Radical cations allylic

The authors, on the basis of their experimental results, pointed out that olefin radical cations-allylically methylated and/or stabilized by heteroatoms or n conjugation [136] do not react with an active oxygen species, suggesting, in spite of their low oxidation potentials, that several factors may contribute to determine this behavior. Thus, one of the important factors is the presence of an independent 7t-system in which odd-electron density is not delocalized, whereas, a second and a third factor can be the insufficient steric hindrance to block the attack of oxygen... 

The addition of a free radical to the carbon-carbon double bond of an allylic group that forms part of an onium ion can induce disintegration of the onium salt, thus giving rise to the release of an inert compound and a reactive radical cation. Allylic compounds employed for this purpose are presented in Chart 10.6, and the reaction mechanism for a typical case is presented in Scheme 10.14 [55]. 

Are the carbon-carbon bond distances in allyl cation, allyl radical allyl anion all similar, or are they significantly... 

Allyl cation, allyl radical and allyl anion differ in the number of electrons contained in a nonbonding 7i-type orbital, the LUMO in the cation and the HOMO in the radical and anion. 

Allyl (27, 60, 119-125) and benzyl (26, 27, 60, 121, 125-133) radicals have been studied intensively. Other theoretical studies have concerned pentadienyl (60,124), triphenylmethyl-type radicals (27), odd polyenes and odd a,w-diphenylpolyenes (60), radicals of the benzyl and phenalenyl types (60), cyclohexadienyl and a-hydronaphthyl (134), radical ions of nonalternant hydrocarbons (11, 135), radical anions derived from nitroso- and nitrobenzene, benzonitrile, and four polycyanobenzenes (10), anilino and phenoxyl radicals (130), tetramethyl-p-phenylenediamine radical cation (56), tetracyanoquinodi-methane radical anion (62), perfluoro-2,l,3-benzoselenadiazole radical anion (136), 0-protonated neutral aromatic ketyl radicals (137), benzene cation (138), benzene anion (139-141), paracyclophane radical anion (141), sulfur-containing conjugated radicals (142), nitrogen-containing violenes (143), and p-semi-quinones (17, 144, 145). Some representative results are presented in Figure 12. 

The energy level diagram including electron populations for the allyl radical, cation, and anion can be shown as illustrated in Figure 5.16. The orbital diagram and energy levels for the allyl system is shown in Figure 5.17. 

I FIGURE 5.16 The energy level diagram for the allyl radical, cation, and anion species. 

In type A reactions one electron is removed from one of the two double bonds to form a cation radical, and allylic substitution and oxidative addition take place as the following reactions. On the other hand, in type B reactions the initial electron transfer from the double bond is accompanied by a transannular reaction between the two double bonds. 

The second is that the initiation consists of the addition to the monomer of some or all of the cationic species thus formed to give the growing carbenium ion (I) (reaction (8)). Even if the first part is proved, the second part must be tested separately, for it is possible, though at present it seems unlikely, that the cations in the initiator solution could generate cations from the monomer by reactions other than the addition reaction (8). For instance, they could generate an allylic cation (II) by abstracting H" (reaction (12)), or they could form a radical-cation (III) by abstracting an electron (reaction (13)), from the monomer ... 

In a rare example of the use of phenylselenides as radical precursors in the generation of alkene radical cations by the fragmentation approach, Giese and coworkers generated a thymidine C3/,C4/ radical cation by expulsion of diethyl phosphate. Trapping experiments were conducted with methanol and with allyl alcohol (Scheme 16), when nucleophilic attack was followed by radical cyclization [66]. 

A series of N-allyl sulfamates, phosphoramides, and phosphorimidates was prepared to explore the possibility of O- N rearrangements via the intermediacy of the contact alkene radical cation/anion pair, followed by 5-exo-trigonal radical cyclizations (Fig. 4) [142],... 

