Synthesis of A2B-type 22-oxacorroles bearing two different five-membered heterocycles at meso positions

Article abstract of DOI:10.1142/S108842461950144X

A series of 22-oxacorroles containing mixed substituents such as either two five-membered heterocycles and one six-membered aryl group or three five-membered heterocycles at three meso positions were synthesized by 3 + 2 oxidative coupling of appropriate

Full text of DOI:10.1142/S108842461950144X

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2019; 23: 1–8
DOI: 10.1142/S108842461950144X
Published at http://www.worldscinet.com/jpp/
Synthesis of A2B-type 22-oxacorroles bearing two different
five-membered heterocycles at meso positions
Umasekhar Booruga and Mangalampalli Ravikanth*
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India
Dedicated to Professor Atsuhiro Osuka on the occasion of his 65th birthday.
Received 16 August 2019
Accepted 23 September 2019
ABSTRACT: A series of 22-oxacorroles containing mixed substituents such as either two five-
membered heterocycles and one six-membered aryl group or three five-membered heterocycles at three
meso positions were synthesized by 3 + 2 oxidative coupling of appropriate 16-oxatripyrrane and meso-
substituted dipyrromethane under mild acid catalyzed conditions. The identities of the 22-oxacorroles
were confirmed by the respective molecular ion peaks in HR-MS spectra and the structures were deduced
by detailed 1D and 2D NMR spectroscopy. NMR studies clearly showed upfield or downfield shifts of
core pyrrole and furan protons and inner NH protons depending on the type of substituent present at the
meso positions. All 22-oxacorroles absorb in the 410–655 nm region, and the position of the absorption
bands depends on the type of meso substituents. The A2B oxacorroles showed one broad and weak
fluorescence band in the 600–700 nm region with low fluorescence quantum yields in the 0.05–0.12
range. Electrochemical studies revealed that the A2B-type 22-oxacorroles are easier to oxidize and also
easier to reduce than triphenyl 22-oxacorroles. Thus, the electronic properties of 22-oxacorroles were
significantly altered when six-membered aryl groups were replaced with five-membered heterocycles as
reflected in spectral and electrochemical measurements.
KEYWORDS: corroles, 22-oxacorrole, five-membered heterocycle, electrochemical studies,
fluorescence.
complexes such as P(V) and Ge(IV) corroles are strongly
INTRODUCTION
fluorescent compounds with high fluorescence quantum
Corroles are aromatic tetrapyrrolic macrocycles
yields [6]. Corroles can be modified further by replacing
where four pyrrole rings are connected with three meso
one or two pyrrole rings with other heterocycles such
carbons and possess one direct pyrrole–pyrrole linkage
as thiophene, furan, selenophene and telluorophene and
[1, 2]. Thus, corroles have one meso carbon fewer than
the resulting core-modified corroles or heterocorroles
porphyrins but contain three inner pyrrole NH atoms,
are expected to possesses different physicochemical
unlike porphyrins which have two inner NH atoms.
properties from regular tetrapyrrolic corroles [7–10].
Hence, corroles have a greater ability to stabilize metals
Interestingly, there are very few reports on heterocorroles
in higher oxidation states than porphyrins [3–5]. This
because of lack of proper synthetic strategies and also
is indeed shown by several studies demonstrating that
partly due to their ‘inherent instability.’In fact, to the best
corroles have ability to stabilize metals in higher oxidation
of our knowledge, only oxa- and thiacorroles have been
states than porphyrins. Corroles, like porphyrins, have
synthesized and characterized so far, but the synthesis
rich coordination chemistry, and many coordination
of many other heterocorroles are yet to be developed
[11–12]. Among oxa- and thiacorroles, the chemistry
of oxacorroles is relatively more explored because of
their better-developed synthetic strategies and their high
stability. Meso-tri(p-tolyl) 22-oxacorrole 1 was obtained
*Correspondence to: Mangalampalli Ravikanth, Department
of Chemistry, Indian Institute of Technology Bombay, Powai,
Mumbai 400076, India, Email: ravikanth@chem.iitb.ac.in.
