Studies on selective β/β bromination of π-extended porphyrins and subsequent coupling reactions

Article abstract of DOI:10.1007/s11164-013-1048-9

Monobromination of β, β-π-extended porphyrins was found to selectively occur at the β or β position of the porphyrins which is antipodal to the fused aromatic ring. Subsequent Sonogashira or Heck coupling of the resultant bromoporphyrin introduced a carboxylphenylethynyl group or an acrylic acid group to the π-extended porphyrin. The optimal reaction conditions were found for the Sonogashira and Heck coupling reaction. All of the coupling products have shown a broadening and red-shift of the Soret band and Q bands in the UV-Vis absorption spectra compared with the π-extended porphyrin starting materials and the original unmodified porphyrins.

Full text of DOI:10.1007/s11164-013-1048-9

Res Chem Intermed
DOI 10.1007/s11164-013-1048-9
Studies on selective b/b0 bromination of p-extended
porphyrins and subsequent coupling reactions
•
•
•
Jiaying Yan Yaqing Feng Nuonuo Zhang
Bao Zhang
Received: 10 October 2012 / Accepted: 11 January 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract Monobromination of b, b0-p-extended porphyrins was found to selec-
tively occur at the b or b0 position of the porphyrins which is antipodal to the fused
aromatic ring. Subsequent Sonogashira or Heck coupling of the resultant bro-
moporphyrin introduced a carboxylphenylethynyl group or an acrylic acid group to
the p-extended porphyrin. The optimal reaction conditions were found for the
Sonogashira and Heck coupling reaction. All of the coupling products have shown a
broadening and red-shift of the Soret band and Q bands in the UV–Vis absorption
spectra compared with the p-extended porphyrin starting materials and the original
unmodified porphyrins.
Keywords Large p-aromatic ꢀ Porphyrin ꢀ Bromination ꢀ Selectivity ꢀ Sonogashira ꢀ
Heck
Introduction
Porphyrin derivatives have been widely used in catalysis [1], photodynamic therapy
[2], organic light-emitting diodes [3], and dye-sensitized solar cells (DSSCs) [4] due
to their vital roles in natural photosynthetic system and strong absorption in the
visible region, as well as the readily tunable optical, photophysical, and electro-
chemical properties by peripheral substitutions [5]. In recent years, intensive efforts
have been directed at developing efficient porphyrin sensitizers used in DSSCs.
A light-to-electric power-conversion efficiency (g) of over 12 % has been realized
Electronic supplementary material The online version of this article (doi:
10.1007/s11164-013-1048-9) contains supplementary material, which is available to authorized users.
J. Yan ꢀ Y. Feng ꢀ N. Zhang ꢀ B. Zhang (&)
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
e-mail: baozhang@tju.edu.cn
123

