Synthesis of 1,6’-Diazulenylketone

Abstract

1,6’-Diazulenylketone (1) was synthesized from 6-azulenecarboxylic acid (2) and its redox properties were evaluated by cyclic voltammetry. Some fundamental compounds in azulene chemistry were effectively derived from 2.

Key words

azulene - 1,6’-diazulenylketone - capto-dative - Friedel-Crafts acylation - cyclic voltammetry

References

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14

Ethyl 2-(1-Azulenyl)-6-azulenecarboxylate (6): To a solution of 6-azulenecarboxylic acid (2; 58.8 mg, 0.342 mmol) in Et₂O (5 mL) was added dropwise ethyl chloroformate (0.0365 mL, 0.382 mmol) followed by Et₃N (0.0537 mL, 0.385 mmol) at 5 °C and the resulting suspension was stirred for 1 h at r. t. The precipitate was filtered off and the filtrate was concentrated to leave the corresponding activated ester as a green solid. The solid and azulene (49.1 mg, 0.384 mmol) were dissolved in nitromethane (1.5 mL), followed by addition of scandium triflate (18.7 mg, 0.0380 mmol) to the solution. The resulting mixture was stirred for 10 h at 45 °C and the reaction was quenched with sat. aq NaHCO₃ solution. The mixture was extracted with CHCl₃ (3 × 10 mL) and the extracts were dried (Na₂SO₄) and concentrated to leave a residue, which was chromatographed (alumina, hexane-EtOAc, 5:1 as eluent) to give 6 as a reddish purple solid (13 mg, 10%); mp 130-135 °C (dec.). ¹H NMR: d = 1.47 (3 H, t, J = 7.1 Hz, CH₃), 4.45 (2 H, q, J = 7.1 Hz, CH₂), 7.25 (1 H, t, J = 9.6 Hz, 5’-H), 7.34 (1 H, t, J = 9.5 Hz, 7’-H), 7.50 (1 H, d, J = 4.3 Hz, 3’-H), 7.67 (1 H, t, J = 9.6 Hz, 6’-H), 7.81 (2 H, s, 1,3-H), 8.06 (2 H, d, J = 11.0 Hz, 5,7-H), 8.30 (2 H, d, J = 11.0 Hz, 4,8-H), 8.36 (1 H, d, J = 10.1 Hz, 4’-H), 8.37 (1 H, d, J = 4.3 Hz, 2’-H), 9.10 (1 H, d, J = 9.8 Hz, 8’-H).¹³C NMR: d = 14.35, 61.81, 116.87, 119.49, 124.24, 124.93, 125.14, 126.64, 132.05, 133.64, 136.24, 136.93, 137.48, 138.77, 138.82, 142.51, 144.25, 149.65, 168.45 (CO). IR: n = 1698 cm^-1 (CO). MS: m/z (%) = 126 (C₁₀H₆ ⁺, 36), 149 (34), 252 (96), 326 (M⁺, 100). UV/Vis (MeCN) l_max (log e) = 206 (4.41 sh), 238 (4.33), 274 (4.58), 288 (4.51 sh), 310 (4.42), 498 nm (4.12).

19

Measured against Ag/Ag⁺ in MeCN with 0.1 M Bu₄NClO₄ and 1 mM AgNO₃ as supporting electrolyte at r. t., C working electrode, n = 10 mVs^-1. The peak separations are 60, 50 and 120 mV for the first, second and third reduction waves, respectively.