Cations Formed by Electron Transfer

The conversion of free radicals by electron transfer into carbocations and subsequent initiation of the cationic polymerization has recently been reviewed The electron transfer process 

This s em initiates polymerization of THF giving, upon irradiation, a high polymer (M — 5.0 10 ). 

In the second group of systems diarylhalonium salts, failing to initiate polymerization generate the carbenium salts by decomposition induced by free radicals formed photochemk Benzophenone, benzil or maleic anhydride were used as sources of free radicals  

Depending on the structure of the carbocation formed and monomer used, initiation can proceed either by hydride abstraction ) or by direct addition. 

The photochemical decomposition of sulfonium or diarylhalonium salts can also lead to the formation of protonic acids subsequently initiating polymerization )  

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The intermediacy of a cation, formed by electron transfer within a photochemically created radical pair, was also invoked to explain the results obtained upon photolysis of 2-bromo-, 2-chloro- and 2-iodopyridine in methanol, ethanol and acetonitrile-water376. The major products are 2-methoxypyridine, 2-ethoxypyridine and 2-acetamidopyridine + 2-hydroxypyridine. In all cases pyridine was the minor reaction product, in contrast with the 3- and 4-halopyridines which produce pyridine exclusively, via a radical process. It is proposed that the unshared electron pair on the nitrogen atom assists in the formation of the 2-pyridyl cation. The presence of cupric salts increases the relative amounts of products formed via ionic reactions because Cu2+ can accept an electron from the 2-pyridyl radical. 

In polar solvents the excited state of sufficiently electron deficient arenes will accept an electron from donors. The fates of the radical ion pairs produced include formation of products of addition to the arene ring. A new example of this mode of reactivity is the photochemical reaction of 1,4-dicyanonaphthalene with benzyl methyl ether in acetonitrile. This yields stereoisomers of the addition product (120). The reaction most likely involves electron transfer from the ether to the naphthalene excited state and subsequent ionisation of a proton from the benzyl ether radical cation. This produces a benzyl ether radical which adds to the naphthalene derivative. An analogous sequence is proposed to explain the photochemical formation of (121)-(124) from ultra-violet light irradiated solutions of naphthalene-1,2-dicarboxylic acid anhydride in methanolic benzene or acetonitrile containing isobutene, 2-butene or 2-methyl-2-butene. Here it is suggested that the alkene radical cation, formed by electron transfer to the excited state of the naphthalene, is attacked by methanol deprotonation... 

Several groups have studied photoreactions of dicyano-aromatic compounds with alkylbenzenes as the electron donors [8]. Efficient proton transfer from the benzylic position of the alkylbenzene radical-cation, formed by electron transfer to excited DCN, to the counter anion (DCN -) is reported to produce a benzylic radical and... 

For practical applications, initiator systems functioning on the basis of dye reduction are most important. Scheme 10.7 illustrates how free radicals are formed with the aid of a co-initiator of the tertiary amine type. In this case, the amino radical cation, formed by electron transfer, loses a proton to give an a-aminoalkyl radical, which initiates the polymerization. 

Oxidative Polymerization Reactions. Clays can initiate polymerization of unsaturated compounds through free radical mechanisms. A free radical R", which may be formed by loss of a proton and electron transfer from the organic compound to the Lewis acid site of the clay or, alternatively, a free radical cation, R+, which may be formed by electron transfer of an electron from the organic compound to the Lewis acid site of the clay, can attack a double bond or an aromatic ring in the same manner as an electrophile. The intermediate formed is relatively stable because of resonance, but can react with another aromatic ring to form a larger, but chemically very similar, species. Repetition of the process can produce oligomers (dimers, trimers) and, eventually, polymers. 

In order to measure the absorption spectra, the radical anions were generated electrochemically in the optical path of a spectrophotometer. The absorption spectrum of 3,5-dinitroanisole radical anion (Figure 11, curve c) is very similar to that of the 550-570 nm species produced photochemically. So we believe this species to be the radical anion formed by electron transfer from the nucleophile to the excited 3,5-dinitroanisole and decaying by interaction with its surroundings including the nucleophile radical cation. The behaviour described seems to be rather general for aromatic nitro-compounds since it is observed with a series of these compounds with various nucleophilic reagents. 

Some electroinitiated polymerizations proceed via monomer radical-cations (VII) formed by electron transfer... 

The positive and negative ions formed by electron transfer are held together strongly by the force of electrostatic attraction. They may however separate to form free, solvated ions in polar solvents and then various secondary reactions can take place. Radical cations often undergo cycloadditions with the neutral (Figure 4.9). 

The photonucleophilic substitution of the monochloroanisoles in alcoholic solvents had been studied earlier378 and it was then concluded that 4-chloroanisole reacts via a radical anion formed by electron transfer from the solvent to an excited molecule, whereas 3-chloroanisole undergoes substitution via an aryl cation. In another study379, it was found, on the basis of quenching and sensitization experiments and on the basis of the ratios of... 

Radical ions are created in solution by chemically or electrochemically induced electron transfer to or from a conjugated ir-system. Even if these ions are thermodynamically stable they are only of limited persistence since they are susceptible to reactions with electrophiles and nucleophiles or undergo other processes like dimerization or electron-transfer induced bond cleavage [9, 10]. Pairs of radical anions and radical cations can also be formed by electron transfer between neutral donors and acceptors either in the ground state or upon photochemical excitation [11, 12]. 