J. Yan et al.
recently for porphyrin-based DSSCs [6], which is the highest g ever reported for
DSSCs. However, it is believed that there is still room for further improvement of g
for porphyrin-sensitized solar cells, since the strong Soret (400–450 nm) and
moderate Q bands (550–600 nm) absorptions of porphyrins only cover a small part
of the whole solar light spectrum. A strategy to increase the light-harvesting
capabilities of porphyrins is to expand the porphyrin p-systems via aromatic ring
fusion. For example, Imahori [7] has reported the construction of meso- and
b-naphthalene-fused porphyrins, and aromatic b, b0 edge-fused porphyrins;
significant improvements of the light-harvesting capabilities by these p-extension
strategies have been recorded. Most studies on the p-extended porphyrins reported
to date involved porphyrin sensitizers with the fused aromatic ring also as the
carboxyl linkage. However, the steric hindrance introduced by the fused aromatic
ring may prohibit the efficient adsorption of sensitizers on the surface of the TiO2
particles, which is detrimental to the conversion efficiency of the resultant DSSCs.
Therefore, in an ongoing project on the synthesis of porphyrin-based sensitizers and
their application in DSSCs, we decided to design a b, b0- p-extended porphyrin
sensitizer with the fused ring and the carboxylic acid anchoring group on the
opposite sites of porphyrin. Furthermore, the p-system can be further elongated via
the unsaturated double or triple bond linkage. To realize this goal, a strategy
involving bromination of the p-extended porphyrins and subsequent Pd(0)-mediated
C–C coupling reaction such as Sonogashira or Heck coupling can be employed.
However, to the best of our knowledge, studies on the modification of the
p-extended porphyrins [8, 9] are limited. In particular, the Sonogashira and Heck
coupling of the selectively monobromo-substituted porphyrins with aromatic fused
rings have not been reported. We have investigated the selective b or b0 monobro-
mination of the p-extended porphyrins, and the subsequent Sonogashira and Heck
couplings, and here we want to report our preliminary results. A linear carboxylphen-
ylethynyl group or a less linear acrylic acid group was introduced to the b or b0
position of the porphyrin, which is antipodal to the fused aromatic ring. The
UV–Vis absorption spectra of the coupling products, the p-extended porphyrins, and
the original unmodified porphyrins were compared, which shows a broadened and
red-shifted Soret band and Q bands in the UV–Vis absorption for the coupling
products.
Results and discussion
It was determined that the p-extended porphyrin 1a reported by Cavaleiro [10, 11] is
a good compound with which to start our studies since it can be easily obtained by
Diels–Alder reaction of Ni(II)-2-vinyl-5,10,15,20-tetraphenylporphyrin with 1,4-
naphthoquinone and subsequent demetallation. Ni(II)-2-vinyl-5,10,15,20-tetraphen-
ylporphyrin was prepared according to a literature procedure [12, 13]. Interestingly,
it was found that the Diels–Alder reaction also proceeded very well with
the freebase and 1,4-naphthoquinone. The total yield is similar for these two
approaches starting from 5,10,15,20-tetraphenylporphyrin. Furthermore, it was
found that the reaction can also be applied to 2-vinyl-5,10,15,20-tetra- (4-tert-
123

Studies on selective b/b0 bromination
Scheme 1 Synthesis of monobromo-substituted porphyrin 2
butylphenyl)porphyrin or its Ni(II) complex as shown in Scheme 1. The yield of 1d
starting from the Ni(II) complex of 2-vinyl porphyrin is 25 % and removal of the
metal afforded compound 1b quantitatively.
Bromination of the p-extended porphyrin 1 with 1.1 eq. N-bromosuccinimide
(NBS) in refluxing chloroform afforded monobrominated porphyrin 2 as a
mixture of two inseparable regioisomers in 50 % combined yield. Monobromin-
ation selectively occurred to the b or b0 position of the porphyrin which is
opposite to the fused aromatic ring.1 It is believed that the metal-free porphyrin
is required for the selective b bromination, which results from the bond-fixing
entity of porphyrin caused by the b-b0-fused aromatic ring [14, 15]. In order to
1
General procedure for bromination: Compound 1 (25.2 lmol) with N-bromosuccinimide (NBS)
(4.5 mg, 25.2 l mol) were dissolved in CHCl3 (5 mL) and the resulting solution was refluxed for 1 h.
After the reaction, the solvent was removed in vacuuo and the residue was then submitted to preparative
TLC using DCM/PE as eluent. The two regioisomeric monobromo porphyrins were obtained as an
inseparable mixture in CHCl3 and the resulting solution was heated at reflux for 1 h. The resulting
mixture was submitted to preparative TLC using dichloromethane (DCM)/petroleum (PE) as eluent, and
we get a mixture of momobromination porphyrin. Compound 2b: Column chromatography DCM:PE =
1:4–1:1, yield:46.5 %, UV/Vis (THF): kmax(nm)(e, 103 M-1 cm-1): 465 (144.5), 635 (12), 707 (8). 1H
NMR (400 MHz, CDCl3) d 9.92 (s, 1H), 9.71 (d, J = 4.7 Hz, 1H), 9.21 (s, 1H), 9.06 (d, J = 8.4 Hz, 1H),
8.91 (d, J = 4.7 Hz, 1H), 8.75–8.65 (m, 3H), 8.65–8.43 (m, 3H), 8.40 (d, J = 7.2 Hz, 1H), 8.10
(d, J = 6.8 Hz, 1H), 7.92 (s, 3H), 7.86–7.67 (m, 5H), 7.65–7.50 (m, 3H), 7.22 (d, J = 8.0 Hz, 1H), 1.74
(s, 9H), 1.62 (m, 18H), 1.57 (s, 9H), -1.07 (s, 1H), -1.24 (s, 1H). MALDI-TOF: m/z calcd for
C72H63BrN4O2 (M?), 1,094.4134; found, 1,094.1184
123

