The Synthesis of Triazadibenzo[cd,g]azulenes: How To Connect Four Different Rings in Just One Step?

Abstract

Deprotonation at the methyl group of 6-methyl-4H-imid­azo[1,2-a]pyridines yields dimeric triazadibenzo[cd,g]azulenes in just one step. This reaction is interpreted as a cascade reaction involving an anionic electrocyclization reaction, a redox sequence and a subsequent dimerization process. The X-ray structural analysis revealed an anti arrangement of both tetracyclic subunits. By quantum chemical calculations the unexpected ring-closure reaction is interpreted in terms of electronic destabilization of the anionic intermediates 8 by configurational strain enabling the otherwise thermodynamically unfavorable cyclization process.

Key words

electrocyclic reactions - quantum chemical calculations - oxidations - fused ring systems - pyridines

References and Notes

• 1a Beckert R. Bauer W. J. Prakt. Chem.  1991,  333:  555 
• 1b Käpplinger C. Beckert R. Günther W. Görls H. Liebigs Ann./Recl.  1997,  617 
• 1c Käpplinger C. Beckert R. Imhof W. J. Prakt. Chem.  1998,  340:  323 
• 1d Käpplinger C. Beckert R. Darsen A. Günther W. Sulfur Lett.  2001,  281 
• 2a Brandenburg J. Käpplinger C. Beckert R. Synthesis  1996,  1302 
• 2b Käpplinger C. Gebauer T. Beckert R. Weiß D. Günther W. Görls H. Friedrich M. Tetrahedron  2004,  60:  3847 
• 3 Stöckner F. Beckert R. Gleich D. Birckner E. Günther W. Görls H. Vaughan G. Eur. J. Org. Chem.  2007,  1237 
• 4 Matschke M. Blumhoff J. Beckert R. Tetrahedron  2008,  64:  7815 
• 5a Atzrodt J. Brandenburg J. Käpplinger C. Beckert R. Günther W. Görls H. Fabian J. J. Prakt. Chem./Chem.-Ztg.  1997,  339:  729 
• 5b Müller D. Beckert R. Görls H. Synthesis  2001,  601 
• 5c Beckert R. Hippius C. Gebauer T. Stöckner F. Lüdigk C. Weiß D. Raabe D. Günther W. Görls H. Z. Naturforsch., B  2006,  61:  437 
• 6a Atzrodt J. Beckert R. Görls H. Heterocycles  1999,  51:  763 
• 6b Gebauer T. Beckert R. Weiß D. Knop K. Käpplinger C. Görls H. Chem. Commun.  2004,  1860 
• 6c Matschke M. Käpplinger C. Beckert R. Tetrahedron  2006,  62:  8586 
• 9a Frisch MJ. Trucks GW. Schlegel HB. Scuseria GE. Robb MA. Cheeseman JR. Montgomery JA. Vreven T. Kudin KN. Burant JC. Millam JM. Iyengar SS. Tomasi J. Barone V. Mennucci B. Cossi M. Scalmani G. Rega N. Petersson GA. Nakatsuji H. Hada M. Ehara M. Toyota K. Fukuda R. Hasegawa J. Ishida M. Nakajima T. Honda Y. Kitao O. Nakai H. Klene M. Li X. Knox JE. Hratchian HP. Cross JB. Bakken V. Adamo C. Jaramillo J. Gomperts R. Stratmann RE. Yazyev O. Austin AJ. Cammi R. Pomelli C. Ochterski JW. Ayala PY. Morokuma K. Voth GA. Salvador P. Dannenberg JJ. Zakrzewski VG. Dapprich S. Daniels AD. Strain MC. Farkas O. Malick DK. Rabuck AD. Raghavachari K. Foresman JB. Ortiz JV. Cui Q. Baboul AG. Clifford S. Cioslowski J. Stefanov BB. Liu G. Liashenko A. Piskorz P. Komaromi I. Martin RL. Fox DJ. Keith T. Al-Laham MA. Peng CY. Nanayakkara A. Challacombe M. Gill PMW. Johnson B. Chen W. Wong MW. Gonzalez C. Pople JA. Gaussian 03, Revision C.02   Gaussian, Inc.; Wallingford CT: 2004. 
• 9b

  Details of the quantum chemical calculations may be obtained from E.-U. Würthwein upon request.

