Synthetic substituted boronates of dihydroxy-bacteriochlorin absorbing and emitting far-red to near-infrared light as bacteriopheophytin-a analogs

Article abstract of DOI:10.1016/j.dyepig.2019.108155

Several substituted boronates of methyl cis-7,8-dihydroxy-pyrobacteriopheophorbide-a possessing the same 3-acetyl-13¹-oxo-bacteriochlorin π-conjugated system as bacteriopheophytin-a found in type-II reaction centers of anoxygenic photosynthetic

Full text of DOI:10.1016/j.dyepig.2019.108155

Dyes and Pigments 175 (2020) 108155
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Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Synthetic substituted boronates of dihydroxy-bacteriochlorin absorbing and
emitting far-red to near-infrared light as bacteriopheophytin-a analogs
,
,*
Daichi Funakoshia, Yosaku Nomuraa, Sunao Shojia b, Hitoshi Tamiakia
a Graduate School of Life Sciences, Ritsumeikan University, Kusatsu, Shiga, 525-8577, Japan
b Division of Applied Chemistry, Faculty of Engineering, Hokkaido University, Sapporo, Hokkaido, 060-8628, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords:
Several substituted boronates of methyl cis-7,8-dihydroxy-pyrobacteriopheophorbide-a possessing the same 3-
acetyl-131-oxo-bacteriochlorin
π-conjugated system as bacteriopheophytin-a found in type-II reaction centers of
Chlorophyll-a
Fluorescence emission
Qy absorption
Semisynthesis
anoxygenic photosynthetic bacteria were prepared by chemical modification of chlorophyll-a. The semisynthetic
bacteriochlorins are less oxidizable to chlorins and more readily available from natural phototrophs than the
above natural bacteriochlorin, while the formers show similar optical properties in solution as the latter. All the
boronate pigments efficiently absorb near ultraviolet, green, and far-red light and strongly emit light in a far-red
to near-infrared region. Their electronic absorption and fluorescence emission data are nearly independent of the
alkyl and (hetero)aromatic substituents at the boron atom.
Substitution effect
1. Introduction
to-NIR light including photodynamic therapy, imaging, and solar cells
[6–13]. Natural bacteriochlorins are chemically labile and readily
Anoxygenic photosynthetic bacteria have single charge-separating
apparatuses called reaction centers (RCs), which are composed of pig-
ments and peptides. In particular, purple photosynthetic bacteria have
pseudo-C2-symmetric type-II RCs, some of which have been already
revealed at an atomic level by crystallographic analyses [1–3]. In the
type-II RCs, two bacteriopheophytin(BPhe)-a or b molecules are found
per apparatus, one of which is used as a primary electron acceptor for
the photoinduced electron-transfer process. Both BPhe-a and b pigments
transformed into the corresponding chlorins with a 17,18-dihydropor-
phyrin π-skeleton to show hypsochromically shifted Qy bands at below
700 nm (a red region). For example, BPhe-a is oxidized via the 7,8-dide-
hydrogenation under aerobic conditions and BPhe-b is isomerized via
hydrogen shift from the C7 to C81 position by the action of a weak acid
to afford the same 3-acetyl-pheophytin-a molecule having a chlorin
π
-system [14,15]. To enhance the chemical stability of bacteriochlorins,
a variety of synthetic analogs have been prepared [16–19]. Thus,
Lindsey and his colleagues synthesized nonoxidized and nonisomerized
bacteriochlorins based on gem-β,β-dimethylpyrrolines [20], and Pandey
and his colleagues modified BPhe-a to less oxidizable bacteriopurpur-
inimides [21]. In this context, we previously reported that cis-7,
8-dihydroxy- [22] and 7,7-dialkyl-8-alkylidne-chlorophyll(Chl)-a de-
rivatives [15] were chemically stable and readily available BPhe-a and b
analogs, respectively.
possess a bacteriochlorin π-system bearing two single Cβ–Cβ bonds in the
opposite pyrrole rings (see Fig. 1, left) [4]. Filamentous anoxygenic
phototrophs including green nonsulfur bacteria and Gemmatimonas
phototrophs also utilize type-II RCs containing BPhe-a [5]. In the RC of
the former phototrophs, one extra BPhe-a molecule is situated between
the primary electron donor (named
a
special pair) and
non-electron-accepting BPhe-a.
Naturally occurring bacteriochlorin pigments including BPhes-a/b
are monomeric in conventional organic solvents to give redmost (Qy)
absorption bands at around 800 nm in a far-red (FR) to near-infrared
(NIR) region. In photosynthetic apparatuses, some of these pigments
interact with each other to form dimers and oligomers with bath-
ochromically shifted Qy bands in 800–1100 nm (NIR). Therefore, bac-
teriochlorins are promising sensitizers for various systems driven by FR-
Herein, we report the synthesis of several boronates 1a–i with a less
oxidizable bacteriochlorin
π-system (Fig. 1, right) by chemically
modifying natural Chl-a. These synthetic bacteriochlorins exhibit
almost the same optical properties in solution, and their electronic
absorption and emission data are nearly independent of the sub-
stituents (R) on the boron atom. Readily available semisynthetic
3-acetyl-131-oxo-bacteriochlorins 1a–i are structurally related to
* Corresponding author.
