HETEROCYCLES

Paper | Special issue | Vol. 77, No. 2, 2009, pp. 1235-1248
Received, 9th August, 2008, Accepted, 30th September, 2008, Published online, 2nd October, 2008.
DOI: 10.3987/COM-08-S(F)105

■ Synthesis of Mono-, Di- and Tribenzoporphyrins from their Soluble Precursors

Tetsuo Okujima,* Yusuke Hashimoto, Guangnan Jin, Hiroko Yamada, and Noboru Ono

Department of Chemistry and Biology, Graduate School or Science and Engineering, Ehime University, Matsuyama 790-8577, Japan

Abstract

Soluble precursors of β-, meso-unsubstituted mono-, di- and tribenzoporphyrins were prepared by the reaction of dimethylbicyclo[2.2.2]octadiene-fused pyrrole with appropriate β-free pyrroles. The retro Diels-Alder reaction of these precursors afforded the corresponding benzoporphyrins in quantitative yields.

INTRODUCTION
Recently, much attention has been focused on organic semiconducting π-conjugated molecules due to their potential applications as large-area, low-cost and flexible materials, for example, in light emitting diodes, organic photovoltaics, or organic field-effect transistors (OFETs).1 Benzoporphyrins and their derivatives have the advantage of easy optimization of molecular structure; low HOMO-LUMO energy gap; and thermal and chemical stability. However, these porphyrins have low solubility and therefore have not been used in solution process suitable for fabricating large-area, low-cost thin-film devices. Improvements in the fabrication of solution-processed thin-film devices are currently driven by two different approaches: introduction of solubilizing groups and of thermally or photochemically removable groups. For example, pentacene is one of the most widely studied organic semiconductors due to its high charge mobility (up to 3 cm2/Vs).2 Pentacenes with solubilizing groups have afforded solution-processed OFETs as described by many research groups.3 Recently, the synthesis of Diels-Alder adducts of pentacene, their thermal conversion to pentacene and application to OFETs were reported by Afzali et al.4 and Müllen et al.5 We have also reported the preparation of an α-diketone precursor of pentacene and its photochemical conversion.6,7
Tetrabenzoporphyrins (TBPs) are also promising semiconductors for practical OFETs. However, only a few examples of soluble precursors of TBPs synthesized utilizing the masked isoindole method have been reported by Cavaleiro and Vinogradov.8 Recently, we have reported a breakthrough methodology for the preparation of TBPs as shown in Scheme 1, and the fabrication of TBP-based OFETs by a solution process.7,9–11 Mono-, di- and tribenzoporphyrins are attractive molecules, which are expected to lead to thin-film devices with reduced π-conjugation compared to TBPs and fine tuning of HOMO-LUMO gaps of benzoporphyrins.9,12,13 Although β-alkyl monobenzoporphyrins9 and meso-aryl dibenzoporphyrins13 have been prepared starting from pyrroles fused with bicyclo[2.2.2]octadiene (BCOD), the corresponding parent compounds have not been obtained so far.

In this paper, we report the synthesis of parent mono-, di- and tribenzoporphyrins 3 – 6 from their soluble precursors by the retro Diels-Alder reaction. Since TBP precursors 2 were found to be more soluble than 1,10 soluble precursors of β-, meso-unsubstituted benzoporphyrins were expected to be derived from pyrroles fused with dimethylBCOD. Therefore we used dimethylBCOD-fused pyrroles as masked isoindoles in the synthesis of the precursors of these porphyrins.

RESULTS AND DISCUSSION
4,7-Dihydro-8,8-dimethyl-4,7-ethano-2H-isoindole (7a)10 was treated with methyl orthoformate in TFA to give dialdehyde 7b in 55% yield. The condensation of tripyrrane 814 with 7b in CHCl3 in the presence of TFA followed by oxidation with DDQ gave mono(dimethylBCOD)porphyrin 9 in 7% yield (Scheme 2).

Syntheses of dibenzoporphyrins are shown in Schemes 3 and 4. There are two stereoisomers of dibenzoporphyrin: adj-type 4 and opp-type 5. To prepare the adj-type precursor, a reaction was carried out using dipyrromethane15 with two equimolar amounts of (hydroxymethyl)pyrrole fused with dimethylBCOD and an equimolar amount of formaldehyde (Scheme 3). Reduction of dimehylBCOD-fused pyrrole 7c with LiAlH4 gave the corresponding (hydroxymethyl)pyrrole, which was treated with paraformaldehyde and dipyrromethane in the presence of p­-TsOH and then oxidized with p-chloranil to give adj-bis(dimethylBCOD)porphyrin 10 as a mixture of diastereomers in 18% yield. These isomers could not be isolated by column chromatography or recrystallization. Since the yield in the deesterification of α-bis(ethoxycarbonyl)dipyrromethane fused with dimethylBCOD was very low, 2+2 condensation of α-free dimethylBCOD-fused dipyrromethane with α-diformyldipyrromethane was not carried out.

