HETEROCYCLES

Paper | Special issue | Vol. 80, No. 2, 2010, pp. 1187-1196
Received, 3rd August, 2009, Accepted, 31st August, 2009, Published online, 4th September, 2009.
DOI: 10.3987/COM-09-S(S)102

■ Thermal Behavior of Bicyclo[2.2.2]octadiene-Installed Precursors for 2H-Anthra[2,3-c]pyrroles and Anthra[2,3-c]thiophene

Hiroki Uoyama, Cai Chenxin, Hiroyuki Tahara, Yusuke Shimizu, Hideki Hagiwara, Yasuaki Hanasaki, Hiroko Yamada, Tetsuo Okujima, and Hidemitsu Uno*

Integrated Center for Science, Ehime University, Bunkyo-cho 2-5, Matsuyama 790-8577, Japan

Abstract

4,11-Ethano-4,11-dihydro-2H-anthra[2,3-c]pyrrole and 4,11-ethano-4,11-dihydroanthra[2,3-c]thiophene derivatives were prepared from the corresponding bicyclo[2.2.2]octadiene-fused five-membered heterocycles by the Diels-Alder reaction with in situ generated α,α’-dibromo-o-xylylene derivatives. Their thermogravimetric analyses showed evapolation without formation of 2H-anthra[2,3-c]pyrrole and anthra[2,3-c]thiophene derivatives under the atmospheric pressure.

INTRODUCTION
Higher benzo analogs of [c]-fused five-membered heterocycles such as 2H-isoindole and isothianaphthene have attracted much attention from synthetic and theoretical chemists,1 because these compounds showed clear contrast to the corresponding higher analogs of [b]-fused five-membered heterocycles (indoles and thianaphthenes) in the sense of aromaticity.2 Local aromaticity is expected in addition to grobal aromaticity in [b]-fused five-membered heterocycles, while there is only grobal aromaticity in the [c]-fused five-membered hetrerocycles. These compounds are very unstable and readily decompose to form intractable resinous material. Moreover, existence of proton tautomerism in N-free isoindole analogs makes this problem worse. The 1- and 3-positions of 2H-isoindole are very reactive toward electrophiles, while the 3-position of tautomeric isoindolenine (1H-isoindole) is highly electrophilic. Thus, self-condensation of isoindoles between the tautomers easily occurs especially in solution. Therefore, preparation of these compounds was limitted for the N-substituted derivatives such as naphtho[2,3-c]pyrrole and dibenzo[e,g]isoindoles.3 However, the instability of isoindole, isothianaphthene, and their higher benzo analogs is not due to aromaticity,1,2 but attributable to their high HOMO and low LUMO energy levels and very large coefficients at 1- and 3-positions in their HOMO and LUMO. In other words, these compounds must not be unstable if these molecules are separated from themselves in vacuum or in an inert matrix, because they have the similar heat formation energies to the corresponding [b]-fused heterocycles. During our continuous studies for the preparation and application of highly π-conjugated aromatics, we have explored the pericyclic cycloreversion method toward these highly reactive compounds either by thermal extrusion of ethylene from bicyclo[2.2.2]octadiene moiety (BCOD)4 or by photo decomposition of an oxalo bridging moiety.5 The latter method is proved to be extremely useful for the preparation of OTFT fabrication by a solution process6 and for highly unstable π systems such as higher acenes.7 However, this method has an inevitable drawback: difficult and tedious preparation of the precursors having the oxalo bridging moiety. Therefore, the thermal conversion method still deserves a certain advantage in order to roughly estimate fundamental stability and sensitivity of a new π system. In this paper, we will discuss about the results on our effort toward preparation of 2H-anthra[2,3-c]pyrroles (1) and anthra[2,3-c]thiophene (2) (Figure 1) by the thermal conversion of the BCOD-installed precursors.