J. Yan et al.
Scheme 2 Sonogashira and Heck coupling reaction of bromo-substituted porphyrin 3
introduce a carboxylic group which can act as an anchorage in the later studies
on the DSSC and further elongate the porphyrin p-system, we investigate the
Sonogashira or Heck coupling reactions of the prepared monobromo-substituted
p-extended porphyrins with methyl 4-ethynylbenzoate and methyl acrylate, res-
pectively (Scheme 2).
We first examined the reaction of metal-free monobromo-substituted p-extended
porphyrin 2a or 2b with methyl 4-ethynylbenzoate under standard Sonogashira
coupling reaction conditions [16–18]. The reaction was carried out by using
PdCl2(Ph3P)2 as catalyst and CuI as co-catalyst in the presence of Et3N in
refluxing THF [19]. It was found by thin layer chromatography (TLC) analysis that
a complex mixture resulted and no starting material remained. Furthermore, no
major products can be isolated from the complex mixture. It was then decided
to make a porphyrin metal complex. After metallation of the porphyrin with
Zn(OAc)2 2H2O, the resultant porphyrin complex 3a was treated with methyl
4-ethynylbenzoate under the above-mentioned Sonogashira coupling reaction
conditions. Trace amounts of coupling products were produced after 24 h and most
of the starting materials were recovered. Interestingly, by changing PdCl2(Ph3P)2
to Pd2(dba)3 and using AsPh3 as ligand [20], in the absence of CuI, the coupling
product 4a was obtained in about 30 % yield. Base hydrolysis of 4 with KOH in
the mixed solvent (THF/EtOH/H2O = 4:4:1) furnished the carboxylic acid group
123