The synthesis of the precursor of opp-dibenzoporphyrin 5 is summarized in Scheme 4. Acid-catalyzed condensation of 7a with 2,5-bis(hydroxymethyl)pyrrole (11) afforded opp-bis(dimethylBCOD)porphyrin 12 as a mixture of diastereomers in 5% yield (route A). Porphyrin 12 was also prepared by 3+1 condensation of tripyrrane 14 with 13 (route B). (Acetoxymethyl)pyrrole 7g was prepared by the Barton-Zard reaction followed by a Vilsmeier-Haack reaction, reduction and acetylation as shown in Scheme 5.16 The condensation of the parent pyrrole with 7g catalyzed by montmorillonite K-10 afforded tripyrrane 14, which was used in the reaction with 13 without further purification after work-up.16,17 Tripyrrane 14 was deesterified by TFA. The resulting mixture was diluted with CHCl3 and treated with 13. After oxidation of the porphyrinogen with DDQ, work-up of the reaction mixture furnished opp-bis(dimethylBCOD)porphyrin 12.

A similar 3+1 porphyrin synthesis afforded the tribenzoporphyrin precursor. DimethylBCOD-fused tripyrrane 16 was prepared by montmorillonite-catalyzed condensation of 7a with 7g, and a subsequent 3+1 reaction of 16 with 13 in the presence of TFA followed by oxidation with DDQ gave tris(dimethylBCOD)porphyrin 17 (Scheme 6). These porphyrins fused with dimethylBCOD 9, 10, 12 and 17 were stable and soluble in CHCl3. They were fully characterized by their spectral data as shown in the experimental section.
Thermogravimetric analysis (TGA) curves of dimethylBCOD-fused porphyrins are shown in Figure 1. The weight loss of 9 started at around 150 ºC and ceased after 180 ºC. The loss of weight was ca. 14%, consistent with the calculated value of 13.5%. Similarly, the retro Diels-Alder reactions of 10, 12 and 17 started at 140 – 150 ºC and were completed by 180 – 190 ºC. The loss of weight was 22% (for 10), 30% (12), and 38% (17), corresponding to the elimination of two or three isobutene molecules and any included solvents. The retro Diels-Alder reaction of 9 was carried out at 200 ºC for 10 min in vacuo in a glass tube oven to give β-, meso-unsubstitued benzoporphyrin (3) in nearly quantitative yield.

The absorption spectra of 9 and 3 are shown in Figure 2. The Soret band of 3 appeared at 402 nm, while that of 9 appeared at 397 nm. Heating 10, 12, and 17 at 200 ºC also resulted in clean formation of 4, 5, and 6, respectively. The Soret bands showed a gradual bathochromic shift as the number of fused benzene rings increased (Figure 3). The absorption and fluorescence data and spectra are contained in Table 1 and Figure 4, respectively. The HOMO-LUMO energy gaps Eg of 3–6, which were estimated from the longest absorption maxima and emission maxima in CHCl3 solution, were 1.98, 1.97, 1.92 and 1.91 eV, respectively. The longest absorption maxima of 4 and 5 were about the same as for 3 and 6, respectively, though both 4 and 5 were dibenzoporphyrins. On the other hand, in 1H NMR spectroscopy of 3–6, signals of the β-pyrrolic protons in 3 were observed at 9.58 ppm for two adj-pyrroles to isoindole and 9.38 ppm for a opp-pyrrole. Benzo protons of 3 showed two multiplet signals at 9.36 and 8.13 ppm. These porphyrins 3 – 6 exhibited the ring current effect similar to that reported for benzoporphyrins fused with BCODs.18

In summary, we have shown that β-, meso-unsubstituted mono-, di-, and tribenzoporphyrins were synthesized from their soluble dimethylBCOD-fused precursors by the retro Diels-Alder reaction in nearly quantitative yield. Further work on fabricating OFETs based on these porphyrins by a solution process is under way.

EXPERIMENTAL
General. Melting points were determined on a Yanaco micro melting point apparatus MP500D and are reported here uncorrected. DI-EI and FAB mass spectra were measured on a JEOL JMS-700. TG analyses were performed on an SII Exstar 600 TG/DTA 6200. IR spectra were measured on a Horiba FT-720 infrared spectrophotometer, and UV-vis spectra on a JASCO V-570 spectrophotometer. 1H NMR spectra (13C NMR spectra) were recorded on a JEOL AL-400 at 400 MHz (100 MHz). Elemental analyses were performed at the Integrated Center for Sciences, Ehime University.