RESULTS AND DISCUSSION
Preparation of BCOD-installed Precursors
We successfully prepared the lower analog of anthra[2,3-c]pyrrole, namely naphtha[2,3-c]pyrrole, by construction of the pyrrole ring at an olefinic double bond of 1,4-ethano-1,4-dihydronaphthalene based on the modified Barton-Zard reaction.8 In the case of anthra derivatives, however, preparation of hitherto unknown 1,4-ethano-1,4-dihydroanthracene is not thought to be easy due to the low reactivity of 1,3-cyclohexadiene toward dienophiles such as 2,3-naphthalyne.9 Therefore, we decided to construct the naphthalene ring at the double bonds of 4,7-ethano-4,7-dihydroisoindole10 and 4,7-ethano-4,7-dihydroisothianaphthene11 derivatives (Scheme 1).
The reactions of ethyl 4,7-ethano-4,7-dihydroisoindole-1-carboxylate (3) with o-xylylene generated from some precursors were examined. Since BCOD-fused pyrrole 3 gradually decomposed to the corresponding isoindole over 180 °C in solution, a precursor which generated o-xylylene at the lower temperature must be required. 1,3-Dihydroisothianaphthene S,S-dioxide (10) was reported as a thermal precursor for o-xylylene.12 BCOD-fused pyrrole 3 and sulfone 10 were heated at 100 °C for several hours and the reaction was monitored by TLC. As no other compound than the starting materials was observed, the temperature was gradually raised finally to 170 °C. The desired Diels-Alder product was not formed at all and the presence of isoindole 9 was detected. Sultine derivative 8 prepared from the reaction of α,α’-dibromoxylene with Longalite (HOCH2SO2Na·2H2O) was reported to be a good precursor for o-xylylene.12,13 The reaction of 3 with sultine 8 was conducted at 150 °C and the reaction was stopped after disappearance of 3. The identifiable products were, however, isoindole 9 and sulfone 10, and none of the desired Diels-Alder product was obtained (Eq 1). In order to suppress the undesired retro-Diels-Alder reaction of 3, the reaction temperature was lowered. However, decomposition prior to the desired Diels-Alder reaction or no reaction took place.

After several trials for the construction of the naphthalene ring, α,α’-dibromoxylylene in situ generated from α,α,α’,α’-tetrabromo-o-xylene and NaI was proved to be a good reagent for this purpose.14 Preparation of the BCOD-installed precursors for anthra[2,3-c]pyrroles and anthra[2,3-c]thiophene is shown in Scheme 1. BCOD-fused pyrrole 3 was reacted with α,α,α’,α’-tetrabromo-o-xylene in DMF at 65 °C with the aid of NaI to give ethanoanthra[2,3-c]pyrrole 4a in a 18% yield.15 Similarly, brominated ethanoanthra[2,3-c]pyrroles 4b and 4c were obtained by using 4,α,α,α’,α’-pentabromo-o-xylene and 4,5,α,α,α’,α’-hexabromo-o-xylene in 46% and 52% yields, respectively. The ester moiety of these compounds was easily removed by the alkaline treatment in ethylene glycol at 175 °C to afford α-free pyrroles 5a, 5b, and 5c in good yields. BCOD-installed thiophene 7 was also obtained from 4,7-ethano-4,7-dihydroisothianaphthene (6)11 in a 31% yield.

Thermal Behavior of the BCOD-installed Precursors
The sample compositions of precursors were first examined by combustion and X-ray analyses. The combusion analysis showed that a tiny amount of water was included in the purified samples of 4a, 4b, 5a, and 7 (see experimental section). In the cases of 5a and 7, single crystals for the X-ray analysis were obtained by slow diffusion of methanol into solutions of 5a and 7 in chlorobenzene.16 In both crystals, no solvent molecule was found. Figure 2 shows the Ortep drawings of 5a and 7. In the asymmetric unit cell of 5a, two crystallographically independent molecules were found. Dihedral angles of the BCOD moiety in 5a and 7 (plane[C3a,C4,C11,C11a] vs plane[C4,C4a,C10a,C11]) were 120.23(17)°, 121.12(17)° and 122.05(13)°. These values were quite similar to those obverved in ethanoisoindoles (121.1(1)°~122.8(2)°).17 Interestingly, the angles between the mean planes of pyrrole and naphthalene moieties were narrowed to be 113.86(9)° and 119.09(9)°, while that of thiophene and naphthalene was widened to be 123.97(5)°. This difference would be ascribed to the crystal packing effect.