Copyright © 2019 World Scientific Publishing Company

2
U. BOORUGA AND M. RAVIKANTH
as a side product along with expanded porphyrin,
25-oxasmaragdyrin by (3 + 2) oxidative coupling
of 16-oxatripyrrane and meso-tolyl dipyrromethane
under mild acid-catalyzed conditions. Meso-triaryl
22-oxacorroles such as 1 are strongly fluorescent and
show an ability to form coordination complexes [8].
Recently, we reported the synthesis of 22-oxacorroles
containing one meso-pyrrolyl group such as compound
2 under the same (3 + 2) oxidative coupling conditions
[12] and we used these corroles to prepare novel
BODIPY-bridged 22-oxacorrole dyads by utilizing the
reactivity of the α position of the meso-pyrrolyl group of
22-oxacorroles. We further developed a straightforward,
simple strategy to prepare ABC-type 22-oxacorroles
bearing three different five-membered heterocycles at
the meso positions [13]. Our studies indicated that the
electronic properties of 22-oxacorroles were significantly
altered by replacing the six-membered aryl groups with
three different five-membered heterocycles such as
pyrrole, thiophene and furan [13]. In continuation of
our work on 22-oxacorroles, herein we report synthesis
and properties of different 22-oxacorroles containing
mixed substituents such as a combination of a six-
membered aryl group and five-membered heterocycles or
a combination of different five-membered heterocycles at
the meso positions and we compare the properties with
appropriate meso-substituted 22-oxacorroles. The studies
show that the properties of 22-oxacorroles are dependent
on the type of substituents present at the meso positions.
addition of ferrocene as an internal standard, taking E1/2
(Fc/Fc+) = 0.42 V vs. SCE. All the solutions were purged
with argon gas prior to electrochemical and spectral
measurements. High resolution mass spectra (HRMS)
were recorded with a Bruker Maxis Impact and Q-TOF
micro mass spectrometer. For UV-vis and fluorescence
titrations, the stock solutions of all compounds (1 × 10-3
M) were prepared by using a HPLC grade chloroform
solvent.
Synthesis of compounds 3–7
Samples of 16-oxaripyrrane (500 mg, 1.58 mmol) 8, 12
and13andmeso-heterocyclic-substituteddipyrromethane
9–11 and 14–16 were dissolved in CH2Cl2 (150 mL) and
stirred under a nitrogen atmosphere for 5 min. A catalytic
amount of TFA (12 mL, 0.106 mmol) was added and
stirring was continued for a further 1.5 h under an inert
atmosphere. DDQ (1.077 g, 4.74 mmol) was added and
stirring was continued in open air for an additional 2 h.
The solvent was removed under reduced pressure and
the crude compound was subjected to silica gel column
chromatography purification. The desired 22-oxacorrole
was collected using petroleum ether/dichloromethane
(3:1) to afford pure 22-oxacorroles 3–7 as purple solids
in 6–12% yield.
Compound 3. Yield 8%. 1H NMR (400 MHz; CDCl3;
Me4Si): dH in ppm -1.55 (s, 2H, –NH), 2.9 (s, 3H, CH3),
6.75 (d, 1H, J = 4.2 Hz, pyrrole ring), 6.80 (d, 1H, J =
3.0 Hz, pyrrole ring), 7.09 (s, 1H, pyrrole ring), 7.19 (s,
1H, pyrrole ring), 7.21 (d, 1H, J = 4.2 Hz, pyrrole ring),
7.50 (s, 1H, pyrrole ring), 7.65 (d, 2H, J = 6.32 Hz,
aryl ring), 8.15 (d, 2H, J = 6.32 Hz, aryl ring), 8.49 (d,
1H, J = 4.3 Hz, pyrrole core), 8.51 (d, 1H, J = 5.04 Hz,
pyrrole ring), 8.91–9.00 (m, 3H, pyrrole core, b-furyl),
8.95 (d, 1H, J = 4.5 Hz, pyrrole core), 9.15 (m, 2H,
b-furyl and b-pyrrole), 9.09 (br.s, 1H, –NH), 9.39 (br.s,
1H, –NH).; 13C NMR (100 MHz, CDCl3; Me4Si): dC in
ppm 108.7, 110.0, 111.4, 116.0, 119.6, 119.5, 122.2,
123.5, 125.5, 126.5, 126.8, 130.2, 136.1, 137.7, 139.8,
141.5, 144.5, 151.7, 161.9; HR-MS calcd for C34H26N5O
(M + H)+ m/z 520.2132, observed 520.2132.
