Synthesis of a doubly SO2-fused phlorin: Tuning the structure and properties by the SO2 groups

Article abstract of DOI:10.1142/S1088424618500657

A doubly SO₂-fused phlorin 4 has been synthesized by the [2 + 2] condensation of dipyrromethanecarbinol 2 and SO₂-fused dipyrromenthane 3 in the presence of TFA, followed by DDQ oxidation. The SO₂-fused phlorin 4 has been characterized by absorption, fluorescence, mass and NMR spectra, as well as X-ray analysis. Compared to the β-unsubstituted phlorin 5, the SO₂-fused phlorin 4 exhibits a red-shifted absorption spectrum (around 12 nm), a more distorted molecular conformation, as well as nice photostability even with an electron-donating meso-3,5-di-tert-butylphenyl group. The titration of 4 and 5 with TBAF has been monitored by absorption spectroscopy. The deprotonated phlorin 4 shows a peak at 870 nm which is red shifted by 26 nm compared to that of deprotonated 5.

Full text of DOI:10.1142/S1088424618500657

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2018; 22: 1–8
DOI: 10.1142/S1088424618500657
Published at http://www.worldscinet.com/jpp/
Synthesis of a doubly SO2-fused phlorin:Tuning the structure
and properties by the SO2 groups
Chengjie Lia◊, Xiujun Liub, Jiewei Shaoa, Guangxian Sua andYongshu Xiea,*◊
a Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry & Molecular
Engineering, East China University of Science & Technology, 200237 Shanghai, China
bResearch Center of Analysis and Test, East China University of Science & Technology, 200237 Shanghai, China
Dedicated to Professor Naisheng Chen on the occasion of his 80th birthday
Received 19 March 2018
Accepted 25 April 2018
ABSTRACT: A doubly SO2-fused phlorin 4 has been synthesized by the [2 + 2] condensation of
dipyrromethanecarbinol 2 and SO2-fused dipyrromenthane 3 in the presence of TFA, followed by DDQ
oxidation. The SO2-fused phlorin 4 has been characterized by absorption, fluorescence, mass and NMR
spectra, as well as X-ray analysis. Compared to the b-unsubstituted phlorin 5, the SO2-fused phlorin 4
exhibits a red-shifted absorption spectrum (around 12 nm), a more distorted molecular conformation,
as well as nice photostability even with an electron-donating meso-3,5-di-tert-butylphenyl group. The
titration of 4 and 5 with TBAF has been monitored by absorption spectroscopy. The deprotonated phlorin
4 shows a peak at 870 nm which is red shifted by 26 nm compared to that of deprotonated 5.
KEYWORDS: phlorin, sulfolenopyrrole, photostability, crystal structure.
interruption of the conjugation framework by the sp3-
INTRODUCTION
carbon, a longer wavelength absorption when compared
Porphyrins exist widely in natural functional systems
to that of the corresponding porphyrin, interesting redox
such as heme, chlorophyll and Vitamin B12, and they are
properties showing two irreversible reduction and three
called life pigments owing to their essential biological
quasi-reversible oxidation signals at mild potentials, and
functions in respiration and photosynthesis. Various
porphyrins, porphyrinoids and oligopyrroles have been
synthesized to explore their structural diversities and
anion-recognition capability [39–42]. In addition, the
one saturated meso-carbon in phlorin causes the presence
of three inner NHs instead of two in porphyrin, which
corresponding applications [1–37]. Among them, calix-
resembles the structure of corrole and may chelate high
phyrins exhibit nonplanar conformations, because they
valent transition metal ions [43–45]. Thus, phlorins have
attracted considerable attention, and many synthetic
approaches have been developed. For example, one-pot
two-step [46], [2 + 2] condensation [39, 47], stepwise
methods [29], and nucleophilic substitution reactions at
one of the meso-carbons have been employed to construct
phlorins and their analogues, such as N-confused phlorins
[7, 48–50], core-modified phlorins [51, 52] and phlorin-
type expanded porphyrins [25, 53–55]. However, phlorins
may be labile to oxidation to give linear tetrapyrroles
contain one or more sp3 meso-carbons, which is in contrast
to the presence of four sp2 meso-carbons in porphyrins,
and thus they have been found to be excellent ligands for
anion binding [38]. As a special member of calixphyrins,
phlorin features one sp3 and three sp2 meso-carbons,
and was first reported by Woodward in the course of
chlorophyll synthesis in 1960 [9]. The existence of one
sp3 meso-carbon in phlorin endows it unique properties,
such as a twisted macrocycle conformation owing to
[11, 56], which restricts their further modifications. In
the past two decades, the stability of phlorins has been
◊ SPP full member in good standing
improved by incorporating bulk groups at the inner
NH groups [57, 58] or by introducing bulky groups at
*Correspondence to: Yongshu Xie, fax: +86-21-64252758,
email: yshxie@ecust.edu.cn.