The thermal behaviors of the BCOD-installed compounds were examined by thermogravimetric analysis (TG), conditions of which were as follows: About 2 mg of the sample were weighed and placed on an aluminium pan, and the temperature was slowly raised to 450 °C at a rate of 10 °C/min under a stream of nitrogen. The TG curves of ethyl anthra[2,3-c]pyrrole-1-carbonylates were shown in Figure 3-a. In all cases, weight losses started at ca. 230 °C and stopped at ca. 300 °C for 4b, 325 °C for 4a, and 350 °C for 4c. The total weight losses of 4a, 4b, and 4c were 65-95%. Simultaneous differential thermal analysis (DTA) revealed the existence of isotropic phase transition during the weight losses. The peak temperatures of DTA in 4a, 4b, and 4c were 245 °C, 213 °C, and 279 °C, respectively. The lowest isotropic phase transition temperature recorded in 4b was ascribed to the diastereomeric nature of this sample. Almost all amounts of the samples were lost in the cases of 4a and 4b, while some amounts of a decomposed material remained in 4c. As the decomposition of the ethyl ester moiety at the 2-position of pyrroles started at ca. 250 °C,17 the retro-Diels-Alder reaction of the BCOD moiety would not take place and the compounds would evapolate simultaneously with the ester decomposition during this temperature range. This was proved by the TG experiment of BCOD-installed α-free pyrrole derivatives 5 and thiophene 7. The TG results are shown in Figure 3-b. The weight losses started at ca. 180 °C for 5a, 230 °C for 5b, 280 °C for 5c, and 180 °C for 7. From DTA, the temperatures of isotropic phase transition in 5a, 5b, 5c, and 7 were 220 °C, 197 °C, 213 °C, and 240 °C, respectively. Since the BCOD moiety fused to [c]thiophene decomposed at the higher temperature than that fused to [c]pyrrole,11,17,18 these heterocycles were concluded to evapolate without forming anthra[2,3-c] five-membered heterocycles 1 and 2.
In conclusion, we prepared 4,11-ethano-4,11-dihydro-2H-anthra[2,3-c]pyrrole and 4,11-ethano-4,11-dihydroanthra[2,3-c]thiophene derivatives from the corresponding bicyclo[2.2.2]octadiene-fused five-membered heterocycles by the Diels-Alder reaction of

α,α’-dibromo-o-xylylenes, which were in situ generated from α,α,α’,α’-tetrabromo-o-xylene derivatives and NaI in DMF at 65 °C. By TG and DTA analyses, no conversion to fully conjugated 2H-anthra[2,3-c]pyrrole and anthra[2,3-c]thiophene derivatives was observed. The precursors bearing an ethano bridge are proved to be insufficient for the preparation of 2H-anthra[2,3-c]pyrrole and anthra[2,3-c]thiophene. Precursors with an oxalo bridge are necessary for this purpose. Further studies along this line are underway.

EXPERIMENTAL
General
Melting points were measured on a Yanagimoto micromelting point apparatus and are uncorrected. NMR spectra were obtained with a JEOL AL-400 or EX-400 spectrometer at the ambient temperature by using tetramethylsilane as an internal standard for 1H and 13C. IR spectra were measured with a Horiba FT-720 infrared spectrophotometer. EI MS spectra were measured with a JEOL JMS-700. Elemental analysis was performed with a Yanaco MT-5 elemental analyzer. TG analysis was preformed with JII EXSTAR 6000 TD/DTA6200. X-Ray diffraction was measured with a Rigaku AFC5R (for 7) or Rigaku AFC8S Mercury-CCD (for 5a) diffractometor. All solvents and chemicals were reagent grade quality, obtained commercially, and used without further purification except for otherwise noted. Dry DMF was distilled under a reduced pressure and then stored on Molecular Sieves 13X. Solvents for chromatography were purified by distillation. Thin-layer (TLC) and column chromatography with silica gel was performed on Art. 5554 (Merck KGaA) or Silica Gel 60N (Kanto Chemical Co.). BCOD-fused pyrrole10 and thiophene11 were prepared according to the literatures.

General procedure for construction of a naphthalene moiety with α,α,α’,α’-tetrabromo-o-xylene
Sodium iodide (24 mmol) was added to a stirred solution of α,α,α’,α’-tetrabromo-o-xylene derivative (4 mmol) and BCOD-fused pyrrole or thiophene (3 mmol) in dry DMF (20 mL) under N2. The mixture was heated at 65 °C for 8 h. After cooling, saturated aqueous NaHSO3 was added to the reaction mixture until the color turned to light yellow. The resulted white solid was collected by filtration. Recrystallization of the crude product from CHCl3/hexane afforded an analytically pure sample.

Ethyl 1,4-dihydro-1,4-ethanoanthra[2,3-c]pyrrole-1-carboxylate (4a):15 Yield 18%, white powder, mp 249-252 °C, Rf = 0.25 (CHCl3/hexane: 70%); 1H NMR (400 MHz, CDCl3) δ 1.42 (3H, t, J = 7.2 Hz), 1.78-1.85 (4H, m), 4.35 (2H, q, J = 7.2 Hz), 4.40 (1H, m), 4.91 (1H, m), 6.70 (1H, d, J = 2.4 Hz), 7.38 (2H, m), 7.61 (1H, s), 7.68 (1H, s), 7.74 (2H, m), and 8.45 (1H, br s). Anal. Calcd for C21H19NO2+1/8H2O: C, 78.91; H, 6.07; N, 4.38. Found: C, 79.01; H, 6.08; N, 4.45%.

