Benzylic and Allylic Oxidations with Bis(trifluoroacetoxyiodo)benzene and tert-Butyl Hydroperoxide

Abstract

Oxidation of benzylic and allylic substrates with a bis(trifluoroacetoxyiodo)benzene/tert-butyl hydroperoxide system to the corresponding α,β-unsaturated enones was investigated. The scope and reaction mechanism are discussed.

Key words

hydrocarbons - hypervalent iodine - tert-butyl hydroperoxide - radical reaction - oxidation - ketones

References and Notes

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8

General Procedure for Oxidation of Benzylic and Allylic Substrates with PIFA/TBHP System
(Caution! Although we have never experienced an explosion, the oxidation of substrates with PIFA/TBHP system should be carried out behind shields.) To a solution of substrate (2 mmol) and anhyd TBHP (40 mmol) in CH₂Cl₂ (10 mL) at -30 ˚C was added NaHCO₃ (20 mmol). Then a freshly prepared solution of PIFA (5.0 mmol) in CH₂Cl₂ (10 mL) was added over 2 h. The temperature was slowly increased to r.t. over 4 h. On completion of reaction, the excess TBHP was reduced by adding DMS^¹8 (40 mmol) and Ti(Oi-Pr)₄ (0.15 mmol) at r.t. while the reaction proceeding was monitored by peroxide testing (KI, AcOH). 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 (20 ˚C/50 mbar). The products were purified on a silica gel column (40 g) by eluting with hexane-EtOAc (95:5). The first fractions gave iodobenzene. Further elution afforded analytically pure ketone. The ketones 7a-l are known, and analytical data were found to be identical with those reported.
9 H -Fluoren-9-one (7b)
Yellow solid, mp 81-82 ˚C (CH₂Cl₂-hexane). ^¹H NMR (200 MHz, CDCl₃): δ = 7.63 (dd, J = 7.3, 1.0 Hz, 2 H), 7.30-7.20 (m, 2 H), 7.48-7.45 (m, 4 H). ^¹³C NMR (50 MHz, CDCl₃): δ = 195.8, 146.4, 136.6, 136.1, 131.0, 126.2, 122.2.
3,7,7-Trimethylbicyclo[4.1.0]hept-3-ene-2,5-dione (7l)
Pale yellow solid; mp 94-96 ˚C (CH₂Cl₂-hexane). ^¹H NMR (400 MHz, CDCl₃): δ = 6.50-6.48 (m, 1 H), 2.33-2.29 (m, 2 H), 1.96 (d, J = 1.8 Hz, 3 H), 1.31 (s, 3 H) 1.30 (s, 3 H). ^¹³C NMR (100 MHz, CDCl₃): δ = 195.2, 194.6, 150.2, 137.8, 40.0, 39.1, 33.9, 29.3, 16.4, 15.6.