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
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.
7 The reaction did not proceed with the irradiation in short time, for example 1 h, for the initiation of the 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.
