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Z. Xue et al. / Tetrahedron 67 (2011) 6030e6035
Cary 300 spectrophotometer. Fluorescence spectra were recorded
on a Jasco FP-6300 spectrometer.
4.2. Synthetic procedures and analytical data
4.2.1. Synthesis of 5,10,15,20-tetrakis(4-nitrophenyl)-21H,23H-por-
phyrin (5).
4.2.1.1. Via nitration method. A typical experimental procedure
(e.g., H2TPP/fuming nitric acid¼1:35) is given below. Under an ar-
gon atmosphere, red fuming HNO3 (2.4 mL, 56.7 mmol,) was added
dropwise over a period of 20 min at 0 ꢁC to a 150 mL chloroform
solution of H2TPP (1.0 g, 1.62 mmol). The reaction was kept for
30 min and then quenched with an aqueous ammonia solution
slowly. The organic layer was extracted and evaporated to dryness
under reduced pressure. The residual brown powder was washed
with boiling water and then subjected to a column separation
process. In the process, the powder was first absorbed onto silica
gel followed by an elution with ethyl acetate. The remained dark
red band was collected and added into a KOH solution (5% w/v,
molar ratio of KOH to silica gel is 2) to remove silica gel. Centrifu-
gation along with repetitive washing with D. I. water afforded the
pure product in 87% yield (1.12 g). 1H NMR (CDCl3, 400 MHz, ppm):
Fig. 4. Proposed reaction mechanism.
which may stand for such impurities from oxidative degradation.
Because of this, we have probed the change of the reaction with
relation to some extreme conditions. For example, we added a bit
largely excess of fuming nitric acid to a chloroform solution of
H2TPP at room temperature and observed that the dark red solu-
tion gradually turned into clearly light red. Its UVevis spectrum
exhibited no typical Soret band absorption at all, suggesting a result
of degradation of the porphyrin macrocycle. It was realized that
only in an ice-bath under an inert atmosphere was the aryl nitra-
tion of H2TPP largely controlled.
d
8.83 (s, 8H, pyrrole alkene protons), 8.68 (d, J¼8.0 Hz, 8H, protons
ortho to NO2), 8.40 (d, J¼8.0 Hz, 8H, protons meta to NO2), ꢀ2.83 (s,
2H, protons inside porphyrin macrocycle). LR-MS (FAB, m/z): calcd
Mþ: 794.2; found: no obvious identifiable peak. MALDI-TOF (m/z):
calcd Mþ: 794.1868; found [MþH]þ: 795.467. UVevis (CHCl3, at
saturated concentration, lmax, nm): 424, 517, 552, 591, 647. Fluo-
rescence (CHCl3, at saturated concentration, nm): lex 424, lem 652.
3. Conclusion
Synthetic approaches to functionalized porphyrins based on
condensation (e.g., Rothemund and Lindsey’s) areusually multi-step
processes.5,6,12 On the contrary, nitration of tetra-arylporphyrins
leading to highly substituted derivatives is of great importance
particularly in view of its simplicity, yield, and well regiospecific
control. In this paper, we have made a further insight into a very
important nitration process of H2TPP toward a series of useful nitro-
porphyrin compounds. We confirmed that the tetra-substituted
derivative is in fact a major product rather than being degraded in
the process. The porphyrin product can be isolated with a remark-
able yield of nearly 90% based on an unusual separation technique
(practical for its large scale production). Because of its low solubility,
previous characterizations were usually vague. In this work, a sub-
stantial characterization of both the nitration and condensation
products was presented and evaluated. The proposed ortho-effect
model can be used to interpret the progress of the nitration reaction
reasonably. More detailed mechanism study is now underway.
4.2.1.2. Via condensation of 4-nitrobenzaldehyde and pyrro-
le. The synthesis was based on the condensation of nitro-
benzaldehyde with pyrrole according to the method described in
the literature.12 The product yield was w4%. 1H NMR (CDCl3,
400 MHz, ppm):
d 8.83 (s, 8H, pyrrole alkene protons), 8.68 (d,
J¼8.0 Hz, 8H, protons ortho to NO2), 8.41(d, J¼8.0 Hz, 8H, protons
meta to NO2), ꢀ2.84 (s, 2H, protons inside porphyrin macrocycle).
LR-MS (FAB, m/z): calcd Mþ 794.2; found: no identifiable peak.
MALDI-TOF (m/z): calcd Mþ: 794.1868; found [MþH]þ: 795.1970.
UVevis (CHCl3, at saturated concentration, lmax, nm): 425, 518, 552,
593, 648. Fluorescence (CHCl3, at saturated concentration, nm): lex
424, lem 651.
4.2.2. Synthesis of 5,10,15,20-tetrakis(4-aminophenyl)-21H,23H-por-
phyrin (H2TAPP) via reduction of 5. The synthesis followed pub-
lished procedures.13 Here, we report its 1H NMR and FAB-mass
characterization data only.
4. Experimental section
4.1. General
4.2.2.1. From nitration method. 1H NMR (DMSO-d6, 400 MHz,
ppm):
d
8.88(s, 8H, pyrrole alkene protons), 7.84(d, 8H, J¼5.1 Hz,
protons ortho to NH2 groups), 7.00(d, 8H, J¼5.1 Hz, protons meta to
NH2 groups), 5.56 (s, 8H, anilinic protons), ꢀ2.74(s, 2H, protons
inside porphyrin macrocycle). LR-MS (FAB, m-nitrobenzyl alcohol,
m/z): calcd Mþ: 674.3; found [MþH]þ: 675.0.
Chloroform was pre-washed with water to remove trace
amount of ethanol stabilizer, dried with magnesium sulfate and
then distilled over calcium hydride. Pyrrole was freshly distilled
from calcium hydride before use. Tetraphenylporphyrin (H2TPP)
was prepared according to standard procedures. Other starting
materials were used as received. 1H NMR spectra were recorded on
a Bruker Avance DRX-400 NMR spectrometer in deuterated sol-
vents. Tetramethylsilane (TMS) and the protic residues of the
solvents were used as internal references for the NMR analyses.
Low-resolution mass spectra (LR-MS) were recorded on a Finnigan
MAT SSQ-710 mass spectrometer, in positive-ion mode, where m-
nitrobenzyl alcohol was chosen as the matrix. High-resolution mass
spectra (HR-MS) were recorded on a Bruker Biflex III matrix-
assisted laser desorption ionization time-of-flight (MALDI-TOF)
mass spectrometer, in m/z. UVevis Spectra were recorded on a OLIS
4.2.2.2. From condensation method. 1H NMR (DMSO-d6,
400 MHz, ppm):
d 8.88 (s, 8H, pyrrole alkene protons), 7.85 (d, 8H,
J¼5.1 Hz, protons ortho to NH2 groups), 7.00 (d, 8H, J¼5.1 Hz, pro-
tons meta to NH2 groups), 5.56 (s, 8H, anilinic protons), ꢀ2.74(s, 2H,
protons inside porphyrin macrocycle). LR-MS (FAB, m-nitrobenzyl
alcohol, m/z): calcd Mþ: 674.3; found [MþH]þ: 675.1.
Acknowledgements
The work described in this paper was partially supported by the
Research Grants Council of Hong Kong (Project Nos.: PolyU 5140/06E