E-mail address: tamiaki@fc.ritsumei.ac.jp (H. Tamiaki).
https://doi.org/10.1016/j.dyepig.2019.108155
Received 30 September 2019; Received in revised form 17 December 2019; Accepted 18 December 2019
Available online 19 December 2019
0143-7208/© 2019 Elsevier Ltd. All rights reserved.

D. Funakoshi et al.
Dyes and Pigments 175 (2020) 108155
natural and fragile BPhe-a and, similarly to the latter, intensively
absorb and emit light in a FR-to-NIR region. Since a variety of the R
groups are easily introduced in the present bacteriochlorins without
any alteration of their optical properties, such functionalized boronate
pigments are promising for applications including intramolecular
electron- and energy-transferring systems from the photoexcited bac-
teriochlorin moiety to an electron- and energy-donating/accepting
moiety in the R group.
the methyl group. In the case of isobutylboronic acid, no esterification
occurred upon refluxing in THF for 1 day, and desired 1c was obtained
(81%) after refluxing in toluene for 2 h. The lower reactivity with iso-
butylboronic acid can be explained by the greater steric bulkiness of the
branched primary alkyl group. Isopropylboronic acid could not react
with 1j even after refluxing in toluene for 1 day, which can be ascribed
to the presence of the more sterically hindered secondary alkyl group.
On the other hand, the reaction of 1j with cyclopropylboronic acid in
toluene afforded desired boronate 1d in a lower 34% yield after pro-
longed reflux (1 day). This is ascribable to the less sterically demanding
cyclopropyl group which is corresponded to the dehydrogenatively
cyclized form of an isopropyl group. Therefore, it can be concluded that
the above esterification is largely dependent on the steric factor of the
alkyl groups in the boronic acids. The present steric effect is comparable
to that for the esterification of a diol with alkyl boronic acids reported
previously [27].
2. Results and discussion
2.1. Synthesis of boronates of bacteriochlorin-diol
Naturally occurring Chl-a was extracted from cyanobacterial cells
and was chemically modified to methyl pyropheophorbide-a (2a) [step
(i) in Scheme 1] [23]. The vinyl group at the 3-position of 2a was hy-
drated via a Markovnikov fashion [step (ii)] [23,24], and the resulting
3-(1-hydroxyethyl) group of 2b was subjected to Ley–Griffith oxidation
to give the corresponding 3-acetyl-chlorin, i.e., methyl 7,8-didehy-
dro-pyrobacteriopheophorbide-a (2c) [step (iii)] [25]. The C7¼C8
double bond of 2c was regioselectively oxidized by osmium tetroxide
and the resulting osmate was cleaved by hydrogen sulfide to afford cis-7,
Phenylboronic acid readily reacted with 1j in THF at room temper-
ature to afford phenylboronate 1e in 77% yield after stirring for 2 days.
p-Anisylboronic acid was more reactive with 1j to give 1f in a higher
yield (97%) after stirring at room temperature for a shorter reaction time
(1 day). The electron-donating p-methoxy group accelerated the esteri-
fication. By contrast, the reaction of 1j with 3,5-bis(trifluoromethyl)phe-
nylboronic acid could not be completed after stirring in THF for 1 day,
and harsh reaction conditions (refluxing in toluene for 2 h) were
necessary to consume all 1j and give the corresponding boronate 1g
(78%). The double electron-withdrawing trifluoromethyl groups
reduced the esterifying reactivity. Therefore, the acidity of boronic acids
affected the above esterification, i.e., the higher acidic boronic acids are,
the lower reactive they are with cis-diol 1j. 2,6-Dimethylphenylboronoic
acid was so sterically hindered that no esterification with 1j was
observed in THF at room temperature. After refluxing in toluene for 1
day, boronate 1h was produced (73%). The esterification of 1j with
arylboronic acids was controlled by the steric factor of the aryl group as
aforementioned in alkylboronic acids. Additionally, 2-thienylboronic
acid reacted with 1j to give 1i (51%) after refluxing in toluene for 1 h.
Such a heteroaromatic group in the boronic acid was effective for the
preparation of boronates of cis-diol 1j. All the synthetic boronates 1a–i
as structural analogs of BPhe-a were mixtures of the 7,8-diastereomers
as in 1j (see Fig. 1, right), which were used for the measurements of
the following optical properties.
