HETEROCYCLES

Paper | Special issue | Vol. 88, No. 1, 2014, pp. 223-231
Received, 13th April, 2013, Accepted, 17th June, 2013, Published online, 20th June, 2013.
DOI: 10.3987/COM-13-S(S)5

■ Palladium-Catalyzed Tetraarylation of 5,15-Dialkylporphyrins with Aryl Bromides

Yutaro Yamamoto, Sumito Tokuji, Takayuki Tanaka, Hideki Yorimitsu,* and Atsuhiro Osuka*

Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo, Kyoto 606-8502, Japan

Abstract

Nickel complexes of 5,15-dialkylporphyrins are subjected to palladium-catalyzed direct arylation under the modified Fagnou conditions. The arylation takes place still exclusively at the four less hindered β positions although the meso-nonyl, hexyl, and propyl groups are considered to impose less steric hindrance than the meso-3,5-di-tert-butylphenyl group in the previous report.

INTRODUCTION
Due to the important roles that porphyrins adopt in a variety of pivotal biological processes, chemists have devoted much time to the design and synthesis of new artificial porphyrins that can be utilized in advanced functional materials.1 Peripheral functionalizations of a porphyrin core is an effective strategy that allows for the systematic construction of porphyrin-based architechtures.2 Palladium-catalyzed cross-coupling reactions3 can be used to successfully introduce a direct carbon–carbon bond at the periphery of a porphyrin.1,2,4 However, these reactions generally take place in moderate yield. Unlike benzene-based building blocks, which are cheap and readily available, prefunctionalized porphyrins such as bromoporphyrins and borylporphyrins are far more precious. It is therefore essential that extensive efforts be made to develop much more efficient, scalable, and reliable synthetic methodologies for achieving these highly desirable molecules.
A recent dramatic growth in reports of transition-metal-catalyzed direct C–H arylations has changed the landscape of biaryl synthesis.5 Direct arylation does not require either an aryl metal reagent or an aryl halide and therefore represents the ideal arylation. We have been interested in the modification of functional aromatics by direct arylation6,7 and we have recently developed conditions for palladium-catalyzed β-selective direct arylation of porphyrins8,9 based on procedures originally reported by Fagnou.10,11 Our direct β-arylation has proved to be applicable to nickel complexes of 5,10,15-triarylporphyrin, 5,15-diarylporphyrin, and 5,10,15-triaryl-20-formylporphyrins bearing bulky 3,5-di-tert-butylphenyl groups as the aryl groups at their meso positions. Since the regioselective outcome of porphyrin arylation is heavily governed by sterics, the size of the group that is present at the meso position is a critical factor. To test this hypothesis, we have synthesized a small series of porphyrins bearing alkyl chains of various sizes at the meso positions. The regioselective outcome of direct arylation reactions of this small series of porphyrins is reported.

RESULTS AND DISCUSSION
Three dialkylporphyrins, 1a (R = C9H19), 1b (R = C6H13), and 1c (R = C3H7), were prepared and subjected to the palladium-catalyzed tetraarylation8 (Table 1). Tetraarylation proceeded very smoothly to afford the corresponding tetraarylated products 2 as the sole isolated products with exclusive regioselectivity. Considering the reaction involves the simultaneous formation of four C–C bonds, the yields are extremely high.12 The presence of Davephos (2-dicyclohexylphosphino-2'-dimethylaminobiphenyl) led to slightly better yields of 2 (Entries 1, 3, 5, 7 vs. 2, 4, 6, 8). Arylation with bulky 2-bromotoluene also proceeded

