Radical paths reactions with neutrals

Recombination of the ion radicals within the cage is thought of as forming the path to rearrangement whilst escape of the radicals and subsequent reaction with the hydrazo compound leads to the formation of disproportionation products often observed. The theory is mainly directed at the two-proton mechanism and does not accommodate well the one-proton mechanism, since this requires the formation of a cation and a neutral radical, viz. 

The photochemical dimerization of p-methoxystyrene does not occur in non-polar solvents or in the absence of electron acceptors. This observation has been rationalized in terms of a reaction path in which the first step is photochemically induced electron transfer to give the styrene cation radical. This species reacts with a ground-state partner to give the cyclobutane cation radical which is then neutralized. 

DPB as well as other DPP molecules (t-stilbene, diphenyl-hexatriene) with relatively low ionization potential (7.4-7.8 eV) and low vapor pressure was successfully incorporated in the straight channel of acidic ZSM-5 zeolite. DPP lies in the intersection of straight channel and zigzag channel in the vicinity of proton in close proximity of Al framework atom. The mere exposure of DPP powder to Bronsted acidic ZSM-5 crystallites under dry and inert atmosphere induced a sequence of reactions that takes place during more than 1 year to reach a stable system which is characterized by the molecule in its neutral form adsorbed in the channel zeolite. Spontaneous ionization that is first observed is followed by the radical cation recombination according to two paths. The characterization of this phenomenon shows that the ejected electron is localized near the Al framework atom. The reversibility of the spontaneous ionization is highlighted by the recombination of the radical cation or the electron-hole pair. The availability of the ejected electron shows that ionization does not proceed as a simple oxidation but stands for a real charge separated state. 

One electron transfer from the highest filled MO of a neutral substrate 170 (Eq. (236) ) to the anode yields a radical cation 171 as product. This may be either a transient intermediate or a stable, long-lived species depending on its substituents and the nucleophilicity of the solvent. The reaction paths of radical cations have been expertly and comprehensively reviewed by Adams 2 5 2 9 so that a short summary seems sufficient at this place. Deprotonation and 1 e-oxidation of 171 with a subsequent Sj l reaction of the resulting cation yields side-chain substitution products 172 (path a), see 9.1. Solvolysis of 171 followed by le-oxidation... 

As illustrated in Fig. 1, one prominent path followed by electrogenerated cation radicals involves loss of a proton to form a neutral radical (R). Such radicals are easier to oxidize than the initial alkene. This second one-electron oxidation produces a carbocation (R - e R" ). If the electrochemical oxidation is carried out in an inert solvent such as dichloromethane, the carbocation is highly reactive. Many useful reactions have been carried out using carbocations generated in this manner (Fig. 4) [1, 6, 7]. Sometimes the carbocation may be unstable at temperamres near ambient, decomposing before it can react with an added reactant. Concern over this problem has led to the very useful concept known as the cation pool method in which the electrochemical reaction is carried out at a low temperature (typically 78 °C) at which the carbocation is stable. The resulting solution is known as the cation pool. 

The activation energies for the reaction LVI- LVII calculated by the MINDO/3 and MNDO methods amount, respectively, to 34 and 31 kcal/mol, while the experimental value for this transformation lies within 7-18 kcal/mol and for neutral cyclobutene it is 33 kcal/mol [47]. The calculations [97] have shown that a more advantageous reaction path is represented by the two-step mechanism LVI-> LVIIILVII. The MINDO/3 and ab initio (6-3IG basis set) methods predict, in good agreement with each other, the energy barrier of the rearrangement of LVI into the cyclopropylcarbinyl cation-radical to be 20-21 kcal/mol and a mere 2.6 kcal/mol for the step LVIII- LVII. 

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