4,7-Dihydro-8,8-dimethyl-4,7-ethano-2H-isoindole-1,3-dicarbaldehyde (7b)
TFA (10 mL) was added slowly to 7a (879 mg, 5.08 mmol) at 0 ºC under an Ar atmosphere and the mixture was stirred for 5 min. After slow addition of methyl orthoformate (12 mL), stirring was coninued at 0 ºC for 2 h. After neutralization with 20% aqueous NaOH, the mixture was extracted with CHCl3. The organic layer was washed successively with water and brine, dried over Na2SO4, and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel with CHCl3 and CHCl3/EtOAc to give 7b (644 mg, 55%).
colorless oil; MS (70 eV) m/z (relative intensity) 230 (M++1, 45%) and 174 (100); IR (neat) νmax 3260, 1684, 1647 and 1573 cm-1; 1H NMR (400 MHz, CDCl3) δ 9.82 (s, 1H, 1- or 3-CHO), 9.80 (s, 1H, 1- or 3-CHO), 9.14 (br, 1H, NH), 6.60 (ddd, 1H, J = 6.3, 6.0, 1.0 Hz, H6), 6.54 (ddd, 1H, J = 6.3, 6.0, 1.0 Hz, H5), 4.20 (m, 1H, H4), 3.73 (dd, 1H, J = 6.0, 1.0 Hz, H7), 1.55 (dd, 1H, J = 12.0, 2.7 Hz, H9), 1.34 (dd, 1H, J = 12.0, 2.7 Hz, H9), 1.13 (s, 3H, 8-Me), and 0.76 (s, 3H, 8-Me); 13C NMR (100 MHz, CDCl3) δ 179.38 (1- or 3-CHO), 179.30 (1- or 3-CHO), 140.50 (C7a), 139.13 (C3a), 135.73 (C6), 134.02 (C5), 128.67 (C1 or C3), 126.54 (C1 or C3), 45.32 (C7), 42.78 (C9), 37.86 (C8), 33.81 (C4), 30.56 (8-Me), and 30.53 (8-Me). Anal. Calcd for C14H15NO2·1/4CHCl3: C, 66.05; H, 5.93; N, 5.41. Found: C, 65.98; H, 5.98; N, 5.42.

t-Butyl 4,7-dihydro-8,8-dimethyl-4,7-ethano-2H-isoindole-1-carboxylate (7d)
A solution of potassium t-butoxide (8.52 g) in dry THF (75 mL) was added dropwise to a stirred solution of 15 (10.44 g, 25.07 mmol) and t-butyl isocyanoacetate (5.3 mL) in dry THF (300 mL) at below 0 °C under an Ar atmosphere. The resulting mixture was stirred at rt for 1 d. The reaction mixture was poured into 1 M HCl, evaporated and extracted with CHCl3. The organic layer was washed successively with water and brine, dried over Na2SO4, and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel with CHCl3 followed by recrystallization with CHCl3/hexane to give 7d (6.27 g, 91%).
white powder; mp 180.1–181.9 °C; MS (70 eV) m/z (relative intensity) 274 (M++1, 2%), 217 (22), and 161 (100); IR (KBr disk) νmax 3321 and 1682 cm-1; 1H NMR (400 MHz, CDCl3) δ 8.43 (br, 1H, NH), 6.53 (ddd, 1H, J = 6.5, 6.1, 1.4 Hz, H6), 6.51 (d, 1H, J = 2.7 Hz, H3), 6.48 (ddd, 1H, J = 6.5, 6.1, 1.4 Hz, H5), 3.77 (d, 1H, J = 6.1 Hz, H7), 3.72 (m, 1H, H4), 1.57 (s, 9H, 1-CO2t-Bu), 1.39 (dd, 1H, J = 11.7, 2.7 Hz, H9), 1.21 (dd, 1H, J = 11.7, 2.7 Hz, H9), 1.06 (s, 3H, 8-Me), and 0.71 (s, 3H, 8-Me); 13C NMR (100 MHz, CDCl3) δ 161.21 (1-CO2t-Bu), 136.05 (C6), 135.67 (C7a), 135.19 (C5), 129.80 (C3a), 117.49 (C1), 112.07 (C3), 80.12 (1-CO2t-Bu), 46.46 (C7), 43.86 (C9), 37.77 (C8), 34.63 (C4), 30.90 (8-Me), 30.28 (8-Me), and 28.60 (1-CO2t-Bu). Anal. Calcd for C17H23NO2: C, 74.69; H, 8.48; N, 5.12. Found: C, 74.69;
H, 8.38; N, 5.07.