favorably (Entries 3, 4). The reactivities of 1a and 1b are comparable to those of meso-diarylporphyrins. On the other hand, the yields of 2c were slightly lower (Entries 7, 8). We speculate this is because 1c is equipped with short propyl groups that exhibit poor solubility in N,N-dimethylacetamide (DMA).
Unfortunately, we could not obtain crystals of 2 suitable for X-ray crystallographic analysis. Nevertheless, we can safely assume that the four aryl groups are located at the less hindered β positions (2, 8, 12, 18 positions), as indicated. It is improbable for the four aryl groups to be introduced at the more crowded β positions (3, 7, 13, 17 positions) adjacent to the alkyl groups since Fagnou’s direct arylation has been established to follow steric control.10 The anticipated regioselectivity is additionally confirmed by 1H NMR spectroscopy (Table 2). The meso-H signal of porphyrin 2a bearing four 3,5-dimethylphenyl groups appear at 9.83 ppm and is shifted downfield by 0.13 ppm compared to that of starting material 1a (9.70 ppm). In contrast, porphyrin 2a' bearing four 2-methylphenyl groups shows its meso-H signal at 9.15 ppm, significantly shifted upfield by 0.55 ppm. This dramatic upfield shift most likely originates from the diatropic ring current of the β-benzene rings, which would be nearly perpendicular to the porphyrinic plane due to the steric crash with the ortho methyl group and would thus cover the meso protons. Notably, the arylation of 1a did not induce drastic changes in the chemical shifts of the signals corresponding to the methylene protons that are in closest proximity to the porphyrin core, as illustrated by limited upfield shifts, 0.14 ppm in 2a and 0.07 ppm in 2a'. The significant changes in the chemical shifts for the meso protons and the small changes for the methylene protons are strong evidence that the aryl groups introduced are located at the less hindered β positions next to the unsubstituted meso positions. Furthermore, similar changes in the chemical shifts for meso protons were observed in the previous β-diarylation of 5,10,15-triarylporphyrin at the 2 and 18 positions,8 where the introduction of 3,5-dimethylphenyl groups and of 2-methylphenyl groups gave rise to a downfield shift (0.25 ppm) and an upfield shift (0.47 ppm), respectively.
The UV-visible absorption spectra of 1a, 2a, and 2a' are illustrated in Figure 1. The introductions of the aryl groups in 2a and 2a' obviously resulted in red-shifted absorptions. Notably, 2a bearing 3,5-dimethylphenyl groups shows more red-shifted absorption than 2a' bearing 2-methylphenyl groups. The difference in the absorption exhibits that 2a' has less effective conjugation than 2a and implies that the 2-methylphenyl groups in 2a' would be more tilted to the porphyrin plane than the 3,5-dimethylphenyl groups in 2a. This implication is consistent with the significant upfield shift of the meso protons in the 1H NMR analysis of 2a'.
In conclusion, 5,15-dialkylporphyrin nickel complexes undergo palladium-catalyzed direct tetraarylation under modified Fagnou conditions using pivalic acid as a mediator. The arylation is high-yielding and takes place exclusively at the four less hindered β positions as observed in the arylation of meso-3,5-di-tert-butylphenyl-substituted porphyrins.

