Radical chain reactions addition transition state

Because the addition steps are generally fast and consequently exothermic chain steps, their transition states should occur early on the reaction coordinate and therefore resemble the starting alkene. This was recently confirmed by ab initio calculations for the attack at ethylene by methyl radicals and fluorene atoms. The relative stability of the adduct radicals therefore should have little influence on reacti-vity 2 ). The analysis of reactivity and regioselectivity for radical addition reactions, however, is even more complex, because polar effects seem to have an important influence. It has been known for some time that electronegative radicals X-prefer to react with ordinary alkenes while nucleophilic alkyl or acyl radicals rather attack electron deficient olefins e.g., cyano or carbonyl substituted olefins The best known example for this behavior is copolymerization This view was supported by different MO-calculation procedures and in particular by the successful FMO-treatment of the regioselectivity and relative reactivity of additions of radicals to a series of alkenes An excellent review of most of the more recent experimental data and their interpretation was published recently by Tedder and... 

The most active elements for the oxygen transfer are the transition metals to the left of the Periodic Table. The order of activity of these is Mo > W > Ti > V > U > Th > Zr, Nb [465]. In addition, several nontransition metal compounds are effective in the reaction, most notably SeO2 and borate esters (See Section 11). The catalytic elements are typically in their highest attainable oxidation state, and have the essential feature of not having a readily accessible lower oxidation state. This is necessary in order not to promote the metal-catalyzed decomposition of the peroxides, which could initiate radical chain reactions. Elements such as Mn, Fe, Co, Rh, Ni, Pt, and Cu are ineffective for this reason. 

One additional H abstraction reaction must be mentioned. Internal 1,5 (Reaction 54) or 1,6 (Reaction 55) hydrogen abstraction generates an alcohol and a radical (21) in a position that may or may not be normal for autoxidation. Intramolecular H abstraction involving a six-membered transition state (Reaction 55) has been identified in saturated alkyls with long side chains (304). Occurrence of the corresponding reaction in unsaturated fatty acids would produce oxidation at sites previously attributed to HO attack (314). 

The addition of polyhaloalkanes and related halo-genated compounds to alkenes can occur via a classical radical chain process (Scheme 13), which is often called the Kharasch reaction.38 In 1961, Minisci et al.39 and Asscher and Vofsi40 discovered that this reaction is catalyzed by transition metal ions in their lower valent state such as Cu+ and Fe2+, and they formulated the mechanism in Scheme 14. The catalysis of the additions by simple metal salts or complexes such as Cu(I)-2,2 -bipyridyl41a and ruthe-... 

Most of the useful iodine transfer radical reactions arise from the addition of alkyl iodides, which have been activated by one or more adjacent carbonyl or nitrile substituents, to unactivated olefins. This both labilizes the initial iodide, facilitating chain initiation, and helps ensure that the atom transfer step is exothermic. The requisite iodides are typically synthesized by deprotonation with EDA or NaH, followed by iodination with I2 or A-iodosuccinimide. Cyclization of an iodoester yields primarily lactone product, proceeding through the intermediacy of the I-transfer products as shown in Scheme 5 [19]. Reactions in which a-iodoesters cyclized with alkynes also proved efficient. Similar ketones yielded less synthetically useful mixtures of cyclopentyl and cyclohexyl (arising from 6-endo transition states) products. 

The good yields obtained in the cyclization of appropriate radical precursors show that this is a convenient method for the synthesis of branched-chain cyclitols from carbohydrates. In addition, an interesting stereoelectronic effect in the cyclization of these acychc sugar derivatives is demonstrated the stabilizing effect of electron-attracting groups vicinal to carbon-centered radicals determines the preferred conformation in the transition state and the stereochemical outcome of the reaction. 

The transition states for the steps of propagation are formed repeatedly in liquid medium systems, containing monomer, initiator, the formed polymer, and frequently a solvent. There are many different types of initiating reactions. These polymerization, however, never terminate by combination or by disproportionation as they do in free-radical chain-growth polymerizations. Instead, terminations of chain growths are results of unimolecular reactions, or transfers to other molecules, like monomers or solvents, or impurities, like moisture. They can also result from quenching by deliberate additions of reactive terminating species. 

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