Studies on selective b/b0 bromination
Table 1 Heck coupling reaction of 3a with methyl acrylate
Entry
T(°C)
Catalyst (ligand)
Isolated yield (%)
Material recovered (%)
1
2
3
4
5
6
40
60
Pd2(dba)3 (AsPh3)
Pd2(dba)3 (AsPh3)
Pd2(dba)3 (AsPh3)
Pd2(dba)3 (AsPh3)
Pd(PPh3)4
8
9
90
88
90
54
0
36
120
90
0
47
82
44
90
Pd(OAc)2 (PPh3)
Trace
DMF was used as solvent and NEt3 as the base
in 5 as shown in Scheme 2.2 Unfortunately, the two isomers in 5a or 5b are still
inseparable at this stage.
To introduce a less linear double bond linkage, we next investigated the Heck
coupling reaction of 3 with methyl acrylate [21–23]. On the basis of the results
obtained from the above Sonogashira coupling reaction, the Heck coupling reaction
was first examined by employing Pd2(dba)3 as catalyst, AsPh3 as additional ligand,
and NEt3 as the base. In this reaction, DMF was used as the solvent. The studies
began with the determination of the optimal temperature for the coupling reaction of
3a as shown in Table 1 (entries 1–4). It was found that the best result was obtained at
90 °C; the coupling product 6a was obtained in 54 % yield and 36 % starting
material recovered.3 At low temperature, such as 40 and 60 °C, most starting
materials were recovered and only trace amounts of the coupling product produced.
At the optimal temperature, we examined the influence of the catalyst on the
coupling reaction. Both Pd(0) and its precursor Pd(II) were evaluated in the presence
of a proper base at 90 °C (Table 1, entries 5 and 6).4 The highest yield (82 %) of
coupling product was achieved with air-stable Pd(OAc)2 as pre-catalyst, PPh3 as both
2
General procedure for Sonogashira reaction: To
a solution of 3 (30.0 l mol) and methyl
4-ethynylbenzoate (48.0 mg, 0.3 mmol) in 10 ml dry THF was added Pd2(dba)3 (13.8 mg, 15 l mol),
AsPh3(3.7 mg, 12.0 l mol) in a glove box under N2. NEt3 (2 mL) was then added respectively via a
syringe under N2. After the resulting mixture was refluxed for 24 h under N2, the solvent was removed in
vacuo. The residue was submitted to preparative TLC using dichloromethane as eluent to provide the
coupling product. Representative data: compound 4a (9.1 mg, 30.0 % yield). UV/Vis (THF):
k
max(nm)(e,103 M-1 cm-1): 476 (171), 612 (14.7), 678.4 (11). 1H NMR (400 MHz, DMSO-d6) d 9.97
(s, 1H), 9.60 (d, J = 4.6 Hz, 0.66 H), 9.54 (d, J = 4.6 Hz, 0.33H), 9.43 (d, J = 8.4 Hz, 1H), 9.19–10 (m,
1H), 8.81 (d, J = 14.3 Hz, 1H), 8.75–8.49 (m, 5H), 8.40 (d, J = 7.3 Hz, 1H), 8.30 (d, J = 6.8 Hz, 1H),
8.18–8.05 (m, 2H), 8.04–7.53 (m, 15H), 7.48 (d, J = 8.2 Hz, 2H), 7.35–7.26 (m, 1H), 7.25–7.16 (m, 1H),
3.91 (s, 3H). MALDI-TOF: m/z calcd for C66H36N4O4Zn (M?), 1,012.2028; found, 1,012.0293
3
General procedure for Heck reaction: A solution of 3 (11.9 l mol), Pd(OAc)2 (0.11 mg, 0.5 l mol),
PPh3(0.83 mg, 2.7 l mol), methyl acrylate (0.3 mmol) in 5 ml dry DMF and 2 ml NEt3, and the resulting
solution was heated for 12 h under the nitrogen atmosphere. Then, the solvent was evaporated to dryness.
The resulting mixture was submitted to preparative TLC using dichloromethane as eluent to give the
coupling product. Representative data: compound 6a (9.1 mg, 82.0 % yield). UV/Vis (THF): kmax(nm)(e,
1
103 M-1 cm-1): 474 (104.7), 612 (11), 692 (9). H NMR (400 MHz, DMSO) d 9.96 (s, 1H), 9.67–9.49
(m, 1H), 9.46–9.35 (m, 1H), 9.25–9.06 (m, 1H), 8.95–8.42 (m, 6H), 8.42–8.33 (m, 1H), 8.30
(d, J = 7.4 Hz, 1H), 8.17–7.42 (m, 16H), 7.32–7.18 (m, 2H), 6.36 (d, J = 16.0, 15.8 Hz, 1H), 3.88–3.63
(m, 3H). MALDI-TOF: m/z calcd for C60H34N4O4Zn (M?), 938.1872; found, 938.1633
4
See supplementary data for the detail
123