EXPERIMENTAL
1H NMR (600 MHz) and 13C NMR (151 MHz) spectra were taken on a JEOL ECA-600 spectrometer. Chemical shifts are reported on a delta scale in ppm relative to residual CHCl3 (δ = 7.26 ppm) for 1H NMR and to CDCl3 (δ = 77.16 ppm) for 13C NMR. Spectroscopic grade solvents were used for all spectroscopic studies without further purification. UV-Visible absorption spectra were recorded on a Shimadzu UV-2550 and Shimadzu UV-3600PC spectrometer. ESI-TOF-MS spectra were recorded on a Bruker Daltonics micrOTOF II instrument using a positive-ion mode. TLC analyses were performed on commercial glass plates bearing a 0.25-mm layer of Merck Silica gel 60F254. Preparative separations were performed by silica-gel column chromatography (Wako gel C-200, C-300, or C-400).
Unless otherwise noted, materials obtained from commercial suppliers were used without further purification. DMA was distilled from barium oxide and kept over Molecular Sieves 4Å under argon. Palladium acetate, nickel acetylacetonate, and Davephos were purchased from Wako Pure Chemicals, nacalai tesque, and Aldrich, respectively.
Starting Materials. Free base 5,15-dialkylporphyrins were prepared according to the literature.13 Subsequent nickelation proceeded quantitatively by treatment of the free base porphyrins with an excess amount of nickel acetylacetonate in boiling toluene overnight.
(5,15-Dinonylporphyrinato)nickel (1a): 1H NMR (600 MHz, CDCl3) δ = 9.70 (s, 2H, meso), 9.43 (d, 4H, J = 4.8 Hz, β), 9.17 (d, 4H, J = 4.8 Hz, β), 4.67 (t, 4H, J = 8.3 Hz, nonyl), 2.36 (m, 4H, nonyl), 1.65 (m, 4H, nonyl), 1.46 (m, 4H, nonyl), 1.35–1.21 (m, 16H, nonyl), 0.86 (t, 6H, J = 6.9 Hz, nonyl) ppm; 13C NMR (151 MHz, CDCl3): δ = 142.68, 141.55, 132.27, 129.56, 117.45, 104.27, 37.80, 34.38, 32.04, 30.62, 29.76 (overlap), 29.50, 22.81, 14.25 ppm; UV-vis (CH2Cl2): λmax (ε [M−1cm−1]) = 402 (190000), 519 nm (12000); MS (ESI, positive): m/z = 619.3279. Calcd for C38H49N4Ni: 619.3305 [M+H]+.
(5,15-Dihexylporphyrinato)nickel (1b): 1H NMR (600 MHz, CDCl3) δ = 9.71 (s, 2H, meso), 9.45 (d, 4H, J = 4.6 Hz, β), 9.18 (d, 4H, J = 4.6 Hz, β), 4.68 (t, 4H, J = 8.3 Hz, hexyl), 2.37 (m, 4H, hexyl), 1.66 (m, 4H, hexyl), 1.45 (m, 4H, hexyl), 1.35 (m, 4H, hexyl), 0.91 (t, 6H, J = 7.8 Hz, hexyl) ppm; 13C NMR (151 MHz, CDCl3): δ = 142.65, 141.52, 132.23, 129.50, 117.40, 104.01, 37.77, 34.36, 31.96, 30.29, 22.85, 14.29 ppm; UV-vis (CH2Cl2): λmax (ε [M–1cm–1]) = 402 (210000), 518 (13000), 550 nm (4000); MS (ESI, positive): m/z = 535.2345. Calcd for C32H37N4Ni: 535.2366 [M+H]+.