EXPERIMENTAL
All chemicals were used as received unless otherwise
noted. All solvents were of at least reagent grade and
dried if necessary. The 1H, 11B, 19F and 13C NMR spectra
were recorded in CDCl3 on Bruker 400 or 500 MHz
instruments. The frequency of 101 and 126 MHz is used
for the 13C nucleus.Tetramethylsilane [Si(CH3)4] was used
as an internal standard for 1H and 13C NMR. Absorption
and steady-state fluorescence spectra were obtained
with a Cary series UV-vis/NIR spectrophotometer and
a Varian–Cary Eclipse spectrofluorometer, respectively.
The fluorescence quantum yields (Ff) were estimated
from emission and absorption spectra by the comparative
method at an excitation wavelength of 440 nm using
H2TTP (Ff = 0.11) as the standard. Cyclic voltammetric
studies were carried out with a BAS electrochemical
system utilizing a three electrode configuration consisting
of glassy carbon working electrode, a platinum wire
auxiliary electrode and a saturated calomel reference
electrode. The experiments were done in dry CH2Cl2
using 0.1 M tetrabutylammonium perchlorate (TBAP)
as supporting electrolyte. Half-wave potentials were
measured using DPV (differential pulse voltammetry)
and also calculated manually by taking the average of
the cathodic and anodic peak potentials. All potentials
were calibrated vs. the saturated calomel electrode by the
Compound 4. Yield 12%. 1H NMR (400 MHz;
CDCl3; Me4Si): dH in ppm -1.99 (s, 2H, –NH), 2.50 (s,
3H, –CH3), 6.85 (d, 1H, J = 3.0 Hz, pyrrole ring), 6.95
(s, 1H, J = 4.6 Hz, pyrrole ring), 7.10 (s, 1H, pyrrole
ring), 7.35 (d, 1H, J = 3.6 Hz, furyl ring), 7.40 (s, 1H,
furyl ring), 7.60 (d, 2H, J = 6.32 Hz, aryl ring), 8.19 (d,
2H, J = 6.32 Hz, aryl ring), 8.55 (d, J = 4.0 Hz, 1H, furyl
ring), 8.60 (d, 1H, J = 5.04 Hz, pyrrole ring), 8.95–9.00
(m, 3H, pyrrole core, b-furyl), 9.01 (d, 1H, J = 4.6 Hz,
pyrrole core), 9.15 (m, 2H, b-furyl and b-pyrrole), 9.21
(d, 1H, J = 4.3, pyrrole core), 9.40 (br.s, 1H, –NH).;
UV-vis (in CH2Cl2, lmax/nm (log e): 428 (5.7), 522 (4.2),
562 (4.1), 601 (4.5), 659 (4.9); lem (nm): 665. HR-MS
calcd for C34H24N4O2 (M + H)+ m/z 521.1972, observed
521.1970.
Copyright © 2019 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2019; 23: 2–8

SYNTHESIS OF A2B-TYPE 22-OXACORROLES BEARING TWO DIFFERENT FIVE-MEMBERED HETEROCYCLES
3
Compound 5. Yield 9%. 1H NMR (400 MHz; CDCl3;
Me4Si): dH in ppm -2.0 (br s, 2H, –NH), 2.90 (s, 3H,
–CH3) 6.78 (d, 1H, J = 4.1 Hz, pyrrole ring), 7.00 (d,
1H, J = 3.0 Hz thiophene ring), 7.29 (d, 1H, J = 4.0 Hz,
pyrrole ring), 7.50 (d, 1H, J = 4.6 Hz, pyrrole ring), 7.57
(d, 1H, J = 3.2 Hz, thiophene ring), 7.49 (d, 1H, J = 3.8
Hz, thiophene ring), 7.55 (d, 1H, J = 3.9 Hz, pyrrole
core), 7.62 (d, 2H, J = 7.2 Hz, aryl ring), 7.99 (m, 1h,
pyrrole core), 8.15 (d, 2H, J = 7.6 Hz, aryl), 8.59 (d, 1H,
J = 4.2 Hz, core), 9.05–9.10 (multiplet, 4H, core), 9.09 (d,
1H, J = 4.0 Hz b-furyl), 9.11 (d, 1H, J = 4.4 Hz, pyrrole),
9.25 (br s, 1H, NH); UV-vis (in CH2Cl2, lmax/nm (log e):
423 (5.7), 505 (2.15), 537 (1.50), 593 (1.90), 645 (2.00):
HR-MS calcd for C31H21N4O2S (M + H)+ m/z 537.1744,
observed 537.1742.
