Naphthalene radical attack

In alternant hydrocarbons (p. 55), the reactivity at a given position is similar for electrophilic, nucleophilic, and free-radical substitution, because the same kind of resonance can be shown in all three types of intermediate (cf. 20,22, and 23). Attack at the position that will best delocalize a positive charge will also best delocalize a negative charge or an unpaired electron. Most results are in accord with these predictions. For example, naphthalene is attacked primarily at the 1 position by NOj, NHJ, and Ph, and always more readily than benzene. 

These results indicate that the naphthalene radical anion is not stable to the solvent tetrahydrofuran at room temperature on a time scale of 100 hours. Decomposition pathways are alkali metal dependent. Sodium and potassium naphthalene attack THF through a proton abstraction, cycloreversion mechanism, as previously described by Bates for the butyllithium/THF system (27). Lithium naphthalenide attacks the THF not only by the Bates mechanism but also by a nucleophilic ring opening, as is implicit in earlier high temperature work on lithium naphthalenide in THF (28) and in work on the attack of THF by tritylmagnesium bromide (29). The two smaller alkali metals, lithium and sodium, leave behind a... 

The most vexed subject in this field is the site of radical attack on substituted aromatic rings. Some react cleanly where we should expect them to. Phenyl radicals add to naphthalene 7.36, to anthracene 7.37 and to thiophene 7.38, with the regioselectivity shown on the diagrams. In all three cases, the frontier orbitals are clearly in favour of this order of reactivity (because of the symmetry in these systems, both HOMO and LUMO have the same absolute values for the coefficients). 

Electrophilic and nucleophilic substitution as well as radical attack occur predominantly in the a-positions [193]. Consequently, since PCN is commercially produced by chlorination of naphthalene the CNs formed are predominately chlorinated in the a-positions. For example, among the hexaCNs in Halowax 1014 the major compounds are 1,2,4,5,6,8-hexaCN (CN-71) and 1,2,4,5,7,8-hexaCN (CN-72) while the minor compounds are 1,2,3,4,6,7-hexaCN (CN-66) and 1,2,3,5,6,7-hexaCN (CN-67) [194,195], cf. Fig. 1. 

Acetylenic Compounds.— The reductive cyclization of acetylenic ketones with naphthalene radical-anions has been studied for a variety of 4,5-secocholest-3-yn-5-one analogues (139). Products vary with structural type three examples are illustrated in Scheme 5. Results are interpreted in terms of initial electron transfer from the reagent to form a ketyl radical-anion, followed by intramolecular radical attack on the triple bond. ... 

The most vexed subject in this field is the site of radical attack on substituted aromatic rings. Some react cleanly where we should expect them to. Phenyl radicals add to naphthalene (399), to anthracene (400)323 and to thiophene... 

Figure 9 drastically simplifies the major reaction paths of alkyl-naphthalene components. Via H-abstraction and successive decomposition reactions, they can easily form, either naphthalenes with unsaturated side chains (vinyl, allyl or alkenyl side chains) or RSR and smaller decomposition products. The preferential radical attack on the alkyl side chain is in the benzyl position due to the weak hydrogen bond. This makes it easy to justify either the formation of RSR or the successive / -decomposition reaction to form vinylnaphthalene. The net result of the successive recombination and condensation reactions of these aromatic species is the formation of PAH of increasing molecular weight with a progressively lower hydrogen to carbon ratio. 

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... 

The first reaction was found by Levy and Szwarc to be predominant when methyl radicals attacked isooctane. The second reaction is predominant, however, for aromatic hydrocarbons. The free radicals formed in the above two reactions will react with each other, with other free radicals, or with impurities. The affinity of the methyl radical to attack an aromatic increases in the following order benzene, diphenyl ether, pyridine, diphenyl, benzophenone, naphthalene, quinoline, phenanthrene, pyrene, and anthracene. The ability of free alkyl radicals to interact with isopropylbenzene and cyclohexene decreases in the following order methyl, ethyl, propyl, butyl, isopropyl, sec-butyl, and tertiary butyl. 