(5,15-Dipropylporphyrinato)nickel (1c): 1H NMR (600 MHz, CDCl3) δ = 9.70 (s, 2H, meso), 9.44 (d, 4H, J = 4.8 Hz, β), 9.17 (d, 4H, J = 4.8 Hz, β), 4.66 (t, 4H, J = 7.8 Hz, propyl), 2.40 (m, 4H, propyl), 1.21 (t, 6H, J = 7.3 Hz, propyl) ppm; 13C NMR (151 MHz, CDCl3): δ = 142.81, 141.62, 132.35, 129.71, 117.22, 104.08, 36.34, 30.69, 15.03 ppm; UV-vis (CH2Cl2): λmax (ε [M–1cm–1]) = 401 (210000), 518 (14000), 550 nm (5000); MS (ESI, positive): m/z = 451.1419. Calcd for C26H25N4Ni: 451.1427 [M+H]+.
Direct Arylation of 5,15-Dinonylporphyrin: 5,15-Dinonylporphyrin 1a (0.050 mmol, 31.1 mg), Pd(OAc)2 (0.010 mmol, 2.2 mg), Davephos (0.020 mmol, 7.9 mg), K2CO3 (0.50 mmol, 69.1 mg), and tBuCO2H (0.25 mmol, 25.6 mg) were added to a reaction flask. The reaction flask was purged with argon, and DMA (1.0 mL) and aryl bromide (0.50 mmol, 70 µL for 1-bromo-3,5-dimethylbenzene or 60 µL for 2-bromotoluene) were added. The reaction mixture was then stirred at 100 ˚C for 20 h. The resulting mixture was allowed to cool to room temperature, diluted with CH2Cl2, and passed through a pad of Celite with copious washings with CH2Cl2. After evaporation of the solvent, the product was separated by silica-gel column chromatography (CH2Cl2/hexane). Further purification by recrystallization from CH2Cl2/MeOH yielded the corresponding product (2a, 50.2 mg, 0.0484 mmol, 97%; 2a', 48.0 mg, 0.0490 mmol, 98%).
Direct Arylation of 5,15-Dihexyl- or 5,15-Dipropylporphyrin: 5,15-Dihexylporphyrin 1b (0.025 mmol, 13.4 mg) or 5,15-dipropylporphyrin 1c (0.025 mmol, 11.3 mg), Pd(OAc)2 (0.005 mmol, 1.1 mg), Davephos (0.010 mmol, 3.9 mg), K2CO3 (0.25 mmol, 34.5 mg), and tBuCO2H (0.125 mmol, 12.8 mg) were placed in a reaction flask under argon. DMA (1.0 mL) and 1-bromo-3,5-dimethylbenzene (0.25 mmol, 35 µL) were added, and the whole mixture was heated at 100 ˚C for 20 h. The resulting mixture was allowed to cool to ambient temperature, diluted with CH2Cl2, and filtered through a pad of Celite with copious washings with CH2Cl2. The filtrate was concentrated in vacuo. Chromatographic purification on silica gel (CH2Cl2/hexane) followed by recrystallization from CH2Cl2/MeOH afforded 2b (22.8 mg, 0.0240 mmol, 96%) or 2c (16.6 mg, 0.0191 mmol, 77%).
[2,8,12,18-Tetrakis(3,5-dimethylphenyl)-5,15-dinonylporphyrinato]nickel (2a): 1H NMR (600 MHz, CDCl3) δ = 9.83 (s, 2H, meso), 9.27 (s, 4H, β), 7.68 (s, 8H, Ar-o), 7.20 (s, 4H, Ar-p), 4.53 (t, 4H, J = 8.7 Hz, nonyl), 2.53 (s, 24H, Ar–Me), 2.34 (m, 4H, nonyl), 1.60 (m, 4H, nonyl), 1.42 (m, 4H, nonyl), 1.32–1.18 (m, 16H, nonyl), 0.84 (t, 6H, J = 6.9 Hz, nonyl) ppm; 13C NMR (151 MHz, CDCl3): δ = 146.29, 140.93, 139.73, 138.51, 136.42, 129.24, 128.74, 127.30, 116.55, 103.95, 37.35, 34.05, 32.02, 30.54, 29.76, 29.71, 29.51, 22.79, 21.73, 14.23 ppm; UV-vis (CH2Cl2): λmax (ε [M–1cm–1]) = 427 (170000), 539 (15000), 565 nm (10000); MS (ESI-MS, positive): m/z = 1057.5657. Calcd for C70H80N4NiNa: 1057.5629 [M+Na]+.