containing two five-membered heterocycles and one
six-membered aryl group present at the meso-positions
(3–5) were synthesized by condensing one equivalent
of 5-(p-tolyl)-10,15,17-trihydro-16-oxatripyrrane 8 with
one equivalent of appropriate dipyrromethane, 5-pyrrolyl
dipyrromethane 9, 5-furyl dipyrromethane 10 and
5-thienyl dipyrromethane 11, whereas 22-oxacorroles
containing three mixed five-membered heterocycles at
the meso positions (6–7) were synthesized by condensing
appropriate 16-oxatripyrrane, 5-(furyl)-10,15,17-tri-
hydro-16-oxatripyrrane 12 or 5-(thienyl))-10,15,17-
trihydro-16-oxatripyrrane13with5-furyldipyrromethane
14 and 5-thienyl dipyrromethane 15 under mild acid-
catalyzed inert conditions for 1 h followed by oxidation
with DDQ in open air for an additional 1 h. The
progress of the reaction was followed by TLC analysis
and absorption spectroscopy. The crude compounds
were subjected to alumina column chromatographic
purification and afforded pure 22-oxacorroles containing
mixed five/six-membered heterocycles/aryl groups at
meso positions 3–7 in 8–9% yields. The identities of
22-oxacorroles 3–7 were confirmed by corresponding
molecular ion peaks in HR-MS spectra and characterized
in detail by 1D and 2D NMR spectroscopy.
Compound 6. Yield 8%. 1H NMR (400 MHz; CDCl3;
Me4Si): dH in ppm -2.60 (s, 2H, –NH), 6.79 (m, 1H,
pyrrole), 7.10 (s, 2H, pyrrole ring, furyl ring), 7.30
(s, 2H, furyl ring), 7.59 (s, 2H, pyrrole), 7.89 (s, 1H,
pyrrole), 7.99 (s, 1H, furyl) 8.19 (d, 2H, b-furyl, pyrrole
ring), 8.25 (d, 2H, b-furyl, pyrrole ring), 8.39 (d, 2H,
b-furyl, pyrrole ring), 9.10 (d, 2H, b-furyl, pyrrole), 9.19
(s, 1H, NH); 13C NMR (100 MHz, CDCl3; Me4Si): dC
in ppm 108.0, 108.3, 110.6, 115.2, 118.8, 121.1, 122.6,
124.7, 125.7, 126.1, 126.3, 128.0, 129.5, 135.4, 136.9,
139.0, 140.7, 143.7, 151.0, 161.2: UV-vis (in CH2Cl2,
The 1H NMR and 1H–1H COSY NMR spectra of meso-
bis-pyrrole-substituted 21-oxacorrole 3 are presented in
Fig. 1. Compound 3 showed four sets of resonances in the
region of 8.50–9.15 ppm corresponding to six b-pyrrole
protons and two b-furan protons. The resonance at 9.15
ppm was identified as a type-e proton and showed cross-
peak connectivity with a type-d proton of b-pyrrole
appearing at 9.13 ppm. The resonance at 9.00 ppm,
due to a type-j proton showed cross-peak connectivity
with a type-k proton of the b-furyl ring. The resonance
at 8.50 ppm, identified as type-f protons, showed cross-
peak relation with a type-g proton. Furthermore, we also
identified and assigned all the meso-pyrrolic protons
which appeared in the region of 6.60–7.50 ppm by using
1D and 2D NMR spectroscopy. The inner NH resonance
appeared as broad resonance at -1.70 ppm whereas
the meso-pyrrolic NH protons appeared as two broad
resonances at 9.09 and 9.39 ppm (Fig. 1a). Compounds
4–7 also exhibited similar NMR features and all
resonances were identified and assigned using 1D and 2D
NMR spectroscopy. Thus, 1D and 2D NMR techniques
were very useful in deducing the molecular structures of
22-oxacorroles 3–7.
l
max/nm (log e): 425 (5.7), 522 (4.0), 565 (3.9), 601
(3.5), 658 (4.9). LR-MS calcd for C31H20N4O3 (M + H)+
m/z, observed 496.2975.
