Allyl radicals trapping

In PE, these trapped radicals have been identihed as, mainly, alkyl and allyl radicals with the stmctures (—CH2CHCH2—) and (—CH—CH=CH—) [134,135]. In the presence of oxygen, the polymeric radicals will react to form diperoxides and hydroperoxides, as well as certain amount of less stable peroxy radicals (—CH2OO ). 

The allyl radical [115] trapped in an argon matrix can be photolytically (A = 410 nm) converted into the cyclopropyl radical [116] (Holtzhauer er a/., 1990). Dicyclopropane and cyclopropane were formed when the photolysed matrix was warmed from 18 to 35 K. The intermediate [116] was shown to be a cr-type (Cs symmetry) and not a rr-type symmetry) radical. Normal coordinate analysis of the radical [116] has been carried out and the IR band at 3118 cm has been assigned to the stretching vibration of the C—H bond at the radical centre. 

Recently, we have demonstrated another sort of homogeneous sonocatalysis in the sonochemical oxidation of alkenes by O2. Upon sonication of alkenes under O2 in the presence of Mo(C0) , 1-enols and epoxides are formed in one to one ratios. Radical trapping and kinetic studies suggest a mechanism involving initial allylic C-H bond cleavage (caused by the cavitational collapse), and subsequent well-known autoxidation and epoxidation steps. The following scheme is consistent with our observations. In the case of alkene isomerization, it is the catalyst which is being sonochemical activated. In the case of alkene oxidation, however, it is the substrate which is activated. 

The dependence of relative rates in radical addition reactions on the nucleophilicity of the attacking radical has also been demonstrated by Minisci and coworkers (Table 7)17. The evaluation of relative rate constants was in this case based on the product analysis in reactions, in which substituted alkyl radicals were first generated by oxidative decomposition of diacyl peroxides, then added to a mixture of two alkenes, one of them the diene. The final products were obtained by oxidation of the intermediate allyl radicals to cations which were trapped with methanol. The data for the acrylonitrile-butadiene... 

Finally, allyl radicals have successfully been employed in macrocyclization reactions, in which the slower rate of reaction of allyl radicals with hydrogen donors turned out to be advantageous46. Thus, radical 11 cyclizes in 1 A-endo mode to provide, after trapping with tin hydrogen, the product 12 as a fi -mixture of the C2/C3 double bond. No products derived from 6-exo or 10-exo cyclizations could be found (equation 8). This can be rationalized by assuming a faster rate of addition of the nucleophilic allyl radical to the electron-deficient terminal double bond than to the C6 or CIO double bonds. 

TABLE 12. Examples for the trapping of allyl radicals with thiols and tin hydrides... 

Only a few examples exist for the intermolecular trapping of allyl radicals with alkenes68,69. The reaction of a-carbonyl allyl radical 28 with silyl enol ether 29 occurs exclusively at the less substituted allylic terminus to form, after oxidation with ceric ammonium nitrate (CAN) and desilylation of the adduct radical, product 30 (equation 14). Formation of terminal addition products with /ram-con figuration has been observed for reaction of 28 with other enol ethers as well. 

Intramolecular trapping of allyl radicals by carbon-carbon double bonds has, of course, been observed to occur readily and with high selectivity (see Sections in and IV). 

The trapping of allyl radicals with other open-shell species can be studied in all reactions in which a sufficiently high concentration of radicals is created and in which the concentration of nonradical trapping agents is low. This prerequisite has been met in Kolbe electrolysis reactions, in which radicals are generated by one-electron oxidation of carboxylate anions. One of the simplest systems, the reaction of methyl radicals with... 

The mechanistic proposal of rate-limiting hydrogen atom transfer and radical recombination is based on the observed rate law, the lack of influence of CO pressure, kinetic isotope effects [studied with DMn(CO)s] and CIDNP evidence. In all known cases, exclusive formation of the overall 1,4-addition product has been observed for reaction of butadiene, isoprene and 2,3-dimethyl-l,3-butadiene. The preferred trapping of allyl radicals at the less substituted side by other radicals has actually been so convincing that its observation has been taken as a mechanistic probe78. 

In summary, it appears that the trapping of allyl radicals with closed-shell trapping agents is quite selective, especially in those cases in which the allyl radical contains one substituted and one unsubstituted terminus. Trapping with radicals appears to produce mixtures of isomers, especially in the dimerization of allyl radicals. The observed regio-selectivities do, however, depend on the reaction conditions, allowing for some control of the reaction outcome for a given substrate. 

Further variations of the general scenario described in Scheme 4 consist in trapping adduct radical 48 before oxidation occurs7. This can be achieved if intramolecular radical additions are possible, as is the case in radical 62. Oxidation of 62 to the corresponding allyl cation is slower than 6-ew-cyclization to the chlorobenzene ring to form radical 63, which subsequently is oxidized to yield tetrahydronaphthalene 64 as the main product (equation 27). This sequence does not work well for other dienes such as 2,3-dimethyl-1,3-butadiene, for which oxidation of the intermediate allyl radical is too rapid to allow radical cyclization onto the aromatic TT-system. 

