Synthesis of β-Thiophene-Substituted 21,23-Dithiaporphyrins

Article abstract of DOI:10.1246/cl.2003.744

β-Thiophene-substituted 21,23-dithiaporphyrins with substituents like methyl and phenyl groups were synthesized and characterized.

Full text of DOI:10.1246/cl.2003.744

744
Chemistry Letters Vol.32, No.8 (2003)
Synthesis of ꢀ-Thiophene-Substituted 21,23-Dithiaporphyrins
Neeraj Agarwal, Sarada P. Mishra, A. Kumar, and M. Ravikanthꢀ
Department of Chemistry, Indian Institute of Technology, Powai, Mumbai-400076, India
(Received May 15, 2003; CL-030424)
b-Thiophene-substituted 21,23-dithiaporphyrins with sub-
work up with ethyl acetate. The selective debromination of 5
to get 3,4-dibromo thiophene 6 was carried out by treating 5
with Zn powder in glacial acetic acid/water mixture (3:2) and
purified by vacuum distillation. The 3,4-dimethyl thiophene 7
was obtained by treating 6 with n-BuLi followed by addition
of excess cold dimethyl sulphate. Treating 6 with phenyl mag-
nesium bromide in ether in the presence of [1,3-bis(diphenyl-
phosphino) propane]dichloronickel(II) (NidpppCl2) gave 3,4-di-
phenylthiophene 8. The unknown diols 9 and 10 were
synthesized4 by treating the 2,5-dilithiated salt of 7 or 8 with
benzaldehyde in THF at 0 ꢁC. The diols were purified by silica
gel column chromatography using petroleum ether/ethyl acetate
and afforded diol 9 in 17% and diol 10 in 28% yields as white
crystalline solids. The diols 9 and 10 were characterized by mp,
IR, mass and NMR spectroscopic techniques.
stituents like methyl and phenyl groups were synthesized and
characterized.
The electronic properties of porphyrins can be tuned at will
by introducing suitable substituents at meso- or b-positions. The
porphyrins synthesized till today possess aryls groups at meso
positions because they can be synthesized easily and they are
very stable to handle for various applications. The introduction
of substituents at b-positions alters the electronic properties tre-
mendously since the substituents at b-positions are in direct
conjugation with the porphyrin ring. There are several reports
on N4 porphyrins having both electron withdrawing and elec-
tron releasing substituents at b-positions and investigated the
spectral and catalytic properties in detail.1 Although there is ex-
tensive literature available on b-substituted N4 porphyrins, the
reports on b-substitution of core-modified porphyrins having
porphyrin cores such as N3O, N3S, N2S2, N2O2 etc are very
scarce. The N2S2 porphyrin such as 1 (S2TPP) has two kinds
of b-positions; b-pyrroles and b-thiophenes at which the sub-
stituents can be introduced. We recently synthesized some b-
pyrrole-substituted 21,23-dithiaporphyrins2 such as 2 and com-
pared the electronic properties with b-unsubstituted S2TPP 1. In
this paper, we report the synthesis of b-thiophene-substituted
N2S2 porphyrins 3 and 4. The comparision of electronic proper-
ties of 3 and 4 with that of b-pyrrole-substituted 21,23-dithia-
porphyrin 2 indicates that the electronic properties were more
altered by introducing substituents at b-thiophene carbons than
b-pyrrole carbons.
N
S
N
S
S
N
S
N
1
2
H3C
H3C
N
S
S
N
N
S
S
N
CH3
CH3
3
4
The N2S2 porphyrins 3 and 4 were synthesized by condens-
ing one equivalent of corresponding diol 9 and 10 respectively
with one equivalent of pyrrole in CH2Cl2 in the presence of cat-
alytic amount of BF3ꢂOEt2 at room temperature followed by ox-
idation with DDQ (Scheme 1).
The required thiophene precursors 3,4-dimethyl thiophene
and 3,4-diphenyl thiophene were synthesized3 as shown in
Scheme 1. The 2,3,4,5-tetrabromothiophene 5 was synthesized
by refluxing thiophene in CHCl3 with excess Br2 followed by
The crude porphyrins 3 and 4 were purified by silica gel
column chromatography and characterized by NMR, mass,
CH3
H3C
Br
Br
Br
Br
n-BuLi/ether
5
1
Zn powder
AcOH/H2O
C,H,N,S analysis and absorption spectroscopy. The H NMR
spectrum of 3 in which methyl groups were present at b-thio-
phene carbons showed a singlet for pyrrole protons which
was as upfield shifted by 0.42ppm (appears at 8.26 ppm) com-
pared to the pyrrole protons of 1 (appears at 8.67 ppm)4 indicat-
ing that the methyl groups at b-thiophene carbons alters the p-
delocalization (Table 1). The pyrrole protons of 4 in which phe-