Ethyl 7-bromo-4,11-dihydro-4,11-ethanoanthra[2,3-c]pyrrole-1-carboxylate and ethyl 8-bromo-4,11-dihydro-4,11-ethanoanthra[2,3-c]pyrrole-1-carboxylate (4b): Yield 46%, white powder, mp 210.5-213.5 °C, Rf = 0.4 (CHCl3/hexane: 70%); 1H NMR (400 MHz, CDCl3) δ 1.41 (3H, t, J = 7.4 Hz), 1.78-1.84 (4H, m), 4.33-4.39 (3H, m), 4.90 (1H, m), 4.90 (1H, m), 6.70 (m, 1H), 7.43-7.51 (2H, m), 7.56-7.64 (2H, m), 7.90 (1H, m), and 8.49 (1H, br s,). 13C NMR (100 MHz, CDCl3, typical signals) δ 14.59, 27.43, 28.02, 37.41, 37.69, 60.13, 113.76, 113.84, 115.00, 115.04, 118.81, 118.87, 119.95, 120.60, 120.73, 121.39, 128.34, 128.38, 128.38, 129.34, 129.41, 129.72, 130.54, 133.28, 133.30, 134.58, 142.89, 143.51, 143.54, 144.17, and 161.35; IR (KBr) νmax 3325, 1680, 1421, 1319, 1292, and 1143 cm-1; MS (EI+) 398 for C21H19O2N81Br. Anal. Calcd for C21H19BrNO2+1/4H2O: C, 62.93; H, 4.65; N, 3.49. Found: C, 62.95; H, 4.86; N, 3.46%.

Ethyl 7,8-dibromo-4,11-dihydro-4,11-ethanoanthra[2,3-c]pyrrole-1-carboxylate (4c): Yield 52%, white powder, mp 211-214 °C, Rf = 0.45 (CHCl3/hexane: 70%); 1H NMR (400 MHz, CDCl3) δ 1.42 (3H, t, J = 7.4 Hz), 1.79 (4H, m), 4.35 (2H, q, J = 7.4 Hz), 4.39 (1H, m) 4.89 (1H, m), 6.71 (1H, d, J = 2.7 Hz), 7.48 (1H, s), 7.56 (1H, s), 8.02 (2H, d, J = 6.1 Hz), and 8.49 (br. s, 1H); 13C NMR (100 MHz, CDCl3) δ 14.72, 27.23, 27.89, 37.30, 37.68, 60.07, 113.70, 115.02, 119.66, 120.33, 120.71, 120.77, 129.37, 131.50, 131.59, 131.81, 131.83, 143.83, 144.48, and 161.16; IR (KBr) νmax 3309, 1676, 1421, 1319, 1294, 1142, 1190, and 1043 cm-1; MS (EI+) 475 for C21H1779Br81BrNO2. Anal. Calcd for C21H19Br2NO2: C, 53.08; H, 3.61; N, 2.95. Found: C, 52.96; H, 3.70; N, 2.94%.

4,11-Dihydro-4,11-ethanoanthra[2,3-c]thiophene (7): Yield 31%, white powder, mp 244-245 °C, Rf = 0.40 (CHCl3/hexane: 10%); 1H NMR (400 MHz, CDCl3) δ 1.84 (4H, m), 4.44 (2H, m), 6.92 (2H, s), 7.39 (2H, m), 7.63 (s, 2H), and 7.74 (2H, m); 13C NMR (100 MHz, CDCl3) δ 27.36, 40.69, 114.27, 121.24, 125.12, 127.30, 132.15, 141.71, and 144.61; IR (KBr) νmax 1697, 1647, 1558, 1541, 1496, 1458, and 1386 cm-1. MS (EI+) 262. Anal. Calcd for C18H14S+1/8H2O: C, 81.70; H, 5.43. Found: C, 81.72; H, 5.68%. The single crystals were obtained by slow vapor diffusion of methanol into a solution of 7 in chlorobenzene. Crystal formula: C18H14S, 0.30 x 0.25 x 0.25 mm, monoclinic, space group P21/c, a = 6.301(5), b = 7.872(5), c = 26.269(4) Å, β = 93.81(4)º, V = 1300.0(13) Å3, Mo Kα, T = 298 K, Z = 4, ρcalcd = 1.340 g·cm-3, µ = 0.230 mm-1, F(000) = 552. 3244 diffractions measured, 2976 unique, 1980 observed [I > 2σ(I)]; Requiv = 0.0281, R1 = 0.0459 [I > 2σ(I)], wR2 = 0.1215 (all); GOF = 1.017. CCDC No. 739265.