t-Butyl 4,7-dihydro-8,8-dimethyl-4,7-ethano-3-formyl-2H-isoindole-1-carboxylate (7e)
A mixture of DMF (1.5 mL) and phosphoryl chloride (1.3 mL) was stirred at 0 ºC for 30 min under an Ar atmosphere. A solution of 7d (2.73 g, 10.0 mmol) in dry CH2Cl2 (50 mL) was added dropwise to this mixture over 30 min at 0 °C. The mixture was stirred at rt for 3 h. After addition of aqueous NaOAc (2.00 g/20 mL) to the reaction mixture, it was stirred at rt for 30 min. The organic layer was washed succesively with water, sat. aqueous NaHCO3, and brine; dried with Na2SO4; and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel with CHCl3 followed by recrystallization with CHCl3/hexane to give 7e (2.92 g, 97%).
white powder; mp 130.5 – 132.0 °C; MS (70 eV) m/z (relative intensity) 302 (M++1, 100) and 245 (64); IR (KBr disk) νmax 3301, 1684 and 1666 cm-1; 1H NMR (400 MHz, CDCl3) δ 9.69 (s, 1H, 3-CHO), 9.00 (br, 1H, NH), 6.59 (ddd, 1H, J = 6.5, 6.1, 1.0 Hz, H6), 6.48 (ddd, 1H, J = 6.5, 6.1, 1.0 Hz, H5), 4.14 (m, 1H, H4), 3.83 (dd, 1H, J = 6.1, 1.0 Hz, H7), 1.59 (s, 9H, 1-CO2t-Bu), 1.50 (dd, 1H, J = 11.7, 2.9 Hz, H9), 1.28 (dd, 1H, J = 11.7, 2.9 Hz, H9), 1.09 (s, 3H, 8-Me), and 0.73 (s, 3H, 8-Me); 13C NMR (100 MHz, CDCl3) δ 178.13 (3-CHO), 160.09 (1-CO2t-Bu), 139.89 (C3a), 136.63 (C6), 136.22 (C7a), 133.52 (C5), 124.57 (C3), 122.54 (C1), 81.97 (1-CO2t-Bu), 46.11 (C7), 43.03 (C9), 37.94 (C8), 33.93 (C4), 30.65 (8-Me), 30.18 (8-Me), and 28.41 (1-CO2t-Bu). Anal. Calcd for C18H23NO3: C, 71.73; H, 7.69; N, 4.65. Found: C, 71.43; H, 7.72; N, 4.57.

t-Butyl 4,7-dihydro-8,8-dimethyl-4,7-ethano-3-hydroxymethyl-2H-isoindole-1-carboxylate (7f)
Sodium borohydride (802 mg) was added slowly to a solution of 7e (2.11 g, 7.00 mmol) in THF (35 mL) and MeOH (15 mL) at 0 °C. The resulting mixture was stirred at the same temperature for 2 h. After addition of water, the mixture was extracted with CHCl3. The organic layer was washed succesively with water and brine, dried over Na2SO4, and concentrated under reduced pressure. A mixture of the residue with hexane was cooled in a refrigerator, and the resulting precipitate was collected by filtration and washed with cold hexane to give 7f (1.99 g, 94%).
white powder; mp 140.7 –141.5 °C; MS (70 eV) m/z (relative intensity) 304 (M++1, 6%), 247 (76), and 191 (100); IR (KBr disk) νmax 3508, 3406, 2970, 2941, 2862 and 1664 cm-1; 1H NMR (400 MHz, CDCl3) δ 8.69 (br, 1H, NH), 6.52 (ddd, 1H, J = 6.6, 6.1, 1.2 Hz, H6), 6.46 (ddd, 1H, J = 6.6, 6.1, 1.2 Hz, H5), 4.61 (d, 2H, J = 5.9 Hz, 3-CH2OH), 3.74 (dd, 1H, J = 6.1, 1.2 Hz, H7), 3.72 (m, 1H, H4), 2.11 (br, 1H, 3-CH2OH), 1.56 (s, 9H, 1-CO2t-Bu), 1.39 (dd, 1H, J = 11.7, 2.7 Hz, H9), 1.19 (dd, 1H, J = 11.7, 2.7 Hz, H9), 1.05 (s, 3H, 8-Me), and 0.71 (s, 3H, 8-Me); 13C NMR (100 MHz, CDCl3) δ 161.32 (1-CO2t-Bu), 136.13 (C6), 136.10 (C7a), 134.92 (C5), 127.37 (C3a), 125.06 (C3), 116.94 (C1), 80.33 (1-CO2t-Bu), 56.43 (3-CH2OH), 46.55 (C7), 43.74 (C9), 37.74 (C8), 33.72 (C4), 30.86 (8-Me), 30.28 (8-Me), and 28.61
(1-CO2t-Bu). Anal. Calcd for C18H25NO3: C, 71.26; H, 8.31; N, 4.62. Found: C, 71.03; H, 8.32; N, 4.60.