8-dihydroxy-bacteriochlorin 1j [step (iv)] [15,26]. In a chlorin π-system,
the pyrrole moiety (B-ring) opposite to the reduced pyrrole ring (D-ring)
was more reactive than any other adjacent ones (A- and C-rings), so the
B-ring was predominantly reacted in the above oxidation. The obtained
cis-diol 1j was a 1:1 diastereomeric mixture at the 7,8-positions, namely
(7R,8S)- and (7S,8R)-stereoisomers. This observation is ascribable to the
non-face-selective attack of an OsO4 species to the C7¼C8 moiety far
from the chiral (17S,18S)-positions in a molecule. The stereochemical
mixture of 1j was used for further experiments. It is noted that 1j is less
oxidized to chlorin due to the lack of the 7,8-dihydrogen atoms [22] and
also is one of readily accessible bacteriochlorins because the starting
material Chl-a is easily obtained in large amounts from commercially
available spirulina [23].
Next, a dry THF solution of cis-diol 1j was treated with methylbor-
onic acid in the presence of magnesium sulfate as a dehydrating reagent
under nitrogen in the dark [step (v) in Scheme 1]. After stirred at room
temperature for 1 day, the corresponding methylboronate 1a was pro-
duced, but a part of 1j yet remained in the reaction mixture. When the
reaction mixture was refluxed for 1 day, 1j was consumed completely,
and 1a was obtained in 80% isolated yield after purification with flash
column chromatography (FCC) and recrystallization. Under the same
conditions (reflux in THF for 1 day), 1j partially reacted with dode-
cylboronic acid to give 1b, but some 1j was visible in the reaction
mixture. After refluxing at higher temperature in dry toluene for 2 h, 1j
was fully consumed, and 1b was produced in 90% yield. The difference
in the reactivities of methyl- and dodecylboronic acids was most likely
due to the greater steric hindrance of the long dodecyl chain than that of
2.2. Electronic absorption properties of boronated bacteriochlorins
Bacteriochlorins have three typical electronic absorption bands, i.e.,
intense Qy, Qx, and Soret bands, which are situated at 800–700,
600–500, and 400–300 nm, respectively. The Qy and Qx bands are
dependent on the transition moments along the molecular y- and x-axes,
respectively (see Fig. 2), and the Soret band is composed of By and Bx
bands. As shown in Fig. 3, methyl pyrobacteriopheophorbide-a (1k)
Fig. 1. Molecular structures of naturally occurring bacteriopheophytins-a/b (left) and synthetic bacteriochlorins (right).
2

D. Funakoshi et al.
Dyes and Pigments 175 (2020) 108155
Scheme 1. Synthesis of methyl cis-7,8-dihydroxy-pyrobacteriopheophorbide-a (1j) and its boronates 1a–i by chemical modification of naturally occurring Chl-a: (i)
H2SO4/MeOH, rt and collidine, reflux; (ii) HBr/AcOH, H2O/CH2Cl2, and CH2N2/Et2O; (iii) O(CH2CH2)2NMeO–Pr4NRuO4/CH2Cl2; (iv) OsO4–C5H5N/CH2Cl2 and
H2S/MeOH; (v) RB(OH)2–MgSO4/dry THF or PhMe.
with a trans-7,8-dihydrogenated form showed three maxima at 754
(Qy), 533 (Qx), and 361 nm (Soret) in dichloromethane, while cis-diol 1j
exhibited the corresponding maxima at 746, 535, and 362 nm. The
transformation of trans-dihydrogenated to cis-dihydroxylated form at
the 7,8-positions as in 1k to 1j induced a blue shift and broadening of the
Qy bands: Δλabs ¼ 8 nm and ΔFWHM(full width at half maximum) ¼
100 cmÀ 1 (see Table 1). The broadening reduced the relative intensity
(Irel) of the Qy band over the major Soret band from 61% to 53%. The
same modification resulted in a red shift of the Qx bands (Δλabs ¼ 2 nm)
and no significant change of the band shapes. By the structural alteration
of 1k to 1j, the main Soret maxima slightly moved to a longer wave-
length (Δλabs ¼ 1 nm) and the band broadened apparently, whereas the
minor Soret band at the red side shifted to a shorter wavelength (387 →
�385 nm). The broadening of the main Soret band is comparable to that
of the Qy bands, so the intense Soret band at a shorter wavelength would
be assigned to the By band, and the Soret band at a longer wavelength
could correspond to the Bx band. This characterization is consistent with
the Gouterman’s four-orbital model [28]: Qy, Qx, Bx, and By bands in a
Fig. 2. Molecular structure of methyl pyrobacteriopheophorbide-a (1k) pre-
bacteriochlorin are situated from longer to shorter wavelengths.
pared from natural BChl-a with molecular x/y-axes.
Methylboronation of cis-diol 1j to 1a blue-shifted the Qy maxima by
10 nm and sharpened the band: ΔFWHM ¼ À 140 cmÀ 1 and Irel ¼ 53% →
62% (see Fig. 3 and Table 1). The methylboronation also affected the Qx
band in a similar manner, giving a small hypsochromic shift (Δλabs ¼ À 3
Table 1
Electronic absorption maxima λabs (nm) of bacteriochlorins 1a–k in
dichloromethane.
