[28276-78-0] · C9H8O5 · p-Methoxycarbonylperbenzoic Acid · (MW 196.17)
(electrophilic reagent which delivers oxygen to alkenes1)
Alternate Name: PMCPBA.
Physical Data: white crystalline solid; dec. 125 °C.
Solubility: sol ether, dioxane, CH2Cl2, CHCl3; slightly sol benzene; insol cyclohexane, CCl4.
Analysis of Reagent Purity: iodometric titration.
Preparative Methods: a mixture of aldehyde (1) (98% pure, 2 g) and CCl4 (50 mL) is stirred under oxygen in a glass reactor at 25 °C and irradiated with a 2 kW high-pressure mercury lamp until about 95% of the theoretical amount of oxygen is absorbed (0.5 h) (eq 1). The product, which separates as a white crystalline powder, is collected and dried (yield 2.0 g, 84%; peroxy acid content 90%); this material can be used for general oxidation reactions.1
Purification: recrystallization from methanol.
Handling, Storage, and Precautions: the peroxy acid (2) is as stable as m-Chloroperbenzoic Acid and shows less than 5% decomposition after storage at 10 °C for 1 year. (2) irritates nasal membranes and causes sneezing. Since the reagent is a peroxide and potentially explosive, care is required in carrying out reactions with it. During workup, check for peroxides before solvent removal. This reagent should be handled in a fume hood.
A number of alkenes have been epoxidized using the peroxy acid (2); the yields of epoxides obtained are comparable to the yields obtained using Perbenzoic Acid. The following is a representative procedure. To a stirred mixture of peroxy acid (2) (6.112 g, 0.03 mol) and 200 mL of CHCl3 maintained below 20 °C, is added dropwise a solution of cyclohexene (3) (2.544 g, 0.03 mol) in 20 mL of CHCl3. The reaction mixture is stirred at 20 °C for 12 h. The precipitate of p-methoxycarbonylbenzoic acid (4.650 g) is removed by filtration. The filtrate is washed with aq Na2CO3 solution, aq NaHSO3, and brine and then dried. The residue obtained after solvent removal is distilled under reduced pressure to furnish the epoxide (4) (yield 90%) (eq 2).1
The epoxidation of the triene (5) with the peroxy acid (2) is regio- and stereoselective, furnishing almost exclusively the epoxide (6) (eq 3).2 Epoxidation of the triene (5) with MoO5.2HMPA furnishes a 5:1 mixture of epoxides (6) and (7); epoxidation of (5) with Hydrogen Peroxide-Peracetic Acid furnishes a 2:1 mixture of epoxides (6) and (7).
Epoxidation of cholesteryl acetate (8) in CH2Cl2 furnishes a mixture of a- and b-epoxides in 65:35 ratio. The influence of solvents on the stereochemistry of epoxidation of (8) has been examined. The highest amount of b-epoxide (58%) is obtained when CCl4 (in which the reagent is insoluble) is used as solvent; the highest amount of a-epoxide (76%) is obtained when t-butanol is used as solvent.3
Ketones undergo oxygen insertion reaction when reacted with the peroxy acid (2) (eq 4).1 The yields obtained are comparable with perbenzoic acid oxidations.
The oxidation of N-heterocycles to N-oxides proceeds smoothly when they are treated with a suspension of (2) in CHCl3. The oxidation of pyridine, 3,4-dimethylpyridine, and 2-methylquinoline has been carried out employing both m-CPBA and acid (2); reagent (2) is as effective as m-CPBA.4
In the epoxidation of alkenes which are moderately or highly reactive, the yields obtained with (2) are comparable with the yields obtained using m-CPBA or MeCO3H. However, m-CPBA and MeCO3H are available commercially; (2) is not available commercially but can be prepared easily in a short time and has a long shelf life.
A. Somasekar Rao & H. Rama Mohan
Indian Institute of Chemical Technology, Hyderabad, India