Compound 7. Yield 9%. 1H NMR (400 MHz; CDCl3;
Me4Si): dH in ppm -2.55 (s, 2H, –NH), 7.15 (m, 4H,
pyrrole, thianyl ring), 7.39 (s, 1H, pyrrole), 7.58 (d, 4H,
pyrrole, thianyl ring), 8.30 (m, 2H, pyrrole), 8.39 (m, 2H,
b-furyl, pyrrole ring), 8.61 (br.s, 4H, b-furyl, pyrrole),
9.05 (s, 1H, NH); 13C NMR (100 MHz, CDCl3; Me4Si):
dC in ppm 109.7, 117.0, 122.8, 123.2, 124.5, 126.5,
127.56, 127.85, 131.2, 131.5, 133.5, 134.8, 137.2, 140.8,
142.5, 152.7; UV-vis (in CH2Cl2, lmax/nm (log e)): 413
(5.4), 496 (3.1), 528 (3.1), 567 (3.5), 633 (4.2). HR-MS
calcd for C31H20N4OS2 (M + H)+ m/z 528.1102, observed
528.1106.
RESULTS AND DISCUSSION
The 22-oxacorroles containing mixed meso-
substituents 3–7 (Chart 1) were synthesized as presented
in Scheme.1. The required precursors, different dipyrro-
methanes such as 5-(p-tolyl)-dipyrromethane [12],
Furthermore, it is noted that the electronic properties
of 22-oxacorroles varied depending on the kind of
substituents present at the meso positions, reflected in the
upfield or downfield shifts in the proton resonances of
certain core protons of macrocycles 3–7. The comparison
5-pyrrolyldipyrromethane
9
[15], 5-thienyldipyrro-
methane 10 [13] and 5-furyldipyrromethane 11 [13]
and 16-oxatripyrranes such as 5-(p-tolyl)-10,15,17-
trihydro-16-oxatripyrrane 8 [12], 5-(furyl)-10,15,17-16-
oxatripyrrane 12 [13] and 5-(thienyl)-10,15,17-trihydro-
16-oxatripyrrane 13 [13] were synthesized by following
the literature procedures and the characterization data
was matched with the reported data. 22-Oxacorroles
1
of H NMR spectra of 22-oxacorroles 3–5 is shown in
Fig. 1a and the relevant NMR data for macrocycles 3–7
is presented in Table 1. As noted from Fig. 2 and the
data in Table 1, the protons experienced slight upfield or
downfield shifts depending on the type of five-membered
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U. BOORUGA AND M. RAVIKANTH
n-BuLi, TMEDA
(i)
CH2OH
O
O
THF, Et2O, N2
OH
OH
CHO
2
2
2
(45%)
NH
(65%)
NH
O
(I)CH2Cl2, TFA,
1.5 Hr
NH
NH
8
NH
N
O
X
(II) DDQ, 1Hr
(8%)
NH NH
NH
O
X= NH:3 (8%)
= O :4 (12%)
= S :5 (9%)
X= NH:9
= O :10
= S :11
n-BuLi, TMEDA
(ii)
X
CH2OH
O
O
THF, Et2O, N2
OH
OH
X= O ,S
CHO
O
(40%)
2
2
2
NH
(65%)
NH
O
O
(I)CH2Cl2, TFA,
1.5 Hr
NH
O
X
NH
X'
NH
X= O :12
= S :13
(II) DDQ, 1Hr
(12%)
N
NH
NH NH
X, X'=O: 6 (8%)
X, X'=S : 7 (9%)
X
X= O :14
= S :15
Scheme 1. Synthesis of oxacorroles 3–7
NH
HN
HN
O
O
HN
X
HN
O
N
NH
N
NH
N
NH
X= NH: 3
X= O : 4
X= S : 5
HN
HN
O
X
X'
N
NH
X, X'= O: 6
X, X'= S : 7
Chart 1. Structures of 22-oxacorroles
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SYNTHESIS OF A2B-TYPE 22-OXACORROLES BEARING TWO DIFFERENT FIVE-MEMBERED HETEROCYCLES
5
1
1
Fig. 1. Comparison of (a) H NMR spectra of compound 3–5 in the selected region; (b) H–1H COSY spectrum of compound 3
recorded in CDCl3 at room temperature
meso substituents present. For example, the inner NH
proton in 3 was observed at -1.60 ppm which experienced
upfield shift in 4 and 5 and appeared at ~ -2.00 ppm.
