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Sodium Bromide-Catalyzed Oxidation of Secondary Benzylic Alcohols Using Aqueous Hydrogen Peroxide as Terminal Oxidant
Received: 18 September 2015
Accepted after revision: 23 October 2015
09 December 2015 (online)
A halide salt, hydroperoxide and AcOH catalyst system was applied to the oxidation of secondary benzylic alcohols. This simple system can be applied to a variety of secondary benzylic alcohols and scaled up for gram-scale preparation. High secondary benzylic alcohol selectivity of the present method is demonstrated in hydroxyketone synthesis. Based on several experimental results, a catalytic cycle for our oxidation is proposed.
References and Notes
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- 29 General Procedure for NaBr-Catalyzed Oxidation: Under a nitrogen atmosphere, to a solution of substrate alcohol (0.5 mmol) in AcOH (1.0 mL) or AcOH–EtOAc (3:7, 2.0 mL) was added a stock solution of aq NaBr solution (1.94 M, 25 μL) and 30% aq H2O2 (50 μL, 0.5 mmol). After stirring the mixture for 1 h at 60 °C, additional 30% aq H2O2 (50 μL, 0.5 mmol) was added, and stirring was continued for another 1 h. After cooling, the mixture was poured into a sat. aq NaHCO3 solution (ca. 30 mL) with the aid of CH2Cl2, and the resulting mixture was extracted with CH2Cl2. The combined organic layers were dried over anhyd MgSO4, filtered and concentrated in vacuo. The residue was chromatographed on silica gel (flash column or preparative TLC) to afford the corresponding ketone. Caution: When the reaction is carried out on a large scale, treatment of the combined organic layers with aq Na2S2O3 solution is recommended to avoid unexpected explosion. 1-Phenylnonan-1-one (2a): Compound 2a was obtained according to the general procedure and purified by preparative TLC (hexane–EtOAc, 20:1) as a colorless oil. 1H NMR (500 MHz, CDCl3): δ = 7.96 (d, J = 7.5 Hz, 2 H), 7.54–7.57 (m, 1 H), 7.45–7.48 (m, 2 H), 2.96 (t, J = 7.5 Hz, 2 H), 1.74 (m, 2 H), 1.27–1.44 (m, 10 H), 0.88 (t, J = 6.9 Hz, 3 H). 13C NMR (125 MHz, CDCl3): δ = 200.6, 137.0, 132.8, 128.5, 128.0, 38.6, 31.8, 29.4, 29.3, 29.1, 24.3, 22.6, 14.1. The NMR data are in agreement with those previously reported in literature (see ref. 30).
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For selected pioneering works, see:
For recent reviews on oxidative coupling using halide salts/ROOH system, see:
For selected examples, see:
For recent reviews on catalytic oxidation of alcohols, see:
For the combination of stoichiometric amounts of alkali bromide salts and oxidants for alcohol oxidation, see:
Catalytic amounts of alkali bromide salts were frequently used in TEMPO-based and related catalyst system. However, in these catalyst systems, redox cycle between Br– and [Br+] works for re-oxidation of TEMPO to the corresponding oxoammonium salt which is active species, see:
For the combination of stoichiometric amounts of oxidant and catalytic amounts of alkali bromide salts for oxidation of alcohols, see:
For the HBr- or Br2-catalyzed oxidation of alcohol, see: