Intermediates bridged radicals

Further evidence for a bromine-bridged radical comes from radical substitution of optically active 2-bromobutane. Most of the 2,3-dibromobutane which is formed is racemic, indicating that the stereogenic center is involved in the reaction. A bridged intermediate that can react at either carbon can explain the racemization. When the 3-deuterated reagent is used, it can be shown that the hydrogen (or deuterium) that is abstracted is replaced by bromine with retention of stereochemistry These results are also consistent with a bridged bromine radical. 

The more facile migration of aryl and other unsaturated groups involves bridged intermediates formed by an addition process. In the case of aryl migration, the intermediate is a cyclohexadienyl radical ... 

The bridged ion-radicals are usually subdivided into three classes (Robin and Day 1967, see also Dumur et al. 2004). Class I—the redox centers are completely localized and behave as separated entities, class II—intermediate coupling between the mixed-valence centers exists class III— the redox centers are completely equivalent being enriched with an unpaired electron by the half-to-half manner. For instance, the cation-radical of A,A,A, A -tetraphenyl-p-phenylenediamine belongs to class III (Szeghalmi et al. 2004). 

Figure 20.6. Comparisons of experimental and theoretical electron diffraction radial distribution curves based on ab initio geometries for the bridged C2F4I radical structure (on left) and a sum of classical anti and gauche structures (on right)." The intermediate present 5 ps after the pump pulse was defined structurally through 2D diffraction difference images. [Reproduced with permission from H. Ihee, V. A. Lobastov, U. M. Gomez, B. M. Goodson, R. Srinivasan, C.-Y. Ruan, and A. H. Zewail, Science 2001, 291, 458. Copyright 2001 American Association for the Advancement of Science.]...

The mechanism by which a, /j-unsaturated ketones (see Scheme 16), jS-keto esters, and uracil derivatives react with iodine in the presence of bis(tctra-w-butylammonium) peroxydisulfate (318) in acetonitrile to give the appropriate iodinated products in good yields is unclear.289 The mechanism may involve the cleavage of (318) to give an n-butylammonium sulfate radical, which can react to fonn a cationic iodine radical and sulfate anion the substrate then reacts with the iodine radical to form an iodine-bridged intermediate. 

Why does a stable bis-allyl analog exist on the Cope reaction surface of 27 In the prototype Diels-Alder reaction of 1,5-hexadiene, the possible bis-allyl intermediate (i.e., two isolated allyl radicals) is about 26 kcal mol higher in energy than the Cope transition state. Only with significant radical stabilization might one expect a bis-aUyl intermediate to occur. One can consider 28 as composed of two bridged phenalenyl radicals (29), which affords a rather dramatic stabilization of each radical. 

Recall that Van Leeuwen et al. (24) had shown that only the most efficient radical scavengers would intercept the photogenerated CH3 radicals. A more probable mechanism involves bimolecular reaction between the transition metal complex and a photogenerated Ti species such as 15 in Scheme 2. This could lead to CHs/Br exchange via a bridged intermediate such as 25. Yields of these reactions were not reported and thus their... 

During the course of biomethylation the methyl group is most likely transferred as a bridging intermediate rather than a free entity. Such an intermediate is assumed to form during an associative mechanism [4]. The methyl group may be electrophilic (cationic), radical or nucleophilic (anionic), depending on the specific donor moiety. A broad variety of methyl transfer reactions are therefore possible. Besides the two biological donors, methylcobalamin (see below) and S-adenosyl-methonine (1) nonenzymatic transmethylation is also possible in the natural environment, probably also very important for the formation and decomposition of metal methyl compounds [3b],... 

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