Aerobic Photooxidation of Methyl Group at Aromatic Nucleus with LiBr

Abstract

A methyl group at the aromatic nucleus was found to be oxidized to the corresponding carboxylic acid directly in the ­presence of lithium bromide under aerobic photoirradiation.

Key words

photooxidation - methyl group - 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. Kodama T. Inagaki S. Masaki Y. Org. Lett.  2000,  2:  2455 
• 3 Photooxidation of alkylbenzene in the presence of alkyl bromides or hydrogen bromide under oxygen has been previously reported, see: Nakada M. Fukushi S. Hirota M. Bull. Chem. Soc. Jpn.  1990,  63:  944 
• 4 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 
• 7 The reaction did not proceed with the irradiation in short time, for example 1 h, for the initiation of the reaction

5

A typical procedure follows: a solution (5 mL) of the 4-tert-butyltoluene (50 mg) and LiBr 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 aqueous solution was added. The aq soln was washed with Et₂O, and acidified with 2 N HCl aq soln, and then extracted again 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.

6

No product (2) was detected when the reaction was carried out in the absence of UV irradiation or LiBr, which indicates the necessity of both LiBr and light for this reaction.

8

A typical procedure follows: a solution (5 mL) of the substrate (0.269 mmol) and LiBr (0.5 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 soln was added. The aq soln was washed with Et₂O, acidified with 2 N HCl aq soln, and then extracted again 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.

9

Catalytic bromine oxidized 1 to 2 in 21% yield under similar conditions.