The carbamates 327 (R R2 Pr or Bu) react with t-butylthiol, malonic acid and ethoxyethylene under irradiation to yield the (ethoxyethyl)amines 333 via the radical cations 332. The A-allyl analogues 334 (R = Pr or PhCH2) cyclize to pyrrolidines 335382. 

In this context, the striking difference of the standard 70 eV El mass spectra of the isomeric cyclooctadienes may be mentioned here (see below)87. Whereas the radical cations of the stereoisomeric 1,5-cyclooctadienes, containing two bis-allylic C—C bonds, give the products of the two-fold allyl cleavage as the base peak ([C4H6]+ /[M]+ s=ss 10 1), the cis,cis-l,4 and cis.cis-].3-cyclooctadicnc ions are reluctant to do so (IC4II511 / [M]+ 1 3). Clearly again, 1,2- and 1,3-H shifts cannot efficiently compete with dissociation of the bis-allylic C—C bond. 

Allylic CH bonds Aliphatic alkenes frequently undergo allylic substitution by oxidation of the double bond to a radical cation that undergoes deprotonation at the allylic position and subsequent oxidation of the resulting allyl radical to a cation, which finally combines with the nucleophiles from the electrolyte [21, 22]. The selectivity is mostly low. Regioselec-tive allylic substitution or dehydrogenation is, however, found in some cases with activated alkenes, for example, -ionone that reacts to (1) (Fig. 5) as a major product [23], menthone enolacetate that yields 90% (2) [24], and 3,7-dimethyl-6-octen-l-ol... 

Acetoxylation proceeds mostly via the radical cation of the olefin. Aliphatic alkenes, however, undergo allylic substitution and rearrangement predominantly rather than addition [224, 225]. Aryl-substituted alkenes react by addition to vic-disubstituted acetates, in which the dia-stereoselectivity of the product formation indicates a cyclic acetoxonium ion as intermediate [226, 227]. In acenaphthenes, the cis portion of the diacetoxy product is significantly larger in the anodic process than in the chemical ones indicating that some steric shielding through the electrode is involved [228]. 

A wide application of Newcomb s method provides a variety of N-heterocyc-lic systems, such as perhydroindoles, pyrrolizidines and aza-brigded bicycles [59, W, 146], The mild reaction conditions are compatible with several funtional groups of the substrate and several trapping agents to functionalize the cyclized product. 2-Substituted pyrrolizidines 132 are accessible by tandem cyclization of iV-allyl-substituted PTOC carbamate 131. In this case the allyl group on the nitrogen serves as an internal trap for the intermediate carbon radical. The Af-methylcyclohept-4-enaminium radical cation, produced from the corres-... 

Small changes in the solvent or in the conditions of oxidation can lead to changes in the electronic and molecular structure of aryldiazo radical cations, from a linear allylic 7T- to a bent a-radical state. Both states have been observed in the radical cations of diphenyldiazomethane (49) and 5-diazo-10,ll-dihydro-5//-dibenzo[a, /]cycloheptene... 

In 1993, Blatter and Frei [34] extended the Aronovitch and Mazur [28] photo-oxidation into zeolitic media, which resulted in several distinctive advantages as described below. Irradiation in the visible region (633 nm) of zeolite NaY loaded with 2,3-dimethyl-2-butene, 16, and oxygen resulted in formation of allylic hydroperoxide, 17, and a small amount of acetone. The reaction was followed by in situ Fourier-transform infrared (FTlR) spectroscopy and the products were identified by comparison to authentic samples. The allylic hydroperoxide was stable at - 50°C but decomposed when the zeolite sample was warmed to 20°C [35]. In order to rationalize these observations, it was suggested that absorption of light by an alkene/Oi charge-transfer complex resulted in electron transfer to give an alkene radical cation-superoxide ion pair which collapses... 

Electrochemical oxidation of alkenes results in the removal on one electron from the alkene function to give a 7t-radical-cation where the electron deficiency is delocalised over tire conjugated system. The majority of alkene radical-cations cannot be characterised because they readily lose an allylic proton in aprotic sol-... 

---