t-Butyl 3-acetoxymethyl-4,7-dihydro-8,8-dimethyl-4,7-ethano-2H-isoindole-1-carboxylate (7g)
To a solution of 7f (1.73 g, 5.71 mmol) in CHCl3 (25 mL) were added acetic anhydride (0.9 mL) and 4-(dimethylamino)pyridine (21 mg) at rt. After stirring at rt for 2 h, the reaction mixture was poured into sat. aqueous NaHCO3. The organic layer was washed succesively with water and brine, dried over Na2SO4, and concentrated under reduced pressure. The residue was washed with CHCl3/hexane to give 7g (1.89 g, 96%).
white powder; mp 102.5 – 103.8 °C; MS (70 eV) m/z (relative intensity) 346 (M++1, 3%), 289 (57), and 233 (100); IR (KBr disk) νmax 3303, 1745 1670 cm-1; 1H NMR (400 MHz, CDCl3) δ 8.48 (br, 1H, NH), 6.53 (dd, 1H, J = 6.9, 6.2 Hz, H6), 6.47 (dd, 1H, J = 6.9, 6.2 Hz, H5), 5.03 (d, 1H, J = 12.9 Hz, 3-CH2OAc), 4.96 (d, 1H, J = 12.9 Hz, 3-CH2OAc), 3.77 (m, 1H, H4), 3.75 (d, 1H, J = 6.2 Hz, H7), 2.06 (s, 3H, 3-CH2OAc), 1.56 (s, 9H, 1-CO2t-Bu), 1.40 (dd, 1H, J = 11.6, 2.7 Hz, H9), 1.19 (dd, 1H, J = 11.6, 2.7 Hz, H9), 1.06 (s, 3H, 8-Me), and 0.71 (s, 3H, 8-Me); 13C NMR (100 MHz, CDCl3) δ 171.32 (3-CH2OAc), 160.89 (1-CO2t-Bu), 136.17 (C6), 135.36 (C7a), 134.80 (C5), 129.79 (C3a), 120.19 (C3), 117.69 (C1), 80.38 (1-CO2t-Bu), 57.09 (3-CH2OAc), 46.47 (C7), 43.60 (C9), 37.73 (C8), 33.76 (C4), 3.82 (8-Me), 30.27 (8-Me), 28.55 (1-CO2t-Bu), and 21.03 (3-CH2OAc). Anal. Calcd for C20H27NO4: C, 69.54; H, 7.88; N, 4.05. Found: C, 69.31; H, 7.79; N, 4.03.

Mono(dimethylBCOD)porphyrin 9
A solution of 7b (238 mg, 1.04 mmol) and 8 (234 mg, 1.04 mmol) in CHCl3 (25 mL) was added dropwise over 30 min to a solution of TFA (8.0 mL) in CHCl3 (100 mL) at rt under an Ar atmosphere in a shaded vessel, and the resulting mixture was stirred for 10 min. The reaction mixture was treated with DDQ (68 mg, 0.30 mmol) for 10 min with stirring at rt, and subsequently neutralized with triethylamine. The resulting black precipitate was removed by filtration with Celite. The filtrate was washed succesively with aqueous NaHCO3 and brine, dried over Na2SO4, and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel with CHCl3 followed by recrystallization with CHCl3/MeOH to give 9 (32 mg, 7%).
red-violet crystals; MS (FAB) m/z 417 (M++1) and 360; UV-vis (CHCl3) λmax, nm 397, 492, 523, and 562; 1H NMR (400 MHz, CDCl3) δ 10.35 (s, 1H, meso-H), 10.34 (s, 1H, meso-H), 10.33 (s, 2H, meso-H), 9.56 (s, 2H, β-H), 9.45 (m, 4H, β-H), 7.23 (dd, 1H, J = 6.6, 5.9 Hz, olefin), 7.17 (dd, 1H, J = 6.6, 5.9 Hz, olefin), 5.65 (m, 1H, bridge head), 5.18 (d, 1H, J = 5.9 Hz, bridge head), 2.10 (dd, 1H, J = 11.7, 2.7 Hz, bridge CH2), 1.75 (dd, 1H, J = 11.7, 2.7 Hz, bridge CH2), 1.55 (s, 3H, Me), 0.64 (s, 3H, Me), and -4.22 (br, 2H, NH). Anal. Calcd for C28H24N4·1/4H2O: C, 79.88; H, 5.86; N, 13.31. Found: C, 80.16; H, 5.94; N, 13.27.
adj-Bis(dimethylBCOD)porphyrin 10
To a solution of 7c (490 mg, 2.00 mmol) in dry THF was added slowly LiAlH4 (349 mg) at 0 ºC under an Ar atmosphere in a shaded vessel. The resulting mixture was stirred at the same temperature for 3 h. The reaction mixture was poured into water, filtered with Celite, and extracted with CHCl3. The organic layer was washed successively with water and brine, dried over Na2SO4, and concentrated under reduced pressure. The residue was diluted with CHCl3 (300 mL), and dipyrromethane (110 mg, 0.755 mmol) was added at rt under an Ar atmosphere in a shaded vessel. After addition of a solution of p-TsOH·H2O (24 mg) and paraformaldehyde (45 mg) in CHCl3 (100 mL), the mixture was stirred for 25 h, after which p-chloranil (214 mg, 0.872 mmol) was added. After stirring at rt for 1 h, the reaction mixture was poured into water. The organic layer was washed successively with sat. aqueous NaHCO3, water, and brine; dried over Na2SO4; and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel with CHCl3 to give 10 (69 mg, 18%).
maroon powder; MS (FAB) m/z 523 (M++1), 466, and 410; UV-vis (CHCl3) λmax, nm 394, 493, 523, 563, and 615; 1H NMR (400 MHz, CDCl3) δ 10.28 – 10.34 (m, 4H, meso-H), 9.45 – 9.49 (m, 4H, β-H), 7.10 – 7.20 (m, 4H, olefin), 5.55 – 5.61 (m, 2H, bridge head), 5.08 – 5.16 (m, 2H, bridge head), 2.04 – 2.08 (m, 2H, bridge CH2), 1.73 – 1.78 (m, 2H, bridge CH2), 1.52 – 1.53 (m, 6H, Me), 0.64 – 0.74 (m, 6H, Me), and -4.26 (br, 2H, NH). Anal. Calcd for C36H34N4·1/2H2O: C, 81.32; H, 6.64; N, 10.54. Found: C, 81.12; H, 6.60; N, 10.44.

