Methallyl radical

Attempts to polymerise isobutene by free radical catalysis have all failed [16,17] and copolymerisation experiments show that the t-butyl radical has no tendency to add to isobutene. The reasons for these facts are not at all obvious. Evidently, they cannot be thermodynamic and therefore they must be kinetic. One factor is probably that the steric resistance to the formation of polymer brings with it a high activation energy [17], and that the abstraction by a radical of a hydrogen atom from isobutene, to give the methallyl radical, has a much smaller activation energy. This reaction will also be accelerated statistically by the presence of six equivalent hydrogen atoms. 

Recently Burrell and Bhattacharyya (4) pulse radiolyzed a solution of allyl bromide in cyclohexane and observed a transient ultraviolet absorption band at 310 m/x which they attributed to the allyl free radical. Their optical technique did not permit them to study the absorption bands below 290 m/x. Another recent investigation was that of Callear and Lee (5) who flash-photolyzed gas mixtures of 1-butene with argon and isobutylene with argon. The most intense band observed by them in the case of 1-butene was at 225 m/x which they attributed to the allyl free radical in essential agreement with the work of Hamill et al. (II) mentioned above. For the isobutylene photolysis, the most intense observed band was at 238 m/x and this was assigned to the /J-methallyl radical, see Table I. 

Remarkably, when the syn- and a/i /-7-ethylidenebicyclo[3.2.0]hept-2-enes were heated, they both gave —25% more endo-6-mQthy 3,3-shift product. So it would appear that there must be a pathway for methallyl radical geometric equilibration. 

Free-radical multicomponent copolymerization of dialkylstannyl maleates or dialkylstannyl dimethacrylates with methallyl alcohol (or (J-hydroxyalkyl acrylates) and vinyl monomers (sryrene, methacrylic acid or methacrylamide) yields polymeric powders. Due to their storage and thermal stability and impact strength they are used as protective coatings 79). 

A superficially similar rearrangement has been observed upon treatment of ruthenium methallyl complex 163 with trimethylphosphine, which induces (reversible) methyl migration, providing /3,/3-dimethylruthenacyclobutane complex 117 in high yield (Scheme 37). The reaction is not considered to involve radical intermediates <1995JA3625, 1997JA11244>. 

A number of sulfur-centered radical scavengers have been employed for Meerwein type carbothiolation reactions [109, 110]. The most prominent of those are certainly xanthates [111-113] and thiocyanates, among which the latter have received special attention recently. As shown in Scheme 21, thiocyanates are well-suited for the functionalization of activated and non-activated alkenes [114, 115]. Remarkably, the reaction of 56 with 2-methallyl chloride to give 57 is not significantly impeded by the possible (3-fragmentation of a chlorine radical, which would lead to allylation products [116]. With an activated and a non-activated alkene present in a substrate... 

The radical cation of frawi-hex-2-ene cleaves at an allylic bond to give a resonance-stabilized methallyl cation, m/z 55. 

Radicals generated thermally with photochemical AIBN initiation from sugars containing halogen, phenylthio, phenylseleno, thiono-carbonate, or xanthate substituents can react with allyl- or methallyl-tributylstannane, leading to extended chain compounds, branched chain compounds and allyl C-glycosldes (e g. 

MA-allyl alcohol (Table 10.25) and MA-methallyl alcohol (Table 10.26) pairs have been combined with a variety of other donor-type monomers and polymerized with radical initiators. In all these cases, classical terpolymer copolymerization equations also failed to describe the resultant polymers. Sackman and Kolb " suggest their studies on MA-allyl alcohol (Table 10.25) or methallyl alcohol (Table 10.26) copolymerizations, with other donor monomers, supported the assumption that a CTC may participate in the propagation reactions. 

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