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A dendron of subphthalocyanine trefoil
Tetrahedron Letters 53 (2012) 313–316
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A dendron of subphthalocyanine trefoil Mitsuhiko Morisue ⇑, Wataru Suzuki, Yasuhisa Kuroda Department of Biomolecular Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan
a r t i c l e
i n f o
Article history: Received 3 October 2011 Revised 1 November 2011 Accepted 7 November 2011 Available online 15 November 2011 Keywords: Dendrimer Subphthalocyanine Axial ligand Hyperbranched polymer Chromophore array
a b s t r a c t The synthesis of a dendron composed of tetrameric subphthalocyanine (SubPc) is accomplished by substituting the chlorine groups with phenoxy groups at the axial positions of SubPc with SubPc-triol. The present molecular design of the SubPc-triol introduces three phenol groups at the peripheral positions of the SubPc macrocycle as a tritopic template to construct SubPc dendrons. The self-polycondensation of SubPc-triol as a ‘divergent’ synthesis only gave a trace amount of the hyperbranched arrays due to poor solubility of the SubPc-triol. In contrast, a ‘convergent’ synthesis with the terminal SubPc improved the solubility throughout the reaction and a tetrameric SubPc dendron was obtained in moderate isolated yield. Ó 2011 Elsevier Ltd. All rights reserved.
Dendritic or hyperbranched multichromophoric arrays are emerging at the forefront of material chemistry due to their excellent photoelectronic properties.1–4 Among them, ligand-to-metal coordination provides a straightforward strategy to construct arborescent structures composed of a metallocomplex or chromophore.4 A non-centrosymmetric dendron of chromophoric unit, on which a highly branched structure converges to the focal core unit through branching p-conjugation or inter-unit electronic interactions, is a promising entity for the development of an energy cascade or a potential gradient aimed at artificial photosynthesis, organic electronics, and nonlinear optics.1–6 A three-forked node should be effective at designing a highly branched dendron in order to achieve further enhanced effects of the dendritic arrangement. The topologically fascinating concave trefoil framework of subphthalocyanine, SubPc, was chosen to triplicate the polymer backbone in a three-dimensional fashion. SubPc is a boron(III) complex of a macrocyclic ligand with a 14p electron system.7–10 The labile chlorine atom bound to the central boron atom at the axial position is readily substituted by a nucleophilic phenoxy group.8 The new axial substitution is able to deliver functional units into SubPc.8 Recently, we have successfully established a template-directed protocol to provide multi-SubPc arrays by employing a multiple phenoxy precursor. Thus, hexakis(4-hydroxyphenyl)benzene as the multiple phenoxy template gave a hexameric SubPc array in moderate isolated yield despite the presence of excessively congested multiple reaction sites.9 We now disclose the synthesis of a novel tetrameric SubPc dendron composed of three peripheral SubPc units converging into the focal SubPc bearing tritopic ⇑ Corresponding author. Tel./fax: +81 75 724 7806. E-mail address: [email protected] (M. Morisue). 0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2011.11.039
4-hydroxyphenylethynyl side-arms, SubPc-triol 1, via radial phenoxy-to-boron(III) bonds. SubPc-triol 1 was derived from triiodo-SubPc and 4-ethynylphenol by the Sonogashira cross-coupling reaction according to literature procedures (Scheme 1).10 Under the reaction conditions, the undesired axial substitution proceeded in part and gave an insoluble precipitate presumably including polycondensed byproducts. The monomeric SubPc-triol 1 was obtained as a bluish purple powder in a 12% isolated yield.11 In a similar way, SubPc-trianisole 2 was prepared as the reference compound in a 56% isolated yield.12 The following discussion concerns the synthetic investigations of the dendritic SubPc arrays. For the first trial, one-pot self-polycondensation of SubPc-triol 1 as a ‘divergent’ approach was attempted under refluxing conditions using anisole, pyridine, or THF as the solvent (Scheme 2). However, self-polycondensation was restricted presumably by very poor solubility of polycondensed 1 even in such polar solvents. Therefore, no polymeric products higher than the dimeric or trimeric species were obtained (entries 1–6 in Table 1).
Scheme 1. Synthetic scheme of SubPc-triol 1 and SubPc-trianisole 2. Reagents and conditions: the corresponding ethynylaryl precursor, CuI, Pd(PPh3)4, Et3N, toluene at 35 °C.
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Scheme 2. A ‘divergent’ approach: one-pot self-polycondensation of SubPc-triol 1 toward the hyperbranched arrays under the reflux conditions of anisole, pyridine, or THF. The results are summarized in entry 1–6 in Table 1.
Table 1 Summary of the reactions of SubPc-triol 1
a B c
Entry
SubPc-tBu3 3
Conditions
Yield (%)
1 2 3 4 5 6 7
0 equiv 0 equiv 0 equiv 0 equiv 0 equiv 0 equiv 6.5 equiv
Anisole, rt, 170 h Anisole, 60 °C, 100 h Anisole, reflux, 100 h THF, rt, 170 h THF, reflux, 100 h Pyridine, reflux, 100 h Toluene/Et3N, reflux, 22 h
Tracea Tracea Tracea Tracea Tracea Tracea 57%c
b
No hyperbranched arrays were observed. Compound 1 was soluble in toluene/Et3N only when 3 was added. Isolated yield based on SubPc-triol 1.
Additionally, the prolonged reaction times were accompanied by partial ring-expansion to afford phthalocyanine as the byproduct.7b Hyperbranched arrays were only obtained in a trace amount by the self-polycondensation of 1. However, this limited result suggests that the reactivity of SubPc-triol 1 might be controlled by the solubility of 1 under appropriate conditions. In this case, SubPc-triol 1 would be suitable for the application of a ‘convergent’ synthesis with SubPc terminal units in the dendritic structure. In the second stage, a ‘convergent’ synthesis was examined. To achieve the synthesis of the dendritic SubPc array, a terminal SubPc unit was designed to improve the solubility throughout the reaction. Thus, we introduced SubPc-tBu3 3, which bears the peripheral tert-butyl groups, as the terminal unit (Scheme 3). Then,
Scheme 3. A ‘convergent’ approach: the mixture of SubPc-triol 1 and SubPc-tBu3 3 was refluxed in toluene/truethylamine (4/1, v/v) to give the tetrameric dendron [SubPc]4 4. The result is summarized in entry 7 in Table 1.
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Figure 1. Representative 1H NMR spectra (300 MHz) of SubPc-tBu3 3, (a) the focal SubPc-triol unit (SubPc-trianisole 2 as the model compound), (b) and [B(SubPc)]4 4 (c) in CDCl3. Asterisk indicates residual solvent.
SubPc-triol 1 was then dissolved as a mixture with a large excess of SubPc-tBu3 3 in toluene/triethylamine (4/1, v/v). After reflux in the dry mixed solvent under the nitrogen atmosphere, the multiple axial substitution reaction proceeded smoothly and gave the tetrameric dendron, [SubPc]4 4, in a 57% isolated yield (entry 7 in Table 1).13,14 Therefore, this ‘convergent’ synthesis was successful in providing a dendritic SubPc array by employing SubPc-triol 1. The 1H NMR spectrum of 4 exhibited a complete upfield shift of the a-phenyl protons of the hydroxyphenyl side-arm from 6.9 to 5.4 ppm (Fig. 1). A similar upfield shift was observed for the b-phenyl protons, indicating that all the phenoxy groups were strongly shielded by the proximal SubPc macrocycle. Moreover, the desired peak for [SubPc]4 4 was observed in a MALDI-TOF MS measurement. These results clearly indicate that full accommodation of the focal SubPc-triol unit was successful. The light-harvesting antenna function of 4 was next studied. The absorption spectrum of [SubPc]4 4 bears a close resemblance to the summation of that of SubPc-triol 1 and SubPc-tBu3 3, whereas the extinction coefficient of the focal unit of 4 is reduced to approximately half the magnitude of the monomeric 2, that is, a hypochromic effect was present (Fig. 2), similar to that observed for the hexameric SubPc array in our previous study.9 The fluorescence behavior indicates a light-harvesting antenna function. The absorption and fluorescence properties of 4 remain as the individual characters of the SubPc constituents. The focal SubPc-triol unit possesses a lower energy level (kmax = 594 nm and kem = 604 nm) compared to that of the peripheral SubPc-tBu3 units (kmax = 567 nm and kem = 574 nm). Then, [SubPc]4 4 predominantly fluoresces at 599 nm due to the focal SubPc-triol unit, even if the peripheral SubPc-tBu3 units are excited (Fig. 2). This result
Figure 2. Absorption (thick line) and fluorescence (thin line) spectra of the peripheral SubPc-tBu3 unit (SubPc-tBu4 bearing the axial phenoxy group as the model compound9) (UF = 0.11) (a), the focal SubPc-triol unit (SubPc-trianisole 2 as the model compound) (UF = 0.11) (b), and [SubPc]4 4 (UF = 0.07) (c) in toluene (25 °C).15 The fluorescence spectra are normalized to the corresponding absorbance at 510 nm of the excitation wavelength.
indicates that photoexcited energy is transferred along an energy cascade into the focal unit of [SubPc]4 4.
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In conclusion, we have successfully developed a synthetic strategy for the dendron composed of a SubPc trefoil by employing a chlorine-to-phenoxy substitution reaction of SubPc-tBu3 3. The SubPc dendron possesses octupolar chromophoric configurations as each generation of the dendron. Such hierarchical octupolar configurations might be a promising motif in the context of material science including non-linear optics, liquid crystals, etc. The remaining axial chlorine of [SubPc]4 4 is available for successive ‘convergent’ syntheses to provide higher generations of the present dendron series by employing SubPc-triol 1. Acknowledgments This work was financially supported in part through a Grantin-Aid for Young Scientists (B) (No. 21750123) from JSPS (Japan Society for the Promotion of Science) and through the Seeds Contest program from Kyowa Hakko Chemical Co., Ltd. This work was also supported by the Kyoto-Advanced Nanotechnology Network program (Graduate School of Materials Science, Nara Institute Science and Technology). References and notes 1. (a) Grayson, S. M.; Fréchet, J. M. J. Chem. Rev. 2001, 101, 3819–3867; (b) Fréchet, J. M. J. J. Polym. Sci. A. 2003, 41, 3713–3725; (c) Newkome, G. R.; Shreiner, C. Chem. Rev. 2010, 110, 6338–6442. 2. Moore, J. S. Acc. Chem. Res. 1997, 30, 402–413. 3. (a) Li, W.-S.; Aida, T. Chem. Rev. 2009, 109, 6047–6076; (b) Choi, M.-S.; Aida, T.; Yamazaki, T.; Yamazaki, I. Chem. Eur. J. 2002, 8, 2667–2678; (c) Guo, M.; Yan, X.; Kwon, Y.; Hayakawa, T.; Kakimoto, M.-A.; Goodson, T., III J. Am. Chem. Soc. 2006, 128, 14820–14821. 4. (a) Harriman, A.; Ziessel, R. Chem. Commun. 1996, 1707–1716; (b) Frey, H.; Lach, C.; Lorenz, K. Adv. Mater. 1998, 10, 279–293; (c) Newkome, G. R.; He, E.; Moorefield, C. N. Chem. Rev. 1999, 99, 1689–1746; (d) Balzani, V.; Campagna, S.; Denti, G.; Juris, A.; Serroni, S.; Venturi, M. Acc. Chem. Res. 1998, 31, 26–34. and references cited therein. 5. (a) Ma, C.-Q.; Mena-Oseritz, E.; Debaerdemaker, T.; Wienk, M. M.; Janssen, R. A. J.; Bäuerle, P. Angew. Chem., Int. Ed. 2007, 46, 1679–1683; (b) Albrecht, K.; Yamamoto, K. J. Am. Chem. Soc. 2009, 131, 2244–2251. 6. NLO: (a) Yokoyama, S.; Nakahama, T.; Otomo, A.; Mashiko, S. J. Am. Chem. Soc. 2000, 122, 3174–3181; (b) Harpham, M. R.; Süzer, Ö.; Ma, C.-Q.; Bäuerle, P.; Goodson, T., III J. Am. Chem. Soc. 2009, 131, 973–979. 7. (a) Geyer, M.; Plenzig, F.; Rauschnabel, J.; Hanack, M.; del Rey, B.; Sastre, A.; Torres, T. Synthesis 1996, 1139–1151; (b) Kobayashi, N.; Ishizaki, T.; Ishii, K.; Konami, H. J. Am. Chem. Soc. 1999, 121, 9096–9110; (c) Claessens, C. G.; González-Rodríguez, D.; Torres, T. Chem. Rev. 2002, 102, 835–853; (d) Torres, T. Angew. Chem., Int. Ed. 2006, 45, 2834–2837. 8. PhO-SubPc: (a) Claessens, C. G.; González-Rodríguez, D.; del Rey, B.; Torres, T.; Mark, G.; Schuchmann, H.-P.; von Sonntag, C.; MacDonald, J. G.; Nohr, R. S. Eur. J. Org. Chem. 2003, 2547–2551; (b) González-Rodríguez, D.; Torres, T.; Guldi, D. M.; Rivera, J.; Herranz, M. Á.; Echegoyen, L. J. Am. Chem. Soc. 2004, 126, 6301– 6313; (c) Iglesias, R. S.; Claessens, C. G.; Torres, T.; Rahman, G. M. A.; Guldi, D. M. Chem. Commun. 2005, 2113–2115; (d) González-Rodríguez, D.; Torres, T.; Olmstead, M. M.; Rivera, J.; Herranz, M. A.; Echegoyen, L.; Castellanos, C. A.; Guldi, D. M. J. Am. Chem. Soc. 2006, 128, 10680–10681; (e) Liu, J.-Y.; Yeung, H.S.; Xu, W.; Li, X.; Ng, D. K. P. Org. Lett. 2008, 10, 5421–5424; Very recently, an improved procedure for the substitution reaction of the axial chlorine to a phenoxy group has been reported by Torres and coworkers: (f) Guilleme, J.; González-Rodrígues, D.; Torres, T. Angew. Chem., Int. Ed. 2011, 50, 3506–3509.
9. Morisue, M.; Suzuki, W.; Kuroda, Y. Dalton Trans. 2011, 40, 10047–10054. 10. del Rey, B.; Torres, T. Tetrahedron Lett. 1997, 30, 5351–5354. 11. Synthesis of SubPc-triol 1, chloro[2,9(10),16(17)-tris(p-hydroxyphenylethynyl) subphthalocyaninato]boron(III): A mixture of chloro 2,9(10),16(17)-triiodosubphthalocyaninatoboron(III) (83 mg, 0.10 mmol) and phydroxyphenylacetylene (65 mg, 0.56 mmol) in triethylamine (1 mL) and toluene (4.5 mL) was deaerated by freeze-pump-thaw cycles in a Schlenk tube. To the mixture were added Pd(PPh3)4 (22 mg, 19 lmol) and CuI (18 mg, 97 lmol) as the catalyst. The mixture was stirred at 35 °C under argon atmosphere for 43 h. After the reaction mixture was passed through Florisil, the crude material was precipitated in petroleum ether. The precipitate, redissolved in ethyl acetate, was reprecipitated from chloroform. The crude material was purified by size-exclusion chromatography (Bio-Beads S-X1; 200–400 mesh, Bio-Rad Co.) with THF. The titled compound was further purified by reprecipitation in petroleum ether and then chloroform. A bluish purple solid was obtained as a C1 and C3 regioisomeric mixture (9.6 mg, 12 lmol; yield 12%). 1H NMR (300 MHz, THF-d8): d = 8.87, 8.85 (s, s, 3H; SubPca(1)), 8.71, 8.69 (d, d, J = 8.0 Hz, 3H; SubPc-a(4)), 7.94 (d, J = 8.0 Hz, 3H; SubPcb), 7.47 (d, J = 8.4 Hz, 6H; Ph b to OH), 6.81 ppm (d, J = 8.4 Hz, 6H; Ph a to OH). 13 C NMR (75 MHz, THF-d8): d = 158.6, 151.7, 151.2, 137.2, 133.1, 131.8, 131.1, 129.5, 127.8, 125.2, 124.9, 124.3, 121.5, 115.5, 113.4, 92.5, 87.5 ppm. 11B NMR (96 MHz, THF-d8): d = 16.2 ppm (br s). MALDI-TOF MS: m/z: calcd for C48H24BClN6O3: 778.17; found: 778.26 [M]+. UV–vis (THF): kmax = 588 nm (Q band). 12. Synthesis of SubPc-trianisole 2, chloro[2,9(10),16(17)-tris(p-methoxyphenylethynyl)subphthalocyaninato]boron(III): A mixture of chloro 2,9(10),16(17)-triiodosubphthalocyaninatoboron(III) (108 mg, 0.14 mmol) and p-methoxyphenyl acetylene (75 mg, 0.57 mmol) was dissolved in triethylamine (1 mL) and toluene (4.8 mL) and deoxygenated by freeze-pump-thaw cycles in a Schlenk flask. The mixture was stirred at 35 °C under argon atmosphere for 18 h. After passing through a Florisil pad, the crude material was subjected to silica-gel column chromatography (toluene/ethyl acetate, 3/1, v/v). The titled material was obtained as a bluish purple solid from reprecipitation in petroleum ether (63 mg, 77 mmol; yield 56%). 1H NMR (300 MHz, CDCl3): d = 9.02 (d, J = 3.0 Hz, 3H; SubPc-a(1)), 8.81 (dd, J = 8.3, 3.0 Hz, 3H; SubPc-b), 8.04 (d, J = 8.3 Hz, 3H; SubPc-a(4)), 7.58 (d, J = 8.8 Hz, 6H; Ph b to –OMe), 6.95 (d, J = 8.8 Hz, 6H; Ph a to –OMe), 3.87 ppm (s, 9H; –OMe) (Fig. 1b). 13C NMR (75 MHz, CDCl3): d = 160.11, 149.9, 149.7, 133.4, 132.9, 131.14, 131.07, 129.6, 129.4, 125.9, 122.3, 114.8, 114.2, 93.0, 88.3, 55.4 ppm. 11B NMR (96 MHz, CDCl3): d = 13.5 ppm. MALDI-TOF MS: m/z: calcd for C51H30BClN6O3: 820.22; found: 820.28 [M]+. UV–vis (toluene): kmax = 594 nm (Q band) (Fig. 2b). 13. Synthesis of [SubPc]4 4: A mixture of SubPc-triol 1 (13 mg, 0.017 mmol) and SubPc-tBu3 3 (66 mg, 0.11 mmol) was refluxed in 2.0 mL of toluene and 0.5 mL of triethylamine under nitrogen atmosphere for 22 h. The solvent was removed under reduced pressure, and the crude material extracted with chloroform was washed with brine. The tetrameric dendron [SubPc]4 4 was purified by size-exclusion chromatography (Bio-Beads S-X1) with chloroform and was obtained as a bluish purple solid (25 mg, 0.010 mmol) in 57% isolated yield. 1H NMR (300 MHz, CDCl3): d = 8.88 (br s, 12H; SubPc-a(1) and a(1)’), 8.79 and 8.77 (br m, 12H; SubPc-a(4) and a(4)’), 8.00–7.98 (br m, 12H; 8.88 (br s, 12H; SubPcb and b’), 7.03 (br d, J = 7.2 Hz, 6H, Ph b to OSubPc), 5.43 (br d, J = 7.2 Hz, 6H, Ph a to OSubPc), 1.55 ppm (s, 81H; t-Bu) (Fig. 1c). 13C NMR (75 MHz, CDCl3): d = 153.98, 151.32, 132.75, 131.18, 128.68, 128.09, 121.79, 119.21, 118.43, 35.84, 31.65 ppm. MALDI-TOF MS: m/z: calcd for C156H129B4ClN24O3: 2465.07; found: 2465.92 [M+H]+. UV–vis (toluene): kmax = 568 nm (Q band) (Fig. 2c). 14. We note that the reaction yield of 4 was susceptible to residual water. The flask and equipped reflux condenser were dried with heating under vacuum prior to the reaction. Toluene was distilled over molten sodium and triethylamine was distilled over sodium hydride immediately before use. 15. Fluorescence quantum yield (UF) in toluene was estimated based on the reported value for 3 (UF = 0.16) in chloroform as described in del Rey, B.; Keller, U.; Torres, T.; Rojo, G.; Agulló-López, F.; Nonell, S.; Martí, C.; Brasselet, S.; Ledoux, I.; Zyss, J. J. Am. Chem. Soc. 1998, 120, 12808–12817.