The lithium naphthalene has been replaced by an even more reactive lithium 4,4 -di-fer -butylbiphenylyl (LDTBB) or lithium dimethylaminonaphthalene (LDMAN). LDTBB not only is a more powerful reducing reagent but is less susceptible to radical attack in addition, it can be applied in a catalytic amount (5 mol %) by suspension of an excess amount of lithium powder in THF. For comparison, the use of LDMAN has the advantage of easy removal of DMAN by a dilute acid wash, although LDMAN may decompose when the reaction temperature is above -45°... 

Aromatic substitution reactions are often complicated and multistep processes. A correlation, however, in many cases can be found between the charged attacking species and the electron density distribution in the molecule attacked during electrophilic and nucleoph c substitution. No such correlation is expected in radical substitution where the attacking particles are neutral, rather a correlation between the reactivities of separate bonds and a free valency index of the bond order. This allows the prediction of the most reactive bonds. Such an approach has been used by researchers who applied quantum calculations to estimate the reactivities of the isomeric thienothiophenes and to compare them with thiophene or naphthalene. " Until recently quantum methods for studying reactivities of aromatics and heteroaromatics were developed mainly in the r-electron approximation (see, for example, Streitwieser and Zahradnik ). The M orbitals of a sulfur atom were shown not to contribute substantially to calculations of dipole moments, polarographic reduction potentials, spin-density distribution, ... 

To demonstrate the intervention of acetoxy radicals, an aromatic substrate (anisole or naphthalene) was added to the electrolyte and the reaction run under constant current conditions. The isolation of aryl acetates from this reaction was considered as evidence for the intermediacy of acetoxy radicals, the aryl acetate being formed via a homolytic attack of the acetoxy radical on the aromatic compound 4S"47). 

The outcome is more complex in the naphthalene series, where both the position of attack and the competition by rearomatization depend on the reagent chosen and conditions. Monocyanonaphthalenes are benzylated only by />-methoxytoluene, or by m- or />-methoxyphenylacetic acids, not by (poly)methylbenzenes [23,199]. 1,4-Dicyanonaphthalene reacts easily with toluene and a variety of precursors of the benzyl radical (alkylbenzenes [200-205], benzylsilanes, stannanes [206], and borates [56], as well as phenylacetic acids) [207] giving three types of products, viz. l-benzyl-4-cyanonaphthalenes, 1- or 2-benzyl-l,2-dihydro-l,4-dicyanonaphthalenes and dibenzobicyclo[3.3.1]nonanes resulting from the formation of a second carbon-carbon bond (Schs. 12 and 13). 

Solution-phase studies are more important preparatively. Two main mechanisms seem to operate in solution. The first is attack of the radical cation of a heteroaromatic donor on a tz nucleophile, as happens in the arylation reactions reported above. Other examples include photochemical reactions in which the heterocycle participates as a donor—for example the formation of 2- and 3-(l,2-diphenylethyl)-pyrroles (yield 44 and 10 %, respectively) from the irradiation of ( )-stilbene in the presence of pyrrole, a reaction which evidence implies is initiated by SET from pyrrole [88]. 2-(2, 2 -Diphenylethyl)furans are cleanly formed on irradiation of the corresponding furans in the presence of 1,1-diphenylethylene and an electron-accepting sensitizer [89]. Likewise, irradiation of naphthalene and benzothiophene in the presence of pyrrole results in electron transfer from the latter and leads eventually to pyrrolyldihydronaphthalene or benzothiophene, 44, respectively (Scheme 29) [90]. 

Formation by a radical addition reaction The presence of methyl and ethenyl groups in the products indicates that shock waves destroyed the structure of benzene and formed some lower-molecular-weight radicals (e.g., methyl and ethenyl radicals). If we assume Uiat the formation of a MeNap or PhNap molecule is a result of attack by a methyl or a phenyl radical against naphthalene molecule, respectively, the yield relations in isomers estimated from tire reactivity indices would be 1-MeNap >2-MeNap and 1-PhNap >2-PhNap. These relative amounts are inconsistent with Uiose of the shock products. Therefore, it is unreasonable to invoke a radical reaction only for the reaction of MeBip and PhNap. 

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