Similarly, the b-furan protons in 3 appeared as two
doublets at 9.13 and 9.15 ppm experienced downfield
shifts in compounds 4 and 5, appearing at ~9.20 and ~9.25
ppm respectively. Thus, the five-membered heterocycles
at the meso positions alter the electronic properties of
22-oxacorroles.
in Table 2. In general, meso-substituted 22-oxacorroles
showed one strong Soret band in the region of 410–
425 nm and four weak Q bands in the region of 480–
680 nm. It is clear from the data presented in Table 2 that
compounds1and2exhibitedoneSoretbandat412nmand
four well-defined Q bands at 496, 529, 582 and 634 nm.
This indicates that the presence of one pyrrole group in
place of the aryl group at the meso position doesn’t alter
the electronic properties. However, upon the introduction
of two or three five-membered heterocycles such as
pyrrole/thiophene/furan in place of the six-membered
aryl groups at the meso positions, bathochromic shifts
were shown in both Soret and Q-band absorption maxima,
indicating the alteration of the electronic properties of the
22-oxacorrole macrocycles.
Photophysical and electrochemical properties
The absorption, fluorescence and electrochemical
properties of meso-substituted 22-oxacorroles 3–7
were studied along with 22-oxacorroles 1 and 2. The
comparison of absorption spectra of compounds 3–5 is
presented in Fig. 2a and the data of all 22-oxacorroles 3–7
along with reference 22-oxacorroles 1 and 2 are presented
The steady state fluorescence spectra of 22-oxacorroles
1–7 were recorded in CHCl3 and the data are included in
Table 2. A comparison of 22-oxacorroles 1–5 is presented
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6
U. BOORUGA AND M. RAVIKANTH
Fig. 2. (a) Comparison of absorption spectra of 3, 4 and 5 recorded in CHCl3. The concentrations used were 1 × 10-5 M; (b) comparison
of fluorescence spectra of 1–5 recorded in CHCl3. The concentrations used were 1 × 10-5 M; (c) electrochemical redox data (V) of
compounds 1 and 3 recorded in CH2Cl2 containing 0.1 M TBAP as supporting electrolyte using a scan rate of 50 mV/s. E1/2 values
reported are relative to SCE
Table 1. Comparison of 1H NMR chemical shift values (in ppm) of compound 1–7 recorded in CDCl3 at room temperature
Com
β-furyl
β-pyrrole
Inner
–NH
Hd
He
Hf7
Hg6
Hh5
Hi8
Hj3
Hk4
1
2
3
4
5
6
7
9.03 (d)
9.02 (d)
9.15 (s)
9.20 (d)
9.11 (m)
9.10 (d)
8.61 (s)
8.82 (d)
9.10 (d)
8.95 (d)
9.18 (d)
9.09 (d)
8.39 (d)
8.39 (d)
9.05 (d)
8.52 (d)
8.50 (d)
8.91 (d)
8.55 (d)
9.10 (d)
8.61 (s)
8.57 (d)
8.79 (m)
8.61 (d)
8.99 (m)
9.02 (m)
8.25 (m)
8.61 (s)
8.80 (d)
8.91 (d)
8.85 (d)
8.99 (m)
9.01 (d)
8.39 (d)
8.39 (d)
8.82 (d)
8.79 (d)
8.43 (d)
8.59 (d)
8.56 (m)
8.40 (d)
8.29 (d)
9.09 (d)
8.20 (d)
8.90 (d)
9.05 (d)
9.02 (m)
8.25 (d)
8.29 (d)
8.78 (d)
8.19 (m)
8.89 (d)
8.99 (m)
9.02 (m)
8.19 (d)
8.61 (s)
-1.91 (bs)
-1.85 (bs)
-1.70 (bs)
-1.99 (bs)
-1.92 (bs)
-2.00 (bs)
-1.90 (bs)
in Fig. 2b. Compound 1 is decently fluorescent and
exhibits one band at 643 nm with a fluorescence quantum
yield of 0.34. Upon introduction of one pyrrole in place
of the aryl group at the meso position of 22-oxacorrole,
the fluorescence maxima remained unaltered but the
fluorescence quantum yield was reduced by ~50%.
