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.

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

References and Notes

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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.

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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 

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