Ring closures of peroxy radicals

Significant amounts of epoxides, oxetanes and tetrahydrofurans can be formed during oxidation of olefins and alkanes through the intermediate alkylperoxyalkyl or hydroperoxyalkyl radicals formed by addition of R02 or by internal H-atom transfer followed by C—O ring closure [96] 

Unlike H-atom transfer, C—O ring closures are highly exothermic processes because of stronger C—O bonds formed from cleavage of weaker 0—0 bonds, less the ring strain of the ether. For the simplest cases of epoxides or oxetanes, in which ring strain is 26—28 kcal mole-1 [13] 

AHr------23 kcal mole-1. Values for AHr for 5- and 6-center rings in which 

The kinetics of these reactions have been examined solely in the context of the competition between ring closure [reaction (150)] and addition of oxygen to the peroxyalkyl radical [reaction (151)] 

Reaction (151) leads to a mixture of alkylperoxyalkylhydroperoxide and polyperoxide depending on the relative ease of H-atom transfer or addition to the parent olefin. 

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More recently, Bachi and coworkers extended and adapted the TOCO reaction to the synthesis of 2,3-dioxabicyclo[3.3.1]nonane derivatives hke 228 (Scheme 52) ° ° . As detailed in Scheme 53a, the bridged bicyclic hydroperoxide-endoperoxides hke 229 are obtained, from (S )-limonene (227), in a 4-component one-operation free-radical domino reaction in which 5 new bonds are sequentially formed. Particular experimental conditions are required in order to reduce the formation of by-products 230 and (PhS)2, and to favor the critical 6-exo-ring closure of peroxy-radical 231 to carbon-centered radical 232206 chemoselective reduction of bridged bicyclic hydroperoxide-endoperoxides... 

FIGURE 10.5 Mechanistic representation of the ring-closure reactions for the -hydroxy-peroxy radicals arising from the OH-isoprene reactions. 

Furost-20(22)-enes (384) react with peroxy-acid with introduction of a hydroxy-group at the 20a-position, and ring closure to give the 20a-hydroxy-spirostan (385). The isolation of up to 25% of the 20,23-diol (386) is attributed to contamination of the furost-20(22)-ene with the 22(23)-unsaturated isomer. Almost all 3,5-dienolic esters and ethers react with electrophiles at C-6, apart from rare instances of reaction at C-4 or C-2 (see above). Reaction between the dienolic ethers (387) and tetranitromethane in ether has now been shown to afford the 2 -nitro-derivatives (388). No mechanistic details are available, but it is tempting to suggest the involvement of free-radicals derived from the reagent. 

Most oxidation reactions proceed by way of elementary steps involving alkylperoxy and alkoxy radicals therefore quantitative descriptions of oxidation processes require reliable absolute rate coefficients for all important elementary steps. This section provides a compilation of rate coefficients and rate parameters for H-atom transfer (abstraction), addition, ring closures and combinations by peroxy radicals, and for abstraction and cleavage by alkoxy radicals. 

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