Epoxidation of alkenes by dioxirane intermediates generated in the reaction of potassium caroate with ketones

TLDR: In this article, it has been shown that the caroate-acetone system is capable of oxidizing nucleophilic organic and inorganic substrates (S:, path ii).

Abstract: Kinetics and 180-labeling experiments have provided evidence for the involvment of dioxirane intermediates 2a (Scheme I) in the ketone-catalyzed decomposition of potassium peroxomonosulfate (KHS05, hereafter called caroate) .1-3 It has been shown that, under the reaction conditions (in water, pH 7.5, 2-10 “C), the side reaction involving Bayer-Villiger (BV) oxidation of the ketone (path iii) is negligible with most ketones (acetone, dialkyl ketones, acetophenones, etc.), whereas it becomes significant a t lower pH values or with ketones having certain structural features (e.g., cyclobutanone, cy~lopentanone).l-~ In competition with path i, amounting to ketone catalysis of caroate decomposition, the dioxirane intermediate is capable of oxidizing nucleophilic organic and inorganic substrates (S:, path ii).1-3 As an example, oxidation of phenylpropiolic acid, PhC=CC02H, a substrate representative of weakly nucleophilic alkynes, could be achieved by the caroate--ketone s y ~ t e m ; ~ ? ~ this is indicative of the high reactivity of intermediate 2a, as the mentioned alkyne resisted all attempts a t oxidation by peroxoacids (as well as by caroate in absence of ketones) under the given conditions. Similarly, trans-cinammic acid (3a) failed to react with alkaline hydrogen peroxide, m-chloroperoxobenzoic acid (MCPBA) in CHzClz or with caroate alone;3 however, as we have already reported, reaction of 3a with caroateacetone in water affords the trans-epoxide 3b in high yield. As the cis-epoxide 4b could also be obtained in high yield upon reaction of cis-cinammic acid (4a) with caroateacetone, the reaction appears to be highly stereospecific.2 We report here further data which illustrate the versatility of the caroate-acetone system as a generalized epoxidation method. These are shown in Table I. The synthesis of 4,5-epoxy-2-hexenoic acid (5b) in high yield, as the sole reaction product from sorbic acid (5a), shows that epoxidation via dioxirane can be remarkably regioselective. Notice that the sorbic acid 4,5 double bond, which is more nucleophilic, is selectively oxidized. Treatment of 5-hexen-2-one (6a) with caroate and acetone also affords the corresponding epoxide in high yield. It is not surprising, in this case, that caroate in the absence of acetone can also convert the alkene into the epoxide in high yield (although substrate conversion is lower). In fact,