Reactions of BODIPY Fluorophore with Cupric Nitrate

Abstract

4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) and corresponding 4,4-dimethyl and 4,4-diphenyl analogues were treated with cupric nitrate trihydrate under different conditions. Corresponding 3-nitro-, nitromethyl-, hydroxymethyl BODIPY, and BODIPY 3-carboxyaldehyde were obtained. The UV/vis and fluorescent properties of these BODIPY analogues were determined.

Key words

BODIPY - fluorophore - cupric nitrate - X-ray structure - fluorescent quantum yield

• References and Notes

• 1 These authors contributed equally to this work.
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• 5 Procedure for the Synthesis of 4,4-Difluoro-3-nitromethyl-1,5,7,8-tetramethyl-2,6-diethyl-4-bora-3a,4a-diaza-s-indacene (2) To a solution of 4,4-difluoro-1,3,5,7,8-pentamethyl-2,6-diethyl-4-bora-3a,4a-diaza-s-indacene (1, 100 mg, 0.31 mmol) in CH₂Cl₂ [20 mL, CH₂Cl₂ was first dried by heating in the presence of P₄O₁₀ and distilled, and then 0.004% H₂O (v/v) was added], a solution of Cu(NO₃)₂·3H₂O in MeCN [0.41 M, 0.38 mL, 0.5 mol equiv, MeCN was first dried by heating in the presence of CaH₂ and distilled, and then 0.004% H₂O (v/v) was added] was added. After the mixture was stirred for 3 h, the products were evaporated under reduced pressure. The residue was redissolved in CH₂Cl₂ (20 mL) and extracted with H₂O (3 × 15 mL). The organic layer was separated, dried (MgSO₄) and evaporated under reduced pressure. The residue was purified by column chromatog-raphy on silica gel. The appropriate fractions, which were eluted with CH₂Cl₂–hexane (50:50, v/v), were combined and evaporated under reduced pressure to give the title compound as a red solid (61 mg, 54%); mp 196–200 °C (CH₂Cl₂–hexanes). R_f = 0.87 (CH₂Cl₂). ¹H NMR (300.1 MHz, CDCl₃): δ = 1.07 (3 H, t, J = 7.6 Hz), 1.10 (3 H, t, J = 7.6 Hz), 2.39 (3 H, s), 2.40 (3 H, s), 2.43 (2 H, q, J = 7.6 Hz), 2.48 (2 H, q, J = 7.6 Hz), 2.55 (3 H, s), 2.70 (3 H, s), 5.72 (2 H, s). ¹³C NMR (75.5 MHz, CDCl₃): δ = 13.0, 14.1, 14.5,14.7, 14.9, 17.1, 17.2, 17.3, 69.4, 131.8, 133.6, 133.9, 134.6, 135.6, 140.9, 142.2, 159.9. ¹¹B NMR (96.3 MHz, CDCl₃): δ = 0.47 (t, J = 33.3 Hz). ¹⁹F NMR (282.4 MHz, CDCl₃): δ = –143.6 (q, J = 33.3). HRMS (EI): m/z calcd for C₁₈H₂₄BF₂N₃O₂: 363.19296; found: 363.19288.
• 6 Procedure for the Synthesis of 4,4-Difluoro-3-nitro-1,5,7,8-tetramethyl-2,6-diethyl-4-bora-3a,4a-diaza-s-indacene (3) To a solution of 4,4-methyl-1,3,5,7,8-pentamethyl-2,6-diethyl-4-bora-3a,4a-diaza-s-indacene (1, 100 mg, 0.31 mmol) in dry CH₂Cl₂ (20 mL), a solution of Cu(NO₃)₂·3H₂O (380 mg, 5 mol equiv) in dry MeCN (10 mL) was added. After the mixture was stirred for 5 min, the products were evaporated under reduced pressure. The residue was redissolved in CH₂Cl₂ (30 mL) and extracted with H₂O (3 × 20 mL). The organic layer was separated, dried (MgSO₄), and evaporated under reduced pressure. The residue was purified by column chromatography on silica gel. The appropriate fractions, which were eluted with CH₂Cl₂–hexane (70:30, v/v), were combined and evaporated under reduced pressure to give the title compound as a red solid (42 mg, 38%); mp 212–216 °C (CH₂Cl₂–hexanes). R_f = 0.42 (system A). ¹H NMR (300.1 MHz, CDCl₃): δ = 1.09 (3 H, t, J = 7.5 Hz), 1.16 (3 H, t, J = 7.5 Hz), 2.37 (3 H, s), 2.41 (3 H, s), 2.46 (2 H, q, J = 7.5 Hz), 2.64 (3 H, s), 2.72 (2 H, q, J = 7.5 Hz), 2.73 (3 H, s). ¹³C NMR (75.5 MHz, CDCl₃): δ = 13.4, 13.7, 14.1, 14.2, 14.9, 17.1, 17.9, 18.3, 130.0, 130.2, 130.4, 137.6, 138.8, 141.5, 142.7, 144.3, 166.8. ¹⁹F NMR (282.4 MHz, CDCl₃): δ = –144.6 (q, J = 29.2 Hz). ¹¹B NMR (96.3 MHz, CDCl₃): δ = 0.30 (t, J = 28.2 Hz). HRMS (EI): m/z calcd for C₁₇H₂₂BF₂N₃O₂: 349.17731; found: 349.17713. It is noted that this reaction gave a mixture of 4,4-difluoro-3-nitro-1,5,7,8-tetramethyl-2,6-diethyl-4-bora-3a,4a-diaza-s-indacene (3) and 4,4-difluoro-1,5,7,8-tetramethyl-2,6-diethyl-4-bora-3a,4a-diaza-s-indacene-3-carboxyaldehyde (4, with a total mass of 78 mg) in a ratio of 6.3:1 as determined by ¹H NMR spectroscopy. Therefore, the overall yield of the title compound is 62%. In addition to the pure 4,4-difluoro-3-nitro-1,5,7,8-tetramethyl-2,6-diethyl-4-bora-3a,4a-diaza-s-indacene (3, 42 mg), separation of the rest of product 3 from contaminating aldehyde 4 was rather difficult.
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