opp-Bis(dimethylBCOD)porphyrin 12
Route A: After bubbling Ar through a solution of 7a (222 mg, 1.28 mmol) and 11 (163 mg, 1.28 mmol) in CHCl3/MeOH (200 mL/40 mL) in a shaded vessel, BF3·Et2O (20 µL) was added at rt under an Ar atmosphere. After the resulting mixture was stirred at the same temperature for 22.5 h, p-chloranil (235 mg, 0.956 mmol) was added, and the reaction mixture was poured into water after stirring for 1 h. The organic layer was washed successively with aqueous NaHCO3, water, and brine; dried over Na2SO4; and concentrated under reduced pressure. The residue was purified by column chromatography on alumina with CHCl3 and then on silica gel with CHCl3, followed by recrystallization CHCl3/MeOH, to give 12 (16 mg, 5%).
Route B: Tripyrrane 14 was prepared by the reaction of 7g (345 mg, 1.00 mmol) with pyrrole (35 µL) in the presence of montmorillonite K-10 clay (0.5 g) which was dried by heating at 100 ºC for 30 min in vacuo, according to a literature procedure.16,17 A solution of 14 in TFA (3 mL) was stirred at rt for 10 min under an Ar atmosphere in a shaded vessel. After dilution with CHCl3 (200 mL), 13 (62 mg, 0.50 mmol) was added to the mixture, which was stirred at the same temperature for 22 h. The reaction mixture was neutralized with triethylamine and treated with DDQ (68 mg, 0.30 mmol) for 1 h with stirring at rt. The mixture was washed successively with aqueous NaHCO3, water, and brine; dried over Na2SO4; and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel with CHCl3 to give 12 (49 mg, 19%).
red-violet powder; MS (FAB) m/z 523 (M++1), 465, and 410; UV-vis (CHCl3) λmax, nm 398, 496, 529, and 562; 1H NMR (400 MHz, CDCl3) δ 10.37 (s, 2H, meso-H), 10.35 (s, 2H, meso-H), 9.46 (s, 2H, β-H), 9.45 (s, 2H, β-H), 7.25 (ddd, 2H, J = 6.3, 5.9, 1.2 Hz, olefin), 7.19 (ddd, 2H, J = 6.3, 5.9, 1.2 Hz, olefin), 5.68 (m, 2H, bridge head), 5.20 (dd, 2H, J = 5.9, 1.2 Hz, bridge head), 2.12 (dd, 2H, J = 11.7, 2.7 Hz, bridge CH2), 1.76 (dd, 2H, J = 11.7, 2.7 Hz, bridge CH2), 1.56 (s, 6H, Me), 0.63 (s, 6H, Me), and -4.51 (br, 2H, NH). Anal. Calcd for C36H34N4·CHCl3: C, 69.21; H, 5.49; N, 8.73. Found: C, 68.98; H, 5.56; N, 8.73.

Tris(dimethylBCOD)porphyrin 17
Tripyrrane 16 was prepared by the reaction of 7g (345 mg, 1.00 mmol) with 7a (87 mg, 0.50 mmol) in the presence of montmorillonite K-10 clay (0.5 g) which was dried by heating at 100 ºC for 30 min in vacuo, as before.16,17 A solution of 16 in TFA (3 mL) was stirred at rt for 10 min under an Ar atmosphere in a shaded vessel. After dilution with CHCl3 (200 mL), 13 (62 mg, 0.50 mmol) was added to the mixture, which was stirred at the same temperature for 4 h. The reaction mixture was neutralized with triethylamine and treated with DDQ (68 mg, 0.30 mmol) for 30 min with stirring at rt, after which it was poured into aqueous NaHCO3. The organic layer was washed successively with water and brine, dried over Na2SO4, and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel with CHCl3 followed by recrystallization with CHCl3/MeOH to give 17 (66 mg, 20%).
dark violet powder; MS (FAB) m/z 629 (M++1), 572, 515, and 460; UV-vis (CHCl3) λmax, nm 397, 496, 528, 563, and 614; 1H NMR (400 MHz, CDCl3) δ 10.28 – 10.35 (m, 4H, meso-H), 9.45 – 9.46 (m, 2H, β-H), 7.08 – 7.26 (m, 6H, olefin), 5.67 – 5.73 (m, 2H, bridge head), 5.53 – 5.58 (m, 1H, bridge head), 5.16 – 5.20 (m, 2H, bridge head), 5.05 – 5.08 (m, 1H, bridge head), 2.05 – 2.14 (m, 3H, bridge CH2), 1.72 – 1.88 (m, 3H, bridge CH2), 1.54 – 1.57 (m, 9H, Me), 0.57 – 0.81 (m, 9H, Me), and -4.54 (br, 2H, NH). Anal. Calcd for C44H44N4·H2O: C, 81.70; H, 7.17; N, 8.66. Found: C, 81.55; H, 6.90; N, 8.38.