Compound (R)
Soreta
Qxb
Qyb
1a (Me)
363
363
363
363
363
363
363
363
363
362
361
532 (25) [830]
532 (25) [850]
532 (25) [870]
533 (25) [850]
533 (26) [840]
533 (26) [840]
533 (27) [850]
532 (26) [840]
533 (26) [850]
535 (25) [940]
533 (24) [910]
736 (62) [590]
737 (61) [600]
736 (60) [600]
737 (61) [600]
736 (61) [600]
736 (61) [600]
735 (62) [580]
736 (61) [590]
736 (61) [590]
746 (53) [730]
754 (61) [630]
1b (C12H25
1c (iBu)
)
1d (cycloPr)
1e (Ph)
1f (Ph(4-OMe))
1g (Ph(3,5-(CF3)2))
1h (Ph(2,6-Me2))
1i (2-thienyl)
1j
1k
Fig. 3. Electronic absorption spectra of bacteriochlorins 1a/g/j/k in
dichloromethane (ca. 10 M). All the spectra were normalized at their intense
peaks in the Soret region.
μ
a
Main Soret maxima corresponding to By maxima.
b
The values in parentheses and brackets indicate Irel (%) and FWHM of bands
(cmÀ 1), respectively.
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D. Funakoshi et al.
Dyes and Pigments 175 (2020) 108155
nm) and sharpening of the band (ΔFWHM ¼ À 110 cmÀ 1). The Soret
bands of 1a exhibited a similar but slightly sharp shape as that of 1j, and
the main peak of the former was shifted to a longer wavelength by 1 nm
than the latter. Comparing with the three bands of 1k, 1a showed a
similar shape of the Qy band with a 18-nm blue shift, a slightly sharp Qx
band with a small blue shift (Δλabs ¼ À 1 nm), and a 2-nm red-shifted
Soret maximum. The observed shifts are comparable to those by sub-
stitution with electron-withdrawing groups at the 7- or 8-position of
Table 2
Fluorescence emission data of bacteriochlorins 1a–k in aerated dichloromethane
at room temperature.a
Compound (R)
λem/nmb
Δ/cmÀ 1
Φem/%
1a (Me)
747 [490]
747 [490]
747 [490]
748 [490]
748 [490]
747 [480]
745 [470]
747 [490]
747 [480]
759 [550]
767 [500]
190
200
210
210
210
200
190
200
200
230
210
14
18
16
17
13
18
19
14
18
14
16
1b (C12H25
1c (iBu)
)
1d (cycloPr)
1e (Ph)
chlorin π-skeleton reported previously [29,30]. Since the cis-7,8-dihy-
1f (Ph(4-OMe))
1g (Ph(3,5-(CF3)2))
1h (Ph(2,6-Me2))
1i (2-thienyl)
1j
droxy moiety and its boronate as in 1a and 1j, respectively, were
electron-withdrawing groups, the above substitution effect on electronic
absorption bands could be explained by their electronic factor.
Substitution of the methyl group at the boron atom of 1a with alkyl
and aryl groups as in 1b–i did not induce any apparent changes in the
corresponding absorption spectra except for the Qy band of 1g bearing a
3,5-bis(trifluoromethyl)phenyl group (see Fig. 3 and S1). The average
values of absorption data of boronates 1a–i with a standard deviation
were calculated as follows (except for the Qy data of 1g): λabs(Qy/Qx/
Soret) ¼ 736.2 � 0.2/532.4 � 0.2/362.9 � 0.2 nm, Irel(Qy/Qx) ¼ 61.0
� 0.6/25.6 � 0.5, and FWHM(Qy/Qx) ¼ 595 � 4/846 � 9 cmÀ 1 (see
Table 1). The effect of boronated substituents (R) on their electronic
absorption spectra was limited, although the strong electron-
withdrawing 3,5-bis(trifluoromethyl)phenyl group in 1g slightly blue-
shifted the Qy maximum (Δλabs ¼ À 1 nm) and sharpened the Qy band
(ΔFWHM ¼ À 15 cmÀ 1).
1k
a
λem, emission maximum (excited at Soret maxima); Δ, Stokes shift ¼ [1/
λabs(Qy) – 1/λem] x 107; Φem, emission quantum yield (excited at main Soret
maxima).
b
The values in brackets indicate FWHM of bands (cmÀ 1).
Similarly as in the changes of the Qy bands, the shapes of the emission
bands of 1a and 1k were the same, and the λem of 1a significantly moved
to a shorter wavelength by 20 nm than that of 1k. Both the Stokes shift
and Φem of 1a were slightly smaller than those of 1k.
The λem and FWHM values of boronates 1a–i were independent of the
substituents (R) except for 1g (see Table 2 and Fig. S2), as aforemen-
tioned for their Qy absorption data. The average value of the λem of
1a–f/h/i is 747.3 � 0.4 nm and larger than 745 nm for 1g, while the
mean FWHM of the formers is 488 � 4 cmÀ 1 and larger than 470 cmÀ 1
for the latter. The slightly blue-shifted and sharp emission band was
observed in 1g (Fig. 4). The Stokes shifts of all boronates 1a–i are
comparable to each other, giving 200 � 7 cmÀ 1 for the average value.
