Oxidation of Carboxylic Acids and Their Esters

The oxidation of carboxylic acids to peroxy acids is carried out by hydrogen peroxide. Laurie acid heated with 90% hydrogen peroxide in the presence of p-toluenesulfonic acid and a detergent for 2.25 h furnishes 95% of peroxylauric acid [174]. Benzoic acid is converted into peroxybenzoic acid in 85-90% yield on treatment with 70% hydrogen peroxide in the presence of methanesulfonic acid at 25-30 °C for 2.5 h [176.  

Esters of carboxylic acids undergo oxidative coupling in the a positions with respect to ester groups when treated in the enolate form with ferric chloride. Thus ethyl acetate stirred with lithium diisopropylamide in tet-rahydrofuran at -78 °C for 10 min and subsequently with anhydrous ferric chloride in dimethylformamide yields 69% of diethyl succinate [911]. 

Under similar conditions but with different oxidants, esters are hy-droxylated in the a positions with respect to the ester groups. Ethyl phen-ylacetate in tetrahydrofuran treated with lithium diisopropyl amide at -78 °C followed by oxidation with molybdenum oxide complex gives ethyl mandelate in 58% isolated yield [531]. Another way to obtain a-hydrox-ylated (or methoxylated) esters is by oxidation with iodobenzene diacetate in the presence of potassium hydroxide (or sodium methoxide) (equation 466) [794]. 

When chiral oxidants such as (+ )-2R,8aS and (-)-2S,8a/ camphor-ylsulfonyloxaziridines are used, the hydroxylation of enolates in the a positions with respect to the ester groups occurs enantioselectively, with enantiomeric excesses ranging from 12 to 85.5% and yields ranging from 35 to 88.5% (equation 467) [1032]. 

Methylene groups in the a positions with respect to carboxylic groups are changed to carbonyl groups thus oi-keto esters are formed. Methyl phenylacetate is oxidized with air in the presence of cobalt benzoate at 110-115 °C to methyl phenylglyoxylate in 86% yield [8], Diethyl malonate is converted by nitrogen sesquioxide (N2O3) into diethyl mesoxalate in 74-76% yield [450], 

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