Carbon radicals radical chain process

N-Alkoxylamines 88 are a class of initiators in "living" radical polymerization (Scheme 14). A new methodology for their synthesis mediated by (TMSlsSiH has been developed. The method consists of the trapping of alkyl radicals generated in situ by stable nitroxide radicals. To accomplish this simple reaction sequence, an alkyl bromide or iodide 87 was treated with (TMSlsSiH in the presence of thermally generated f-BuO radicals. The reaction is not a radical chain process and stoichiometric quantities of the radical initiator are required. This method allows the generation of a variety of carbon-centered radicals such as primary, secondary, tertiary, benzylic, allylic, and a-carbonyl, which can be trapped with various nitroxides. 

The readily available organotin compounds include tin hydrides (stannanes) and the corresponding chlorides, with the tri-n-butyl compounds being the most common. Trialkylstannanes can be added to carbon-carbon double and triple bonds. The reaction is usually carried out by a radical chain process,137 and the addition is facilitated by the presence of radical-stabilizing substituents. 

Diphenyl-4//-pyran (151a R = Ph) undergoes a free-radical chain process with trichloromethyl radicals generated from carbon tetrachloride, affording pyrylium radical cation 378a353 (see Eq. 19). 

Our early work examined the reaction of PCTFE with sulfur, selenium and phosphorous nucleophiles 9 to achieve high levels of functionalization through a well-precedented (in the case of perfluoroalkyl iodides)20"24 one electron transfer, radical anion chain process. While such a reaction demonstrated the feasibility of using one-electron processes for the functionalization of PCTFE, the carbon-sulfur linkage remained susceptable to oxidation. 

The structural isomerization of cyclopropane to propene does not involve radical chain processes if a trimethylene diradical intermediate is involved, it must be of such a short lifetime that it may not be easily trapped through gas-phase intermolecular reactions Yet the trimethylene diradical hypothesis does account for the thermal interconversion of cis and trans 1,2-d2-cyclopropanes in a most plausible manner. Homolytic cleavage of one carbon-carbon bond would form a 1,3-diradical intermediate rotations of terminal methylene groups in this trimethylene diradical followed by reformation of the cyclopropane ring would rationalize the isomerization According to this model, the net outcome of a stereomutation process would be dictated by the relative magnitudes of rate... 

Some nucleophilic displacement reactions (particularly those involving ketone enolates and other carbon nucleophiles) proceed routinely in low yield, with poor material balance. There is increasing evidence that these reactions are actually radical chain processes catalyzed, in principle, by a single electron transfer from the anion to the heterocycle. In such cases... 

Essentially all the simple geminal di- or poly halides studied have been derivatives of methane. Reaction of diiodomethane (25,103) with triethyl phosphite proceeds normally, to furnish tetraethyl methanedi-phosphonate and iodomethanephosphonate. However, the reaction between carbon tetrachloride and trialkyl phosphite, first investigated by Kamai and Egorova (156,181), is catalyzed by peroxides or ultraviolet light, and accordingly it has been formulated by Kamai and Kharrasova (157) as a radical-chain process. 

Since the total reaction is basically initiated by generation of an organic radical, the reaction rate is closely related to the stability of the resulting radical. This type of oxidative addition therefore takes place rapidly for tertiary alkyl halides and quite slowly for tosylate regardless of steric repulsion. The stereochemistry of the O -carbon is completely lost. When the radical ion pair is not stable enough in the cage, the radical escapes to the solution leading to radical chain process... 

However, Willcott found evidence for a radical chain process in these reactions. Thus,l,5-dimethyl-5-deuteriomethyl-CPD gives 23% do and 15% d2 product in addition to the expected di material. Nonetheless, spiro[4.4]nona-1,3-diene undergoes ring expansion to bicyclo[4.3.0]nonadienes with allowed retention of stereochemistry at the migrating carbon, so the reaction appears to be concerted in these cases (see Chapter 10, Section 3). 

The reaction scheme 1 depicts a radical chain process. Anionic and cationic processes are similar, but with a full negative (-) or positive (+) charged carbon initiating center. In the initiation step, the monomers are activated by the initiators. During propagation, styrene monomer units are added to the growing polymer chains contributing to the macromolecular properties of the polymers. 

This is a radical chain process involving initiation, propagation and termination reactions as set out in Scheme 10.4 where RH represents an unsaturated fatty acid or ester with H attached to an allylic carbon atom ... 

Free-Radical-lnitiated Hydrosilylation. In early reports, it was suggested that addition of hydrosilanes to multiple bonds could proceed as free-radical chain process because of a relatively low energy of the Si—H bond in comparison with that of the C—H bond. For this reason, many synthetic and mechanistic studies have been devoted to the hydrosilylation of carbon-carbon (C=C, CM])) bond initiated by free radicals generated in the reaction mixture (3,6,10,16). Free-radical addition of hydrosilanes resembles the addition of hydrogen bromide to alkenes and always occurs according to the anti-Markownikov rule (3). 

In the photolysis of polycaproamide, hydrogen and carbon monoxide are liberated at a practically constant rate, which indicates the unbranched character of the radical-chain processes [74],... 

Mechanistically, radical polymerizah ons are addition polymerizations, and they follow the exact same radical chain process discussed in Chapters 10 and 11. The key steps of initiation, propagation, and termination are again involved, as recapitulated in Figme 13.13 B. Based upon studies of substituents on the carbon beta to the site of radical addition, it has been concluded that the adding radical has almost completely formed a bond at the transition state. Secondary isotope effects support this notion of a late transition state, with values between 1.05 to 1.17 for deuterium at the site beta to addition. 

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