Facile Aerobic Photooxidation of Alcohols in the Presence of Catalytic Lithium Bromide

Abstract

Alcohols were found to be oxidized to the corresponding carboxylic acids in the presence of catalytic lithium bromide under photoirradiation.

Key words

photooxidation - alcohols - lithium bromide - aerobic - carboxylic acid

References

• 1 Comprehensive Organic Transformations: A Guide to Functional Group Preparations   Larock RC. Wiley-VCH; New York: 1999. 
• 2 Itoh A. Hashimoto S. Kodama T. Masaki Y. Synlett  2005,  2107 
• 3 Regarding photooxidation with alkali metal halides, the conversion of CO to CO₂ and H₂ to H₂O in the presence of hydrogen exposed to over 200 nm of ultraviolet light has been previously reported, see: Ryabchuk V. Catal. Today  2000,  58:  89 
• 4a

  For direct oxidations of alcohols to carboxylic acid without photoirradiation, see ref. 1. See also:

• 4b Zhao M. Li J. Song Z. Desmond R. Tschaen DM. Grabowski EJJ. Reider PJ. Tetrahedron Lett.  1998,  39:  5323 
• 4c Zhao M. Li J. Mano E. Song Z. Tschaen DM. Grabowski EJJ. Reider PJ. J. Org. Chem.  1999,  64:  2564 
• 4d Ji H. Mizugaki T. Ebitani K. Kaneda K. Tetrahedron Lett.  2002,  43:  7179 
• 4e Yasuda K. Ley SV. J. Chem. Soc., Perkin Trans. 1  2002,  1024 

5

We believe an effective wavelength of light is 365 nm.

6

When using primary alcohol as substrate, a typical procedure follows: a solution (5 mL) of the substrate (50 mg) and LiBr (0.3 equiv) in dry EtOAc was stirred and irradiated at r.t. with a 400 W high-pressure mercury lamp externally for the indicated time. The reaction mixture was concentrated under reduced pressure, and 1% NaOH aq solution was added. The aqueous solution was washed with Et₂O, and then acidified with 2 N HCl aq solution, which was extracted with Et₂O. The organic layer was washed with brine and dried over Na₂SO₄, and concentrated under reduced pressure. The product was pure without further purification.
When using secondary alcohol as substrate, a typical procedure follows: a solution (5 mL) of the substrate (50 mg) and LiBr (0.3 equiv) in dry EtOAc was stirred and irradiated at r.t. with a 400 W high-pressure mercury lamp externally for the indicated time. The reaction mixture was concentrated under reduced pressure, and 1% NaOH aq solution was added. The aqueous solution was washed with Et₂O and the organic layer was concentrated, and the residue was purified by preparative TLC. The aqueous layer was then acidified with 2 N HCl aq solution and extracted with Et₂O the same as in the former cases.

7

Catalytic bromine oxidized 1 to 2 in 59% yield under similar conditions.