Carbocations radical cations

In electroorganic synthesis, radical ions preponderate as useful intermediates (Scheme la). Radical cations can, in subsequent deprotonations and further electron transfers (ET), be transformed into carbocations or vice versa as radical anions can be converted by protonation and further reduction into carbanions. Carbanions can also be generated by reductive cleavage of R-X (X = Cl, Br, I, OTos), and subsequent reduction of the intermediate radical R (Scheme Id). Radical anions can also be used as elec-trogenerated bases (ECBs, see Chapter 14) for the deprotonation of C-H bonds to form carbanions. 

By analogy with their behavior in mass spectrometry, branched hydrocarbons are cleaved when oxidized in CH3 CN/TEABF4 at —45 °C. The resulting acetamides of the fragments (Table 6) are formed by cleavage of the initial radical cation at the C,C bond between the secondary and tertiary C atom, to afford after a second electron transfer, carbocations, which react in a Ritter reaction with acetonitrile [29]. 

The reaction occurs by oxidation of the CH bond to a radical cation that is de-protonated to a radical. This is further oxidized to a carbocation that reacts with the nucleophiles in the electrolyte. The regiochemistry is controlled by inductive deactivation (—I-substituents) as well as by activation (+I-substituents), which leads to a reactivity tert.H > sec.H > prim.H. In steroids, a preferential adsorption appears to play a role. 

Chemoselective oxidation of 3-arylsul-fenylmethyl-A -cephem affords 3-meth-oxymethyl-A -cephem in an anodic substitution. From the two thioethers, the arylthio group is more easily oxididized to a radical cation, that then undergoes cleavage to a thiyl radical and a carbocation (Fig. 31) [152]. 

An anodic azacyclization, producing tropane-related 11-substituted dibenzo[a,d]cycloheptimines 123, was recently developed by Karady et al. [136, 137]. This two-electron process is initiated by anodic oxidation of the O-substituted hydroxylamine 119 in nucleophilic solvent. It is proposed that the first one-electron oxidation leads to the aminium radical cation 120 which adds rapidly to the double bond. The electron-rich carbon radical 121 is readily oxidized to the carbocation 122. Selective nucleophilic attack on 122 from the less hindered exo-side yields the 11- substituted product 123. Depending on the... 

Oxidation of aliphatic ketones in trifluoroacetic acid leads to hydrogen abstraction by the carbonyl oxygen radical-cation fonning a carbon radical, then further oxidation of the radical to the carbocation and migration of this centre along the carbon chain by a series of hydride transfer steps. Long chain ketones yield a mixture of alcohol trifluoToacetates by reaction of the carbocation centres with the solvent [3]. 

A cation in which significant positive charge is located on at least one carbon atom, itself having an even number of electrons. Both carbenium ions and pentavalent species (such as the methanonium ion, CH5+) are carbo-cations. However, radical cations and carbynium ions are not considered to be carbocations. See Carbenium Ion Bridged Carbocation Carbonium Ion... 

The actual species responsible for cationic polymerizations initiated by ionizing radiation is not established. The most frequently described mechanism postulates reaction between radical-cation and monomer to form separate cationic and radical species subsequently, the cationic species propagates rapidly while the radical species propagates very slowly. The proposed mechanism for isobutylene involves transfer of a hydrogen radical from monomer to the radical-cation to form the r-butyl carbocation and an unreactive allyl-type radical ... 

The expressions (Eqs. 5-34 and 5-42) for Rp in cationic polymerization point out one very significant difference between cationic and radical polymerizations. Radical polymerizations show a -order dependence of Rp on while cationic polymerizations show a first-order depenence of Rp on R,. The difference is a consequence of their different modes of termination. Termination is second-order in the propagating species in radical polymerization but only first-order in cationic polymerization. The one exception to this generalization is certain cationic polymerizations initiated by ionizing radiation (Secs. 5-2a-6, 3-4d). Initiation consists of the formation of radical-cations from monomer followed by dimerization to dicarbo-cations (Eq. 5-11). An alternate proposal is reaction of the radical-cation with monomer to form a monocarbocation species (Eq. 5-12). In either case, the carbocation centers propagate by successive additions of monomer with radical propagation not favored at low temperatures in superpure and dry sytems. 

A number of interesting correlations have been made on the basis of Koopmans theorem.107 However, it is important to recognize that a cation is formed in the process, and that the factors that lead to stabilization of ordinary carbocations are equally as important with radical cations. Thus, for example, hyperconjugation is important with these ions just as it is with ordinary carbocations. 

In this, the adsorbed organic substrate is electrochemically oxidised to form a radical cation, a proton is eliminated in a chemical step to form a radical, which is then further electrochemically oxidised to a carbocation, which reacts with a fluoride ion in the double layer to produce the fluoroorganic compound. 

The proposed mechanism of oxidative fluorination of unsaturated compounds by halogen fluorides [84-86] and VF5 [33] includes electron transfer from substrate to halogen fluoride or VF5 as a first step, followed by addition of F" to a radical-cation, leading to formation of a radical and its further oxidation to carbocation (see Eq. 12, pathways A,B). It should be pointed out that this is not the only direction, and the actual mechanism may depend strongly on the substrates and reaction conditions. Other mechanisms, such as a radical process (pathway C), cannot be ruled out. 

Anodic oxidation of alkanes is a viable alternative means of generating ej-radical cations from alkanes [23], In contrast with oxidations with short-lived photoexcited species, electrooxidation of the substrate absorbed on the anode involves two consecutive ET steps - oxidation of the alkane then deprotonation to an alkyl radical (Eq. 8) and further oxidation of the alkyl radical to a carbocation (Eq. 9). 

L. Eberson, M. P. Hartshorn, F. Radner, Electrophilic Aromatic Nitration Via Radical Cations Feasible or Not , in Advances in Carbocation Chemistry (J. M. Coxon, Ed.) JAI, Greenwich, CT, 1995. 

Most carbocations have a three-bonded carbon atom with six paired electrons in its valence shell. The radical cation just shown is not a normal carbocation. The carbon atom has seven electrons around it, and they bond it to four other atoms. This unusual cation is represented by the formula [CH4]+, with the + indicating the positive charge and the indicating the unpaired electron. 

The first one electron oxidation produces a radical cation on the sugar phosphate (SP +). The radical cation subsequently deprotonates yielding a neutral carbon centered radical SP(-H). The second oxidation involves an electron transfer from SP(-H) to a nearby guanine radical cation G +. This step requires that the hole on the guanine have some mobility. It is known that a hole located on guanine at 4K is mobile, with a range of ca. 10 base pairs [76], The result of this second oxidation is a a deoxyribose carbocation SP(-H)+. 

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