Retro Diels-Alder reaction
(DimethylBCOD)porphyrins 9, 10, 12, and 17 (ca. 10 mg each) were heated at 200 ºC under reduced pressure for 10 min in a glass tube to give benzoporphyrins 3 – 6 in quantitative yields.
3: red-violet powder; MS (FAB) m/z 360 (M+); UV-vis (CHCl3) λmax, nm 402, 495, 525, 570, and 626. 1H NMR (400 MHz, CDCl3) δ 10.59 (s, 2H, meso-H), 10.30 (s, 2H, meso-H), 9.56 – 9.60 (m, 4H, pyrrole-β), 9.38 (s, 2H, pyrrole-β), 9.36 (m, 2H, benzo), 8.13 (m, 2H, benzo), and -3.57 (br, 2H, NH); Anal. Calcd for C24H16N4: C, 79.98; H, 4.47; N, 15.55. Found: C, 79.77; H, 4.69; N, 15.32.
4: purple powder; MS (FAB) m/z 410 (M+); UV-vis (CHCl3) λmax, nm 407, 520, 551, 572, and 627. 1H NMR (400 MHz, CDCl3) δ 10.63 (s, 1H, meso-H), 10.39 (s, 2H, meso-H), 10.19 (s, 1H, meso-H), 9.52 (m, 2H, benzo), 9.44 (m, 2H, benzo), 9.38 (d, 2H, J = 4.2 Hz, pyrrole-β), 9.32 (d, 2H, J = 4.2 Hz, pyrrole-β), 8.24 (m, 4H, benzo), and -2.52 (br, 2H, NH); Anal. Calcd for C28H18N4: C, 81.93; H, 4.42; N, 13.65. Found: C, 82.20; H, 4.72; N, 13.36.
5: red-violet powder; MS (FAB) m/z 410 (M+); UV-vis (CHCl3) λmax, nm 401, 409, 492, 529, 586, 593, and 645. 1H NMR (400 MHz, CDCl3) δ 10.60 (s, 4H, meso-H), 9.60 (s, 4H, pyrrole-β), 9.36 (m, 4H, benzo), and 8.12 (m, 4H, benzo); Anal. Calcd for C28H18N4: C, 81.93; H, 4.42; N, 13.65. Found: C, 81.76; H, 4.58; N, 13.50.
6: dark violet powder; MS (FAB) m/z 460 (M+); UV-vis (CHCl3) λmax, nm 391, 412, 525, 557, 588, and 646. 1H NMR (400 MHz, CDCl3) δ 10.51 (s, 2H, meso-H), 10.38 (s, 2H, meso-H), 9.62 (m, 2H, benzo), 9.35 (s, 2H, pyrrole-β), 9.33 (m, 2H, benzo), 9.27 (m, 2H, benzo), 8.36 (m, 2H, benzo), 8.10 (m, 4H, benzo), and -2.35 (br, 2H, NH); Anal. Calcd for C32H20N4: C, 83.46; H, 4.38; N, 12.17. Found: C, 83.54; H, 4.50; N, 11.95.

ACKNOWLEDGEMENTS
This work was partially supported by Grants-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology, Japan (No. 18550036 to NO and No. 18550037 to HY) and the JGC-S Scholarship Foundation (TO). We used ethyl isocyanoacetate from the Nippon Synthetic Chem. Ind. (Osaka Japan).


Dedicated to Professor Emeritus Keiichiro Fukumoto on the occasion of his 75th birthday.