These small values indicate that the structural difference between their
ground and excited states is not so large. Their Φem values partially
fluctuated, and 1g gave the maximum value of 19%. Therefore, the ef-
fect of the R groups on the fluorescence properties is limited.
2.3. Fluorescence emission properties of boronated bacteriochlorins
Synthetic bacteriochlorins 1a–k were highly fluorescent in a diluted
dichloromethane solution. When 1k was excited at the main Soret
maxima (361 nm), fluorescence emission bands were observed as the
mirror image of its Qy absorption bands (see the red line of Fig. 4). The
main emission peak (λem) was located at 767 nm, and the Stokes shift (Δ)
was 210 cmÀ 1 (Table 2). The modification of the peripheral substituents
at the B-ring from 1k to 1j resulted in an 8-nm blue shift of λem and a
slight broadening of the main emission bands (ΔFWHM ¼ 50 cmÀ 1).
This fluorescence observation is comparable to the changes of their Qy
absorption data (vide supra). By the above modification, the Stokes shift
slightly enhanced (210 → 230 cmÀ 1), and the fluorescence emission
quantum yields (Φem) rather reduced (16% → 14%).
3. Experimental
3.1. General
Electronic absorption spectra were measured with a Hitachi U-3500
spectrophotometer. The fluorescence emission spectra and quantum
yields excited at Soret maxima were obtained by a Hamamatsu Pho-
tonics C9920-03G spectrometer. 1H NMR spectra were recorded on a
JEOL ECA-600 (600 MHz) or AL400 (400 MHz) spectrometer; residual
CHCl3 (δ ¼ 7.26 ppm) was used as an internal reference. High resolution
mass spectra (HRMS) were recorded on a Bruker micrOTOF II spec-
trometer: atmospheric pressure chemical ionization (APCI) and positive
mode in a methanol solution. FCC and TLC were performed with silica
gel (Wakogel C-300 and Merck Kieselgel 60 F254).
The methylboronation of 1j to 1a induced a substantial blue shift of
λem (759 → 747 nm) and sharpening of the emission band (FWHM ¼
550 → 490 cmÀ 1) (see Fig. 4 and Table 2), which corresponded to the
changes of their Qy bands (vide supra). The Stokes shift decreased by the
methylboronation (230 → 190 cmÀ 1). The suppression is ascribable to
the conformational fixation of the 7,8-substituents by the boronated
cyclization of the cis-diol. The Φem of 1a was the same as that of 1j.
Methyl 7,8-didehydro-cis-7,8-dihydroxy-pyrobacteriopheophorbide-
a (1j) [26] and methyl pyrobacteriopheophorbide-a (1k) [14] were
prepared according to reported procedures. All the reaction reagents
and solvents were obtained from commercial suppliers and utilized as
supplied. Dry THF and toluene was obtained by reflux with calcium
chloride and distillation. Dichloromethane for optical spectroscopy was
purchased from Nacalai Tesque as a reagent prepared specially for
spectroscopy and used without further purification.
3.2. Synthetic procedures for boronation
A 1:1 7,8-diastereomeric mixture of cis-diol 1j (9.0 mg, 15
μ
mol) was
dissolved in dry THF or toluene (15 ml), to which were added boronic
acids (90 μmol, 6 eq) and magnesium sulfate (2.4 mg, 20 μmol). The
Fig. 4. Fluorescence emission spectra of bacteriochlorins 1a/g/j/k in
mixture was stirred at room temperature or refluxed under nitrogen in
the dark. After the starting diol was consumed (monitored by TLC), the
dichloromethane (ca. 1 μM, excitation at main Soret maxima). All the spectra
were normalized at their intense peaks.
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D. Funakoshi et al.
Dyes and Pigments 175 (2020) 108155
reaction mixture was filtrated, the solvent was evaporated, and the
residue was purified by FCC (0–1% MeOH/CH2Cl2 or 0–10% Et2O/
CH2Cl2) and recrystallization (CH2Cl2/hexane) to give a (7R,8S)- and
(7S,8R)-diastereomeric mixture of the corresponding boronates 1a–i as
black solid (see their spectral data below and their 1H NMR spectra in
Figs. S3–S11): isolated yields ¼ 80% (1a: R ¼ Me, THF, reflux, 1 d, 1:1
stereoisomers), 90% (1b: dodecyl, toluene, reflux, 2 h, 1:1), 81% (1c:
isobutyl, toluene, reflux, 2 h, 1:1), 34% (1d: cyclopropyl, toluene, reflux,
1 d, 1:1), 77% (1e: phenyl, THF, rt, 2 d, 1:1), 97% (1f: p-anisyl, THF, rt,
1 d, 1:1), 78% (1g: 3,5-bis(trifluoromethyl)phenyl, toluene, reflux, 2 h,
1:1), 73% (1h: 2,6-dimethylphenyl, toluene, reflux, 1 d, 5:4), and 51%
(1i: 2-thienyl, toluene, reflux, 1 h, 1:1).