Iron(II) salts, usually in conjunction with catalytic amounts of copper(II) compounds, have also been used to mediate radical additions to dienes91,92. Radicals are initially generated in these cases by reductive cleavage of peroxyesters of hydroperoxides to yield, after rearrangement, alkyl radicals. Addition to dienes is then followed by oxidation of the allyl radical and trapping by solvent. Hydroperoxide 67, for example, is reduced by ferrous sulfate to acyclic radical 68, which adds to butadiene to form adduct radical 69. Oxidation of 69 by copper(H) and reaction of the resulting allyl cation 70 with methanol yield product 71 in 61% yield (equation 29). 

This scheme can be extended by using mixtures of dienes with electron-deficient alkenes such as acrylonitrile. Due to its nucleophilic nature, addition of radical 68 to acrylonitrile is faster than addition to butadiene. The resulting ambiphilic adduct radical then adds to butadiene to form a relatively unreactive allyl radical. Oxidation and trapping of the allyl cation by methanol lead, as before, to products such as 72 and 73, which are composed of four components the radical precursor 67, acrylonitrile, butadiene and methanol (equation 30)17,94. 

The allyl alcohol trapping reaction was further studied by Crich and coworkers, who applied the Barton decarboxylation reaction as radical source,... 

The dimer 352 of 351 was isolated from the product mixtures of two experiments conducted to trap 351 by alkenes, one with 350 and the other with 354 as substrate. Although no cycloadduct with the alkene was observed in one case, the yield of 352 amounted to only 0.8%. Nevertheless, the structure of 352 is interesting, since it suggests that the tetramethyleneethane diradical assumed to be the intermediate undergoes ring closure preferentially between two different allyl-radical termini. 

If photolyzed with light of the intensity I, HBr adds to propadiene (la) in the gas phase with a rate given by v=kexp[HBr]I<). This transformation affords within the detection limit (GC) 2-bromo-l-propene (5a) as sole reaction product (Table 11.1). The conversion of methyl-substituted allenes, such as lc and If, under these conditions follows the same kinetic expression [37]. Results from competition experiments indicate that the reactivity of an allene towards HBr increases progressively with the number of methyl substituents from propadiene (la) (= 1.00) to 2,4-dimethylpenta-2,3-diene (If) (1.65). In all instances, Br addition occurs exclusively at Cp to furnish substituted allyl radicals, which were trapped in the rate determining step by HBr. 

The phenylselenyl radical adds irreversibly to the central carbon atom of 2-methylbuta-l, 2-diene (Id) with a rate constant of 3 x 106 M-1 s-1 (23 1 °C) (Scheme 11.7) [45], On a synthetic scale, PhSe addition to cumulated Jt-bonds has been investigated by oxidizing phenylselenol with air in the presence of mono-, 1,1-di- or 1,3-di-substituted allenes to provide products of selective fi-addition. Trapping of 2-phenyl -selenyl-substituted allyl radicals with 02 did not interfere with the hydrogen atom delivery from PhSeH (Scheme 11.7) [31]. 

Addition of the dicyanomethyl radical to propadiene (la) occurs exclusively at Q (not shown in Scheme 11.8) [60]. On the other hand, methyl-substituted allenes, e.g. Id, undergo /3-selective reactions with 2-bromomalodinitrile (15). The significant /3-selectivity has been associated with the steric demand of the incoming radical 16, which favors addition to the sterically least hindered site at the diene Id to provide allylic radical 17. However, it seems likely that a stabilization of an intermediate allylic radical, e.g. 17, by methyl substituents contributes significantly to the observed regioselectivity of product formation. Trapping of intermediate 17 with bromine atom donor 15 proceeds at the least substituted carbon to afford allylic bromide 18. 

The 5-dig-mode of cyclization has been applied in the synthesis of N-heterocycles. For example, treatment of the /i-allenyl dithiosemicarbazide 37 with Bu3SnH and AIBN in hot benzene furnishes the substituted 3H-pyrrole 38 in 41% yield and the isomeric heterocycle 39 in 30% yield (Scheme 11.13) [68], Iminyl radical 40 is formed via Bu3Sn addition to the thiocarbonyl group of the radical precursor 37 and fragmentation of the adduct (not shown). Nitrogen-centered radical 40 adds 5-dig-selectively to provide substituted allyl radical 41. The latter intermediate is trapped by Bu3SnH to furnish preferentially product 38 with an endocydic double bond. 

Naturally, homometallic molybdenum compoimds cannot model the first step in the catalytic cycle—the primary H atom abstraction, which undoubtedly proceeds with bismuth participation (Bi = O, Bi - O - Bi, Bi - O - Mo) [ 11 ]. Bi might be involved in another step, too it is not clear yet whether the allyl radical generated is trapped by a Mo = 0, a Mo - O - Mo or else by a Mo - O - Bi moiety [ 11 ]. In so far it seemed of course very interesting to gain access to molecular Mo - O - Bi compoimds in order to see how those behave and whether any conclusions can be drawn for the corresponding surface sites. 

A second important reaction observed in irradiated polyethylene and certain other polymers as well as paraffins is the formation of trans-vinylene. This product is far less influenced by such factors as temperature or radical scavengers than is crosslinking, and it is therefore often assumed to be formed directly by a molecular process—i.e., molecular detachment of hydrogen. Even this simple explanation cannot be fully sustained. Long after radiation has ceased, the frarw-vinylene concentration continues to rise or fall, whether the specimen is exposed to oxygen or not. This behavior may possibly be correlated with the reaction of trapped alkyl and allylic radicals, which, being slightly mobile, can add to frarw-vinylene or each other over a period of days. 

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