• 10 Energies of the DFT-optimized structures were calculated using the SCS-MP2-method. They include DFT zero point correction. See: Grimme S. J. Chem. Phys.  2003,  118:  9095 
• 11 Goumans TPM. Ehlers AW. Lammertsma K. Würthwein E.-U. Grimme S. Chem. Eur. J.  2004,  10:  6468 
• 12 Klötgen S. Fröhlich R. Würthwein E.-U. Tetrahedron  1996,  52:  14801 
• 13 Klötgen S. Würthwein E.-U. Tetrahedron Lett.  1995,  36:  7065 
• 14 Gerdes K. Sagar P. Fröhlich R. Wibbeling B. Würthwein EU. Eur. J. Org. Chem.  2004,  3465 

7

Crystal Data for Compound 10: C₄₄H₃₄N₈˙2CHCl₃, Mr = 913.53 gmol^-¹, blue prism, size 0.04 × 0.04 × 0.04 mm^³, triclinic, space group Pi, a = 11.4589 (4), b = 12.5571 (5), c = 15.8154 (9) Å, α = 82.756 (2)˚, β = 68.797 (3)˚, γ = 79.149 (2)˚, V = 2079.51 (16) Å^³, T = -90 ˚C, Z = 2, ρ_calcd. = 1.459 gcm^-³, µ(Mo-K_α) = 4.59 cm^-¹, F(000) = 940, 14823 reflections in h(-14/14), k(-16/16), l(-16/20), measured in the range 1.95˚≤ Θ ≤27.50˚, completeness Θ_max = 99.3%, 9491 independent reflections, R_int = 0.0415, 6118 reflections with F_o >4σ(F_o), 553 parameters, 0 restraints, R1_obs = 0.0663, wR^² _obs = 0.1607, R1_all = 0.1129, wR^² _all = 0.1922, GOOF = 0.983, largest difference peak and hole: 1.255/-0.416 e Å^-³. CCDC 686207 (10) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: [phone]; or [e-mail]].

8

Preparation of 10 Starting from 1 and 2-Amino-6-methylpyridine (6; R = Me); Typical Procedure: A solution of 2-amino-6-methylpyridine (6; 0.216 g, 2.00 mmol) in MeCN (20 mL) was added to a solution of oxalic acid bisimidoylchloride 1 (0.610 g, 2.00 mmol) in MeCN (20 mL). After addition of Et₃N (1.4 mL, 10 mmol) the reaction mixture was heated under reflux for about 4-5 h. The purple solution was cooled to r.t., evaporated to dryness and the product was isolated by column chromatography (SiO₂, toluene-acetone, 100:1). The bis(2,11,11b-triazadibenzo[cd,g]dihydroazulene) 10 was obtained as dark blue crystalline solid. Selected data for 10: yield: 0.20g, 30%; mp >250 ˚C (dec.). MS (DEI): m/z (%) = 675 (5) [M⁺], 674 (8) [M⁺ - 1], 541 (12), 486 (26), 441 (11), 338 (16), 337 (18), 234 (31), 205 (21), 149 (35), 133 (47), 106 (100), 78 (27). ^¹H NMR (CDCl₃): δ = 2.17 (s, 6 H), 2.25 (s, 6 H), 3.99 (s, 2 H), 6.48 (s, 2 H), 6.65 (d, ^³ J = 9.0 Hz, 2 H), 6.83 (d, ^³ J = 7.0 Hz, 2 H), 6.87 (d, ^³ J = 7.0 Hz, 2 H), 7.05 (m, 2 H), 7.20 (d, ^³ J = 9.5 Hz, 4 H), 7.65 (d, ^³ J = 8.0 Hz, 4 H), 7.74 (d, ^³ J = 8.3 Hz, 2 H). ^¹³C NMR (CDCl₃): δ = 20.8, 21.2, 41.6, 116.0, 119.9, 120.2, 125.3, 128.5, 129.3, 130.2, 130.4, 132.3, 133.1, 135.6, 139.8, 141.3, 141.7, 142.6, 146.1, 162.3. UV-Vis (CHCl₃): λ_max (log ε) = 272 (4.4), 457 (3.9), 537 (4.0), 571 (3.9) nm.