3.62 (3H, s, 172-COOCH3), 3.53, 3.52, 3.52, 3.51 (each half of 3H, s, 2-,
12-CH3), 3.21 (3H, s, 31-CH3), 2.84–2.69 (2H, m, 8-CH2), 2.67–2.51
(2H, m, 171-CH2), 2.33/2.30 (3H, s, 7-CH3), 2.28–2.22 (2H, m, 17-CH2),
1.75 (3H, d, J ¼ 7 Hz, 18-CH3), 0.97/0.88 (3H, t, J ¼ 8 Hz, 81-CH3),
0.53–0.45 (4H, m, BC(CH2)2), À 0.07/–0.14, À 1.38/–1.42 (each 1H, s,
NH x 2), À 0.2 (1H, m, BCH); HRMS (APCI) found: m/z 649.3194, calcd
for C37H42N4O6B: MHþ, 649.3192.
3.3.5. Phenylboronate of methyl 7,8-didehydro-cis-7,8-dihydroxy-
pyrobacteriopheophorbide-a (1e)
UV-VIS (CH2Cl2) λmax 363 (ε, 100,000), 533 (26,000), 736 nm
(61,000); 1H NMR (400 MHz, CDCl3) δ(1/1) 9.41/9.37 (1H, s, 5-H),
8.90/8.89 (1H, s, 10-H), 8.56/8.54 (1H, s, 20-H), 7.84 (2H, dt, J ¼ 8,
1 Hz, Ph-2,6-H), 7.36 (1H, tt, J ¼ 8, 1 Hz, Ph-4-H), 7.27 (2H, br-t, J ¼ 8
Hz, Ph-3,5-H), 5.15/5.14, 5.00/4.98 (each 1H, d, J ¼ 20 Hz, 131-CH2),
4.40–4.32 (1H, m, 18-H), 4.21–4.16 (1H, m, 17-H), 3.63/3.60 (3H, s,
172-COOCH3), 3.552/3.543 (3H, s, 12-CH3), 3.542/3.532 (3H, s, 2-
CH3), 3.235/3.233 (3H, s, 31-CH3), 2.96–2.82 (2H, m, 8-CH2), 2.67–2.48
(2H, m, 171-CH2), 2.46/2.43 (3H, s, 7-CH3), 2.32–2.18 (2H, s, 17-CH2),
1.75 (3H, d, J ¼ 8 Hz, 18-CH3), 1.04/0.95 (3H, t, J ¼ 8 Hz, 81-CH3),
À 0.07/–0.15, À 1.38/–1.43 (each 1H, s, NH x 2); HRMS (APCI) found:
m/z 685.3199, calcd for C40H42N4O6B: MHþ, 685.3192.
3.3. Spectral data of boronates
3.3.1. Methylboronate of methyl 7,8-didehydro-cis-7,8-dihydroxy-
pyrobacteriopheophorbide-a (1a)
UV-VIS (CH2Cl2) λmax 363 (ε, 100,000), 532 (25,000), 736 nm
(62,000); 1H NMR (600 MHz, CDCl3) δ(1/1) 9.32/9.28 (1H, s, 5-H),
8.83/8.82 (1H, s, 10-H), 8.57/8.56 (1H, s, 20-H), 5.15, 5.02/5.00
(each 1H, d, J ¼ 20 Hz, 131-CH2), 4.38/4.36 (1H, dq, J ¼ 2, 7 Hz, 18-H),
4.21/4.19 (1H, dt, J ¼ 7, 2 Hz, 17-H), 3.63/3.62 (3H, s, 172-COOCH3),
3.541/3.535 (3H, s, 2-CH3), 3.53/3.52 (3H, s, 12-CH3), 3.21 (3H, s, 31-
CH3), 2.85–2.74 (2H, m, 8-CH2), 2.66–2.61, 2.58–2.50 (each 1H, m, 171-
CH2), 2.40/2.37 (3H, s, 7-CH3), 2.32–2.21 (2H, m, 17-CH2), 1.755/
1.752 (3H, d, J ¼ 7 Hz, 18-CH3), 0.96/0.88 (3H, t, J ¼ 7 Hz, 81-CH3),
0.27 (3H, s, BCH3), À 0.13/–0.20, À 1.43/–1.46 (each 1H, s, NH x 2);
HRMS (APCI) found: m/z 623.3042, calcd for C35H40N4O6B: MHþ,
623.3035.
