Oxidation of Secondary Alcohols with Phenyltrimethylammonium Tribromide in the Presence of a Catalytic Amount of Antimony(III) Bromide or Copper(II) Bromide

Abstract

The oxidation of alcohols was carried out with phenyl­trimethylammonium tribromide in the presence of a catalytic amount of SbBr₃ or CuBr₂. 1,2-Diols, such as hydrobenzoin, were converted into 1,2-diketones or α-ketols without oxidative cleavage of the glycol C-C bond at room temperature. A variety of secondary alcohols were also oxidized to the corresponding carbonyl compounds in a chemoselective manner.

Key words

oxidation - alcohol - phenyltrimethylammonium tribromide - antimony(III) bromide - copper(II) bromide

References

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26

In particular, the oxidation of 1,2-diols 5, 8 to α-ketols with PTAB-CuBr₂-pyridine proceeded slowly at r.t. For example, compound 8 was converted to 11 in 42% yield for 14 h. 1,2-Dicyclohexyl-1,2-ethanediol (8) was recovered unchanged in 49% yield. It was assumed that the coordination of CuBr₂ to 1,2-diols or α-ketols was stronger than that of SbBr₃. Consequently, it required long reaction time to oxidize 1,2-diols to ketols with PTAB-CuBr₂-pyridine at r.t.

36

Mandelic acid methylester and ethyl 3-hydroxybutyrate were recovered unchanged in 92-95% yields at r.t. for 17-18 h. 1,4-Dichloro-2-butanol was also recovered unchanged in 90% yield at r.t. for 17 h.

37

Benzyl alcohol was recovered in 60-75% yields for 17-24 h. Benzaldehyde dimethylacetal was gradually afforded in 13-32% yields for 48-72 h.

38

A typical procedure is described for the oxidation of meso-hydrobenzoin 1. To a solution of SbBr₃ (18 mg, 0.05 mmol) in MeOH (8 mL) were added pyridine (60 µL) and PTAB (281 mg, 0.75 mmol) at r.t. After stirring for 5 min meso-hydrobenzoin 1 (53 mg, 0.25 mmol) was added. The reaction mixture was treated with 0.5 M aq Na₂S₂O₃ after stirring for 16 h at r.t. and extracted with EtOAc. The organic layer was washed by 0.5 M aq Na₂S₂O₃ and successively sat. aq NaCl and dried by MgSO₄. After removal of the solvent in vacuo, the residue was purified by column chromatography on silica gel (Wakogel C-200) with CCl₄ and CHCl₃ (3:1 v/v). Benzil (3, 50 mg, 0.24 mmol) was obtained in 96% yield.