Copyright © 2018 World Scientific Publishing Company

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C. LI ET AL.
the meso-carbons [59]. Recently, Bruce and coworkers
obtained stable phlorins by introducing electron-deficient
groups like pentafluorophenyl groups to the meso-
positions [39]. However, most of the trials are focused
on modification of the meso-positions, and b-substituted
phlorins remain relatively unexplored. Herein, we would
like to report the synthesis of a b-SO2-fused phlorin 4
(Scheme 1). The effects of the SO2 group on molecular
conformation, absorption and photostability have been
investigated. In addition, the titration of SO2-phlorin 4
with tetrabutylammonium fluoride (TBAF) has been
investigated using absorption and 1H NMR spectra.
the stability of the molecule. However, after exposure
to ambient light for 4 days, only slight changes in the
absorption spectra of 4 and 5 were observed, indicating
that 4 exhibits excellent photostability similar to that
of phlorin 5, which contains an additional electron-
withdrawing meso-C6F5 substituent [39]. These results
imply that the electron-deficient SO2 groups are favorable
for reducing the electron-density on the phlorin core,
and thus can prevent its oxidative ring opening reaction.
Hence, the introduction of electron-withdrawing SO2
groups at the b positions could be an effective strategy for
stabilizing phlorin, complementary to that of introducing
C6F5 groups to the meso-positions. Corresponding to the
absorption peak at 678 nm, the emission band of 4 was
observed at 758 nm when its CH2Cl2 solution was excited
at 684 nm. Mass spectra were also recorded to confirm
the identity of 4. As a result, the pseudo-molecular ion
[M - H]- corresponding to C62H46F10N4S2O4 was observed
at m/z = 1163.3 in the ESI mass spectrum. In the MALDI-
TOF spectrum, a corresponding characteristic fragment
was observed at m/z = 1036.3, owing to the loss of the
two SO2-groups [32, 61].
RESULTS AND DISCUSSION
The SO2-phlorin 4 was prepared according to the
Lindsey-modified MacDonald [2 + 2] approach [8, 60],
relying on the optimized procedure by condensation of the
corresponding bis(b,b′-sulfoleno)pyrromethane 3 [61]
and dipyrromethanecarbinol 2 [48] in the presence of
trifluoroacetic acid (TFA), followed by DDQ oxidation
(Scheme 1). [62] In detail, the a,a′-di-pentafluoro-
benzoyl-5,5-diphenyldipyrromethane 1 was synthesized
according to the literature [48] and was reduced with
NaBH4 to afford the intermediate dipyrromethanecarbinol
2. The mixture of 2 and 3 in CH2Cl2 was treated with
TFA for 18 h, and then DDQ was added followed by
quenching the reaction with triethylamine (TEA). After
working up, SO2-phlorin 4 was obtained with a yield of
27% as dark green crystals after purification with silica
gel column chromatography and recrystallization from
CH2Cl2/n-C5H12.
The absorption spectrum of 4 in CH2Cl2 (Fig. 1)
exhibits a typical absorption of phlorin with a Q band
and a Soret band at 678 and 442 nm, respectively,
showing a red shift of 12 nm and 14 nm, respectively,
as compared to those of phlorin 5 (666 nm for Q band,
and 428 nm and 411 nm for Soret band). It is known that
electron-withdrawing meso-substituents may enhance
the stability of phlorins [39]. The SO2-phlorin 4 contains
an electron-donation 3,5-di-tert-butyl group at one of the
meso-positions, which is expected to be unfavorable for
The structure of 4 was further established by detailed
analysis with one- and two-dimensional NMR spectra
1
(1H,1H COSY and NOESY, and H,13C HSQC spectra).
The symmetric sulfolenophlorin 4 showed clear signals
1
in the H NMR spectrum, including one singlet at the
high field, two singlets and one broad singlet at the
intermediate field, as well as three doublets, two broad
multiplets and one triplet at the low field (see Fig. 2).
To complete the assignments, two-dimensional NMR
spectroscopy was employed. In the1H,1H COSY spectrum
of 4, the triplet at 7.65 ppm for one proton is coupled
with the doublet at 7.33 ppm for two protons, and they
are respectively assigned to the protons attached to the
para- and ortho-positions of the 3,5-di-tert-butylphenyl
substituent. The two doublets, at 6.97 ppm and 6.93 ppm,
correlate with each other and are assigned to the pyrrolic
b-protons. In the 1H,13C HSQC spectrum, the singlets at
3.41 ppm and 3.83 ppm are correlated with the carbons
at 55.9 ppm and 55.3 ppm, respectively, indicating the
typical chemical shifts of the methylene units connected
Scheme 1. Synthesis of SO2-phlorin 4 by the Lindsey-modified MacDonald [2 + 2] approach
Copyright © 2018 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2018; 22: 2–8

SYNTHESIS OF A DOUBLY SO2-FUSED PHLORIN: TUNING THE STRUCTURE AND PROPERTIES BY THE SO2 GROUPS
3
Fig. 1. Left, absorption spectra of SO2-phlorin 4 (solid line) and 5 (dashed line) in CH2Cl2 (2 × 10-5 M) and emission spectrum of 4
in CH2Cl2 (5 × 10-6 M). Right, absorption spectra of 4 and 5 in CH2Cl2 (2 × 10-5 M) before/after exposure to ambient light for 4 days
at 23°C under air
Fig. 2. Top, 1H NMR spectrum of 4 in CD2Cl2 (400 MHz, 25°C). Bottom, the 1H,1H NOESY spectrum of 4 in CD2Cl2 (400 MHz,
25°C) with the corresponding coupling labeled with blue and red circles
with the SO2 groups. Then, the 1H,1H NOESY spectrum
assists further detailed assignments (Fig. 2). The ortho-
proton at 7.65 ppm from 3,5-di-tert-butylphenyl group
is coupled with the singlet at 3.41 ppm, and thus this
signal could be assigned to those attached to C81 and
C121, while the signal at 3.83 ppm can be assigned to
the protons of C71 and C131. In addition, the doublet at
6.93 ppm is correlated with the signal from the phenyl
group and could be assigned to the protons attached to
C2 and C18. Accordingly, the signal at 6.97 ppm can be
unambiguously assigned to the protons of C3 and C17.
Single crystals of SO2-phlorin 4 suitable for X-ray
analysis were obtained by diffusion of n-C5H12 into a
solution of 4 in CH2Cl2 at room temperature. Compound
4 crystalized in the triclinic space group P-1. A unit
cell contains two molecules of 4. In the molecular
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C. LI ET AL.