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Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
A dendron of subphthalocyanine trefoil Mitsuhiko Morisue ⇑, Wataru Suzuki, Yasuhisa Kuroda Department of Biomolecular Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan
a r t i c l e
i n f o
Article history: Received 3 October 2011 Revised 1 November 2011 Accepted 7 November 2011 Available online 15 November 2011 Keywords: Dendrimer Subphthalocyanine Axial ligand Hyperbranched polymer Chromophore array
a b s t r a c t The synthesis of a dendron composed of tetrameric subphthalocyanine (SubPc) is accomplished by substituting the chlorine groups with phenoxy groups at the axial positions of SubPc with SubPc-triol. The present molecular design of the SubPc-triol introduces three phenol groups at the peripheral positions of the SubPc macrocycle as a tritopic template to construct SubPc dendrons. The self-polycondensation of SubPc-triol as a ‘divergent’ synthesis only gave a trace amount of the hyperbranched arrays due to poor solubility of the SubPc-triol. In contrast, a ‘convergent’ synthesis with the terminal SubPc improved the solubility throughout the reaction and a tetrameric SubPc dendron was obtained in moderate isolated yield. Ó 2011 Elsevier Ltd. All rights reserved.
Dendritic or hyperbranched multichromophoric arrays are emerging at the forefront of material chemistry due to their excellent photoelectronic properties.1–4 Among them, ligand-to-metal coordination provides a straightforward strategy to construct arborescent structures composed of a metallocomplex or chromophore.4 A non-centrosymmetric dendron of chromophoric unit, on which a highly branched structure converges to the focal core unit through branching p-conjugation or inter-unit electronic interactions, is a promising entity for the development of an energy cascade or a potential gradient aimed at artificial photosynthesis, organic electronics, and nonlinear optics.1–6 A three-forked node should be effective at designing a highly branched dendron in order to achieve further enhanced effects of the dendritic arrangement. The topologically fascinating concave trefoil framework of subphthalocyanine, SubPc, was chosen to triplicate the polymer backbone in a three-dimensional fashion. SubPc is a boron(III) complex of a macrocyclic ligand with a 14p electron system.7–10 The labile chlorine atom bound to the central boron atom at the axial position is readily substituted by a nucleophilic phenoxy group.8 The new axial substitution is able to deliver functional units into SubPc.8 Recently, we have successfully established a template-directed protocol to provide multi-SubPc arrays by employing a multiple phenoxy precursor. Thus, hexakis(4-hydroxyphenyl)benzene as the multiple phenoxy template gave a hexameric SubPc array in moderate isolated yield despite the presence of excessively congested multiple reaction sites.9 We now disclose the synthesis of a novel tetrameric SubPc dendron composed of three peripheral SubPc units converging into the focal SubPc bearing tritopic ⇑ Corresponding author. Tel./fax: +81 75 724 7806. E-mail address: [email protected] (M. Morisue). 0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2011.11.039
4-hydroxyphenylethynyl side-arms, SubPc-triol 1, via radial phenoxy-to-boron(III) bonds. SubPc-triol 1 was derived from triiodo-SubPc and 4-ethynylphenol by the Sonogashira cross-coupling reaction according to literature procedures (Scheme 1).10 Under the reaction conditions, the undesired axial substitution proceeded in part and gave an insoluble precipitate presumably including polycondensed byproducts. The monomeric SubPc-triol 1 was obtained as a bluish purple powder in a 12% isolated yield.11 In a similar way, SubPc-trianisole 2 was prepared as the reference compound in a 56% isolated yield.12 The following discussion concerns the synthetic investigations of the dendritic SubPc arrays. For the first trial, one-pot self-polycondensation of SubPc-triol 1 as a ‘divergent’ approach was attempted under refluxing conditions using anisole, pyridine, or THF as the solvent (Scheme 2). However, self-polycondensation was restricted presumably by very poor solubility of polycondensed 1 even in such polar solvents. Therefore, no polymeric products higher than the dimeric or trimeric species were obtained (entries 1–6 in Table 1).
Scheme 1. Synthetic scheme of SubPc-triol 1 and SubPc-trianisole 2. Reagents and conditions: the corresponding ethynylaryl precursor, CuI, Pd(PPh3)4, Et3N, toluene at 35 °C.
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Scheme 2. A ‘divergent’ approach: one-pot self-polycondensation of SubPc-triol 1 toward the hyperbranched arrays under the reflux conditions of anisole, pyridine, or THF. The results are summarized in entry 1–6 in Table 1.
Table 1 Summary of the reactions of SubPc-triol 1
a B c
Entry
SubPc-tBu3 3
Conditions
Yield (%)
1 2 3 4 5 6 7
0 equiv 0 equiv 0 equiv 0 equiv 0 equiv 0 equiv 6.5 equiv
Anisole, rt, 170 h Anisole, 60 °C, 100 h Anisole, reflux, 100 h THF, rt, 170 h THF, reflux, 100 h Pyridine, reflux, 100 h Toluene/Et3N, reflux, 22 h
Tracea Tracea Tracea Tracea Tracea Tracea 57%c
b
No hyperbranched arrays were observed. Compound 1 was soluble in toluene/Et3N only when 3 was added. Isolated yield based on SubPc-triol 1.