J. Yan et al.
Fig. 1 Part of the 1H-NMR spectrum of 1b
reducing agent and additional ligand, and NEt3 as the base. With Pd(PPh3)4, the
coupling product was produced in 47 % yield and 44 % starting material recovered,
which is similar to the results obtained with Pd2(dba)3 (Table 1,entries 3 vs. 5). Base
hydrolysis of 6 under the above-mentioned conditions yielded compound 7.
The structure of monobromo-substituted p-extended porphyrin 2 was determined
by analysis of 1H-NMR spectra of the starting materials and products. Based on the
studies reported by Cavaleiro, it is known that for 1b, similar to 1a (Fig. 1),
b-pyrrolic protons H-18 and H-17 appear as two doublets at d 9.74 and 8.95 ppm,
respectively, with J value of 4.6 Hz. Pyrrolic protons H-8 and H-7 appear at d 8.66
and 8.64 ppm, and each as a doublet with J value of 4.6 Hz. Protons H-12 and H-13
1
resonate at d 8.84 ppm as a singlet. The H-NMR spectrum of bromo-substituted
porphyrin 2b (Fig. 2) is very similar to that of starting porphyrin 1b. Unfortunately,
the signals for two regioisomers completely overlapped. Therefore, the ratio of the
two isomers cannot be obtained at this stage. However, the structures of the bromo
1
products can still be determined by comparing the H-NMR spectrum of 1b with
that of 2b. As shown in Fig. 1, b-pyrrolic protons H-17, H-18, H-8 and H-7 still
appear as a doublet at d 9.72, 8.91, and 8.73, 8.68 ppm. The coupling constants for
two sets of doublets are 4.8 (JH-18, H-17) and 4.8 Hz (JH-8, H-7), respectively.
Integration of the singlet at d 8.70 ppm indicates that the signal is due to one proton
resonance, which is assigned to b-pyrrolic proton H-12 or H-13. The other pyrrolic
proton (H-12 or H-13) is substituted by bromine, which suggests that bromination
occurred to the b position antipodal to the fused aromatic ring.
1
The structures of the coupling products 4 and 6 were characterized by H-NMR
spectrum [24], which also confirmed the presence of two regioisomers in compound
2. For example, for the Sonogashira coupling product 4a, the ratio of two isomers is
2:1 and calculated based on the integration of H-18 doublets at d 9.60 and 9.54 ppm.
For the two isomers of Heck coupling product 6a, the H-18 doublets appear at d
9.59 and 9.54 ppm, and the ratio of two isomers is calculated as 3:1 based on the
integration. The presence of the methyl singlet due to the COOCH3 group in 4 or 6
is diagnostic and confirms our assignment of the structures for the coupling
products. The two methyl singlets in two isomers of 4a completely overlap and
appear at d 3.91 ppm, whereas for 6a two singlets resonate at d 3.84 and 3.69 ppm.
123

Studies on selective b/b0 bromination
Fig. 2 Part of the 1H-NMR spectrum of 2b
It was also found that the coupling constants between two olefinic protons in two
isomers of Heck coupling product 6a are both 16.0 Hz, which indicates the stereo-
chemistry of the introduced double bond is trans.
The UV–Vis spectra of 1, 5, 7, and TPP in THF were all obtained. Since the
UV–Vis spectra of derivatives 1b, 5b and 7b are similar to the ones obtained for 1a,
5a and 7a, for the purposes of clarity, only the UV–Vis spectra of derivatives 1a, 5a
and 7a are shown in Fig. 3. The hydrolyzed coupling products 5a and 7a have
shown a broadened and a red-shift of the Soret band compared to the p-extended
porphyrin 1a, and the original unmodified porphyrin TPP. The Soret bands of 5a
Fig. 3 UV–Vis spectra of 1a, 5a, 7a, and TPP in THF
123