Fig. 3. Crystal structure of 4 from front view with partial bond lengths in Å and bond angles (top left, the hydrogens attached to
carbons are omitted for clarity) and side view (bottom left, hydrogens attached to carbons and the aryl groups at the 5,10,15-positions
are omitted for clarity). The side view of the crystal structures of 4 and 5, with the latter drawn using the cif file of 5 reported in the
Ref. [39] (top right for 4 and bottom right for 5, the hydrogens, the aromatic substituents at the 5,10,15-positions are omitted for
clarity)
structure (Fig. 3, top left), the pyrroles are conjugated
with alternate single and double bonds, except for the
interruption by the sp3-carbon C20, which shows C–C
bond lengths in the range of 1.525 Å–1.549 Å (Fig. 3),
and the bond angles around C20 lie within 107.2–111.5°.
All these values observed for C20 are typical for sp3-
carbons. The presence of the sp3 meso-carbon leads to a
distortion of the phlorin framework, affording a twisted
conformation. It should be noted that the existence of SO2
groups at the b-positions makes the molecule even more
distorted owing to the steric hindrance, when compared
with 5 (Fig. 3, right). Pyrrolic rings A and B are roughly
coplanar in 4, in sharp contrast to the roughly coplanar
rings of B and C observed in 5 (Fig. 3, right). The other
two pyrrole rings, C and D, are tilted out of the mean
plane composed of A and B with the dihedral angles
of 25.0° and 43.7°, respectively. The nitrogen atoms in
ring C and D are oriented towards opposite directions of
the A/B mean plane. The C6F5 group at the 5-position is
almost vertical to the mean plane of the ring A and B with
a large dihedral angle of 82.6°. The two phenyl groups are
respectively located above and below theA/B mean plane,
with dihedral angles of 84.1° and 60.3°, respectively.
Similar to normal phlorins, the sulfolenophlorin
4 underwent intense interaction with TBAF. Gradual
addition of the TBAF solution (24.2 mM) in CH2Cl2 to
the solution of 4 (29.2 mM) in CH2Cl2 (3 mL) led to a
dramatic color change from green to light brown, with
clear isosbestic points in the absorption spectra (Fig. 4).
During titration with TBAF, the peaks at 371 nm and
678 nm decreased, accompanied with the development
of new peaks at 451 nm and 870 nm (see Fig. 4, left),
which may be caused by the deprotonation of the NH
by F- [48]. Accordingly, in the negative mode ESI-MS
spectrum of the solution of 4 in the presence of TBAF,
a peak was observed at m/z = 1163.3, corresponding
to [M - H]-. In addition, NMR titration analysis also
indicated the deprotonation of the phlorin with the
observation of a broad signal at 14.2 ppm, which is
proposed to be the signal of [FHF]- [63]. The SO2
substitution at the b-positions shows noticeable effect
on the absorption of the deprotonated form by a 26 nm
red shift (870 nm) when compared to that of the normal
phlorin 5 (844 nm, Fig. 4, right) [40, 41].
In summary, a SO2-fused phlorin 4 was synthesized,
and the effect of SO2-fusion on the structures and
properties were systematically investigated. The steric
hindrance of the SO2 group distorted the phlorin core
when compared to that of the normal phlorin, and the
absorption bands are red shifted. Introduction of electron-
withdrawing SO2 groups is favorable for stabilizing the
phlorin core. Hence, 4 shows excellent photostability
comparable to 5, even though it contains an electron-
donating meso-3,5-di-tert-butylphenyl group. Hence,
SO2-fusion could be a new strategy for stabilizing phlorin
against light. The interaction of 4 with TBAF has been
studied. Compared with titration of the b-unsubstituted
phlorin 5, the fusion of SO2 red shifted the absorption
Copyright © 2018 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2018; 22: 4–8

SYNTHESIS OF A DOUBLY SO2-FUSED PHLORIN: TUNING THE STRUCTURE AND PROPERTIES BY THE SO2 GROUPS
5
Fig. 4. Top left, absorption spectral changes during the titration of the solution of 4 (29.2 mM) in CH2Cl2 (3 mL) with TBAF
(24.2 mM, 1 mL per injection). Top right, absorption spectral changes during the titration of the solution of 5 (28.9 mM)
1
in CH2Cl2 (3 mL) with TBAF (24.2 mM, 1 mL per injection). Bottom, H NMR spectra of 4 and of 4 with TBAF (4 eq.) in
DMSO-d6 (400 MHz, 25 °C)
of the deprotonated phlorin by ca. 26 nm (870 nm). In
conclusion, the introduction of SO2 groups is effective
for tuning the conformation, absorption and stability of
phlorin. Considering the easy functionalization of the
sulfolenopyrrole units [61, 64, 65], the SO2-phlorin 4
may open up an efficient pathway for constructing novel
functionalized phlorins.