3.3.6. p-Anisylboronate of methyl 7,8-didehydro-cis-7,8-dihydroxy-
pyrobacteriopheophorbide-a (1f)
UV-VIS (CH2Cl2) λmax 363 (Irel, 100), 533 (26), 736 nm (61); 1H NMR
(400 MHz, CDCl3) δ(1/1) 9.39/9.35 (1H, s, 5-H), 8.89/8.88 (1H, s, 10-
H), 8.55/8.53 (1H, s, 20-H), 7.79/7.78 (2H, d, J ¼ 8 Hz, o-H x 2), 6.80
(2H, d, J ¼ 8 Hz, m-H x 2), 5.15/5.14, 5.00/4.98 (each 1H, d, J ¼ 20 Hz,
131-CH2), 4.38–4.33 (1H, m, 18-H), 4.19–4.17 (1H, m, 17-H), 3.73 (3H,
s, p-OCH3), 3.63/3.60 (3H, s, 172-COOCH3), 3.55, 3.54, 3.54, 3.53 (each
half of 3H, s, 2-, 12-CH3), 3.23 (3H, s, 31-CH3), 2.95–2.81 (2H, m, 8-
CH2), 2.67–2.48 (2H, m, 171-CH2), 2.44/2.41 (3H, s, 7-CH3), 2.32–2.21
(2H, m, 17-CH2), 1.74 (3H, d, J ¼ 7 Hz, 18-CH3), 1.03/0.94 (3H, t, J ¼ 7
Hz, 81-CH3), À 0.05/–0.13, À 1.37/–1.41 (each 1H, s, NH x 2); HRMS
(APCI) found: m/z 715.3302, calcd for C41H44N4O7B: MHþ, 715.3298.
3.3.2. 1-Dodecylboronate of methyl 7,8-didehydro-cis-7,8-dihydroxy-
pyrobacteriopheophorbide-a (1b)
UV-VIS (CH2Cl2) λmax 363 (Irel, 100), 532 (25), 737 nm (61); 1H NMR
(600 MHz, CDCl3) δ(1/1) 9.30/9.26 (1H, s, 5-H), 8.82/8.81 (1H, s, 10-
H), 8.56/8.55 (1H, s, 20-H), 5.15, 5.01/4.99 (each 1H, d, J ¼ 19 Hz, 131-
CH2), 4.38/4.36 (1H, dq, J ¼ 2, 7 Hz, 18-H), 4.20/4.19 (1H, dt, J ¼ 7, 2
Hz, 17-H), 3.629/3.627 (3H, s, 172-COOCH3), 3.54, 3.53, 3.53, 3.52
(each half of 3H, s, 2-, 12-CH3), 3.21 (3H, s, 31-CH3), 2.88–2.73 (2H, m,
8-CH2), 2.67–2.51 (2H, m, 171-CH2), 2.37/2.34 (3H, s, 7-CH3),
2.32–2.20 (2H, m, 17-CH2), 1.754/1.749 (3H, d, J ¼ 7 Hz, 18-CH3),
1.40–1.00 (20H, m, BC(CH2)10), 0.97/0.89 (3H, t, J ¼ 8 Hz, 81-CH3),
0.844/0.840 (3H, t, J ¼ 7 Hz, BC11CH3), 0.77 (2H, t, J ¼ 8 Hz, BCH2),
À 0.08/–0.15, À 1.39/–1.43 (each 1H, s, NH x 2); HRMS (APCI) found:
m/z 777.4766, calcd for C46H62N4O6B: MHþ, 777.4757.
3.3.7. 3,5-Bis(trifluoromethyl)phenylboronate of methyl 7,8-didehydro-cis-
7,8-dihydroxy-pyrobacteriopheophorbide-a (1g)
UV-VIS (CH2Cl2) λmax 363 (Irel, 100), 533 (27), 735 nm (62); 1H NMR
(400 MHz, CDCl3) δ(1/1) 9.47/9.43 (1H, s, 5-H), 8.91/8.90 (1H, s, 10-
H), 8.60/8.58 (1H, s, 20-H), 8.26 (2H, s, Ph-2,6-H), 7.85 (1H, s, Ph-4-H),
5.16, 5.02/5.00 (each 1H, d, J ¼ 20 Hz, 131-CH2), 4.41–4.34 (1H, m, 18-
H), 4.21–4.19 (1H, m, 17-H), 3.63/3.60 (3H, s, 172-COOCH3), 3.58,
3.57, 3.57, 3.56 (each half of 3H, s, 2-, 12-CH3), 3.25 (3H, s, 31-CH3),
2.96–2.88 (2H, m, 8-CH2), 2.67–2.53 (2H, m, 171-CH2), 2.51/2.48 (3H,
s, 7-CH3), 2.32–2.17 (2H, m, 17-CH2), 1.76/1.75 (3H, d, J ¼ 7 Hz, 18-
CH3), 1.03/0.95 (3H, t, J ¼ 7 Hz, 81-CH3), À 0.17/–0.24, À 1.44/–1.48
(each 1H, s, NH x 2); HRMS (APCI) found: m/z 821.2950, calcd for
C42H40N4O6BF6: MHþ, 821.2940.
