Triangulenium Ions: Versatile Organic Photoredox Catalysts for Green-Light-Mediated Reactions

Abstract

The development of tunable organic photoredox catalysts remains important in the field of photoredox catalysis. A highly modular and tunable family of trianguleniums (azadioxatriangulenium, diazaoxatriangulenium, and triazatriangulenium), and the related [4]helicene quinacridinium have been used as organic photoredox catalysts for photoreductions and photooxidations under visible light irradiation (λ = 518–640 nm). A highlight of this family of photoredox catalysts is their readily tunable redox properties, leading to different reactivities. We report their use as photocatalysts for the aerobic oxidative hydroxylation of arylboronic acids and the aerobic cross-dehydrogenative coupling reaction of N-phenyl-1,2,3,5-tetrahydroisoquinoline with nitromethane through reductive quenching. Furthermore, their potential as photoreduction catalysts has been demonstrated through the catalysis of an intermolecular atom-transfer radical addition via oxidative quenching. These transformations serve as benchmarks to highlight that the easily synthesized trianguleniums, congeners of the acridiniums, are versatile organic photoredox catalysts with applications in both photooxidations and photoreductions.

Key words

photoredox organocatalysis - trianguleniums - photoreduction - photooxidation - heterocyclic fused cationic compounds

• References and Notes

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• 28 PC-Catalyzed Oxidative Hydroxylation of (4-Cyanophenyl)boronic Acid under Visible Light (Table [2]); General Procedure (4-Cyanophenyl)boronic acid (37 mg, 0.25 mmol) and the appropriate PC (3 mg, 6 μmol, 2.5 mol%) were dissolved in DMF (2.5 mL) in a 25 mL Schlenk tube, and DIPEA (90 μL, 0.5 mmol, 2 equiv) was added. The solution was stirred at rt under LED irradiation, open to the atmosphere. After 22 h, the solution was cooled to 0 °C, and the reaction was quenched by the addition of 1 M aq HCl (10 mL). The aqueous phase was extracted with Et₂O (3 × 10 mL) and the combined organic phases were washed with sat. aq NaCl (2 × 10 mL) and dried (MgSO₄). The solvent was removed in vacuo to give a crude product. The yield was determined by ¹H NMR analysis with 1,3,5-trimethoxybenzene (1 equiv) as the internal standard. 4-Hydroxybenzonitrile ¹H NMR (500 MHz, CDCl₃): δ = 7.59–7.51 (m, 2 H, ArH), 6.97–6.89 (m, 2 H, ArH), 6.31 (bs, 1 H, ArOH). ¹³C NMR (126 MHz, CDCl₃): δ = 160.0 (CN), 134.5 (ArC), 119.3 (ArC), 116.6 (ArC), 103.6 (ArC).
• 29 APC-Catalyzed Aerobic Cross-Dehydrogenative Coupling Reaction of N-Phenyl-1,2,3,4-tetrahydroisoquinoline with Nitromethane under Visible Light (Table [3]); General Procedure N-phenyl-1,2,3,4-tetrahydroisoquinoline (21 mg, 0.1 mmol) and the appropriate PC (1 mg, 2.5 μmol, 2.5 mol%) were dissolved in MeNO₂ (1 mL) in a 25 mL Schlenk tube, and the solution was stirred at rt under LED irradiation, open to the atmosphere. After 4.5 h, the solvent was evaporated in vacuo to furnish a crude product. The yield was determined by ¹H NMR analysis with 1,3,5-trimethoxybenzene (1 equiv) as the internal standard. 1-(Nitromethyl)-2-phenyl-1,2,3,4-tetrahydroisoquinoline ¹H NMR (500 MHz, CDCl₃): δ = 7.30–7.24 (m, 3 H, ArH), 7.24–7.18 (m, 2 H, ArH), 7.15–7.12 (m, 1 H, ArH), 7.01–6.97 (m, 2 H, ArH), 6.86 (tt, J ₁ = 7.3, J ₂ = 1.0 Hz, 1 H, ArH), 5.56 (t, J = 7.2 Hz, 1 H, CH), 4.89 (dd, J ₁ = 11.9, J ₂ = 7.8 Hz, 1 H, ½ × CH ₂NO₂), 4.57 (dd, J = 11.9, 6.6 Hz, ½ × CH ₂NO₂ 1 H), 3.71–3.60 (m, 2 H, CH ₂), 3.13–3.05 (m, 1 H, ½ × CH ₂), 2.85–2.77 (m, 1 H, ½ × CH ₂). ¹³C NMR (126 MHz, CDCl₃): δ = 148.5 (ArC), 135.4 (ArC), 133.0 (ArC), 129.7 (ArC), 129.3 (ArC), 128.3 (ArC), 127.1 (ArC), 126.9 (ArC), 119.7 (ArC), 115.3 (ArC), 78.9 (CH₂NO₂), 58.4 (CHN), 42.4 (CH₂), 26.6 (CH₂).
• 30 PC-Catalyzed Intermolecular Atom-Transfer Radical Addition between 4-Nitrobenzyl Bromide and Styrene under Visible Light (Table [4]); General Procedure iAn oven-dried 25 mL Schlenk tube was charged with 4-nitrobenzyl bromide (43 mg, 0.2 mmol), LiBr (35 mg, 0.4 mmol, 2 equiv), and the appropriate PC (1 mg, 2 μmol, 1 mol%). The tube was evacuated and refilled with N₂ (×3) before anhyd degassed MeCN (1 mL) and styrene (0.23 mL, 2 mmol, 10 equiv) were added under a positive flow of N₂, and the tube was sealed. The solution was freeze–pump–thaw degassed (3×), then refilled with N₂. The solution was then stirred at rt under LED irradiation for 22 h. The solvent was evaporated in vacuo to furnish the crude product. The yield was determined by ¹H NMR analysis using 1,3,5-trimethoxybenzene (1 equiv) as the internal standard. 1-(3-Bromo-3-phenylpropyl)-4-nitrobenzene ¹H NMR (500 MHz, CDCl₃): δ = 8.18–8.14 (m, 2 H, ArH), 7.40–7.29 (m, 7 H, ArH), 4.87 (dd, J₁ = 8.7, J ₂ = 6.1 Hz, 1 H, CHBr), 3.00–2.91 (m, 1 H, ½ × CH ₂), 2.85–2.78 (m, 1 H, ½ × CH ₂), 2.69–2.57 (m, 1 H, ½ × CH ₂), 2.51–2.40 (m, 1 H, ½ × CH ₂). ¹³C NMR (126 MHz, CDCl₃): δ = 148.4 (ArC), 146.8 (ArC), 141.6 (ArC), 129.5 (ArC), 129.0 (ArC), 128.8 (ArC), 127.4 (ArC), 124.0 (ArC), 54.1 (CHBr), 40.9 (CH₂), 34.3 (CH₂).
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