Of allyl radical

We attributed the decreased bond dissociation energy in propene to stabilization of allyl radical by electron delocalization Similarly electron delocalization stabilizes benzyl rad ical and weakens the benzylic C—H bond... 

The chain propagation step consists of a reaction of allylic radical 3 with a bromine molecule to give the allylic bromide 2 and a bromine radical. The intermediate allylic radical 3 is stabilized by delocalization of the unpaired electron due to resonance (see below). A similar stabilizing effect due to resonance is also possible for benzylic radicals a benzylic bromination of appropriately substituted aromatic substrates is therefore possible, and proceeds in good yields. 

The spectroscopy, structure, photochemistry, and unimolecular reactions of allyl radical have been studied extensively and reviewed recently.145 Possible dissociation channels of allyl radical, their energetics, and the potential energy barriers of the C3H5 system are shown in Figs. 20 and 21.145,146... 

Fig. 20. Possible dissociation channels of allyl radical and their standard heats of formation relative to allyl. The loss of H2 generally proceeds via a high activation barrier and is thus considered unlikely. (From Fischer et ai.14B)...

The reactivity of allyl radicals does, however, appear to be sufficient for intramolecular radical reactions. In a systematic study, Stork and Reynolds investigated the feasibility of allyl radical 5-exo cyclizations41. It was found that cyclization proceeds readily for a variety of systems, especially for those with geminal 3,3-diester substitution. Mixtures of c/s/fraws-cyclopentanes are formed as the major products, while 6-enclo cyclization is hardly observed42. Allyl radicals behave in this respect much like alkyl radicals43. Cyclization is not even hindered by the presence of substituents at the attacked carbon... 

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... 

Allyl radicals can, of course, also be generated by electrolysis of the corresponding /J,y-unsaturated carboxylic acids together with a second carboxylic acid. This mixed Kolbe electrolysis method has been used to study the recombination of allyl radical 32 with the undecyl radical 3370. Recombination leads to the formation of adducts 34 and 35 in a ratio of 72 28, again preferring the product with the higher substituted double bond (equation 16). 

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. 

A number of earlier investigations of the regioselectivity in the reaction of allyl radicals with other radicals has been plagued by severe analytical problems41,80. 

The regioselectivity in the dimerization of allyl radicals has been studied by a variety of methods. One of the earliest investigations into this field employed the Kolbe electrolysis... 

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. 

Reviewing now the last four sections, it is obvious that the major problem in radical chain reactions involving dienes or polyenes is the low reactivity of the diene (or polyene) adduct radicals. This allows for the occurrence of allyl radicals in intramolecular reactions but poses a major problem in intermolecular radical chain reactions. The obvious solution to this problem is to use methods in which radicals are produced stoichiometrically and not... 

The concentration of copper(II) has a pronounced effect on the course of the reaction. In the presence of very low copper(II) concentrations, oxidation of allyl radical 69 is slow and major amounts of allyl radical dimer are formed. In the presence of very high concentrations of copper(II), radical 68 is oxidized rapidly before addition to diene can take place. An optimum yield of product 71 can therefore only be achieved at certain copper(II) concentrations. The metal-ion-promoted addition of chloramines to butadiene appears to follow the same mechanism93. 

Another early acidity investigation of propene by the thermodynamic method involved the determination of the electron affinity of allyl radical by photodetachment from allyl anion34. Extrapolation of the data to a photodetachment threshold gave an electron affinity (EA) of allyl radical of 0.55 eV which, combined with a bond dissociation energy of allyl-H of 89 kcalmol-1, gave A//ac d = 390 kcalmol-1. 

Schafer reported that the electrochemical oxidation of silyl enol ethers results in the homo-coupling products. 1,4-diketones (Scheme 25) [59], A mechanism involving the dimerization of initially formed cation radical species seems to be reasonable. Another possible mechanism involves the decomposition of the cation radical by Si-O bond cleavage to give the radical species which dimerizes to form the 1,4-diketone. In the case of the anodic oxidation of allylsilanes and benzylsilanes, the radical intermediate is immediately oxidized to give the cationic species, because oxidation potentials of allyl radicals and benzyl radicals are relatively low. But in the case of a-oxoalkyl radicals, the oxidation to the cationic species seems to be retarded. Presumably, the oxidation potential of such radicals becomes more positive because of the electron-withdrawing effect of the carbonyl group. Therefore, the dimerization seems to take place preferentially. 

Cope rearrangement. This is a 3, 3 sigma-tropic rearrangement. Here there is an interaction between a pair of allyl radicals. 

The homo of allylic radicals with several carbon atoms can be depicted as follows ... 

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