Radicals biradical intermediates

The enthalpy changes associated with proton transfer in the various 4, -substituted benzophenone contact radical ion pairs as a function of solvent have been estimated by employing a variety of thermochemical data [20]. The effect of substituents upon the stability of the radical IP were derived from the study of Arnold and co-workers [55] of the reduction potentials for a variety of 4,4 -substituted benzophenones. The effect of substituents upon the stability of the ketyl radical were estimated from the kinetic data obtained by Creary for the thermal rearrangement of 2-aryl-3,3-dimethylmethylenecyclopropanes, where the mechanism for the isomerization assumes a biradical intermediate [56]. The solvent dependence for the energetics of proton transfer were based upon the studies of Gould et al. [38]. The details of the analysis can be found in the original literature [20] and only the results are herein given in Table 2.2. 

Calculations based on this second model give the observed value for the entropy of activation. In addition, this model may be used to account for the observed isotope effect (Benson and Nangia, 1963). If the tetra-methylene biradical is involved then it is to be expected that appropriately substituted cyclobutanes might undergo cis-trans isomerization reactions. This will be referred to again later. One final point should be mentioned in connection with biradical intermediates in both cyclopropane and cyclobutane reactions. This concerns the absence of any effect of radical inhibitors on these systems, when it might be expected that they would interact with the biradicals. In fact calculations show that, under the conditions of formation, the biradicals have extremely short lifetimes sec) and hence, unless radical inhibitors are... 

Schindler and coworkers verified the formation of hydroxyl radicals kinetically and further RRKM calculations by Cremer and coworkers placed the overall concept on a more quantitative basis by verifying the measured amount of OH radical. An extensive series of calculations on substituted alkenes placed this overall decomposition mechanism and the involvement of carbonyl oxides in the ozonolysis of alkenes on a firm theoretical basis. The prodnction of OH radicals in solution phase was also snggested on the basis of a series of DFT calculations . Interestingly, both experiment and theory support a concerted [4 4- 2] cycloaddition for the ozone-acetylene reaction rather than a nonconcerted reaction involving biradical intermediates . 

Experimental evidence of the involvement of a biradical intermediate in the decomposition of 3,3-dimethyl-l,2-dioxetane (10) has been obtained by radical trapping with 1,4-cyclohexadiene (CHD). Decomposition of 10 in neat CHD was shown to result in the formation of the expected 1,4-dioxy biradical trapping product, 2-methyl-1,2-propanediol (11) ° . However, more recently, it has been shown that the previously observed trapping product 11 was formed by induced decomposition of the dioxetane, initiated by the attack of the C—C double bond of the diene on the strained 0—0 bond of the cyclic peroxide (Scheme 9)"°. 

In view of the present calculated results, the SET mechanism would be described as follows. Basically, the polar four-center reaction in Scheme 14 leads to C—C bond formation. However, when the alkyl group is bulky, only the two-center (Mg—O) reaction takes place. The aUcyl-Mg bond is cleaved homolytically owing to the persistent Mg tetravalency and the stability of the resultant radical species. Hence, biradical intermediates are formed not by a single electron transfer but by the C—Mg homolytic scission. 

When thioketones are irradiated alone, a cydodimerizalion may occur to give a 1,3-dithietane <4.98). In the presence of an alkene, different cycloadducts are found, usually ihietanes. If visible radiation is used, electron-rich alkenes are especially effective as addends, and the products can be rationalized on the basis of a two-step mechanism involving the more highly stabilized biradical intermediate 4.99). Sometimes a 1,4-dithiane accompanies the thietane (4.100) as a result of the trapping of the biradical by a further molecule of ground-state thioketones (unlike ketones, thioketones react with radicals quite readily, which is one of the causes of... 

Reactivity in the elimination reaction is probably largely determined by the nature of the substituents (in addition to hydrogen) attached to the y-carbon atom. If the reaction is stepwise, one would expect faster reaction with either 2-hexanone or methoxyacetone than with 2-pentanone, because the biradical intermediates would be more stable in the former two cases. Such a conclusion follows from the similarity shown by various ketone triplets in intermolecular reactions333 to the known selectivity of attack by /-butoxy radicals in both inter-353 and intramolecular reactions.354... 

If a biradical intermediate is involved in the reaction path, it must be the one shown above and clearly is not the most stable possibility. However, the initial addition to the isolated double bond does proceed to form a five-membered ring as would appear to be common with radical and biradical species. 

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