Allylic species free allyl radicals

The mechanism of the Kharasch-Sosnovsky reaction remains unclear. The generally accepted version, as proposed by Kochi and co-workers (94-96) and later improved by Beckwith and Zavitsos (97), is illustrated in Scheme 8. Cuprous ion reduces the perbenzoate to Cu(II)OBz (Bz = benzoyl) and free /-BuO radical. The radical abstracts an allylic hydrogen atom generating an allyl radical that combines with the cupric salt to form an allylcopper(III) species. Reductive elimination with... 

Allylic intermediate, 27 185-187 ii-Allylic nickel intermediates, 33 15-18, 22 Allylic oxidation, see Oxidation, allylic Allylic species, 30 21 formation, isomerization, 30 18-19 free allyl radicals, 30 149 a-hydrogen abstraction, 30 147 Allyl methacrylate oxidation, 41 305 Allyls... 

Nadal and colleagues recently reported a Ni-catalyzed carbonylative Pauson-Khand-like [2+2+1] cycloaddition of allyl halides and alkynes in the presence of carbon monoxide and iron as the stoichiometric reducing agent [148]. The reaction was proposed to occur via reductively generated Ni(I)-radical like species free radicals were, however, considered unlikely. 

The key allyl species in the Grasselli mechanism [5 a] remains unspecified does it coordinate to the high-valent molybdenum in a a/n fashion, or does it occur as radicals What is the role of free allyl radicals Also, the product-forming C-O and C-N connection steps, respectively, leave open questions. For example, are there intramolecular rearrangements like those shown in Scheme 2 responsible for the formation of the new bonds How are the product precursors detached from the metal(s) ... 

The formation of acrolein from free allyl radical and MOO3 can be explained by the formation of a rr-ally 1 surface complex which forms a cr-O-allyl species followed by hydrogen abstraction (Scheme 6) (20). When ammonia is present, the formation of surface NH species followed by the analogous... 

The asterisk indicates some excited state of free radicals. The conversion of the free radical —CH2CHf- to —CH2CHCH3 could easily be supposed when one takes into consideration that the free radicals —CH2CHf might be produced as intermediate species in photo-induced radical conversion from an allylic radical to the free radical —CH2CHCH3 (71). 

Valuable insight of multiple additions of free radicals came from the ESR spectroscopic investigations of benzyl radicals, C-labeled at the benzylic positions [97,98]. These radicals can be prepared in situ by photolysis of saturated solutions of Cgo in labeled toluene containing about 5% di-ferf-butyl peroxide. Thereby, the photochemically generated ferf-butoxy radicals readily abstract a benzylic hydrogen atom from the toluene. Two radical species with a different microwave power saturation behavior can be observed. One radical species can be attributed to an allylic radical 63 and the other to a cyclopentadienyl radical 65 formed by the addition to three and five adjacent [5]radialene double bonds, respectively (Scheme 11). In these experiments no evidence for the radical 61 is found, which is very likely a short-lived species. 

Since such radicals react less rapidly than radicals not stabilized by resonance, the observable polymerization rate decreases. For this reason, the process is termed degradative chain-transfer. In degradative chain-transfer, growing, polymeric free radicals collide with a monomer molecule to form a new, stable free radical which propagates only with difficulty. The allylic radical may terminate growing radicals, dimerize, cause the decomposition of peroxidic initiators, or initiate the formation of new polymerizing species. The first of these possible processes has been termed cross termination [14]. 

Butyl rubber is a copolymer of isobutylene and I -2% isoprene. As a result the polymer chains contain internal double bonds which are expected to participate in cross-linking reactions. However, the overall molecular mass is expected to fall on irradiation due to the predominance of main-chain scission through the isobutylene units. Thus the radiation chemistry of the isoprene units within butyl rubber is accessible to study via solution NMR. In a comprehensive study Hill identified the primary free radical species by electron spin resonance spectroscopy at low temperatures, and the products of their subsequent reaction by C solution-state NMR. A number of new cross-link structures were identified and the mechanisms of cross-linking determined. Initial reaction involves addition of radicals either directly to the isoprene double bonds or to allyl radicals. Further addition of hydrogen atoms results in a mixture of fully-saturated and unsaturated cross-link structures. Cross-links of both H- and Y-type were identified and the yields of products agreed closely with the yields determined from measurement of changes in molecular weight on irradiation. 