References

1. S. R. Forrest, Nature, 2004, 428, 911; CrossRef A. R. Murphy and J. M. J. Fréchet, Chem. Rev., 2007, 107, 1066;; CrossRef C. D. Dimitrakopoulos and P. R. L. Malenfant, Adv. Mater., 2002, 14, 99; CrossRef Y. -L. Loo, AIChE Journal, 2007, 53, 1066; CrossRef K. Takimiya, Y. Kunugi, and T. Otsubo, Chem. Lett., 2007, 36, 578. CrossRef
2. H. Klauk, M. Halik, U. Zschieschang, G. Schmid, and W. Radlik, J. Appl. Phys., 2002, 92, 5259; CrossRef T. W. Kelley, L. D. Boardman, T. D. Dunbar, D. V. Muyres, M. J. Pellerite, and T. P. Smith, J. Phys. Chem. B, 2003, 107, 5877. CrossRef
3. C. D. Sheraw, T. N. Jackson, D. L. Eaton, and J. E. Anthony, Adv. Mater., 2003, 15, 2009; CrossRef F. Würthner and R. Schmidt, ChemPhysChem, 2006, 7, 793; CrossRef M. M. Payne, S. R. Parkin, J. E. Anthony, C. -C. Kuo, and T. N. Jackson, J. Am. Chem. Soc., 2005, 127, 4986; CrossRef Y. Li, Y. Wu, P. Liu, Z. Prostran, S. Gardner, and B. S. Ong, Chem. Mater., 2007, 19, 418. CrossRef
4. A. Afzali, C. D. Dimitrakopoulos, and T. L. Breen, J. Am. Chem. Soc., 2002, 124, 8812; CrossRef K. P. Weidkamp, A. Afzali, R. M. Tromp, and R. J. Hamers, J. Am. Chem. Soc., 2004, 126, 12740; CrossRef C. R. Kagan, A. Afzali, and T. O. Graham, Appl. Phys. Lett., 2005, 86, 193505; CrossRef G. S. Tulevski, Q. Miao, A. Afzali, T. O. Graham, C. R. Kagan, and C. Nuckolls, J. Am. Chem. Soc., 2006, 128, 1788. CrossRef
5. A. R. Brown, A. Pomp, D. M. de Leeuw, D. B. M. Klaassen, P. Herwig, and K. Müllen, J. Appl. Phys., 1996, 79, 2136; CrossRef P. T. Herwig, and K. Müllen, Adv. Mater., 1999, 11, 480. CrossRef
6. H. Uno, Y. Yamashita, M. Kikuchi, H. Watanabe, H. Yamada, T. Okujima, T. Ogawa, and N. Ono, Tetrahedron Lett., 2005, 46, 1981; CrossRef H. Yamada, Y. Yamashita, M. Kikuchi, H. Watanabe, T. Okujima, H. Uno, T. Ogawa, K. Ohara, and N. Ono, Chem. Eur. J., 2005, 11, 6212. CrossRef
7. H. Yamada, T. Okujima, and N. Ono, Chem. Commun., 2008, 2957. CrossRef
8. M. G. V. Vicente, A. C. Tomé, A. Walter, and J. A. S. Cavaleiro, Tetrahedron Lett., 1997, 38, 3639; CrossRef O. S. Finikova, A. V. Cheprakov, I. P. Beletskaya, P. J. Carroll, and S. A. Vinogradov, J. Org. Chem., 2004, 69, 522. CrossRef
9. S. Ito, T. Murashima, H. Uno, and N. Ono, Chem. Commun., 1998, 1661; S. Ito, N. Ochi, T. Murashima, H. Uno, and N. Ono, Heterocycles, 2000, 52, 399. CrossRef
10. T. Okujima, Y. Hashimoto, G. Jin, H. Yamada, H. Uno, and N. Ono, Tetrahedron, 2008, 64, 2405. CrossRef
11. S. Aramaki, Y. Sakai, and N. Ono, Appl. Phys. Lett., 2004, 84, 2085; CrossRef P. B. Shea, J. Kanicki, and N. Ono, J. Appl. Phys., 2005, 98, 014503; CrossRef P. B. Shea, J. Kanicki, L. R. Pattison, P. Petroff, M. Kawano, H. Yamada, and N. Ono, J. Appl. Phys., 2006, 100, 034502; CrossRef A. S. Dhoot, S. Aramaki, D. Moses, and A. J. Heeger, Adv. Mater., 2007, 19, 2914. CrossRef
12. S. H. Lee and K. M. Smith, Tetrahedron Lett., 2005, 46, 2009. CrossRef
13. Y. Inokuma, N. Ono, H. Uno, D. Y. Kim, S. B. Noh, D. Kim, and A. Osuka, Chem. Commun., 2005, 3782. CrossRef
14. S. Taniguchi, H. Hasegawa, M. Nishimura, and M. Takahashi, Synlett, 1999, 73; CrossRef S. Taniguchi, H. Hasegawa, S. Yanagiwa, Y. Tabeta, Y. Nakano, and M. Takahashi, Tetrahedron, 2001, 57, 2103. CrossRef
15. R. Chong, P. S. Clezy, A. J. Liepa, and A. W. Nichol, Aust. J. Chem., 1969, 22, 229; CrossRef P. S. Clezy and G. A. Smythe, Aust. J. Chem., 1969, 22, 239; CrossRef Q. W. Wang and D. W. Bruce, Synlett, 1995, 1267. CrossRef
16. T. Okujima, N. Komobuchi, Y. Shimizu, H. Uno, and N. Ono, Tetrahedron Lett., 2004, 45, 5461; CrossRef T. Okujima, N. Komobuchi, H. Uno, and N. Ono, Heterocycles, 2006, 67, 255. CrossRef
17. A. Boudif and M. Momenteau, J. Chem. Soc., Perkin Trans. 1, 1996, 1235. CrossRef
18. H. Uno, Y. Shimizu, H. Uoyama, Y. Tanaka, T. Okujima, and N. Ono, Eur. J. Org. Chem., 2008, 87; CrossRef J. Mack, M. Bunya, Y. Shimizu, H. Uoyama, N. Komobuchi, T. Okujima, H. Uno, S. Ito, M. J. Stillman, N. Ono, N. Kobayashi, Chem. Eur. J., 2008, 14, 5001 CrossRef