Additionally, the prolonged reaction times were accompanied by partial ring-expansion to afford phthalocyanine as the byproduct.7b Hyperbranched arrays were only obtained in a trace amount by the self-polycondensation of 1. However, this limited result suggests that the reactivity of SubPc-triol 1 might be controlled by the solubility of 1 under appropriate conditions. In this case, SubPc-triol 1 would be suitable for the application of a ‘convergent’ synthesis with SubPc terminal units in the dendritic structure. In the second stage, a ‘convergent’ synthesis was examined. To achieve the synthesis of the dendritic SubPc array, a terminal SubPc unit was designed to improve the solubility throughout the reaction. Thus, we introduced SubPc-tBu3 3, which bears the peripheral tert-butyl groups, as the terminal unit (Scheme 3). Then,
Scheme 3. A ‘convergent’ approach: the mixture of SubPc-triol 1 and SubPc-tBu3 3 was refluxed in toluene/truethylamine (4/1, v/v) to give the tetrameric dendron [SubPc]4 4. The result is summarized in entry 7 in Table 1.
M. Morisue et al. / Tetrahedron Letters 53 (2012) 313–316
315
Figure 1. Representative 1H NMR spectra (300 MHz) of SubPc-tBu3 3, (a) the focal SubPc-triol unit (SubPc-trianisole 2 as the model compound), (b) and [B(SubPc)]4 4 (c) in CDCl3. Asterisk indicates residual solvent.
SubPc-triol 1 was then dissolved as a mixture with a large excess of SubPc-tBu3 3 in toluene/triethylamine (4/1, v/v). After reflux in the dry mixed solvent under the nitrogen atmosphere, the multiple axial substitution reaction proceeded smoothly and gave the tetrameric dendron, [SubPc]4 4, in a 57% isolated yield (entry 7 in Table 1).13,14 Therefore, this ‘convergent’ synthesis was successful in providing a dendritic SubPc array by employing SubPc-triol 1. The 1H NMR spectrum of 4 exhibited a complete upfield shift of the a-phenyl protons of the hydroxyphenyl side-arm from 6.9 to 5.4 ppm (Fig. 1). A similar upfield shift was observed for the b-phenyl protons, indicating that all the phenoxy groups were strongly shielded by the proximal SubPc macrocycle. Moreover, the desired peak for [SubPc]4 4 was observed in a MALDI-TOF MS measurement. These results clearly indicate that full accommodation of the focal SubPc-triol unit was successful. The light-harvesting antenna function of 4 was next studied. The absorption spectrum of [SubPc]4 4 bears a close resemblance to the summation of that of SubPc-triol 1 and SubPc-tBu3 3, whereas the extinction coefficient of the focal unit of 4 is reduced to approximately half the magnitude of the monomeric 2, that is, a hypochromic effect was present (Fig. 2), similar to that observed for the hexameric SubPc array in our previous study.9 The fluorescence behavior indicates a light-harvesting antenna function. The absorption and fluorescence properties of 4 remain as the individual characters of the SubPc constituents. The focal SubPc-triol unit possesses a lower energy level (kmax = 594 nm and kem = 604 nm) compared to that of the peripheral SubPc-tBu3 units (kmax = 567 nm and kem = 574 nm). Then, [SubPc]4 4 predominantly fluoresces at 599 nm due to the focal SubPc-triol unit, even if the peripheral SubPc-tBu3 units are excited (Fig. 2). This result
Figure 2. Absorption (thick line) and fluorescence (thin line) spectra of the peripheral SubPc-tBu3 unit (SubPc-tBu4 bearing the axial phenoxy group as the model compound9) (UF = 0.11) (a), the focal SubPc-triol unit (SubPc-trianisole 2 as the model compound) (UF = 0.11) (b), and [SubPc]4 4 (UF = 0.07) (c) in toluene (25 °C).15 The fluorescence spectra are normalized to the corresponding absorbance at 510 nm of the excitation wavelength.
indicates that photoexcited energy is transferred along an energy cascade into the focal unit of [SubPc]4 4.