J. Yan et al.
and 7a appear at 475 and 474 nm, whereas porphyrins 1a and TPP possess a Soret
band at 458 and 417 nm, respectively. The Q bands of 5a and 7a also red-shifted to
the near infrared region (5a at 707 nm; 7a at 695.5 nm) compared to those of 1a and
TPP as shown in the inset of Fig. 3. These results suggested that the light-
harvesting capabilities of p-extended porphyrin 1a were further enhanced by
introducing a conjugated carboxylphenylethynyl group to the b position antipodal to
the fused aromatic ring via Sonogashira coupling reaction. Therefore, compounds 5
and 7 can be potentially used as efficient dye-sensitizers in DSSCs.
However, interestingly, it was found that the molar extinction coefficients at the
Soret bands for the p-extended porphyrins (1a, 5a and 7a) of the peaks reduced and
are about 30 % of that for TPP. These results are consistent with those previously
reported by Imahori [7].
Conclusions
In summary, we have developed a method for the preparation of p-extended
porphyrin sensitizers with the fused aromatic ring and the carboxylic acid anchoring
group on the opposite sites of porphyrins. The optimal reaction conditions were
determined, which can be employed for the synthesis of various p-extended
porphyrin sensitizers. The UV–Vis absorption spectra of the hydrolyzed coupling
products show a broadened and red-shifted Soret band and Q bands compared with
those of the starting p-extended porphyrins and the original unmodified porphyrins.
The results suggest that the hydrolyzed coupling products can be potentially used in
the fields of DSSC systems. Future studies will be focused on the performance of the
solar cells sensitized by porphyrin 5 or 7.
Acknowledgments This work is supported by National Natural Science Foundation of China (No.
21076147), Natural Science Foundation of Tianjin (No. 10JCZDJC23700), China International Science
and Technology Project (No 2012DFG41980) and Seed Foundation of Tianjin University (Bao Zhang).
References
1. H. Imahori, T. Umeyama, S. Ito, Acc. Chem. Res. 42, 1809 (2009)
2. T.D. Lash, J. Porphyrins Phthalocyanines 5, 267 (2001)
3. S. Yamaguchi, T. Katoh, H. Shinokubo, A. Osuka, J. Am. Chem. Soc. 129, 6392 (2007)
4. N.V. Konovalova, E.V. Kirienko, V.N. Luzgina, Macroheterocycles 5, 115 (2012)
5. S. Baluschev, V. Yakutkin, T. Miteva, Y. Avlasevich, S. Chernov, Angew. Chem. Int. Ed. 46, 7693
(2007)
¨
6. A. Yella, F.W. Lee, H.N. Tsao, E.W.G. Diau, C.Y. Yeh, S.M. Zakeeruddin, M. Gratzel, Science 334,
629 (2011)
7. S. Hayashi, M. Tanaka, H. Hayashi, S. Eu, T. Umeyama, Y. Matano, Y. Araki, H. Imahori, J. Phys.
Chem. C 112, 15576 (2008)
8. K. Matsumoto, S. Kimura, T. Morishita, Y. Misumi, N. Hayashi, Synlett 233 (2000)
9. R. Deshpande, L. Jiang, G. Schmidt, J. Rakovan, X. Wang, K. Wheeler, H. Wang, Org. Lett. 11, 4251
(2009)
´
10. J.A.S. Cavaleiro, M.G.P.M.S. Neves, A.C. Tome, ARKIVOC 14, 107 (2003)
´
11. M.A.F. Faustino, M.G.P.M.S. Neves, A.C. Tome, A.M.S. Silva, J.A.S. Cavaleiro, ARKIVOC 9, 332
(2005)
123

Studies on selective b/b0 bromination
´
12. A.G. Silva, M.A.F. Faustino, T.M.P.C. Silva, M.G.P.M.S. Neves, A.C. Tome, A.M.S. Silva, J.A.S.
Cavaleiro, J. Chem. Soc. Perkin Trans. I 1, 1774 (2002)
13. O.B. Locos, D.P. Arnold, Org. Biomol. Chem. 4, 902 (2006)
14. K.M. Shea, L. Jaquinod, R.G. Khoury, K.M. Smith, Tetrahedron 56, 3139 (2000)
15. L. Jaquinod, R.G. Khouryn, K.M. Sheab, K.M. Smith, Tetrahedron 55, 13151 (1999)
16. K. Sonogashira, in Comprehensive Organic Synthesis, ed by B. M. Trost, I. Fleming, (Pergamon
Press, Oxford, 1991), p. 521
17. A. Elangovan, Y.-H. Wang, T.-I. Ho, Org. Lett. 5, 1841 (2003)
´
18. D. Garcıa, A.M. Cuadro, J. Alvarez-Builla, J.J. Vaquero, Org. Lett. 6, 4175 (2004)
19. C.W. Lee, H.P. Lu, C.M. Lan, Y.L. Huang, Y.R. Liang, W.N. Yen, Chem. Eur. J. 15, 1403 (2009)
˚
20. T. Ljungdahl, T. Pettersson, B. Albinsson, J. Martensson, J. Org. Chem. 71, 1677 (2006)
21. F. Alonso, I.P. Beletskaya, M. Yus, Tetrahedron 61, 11771 (2005)
22. J.A. Mikroyannidis, G. Charalambidis, A.G. Coutsolelos, P. Balraju, G.D. Sharma, J. Power Sources
196, 6622 (2011)
23. O.B. Locos, D.P. Arnold, Org. Biomol. Chem. 4, 902 (2006)
123