)
1.457(6)
1.363(5)
N
1.334(5)
saponification affording
7
quantitatively. Without the
protection of hydroxyl group, the saponification of 5 produced
a complex mixture and corresponding carboxylic acid was not
obtained at all. Through decarboxylation using NaOH and
glycerol at 180 °C, dipyrropyridine 8 was obtained in 42% yield.
Trimer 8 was condensed with pentafluorophenyl-substituted
dipyrromethane dicarbinol 9’, which was prepared from the
reduction of acyl precursor 9. Similarly to our reported
procedure of the synthesis of pyripentaphyrin,11 acid-catalised
Fig. 2. X-ray crystal structure of 3-benzyloxypyripentaphyrin 1•HCl. a) Top view and b)
selected bond lengths of the pyridine unit shown in Å. Solvent molecules and meso-aryl
groups in side view were omitted. The thermal ellipsoids are shown at the 50%
probability level.
2 | J. Name., 2012, 00, 1-3
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O
OH
The structure of 1 was unambiguously determined by X-ray
crystallographic analysis.14 The X-ray crystal structure was
obtained in the protonated form with the counter chloride
anion, as generated by 1,2-dichloroethane employed as the
crystallisation solvent. In the X-ray crystal structure of pyridine
ring of 1, the pyridine ring showed C–C bond lengths in the
range of 1.386(6)–1.417(6) Å, which were typical of aromatic C–
C bonds. The C–N bond lengths of the pyridine ring were
1.334(5) and 1.345(7) Å, indicating its aromatic character. (Fig.
2b).
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N
N
NH
HN
NH
N
HN
HN
C6F5
C6F5
C6F5
C6F5
NH HN
C6F5
C6F5
enol-form
keto-form
Scheme 3. Keto-enol tautomerisation of 2.
We conducted deprotection of benzyl group of 1 using
hydrogenation under Pd/C catalysis, which proceeded almost
quantitatively (Scheme 2). Simple chromatographic separation
followed by recrystallisation furnished 2 in 78% yield. HR-ESI-
TOF-MS analysis of 2 showed a peak at m/z = 946.1691 (calcd
for C46H22N5F15O1: 946.1663), which indicated the expected
benzyl group removal. The NMR spectrum of 2 in CDCl3 clearly
showed downfield-shifted β-protons in the range of 7.8–8.2
ppm, although complete characterisation of them was not easy
because of its broadened peaks (Fig. S13). This broad NMR
spectrum might be attributed to fast equilibrium of keto and
enol forms of 2 (Scheme 3).8d The NMR peaks did not sharpen
even at lower or higher temperature. However, in CD3OD, highly
downfield-shifted peaks belonging to pyrrole-β protons were
clearly observed at 7.87–8.23 ppm, indicating the aromatic
character. The peak for the β-proton of 3-pyridone unit was
highly downfield- shifted to 9.67 ppm owing to the diatropic
ring current of the 22π conjugation circuit and electron-
withdrawing effect of the conjugated carbonyl group (Fig. S14).
These data support the increased contribution of keto-form in
protic solvents, as known in the tautomerism between the 4-
and 2- pyridone derivatives.15 In X-ray crystal structure of 216
grown from the slow diffusion of hexane into chloroform
solution was determined to be keto-form with four amine-type
pyrrolic nitrogen atoms (Fig. 3). All pyrrole and pyridone units
were inward-pointing. The detailed X-ray crystallographic
structure showed a 1.278(6) Å of C–O bond, indicating strong
double-bond character of 2. In the 3-oxypyridine ring, C–N bond
lengths were 1.345(6) and 1.352(6) Å, indicating retained
aromatic bond character, while the bond-length alternation of
C–C bonds was larger than those of 1, at three 1.448(7),
1.439(8), 1.428(7) Å of single C–C bonds and 1.360(7) Å for one
double C–C bond, suggesting the keto-form of 2 in the solid
state. Furthermore, the pyridine–pyrrole C–C bond lengths
were 1.434(7) and 1.433(7) Å, which is shortened to the
aromatic bond length region compared with those in 1 (1.462(6)
and 1.457(6) Å). The harmonic oscillator model of aromaticity
(HOMA)17 value for the overall 22π macrocyclic structure of 2
was 0.71, which was larger than that of the cross-conjugated
macrocyclic pathway of 1 calculated as 0.56 (Fig. S16). The mean
plane deviation (MPD) of the pentaphyrin skeleton of 2 was
calculated as 0.33 Å suggesting relatively planar structure of 2
than that of 1 with 0.46 Å of MPD. (Fig. 4).
O1
1.360(7)
1.439(8)
O
1.278(6)
a)
b)
1.428(7)
1.433(7)
1.448(7)
1.434(7)
1.352(6)
N
1.345(7)
Fig. 3. X-ray crystal structure of 3-oxypyripentaphyrin 2. a) Top view b) selected bond
lengths shown in Å. The solvent molecules and meso-aryl groups in side view were
omitted. The thermal ellipsoids are shown at the 50% probability level.
1.5
1.0
0.5
N1
N4
0.0
N3
N2
N5
– 0.5
– 1.0
– 1.5
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
position
1
Fig. 4. Mean-plane deviation diagram calculated from the 29 core atoms for X-ray crystal
structures of 1(black) and 2(red).