nyls were introduced at b-thiophene carbons were further up-
field shifted (appears at 8.10 ppm) due to more effective altera-
tion of p-delocalization caused by phenyl groups at b-thiophene
carbons. The mass spectra showed Mþ ion peak confirming the
composition of 3 and 4.
Br2, CHCl3
reflux
Me2SO4
S
Br
S
S
Br
S
7
PhMgBr
Ph
Ph
5
6
8
S
R
R
R
R
R
CHO
R
THF
+
+
Li
S
S
S
H
Li
H
H
OH
OH
OH
9 (R=CH3) or 10 (R=Ph)
7 or 8
R
R
BF3.OEt2
+
3 or 4
N
CH2Cl2, RT, DDQ
S
H
H
H
OH
OH
The absorption spectra of 1, 2, 3 and 4 recorded in toluene
for both Q-bands and Soret band are presented in Figure 1 and
the data is tabulated in Table 1.
Scheme 1. Synthetic scheme for the preparation of 3,4-disub-
stituted thiophenes, corresponding diols and b-thiophene substi-
tuted porphyrins 3 and 4.
Copyright Ó 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.8 (2003)
745
Table 1. 1H NMR chemical shifts of thiophene and pyrrole
protons in CDCl3 and absorption data in toluene for porphyrins
1-4
maximum red shifts observed for 4 indicates that the introduc-
tion of bulky substituents at b-thiophene carbons alters the en-
ergy levels more effectively compared to the introduction of the
same substituents at b-pyrrole carbons.
In conclusion, we prepared b-thiophene-substituted 21,23-
dithiaporphyrins under mild conditions. The comparison of
electronic properties of b-thiophene-substituted porphyrins with
that of previously prepared b-pyrrole-substituted 21,23-dithia-
porphyrins indicated that the substituents at b-thiophene car-
bons alter p-delocalization of porphyrin ring more effectively
compared to same substituents at b-pyrrole carbons.
Porphyrin
1HNMR (d in ppm) Soret
Thiophene pyrrole
Absorption
Q-bands lnm (e ꢃ 10ꢄ4
)
I
IV
III
II
1 9.67(s)
8.67(s)
435
(25) (2.6) (0.70) (0.22) (0.45)
514
547
633
696
2 9.23(s)
3 —
—
440
(21.2) (1.9)
444 524
(7.5) (0.85)
454 524
(2.9) (0.69)
519
—
—
—
—
—
—
637
(0.22) (0.31)
640 707
(0.15) (0.15)
654 715
(0.17) (0.12)
700
8.26(s)
8.10(s)
Financial assistance from Department of Science and
Technology to MR is greatfully acknowledged.
4 —
References and Notes
1
a) G. Hariprasad, S. Dahal, and B. G. Maiya, J. Chem. Soc.,
Dalton Trans., 1996, 3429. b) P. E. Ellis and J. E. Lyons,
Coord. Chem. Rev., 105, 181 (1990). c) P. Bhyrappa and
V. Krishnan, Inorg. Chem., 30, 239 (1991). d) F. D’Souza,
G. R. Deviprasad, and M. E. Zandler, J. Chem. Soc., Dalton
Trans., 1997, 3699.
2a) M. Ravikanth, Chem. Lett., 2000, 480. b) N. Agarwal, S.
P. Mishra, A. Kumar, C.-H. Hung, and M. Ravikanth, J.
Chem. Soc., Chem. Commun., 2002, 2642.
3
4
5
K. Tamao, S. Kodama, I. Nakajima, M. Kumada, A. Minato,
and K. Suzuki, Tetrahedron, 38, 3347 (1982).
A. Ulman and J. Manassen, J. Chem. Soc., Perkin Trans. 1,
1979, 106.
Figure 1. Q-bands and Soret band (inset) absorp-
tion spectra of 1-4 recorded in toluene.
Characterization of porphyrin 3: 1H NMR (CDCl3, d in
ppm) 2.61 (s, 12H, CH3), 7.74 (m, 12H, phenyl), 8.12 (m,
8H, phenyl), 8.26 (s, 4H, b-pyrrole). 13C NMR (77.1 MHz,
CDCl3, d in ppm) 18.44, 29.87, 32.10, 127.23, 127.35,
132.19, 133.75, 143.07, 144.67. Anal. Calcd: C, 81.78; H,
5.15; N, 3.97; S, 9.10. Found: C, 80.87; H, 5.27; N, 3.95;
S, 9.01. ESI-MS C48H36N2S2 Calcd, av. mass: 704.94; obsd
Porphyrin 2 in which phenyls were introduced at b-pyrrole
carbons exhibited three well defined Q-bands and one Soret
band which were broad and red shifted compared to unsubstitut-
ed parent S2TPP. The introduction of methyl groups at b-thio-
phene carbons as in 3 resulted in more broadening and further
red shifts of absorption bands. However as shown in Figure 1,
the maximum shifts and broadening of absorption bands were
observed for 4 in which phenyl groups were present at b-thio-
phene carbons. The presence of bulky substituents at the b-po-
sitions (pyrrole and thiophene carbons) alters the energy levels
resulted in broadening and red shifts of absorption bands. The
1
m=z: 704.87. 4: H NMR (CDCl3, d in ppm) 7.54-7.91 (m,
40H, phenyl), 8.10 (s, 4H, b-pyrrole). Anal. Calcd: C, 85.68;
H, 4.65; N, 2.94; S, 6.73. Found: C, 86.28; H, 4.93; N, 3.13;
S, 6.82. ESI-MS C68H44N2S2 calcd av, mass: 953.22; obsd
m=z: 953.33.
Published on the web (Advance View) July 21, 2003; DOI 10.1246/cl.2003.744