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In conclusion, we have successfully developed a synthetic strategy for the dendron composed of a SubPc trefoil by employing a chlorine-to-phenoxy substitution reaction of SubPc-tBu3 3. The SubPc dendron possesses octupolar chromophoric configurations as each generation of the dendron. Such hierarchical octupolar configurations might be a promising motif in the context of material science including non-linear optics, liquid crystals, etc. The remaining axial chlorine of [SubPc]4 4 is available for successive ‘convergent’ syntheses to provide higher generations of the present dendron series by employing SubPc-triol 1. Acknowledgments This work was financially supported in part through a Grantin-Aid for Young Scientists (B) (No. 21750123) from JSPS (Japan Society for the Promotion of Science) and through the Seeds Contest program from Kyowa Hakko Chemical Co., Ltd. This work was also supported by the Kyoto-Advanced Nanotechnology Network program (Graduate School of Materials Science, Nara Institute Science and Technology). References and notes 1. (a) Grayson, S. M.; Fréchet, J. M. J. Chem. Rev. 2001, 101, 3819–3867; (b) Fréchet, J. M. J. J. Polym. Sci. A. 2003, 41, 3713–3725; (c) Newkome, G. R.; Shreiner, C. Chem. Rev. 2010, 110, 6338–6442. 2. Moore, J. S. Acc. Chem. Res. 1997, 30, 402–413. 3. (a) Li, W.-S.; Aida, T. Chem. Rev. 2009, 109, 6047–6076; (b) Choi, M.-S.; Aida, T.; Yamazaki, T.; Yamazaki, I. Chem. Eur. J. 2002, 8, 2667–2678; (c) Guo, M.; Yan, X.; Kwon, Y.; Hayakawa, T.; Kakimoto, M.-A.; Goodson, T., III J. Am. Chem. Soc. 2006, 128, 14820–14821. 4. (a) Harriman, A.; Ziessel, R. Chem. Commun. 1996, 1707–1716; (b) Frey, H.; Lach, C.; Lorenz, K. Adv. Mater. 1998, 10, 279–293; (c) Newkome, G. R.; He, E.; Moorefield, C. N. Chem. Rev. 1999, 99, 1689–1746; (d) Balzani, V.; Campagna, S.; Denti, G.; Juris, A.; Serroni, S.; Venturi, M. Acc. Chem. Res. 1998, 31, 26–34. and references cited therein. 5. (a) Ma, C.-Q.; Mena-Oseritz, E.; Debaerdemaker, T.; Wienk, M. M.; Janssen, R. A. J.; Bäuerle, P. Angew. Chem., Int. Ed. 2007, 46, 1679–1683; (b) Albrecht, K.; Yamamoto, K. J. Am. Chem. Soc. 2009, 131, 2244–2251. 6. NLO: (a) Yokoyama, S.; Nakahama, T.; Otomo, A.; Mashiko, S. J. Am. Chem. Soc. 2000, 122, 3174–3181; (b) Harpham, M. R.; Süzer, Ö.; Ma, C.-Q.; Bäuerle, P.; Goodson, T., III J. Am. Chem. Soc. 2009, 131, 973–979. 7. (a) Geyer, M.; Plenzig, F.; Rauschnabel, J.; Hanack, M.; del Rey, B.; Sastre, A.; Torres, T. Synthesis 1996, 1139–1151; (b) Kobayashi, N.; Ishizaki, T.; Ishii, K.; Konami, H. J. Am. Chem. Soc. 1999, 121, 9096–9110; (c) Claessens, C. G.; González-Rodríguez, D.; Torres, T. Chem. Rev. 2002, 102, 835–853; (d) Torres, T. Angew. Chem., Int. Ed. 2006, 45, 2834–2837. 8. PhO-SubPc: (a) Claessens, C. G.; González-Rodríguez, D.; del Rey, B.; Torres, T.; Mark, G.; Schuchmann, H.-P.; von Sonntag, C.; MacDonald, J. G.; Nohr, R. S. Eur. J. Org. Chem. 2003, 2547–2551; (b) González-Rodríguez, D.; Torres, T.; Guldi, D. M.; Rivera, J.; Herranz, M. Á.; Echegoyen, L. J. Am. Chem. Soc. 2004, 126, 6301– 6313; (c) Iglesias, R. S.; Claessens, C. G.; Torres, T.; Rahman, G. M. A.; Guldi, D. M. Chem. Commun. 2005, 2113–2115; (d) González-Rodríguez, D.; Torres, T.; Olmstead, M. M.; Rivera, J.; Herranz, M. A.; Echegoyen, L.; Castellanos, C. A.; Guldi, D. M. J. Am. Chem. Soc. 2006, 128, 10680–10681; (e) Liu, J.-Y.; Yeung, H.S.; Xu, W.; Li, X.; Ng, D. K. P. Org. Lett. 2008, 10, 5421–5424; Very recently, an improved procedure for the substitution reaction of the axial chlorine to a phenoxy group has been reported by Torres and coworkers: (f) Guilleme, J.; González-Rodrígues, D.; Torres, T. Angew. Chem., Int. Ed. 2011, 50, 3506–3509.