In contrast to the radical cations of strained-ring cycloalkanes, the cyclopentane radical cation, c-CsHio , formed by electron transfer to radiolysis-induced holes in halocarbon matrices, had a simpler spectrum. A triplet with uh = 2.5 mT (2H) was attributed to a localized species with Cj symmetry. The unpaired electron was assigned to a W-shaped cr-orbital, involving C5-C1-C2, and the two equatorial protons at C5 and C2 [80, 88, 89]. At temperatures above 77 K, all ring protons become equivalent, most probably as a result of processes such as ring inversion, or pseudo-rotation around the C5-axis [89]. 

Tin-carbon bonds can be broken by reaction with electrophiles (e.g. protic acids, Lewis acids, halogens), nucleophiles (e.g. RLi), or free radicals (e.g. succimidyl, t-butoxyl), or with certain transition metal (particularly palladium) compounds. Fragmentation can also be induced through the radical cations which are formed by electron transfer. 

For unsubstituted stilbene, azastilbenes, and naphthylethylenes the CT interaction involves only singlet states as initial species. Introduction of a nitro group makes triplet states accessible for electron transfer [513], Because of the larger value of tt as compared to rs, smaller donor concentrations are required for electron transfer in the triplet state. The same Stern-Volmer constants for quenching of 4>, c and 1/tt (or l() indicate that trans - cis photoisomerization and electron transfer compete. This was also found when a positive charge was introduced by quaternization of 4-R-azastilbenes (A+, R = nitro or cyano) [170,489], but not for compounds with R = dimethylamino [171]. Under certain conditions (e.g., in a solvent of medium polarity such as dichloromethane and for X- = I"), a radical pair (A. . X ) is produced by excitation of the ion pair (Figure 20b) [172,489]. The same (neutral) radical can be formed by electron transfer from the amine, (e.g., DABCO) to the cation of a quarternary salt of 4-R-azastilbene in the case R = nitro the electron is transferred to the triplet state, in competition with trans- cis photoisomerization (Figure 21). 

The development of solid-state polymerization was largely due to the use of high-energy radiation such as y-rays 115,123), x-rays 124>, electron beams 125,126) or a-partic-les ll6). y-Rays are used most frequently. It is generally accepted that y-ray-induced polymerization of TXN proceeds by a cationic mechanism. Ions or radical-ions are formed by electron transfer from TXN, the loss of hydrogen atoms or the heterolytic cleavage of the ring 127>. 

Photoreduction of the herbicide paraquat dichloride in aqueous propan-2-ol is more efficient in the presence of a sensitizer such as benzophenone than on direct irradiation.84 Hyde and Ledwith84 propose that the paraquat cation radical is formed by electron transfer from ketyl radicals, in turn produced during the conventional photoreduction of the sensitizer ketone. The suggested mechanism is given in reactions (23)—(25), where PQ2+ is the paraquat dication. The reduction process therefore involves chemical sensitization, rather than electronic energy transfer. 

Photosubstitutions. Selective control of reactivity at different positions of the aromatic ring included in the CD cavity has been obtained in the nucleophilic photohydroxylation and photocyanation of fluoroanisoles (FA) by Liu and Weiss [308,309]. The reaction (see Scheme 27 for 4-FA) involves the FA" " radical cation intermediate formed by electron transfer between a triplet and a ground state molecule. Tables 19 and 20 list the relative quantum yields of the two photoreactions in mixtures of bound and free 4-FA and 2-FA. Both reactions appear to be almost totally inhibited in the... 

The radical cation of (163) can be formed by electron transfer photochemistry and in methanol addition products are formed. A study of the photochemical reactivity of some novel 3-phenylnorbomadienes has been reported. Bichromo-phoric norbomadiene derivatives have also been synthesized and studied photo-chemically. A study of the photoisomerization of some norbomadienes has been carried out within the constrained environment of P-cyclodextrin. The bichromophoric system (164) undergoes intramolecular electron transfer by a through-bond mechanism on irradiation. The transfer is from the benzidine... 

The cycloaddition of l,l-bis(2-thienyl)ethylene (prepared in situ from the ethanol (169) with strong electron acceptors such as TCNE or ddq has been shown to proceed via a radical ion pair formed by electron transfer (Scheme 29) <90H(3i)i873>. The reaction with TCNE is rapid and quantitative, taking place at room temperature in 15 min. With ddq, the initial product (170) is further oxidized to (171). When the ethanol (169) is heated alone in the dark at 150°C, it generates the aromatized cycloadduct (173) in 70% yield. Other minor products possibly result from a radical process <91CB1203>. On the other hand, irradiation of the alcohol (169) generates l,l-bis(2-thienyl)ethylene cleanly, which is subsequently transformed to the cycloadduct (172). A radical cation may be implicated in this photochemical [4 + 2] cycloaddition also. 

The transition state is formed by electron transfer from a C-CH bond into an empty p-orbital of the reacting cation. A similar mechanism can be used to describe 1,3-methyl shift as well as 1,2-ethyl migration ... 

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