Quinoxalinodehydroannulenes: A Novel Class of Carbon-Rich Materials

Abstract

Acetylenic quinoxalinodehydro[12]- and -[24]annulenes have been prepared from Boc-protected aminobenzodehydroannulenes and diethynyl-1,2-diones. For the smaller annulene, theoretical and spectroscopic studies reveal π-delocalisation over the entire aromatic system, including the external acetylenes. In contrast, the larger annulene prevails in a tub-like conformation, which limits π-orbital overlap and gives rise to higher-energy absorption and emission maxima.

acetylenes - dehydroannulenes - MMFFs calculations - π-delocalisation - UV/Vis spectroscopy

References

1

New address: Dr. S. Ott, Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, 10691 Stockholm, Sweden.

2

New address: Prof. Dr. R. Faust, Institute of Chemistry, Faculty of Physical Sciences, University of Kassel, Heinrich-Plett-Str. 40, 34132 Kassel (Germany). Fax: [phone]. E-mail: [e-mail].

For recent reviews in this field see:

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10a Solooki D. Bradshaw JD. Tessier CA. Youngs WJ. Organometallics  1994,  13:  451 

10b Cook MJ. Heeney MJ. Chem.-Eur. J.  2000,  6:  3958 

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11

General Procedure for Compounds 1 and 3: CuCl (99 mg, 1 mmol) and Cu(OAc)₂ (181 mg, 1 mmol) were added to a degassed solution of butadiyne 2 (71 mg, 0.1 mmol) in MeOH and pyridine (1:1, 70 mL). After stirring for 4 h at 60 °C the reaction mixture was concentrated in vacuo to approximately 10 mL, before H₂O (50 mL) and Et₂O (100 mL) were added. After stirring for 15 min, the layers were separated and the organic layer was washed with brine (3 × 50 mL) until the aqueous washings remained colourless. Drying of the organic layer over Na₂SO₄ was followed by filtration and evaporation of the solvents in vacuo. The resulting residue was triturated with EtOAc and hexane for 10 min (1:1, 10 mL) and collected. Hygroscopic, yellow solid, yield 51 mg (72 µmol, 72%). The presence of 3 was confirmed indirectly following condensation reactions with 1,2-diones. From the analysis of these experiments, the ratio between 1 and 3 was judged to be approximately 2:1.
Compound 1: ¹H NMR (400 MHz, CDCl₃): δ = 7.16 (br s, 4 H, Ph), 6.51 (br s, 4 H, NH), 1.48 (s, 36 H, tert-Bu). UV/Vis (CH₂Cl₂): λ_max (ε) = 244 (37000), 274 (30000), 302 (43000), 322 (69000) nm. FAB-MS: m/z (%) = 708 (2) [M⁺]. Accurate mass [HRMS (FAB), m/z]: anal. calcd for C₄₀H₄₅N₄O₈: 709.3237; found: 709.3210.

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13

Representative Procedure for the One-Pot Deprotection/Condensation Sequence between 1,2-Diones and Boc-Protected Annulenes 1 and 3: A mixture of compounds 1 and 3 (2:1, 28 mg; 0.02 mmol of 1 and 0.01 mmol of 3) was added to a degassed solution of 1,2-dione (0.16 mmol) in HOAc (5 mL). Trifluoroacetic acid (2 mL) was added at 60 °C upon which the initially yellow suspension turned into a red solution. The reaction mixture was continuously stirred at this temperature for 24 h. After cooling to 0 °C, 4 and 5 could be removed by filtration. During the filtration the initially yellow solids turned brown. Dehydro[12]annulene 4 could be separated from 5 by filtration through a plug of silica, eluting with CHCl₃. Due to the low stability of 4 and 5, quantitative measurements such as the determination of extinction coefficients were not performed. Yields are given for the isolated products and are relative to the amount of the respective Boc-protected precursor in the mixture of 1 and 3.
Compound 4: yield: 7 mg (0.011 mmol, 53%). ¹H NMR (400 MHz, CDCl₃): δ = 7.94 (s, 4 H, quinoxaline), 7.47 (m, 8 H, Ph), 7.34 (m, 12 H, Ph). UV/Vis (CH₂Cl₂): λ_max = 334 (sh), 357, 405 (sh), 416, 442 nm. FAB-MS: m/z (%): 657 (100) [M⁺]. Accurate mass [HRMS (FAB), m/z): anal. calcd. for C₄₈H₂₅N₄: 657.2079; found: 657.2057.
Compound 5: ¹H NMR (400 MHz, CDCl₃): δ = 8.37 (s, 8 H, quinoxaline), 7.47 (m, 16 H, Ph), 7.34 (m, 24 H, Ph). FAB-MS: m/z (%) = 1314 [M⁺].

14 Faust R. Weber C. Fiandanese V. Marchese G. Punzi A. Tetrahedron  1997,  53:  14655 

15

Compound 6 could be separated from 7 by gel permeation chromatography on a polystyrene resin cross-linked with divinylbenzene, using THF as eluent. Yields are given for the isolated products and are relative to the amount of the respective Boc-protected precursor in the mixture of 1 and 3. Compound 6: yield: 20 mg (0.018 mmol, 86%). ¹H NMR (400 MHz, CDCl₃): δ = 7.79 (s, 4 H, quinoxaline), 1.18-1.13 (m, 84 H, i-Pr). ¹³C NMR (100 MHz, CDCl₃): δ = 141.3, 141.0, 129.8, 129.2 (all aromatic C), 103.1, 101.4, 91.7, 86.1 (all acetylenic C), 18.7 [CH(CH₃)₂], 11.4 [CH(CH₃)₂]. UV/Vis (CH₂Cl₂): λ_max (ε) = 252 (62000), 301 (47000), 365 (171000), 420 (43000), 432 (55000), 445 (34000), 461 (121000) nm. MALDI-TOF: m/z (%) = 1073.6 (100) [M⁺]. Accurate mass [HRMS (FAB), m/z]: anal. calcd for C₆₈H₈₉N₄Si₄: 1073.6164; found: 1073.6178.
Compound 7: yield: 19 mg (0.009 mmol, 86%). ¹H NMR (400 MHz, CDCl₃): δ = 8.20 (s, 8 H, quinoxaline), 1.18-1.13 (m, 168 H, i-Pr). ¹³C NMR (100 MHz, CDCl₃): δ = 141.2, 140.0, 134.2, 126.0 (all aromatic C), 103.2, 101.4, 80.6, 80.0 (all acetylenic C), 18.7 [CH(CH₃)₂], 11.4 [CH(CH₃)₂]. UV/Vis (CH₂Cl₂): λ_max (ε) = 252 (115000), 321 (340000), 379 (90000), 389 (86000), 401 (142000), 425 (236000) nm. MALDI-TOF: m/z (%) = 2148.4 (100) [M⁺].

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17

Calculations were performed with MMFFs using Macromodel. See: www.schrodinger.com.

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