9. Morisue, M.; Suzuki, W.; Kuroda, Y. Dalton Trans. 2011, 40, 10047–10054. 10. del Rey, B.; Torres, T. Tetrahedron Lett. 1997, 30, 5351–5354. 11. Synthesis of SubPc-triol 1, chloro[2,9(10),16(17)-tris(p-hydroxyphenylethynyl) subphthalocyaninato]boron(III): A mixture of chloro 2,9(10),16(17)-triiodosubphthalocyaninatoboron(III) (83 mg, 0.10 mmol) and phydroxyphenylacetylene (65 mg, 0.56 mmol) in triethylamine (1 mL) and toluene (4.5 mL) was deaerated by freeze-pump-thaw cycles in a Schlenk tube. To the mixture were added Pd(PPh3)4 (22 mg, 19 lmol) and CuI (18 mg, 97 lmol) as the catalyst. The mixture was stirred at 35 °C under argon atmosphere for 43 h. After the reaction mixture was passed through Florisil, the crude material was precipitated in petroleum ether. The precipitate, redissolved in ethyl acetate, was reprecipitated from chloroform. The crude material was purified by size-exclusion chromatography (Bio-Beads S-X1; 200–400 mesh, Bio-Rad Co.) with THF. The titled compound was further purified by reprecipitation in petroleum ether and then chloroform. A bluish purple solid was obtained as a C1 and C3 regioisomeric mixture (9.6 mg, 12 lmol; yield 12%). 1H NMR (300 MHz, THF-d8): d = 8.87, 8.85 (s, s, 3H; SubPca(1)), 8.71, 8.69 (d, d, J = 8.0 Hz, 3H; SubPc-a(4)), 7.94 (d, J = 8.0 Hz, 3H; SubPcb), 7.47 (d, J = 8.4 Hz, 6H; Ph b to OH), 6.81 ppm (d, J = 8.4 Hz, 6H; Ph a to OH). 13 C NMR (75 MHz, THF-d8): d = 158.6, 151.7, 151.2, 137.2, 133.1, 131.8, 131.1, 129.5, 127.8, 125.2, 124.9, 124.3, 121.5, 115.5, 113.4, 92.5, 87.5 ppm. 11B NMR (96 MHz, THF-d8): d = 16.2 ppm (br s). MALDI-TOF MS: m/z: calcd for C48H24BClN6O3: 778.17; found: 778.26 [M]+. UV–vis (THF): kmax = 588 nm (Q band). 12. Synthesis of SubPc-trianisole 2, chloro[2,9(10),16(17)-tris(p-methoxyphenylethynyl)subphthalocyaninato]boron(III): A mixture of chloro 2,9(10),16(17)-triiodosubphthalocyaninatoboron(III) (108 mg, 0.14 mmol) and p-methoxyphenyl acetylene (75 mg, 0.57 mmol) was dissolved in triethylamine (1 mL) and toluene (4.8 mL) and deoxygenated by freeze-pump-thaw cycles in a Schlenk flask. The mixture was stirred at 35 °C under argon atmosphere for 18 h. After passing through a Florisil pad, the crude material was subjected to silica-gel column chromatography (toluene/ethyl acetate, 3/1, v/v). The titled material was obtained as a bluish purple solid from reprecipitation in petroleum ether (63 mg, 77 mmol; yield 56%). 1H NMR (300 MHz, CDCl3): d = 9.02 (d, J = 3.0 Hz, 3H; SubPc-a(1)), 8.81 (dd, J = 8.3, 3.0 Hz, 3H; SubPc-b), 8.04 (d, J = 8.3 Hz, 3H; SubPc-a(4)), 7.58 (d, J = 8.8 Hz, 6H; Ph b to –OMe), 6.95 (d, J = 8.8 Hz, 6H; Ph a to –OMe), 3.87 ppm (s, 9H; –OMe) (Fig. 1b). 13C NMR (75 MHz, CDCl3): d = 160.11, 149.9, 149.7, 133.4, 132.9, 131.14, 131.07, 129.6, 129.4, 125.9, 122.3, 114.8, 114.2, 93.0, 88.3, 55.4 ppm. 11B NMR (96 MHz, CDCl3): d = 13.5 ppm. MALDI-TOF MS: m/z: calcd for C51H30BClN6O3: 820.22; found: 820.28 [M]+. UV–vis (toluene): kmax = 594 nm (Q band) (Fig. 2b). 13. Synthesis of [SubPc]4 4: A mixture of SubPc-triol 1 (13 mg, 0.017 mmol) and SubPc-tBu3 3 (66 mg, 0.11 mmol) was refluxed in 2.0 mL of toluene and 0.5 mL of triethylamine under nitrogen atmosphere for 22 h. The solvent was removed under reduced pressure, and the crude material extracted with chloroform was washed with brine. The tetrameric dendron [SubPc]4 4 was purified by size-exclusion chromatography (Bio-Beads S-X1) with chloroform and was obtained as a bluish purple solid (25 mg, 0.010 mmol) in 57% isolated yield. 1H NMR (300 MHz, CDCl3): d = 8.88 (br s, 12H; SubPc-a(1) and a(1)’), 8.79 and 8.77 (br m, 12H; SubPc-a(4) and a(4)’), 8.00–7.98 (br m, 12H; 8.88 (br s, 12H; SubPcb and b’), 7.03 (br d, J = 7.2 Hz, 6H, Ph b to OSubPc), 5.43 (br d, J = 7.2 Hz, 6H, Ph a to OSubPc), 1.55 ppm (s, 81H; t-Bu) (Fig. 1c). 13C NMR (75 MHz, CDCl3): d = 153.98, 151.32, 132.75, 131.18, 128.68, 128.09, 121.79, 119.21, 118.43, 35.84, 31.65 ppm. MALDI-TOF MS: m/z: calcd for C156H129B4ClN24O3: 2465.07; found: 2465.92 [M+H]+. UV–vis (toluene): kmax = 568 nm (Q band) (Fig. 2c). 14. We note that the reaction yield of 4 was susceptible to residual water. The flask and equipped reflux condenser were dried with heating under vacuum prior to the reaction. Toluene was distilled over molten sodium and triethylamine was distilled over sodium hydride immediately before use. 15. Fluorescence quantum yield (UF) in toluene was estimated based on the reported value for 3 (UF = 0.16) in chloroform as described in del Rey, B.; Keller, U.; Torres, T.; Rojo, G.; Agulló-López, F.; Nonell, S.; Martí, C.; Brasselet, S.; Ledoux, I.; Zyss, J. J. Am. Chem. Soc. 1998, 120, 12808–12817.