the protons of the 3,5-di-tert-butylphenyl group, and
PyH indicates the pyrrolic b-H. Mass spectra were
collected using a Waters LCT Premier XE spectrometer
or an AB Sciex 4800 Plus MALDI-TOF Analyzer.
UV-vis absorption spectra were recorded on a Shimadzu
UV2600 spectrophotometer and fluorescence spectra
were recorded on a Varian Cary Eclipse Fluorimeter.
Crystallography
EXPERIMENTAL
Single crystals of 4 were obtained by the slow diffusion
of n-C5H12 to its solution in CH2Cl2. X-ray analyses were
performed on a SMART APEX Area Detector System
equipped with a CCD detector (Bruker) using MoKa
(graphite, monochromated, l = 0.71069 Å) radiation.
The structure was solved by the direct method using
SHELXS-97 and refined using the SHELXL-97 program
[66]. The positional parameters and thermal parameters
of non-hydrogen atoms were refined anisotropically on
F2 by the full-matrix least-squares method. Hydrogen
atoms were placed at calculated positions and refined
riding on their corresponding carbon atoms. CCDC
1827752 contains the supplementary crystallographic
data for this paper, and these data are provided free of
General
Commercially available solvents and reagents were
used without further purification unless otherwise
mentioned. SO2-dipyrromethane 3 was synthesized
according to the literature [61]. Thin-layer chromato-
graphy (TLC) was carried out on glass sheets coated
with silica gel 60 F254 (Qingdao Haiyang Chemical
Co., Ltd). NMR spectra were obtained using a Bruker
AM 400 spectrometer, and chemical shifts were reported
relative to the residual peaks of CD2Cl2 (d = 5.32), CDCl3
(d = 7.26) and DMSO-d6 (d = 2.50) in ppm, where PhH
indicates the protons of the phenyl groups, ArH indicates
Copyright © 2018 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2018; 22: 5–8

6
C. LI ET AL.
charge by the Cambridge Crystallographic Data Centre.
Crystal data for 4: C62H43F10N4O4S2, Mr = 1165.15,
triclinic, space group: P-1, a = 12.3934 (6), b = 16.5275
(12), c = 16.8337 (10), a = 97.173 (2)°, b = 109.082 (2)°,
g = 99.489 (2)°, V = 3154.4 (3)Å3, Z = 2, R indices [I > 2s
(I)] R1 = 0.0668, wR2 = 0.1339, R indices (all data) R1 =
0.1307, wR2 = 0.1558, GOF = 1.041.
7.74 (br, s, 2H, NH). ESI-MS: m/z (%) 1166.3 (11),
1165.3 (38), 1164.3 (70), 1163.3 (100, [M-H]-) calc.
[M-H]- C62H45F10N4O4S2: 1163.3.