Compounds 3–5 are weakly fluorescent. The fluorescence
maxima were bathochromically shifted by 10–35 nm
compared to 1 and 2 and the fluorescence quantum
yields were significantly reduced. The 22-oxacorroles 6
and 7 were non-fluorescent. Thus, the presence of five-
membered heterocycles in place of six-membered aryl
groups at the meso positions decreases the fluorescence
properties of 22-oxacorroles.
The electrochemical properties of 22-oxacorroles
1–7 were investigated by cyclic voltammetry (CV) in
dichloromethane using tetrabutylammonium perchlorate
as supporting electrolyte. A comparison of cyclic
voltammograms of oxacorrole 3 and oxacorrole 1 is
shown in Fig. 2d and the data for compounds 1–5 is
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SYNTHESIS OF A2B-TYPE 22-OXACORROLES BEARING TWO DIFFERENT FIVE-MEMBERED HETEROCYCLES
7
Table 2. Photo physical data of Compounds 3–7 along with 1 and 2
-1
Q band (nm) labs /nm (log e/M-1 cm )
lem
F
.
Compound
Soret band (nm)
412 (5.18)
1
496 (4.03), 529 (3.94), 582 (3.62), 634 (4.01)
643
0.34
2
3
4
5
6
7
412 (5.10)
423 (5.00)
422 (3.25)
423 (2.05)
423 (3.05)
423 (3.29)
456 (4.01), 497 (4.00), 533 (6.50), 634 (3.58)
504 (4.00), 537 (2.99), 599 (3.01), 653 (4.01)
508 (3.56), 538 (2.01), 600 (2.86), 654 (3.21)
505 (2.15), 537 (1.50), 593 (1.90), 645 (2.00)
507 (3.55), 534 (3.62), 589 (3.60), 640 (4.02)
506 (3.21), 533 (2.51), 584 (3.61), 639 (4.00)
643
643
677
655
—
0.15
0.12
0.14
0.07
—
—
—
Table 3. Electrochemical redox data (V) of Compounds 1–7
recorded in dichloromethane containing 0.1 M TBAP as
supporting electrolyte using scan rate of 50 mV/s. E1/2 values
reported are relative to SCE
the region of 410–650 with low fluorescence quantum
yields (0.05–0.12). The absorption maxima were found
to be dependent on the meso-heteroaryl substituents
and the electrochemical studies reveal that the redox
properties of 22-oxacorroles were significantly altered
by replacing six-membered meso-aryl groups with
different heterocycles. The oxacorroles are easier
to oxidize and also easier to reduce than triphenyl
22-oxacorroles.
Compound
I
II&III
I
II
Eox (V)
Eox (V)
Ered (V)
Ered (V)
1
2
3
4
5
6
7
0.90
0.77
0.55
0.53
0.60
—
1.10, 1.47
1.04, 1.24
0.96
-1.70
-1.33
-1.16
-1.14
-1.17
—
-1.93
—
-1.44
-1.10
-1.10
—
Acknowledgments
0.96
UB thanks the UGC, New Delhi and MR thanks the
government of India for funding CSIR project No. 01
(2850)/16/EMR-II.
0.97
—
—
—
—
—
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presented in Table 3. Unlike compound 1, compound 3
showed four redox processes: two reversible oxidations
and two reversible or quasi-reversible reductions. The
comparison of redox potentials of compound 3 with
those of compound 1 indicated that compound 3 is easier
to oxidize and also easier to reduce than compound 1.
Furthermore, compounds 6 and 7 are not stable under the
redox conditions.
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