The aromaticity and strong NIR absorption of 2 was
confirmed by its UV/Vis absorption spectra (Fig. 5). The
absorption spectrum of 1 showed a relatively broad Soret-like
band at 435 nm and almost no Q-like bands, suggesting its
nonaromatic characteristics. However, 2 in dichloromethane,
showed a similar the overall absorption spectrum shape, except
for weak absorption bands at 437, 497, 519, 716 and 842 nm,
which can be assigned as 22π conjugation circuit. Furthermore,
in methanol, all these peaks showed increased intensity, with a
high molar coefficient observed. In particular, the strong Q-like
absorption band at 852 nm with absorption coefficient of
4.2×104 M-1cm-1 suggested the suitability of this compound as
an NIR photosensitiser.
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Fig. 5. UV/Vis absorption spectra of 1 in dichloromethane (black) and 2 in
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dichloromethane (blue) and 2 in methanol (red).
To investigate the aromaticity of 2, we conducted DFT
calculation18 (B3LYP/6-31G level) on the keto- and enol forms of
2. The aromaticity of the keto-form of 2 was supported by the
calculated nucleus-independent chemical shifts (NICS(0))
value19 of –10.91 ppm at the gravity point of the macrocycle. In
contrast, those of 1 and the enol-form of 2 were calculated to
be –0.66 and –0.73 ppm, respectively, indicating nonaromatic
characters. Meanwhile, the NICS(0) values of the six-membered
ring in 1 and the enol-form of 2 were –9.73 and –9.98 ppm,
respectively, owing to their 6π-electron aromaticity, while that
of the keto-form of 2 was +4.08 ppm because the point was not
the centre of a 6π ring but the outside periphery of a 22π
macrocycle. The keto-form of 2 was slightly (16.7 kJ/mol) more
stable than that of enol-form though the HOMO-LUMO gaps of
1 and keto-and enol forms of 2 were very similar (1.97-2.00 eV).
As observed in cyclic voltammograms with similar HOMO-
LUMO gaps of 1 and 2 (1.45-1.40 eV, see ESI).
In conclusion, we have synthesised benzyl-group-protected
pyripentaphyrin 1 and demonstrated its deprotection using
hydrogenation to produce 3-oxypyripentaphyrin 2. The
transformation from nonaromatic 1 to aromatic 2 drastically
induced aromaticity in the 22π conjugation system, preserving
its stability. The aromaticity of 2 was enhanced in methanol,
with strong NIR absorption observed. Therefore, we have
demonstrated the potential of expanded pyriporphyrins as
theoretically promising candidates for photosensitizers.
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Conflicts of interest
There are no conflicts to declare.
13 K. Rachlewicz, L. Latos-Grażyński, A. Hebauer, A. Vivian, J. L.
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Acknowledgements
14 X-ray
data
for
1•HCl:
(C53H29F15N5O1)•HCl•2(1,2-
dichloroethane) (Mr = 1171.22), triclinic, space group P_1 (No.
2), a = 13.2433(3), b = 13.7134(3), c = 15.7312(4) Å, α =
70.848(1)°, β = 80.596(1)°, γ = 66.090(1)°, V = 2465.69(10)
Å3, Z = 2, ρcalcd = 1.577 gcm-3, T = 93(2) K, R1 = 0.0841 (I > 2σ(I)),
wR2 = 0.2495 (all data), GOF = 1.120. CCDC 1996098 contains
the supplementary crystallographic data for this paper. These
data are provided free of charge by the Cambridge
Crystallographic Data Centre.
The work at Chiba was supported by JSPS KAKENHI
This work was supported by a JSPS KAKENHI grant for young
scientists No. 17K14445(B) from MEXT of Japan and The
Inohana Foundatioin.
Notes and references
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15 A. Gordon, A. R. Katritzky, Tetrahedron Lett, 1968, 23, 2767–
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DOI: 10.1039/D0OB01213K
2770.
16 X-ray data for 2: (C46H22F15N5O1)•2(chloroform) (Mr
=
1184.42), monoclinic, space group P-1 (No. 2), a = 12.2776(3),
b = 13.0559(3), c = 16.5083(4) Å, α = 105.241(1)°, β =
110.613(1)° γ = 98.939(2)°, V = 2297.85(10) Å3, Z = 2, ρcalcd
=
1.712 gcm-3, T = 93(2) K, R1 = 0.0965 (I > 2σ(I)), wR2 = 0.2705
(all data), GOF = 0.1049. CCDC 1996099 contains the
supplementary crystallographic data for this paper. These
data are provided free of charge by the Cambridge
Crystallographic Data Centre.
17 (a) T. M. Krygowski, T. M. Cryański, Tetrahedron 1996, 52,
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Wallingford, CT, 2009 (Full citation in SI).
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O
O
N
N
NH
HN
NH
N
HN
C6F5
C6F5
C6F5
C6F5
Deprotection
NH HN
HN
C6F5
C6F5
Nonaromatic Macrocycle
Aromatic Macrocycle
NIR Absorption
We report a deprotection-induced aromaticity switching from nonaromatic to 22π
aromatic macrocycle of 3-oxopyripentaphyrin(0.1.1.1.0) with strong NIR
absorption.