3.3.3. Isobutylboronate of methyl 7,8-didehydro-cis-7,8-dihydroxy-
pyrobacteriopheophorbide-a (1c)
UV-VIS (CH2Cl2) λmax 363 (Irel, 100), 532 (25), 736 nm (60); 1H NMR
(600 MHz, CDCl3) δ(1/1) 9.30/9.26 (1H, s, 5-H), 8.82/8.81 (1H, s, 10-
H), 8.56/8.55 (1H, s, 20-H), 5.15, 5.01/4.99 (each 1H, d, J ¼ 19 Hz, 131-
CH2), 4.38/4.36 (1H, dq, J ¼ 2, 7 Hz, 18-H), 4.20/4.18 (1H, dt, J ¼ 7, 2
Hz, 17-H), 3.63/3.62 (3H, s, 172-COOCH3), 3.535, 3.528, 3.527, 3.517
(each half of 3H, s, 2-, 12-CH3), 3.21 (3H, s, 31-CH3), 2.85–2.72 (2H, m,
8-CH2), 2.66–2.50 (2H, m, 171-CH2), 2.37/2.34 (3H, s, 7-CH3),
2.32–2.20 (2H, m, 17-CH2), 1.85–1.77 (1H, m, BCCH), 1.752/1.749
(3H, d, J ¼ 7 Hz, 18-CH3), 0.98/0.89 (3H, t, J ¼ 8 Hz, 81-CH3), 0.78–0.74
(8H, m, BCH2C(CH3)2), À 0.09/–0.16, À 1.40/–1.44 (each 1H, s, NH x 2);
HRMS (APCI) found: m/z 665.3508, calcd for C38H46N4O6B: MHþ,
665.3505.
3.3.8. 2,6-Dimethylphenylboronate of methyl 7,8-didehydro-cis-7,8-
dihydroxy-pyrobacteriopheophorbide-a (1h)
UV-VIS (CH2Cl2) λmax 363 (Irel, 100), 532 (26), 736 nm (61); 1H NMR
(400 MHz, CDCl3) δ(5/4) 9.38/9.35 (1H, s, 5-H), 8.90/8.89 (1H, s, 10-
H), 8.60/8.58 (1H, s, 20-H), 7.033/7.028 (1H, t, J ¼ 8 Hz, Ph-4-H),
6.81/6.80 (2H, d, J ¼ 8 Hz, Ph-3,5-H), 5.17/5.16, 5.02/4.99 (each
1H, d, J ¼ 20 Hz, 131-CH2), 4.41–4.38 (1H, m, 18-H), 4.23–4.20 (1H, m,
17-H), 3.62/3.64 (3H, s, 172-COOCH3), 3.55/3.54 (3H, s, 2-CH3), 3.53/
3.52 (3H, s, 12-CH3), 3.199/3.195 (3H, s, 31-CH3), 2.92–2.82 (2H, m, 8-
CH2), 2.65–2.54 (2H, m, 171-CH2), 2.51/2.49 (3H, s, 7-CH3), 2.35–2.24
(2H, m, 17-CH2), 2.06/2.04 (6H, s, Ph-2,6-CH3), 1.76/1.78 (3H, d, J ¼ 7
Hz, 18-CH3), 0.93/1.02 (3H, t, J ¼ 8 Hz, 81-CH3), À 0.17/–0.11, À 1.44/
–1.41 (each 1H, s, NH x 2); HRMS (APCI) found: m/z 713.3512, calcd for
3.3.4. Cyclopropylboronate of methyl 7,8-didehydro-cis-7,8-dihydroxy-
pyrobacteriopheophorbide-a (1d)
UV-VIS (CH2Cl2) λmax 363 (Irel, 100), 533 (25), 737 nm (61); 1H NMR
(400 MHz, CDCl3) δ(1/1) 9.27/9.22 (1H, s, 5-H), 8.79/8.78 (1H, s, 10-
H), 8.56/8.54 (1H, s, 20-H), 5.15/5.14, 5.01/4.99 (each 1H, d, J ¼ 20
Hz, 131-CH2), 4.38–4.34 (1H, m, 18-H), 4.21–4.18 (1H, m, 17-H), 3.63/
C
42H46N4O6B: MHþ, 713.3505.
5

D. Funakoshi et al.
Dyes and Pigments 175 (2020) 108155
3.3.9. 2-Thienylboronate of methyl 7,8-didehydro-cis-7,8-dihydroxy-
pyrobacteriopheophorbide-a (1i)
References
UV-VIS (CH2Cl2) λmax 363 (Irel, 100), 533 (26), 736 nm (61); 1H NMR
(600 MHz, CDCl3) δ(1/1) 9.39/9.35 (1H, s, 5-H), 8.90/8.89 (1H, s, 10-
H), 8.56/8.54 (1H, s, 20-H), 7.67 (1H, m, thienyl-5-H), 7.55 (1H, d, J ¼
4 Hz, thienyl-3-H), 7.09 (1H, t, J ¼ 4 Hz, thienyl-4-H), 5.15/5.14, 5.00/
4.99 (each 1H, d, J ¼ 19 Hz, 131-CH2), 4.39–4.33 (1H, m, 18-H),
4.21–4.17 (1H, m, 17-H), 3.63/3.60 (3H, s, 172-COOCH3), 3.55, 3.54,
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1H, s, NH x 2); HRMS (APCI) found: m/z 691.2764, calcd for
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Appendix A. Supplementary data
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Supplementary data to this article can be found online at https://doi.
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6