Phosphorus-Substituted Azulenes Accessed via Direct Hafner Reaction of a Phosphino Cyclopentadienide

Abstract

The Hafner azulene synthesis may be applied to the direct synthesis of phosphorus-substituted azulenes, when a phosphinocyclopentadienide is used as one of the reactants. The azulenyl phosphines produced in this fashion are preferentially isolated as the corresponding phosphine oxides or phosphine borane adducts.

Key words

azulene - phosphine - pyrylium salt - cyclopentadienide - ­borane adduct

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• 17 Procedure for the Preparation of 8 At 0 °C, to a suspension of 2,4,6-trimethylpyrylium tetrafluoroborate (5, 1.10 g, 5.23 mmol, 1.00 equiv) in THF (30 mL) was added a solution of lithium (diphenylphosphino)cyclopentadienide (4, 2.68 g, 10.5 mmol, 2.00 equiv) in THF (30 mL). After stirring at 0 °C for 1 h, the mixture was filtered through a pad of neutral alumina under an atmosphere of argon. To the stirred filtrate, at r.t., was slowly added a solution of borane–THF complex (11.0 mL, 1.0 M in THF, 2.10 equiv). After stirring for 16 h, the reaction was quenched by the addition of MeOH (10 mL). The solution was then concentrated under reduced pressure to a small volume, and added to EtOAc (60 mL). The solution was washed with water (3 × 50 mL) and with sat. brine, the organic layer was dried over anhydrous MgSO₄, and filtered. The filtrate was concentrated under reduced pressure to give the crude product, which was purified by column chromatography (0 → 10% EtOAc in PE) to give boranyldiphenyl(4,6,8-trimethylazulen-2-yl)phosphine (8, 228 mg, 0.621 mmol, 12%) as a purple solid (mp 148–150 °C). R_f = 0.53 (PE–EtOAc, 3:1). ¹H NMR (300 MHz, CDCl₃): δ = 7.73–7.66 (4 H, m), 7.51–7.41 (6 H, m), 7.46 (2 H, d, ³J _PH = 5.1 Hz), 7.12 (2 H, s), 2.83 (6 H, s), 2.65 (3 H, s), 1.44 (3 H, br d); ¹³C NMR (75 MHz, CDCl₃): δ = 149.7, 148.7, 136.5 (d, ³ J _CP = 11.5 Hz), 133.0 (d, ²J _CP = 9.9 Hz), 131.1 (d, ¹ J _CP = 62.7 Hz), 130.8 (d, ⁴ J _CP = 2.5 Hz), 130.6 (d, ¹ J _CP = 58.9 Hz), 128.5 (d, ³ J _CP = 10.2 Hz), 128.3 (d, ⁵ J _CP = 1.2 Hz), 120.8 (d, ² J _CP = 10.5 Hz), 29.0, 25.2. ³¹P NMR (122 MHz, CDCl₃): δ = 12.38–11.65 (m). ¹¹B NMR (96 MHz, CDCl₃): δ = –34.5. IR (film): ν_max = 3675, 2987, 2971, 2901, 2380 (ν_BH), 1578, 1537, 1483, 1468, 1435, 1408, 1394, 1377, 1333, 1290, 1218, 1187, 1140, 1102, 1027, 1066, 907, 882, 847, 809, 797, 766, 740, 690 cm^–1. HRMS (ESI⁺): m/z calcd for [C₂₅H₂₆BP + Na]⁺: 391.1763; found: 391.1796. See Supporting Information for complete NMR assignments.
• 18 Rogano F, Stojnic D, Linden A, Abou-Hadeed K, Hansen H. Helv. Chim. Acta 2011; 94: 1194