Preparation of 5,10,15-tripentafluorophenyl-20,20-
diphenylphlorin 5. Phlorin 5 was synthesized according
to the reported method [39]. UV-vis (CH2Cl2): lmax
,
nm (log e) 666 (4.32), 622 shoulder (4.20), 428 (4.69),
1
411 (4.69), 354 (4.44), 296 (4.31). H NMR (400 MHz,
DMSO-d6, 25°C): dH, ppm 6.13 (dd, J = 2.0, 3.7 Hz, 2H,
PyH), 6.86 (m, 4H, PhH), 6.96 (dd, J = 2.0, 3.7 Hz, 2H,
PyH), 7.07 (d, J = 5.1 Hz, 2H, PyH), 7.32 (m, 6H, PhH),
7.37 (d, J = 5.1 Hz, 2H, PyH), 7.69 (s, 1H, NH), 8.38
(s, 1H, NH).
Synthesis
Preparation of a,a′-di-pentafluorobenzoyl-5,5-di-
phenyldipyrromethane 1. Compound 1 was synthe-
sized according to the literature by acylation of
5,5-diphenyldipyrromethane with C6F5COCl in the
presence of PhMgBr in THF [48], yield: 32%, 1H NMR
(400 MHz, CDCl3, 25°C): dH, ppm 6.22 (dd, J = 2.7,
3.9 Hz, 2H, PyH), 6.71 (dd, J = 2.7, 3.9 Hz, 2H, PyH),
7.10 (m, 4H, PhH), 7.41 (m, 6H, PhH), 9.18 (s, 2H, NH).
Preparation of SO2-phlorin 4. To a solution of 1
(24.0 mg, 35 mmol) in THF/MeOH (6 mL, 5/1, v/v) was
added NaBH4 (13.3 mg, 350 mmol, 10 eq). After 2 h,
compound 1 was completely consumed according to TLC
analysis (on silica gel). The reaction was quenched by
addition of H2O (15 mL), and the raw product dicarbinol
2 was extracted with CH2Cl2 (3 × 10 mL). The organic
phase was filtered through a plug of dry cotton wool.
The filtrate was dried under reduced pressure and the raw
product of 2 was used without further purification.
Acknowledgments
This work was financially supported by NSFC
(21472047, 21772041, 21702062), the Program for
Professor of Special Appointment (Eastern Scholar,
GZ2016006) at Shanghai Institutions of Higher Learning,
Shanghai Pujiang Program (17PJ1401700), and the
Fundamental Research Funds for the Central Universities
(WK1616004, 222201717003, 222201714013). The
authors thank the Research Center ofAnalysis andTesting
of East China University of Science & Technology for
the help with characterization.
Supporting information
Dicarbinol 2 and the sulfolenodipyrromethane 3
(18.1 mg, 35 mmol) were dissolved in Ar-purged CH2Cl2
(14 mL), then TFA (0.11 mL, 1.5 mmol) was added and
the reaction mixture was stirred for 18 h under Ar in the
dark. DDQ (11.2 mg, 19.3 mmol) was added and the
reaction mixture was stirred for 20 min. Triethylamine
(0.5 mL) was used to quench the reaction. The mixture
was washed successively with saturated aq. NaHCO3 (2 ×
10 mL) and H2O (2 × 10 mL). The organic phase passed
through a plug of dry cotton wool and the filtrate was dried
under reduced pressure. The dark residue was purified by
silica gel column chromatography (2 cm × 15 cm). The
product 4 was washed by CH2Cl2/petroleum ether (1/1,
v/v) as a green fraction. Finally, dark green crystalline
sulfolenophlorin 4 was obtained by recrystallization from
CH2Cl2/n-C5H12.Yield: 10.2 mg (27%). UV-vis (CH2Cl2):
Crystallographic data have been deposited at the
Cambridge Crystallographic Data Centre (CCDC) under
number CCDC-1827752. Copies can be obtained on
request, free of charge, via http://www.ccdc.cam.ac.uk/
data_request/cif or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK
(fax: +44 1223-336-033 or email: deposit@ccdc.cam.
ac.uk).
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