Substrate-Selectivity in Catalytic Photooxygenation Processes Using a Quinine-BODIPY System

Published as part of the Special Section 11^th EuCheMS Organic Division Young Investigator Workshop

Abstract

Substrate selectivity by means of synthetic catalysts remains a challenging topic in chemistry. Here, a catalytic system combining an iodo-BODIPY photosensitizer and quinine was evaluated in the competitive photooxygenation of non-hydrogen and hydrogen-bond-donor substrates. The ability of quinine to activate hydrogen-bond-donor substrates towards photooxygenation was reported and the results were benchmarked with photooxygenation experiments in the absence of quinine.

Key words

photooxygenation - singlet oxygen - substrate selectivity - BODIPY - oxidation

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• 14 (R)-((1S,2S,4S,5R)-5-(1-benzhydryl-1H-1,2,3-triazol-4-yl)quinuclidin-2-yl)(6-methoxyquinolin-4-yl)methanol (4) Reaction conditions described for the synthesis of 3 were applied to the preparation of 4 but diphenylmethyl azide (731 mg, 3.5 mmol, 1 equiv) was used instead of azidobenzene. Purification by column chromatography, eluting with EtOAc/MeOH 85:15, gave the quinine derivative 4. Yield: 255 mg (41%); mp 114 °C. IR (ATR): 3143, 2926, 2874, 1506, 1238, 1028, 725, 698 cm^–1. ¹H NMR (300 MHz, CDCl₃): δ = 8.64 (dd, J = 4.5, 1.9 Hz, 1 H), 7.92 (d, J = 9.4 Hz, 1 H), 7.51 (d, J = 4.8 Hz, 1 H), 7.34–7.22 (m, 7 H), 7.22–7.17 (m, 1 H), 7.01 (s, 1 H), 6.96–6.88 (m, 5 H), 5.66 (d, J = 3.9 Hz, 1 H), 3.81 (s, 3 H), 3.70–3.56 (m, 1 H), 3.46–3.32 (m, 2 H), 3.31–3.20 (m, 1 H), 3.08–2.98 (m, 1 H), 2.81–2.66 (m, 1 H), 2.12–2.04 (m, 1 H), 1.87–1.70 (m, 2 H), 1.68–1.53 (m, 1 H), 1.40–1.31 (m, 1 H), 0.96–0.81 (m, 1 H). ¹³C NMR (75 MHz, CDCl₃): δ = 157.9, 150.1, 147.6, 146.9, 144.4, 138.2, 138.1, 131.7, 129.0 (2 C), 128.9 (2 C), 128.6, 128.5, 128.1 (2 C), 127.9 (2 C), 126.7, 121.8, 120.6, 118.6, 101.2, 71.2, 68.1, 59.6, 55.8, 43.4, 33.2, 29.8, 28.1, 27.1, 21.8. HRMS (ESI): m/z [M + H]⁺ calcd for C₃₃H₃₄N₅O₂: 532.2713; found: 532.2716.
• 15 Pair-Wise Competitive Photooxygenation Experiments; Typical Procedure A Schlenk flask was charged with anthracene (37.4 mg, 0.21 mmol, 1 equiv), methyl 2-oxoindane 1-carboxylate (40 mg, 0.21 mmol, 1 equiv), photosensitizer 1 (6 mg, 0.0105 mmol, 5 mol%), methyl phenyl sulfone (16.4 mg, 0.105 mmol, 0.5 equiv) as an internal standard and CDCl₃ (4.2 mL) to give a red solution. The reaction medium was gently bubbled through with oxygen for 5 min and then placed under an oxygen atmosphere. The homogeneous solution was irradiated with two green LEDs (1 W, 75 Lm, 535 nm typical wavelength). The distance from the light source to the irradiation Schlenk vessel was 2 cm without the use of any filters. The reaction was stirred for the appropriate reaction time and an aliquot (0.2 mL) was taken from the reaction mixture. The aliquot was diluted with CDCl₃ (0.4 mL) and nitrogen was bubbled through the solution to remove oxygen. The samples were then analyzed by ¹H NMR spectroscopy to determine the yield and product formation.
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