General procedure for removal of an ester moiety
A stirring mixture of an ester (1.00 mmol) and KOH (5 mmol) in ethylene glycol (10 mL) was heated at 175 °C under N2 in the dark. After 2 h, the reaction mixture was cooled to room temperature, and then extracted with CH2Cl2 (50 mL). The organic extract was washed with saturated NaHCO3, water, and brine, dried over anhydrous Na2SO4, and concentrated in vacuo. The residue was purified by silica-gel chromatography and recrystallization to give the targeted compound.

4,11-Dihydro-4,11-ethanoanthra[2,3-c]pyrrole (5a):15 Yield 67%, white powder, mp 220-223 °C, Rf = 0.15 (CHCl3/hexane: 30%); 1H NMR (400 MHz, CDCl3) δ 1.83 (4H, m), 4.39 (2H, m), 6.70 (1H, d, J = 2.0 Hz), 7.36 (2H, m), 7.61 (2H, s), 7.73 (2H, m), and 7.59 (1H, br s). Anal. Calcd for C18H15N+1/4H2O: C, 86.54; H, 6.25; N, 5.61. Found: C, 86.36; H, 6.36; N, 5.66%. The single crystals were obtained by slow vapor diffusion of methanol into a solution of 5a in chlorobenzene. Crystal formula, C18H15N, 0.60 x 0.20 x 0.04 mm, monoclinic, space group C2/c, a = 35.039(6), b = 6.1755(8), c = 28.703(5) Å, β = 125.559(3)º, V = 5052.6(14) Å3, Mo Kα, T = 100 K, Z = 16, ρcalcd = 1.290 g·cm-3, µ = 0.075 mm-1, F(000) = 2080. 13006 diffractions measured, 5685 unique, 3655 observed [I > 2σ(I)]; Requiv = 0.0444, R1 = 0.0665 [I > 2σ(I)], wR2 = 0.1953 (all); GOF = 1.049. CCDC No. 742678.

7-Bromo-4,11-dihydro-4,11-ethanoanthra[2,3-c]pyrrole (5b): Yield 73%, white powder, mp 182-184 °C; Rf = 0.2 (CHCl3/hexane: 30%); 1H NMR (400 MHz, CDCl3) δ 1.83 (4H, m), 4.38 (2H, m), 6.59 (2H, m), 7.45 (1H, m), 7.50 (2H, m), 7.59 (2H, m), and 7.89 (1H, br s,); 13C NMR (100 MHz, CDCl3) δ 28.44, 37.42, 108.99, 109.07, 118.55, 119.84, 120.62, 127.45, 127.49, 128.08, 128.93, 129.29, 130.53, 133.29, 136.21, 144.33, 144.95, and one carbon signal was not identified probably due to overlap; IR (KBr) νmax 3365, 1592, 1520, 1487, 1047, and 895 cm-1; MS (EI+) 325 for C18H14N81Br. Anal. Calcd for C18H14BrN: C, 66.68; H, 4.35; N, 4.32. Found: C, 66.70; H, 4.74; N, 4.17%.

7,8-Dibromo-4,11-dihydro-4,11-ethanoanthra[2,3-c]pyrrole (5c): Yield 70%, white powder, mp 207-209 °C. Rf = 0.2 (CHCl3/hexane: 30%); 1H NMR (400 MHz, CDCl3) δ 1.82 (4H, m), 4.39 (2H, m), 6.59 (2H, d, J = 2.4 Hz), 7.48 (2H, s), 7.64 (1H, br s), and 8.01(2H, s). 13C NMR (100 MHz, CDCl3) δ 28.31, 37.42, 109.06, 119.64, 120.38, 127.22, 131.53, 131.93, and 145.21; IR (KBr) νmax 3435, 1466, 1394, 1099, and 1030 cm-1; MS (EI+) 403 for C18H13N79Br81Br. Anal. Calcd for C18H13Br2N: C, 53.63; H, 3.25; N, 3.47. Found: C, 53.72; H, 3.27; N, 3.83%.

ACKNOWLEDGEMENTS
This work was partially supported by Grant-in-Aid for Scientific Research C (20550047) from Ministry of Education, Culture, Sports, Science and Technology of Japan.

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