Some perspective is necessary here. As indicated in Chapter 7, Section 4.1 on the Cope rearrangement, the free energy for formation of a cyclohexane-1,4-diyl is 50-53 kcal/mol and that for formation of two allyl radicals is roughly 57 kcal/mol. However, in the current system, the diyl is destabilized by roughly 20 kcal/mol due to the bicyclo[2.2.1]ring system that must be generated. Such a species is kinetically inaccessible due, in part, to a substantial negative entropy despite the fact that the activation enthalpy for its formation would appear to be 50-55 kcal/mol. 

Dole and co-workers have reported yields of alkyl free radicals in polyethylene irradiated at 77 K ranging from 2.7 to 3.7 (141,145,149). Furthermore, Cracco, Arvia, and Dole (49) reported that on warming, alkyl radicals decay by a first-order process, and they attributed this to reactions between alkyl radicals within isolated spurs. The persistent free radicals on warming to room temperature are the allyl radicals II. The impact of long-term stability of radical species on the stability of polyethylene has been underlined by studies of Jahan and co-workers (150-157) of ultrahigh molecular weight polymer used in medical implants. 

That the mechanism of allylic bromination is of the free-radical type was demonstrated by Dauben and McCoy, who showed that the reaction is veiy sensitive to free-radical initiators and inhibitors and indeed does not proceed at all unless at least a trace of initiator is present. Subsequent work indicated that the species that actually abstracts hydrogen from the substrate is the bromine atom. The reaction is initiated by small amounts of Br. Once it is formed, the main propagation steps are... 

Lewis acids such as SnCl4 also catalyze the reaction, in which case the species that adds to the alkenes is H2C —O— SnC. Montmorillonite KIO clay containing zinc(IV) has been used to promote the reaction. The reaction can also be catalyzed by peroxides, in which case the mechanism is probably a free-radical one. Other transition metal complexes can be used to form allylic alcohols. A typical example is. ... 

Evidence indicates [28,29] that in most cases, for organic materials, the predominant intermediate in radiation chemistry is the free radical. It is only the highly localized concentrations of radicals formed by radiation, compared to those formed by other means, that can make recombination more favored compared with other possible radical reactions involving other species present in the polymer [30]. Also, the mobility of the radicals in solid polymers is much less than that of radicals in the liquid or gas phase with the result that the radical lifetimes in polymers can be very long (i.e., minutes, days, weeks, or longer at room temperature). The fate of long-lived radicals in irradiated polymers has been extensively studied by electron-spin resonance and UV spectroscopy, especially in the case of allyl or polyene radicals [30-32]. 

Thermal insertion occurs at room temperature when R is XCH2CHAr-, at 40° C when R is benzyl, allyl, or crotyl (in this case two isomeric peroxides are formed), but not even at 80° C when R is a simple primary alkyl group. The insertion of O2 clearly involves prior dissociation of the Co—C bond to give more reactive species. The a-arylethyl complexes are known to decompose spontaneously into CoH and styrene derivatives (see Section B,l,f). Oxygen will presumably react with the hydride or Co(I) to give the hydroperoxide complex, which then adds to the styrene. The benzyl and allyl complexes appear to undergo homolytic fission to give Co(II) and free radicals (see Section B,l,a) in this case O2 would react first with the radicals. 

At the same time, delocalization of unpaired spin in the free-radical product appears to be important for the course of the substitution reaction. For example hydrogen shift in sabinene radical cation 39a leads to a conjugated system (40 ) nucleophilic attack on l-aryl-2-alkylcyclopropane radical cations 43 or 47 produces benzylic radicals nucleophilic attack on 39a generates an allylic species and attack on the tricyclane radical cations 55 or 56 forms tertiary radicals. Apparently, formation of delocalized or otherwise stabilized free radicals is preferred. 

Certain fuel components can be oxidized by the free-radical process. Benzyl, allyl, and tertiary compounds can all be oxidized at room temperature to yield hydroperoxide species. 

Problem 8.28 (a) Apply the MO theory to the allyl system (cf. Problem 8.26). Indicate the relative energies of the molecular orbitals and state if they are bonding, nonbonding, or antibonding, (b) Insert the electrons for the carbocation C,H, the free radical C,H, and the carbanion CjH, and compare the relative energies of these three species. 

Bromo- and chloro-iodinanes (29 and 31) behave as free radical halogenating agents (79JA3060). They give photoinitiated benzylic halogenation of toluene or allylic halogenation of cyclohexene in high yield. Cyclic 10-C1-3 and 10-F-3 species have not yet been reported. 

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