Bis(trifluoroacetoxyiodo)benzene-Induced Activation of tert-Butyl Hydroperoxide for the Direct Oxyfunctionalization of Arenes to Quinones

Abstract

Various aromatic hydrocarbons were oxidized with bis(trifluoroacetoxyiodo)benzene (PIFA)/tert-butyl hydroperoxide system to afford the corresponding quinones. The reaction conditions and scope have been discussed in detail.

Key words

hypervalent iodine - peroxyiodone - aromatic hydrocarbon - oxidation - quinone

References

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6

General Procedure for Oxidation of Arenes with PIFA/TBHP System: (Caution! Although we have never experienced explosion, the oxidation of arenes with TBHP/PIFA system was carried out behind shields.) To a solution of arene (2 mmol) and anhyd TBHP (10 mmol) in 10 mL of CH₂Cl₂ at -30 °C was added NaHCO₃ (5 mmol). Then, a freshly prepared solution of PIFA (3 mmol) in 10 mL of CH₂Cl₂ was added within 2 h. The temperature was slowly increased to r.t. within 1 h. The suspension was filtered and the solution was washed with sat. NaHCO₃ solution and H₂O. The organic layer was dried over MgSO₄ and the solvent was removed at reduced pressure (5 °C/50 mbar). The products were purified on a silica gel column (40 g) by eluting with hexane-EtOAc (90:10). First fractions gave iodobenzene. Further elution afforded analytically pure quinones. In the case of 5a, 2-methyl- and 6-methyl-1,4-naphthoquinone could not be separated. Compound 5b was obtained from the mixture by crystallization from EtOH. The mixture of both naphthoquinones 5b and 5c was also directly analyzed by ¹H NMR and GC-MS (equipped with a non-polar column using EI ionization at 70 eV). Compound 5b: ¹H NMR (400 MHz, CDCl₃): δ = 2.18 (d, 3 H, J = 1.5 Hz), 6.83 (q, 1 H, J = 1.5 Hz), 7.68-7.75 (m, 2 H), 8.01-8.12 (m, 2 H). ¹³C NMR (100 MHz, CDCl₃): δ = 187.5, 186.9, 150.1, 137.6, 135.6, 135.5, 134.2, 134.1, 128.5, 128.0, 18.4. Compound 6b: ¹H NMR (200 MHz, CDCl₃): δ = 2.11 (s, 6 H), 7.60-7.64 (AA part of AA′BB′ system, 2 H), 8.01-7.97 (BB′ part of AA′BB′ system, 2 H). ¹³C NMR (50 MHz, CDCl₃): δ = 185.6, 144.2, 134.1, 133.0, 127.0, 13.7. Compound 6c: ¹H NMR (200 MHz, CDCl₃): δ = 2.40 (s, 6 H), 6.90 (s, 2 H), 7.82 (s, 2 H). ¹³C NMR (50 MHz, CDCl₃): δ = 186.2, 144.6, 139.4, 130.8, 128.5, 21.1. Compound 7b: ¹H NMR (200 MHz, CDCl₃): δ = 3.89 (s, 3 H), 6.15 (s, 1 H), 7.76-7.65 (m, 2 H), 8.12-8.03 (m, 2 H). ¹³C NMR (50 MHz, CDCl₃): δ = 185.6, 180.9, 161.3, 135.2, 134.2, 132.9, 132.0, 127.5, 127.0, 110.8, 57.3. Compound 10b: ¹H NMR (200 MHz, CDCl₃): δ = 6.62 (s, 2 H), 7.73 (s, 4 H). ¹³C NMR (50 MHz, CDCl₃): δ = 180.4, 157.3, 148.6, 136.1, 124.9, 116.8.