[2,8,12,18-Tetrakis(2-methylphenyl)-5,15-dinonylporphyrinato]nickel (2a’): 1H NMR (600 MHz, CDCl3): δ = 9.30 (s, 4H, β), 9.15 (s, 2H, meso), 7.76 (d, 4H, J = 5.8 Hz, Ar-o), 7.49–7.41 (m, 12H, Ar-m Ar-p), 4.60 (t, 4H, J = 8.0 Hz, nonyl), 2.43 (s, 12H, Ar–Me), 2.38 (m, 4H, nonyl), 1.60 (m, 4H, nonyl), 1.42 (m, 4H, nonyl), 1.30–1.18 (m, 16H, nonyl), 0.83 (t, 6H, J = 7.1 Hz, nonyl) ppm; 13C NMR (151 MHz, CDCl3): δ = 145.87, 141.38, 140.97, 137.56, 135.77, 132.52, 130.61, 129.00, 128.16, 125.70, 117.27, 103.18, 37.76, 34.32, 32.02, 30.60, 29.76, 29.47, 22.78, 21.37, 20.89, 14.23 ppm; UV-vis (CH2Cl2): λmax (ε [M–1cm–1]) = 417 (180000), 531 (11000), 561 nm (6000); MS (ESI-MS, positive): m/z = 979.5140. Calcd for C66H73N4Ni: 979.5183 [M+H]+.
[2,8,12,18-Tetrakis(3,5-dimethylphenyl)-5,15-dihexylporphyrinato]nickel (2b): 1H NMR (600 MHz, CDCl3) δ = 9.83 (s, 2H, meso), 9.28 (s, 4H, β), 7.68 (s, 8H, Ar-o), 7.20 (s, 4H, Ar-p), 4.53 (t, 4H, J = 8.0 Hz, hexyl), 2.53 (s, 24H, Ar–Me), 2.34 (m, 4H, hexyl), 1.61 (m, 4H, hexyl), 1.41 (m, 4H, hexyl), 1.33 (m, 4H, hexyl), 0.89 (t, 6H, J = 7.3 Hz, hexyl) ppm; 13C NMR (151 MHz, CDCl3): δ = 146.38, 140.98, 139.80, 138.54, 136.44, 129.28, 128.77, 127.35, 116.63, 103.96, 37.45, 34.16, 31.96, 30.30, 22.88, 21.74, 14.30 ppm; UV-vis (CH2Cl2): λmax (ε [M–1cm–1]) = 426 (160000), 538 (12000), 568 nm (8000); MS (ESI-MS, positive): m/z = 951.4823. Calcd for C64H69N4Ni: 951.4870 [M+H]+.
[2,8,12,18-Tetrakis(3,5-dimethylphenyl)-5,15-dipropylporphyrinato]nickel (2c): 1H NMR (600 MHz, CDCl3) δ = 9.87 (s, 2H, meso), 9.32 (s, 4H, β), 7.70 (s, 8H, Ar-o), 7.20 (s, 4H, Ar-p), 4.56 (t, 4H, J = 8.0 Hz, propyl), 2.54 (s, 24H, Ar–Me), 2.39 (m, 4H, propyl), 1.20 (t, 6H, J = 7.4 Hz, propyl) ppm; 13C NMR (151 MHz, CDCl3): δ = 146.38, 140.01, 139.80, 138.52, 136.40, 129.27, 128.75, 127.38, 116.37, 104.00, 36.13, 34.16, 30.42, 21.73, 15.08 ppm; UV-vis (CH2Cl2): λmax (ε [M–1cm–1]) = 426 (190000), 537 (15000), 568 nm (10000); MS (ESI-MS, positive): m/z = 867.3888. Calcd for C58H57N4Ni: 867.3931 [M+H]+.

ACKNOWLEDGEMENTS
This work was supported by Grants-in-Aid (nos. 22245006 (A), 20108006 “pi-Space”, 24685007, and 22406721 “Reaction Integration”) from MEXT. T.T. and S.T. acknowledge JSPS Fellowship for Young Scientists. H.Y. thanks financial support from Kinki Invention Center.

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12. Reviews on the importance of one-pot multiple bond formation: S. Suga, D. Yamada, and J. Yoshida, Chem. Lett., 2010, 39, 404 and references cited therein; CrossRef A. J. Bard, 'Integrated Chemical Systems,' Wiley, New York, 1994; 'Multicomponent Reactions,' ed. by J. Zhu and H. Bienaymé, Wiley, Weinheim, 2005. CrossRef
13. Y. Nakamura, S. Y. Jang, T. Tanaka, N. Aratani, J. M. Lim, K. S. Kim, D. Kim, and A. Osuka, Chem. Eur. J., 2008, 14, 8279. CrossRef