Oxidation of Alkenes by Peroxy-acids

Oxidation of Alkenes to Oxirans by Peroxy-acids. An improved procedure for epoxidation using aromatic peroxy-acids has been reported. After a normal epoxidation with 3-chloroperoxybenzoic acid (mCPBA) in CH2CI2, activated KF is added to the crude mixture, and this results in the precipitation of both wCPBA and the aromatic acid by-product, leaving an acid-free reaction mixture for normal work-up. As an alternative, the insoluble mCPBA-KF complex itself may be used for the epoxidation of alkenes overnight at room temperature. After filtration and treatment of the CH2CI2 solution with more KF (to ensure removal of any residual peroxy-acid), normal work-up leads to yields in excess of 95% for cyclohexene and styrene oxides. 

The site-selectivity of oxidations by mCPBA is demonstrated in the conversion of (4 R = Me or Ph) into the corresponding ene epoxide (5). The product is sensitive to acid, so that the conversion is accomplished in a basic two-phase medium. Normal epoxidation of (6) with mCPBA leads to (7), The stereochemistries for such reactions are shown in the predominant formation of the 3-epoxide (8) (81%) from the parent alkene, with 12% of the a-product. Similar epoxidation of the cannabinol (9) leads to a less stereo-specific isomer distribution of 27.3% and 18.2%. Remarkable stereoselectivity has been shown in the epoxidation of the 14,15-unsaturated oestratrienes (10). Whereas oxidation of 17j3-esters and 17/3-ethers gave 14a,15a-epoxides ( 59%), the 17j3-urethane derivatives displayed a s /w-directive effect to yield 14/3,15j3-epoxides ( 87%). 

Meiji Seika Kaisha, Ltd., Jpn. Kokai Tokkyo Koho 82 02 232. 

Yamamoto, S. Narimatsu, K. Watanabe, and H. Yoshimura, Chem. Pharm. Bull., 1981, 29, 3378. 

The rates of epoxidation of cyclododecene with a series of aliphatic peroxy-acids have been correlated, using the Taft equation. The reaction constant (p ) was + 2.0 and the steric constant (6) was found to be essentially zero. A two-parameter correlation has been found for the effect of basicity and polarity of the solvent on the rate of epoxidation of propene with peracetic acid. Rate constants and activation parameters for the epoxidation of a number of cycloalkenes, including (11 R = H or COOMe), (12 R = H, Ph, or 2-furyl), (13), (14), and cyclo-octa-l,5-diene, have been measured. An isokinetic relationship was demonstrated, with the isokinetic temperature of 3 C. There was only a weak dependence of the rate on the structure of the alkene. 

Oxidation of Alkenes to Oxirans by Peroxy-acids. The mechanism of the reaction of m-chloroperbenzoic acid with double bonds has been investigated through a study of the epoxidation of a series of cycloalkenes (of ring sizes 5,6,7,8, and 12) and substituted cyclohexenes.The second-order rate constants were determined in CHCI3 at 0—30°C, and the data support a 1,3-dipolar cycloaddition reaction. 

Interest has been shown in the oxides of dicyclopentadiene, as noted earlier, with reports of epoxidations using permaleic, perphthalic, and peracetic acids. The mechanism of oxidation of fran -stilbene by peroxomonophosphoric acid (H3PO5) in a number of solvents has been investigated. A report on the epoxidation of a number of methoxy- and hydroxy-substituted cinnamaldehydes (24 R = = H, MeO, or OH) concludes with the suggestion that the 

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